How to ensure smooth engagement of moving pinion with fixed rack?
How to ensure smooth engagement of moving pinion with fixed rack?
(OP)
Hi
I have a pinion which is being carried towards a rack. The pinion is turning. I want the pinion to engage smoothly with the rack without stoping the pinion.
Have thought about mounting the pinion on a shaft split with some sort of flexible coupling.
Also what if I tapered off the ends of the rack?
Any other ideas please, anyone done this before?
Thanks
marv
I have a pinion which is being carried towards a rack. The pinion is turning. I want the pinion to engage smoothly with the rack without stoping the pinion.
Have thought about mounting the pinion on a shaft split with some sort of flexible coupling.
Also what if I tapered off the ends of the rack?
Any other ideas please, anyone done this before?
Thanks
marv





RE: How to ensure smooth engagement of moving pinion with fixed rack?
Don
Kansas City
RE: How to ensure smooth engagement of moving pinion with fixed rack?
RE: How to ensure smooth engagement of moving pinion with fixed rack?
Timelord
RE: How to ensure smooth engagement of moving pinion with fixed rack?
The pinion is on a carriage. The carriage drives itself along with a motor which is linked to our pinion. So when the carriage moves, the pinion is turning.
the carriage drives itself up to the rack. The turning pinion then has to engage with the rack, as smoothly as possible, with no stop/start to the carriage motor.
There is no way (that I can see) to maintain the pitch of the pinion in sync. with the rack as it's movement once it is off the rack is independant.
Hope this makes some sense, and many thanks for the ideas so far.
RE: How to ensure smooth engagement of moving pinion with fixed rack?
Could you have the driven pinion on a swing-arm, spring biased towards an engaged position stop, so if it encounters a jamming situation it can ride up, the continuous turning force would tend to pull the rack in until a meshing condition occurs, and then it would come back to the bottom stop. It may require a damper to avoid slamming down and possibly a latch to keep it in mesh if the torque is high.
RE: How to ensure smooth engagement of moving pinion with fixed rack?
Ted
RE: How to ensure smooth engagement of moving pinion with fixed rack?
As the pinion is connected to the motor, can I assume that once it engages with the rack it then becomes the driver?
I think you've already covered this but I'll double check; after having meshed with the rack and then disengaging will the pinion maintain the same angular position relative to the rack?
Ron Volmershausen
Brunkerville Engineering
Newcastle Australia
http://www.aussieweb.com.au/email.aspx?id=1194181
RE: How to ensure smooth engagement of moving pinion with fixed rack?
You could make the teeth thin with a lot of backlash or have a high pressure angle (say 25 degrees) with thin toplands.
RE: How to ensure smooth engagement of moving pinion with fixed rack?
So to reply to you all:
To SincoTC:
For my application, I think the swing-arm mounted pinion may have some mileage but I am worried that if the pinion hits the rack at just the wrong point then it could end in broken teeth or a stalled motor. If you have any real-world experience of making this kind of thing work I would love to know more.
To hydtools
I had considered using a clutch on the pinion shaft, however there is a large variation in torque that the pinion will see so I doubt it would work in all load scenarios.
To gearcutter:
Yes the pinion is under load and is the driver when engaged with the rack. I can't maintain it's mesh with the rack once it has moved away though.
To ExRanger:
A restriction on the design is that the carriage can't be raised or lowered relative to the rack. Thanks for the suggestion of a quill shaft, I have never seen one of those.
Thanks all. I'm thinking I may use a drive tyre instead of a rack and pinion......
RE: How to ensure smooth engagement of moving pinion with fixed rack?
Ted
RE: How to ensure smooth engagement of moving pinion with fixed rack?
ta
marv
RE: How to ensure smooth engagement of moving pinion with fixed rack?
Sorry; plenty of experience with fiddly/complicated friction and gear driven mechanisms, but nothing directly analogous to this!
I would have thought that providing the pinion is always being driven in the same driving direction as the carriage and it is not permitted to drop below the full engaged depth relative to the rack, then it would always be able to "climb out" of a jam/stall situation. I would be inclined to truncate the first rack tooth to about half height and chamfer the inner flank back like a buttress thread to aid engagement and strengthen the tooth.
My biggest concern (afterthought), was that if the carriage was being driven at nominally the gear-driven rate, there would be no speed differential to synchronise the meshing, so if it climbed up tooth top to top, it would just roll all the way along the tops like this, not a lot of use! However, if the carriage drive was slower and the pinion could PULL the carriage at its slightly faster speed, then it would all work OK, however, as you said that the drive was shared, this would mean the wheels would have to skid when the gear was driving (unless they were driven by over-run sprag clutches or similar).
If you are considering a drive tyre, I would recommend a vertical blade in place of the rack and have a vertical spindled drive wheel opposed by a pinch wheel so that your driving force is independant of actual weight on wheels, which, with a vertical wheel would be in conflict with the other carriage wheels, plus it will give you directional control.
RE: How to ensure smooth engagement of moving pinion with fixed rack?
There is an intermediate gear between the carriage drive and the pinion, so the pinion moves faster than the carriage. So the pinion will indeed pull the carriage - this is the intention.
So, you reckon that if I chamfer the rack and drop the first tooth height, it would mesh OK? This was how I planned to overcome the meshing problem but I was not confident it would work, and if it did then I worried there would be a noticable "clunk". What do you think?
Another idea I had was to use a static flat chain instead of the rack and to use a sprocket instead of the pinion. I thought the sprocket might ride up / engage in a more forgiving way. Any thoughts on this?
RE: How to ensure smooth engagement of moving pinion with fixed rack?
Yes, I feel that the with the first tooth modified like this, it moves the "worst case" first contact nearer horizontaly to the pinion's centreline, bringing line from this point to the pinion axis nearer to the vertical so that it lifts the swing-arm and reduces the bending moment on the rack tooth as below:
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There isn't much grip between pinion and rack in this situation, but with the pinion rotating and the carriage driven at a slower rate it should come to a point where it will haul itself onto the rack. How well and quietly it achieves this is difficult to judge. A lot depends on the mass and size of the parts involved, how fast the pinion is turning and the strength of the swing-arm spring, which is a balance between strong enough to maintain mesh when driving, but easy enough to lift up out of trouble on initial meshing. I would fit an hydraulic shock absorber/damper to prevent it slamming down. My gut feeling is that if the speeds are relatively slow, there would be more chance of success.
However, I would definately make some kind of mock-up or scale model to experiment with first, just to make sure it works OK and there are no show-stoppers! Sorry, I've not had any experience with chains, other than that sprockets are usually narrow pointy things that get burred and damaged relatively easily.
Good luck!
RE: How to ensure smooth engagement of moving pinion with fixed rack?
You should know the speed and the amount of time/distance until the pinion engages the rack. If you know what position of the pinion tooth you want to engage the rack, determine the position the pinion needs to be stopped at (or where it starts from). Place a prox. switch to make sure the pinion is oriented in that manner and allow the prox. switch to determine when to stop the motor when coming to a rest. Since you know the speed and distance to the rack, you could position the pinion so it engages at a certain point on the rack teeth.
Just a thought.
Don
RE: How to ensure smooth engagement of moving pinion with fixed rack?
Ted
RE: How to ensure smooth engagement of moving pinion with fixed rack?
Thanks again, sounds like a good plan. I'll give a prototype a go(if I get the chance, depends on client reaction) and see how it goes - will post results here. Might be a few weeks though.
djm883:
Sorry, won't work as I can't stop the pinion rotating.
hydtools:
I have the same thoughts and would like to give this a try. If I get the chance, will post results of prototype here.
Thanks all!
RE: How to ensure smooth engagement of moving pinion with fixed rack?
Leave the pinion in contact with it all the time.
Use a clutch on the pinion shaft.
Make it a dog clutch with balk rings, just like in a synchromesh transmission.
... Which is something you should study. All this stuff with chamfers and weird teeth was studied, tried in production, and found wanting in the last century.
The other choice is some fairly fancy electronic controls.
Mike Halloran
Pembroke Pines, FL, USA
RE: How to ensure smooth engagement of moving pinion with fixed rack?
MikeHalloran:
I agree that on their own, chamfers and weird teeth have proven to be of little or no value, but in combination with a mechanism that allows the pinion to move away from the correct pitch distance, we found that they will substantially reduce the load on the rack's leading and second tooth.
marvincooper:
I was thinking overnight about the relevance of the device I worked on had to your problem, so I will explain what it was and you can form your own opinion.
Quite a few years ago, we were approached to help sort out a piece of Lab equipment that had become unusable and the originsal manufacturer had eventually refused to help. It consisted of several small interchangeable steel blocks that slid in dovetails on a microscope stage. A short length of rack was fixed to the stage between the dovetails and was engaged by a thumbscrew driven pinion in each block, this enabled the blocks to be precisely positioned under the microscope. The parts were quite well made, but the gears were small at 96DP; with the pinions mounted rigidly and on the tight side of the correct centre distance, they needed care and a little "engineers feel" to engage smoothly. This all went OK until an eminent Professor took over the lab and wanted to do some experiments himself. He was a brilliant man, but totally lacked the aforementioned "engineers feel", in fact, his lab technician said that his Prof would break a hammer if he was daft enough to give him one! In under a month, all the gears were in a terrible state, with bent rack teeth and broken pinnions, the manufacturer replaced the gears under warranty and returned them. However, they still jammed, so the lab tech chamfered the rack ends. I actually think that this didn't help the engagement and more likely made it worse! Within a week, all the gears were mashed again and this time the manufacturer told the lab to go away (because they had been tampered with) and that's where we came into the picture.
Unfortunately, other reasons prevented a full length rack, so we were stuck with the short rack drive because a friction drive that would fit in, couldn't cope with the static friction of the dovetails. Therefore, we renewed the gears and mounted the pinion bearings on flexure strips, these permitted enough movement to allow the pinion to climb out of a jam situation and their compliance allowed a tight but smooth mesh. We made a dummy block out of perspex so that we could watch the engagement action with another microscope and came to the conclusion that although it seemed to work well enough with plain ends to the rack, the stubby shape (shown in my last post) protected the first normal tooth from the worst case jam and gave a better lifting action to the pinion. They've been back in service now for over ten years and I hear that they still work very smoothly and that anyone can use them without difficulty.
So, now you can see that although the rack drive part is similar, I expect that there is a vast difference in scale and other details, which when added to the mechanical drive leads to my element of caution.
I do have to ask the question, why do you need the rack drive? We were stuck with it by the required precision, limited traction and lack of space for a pinch wheel. The method we used may well work in your case, but unless the rack is essential or can't be extended to full travel for some reason, then I would think that a friction drive probably has a more predictable outcome and easier development.
Good luck again
RE: How to ensure smooth engagement of moving pinion with fixed rack?
RE: How to ensure smooth engagement of moving pinion with fixed rack?
Crazy idea - what if the rack itself had some play in its travel before it hit a hard stop. In other words, if the whole length of rack could translate +/- one pitch length freely (or more likely with some small resistance) before it hit a stop. Then if a pinion tooth came crashing against a rack tooth it could move over to allow proper engagement at which point the pinion would drive the rack hard over to its stop for normal operation.
RE: How to ensure smooth engagement of moving pinion with fixed rack?
I can't extend the fixed rack - that's why I have this problem. Thanks anyway.
SincoTC:
Thanks again. It does sound like a similar problem though I think I have much higher forces to deal with. I don't really have room (I don't think) for a pinch roller or a large drive wheel, but will have to explore this and see what the implications are on the rest of the machine.
ExRanger:
Yes, that's an idea I had too but there is still the problem of the pinion teeth whacking into the rack. Plus I don't really have space to add more bearings. But it may be worth considering so I will have another think on this. Thanks.
marv
RE: How to ensure smooth engagement of moving pinion with fixed rack?
Or reduce the length of support of the rack and take advantage of the flexing of the rack instead of installing a spring.
No idea of the dimensions of your rack as to which idea would be better or no good at all.
Ted
RE: How to ensure smooth engagement of moving pinion with fixed rack?
Fe
RE: How to ensure smooth engagement of moving pinion with fixed rack?
Split the rack so that you have a pre determined "lead in" or approach length.
Allow this length of rack to pivot off the main rack length in a way so as to maintain the pitch without too large a variation. The longer the "lead in" rack length is the less change in pitch as the length pivots.
Mount a spring on the opposite end of the pivot point or right at the start of the rack assembly. The spring is placed under the rack and should "give" if teeth tips come into contact but provide enough force to resist the radial separation forces of the mesh. You could maybe even use a hydraulic system to replace the spring. Either way, design the system so it is adjustable.
It seems to make sense to use a high pressure angle in this case but I think that a lower pressure angle might be best as this would reduce the separation forces and actually help draw the pinion into mesh. A lower pressure angle would also allow for wider top lands of the teeth making them stronger and less prone to chipping etc.
Ron Volmershausen
Brunkerville Engineering
Newcastle Australia
http://www.aussieweb.com.au/email.aspx?id=1194181
RE: How to ensure smooth engagement of moving pinion with fixed rack?
gearcutter:
I'm liking the sprung pivoting rack idea, will look into this more. I guess there may still be a nasty "clunk" if the pinion hits the rack just wrong but it may be worth a try. Thanks.
RE: How to ensure smooth engagement of moving pinion with fixed rack?
It may be time to consider other concepts besides the gear rack. Why don't you post a decent sketch, so that others can see what you're up against?
RE: How to ensure smooth engagement of moving pinion with fixed rack?
In conjunction with an idea like the spring-loaded pivoting rack or sliding rack, could you make the first few rack teeth out of hard plastic or rubber? Maybe something like PEEK. I don't know how much torque you will have, but if its low enough that the plastic teeth can survive and won't deflect excessively, then it would provide a smoother engagement. I would guess that when the carriage first engages the rack, the carriage wheels will still provide some of the drive momentarily, plus you have some forward momentum of the carriage. Maybe the first 2 or 3 rack teeth will never see the full torque anyway.
RE: How to ensure smooth engagement of moving pinion with fixed rack?
So far you haven't revealed squat about how big this drive has to be, or what forces or torques or masses are involved, or what level of precision is necessary.
You could meet us halfway.
Speaking of halfway, if the pinion is geared to spin at exactly "rack speed", then it will stay almost in sync with the rack over a fair distance of travel away from the rack, and re-engage without much fanfare, provided that the carriage wheels don't slip much.
If on the other hand, the wheels do slip, or the carriage is otherwise started toward the rack while randomly out of phase with it, then the chance of a clunk- free engagement is roughly proportional to the ratio of pinion lash to tooth thickness. I.e., mostly clunky.
So I was thinking of rack teeth that are flexible, not up and down/ in and out of mesh, but in a pitch-wise direction, e.g. formed at the tips of a comb-like structure, perhaps with deeper saw cuts between the teeth at the free end, etc.
Or, you could use a pinion that doesn't really much care about rack phase, e.g. a wire brush.
Mike Halloran
Pembroke Pines, FL, USA
RE: How to ensure smooth engagement of moving pinion with fixed rack?
RE: How to ensure smooth engagement of moving pinion with fixed rack?
Ted
RE: How to ensure smooth engagement of moving pinion with fixed rack?