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Motor anti-reverse-rotation device failure (sliding pin type)
10

Motor anti-reverse-rotation device failure (sliding pin type)

Motor anti-reverse-rotation device failure (sliding pin type)

(OP)
Machine Description: Outdoor vertical motor (700hp, 900rpm) driving a pump through a rigid coupling. Rated torque = 5250*700/900 ~ 4100 ft-lbf. Locked rotor torque is probably around 8000 ft-lbf.

What happened: Motor tripped during start on time overcurrent (51 relay). On-site inspection revealed motor would not rotate by hand, even after uncoupled. Sent to repair shop who provided attached photos showing a damaged anti-reverse-rotation device.

Overview of anti-reverse-rotation device operation: Slide 1 is an overview of the parts of the device. Principle of operation: During start, the vertical pins slide up the ramp on the stationary ratchet plate which pushes them into the holes in the rotating pin-holder where they are held I place against the side of the hole by centrifugal force (and friction). When the motor slows down, the pins fall down onto the stationary ratchet plate again but a portion also remains in the hole. If the motor rotates reverse then the pin is pushed backwards against the flat vertical portion of the ratchet plate preventing reverse rotation. By the way normal rotation is CW viewed from the top. There are 12 equally spaced ratchet ramps and 6 equally spaced pins (why are there 12 holes... there were originally 6 holes but during a previous refurbishment they were found damaged so 6 alternate holes were drilled at that time... that previous repair was associated with reverse rotation damage to the holes and the vertical faces of the ratchets found during proactive refurbishment which had never caused any problems with motor operation in the plant).

Inspection Findings: The photos show that one out of 6 pins is broken roughly in half (not much deformation, maybe a brittle failure?). One pin hole has a wallowed entry (on the side that would suggest that damage occurred while rotating forward with pin stuck on ramp)[revised to remove incorrect info]. One ramp is severely damaged. The vertical portion of another ramp is slightly damaged at the top.

The repair shop’s conclusion is that the pin bound in the hole due to rust and lack of clearance (0.005” clearance on a pin which is maybe 0.5” diameter and 1” long). They plan to increase clearance to 0.020”. They also suggested using stainless steel pins to address the rust, but as far as I can tell from the photos the rust is mostly on the pin holder rather than the pins (I’ll ask them about that). I asked if parts could be lightly coated in oil to reduce rust. They said lubrication is not typically used since it would attract/retain dirt/dust. I suggested some kind of dry powder lubricant and they said they’d consider it.

MAIN QUESTION: Based on the info above, do you have any ideas about the cause of the failure or what are the proper actions to prevent recurrence (I'm preparing for a phone call with the shop Friday 1/22 to finalize our repair plan).

My questions for the shop (not necessarily important for responders to this post). To prepare for my phone call I made a list of questions. This is not the main part of my post, I'm just listing it here because it's convenient for me. You can add questions or weigh in on my queestions if you like, but you don't have to read them (all the important stuff is above):
1 - Is there rust on the pins? (I don't see it). What can we do about rust in the pin holes?
2 - What is the silver pattern on bottom of the pin holder slide 2
3 - Slide 4 why does the brown rub pattern seem to be further out at a larger radius on the ramp and then closer in at a smaller radius on the flat?
4 - Is the rub mark on the non-damaged stationary ramps normal / expected outside of a failure?
5 - What about the slightly damaged vertical wall slide 5 at 10:00 position (two to the left of the obvious damaged ramp). It looks as if the pin was starting to fall and caught the top of the vertical wall and then pin pushed up out of the way. Is that normal?
6 - What materials are the pins, pin-holder, ratchet plate? Are any of these parts painted? (what explains the colors on the ratchet plate).
6a. Regarding pin material - Is it expected to have a brittle looking failure without much pin deformation
6b [new]Were the materials changed during the last repair? Is the stationary ratchet plate now harder such that more of the reverse impact is transferred to the pin?
7. How much clearance is between the top of the ratchet and bottom of the pin holder
8 What are exact dimensions of the pin (I said 0.5" x 1" but that's just my guess). And what is the height of the stationary ramp?
9. If we lined up the pin's plane of the failure with the bottom of the pin holder, where would the bottom of the pin be... resting on the bottom between ramps of resting in the middle of the ramp where the damage is or somewhere else.
10. are those clearances per side or diameteral.
11. You'd think the pin breaking would allow the motor to successfully start. Did the pin break before trip but still slowed it down enough to trip?
12. What is the radius of curvature of the bottom corners of those pins? Would a larger radius of curvature be beneficial to help the pin slide smoothly up the ramp during start?
13. Could the proposed larger clearance allow the pin to tilt enough to score the inside of the holes during starting? Maybe larger radius of curvature on top corner of the pins would help prevent that? And also radius of the hole entry?


miscellaneous note - Since the are exactly twice as many ratchets as pins, all six pins will contact at the same time (assuming dimensions are precise enough to permit them all to contact). Other designs have different odd combinations of pins and ratchets which means the pins wouldn’t all contact at the time (one would hit first to stop motion) but that type of staggered design has the advantage that it significantly limits the arc available for reverse windup before contact is made and is therefore probably a better approach in terms of stresses during stopping reverse rotation. But the distinction doesn't seem important because it appears the damage here occurred during forward rotation.

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(2B)+(2B)' ?

RE: Motor anti-reverse-rotation device failure (sliding pin type)

Just for thought, the anti-rotation should engage immediately the motor comes to a stand-still - possibly the pump started reverse rotation before the pins engaged, ie the pins were in the up position and slow to engage allowing the pump / motor to run in reverse, resulting in very high load on the pins when they finally engaged - - maybe only 1 or 2 engaged.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

RE: Motor anti-reverse-rotation device failure (sliding pin type)

(OP)
> the pins were in the up position and slow to engage allowing the pump / motor to run in reverse, resulting in very high load on the pins when they finally engaged - - maybe only 1 or 2 engaged.

Thanks Artisi. Damage that occurs during start / forward rotation would be on the ramp (where we see most of our damage as in slide 5 12:00). Damage that occurs during reverse rotation would be on the vertical wall (where we see only minor damage slide 5 10:00).

A DETOUR to talk about damage we saw during a previous refurbishment which was fully repaired long before the current event. In that previous refurbishment the location of the damage was on the vertical wall and indicates there was at one time more stress from stopping reverse rotation than the device could handle. photo attached to the current post shows that previous since-repaired damage.

But let's set aside that previous failure (long ago repaired) and focus on the recent event and recent findings:

The first indication of a problem was during start (although maybe that was not time of initial failure).

The main damage we see to the stationary ratchet plate in the current damage is on the ramp (indicating forward rotation) as slide 5 12:00. Although there is also a little bit of less-severe damage on the top of a vertical wall of the stationary ratchet plate (slide 5 10;00 position) which presumably would have occurred during reversing. Maybe that could somehow cascade to later produce the rest of the damage during a subsequent start? I don't rule it out but I can't visualize the sequence of events that would be involved.


The location of the damage on the pin-holder hole (slide 3, remembering normal forward rotation of this pinholder piece would have been CW from top = CCW when viewed from the bottom which is the side this photo is taken from) all suggest that the damage occurred during forward rotation / start*
  • * Note my assumption above in analysing pinhole damage was that the damage occurred while the pin was still intact and in the hole... but maybe there are other scenarios like the broken pin had already broken and damaged the hole on it's way out in which case it may have been pivoting and the top of the pin could have caused damage at that location during reverse rotation... but that scenario seems unlikely to me because I would have expected additional damage on the pivot point at the opposite side of the hole in that case (ovalized hole entrance). Further analysis of height of the pin fracture relative to the other parts (questions 7/8/9) might give a clue where the pin was at the time it broke... it may be that the height of the break suggests pin had to be fully dropped at the time it broke which could argue for damage during reverse rotation rather than while climbing up the ramp in forward rotation. Also I'd think a careful inspection of additional marks inside of that hole would distinguish these two scenarios
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(2B)+(2B)' ?

RE: Motor anti-reverse-rotation device failure (sliding pin type)

Quote (electricpete)

it appears the damage here occurred during forward rotation
I have seen pump shaft broken presumably during pump start while on reverse rotation. Here also it appears brute force was applied on ratchet pins and it broke as a result.
I apprehend that there is a leakage in the discharge check valve so there is certain amount of reverse torque available due to reverse flow before start. As the pump starts, initially motor overcomes this reverse torque. After it starts rotating, there is a high acceleration as it experiences almost no discharge pressure till the back side of NRV becomes pressurized.
During the high acceleration period(fraction of a second) the pin has insufficient time to slide back and may have created a notch due to the impulse which prevents further sliding. As considerable amount of water is getting accelerated, the torque is also high acting on a single pin, which results in breakage.
In order to prevent this I suppose the leakage across NRV to be minimized. Also it is safer to use all 12 pins available in design which reduces the probability of failure. It also may be worthwhile to lubricate the pins and ramps.
It is also to be noted that sliding of pin over ramp may be prevented by presence of surface defects. So the surfaces should be checked periodically.

Engineers, think what we have done to the environment !https://www.linkedin.com/in/goutam-das-59743b30/

RE: Motor anti-reverse-rotation device failure (sliding pin type)

(OP)
> I apprehend that there is a leakage in the discharge check valve so there is certain amount of reverse torque available due to reverse flow before start.

My understanding is there is no check valve or NRV on this machine. We don't use motor anti-reverse rotation devices except on pumps that don't have those (which are generally low speed high flow low dp pumps attached to pipes which are very large, maybe too large for check valve to be practical). The parallel pumps were in operation at the time of our failed start attempt.

> Also it is safer to use all 12 pins available in design which reduces the probability of failure.

I'll discuss it with shop, but original design was 6 pins, 12 ramps. Double the pins might increase wear on those ramps or have some other unintended effect.

> It also may be worthwhile to lubricate the pins and ramps. It is also to be noted that sliding of pin over ramp may be prevented by presence of surface defects. So the surfaces should be checked periodically.

Thanks. I'm definitely with you on the role of lubrication for preventing binding and corrosion (mentioned it in the op). Periodic inspection is something for us to consider.

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(2B)+(2B)' ?

RE: Motor anti-reverse-rotation device failure (sliding pin type)

(OP)
Are there any other ideas / comments?

I have my phone call a little over an hour from now and we will have to finalize our repair plans for this motor at that time.

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(2B)+(2B)' ?

RE: Motor anti-reverse-rotation device failure (sliding pin type)

(OP)
Phone call is done. There was not a lot of time for all my questions. We will proceed with the same repair plan as above (widen the pin clearance, use stainless steel pins possibly larger diameter, use dry lubricant.

I did find out the pinhole that has the obvious damage was one of the unused pinholes... so that pinhole damage was leftover from before, not the recent event. Also the pins are apparently 1.5 - 2" long. The shop said that close examination of the failed pin showed that it had fine crystal structure for the outer 1/8" and coarser grain structure inside, so they think it was made from case hardened dowel rod, and that may have contributed to the brittle failure.

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(2B)+(2B)' ?

RE: Motor anti-reverse-rotation device failure (sliding pin type)

I don't think stainless steel is a wise choice here. Are you going to be using a hardened stainless steel? Is it at least as hard as the existing pin? Stainless steel is extra sensitive to fretting and galling. Drilling the holes oversized and sleeving with oil impregnated bronze bushings would be a better solution.

I suspect fretting is an issue because you mentioned a pin being bound by rust. I see no corrosion (pitting) present in the pictures so the rust could have been the product of fretting corrosion. A bronze or Nitronic (xx) bushing will solve this.

I also suggest milling the holes as opposed to drilling. A drill doesn't provide as precise a location so one pin may end up carrying more load if it contacts first.

Better yet, make the holes square and use square pins. This will give a better bearing surface to carry the load. Drill the corners as connect the dots. This will save you from having to repair the already damaged holes.

RE: Motor anti-reverse-rotation device failure (sliding pin type)

(OP)
Thanks TugBoatEng, those are great comments.

I'll ask about the stainless pin hardness. Harder is less susceptible to galling I gather. But also the difference in hardness between two materials is important, right? I think stainless steel galling is more prevalent when both materials are stainless steel than when stainless is combined with other steel. I'm not very familiar with how to prevent galling and I'm open to any more comments on all of that. At any rate we will use some kind of dry lubricant like moly disulfide or grpahite in isopropyl alchohol to help reduce concerns about galling.

I'm not familiar with oil impregnated brass bushings. Googling shows me they are made with a special process to produce a structure that is porous to allow oil in. I wonder if that structure is still ductile like bronze of if it becomes brittle / susceptible to cracking ? It would need to be able to stand up to a pretty good shock during stopping reverse rotation. Bronze or nitronic bushing are other options to look at. I don't think I have enough time to be able to change the plan for the current motor repair, but it'll be something to look at for the next motor.

I agree precise hole spacing and ratchet spacing is absolutely critical to have a prayer of load sharing in this design (other designs with staggered odd numbers of pins and ramps don't rely on sharing load but still benefit from the other pins in limiting the worst case reverse windup arc). It is interesting that in the photo of a previous damage the pins had deformed the vertical wall part of the stationary ratchet plate. It seems there was some ductile deformation going on that may have ended up helping to equalize the loading (regardless of any previous spacing mismatch on the stationary parts) and also perhaps the deformation energy absorbed a bit of the shock so the pins didn't see as much (those previous pins didn't break). I don't know if that was part of the design intent or not. That previous damage did NOT cause any operational problems... it was only found during proactive refurbishment. The same ratchet plate was repaired by weld buildup and re-used and now we ended up with the pin failure. Maybe the weld repair was not as ductile as original metal (although of course the leading theory is still that the current damage occurred during start so if that's true then it wouldn't be relevant to the current damage).


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(2B)+(2B)' ?

RE: Motor anti-reverse-rotation device failure (sliding pin type)

I think the failure initiated during the previous anti-reversal. The pin broke in the notch before the one with the gouge. The broken pin kept the remains from dropping completely out of the hole. During the start, as the broken pin advanced to the next slot the remains dropped out of the hole and that's when it wedged and bound. It must have happened this way as this would allow the motor to gain some momentum before being abruptly stopped to create this amount of damage.

RE: Motor anti-reverse-rotation device failure (sliding pin type)

(OP)
Thanks. I don't rule out that it occurred during reverse yet but I'm still leaning against that theory. If that were the case you'd think the pin would show some distinctive damage pattern on it like sliding smudge across that fracture surface (or on the side of the pin fragment if it had tipped over). Also during start there is a large electromagnetic torque (4100 ft-lbf) present even without the additional torque from rate-of-change of momentum (in contrast to stop/reversing when the rate of change of momentum is the sole source of torque). But I still have skepticism that we really understand the failure scenario and the factors that contributed. I've asked for some additional vertical measurements (7/8/9) to help piece together the actual failure scenario... let's say when the pin fractured the elevation of that fracture was at the bottom of the pin-holder (there's not much clearance between bottom of rotating pinholder and top of stationary ramp), then work down from that to figure out where the bottom of the pin would've been at the time it failed (is it on the ramp or at the bottom of the ramp against the reverse wall stop). If it ends up that the elevation of the failure would've been inside the hole assuming the bottom of the pin was at the middle of the ramp, then that would seem to contradict the theory of failure during start (I don't think the pin would break at a location that was inside the hole at the time of failure).


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(2B)+(2B)' ?

RE: Motor anti-reverse-rotation device failure (sliding pin type)

Do notice that the gouge is not centered in the ramp. The pin was not in the hole when it jammed up.

RE: Motor anti-reverse-rotation device failure (sliding pin type)

I looked again at "previous since-repaired damage" photo, and cannot believe the amount of damage shown on the vertical face, you can also see damage on the up-ramp where the retaining pin has gouged into the face. Seems the pump unit was running backwards (more than once) prior to the pins dropping into place. Also see the same on the latest failure although not as bad. To me it seems the pins are being held up and very slow to drop, possible made worse as the speed increases (in reverse) the centrifugal force is also assisting - would also look to rounding the end on the bottom of the pins.

added: the pins should drop immediately the pump comes to rest and prior to reverse rotation starting - that's assuming there is some back flow, unlikely not to be - would except there is a pause before the change of direction - back flow needs to develop enough torque to start rotation, same as start-up torque at which time the pin/s should have engaged

So what's holding the pins up?

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

RE: Motor anti-reverse-rotation device failure (sliding pin type)

I think the engaging pins should have a spring assisted mechanism to ensure faster engagement.
Also it is strange not to have a discharge check valve. A controlled check valve can prevent water hammer as well as reduce the reverse rotation or when there is a anti reverse rotation mechanism, it can reduce the severity of impact on the same.

Engineers, think what we have done to the environment !https://www.linkedin.com/in/goutam-das-59743b30/

RE: Motor anti-reverse-rotation device failure (sliding pin type)

electricpete

is it possible to display the actual assembly with 2D cad view in pdf. I having difficulty understanding how this coupling assembles.
problem I see the pins are still engaged while reversing. thus catching the tips of the pins. from previous small designs there is some type of spring
to retain engagement but it is freely allowed to overcome the spring and the jaws of two ratchet faces to disengage with out damage.
normally if it does not disengage properly there is a shear neck on a shaft that is designed to to shear at a predetermined torque.

problem I see here is I don't view the other mating face ratchet that engages with the one shown. the assembly is not dis in gaging properly.
the hole may need counter sinks that allow full disengagement.
edit rounded spherical heads on the pins would also help

RE: Motor anti-reverse-rotation device failure (sliding pin type)

mfgenggear :this is not a coupling, it's a hold-back device to stop reverse rotation of the pump / drive motor, in forward operation there is no connection between the hold-back and pins.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

RE: Motor anti-reverse-rotation device failure (sliding pin type)

Electricpete :,
Question, why is the face of the rotating pin holder and the ratchet hold-back "blued" from high temperature as well as showing signs of rotating contact?

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

RE: Motor anti-reverse-rotation device failure (sliding pin type)

The blue color is layout fluid.

RE: Motor anti-reverse-rotation device failure (sliding pin type)

Tugboat:having looked again on the comp. rather than the phone, you are right, but why, there is no setting out etc on these components?

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

RE: Motor anti-reverse-rotation device failure (sliding pin type)

(OP)
> Do notice that the gouge is not centered in the ramp.

I noticed that too... on all the ramps both the current damage and the previous damage. I think the geometry of the ramp is tricky (it's not something that you would intuitively select if you were going to build this thing yourself). If you were to draw a line of "straight up-hill" (highest elevation gradient) on these ramps, I think that line would not be pointing directly circumferential. Instead it would be pointing somewhere between circumferential and radial. Due to that funky orinetation of the ramp, only the outer part of the pin makes contact with the ramp on the way up the ramp. Then when the pin gets to the level top of the ramp, a larger area of the pin centered on the pin axis makes contact and the mark is further inwards.

What is the purpose of having the ramp slanted in that crazy way? I'm not sure.

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(2B)+(2B)' ?

RE: Motor anti-reverse-rotation device failure (sliding pin type)

(OP)

Quote (Artisi)

I looked again at "previous since-repaired damage" photo, and cannot believe the amount of damage shown on the vertical face, you can also see damage on the up-ramp where the retaining pin has gouged into the face. Seems the pump unit was running backwards (more than once) prior to the pins dropping into place. Also see the same on the latest failure although not as bad. To me it seems the pins are being held up and very slow to drop, possible made worse as the speed increases (in reverse) the centrifugal force is also assisting - would also look to rounding the end on the bottom of the pins.

added: the pins should drop immediately the pump comes to rest and prior to reverse rotation starting - that's assuming there is some back flow, unlikely not to be - would except there is a pause before the change of direction - back flow needs to develop enough torque to start rotation, same as start-up torque at which time the pin/s should have engaged

I don't have a full answer what was going on in the previous photo. That previous photo represented about 15 years of motor service whereas the recent photo represents only 3 years.

With 6 pins and 12 ramps, it is possible to get a full (360/12=) 30 degree windup in the reverse direction between when the pin starts dropping and hits the vertical wall (which creates more momentum for the device to stop). In contrast if you had an odd combination let's say 7 pins and 12 ramps then I think it would be something like 30/7 ~ 4 degrees worst case windup I think. The 6 pins / 12 ramps has the advantage that it's supposed to share load, but does it really? I mentioned the deformation of the stationary parts would help compensate for uneven spacing there, but it wouldn't compensate for uneven spacing of the rotating pin holes. Maybe those pins are way stronger / harder than the stationary parts and the single pin in the worst case hole just kept beating up whatever stationary wall it landed on. New set of holes was drilled last repair 3 years ago to address that one damaged hole so maybe that problem was "fixed" at that time? (wishful thinking, I don't know). Why was new set of holes drilled during refurbishment three years ago... it was because of that one damaged hole. So does that damaged hole give a clue what was going on with that device before the previous damage? (I'll have to ask again if there was pin damage found during the previous refurbishment but I don't think there was). It's strange because there was never any operational evidence of problem prior to that previous refurbishment.

There are 4 pumps, two small (700hp motor) and two large (4000hp motor) in parallel. There can be many different combinations in service when this motor is switched to off (and it's also pumping water uphill so there would be reversing even if no parallel pumps running). Since the damage is on all the ratchets (not just half that engage with pins during a given stop) we can't attribute that previous damage to a single event... must have been multiple.

> So what's holding the pins up?

As we know centrifugal force creates the normal force of pin in hole, which gives friction. I suppose rough surface of pin or hole could increase friction to slow dropping (so the motor has to drop to lower speed before it begins dropping). The pin itself is likely to get rougher over time during this service, the hole seems to be susceptible to corrosion. But it would have to be all 6 pins, which seems maybe less likely. At any rate, the dry lubricant we will add to minimize friction while the pin is travelling up in the hole has the happy side effect to also minimize friction while the pin is dropping down.

I do wonder what happens if the pin is only partially dropped when it makes contact (since all 6 pins are somewhat synchronized they might all be only partially dropped when they make contact during a given event). They could in theory all be 1/8" down below top of the ratchet when they make contact (in which case they might just rip the top of the ratchet a little and perhaps move back up) or 1/4" down or 1/2" down. I think I can see some evidence of this happening by looking at the stationary ratchet damage but I'm not sure.


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(2B)+(2B)' ?

RE: Motor anti-reverse-rotation device failure (sliding pin type)


Quote"
Artisi
mfgenggear :this is not a coupling, it's a hold-back device to stop reverse rotation of the pump / drive motor, in forward operation there is no connection between the hold-back and pins." Unquote

o thanks for the clarification.

RE: Motor anti-reverse-rotation device failure (sliding pin type)

I do see that the ramps are rubbed outboard of centerline which doesn't really mean much but the actual impact from the pin is not inline with the normal path so it could not have been in the hole at the moment of impact. In other words, it was already broken.

RE: Motor anti-reverse-rotation device failure (sliding pin type)

Is the anti-reverse clutch provided by the motor manufacturer ?

I'd be thinking about buying a new A-R clutch assembly, and maybe the latest model upgraded from all the lessons learned.

There should be maintenance info ( including pin lubrication recommendations ) in somebody's O&M manual.
As other said "Rebuilding" the busted one with drilled holes just ain't right.

RE: Motor anti-reverse-rotation device failure (sliding pin type)

(OP)
> I do see that the ramps are rubbed outboard of centerline which doesn't really mean much but the actual impact from the pin is not inline with the normal path so it could not have been in the hole at the moment of impact. In other words, it was already broken.

One thing I said wrong before is that the mark on the top level surface of that ratchet plate is centered upon the pin axis... it is not. You can see where the pins sit by looking for the circles on the ratchet plate where the pins have been resting while stationary... that location is closer to the outside than to the inside. Then why are the marks on the top closer to the inside? I can only guess maybe that top portion of the stationary ratchet plate which is level in the circumferential direction is tilted outward in the radial direction to the pin makes contact on the inner side (although I can't detect that in the pictures). It looks like the circular damage in the ramp is roughly centered at the same radius where the circular mark from the pins resting is, although the angle of the photos makes it tough to judge. I'll ask for a photo taken from directly above the inverted ratchet plate (if they haven't started cleaning it up yet) so it is easier to judge those types of things.





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(2B)+(2B)' ?

RE: Motor anti-reverse-rotation device failure (sliding pin type)

(OP)

Quote (Tmoose)

Is the anti-reverse clutch provided by the motor manufacturer ?

Yes.

Quote (Tmoose)

I'd be thinking about buying a new A-R clutch assembly, and maybe the latest model upgraded from all the lessons learned.
There should be maintenance info ( including pin lubrication recommendations ) in somebody's O&M manual.

The motor OEM is Electric Products - long ago out of business (*). The manual includes a drawing that shows the ratchet, but there are no specific instructions related to it (most of the instructions are more generic).

* Edit I see now the motor has multiple nameplates including EP and Continental. Some of the documentation comes from EP who we bought the motor from and some from Continental (I think Continental subcontracted to EP). Electric Products is out of business but Continental is still in the motor business in New Jersey. I'll try reaching out to them.

Quote (Tmoose)

As other said "Rebuilding" the busted one with drilled holes just ain't right.

I agree the design seems dodgy, and the failure shortly (3 years) after repair casts a lot of suspicion on that last repair. What is it specifically you don't like about drilling a new set of holes assuming their position is well controlled? (Or are you just pointing out an apparent lack of root cause analysis).


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(2B)+(2B)' ?

RE: Motor anti-reverse-rotation device failure (sliding pin type)

I would be careful about oversizing the holes compared to pin diameter to minimize pin cocking within hole. I would chamfer/round the hole and pin edges to minimize friction and improve engagement. I would not use a dry lubricant, but instead consider a good synthetic grease. I have used this product on shaft couplings.
http://powerupusa.net/index.php/products/power-up-...

Did any the pins or hole surfaces show any signs of fretting or chatter that may be caused by torsional vibrations? Did the pin or hole surfaces show any signs of EDM from stray current that could increase sliding friction?

Walt

RE: Motor anti-reverse-rotation device failure (sliding pin type)

(OP)

Quote (mfgenggear)

rounded spherical heads on the pins would also help

I've been wondering about what role the radius at the corner of the pin plays. Large radius (or spherical head) reduces contact area. But is that a good thing? I'm not sure (friction force depends on normal force and coefficient but not contact area in an idealized scenario). Unless there is an obvious answer it seems like we should stick with OEM design.

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(2B)+(2B)' ?

RE: Motor anti-reverse-rotation device failure (sliding pin type)

(OP)
> I would be careful about oversizing the holes compared to pin diameter to minimize pin cocking within hole.

Pin diameter is something like 1", and it currently has 0.005" per side and they're going to 0.020" per side. The length of engagement of pint within the hole I don't know yet but I think it's at least 0.5" at its minimum when dropped. It's hard to visualize cocking in this scenario. On the other hand it's also very hard to visualize the scenario that resulted in the pin getting "stuck" during this failure. The increased clearance can also result in a slightly larger tilt of the pin which may have implications for the sliding behavior of the bottom of the pin on the ramp (and also more uneven sliding velocity as it goes up the ramp if pin is rocking within hole during that period). I don't have experience with selecting this type of clearance but I'm inclined to go with my repair shop recommendation on this unless there is compelling information not to.

> I would chamfer/round the hole and pin edges to minimize friction and improve engagement.

Discussed pin above. Hole edge radius is a similar consideration.

> I would not use a dry lubricant, but instead consider a good synthetic grease.

I don't think this is a good application for grease at all. When it's time for the pins to drop, there's not a lot of force pushing them down, just gravity. It is very easy for me to imagine a grease-filled clearance just keeping the pin up in the hole. If it happens to all 6, I don't want to be standing nearby.

=====================================
(2B)+(2B)' ?

RE: Motor anti-reverse-rotation device failure (sliding pin type)

"I don't think this is a good application for grease at all. When it's time for the pins to drop, there's not a lot of force pushing them down, just gravity. It is very easy for me to imagine a grease-filled clearance just keeping the pin up in the hole. If it happens to all 6, I don't want to be standing nearby."

I am thinking of a grease surface coating and not filling with a lot of grease. The Thixogrease product has a very soft consistency (like hand cream) that stays on surface well, and it appears to act like a EP-rated coupling grease without breakdown/separation over time or with spinning/centrifuge.

Walt

RE: Motor anti-reverse-rotation device failure (sliding pin type)

(OP)

Quote (electricpete)

On the other hand it's also very hard to visualize the scenario that resulted in the pin getting "stuck" during this failure

Maybe pin getting stuck in the hole when dropping down under gravity is more likely failure then pin getting stuck in the hole as the ramp pushed it up.

So a new proposed scenario: during last stop the pin didn't drop fully down (maybe it got stuck 1/2" off the bottom), then when the motor was started the pin got a running start and slammed into the side of that ramp, which created different dynamics unfavorable to pin sliding up into hole.

Quote (strong)

I am thinking of a grease surface coating and not filling with a lot of grease. The Thixogrease product has a very soft consistency (like hand cream) that stays on surface well, and it appears to act like a EP-rated coupling grease without breakdown/separation over time or with spinning/centrifuge.
Maybe that's something we'll consider if we ever decide to add springs above the pins to help force them down as someone else suggested.

=====================================
(2B)+(2B)' ?

RE: Motor anti-reverse-rotation device failure (sliding pin type)

electricpete:

this and its twin pump run in parallel, separate pipe or common pipeline/s? why I'm asking if separate - can the pump be allowed to run backwards until stationary, is the hold-back necessary?.
Any damage ever reported on the second pump hold-back?
Forget spring loaded pins - useless.
Possibly this failure was just back luck - has the failed pin been analysed for failure mode, possible fatigue over many years of use?

just for interest who was the pump manufacturer?

Your tag 2B + 2B suits the problem very well --- "To be, or not to be: that is the question: Whether 'tis nobler in the mind to suffer The slings and arrows of outrageous fortune, Or to take arms against a sea of troubles, And by opposing end them? To die: to sleep; No more; and by a sleep to say we end the heart-ache ...."

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

RE: Motor anti-reverse-rotation device failure (sliding pin type)

If this unit has isolation valves could it not be started and stopped with the isolation valve shut? That should eliminate the wear on the anti-rotation device.

RE: Motor anti-reverse-rotation device failure (sliding pin type)

Pete, are the holes for the pins at an angle (not plumb with shaft) to allow centrifugal force to keep the pin up when rotor is spinning? All pins would have to drop about the same time when rotor speed is near zero to minimize damage. It is not obvious that springs are good, if they do not allow pins to stay above the stationary ramps when rotor is in normal operation.

Walt

RE: Motor anti-reverse-rotation device failure (sliding pin type)

Quote (electripete)

Maybe that's something we'll consider if we ever decide to add springs above the pins to help force them down as someone else suggested.
There are two problems with your existing design:
1. The pin weight (and spring force if you add them) will introduce a permanent rubbing action which will generate heat as well as wear during normal operation. The significant wear marks on the ratchet plate is testimony to this. With this you will lose a significant amount of energy and generated heat is another headache.
2. Without springs you have no control over speed of engagement in presence of friction. So at instant of engagement only a part of the pins may engage and so increasing the instantaneous compressive force on vertical ratchet teeth. The damage marks on vertical surface of ratchet teeth are most probably due to this.

I think the best option will be to use a synchro clutch which engages based on speed. Here, figuratively speaking, the spring assisted pins will be in radial direction which will be clear of rotating parts at normal speeds due to centrifugal force. After a trip when rotor slows down sufficiently, the pins with assistance from springs will engage with the stationary ratchet wheel to prevent reverse rotation. Given large motors (4000 hp), the savings due to increased mechanical efficiency may pay for the additional cost of such a clutch. You should check on this.



Engineers, think what we have done to the environment !https://www.linkedin.com/in/goutam-das-59743b30/

RE: Motor anti-reverse-rotation device failure (sliding pin type)

Pete;
Little late to the party here. (Rarely check this mechanical board for electric motor related topics.)
A lot of wordsmith-ing to read in this thread which I have not scrolled entirely... but here's an observation tossed on the pile with the rest of the theories.

The blueing on the ratchet plate indicates... -"tells me"- someone was trying to solve a problem with this assembly long before it failed.
The purpose of the blueing (or light dusting of paint in some instances) is a tactic a mechanic uses to understand what's rubbing or causing an issue.

By noting the photo from the first failure, you'll see the ratchet plate is in its original factory [insulator red] color with no blueing applied.

This observation does not cure the problem.... "but" it does indicate that whatever symptom originally existed during the first repair,
was likely never adequately solved. The blueing was applied to understand an issue at the time of re-assembly.

Motor is assembled on the shop floor. There's an issue... it has to be dismantled to investigate. Blueing is applied in a search for evidence.

And here's another wild theory: Have any of the parts for the mechanism been accidentally magnetized?
An anti rotation device using balls would not so much be effected... but maybe an assembly using cylindrical parts could be problematic if they
became magnetized.

John

RE: Motor anti-reverse-rotation device failure (sliding pin type)

Holes are to small .015 to .02 clearance, no lubrication required.

A tight fit on pins / hardened dowels will not allow the movement that you need for the ratchet to work properly.

opinion:

As for the extra holes, I would keep them.

no springs, gravity does the work.

re examine the pin size.

RE: Motor anti-reverse-rotation device failure (sliding pin type)

Did anyone stop and consider it might be an anti-reverse device that uses steel balls instead of pins?

RE: Motor anti-reverse-rotation device failure (sliding pin type)

"Did anyone stop and consider it might be an anti-reverse device that uses steel balls instead of pins?"

The OP photos show Pins. That is why it is not working very well; it has no Balls!

Walt

RE: Motor anti-reverse-rotation device failure (sliding pin type)

(OP)

Quote (Artisi)

this and its twin pump run in parallel, separate pipe or common pipeline/s? why I'm asking if separate - can the pump be allowed to run backwards until stationary, is the hold-back necessary?.
Any damage ever reported on the second pump hold-back?
Common pipeline. I don't know much about the system or the sequence that they operate pumps.

Quote (Artisi)

Forget spring loaded pins - useless.
I agree. I only brought that up in the context of looking for a situation when grease would make sense, but the springs wouldn't make sense.

Quote (Artisi)

Possibly this failure was just back luck - has the failed pin been analysed for failure mode, possible fatigue over many years of use?
The pin will be saved. I'll post a closer photo if I get one.

Quote (Artisi)

just for interest who was the pump manufacturer?
Allis Chalmers

Quote (TugBoatEng)

If this unit has isolation valves could it not be started and stopped with the isolation valve shut? That should eliminate the wear on the anti-rotation device.
They have discharge motor-operated butterfly valves. I don't know the full logic for the design, but I'm thinking about the scenario of loss of power... the pumps stop, the valves stay in their open position, you won't want all that water rushing backwards from the reservoir and reverse overspeeding the pumps and draining the reservoir.

Quote (Walt)

Pete, are the holes for the pins at an angle (not plumb with shaft) to allow centrifugal force to keep the pin up when rotor is spinning? All pins would have to drop about the same time when rotor speed is near zero to minimize damage. It is not obvious that springs are good, if they do not allow pins to stay above the stationary ramps when rotor is in normal operation.
Holes are pure vertical. Ratchet ramp lifts them during start, centrifugal force and friction pin them to the side of the hole. I agree, no springs, see previous comments.

Quote (goutam)


There are two problems with your existing design:
1. The pin weight (and spring force if you add them) will introduce a permanent rubbing action which will generate heat as well as wear during normal operation. The significant wear marks on the ratchet plate is testimony to this. With this you will lose a significant amount of energy and generated heat is another headache.
2. Without springs you have no control over speed of engagement in presence of friction. So at instant of engagement only a part of the pins may engage and so increasing the instantaneous compressive force on vertical ratchet teeth. The damage marks on vertical surface of ratchet teeth are most probably due to this.

I think the best option will be to use a synchro clutch which engages based on speed. Here, figuratively speaking, the spring assisted pins will be in radial direction which will be clear of rotating parts at normal speeds due to centrifugal force. After a trip when rotor slows down sufficiently, the pins with assistance from springs will engage with the stationary ratchet wheel to prevent reverse rotation. Given large motors (4000 hp), the savings due to increased mechanical efficiency may pay for the additional cost of such a clutch. You should check on this.
1 - I agree there is potential contact but not much. I don't see it as a problem unless it roughens the bottom of the pin (which is an important sliding surface during start... and if rough can damage the ramp). I'll take a close look at the bottom of those pins if I get a chance.
2. I agree, the pins fall when the time comes, they may not be all the way down by the time they contact that vertical wall and there is evidence of that.

clutch seems like a more complicated modification than I'm interested in at the moment.

Quote (John D)

Little late to the party here. (Rarely check this mechanical board for electric motor related topics.)...

The blueing on the ratchet plate indicates... -"tells me"- someone was trying to solve a problem with this assembly long before it failed.
The purpose of the blueing (or light dusting of paint in some instances) is a tactic a mechanic uses to understand what's rubbing or causing an issue.

By noting the photo from the first failure, you'll see the ratchet plate is in its original factory [insulator red] color with no blueing applied.

This observation does not cure the problem.... "but" it does indicate that whatever symptom originally existed during the first repair,
was likely never adequately solved. The blueing was applied to understand an issue at the time of re-assembly.

Motor is assembled on the shop floor. There's an issue... it has to be dismantled to investigate. Blueing is applied in a search for evidence.

And here's another wild theory: Have any of the parts for the mechanism been accidentally magnetized?
An anti rotation device using balls would not so much be effected... but maybe an assembly using cylindrical parts could be problematic if they became magnetized.
Better late than never! I agree the blueing is a clue to what they were looking at. I'll check if the failed pin shows any evidence of being magnetized... I kind of doubt I'll see any but that could certainly stop or slow it from falling.

Quote (JS)

Holes are to small .015 to .02 clearance, no lubrication required.

A tight fit on pins / hardened dowels will not allow the movement that you need for the ratchet to work properly.

opinion:

As for the extra holes, I would keep them.

no springs, gravity does the work.

re examine the pin size.
Thanks. I'm curious if you have a reference or thought process leading to the pin clearance recommendation. 0.015 to 0.020 per side is exactly what our shop came up with, but I couldn't pin them down on the basis.
No springs - I agree.
The benefit of increasing the pin size is just for the increased strength from increased cross section?

Quote (goutam)

The current anti-rotation device is on the shaft above the upper bearing. Your linked device would have to be at the coupling location (replacement for a coupling). It might be good for a new machine from the ground up but I don't think for a backfit here. That is already somewhat congested and an area we need to access for things like alignment. And I'm not really interested in any big changes if I can avoid it. If I find suitable replacement a/r device for the top I'd be more likely to go that way... at some point in the future (the repairs for this particular motor are already well on their way).

Quote (JS)

Did anyone stop and consider it might be an anti-reverse device that uses steel balls instead of pins?
You lost me on that. There are no balls in our device.

I'd like to thank everyone for the responses so far. I threw a bunch of info out there and I know it was tough to read. Your responses gave me a lot to think about.

=====================================
(2B)+(2B)' ?

RE: Motor anti-reverse-rotation device failure (sliding pin type)

I actually think pin weight is to your advantage here. Once the motor starts and is spinning the pins slide up on the ramps and into the hole and the centrifugal force holds sideways against it's housing and keeps it from dragging with any significant force on the ratchet plate. However a heavier pin once stopped will have more force available to overcome friction and drop into place. I would consider making the pins as long as possible to give them as much weight as possible to encourage them to drop.

RE: Motor anti-reverse-rotation device failure (sliding pin type)

The weight of each pin should be the same within a fraction of a gram to minimize rotor unbalance, and to allow the same gravity force to drop all pins at nearly the same time or rotor position. If the pins extend into a blind hole, then the hole should be vented and kept clean. Any suction pressure created by the dropping pin would slow it down.

I would have a new top plate made instead of drilling holes in the old one. It looks to have a lot of distress. There should be a specified gap or clearance between rotating plate and stationary backstop. If motor provides thrust bearing for pump, then be aware of up and down thrust and rotor vertical movement. The the rotating plate has through holes, then what is the condition of the plate above it? Does it have small holes for venting air from the pin-hole cavity?

Walt

RE: Motor anti-reverse-rotation device failure (sliding pin type)

Quote (electripete)

1 - I agree there is potential contact but not much. I don't see it as a problem unless it roughens the bottom of the pin
But the deep near circular wear marks on ratchet wheel give rise to the suspicion that indeed pins make contact with wheel teeth during normal running.
Actually what might happen is that friction will try to hold back the pins but due to pump vibration the pins will tend to slide back to lowest position. Push from the ratchet teeth will push them to the top again and again it will tend to slide back.
If you have some space and flexibility you can think of attaching 3 balls with springs and connected to the pins through pivots(via a circular ring) which will lift the pins at certain speed and drop the pins to low position at low speed by spring and gravity action. The principle is similar to mechanical governors in steam turbines.
Excuse me if I am asking you too much as I do not have the full information.


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RE: Motor anti-reverse-rotation device failure (sliding pin type)

The pins likely do drop as speed drops. They ride up and down on the ramps until the reversal. That's why you see the wear on top of the ratchet mechanism. This should be quite audible as it happens. It's only for a short time so it shouldn't cause significant wear.

RE: Motor anti-reverse-rotation device failure (sliding pin type)

Electricpete :
Have forwarded an email to what I believe is your address.
Regards

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

RE: Motor anti-reverse-rotation device failure (sliding pin type)

I have not been following this discussion very closely, but it seems that the fundamental problem has not been identified correctly.

One pin has been sheared, causing significant damage. The only way for that to happen is that all the pins were stuck, and the pump reversed and built some speed. Then one pin dropped and was sheared. There is only the weight of the pin causing it to drop when the motor stops. When the motor runs, the pin is under significant centrifugal load pressing it to the side of the hole it is in, and friction force will be much higher than pin weight. So the pins are sitting motionless in their holes while the motor runs, allowing the pins to get slightly stuck by rust.

Only one pin has to drop for the anti-reversal to work. Even when all pins drop as expected, only one pin would be carrying all the load, due to fabrication tolerances.

RE: Motor anti-reverse-rotation device failure (sliding pin type)

(OP)

Quote (goutam)

But the deep near circular wear marks on ratchet wheel give rise to the suspicion that indeed pins make contact with wheel teeth during normal running.
Actually what might happen is that friction will try to hold back the pins but due to pump vibration the pins will tend to slide back to lowest position. Push from the ratchet teeth will push them to the top again and again it will tend to slide back
I tend to agree. If the marks were occuring during stopping I'd expect to see a more uneven pattern from pins dropping somewhere in the middle of the flat part.. with heavier damage toward the ramp end of the flat part. On the other hand I'm not really worried about efficiency and I don't see it as a reliability problem unless it damages the bottom of the pins (I do plan to look at those).

Quote (artisi)

Have forwarded an email to what I believe is your address.
Got it, thanks! Those were some thoughts from an oem rep... I have to go back and read those again.

Quote (CompositePro)

One pin has been sheared, causing significant damage. The only way for that to happen is that all the pins were stuck, and the pump reversed and built some speed. Then one pin dropped and was sheared
I have an open mind that we are completely missing something in our theory of the crime (I said before I'm not sure we correctly identified the causes and contributors). But that particular scenario seems unlikely since it requires 5 pins sticking and more importantly it doesn't explain the damage on the ramp. I requested some vertical measurements from the shop (questions 7/8/9 of op) which I hope will help clarify the position of the pin on the ramp at the time of failure... I'll post them when I get them.


=====================================
(2B)+(2B)' ?

RE: Motor anti-reverse-rotation device failure (sliding pin type)

There is a lot to read in this post, but from 38 years experience in industrial situations, this type of problem happens when there is no discharge check valve installed or it is leaking back through the check valve and you have been fortunate the shaft has not broken or the impeller has not become loose and is sometimes locking up inside the volute.

I have had to repair a few pumps in the past with reversing motors from leaking check valves and loosen impellers.

Good luck

RE: Motor anti-reverse-rotation device failure (sliding pin type)

As @mech-engr-experience has indicated a properly designed discharge valve should be provided. Usually for higher sized pumps a controlled closing butterfly valve is provided with closing time calculated from water hammer analysis.
The reverse rotation ratchet is provided as last line of defense. It should not experience too much impulsive force which may damage the mechanism. The impulse force is softened by discharge valve.
To ensure electric supply to the discharge valve in case of black-out you can provide emergency supply to the valves. We also normally provide reverse rotation detection switches to prevent restarting the pump under reverse rotation.
Also since the impact was severe you need to check integrity and alignment of all the bearings and the impeller.

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RE: Motor anti-reverse-rotation device failure (sliding pin type)

(OP)
There is most definitely not a discharge check valve installed. I should have put that into the op.

I work in a power plant. 95% of our systems have discharge check valve for parallel pumps. The ones that don't are all have motor anti-rotation device (which is my excuse for not including it in op... from my narrow perspective it was obvious, but I now realize not so obvious for others and for all I know there are applications that might include both check valve and non-reverse-rotation device). These applications at our plant that don't include check valves tend to be in very large piping systems (power plant circ water, main coolant, and reservoir makeup which is the application of the 700hp motor I'm posting about here). I assume either large check valves might cause problems with water hammer or else they are too expensive. It is the design we've had for 30 years of operation with relatively few problems. It's certainly beyond my interest to add check valves.

I'm more familiar with our power plant circulating water system (a critical system) than our reservoir makeup system (not as critical). For the power plant circulatign water system we have some complicated interlock logic between the pump motors and the discharge valves that helps prevent challenging the reverse rotation ratchet (for example when the pump handswitch is taken to off, it does not immediately stop the pump but instead begins closing the valve, and then when the valve is near closed a valve limit switch completes the logic to stop the pump motor... so when the the motor stops the fluid system doesn't create as much torque to accelerate it in the reverse direction because the valve is almost closed). We haven't had any problems on those anti-reverse rotation ratchets although those are not a sliding pin type, those are a pivoted arm type. As I said I'm not as familiar with the reservoir makeup application which is subject of this thread, but if I get a chance I'll review to see if it has similar logic circuits which close the valve most of the way before stopping the pump.

At any rate looking at the photos it looks like we may have had some severe forceful engagements of the anti-rotation device in the older days before the previous repair where the round pin shape was visible as an indentation in the vertical wall of the stationary ratchet plate ... I don't see that same evidence in the vertical wall of the stationary ratchet plate after the recent failure. At this point I'm more inclined to view this as a malfunction of the anti-reverse-rotation device rather than a consequence of excess force (still waiting those vertical measurements).

=====================================
(2B)+(2B)' ?

RE: Motor anti-reverse-rotation device failure (sliding pin type)

The damage to the ramp side of the teeth would have occurred when the motor was restarted after the one pin was sheared. The broken pin piece could have taken seconds or minutes to get jammed and cause the damage. It also appears that the "previously damaged" plate had radius faces machined into the teeth.

RE: Motor anti-reverse-rotation device failure (sliding pin type)

Electricpete :

what is the material and hardness of the ratchet? so with repeated cycles the pin was gouging the ratchet. thus it jammed. maybe carburized surface would prevent wear from repeated cycles.
also I said a spherical end on the pins would prevent gouging into the ratchet. but maybe a chamfer to match the angle of the ramp up on the ratchet. simple fix.
because once the pin dug into the ramp it prevent the pin from pushing up in to the hole. this actually happen when the motor started. dug into the ramp and failed.

RE: Motor anti-reverse-rotation device failure (sliding pin type)

Hi Pete
Would be interested in knowing what the final fix entailed.
Hopefully it's years into the future before you can report back that it could have a problem 😉

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

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