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Shaft steels

Shaft steels

Shaft steels

(OP)
Hi, I hope someone can help me.
I have designed some specific application machinery and one of the processes is a twin screw extruder for making powder coatings, I designed this machine two years ago, made it and have been using the machine now for over a year. originally it was only meant to be a prototype just to see if it worked, ie did it make powder coatings, anyway it did and I have developed a business around this extruder.
The problem is now the gearbox, the shafts last about 2 months and then fail. I recently redesigned the mechanics basically beefing up the shafts diameters etc. My question is: Previously (whilst living in England) I always used En24T, turned the shaft, installed and ran and I always had good luck with it. Here in the States I have had trouble with steels, Does anyone know a decent equivelent to En24T (is it 4340 - chemically similar but is it equiv in physical properties?) I also used 1144 on advise from our steel stockist, but I think he had another motive.  OR - does anyone know where I can get En24T from in USA?

RE: Shaft steels

I would consider 4340 low alloy steel, quenched and tempered to strength properties similar to the original shaft material (En24T). The 4340 low alloy steel has excellent hardenability, good notch toughness and good fatigue strength.

RE: Shaft steels

1144 is resulphurized steel, great for machinability, but generally poorer than the equivalent low-carbon steel (1044) in fatigue life.  The sulfur inclusions that make the steel have small chips in machining also act as stress risers that reduce its fatigue properties.

RE: Shaft steels

(OP)
Thanks for the answers.I am going to try elaborate the problem. The gearbox has one shaft in (Drive) and two shafts out and because of the center distance of the twinscrew extruder bit I am very limited on space,ie, diameter of shaft and/or bearing diameter. The twinscrew extruder is also a high torque application and fairly slow speed.
In the perfect world that hasn't arrived here yet I would just over engineer the shaft diameters and probably have enough to not have an issue but I am struggling for diameter, this is why I would like to use the best steel available for low speed high torque applications? Like I said earlier En24T is my fall back but I did use this material from advise of chartered engineer etc but that application was a high speed low torque one. Maybe the material should or could be different??
I really appreciate any advice.

RE: Shaft steels

Quote:

Does anyone know a decent equivelent to En24T (is it 4340 - chemically similar but is it equiv in physical properties?)

4340.

RE: Shaft steels

I agree with all the posts, but I would also throw in a couple of things.  

1) How is it failing?  Is it bending?  Is it breaking?  Where is it breaking?  If you have machined a sharp corner into it or have a rough finished surface, that's going to have a HUGE effect on the life of the shaft.  

2) The manufacturing process of the bar stock also has a lot to do with your strength.  Is it cold drawn?  Is it hot rolled?  Is it forged?  This step in the manufacturing process will change the strength and toughness of the steel as much as the heat treat will.  

3) Your supplier may supply the shaft in any number of conditions.  AISI 4340 can have an extremely wide range of strengths, depending on the temper.  You should find a supplier who can offer the shaft in a fairly wide variety of hardnesses.  For example, here's a link to a list of a few different types of steels, but notice that 4340 comes in a couple of different grades -- 66ksi yield for plain 4340 and 100ksi yield for what they're calling "aircraft alloy".  This is achieved mainly by a different tempering, but look down the list and you can find 4330 aircraft alloy that has a minimum yield of 185ksi.  The difference between 4330 and 4340 is a difference of 0.10% carbon in the chemistry, but that change doesn't account for the strength difference nearly as much as the heat treatment does.  Anyway, the point being made is that two shafts made of 4340 may have wildly different strengths and you need to be armed with this information when you talk to your supplier.  

http://www.emjmetals.com/esl/

About the 17-4PH -- you can get some really good results with this, but it's hardened in a different manner than 4340 and you still need to make sure that you're getting what you want for your application in terms of minimum strength and toughness.  

So, after going through all of that, I would pass that along with the caveat that you need to remember that the harder you go, the more brittle the material.  Just something to keep in mind, but if you don't have sharp corners and stay away from the rough surfaces and aren't subjecting the shaft to sudden impact loading, you might just ask for a harder condition of the same material you've been asking for all along.  

-T

Engineering is not the science behind building things.  It is the science behind not building things.   

RE: Shaft steels

No one does any strength calculations any more? The shaft material and heat traetment selection should be selected based on the torsional and bending analysis of the shaft. This is quite a simple and straight forward calculations. No need for FEA. I do not see any point suggesting a candidate material without tackling the stresses involved not to mention stress concentration points and fatigue analysis.

RE: Shaft steels

I agree up to a certain point, israelkk.  But the OP said that he was limited on space and couldn't go any bigger.  Since it's failing, I assume that calculations would show that something is coming up short.  But we don't know what, which is why I asked how it was failing.  If it is bending, then it's probably not hard/strong enough.  If it's failing in fatigue, it might be a surface finish or a stress concentratin problem.  If it's experiencing shock loading, then the shaft he has may be too hard/strong.  

I agree that there's no substitution for hand calcs, but you could easily show a calculation that you could drop lots of weight off of the frame of a car by making it out of tool steel, but I'm sure you would agree that that's not necessarily the best solution.  

-T
 

Engineering is not the science behind building things.  It is the science behind not building things.   

RE: Shaft steels

If it takes two months to fail, it sounds like fatigue.  I would take a look at your design to make sure that you eliminate notch sensitivity, etc.  Fillet all your interior angles and have the shaft turned, ground and polished (or buy it stock that way).

Don
Kansas City

RE: Shaft steels

You could build a car out of tool steel and remove a lot of weight, but it would fail many tests/requirements other than durability. A car is a very bad example.

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Shaft steels

Hi agpowder

I think like Israelkk and EngineerTex you need to establish the loads on the shaft first and then select a material based on stress calcs and the enviroment.
If your short on space that will also drive material selection.
Can you give details of shaft sizes and loads, then we may be able to assist you further.

regards

desertfox

RE: Shaft steels

(OP)
Thanks for the help.
The shaft failures that were happening every 2 months or so were either bearing race impaction on the shaft,ie diameter reduction from bruising under the bearing race or the shaft would snap at one of the machined shoulders.
Recently I have upsized the shafts (at the expence of the bearing sizing)
The machining work is done by me, I am completely self tought on lathe/ mill and whilst the tolerencing is good the finish and the make quality is probably sub standard for what I am expecting the machine to do now.
The machine failed last afternoon with the bigger shafts but I think I have identified a problem inside the extruder (putting a whip load of the shaft). I am working now to eliviate this problem.
I am begining to think the machine needs some design and manufacturing TLC that I am not capable (or qualified) to do.

RE: Shaft steels

(OP)
Engineertex: your second question, I just buy 1144 from McMasterCarr, machine it and install it.
Desertfox: The shafts are 213mm long, bearing journals are 25dia and 20dia. Spur gear sits on a 21mm dia. The gear is 12 pitch 1.667" Pitch. The shaft has a 5/8" hexagon that I machine onto the end to drive the extruder screw. The hex is 32mm long. The two output shafts are fixed at 1.57" center distance. I have to stagger the driven gears on the two shafts to have everything fit inside the gearbox.

RE: Shaft steels


As mentioned by EngineerTex and others, it is very important to make sure that the "machined shoulders" do not have sharp corners, they should be almost as large as the the radius on the bearing's inner ring (not bigger though or the race will not be located properly against its end face). The surface of this "fillet radius" must be smooth, without scratches or chatter and should blend smoothly to the journal and shoulder without ridges.

Trevor Clarke. (R & D) Scientific Instruments.Somerset. UK

SW2007x64 SP3.0 Pentium P4 3.6Ghz, 4Gb Ram ATI FireGL V7100 Driver: 8.323.0.0
SW2007x32 SP4.0 Pentium P4 3.6Ghz, 2Gb Ram NVIDIA Quadro FX 500 Driver: 6.14.10.7756
 

RE: Shaft steels

(OP)
Desert Fox: As far as loads are concerned, I have thought about this a lot over the last two years and I think the perfect situation the load would be an axial load only where tha product in the extruder transfer the load axially back into the gearbox. I installed a thrust bearing on both output shafts for this. The process screws and kneader blocks of the extruder intermesh with each other and sit into the extruder barrel. I have two 2.060" diameter holes on the 1.57" ctr. The two holes overlap (screws intermesh) The individual screws have a diameter of 2" and should freely rotate in the barrel. The barrel/screw is continuously lubricated and float in the molten product (powder Coating plastic)The screws are 40" long.
I think the screws do whip and this is what is giving the failing load on the gearbox. I do not know how to "work out" this load other than recognise that it can happen especially if the machine runs empty (it shouldn't do this but sometimes does because operator not filling hopper and when the machine has been repaired (and is clean))

RE: Shaft steels

(OP)
SincoTC: I use Tungsten turning inserts that are radiused at slightly less than the relieving radius on the bearings

RE: Shaft steels

Is there any way to eliminate shoulders altogether?  I don't know what your shaft design looks like, but there may be ways to design using clamp collars, snap rings, tapered cotter joints, etc.  

If you don't have a thrust load, you might even consider letting the shaft "float" in the inner race of the bearing with no shoulder.

Don
Kansas City

RE: Shaft steels

Hi agpowder

I think you might find that you also have torsional loads on the shafts in addition to axial ones.
Failure of shafts at shoulders is a known weak spot due to stress concentrations.
By the way when the shaft fails at a shoulder what to the failed edges look like eg:- can you push to failed surfaces together like a jigsaw or is there to much distortion, also are the faces of the shaft parallel to the shaft axis or slightly helical along the shaft axis.

regards

desertfox

RE: Shaft steels

" bearing race impaction on the shaft,ie diameter reduction from bruising under the bearing race "

What is the fit between the shaft and bearing inner race?
It probably should be ~ 0.0005 inch diametral interference, with a ground finish.

If its a ball or roller bearing, and the arrangement is the fairly typical stationary-outer-race/shaft-and-inner-race turning-together with a fairly constant direction radial load, the tendency for the shaft to creep ("spin")inside the race is strong if there is any clearance between race and shaft.  At a mere 100 rpm there will be about 1 million rotations in a week (24/7 service). If the loads are high then even a line-to-line zero clearance fit will wiggle then creep.

RE: Shaft steels

(OP)
Hi desertfox
You are correct the extruder imparts high torque loads, I was thinking of the loads on the bearings, sorry.
The sort of load would be analogious to an auger conveying a really thick syrup. In fact this is exactly what it is.
I think maybe half the snapped shafts have gone at shoulders and the other half have broken inside the gear. all the breaks can be "fitted" back together.Often the key slot is part of the snap. Often when one output shaft broke upon inspection the other one would have a crack in it. and you could probably predict the breaks by a combination of product throughput and running load.
Sorry I dont understand the last question.by faces do you mean surface. The shaft is 213 mm long. the first 87mm is 20dia then 31mm of 21dia then 44mm of 30dia then 54mm of 25dia then 32mm of 5/8" hexagon. There is no machined tapering on the shaft.

RE: Shaft steels

(OP)
Wow replys come quick here. I wish I could type faster.
TMoose.
You say interference? ie the shaft is half thou bigger than the ID of brg?
Deffinately not, I machine to +0.00mm/-0.01mm which is 0.4thou?? I think. I do not have a cylindrical grinder, I usually machine to +0.01 or +0.02 and stand for a couple of minutes with emery paper whilst the lathe is running fast. I then use a bearing to check the fit and aim for one that "slips on with very light pressure.
I probably should go tighter but I worry about damaging these tiny bearings whilst pushing them on or damaging another bearing that would be under load whilst pushing it.

RE: Shaft steels

desertfox has hit the nail on the head concerning the existence of an axial load on your shaft. We had 6 far larger WP twin screw extruders and initially suffered horrific growing pains with getting the torque to the screws. In the larger extruders the problem with any whipping is controlled by the clearance between the screws and the elements.

On certain polymers there is also a stick/slip or windup problem that will put very large impact loads on the shafts and even transmit them to gear box.  

How is the outlet setup on you extruder, like filter, die plate changer, etc?
The reason I ask is that I checked and the only problem anyone could recall was that on some small extruders we had some mechanical failure that were due to a screen changer being too slow.

How are you heating the extruder?
 

RE: Shaft steels

(OP)
UncleSyd.
I do not have a die at all at the end of the extruder. The melt just pours out of the "figure 8" and falls 3" into a chill roll.
If, on start up I havn't heated enough and the kneaders are still stuck then the drive will dead head and bang, bang bang. This scares the crap out of me and I make sure the plastic is really liquid and normally I will turn the gearbox input shaft pulley (sheave) by hand to make sure before starting, ie: it is not really an issue anymore, I also do not clean with PVC as I felt the impact was too great, I now just clean with Polyester resin.
The clearance between the barrel and screw elements is now about 1.5mm on the radius. WP extruders are notoriously tight on clearances. With Powder coatings this is unnecessary.
Thanks for your reply and any additional help would be appreciated.

RE: Shaft steels

Hi agpowder

I think you need to calculate stresses based on the axial and torsional loads the screw actual see's in service, ie a combined stress calculation (Tresca) or (Von Mises)and apply the appropriate stress concentration factor.
If you can fit the shaft ends together that have failed that would suggest a brittle fracture, so it would appear as another poster as suggested that a crack is initiated during operation which grows slowly under repeated cyclic stress untill the reduced area can no longer carry the load and fails rapidly (fatigue).
What I mean by the last part of my previous post is imagine your looking at the longnitudinal length of the shaft, now saw through the shaft dead straight at right angles to the longnitudinal axis, ie the cut faces would be at right angles to longnitudinal axis of the shaft.
Now take the same saw and do the same but this time hold the saw at an angle of say 45 degrees the faces are no longer at right angles to longnitudinal axis.
My guess is your shaft ends looking at the longnitudinal axis have a angle to them, that would be explained by the principle tensile stresses acting along the principle planes due to the combined loading mentioned at the start of this post.
Can you post a sketch of the shaft and its supports along with the axial and torsional loadings that it see's?

regards

desertfox

RE: Shaft steels

(OP)
I will bring my camara to work tomorrow and open the top of the gearbox and photo it. Can you attach photos here?

RE: Shaft steels

Hi agpowder

Yes you can, but make them pdf's otherwise i can't open them.But we still need loadings

regards

desertfox

RE: Shaft steels

(OP)
Ok, I need to come clean here, I have never been to college and I basically left school at 14. I have no idea how to calculate mpg in my car let alone what you are suggesting. I have however "designed" a complete line of powder grinding and classifier machines, cyclone collectors, dust filters and I can muddle my way through electronic controls, I have sold the grinders to over fifty powder coating companies worldwide (they range in price from $40,000 up to $250,000 a piece. In a powder manufacturing line the grinders are only half the storey, the extruder is the real workhorse. They start at about $200,000 and I decided I could make one to my own design that was much lower cost. I made the prototype we are talking about and it works good (the process side) I couldn't find customers so I decided to start making powder coatings myself with my own machines. I found good customers and I have a really nice business doing it.
I know what I want a machine to do and I can design a mechanism to do it but I dont know how to calculate anything to do with it. I suppose my original question is still valid, If you had to take a shot in the dark what steel would you expect to be good for a low speed, high torque application. For example, 1018 would be like a rubber band. Spring steel would be to brittle and impossible to machine? There must be a good go to steel for general purpose low speed (approx500rpm) and high torque?

RE: Shaft steels

Hi agpowder

Well lets look at the photo's anyway, if you can post a pic of a failed shaft it may help.
How did you size the motor's etc if you cannot do calculations I can only assume someone else did them for you in which case can he help you now.
I can't take a shot in the dark I haven't even seen the machine or what it looks like.
Generally ordinary mild steel is a common widely used steel for lots of different applications, but certain components like yours may require a slightly better steel.
I am afraid the only sure fired way is to go back to the design and get someone to do calculations, I suppose you can get the maximum rated torque from the  motors which is a starting point but then we would need still the axial thrust on the screw.

regards

desertfox

RE: Shaft steels

desertfox
I was just about to post concerning the open ended aspect of this extruder. Without some restriction on the outlet like a breaker plate or die the only significant axial forces he will see would come from the inlet section of the screw where the polymer is trying to melt.

Several things mentioned by the OP is the "bang bang" aspect his startup procedure. This sounds like windup or a stick slip situation that can put tremendous loads on the screw which would be passed to the prime mover. On our big WP's we always start with the low melting component of the polymer compound. The PM is interlocked with the barrel temperature sensors to prevent premature powering up

agpowder,
I am wondering how you get effective kneading with no back pressure in the kneading area?

Anecdotal:
You mention that The WP's are know for their very tight internal clearance which is correct. I help set up all the tools necessary to measure the barrel and and elements on a 120 WP that was being used for experimental work. The had run the machine about 3 months on a new product and decided to dimensionally check the elements and barrels while some factory representatives were on site. As they pulled the screws my partner I looked at each other as something was way out whack. As they demounted the screw elements we started checking the dimensions. We were setup to measure to 0.001" on the OD. What we measured was in 0.250" loss on the elements and when we measured the bore it's wear added another 0.125" of loss. The bad part of this we had inspected another machine a month before and had tossed elements that showed 0.020" wear. These elements aren't cheap.  I sure wish I could have been in the meeting between WP and company.


 

RE: Shaft steels

(OP)
No I did not have anyone to calculate stuff. I picked 7.5kW for the motor because when I first decided to make the prototype I knew I would be starting it in my garage and that was the biggest motor I could run on the electric supply going to the garage.
In the past I had my father to check I wasn't doing anything stupid but I am in the states now and he is in England and unfortunately I tend not to be very organised on drawings either and he cannot travel anymore. He is or was a chartered engineer (retired). I always new the 10Hp was too big for the original gearbox but the machine was just made originally to see if it would make powder coat with a view to developing a line of machines to sell. I think now I can make more money by using the machines rather than selling them. Perhaps I should hire someone to design the gearbox proper. You know what as  I have come up against a brick wall maybe I should just use the 4340 steel as discussed earlier and make a load of spare gearboxes just in case.

RE: Shaft steels

Hi agpowder. Make sure you get close-up pictures of the failure - preferably use a camera with macro function so as to get good clear close pics, people want to look at the failed surfaces - not necessarily "the gearbox".

RE: Shaft steels

Hi unclesyd

I know very little about these machines so anything you can add will be very welcome.
In the meantime i'll try find a bit more about them.


regards

desertfox

RE: Shaft steels

(OP)
Thanks Unclesyd
I agree with your first sentance, the axial loading is the reverse thrust from the augers/screw elements.
The bang bang is on new or cleaned screws when we switch the machines off we purge with epoxy resin that melts low and is easy to start, a purged machine does not bang bang.
Effective kneading: I have never seen any "dispersion" problems at all, in fact I originally set the 32 neader blocks (the elliptical beaters) up as 10 30degrees, 10 60degs and 12 90degs. Now I have 31 30degs and 1 0deg. I manage to get 30% more capacity by doing this and I have not seen any degredation in dispersion quality.
I once stripped an APV machine and the config was half screws, half beaters and half of the screws had completely worn away down to the core of the screw flight. This machine was still working. This was one of the reasons I figured an idiot like me would have a chance of making one myself and getting it to work. I also knew the non back pressure would work because I once bought a Thermo Prism extruder and that did not have anything on the end. I also think Powder Coatings is a very easy application for a twinscrew extruder.

RE: Shaft steels

desertfox,
These machines are basically very high shear rate compounders in nearly all plastic manufacturing facilities. There is a lot of science and well as a little black magic. The amount of power required to run the larger machines is quite high. One the WP 120, early model, had 1000 hp motor.

Here is a short article from Coperion who gobbled up Werner Pfiederer. The old WP site did have a lot of technical information, of which I hope Coperion has keep up. If you ever see a booklet called "Technical Manual by WP grab it.
I tried to check the Coperion site but it is extremely slow tonight.

http://www.coperion.com/up_doc/Veroeffentlichungen/2007/CWP/CWP_200703_Masterbatch%20production.pdf

RE: Shaft steels

I was looking on the McMaster-Carr website.  You had mentioned that your shafts are their 1144 round stock.  

According to the catalog page, their 1144 is treated to a condition of 100ksi (100,000 psi).  The 4340 that they offer is in the annealed condition and only offers 68,500 psi.  I would suggest finding another supplier where you can get 4340 heat treated to a higher strength.  4340 heat treated (quenched and tempered) to over 125,000 psi has excellent toughness and impact properties and I would suggest finding a distributor who can offer that.  

Although McMaster offers 8620 at 100,000 psi minimum yield, I would shy against it.  8620 is pretty similar to 4340, but with less carbon and a lot less nickel.  The nickel is what's important here, as adding nickel as an alloying agent is where you get your impact strength properties and 4340 has about 3 times as much nickel as 8620 does.  

FYI -- in the AISI numbering system, the last two digits designate carbon content in hundredths of a percent.  4340 has 0.40% carbon and 4330 has 0.30% carbon.  Carbon is the overriding factor in heat treatability.  An alloy with 0.20% carbon or less (8620, for example) has almost no response to quench and tempering operations.  If you see an "L" in the designation like 12L14, it contains lead, which increases ease of machining significantly, but a huge problem for you if any of your extrusions end up in toys, food processing or certain consumer goods.  Just an FYI.  

-T

Engineering is not the science behind building things.  It is the science behind not building things.   

RE: Shaft steels

I was just looking through the McMaster site and it appears that they have ASTM A564 stainless steel hardened to a number of different conditions.  

Since it's not going to be about the calculations, you might just experiment with different hardnesses.  Their H1150 condition starts at a minimum yield strength of 145,000 psi, which is a 45% strength improvement over your current AISI 1144.  If that's too hard, i.e., it breaks even sooner after installation, then go softer.  On the other hand, if it fatigues again, but after a longer period of time, then go with the H900 condition rod.  

Another question -- are you sure that you have your bearings lined up dead-nuts true?  Even a slight amount of misalignment in your frame will allow a fully-reversed bending on the shaft with every revolution, having a devastating effect on any type of shaft that you install in it.  

Oh, and by the way -- about not having gone to college, remember that an amateur built the Ark while experts built the Titanic.  

-T

 

Engineering is not the science behind building things.  It is the science behind not building things.   

RE: Shaft steels

1144 is terrible in fatigue and impact.  Go with heat treated 4340 to improve both of these.

Regards,

Cory

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.

RE: Shaft steels

(OP)
Thanks Engineertex and corypad, this advise is exactly what I was hoping for.
H1150, I can't find that listed maybe from the yeild strength quoted they now call it15-5PH. It is listed as hard to machine and from the earlier posts I can give myself a much better chance by improving the make quality and smooth the finish better.
I will try to source the 4340 in 125,000PSI yield strength.
I remember speaking to the chief engineer of thremo prism when they come to commission the extruder and he slipped that they used En24T for the gearbox input shaft.
Corypad, sorry if I broke the rules.

Thank you all for taking the time to baby me through this discussion.

Oh by the way I think the titanic was engineered OK it was the driver that hit the iceberg and why did the driver of the arc include mosquitoes?

RE: Shaft steels

agpowder,

Several members like CoryPad have link to FAQ's in the signature, so it appears in all of their responses, and is not directed at any one individual.

RE: Shaft steels

(OP)
OK, I have found a supplier of 4340 with heat treated yield strength of 115K PSI and 112K PSI for 1.75" diameter and 1.25" diameter respectively. The metologist (sorry if spelt wrong) is on vacation until Tuesday.
I wanted to check two things here:
1. Is the steel still machinable?
2. Can the shaft after machining be case hardened and ground at the bearing and gear seats?
I have spoken with the CNC shop next door (they will do a better job than me) and they are happy to machine up to 44 Rockwell but the steel stockist salesman doesn't have this info, (he refers to the metelogist)

RE: Shaft steels

Checkout Castle Metals for preheat treated 4340. Normally on preheat treated bar stock the hardness is in the range of Rc 28-32 and very readily mahined.

http://www.amcastle.com/productFormsBar.aspx

It doesn't matter who you get it from you need the E4340 or "Aircraft Quality" material.

Here is another material that we used extensively for parts such as yours. It is a little pricey but we have had excellant results and the Associated service is/was next to none. We used this material as universal repalcement for all low alloy steels.

http://www.associatedsteel.com/pdf/09-10.pdf

RE: Shaft steels

1)  You can machine anything.  It's just a matter of how long it takes and how much effort you have to go through.  

2)  For 4340, a hardness of 44 Rockwell (c-scale, I assume) is somewhere around 180,000 psi - 185,000 psi yield.  This would probably be the upper limit of any of your candidate materials.  

3)  The hardness of 28-32 Rc is around 115,000 psi - 130,000 psi.  The range of 115,000 psi and above will definitely require a heat treat, which is no problem -- you just purchase it in that condition.  As a side note, it's the temperature of the temper that determines the final hardness of the heat treating of 4340.  

4)  I'm not so sure that you would need to case harden the bearing and gear seats.  Did your machine shop tell you that you would need to?  How are they planning on hardening the part?  

-T

Engineering is not the science behind building things.  It is the science behind not building things.   

RE: Shaft steels

(OP)
Thanks Engineertex.
Yes the shop next door said they could send away to case harden the shaft and then they could grind to size. I think he just said he could do it if I wanted it rather than really suggesting that I needed to do it. I get the impression that you lot know more abot the metal properties than he does. He did also say that case hardening would improve the seats life or durability without taking away from the shafts application requirements. What do you think? case harden or not?

RE: Shaft steels

He is probably correct that it will improve the seat life and durability without taking away from the application requirements.  My view is that it's debatable and I can't definitively say to do it or not.  

The best thing to do is go to the SKF, INA or Timken website, check out the engineering section and see what hardness the bearing manufacturer says you need.  

The experimental side of me says not to do it at all and see what happens.  If your shaft breaks again, then the surface hardening was either wasted on something that failed elsewhere, or the hardening was the cause of the failure.  So my experimental philosophy would say not to change too many things at once.  I could be wrong about that and I'm sure that someone here probably has a good reason why I am.  

Is the hardening to be done by flame hardening?  Carburizing?  Shot-peening?  

-T

Engineering is not the science behind building things.  It is the science behind not building things.   

RE: Shaft steels

I would not carburize the part in question. The shaft material, if 4340, is capable of supporting a bearing with no problems. There is a caveat in that the bearing has to be operating within it's design parameters.

 

RE: Shaft steels

(OP)
Ah. now that we are on load of bearings. The original small dia shaft gearbox bearings never failed, however I always changed every bearing everytime I changed a shaft ~ every two months ish. Now with the new big shaft gearbox I have had to change to a smaller OD/ID difference dimension, I mean the OD stayed approx the same but the ID got larger. The dynamic load ratings for the two deep groove ball bearings are 6890N and 8840N for the small shaft g/box but the Dynamic load ratings for the ball bearings on the big shaft g/box are 6370N and 4360N. This could be an issue. BUT I have been more worried concerning the bearings about the width of the bearings, the new bearings are 7 and 9mm and the old bearings are 11 and 10mm.
Now this is an uneducated guess but if the under bearing race bruising was from nocking or a whiplash load then the shaft damage would presumably be at a single point on the periferal of the shaft and now that I have reduced the width of the bearing does this mean faster damage?
However I think with the new shafts properly tolerenced (slight interference fit) and better machining quality and better shaft material and bigger dia then hopefully there wont be a problem?
I am in the process of ordering everything to make 4 additional g/boxes, two for new machines and two as spares on the shelf. I really would like to get them as right as right can be.
The thrust bearings load rating has pretty much doubled from the increase in shaft dia and so can probably be not regarded for this part of the discussion, or maybe I should worry about "not enough" load.
I forgot all about castle metals, I used to play cricket on the south side of Chicago with one of the supervisers there, small world isn't it.
Oh by the way I machined 0.4mm off of the screw and kneader diameters after you commented about the loss of tolerence on your WP and this really has reduced the load and made the machine sound smoother without any noticable dispersion affect - thanks.

RE: Shaft steels

(OP)
Here is a picture of the intermeshing screws of the extruder. These are an old set but very similar to what is running now. The powder coat premix is intraduced into the auger flighting at the far right hand side and then conveyed towards the kneader blocks on the left side. Only the left side of the extruder is heated really just to start the melt. Once conveyed the premix is forced through the kneaders and is worked on to disperse all the raw materials. Once running the work the kneaders impart on the product self generates sufficient heat that we actually have to chill the extruder barrels to cool them down.

Although the individual components are tightly clamped I did notice when I turned the overall diameter down (between centers on lathe) the screws where definetely whipping, I think it would be a reasonable assumption that they try to do this in the machine also.
The only support that these screws have is the gearbox at the right hand end and the floating natue of the screws in the molten plastic at the left hand end. This is why they chatter and bang around when they are new or have just been cleaned. It takes 20 seconds or so for the epoxy or polyester resin to fully support the kneaders.

RE: Shaft steels

You definitely have fatigue involved though it might not be the whole picture. The number 3 and 4 (left to right) shafts have definitely failed from fatigue, evidenced by beach marks.
The large shaft 3 is interesting as a failure in the keyway area usually indicates a torsional component in the equation.
The number 1 and 2 shafts also look to have fatigue as a major component of the failure.

Considering the age of the gearbox it appears that there is a severe wear problem with the gears. As a first pass it looks like they are way under designed.
There also appear to be heat tint everywhere. You will need to change your lubrication to an oil with a higher temperature limit and very high EP load.

At present you have what I call a basket case. you have to resolve the problem piecemeal. Like fix the lubrication, get a harder/tougher gear and work on getting a better material for screws.

Do you remove any components prior to failure?

Have you had any problems with the couplings?

Do you have any connections to a Metallurgical Lab?

  

RE: Shaft steels

Hi agpowder
Thanks for the photographs.
Well the shaft on the far right with the keyway looks like it failed from fatigue and the fact that the failed face isn't flat suggested that it failed under combined torsional and tensile loading. The other 3 shafts look to me anyway to have failed in torsion quite rapidly looking at the surfaces, on the those 3 shafts there seem to be a machined shoulder just at the point of failure.
Now interesting if I am right and the 3 shafts have failed in torsion ie by shear stress then according to my book "Machine Design" by Paul H Black and O.E.Adams Jnr Fatigue cracks do not grow under shear or compressive stresses but I am happy to be corrected if my failure mode is incorrect.
The 3 shafts that I felt have failed in torsion where abouts is that failure when looking at the gearbox picture?I agree with unclesyd its looks like the design needs beefing up and this is where the number crunching comes in, it may pay you to get someone to analyse your design before making anymore parts. Also as suggested get those failed shafts to a laboratory and they should give you a better idea of what the problems are.

Regards

desertfox

RE: Shaft steels

(OP)
Thanks Guys.
I was concerned about the gears as well and I stripped the box down again with intention of changing them out.
I agree and am going to impliment everything that you guys have advised.
For the short term I have to get the machine running for production. Hopefully the NEW (today) gears will last a week for me to get all the stuff prepared. I am going to get the 125K 4340 and machine a set of shafts really carefully paying attention to the fits and finish. I am going to relook at the design and see if I can increase the width of the gears and get them specially made by a specialist gear maker and take their advise on the steel etc.
After getting the machine rigged up to last a while I do need to find a company that can develop the gearbox.
Do any of you know how I could possibly find an engineering company that may want to make and market the machines? They really do work good at making the product.

RE: Shaft steels

(OP)
Sorry desertfox I didn't answer your question.
The three shafts in question did fail at a shoulder and the two bigger ones (the input drive shaft) failed at the other end to where the pulley (sheave) is.
The smaller shaft failed at the right hand side of the gear - the other side to where the hex is.
These shafts are from the old smaller gearbox.
These shafts represent the three types of failure.
The bearing seat diameter reduction damage is the least common. the shoulder fail and the keyway fail are about equal in occurance.
I think the basket case description is correct, I think I need to make and assemble this whole box perfectly with regard to fits and finish and see what happens.

Excuse my ignorance again, when you say a very high EP Load are you refering to the AGMA Grade. I am using an ISO grade of 46 which I think is about 1EP are you saying I should use say 7EP which has an ISO grade of 460?

RE: Shaft steels

The actual breakages are now well covered, so my comment is regarding the machined finished on the shafts especially in the region of the failures- the surface finish is, if you will excuse an Australian colloquialism,"bloody awful" - from what can be seen on the photo's a lot more attention is required to surface finish - I would not be surprised if one aspect of the failure is "stress raisers" which have been machined into the shafts.

RE: Shaft steels

Yes I would go with a 7EP until you can get things under control. Here is an oil that I've used for many years both as crutch and a standard lubricant. One thing it will do is track the gears and is very slow to drain off on shut down. You never startup metal to metal.
There are other products that have similar characteristics.

http://www.le-inc.com/products/documents/0604-0609_tdb.pdf

agpowder can you give us some information on the input to the gearbox and maybe a little information on the gears, motor hp and rpm and sheave sizes? That is one helluva an input sheave.

desertfox,
I was thinking along the same lines as you but when I went to a better monitor I believe I see beach marks on #3. The edges of the fractures appear to have the look of multiple origin fatigue. The final fracture would be shear in either case. It gets tricky since the shafts are so overload. I would like to see the fracture face with oblique lighting.   

RE: Shaft steels

Hi All

Artisi makes a very good point regarding surface finish.

Unclesyd
You may well be right I tried zooming in on the pictures and
the shaft next to the one with the keyway and it may have some slight beachmarks but I couldm't be sure, I could certainly see some river lines  pointing from bottom right toward top left indicating the source of a crack.
All the failures look quite brittle and it would be interesting to know how long this items lasted in service and are they all made from the same material.

desertfox

RE: Shaft steels

desertfox,
As you posted I was sitting here thinking about my old metallurgical lab with 2 Axiomat Metallurgical Microscopes, 2 high power stereo microscopes, and all the accessories. Just around the corner from the lab there are 3 SEM's. All this equipment is going unused as they have no one that is interested. I surely would like to have the fractures and about one hour.

agpowder I will check tomorrow if I can find the name of the company that built some multi output gearboxes for us.  

RE: Shaft steels

Better surface finish and a stronger steel will help you I am sure.
One specific design change you should do is to change the shape of the keyway. your current keyway design is a constant depth and ends with a radius equal to its width. that is not a really bad keyway but you can do better.
make your keyway the width and depth you have it, but at the end it should continue straight as the depth of the keyway shallows up along the radius of the cutting tool you use. this will greatly increase the fatigue strength of that portion of your part.
The machine shop will have no problem doing this or if you choose to do it you can use a woodruff cutter cutting in from the side (assuming a vertical mill)

http://www.abmtools.com/Cutting_Tools/Woodruff_Cutter_Key_Way.htm

the picture on the left in this is what I meant by the shape of the keyway. (took 15 pages of Google images to find a good drawing)
http://www.me.metu.edu.tr/me307/Standards/keys/TS%20147-ISO%202491.pdf

RE: Shaft steels

agpower

The most important thing you can do at the moment is to have a proper analysis undertaken of the shafts to establish the failure mode.  Once this is known you can then look into what is causing this type of failure, at the moment you seem to be reacting without any proper review as to why you have been changing things - this just leads to going round and round circles - unless of course you unknowingly hit on the problem and cure.  

One other point you need to very carefully look into is the alignment between g/box and extruder - why I say this is because of the 2 hacksaw blade pieces shown in your photo' which appear to be used as shims - need I say any more - also, the rigid coupling between shafts is other than ideal as it doesn't allow for any misalignment, runout or thermal change which can lead to problem. I realise there is little room between shafts but I would search / inquire for a flexible connection for use to transmit the drive from g/box to extruder, it could be as simple as a small swivel joint.  

It is very possible that you don't have a "steel problem" but rather a design / configuration / assembly problem.

 

RE: Shaft steels

what about self aligning bearings? If they could be incorporated they would make the unit less sensitive to a small misalignment.
Again please consider changing the shape of the keyway as I posted above, It will help with or eliminate the fatigue failures originating from the keyway. Eliminating this problem should allow your unit to last long enough to consistently fail at its next weakest point and simplify your problem solving task.

RE: Shaft steels

Shaft failures 1, 2, and 3 (from left to right) are all due to stress concentration at the sharp transition between the diameters.  The concave appearance of the shaft ends shows that the reduced diameter actually pulled material away from the shaft as it cracked.  This is classic stress concentration.  Poor machine finish in this area may also contribute, but modifying the transition will fix this problem.  Increase the radius between the large and small diameter, or use one of the other established techniques for reducing stress concentrations.

The keyway failure is, as others have indicated, a fatigue failure related to the design of the keyway.

Get rid of the stress concentrations and poor fatigue details, make sure everything is properly aligned (i.e. eliminate bending), and you'll go a long ways towards solving your problems.

RE: Shaft steels

(OP)
OK, thanks everyone for the advise.
As I said earlier the shaft photos were of the old ones. On the new design I have consistantly doubled the cross sectional area of the shafts in almost all areas. I can change the keyway spec. What are you general feelings on doing away with key and using Heavy duty keyless bushings,ie the double taper sort that when tightened expands the OD and grip the ID?
The hacksaw blades are actuall feeler guages that I am using as shims, I spend quite a lot of time adjusting the position of the extruder barrel and probably more importantly the position of the gearbox. I use feelers so that I have a really decent assortment of thicknesses.
Couplings. Yes they are solid. I have to be able to broach a hex into the coupling. I have to start with a 5/8" through hole. Also I have worked on two other makes of extruder and they all use rigid couplings. Thermal expansion wouldn't be a prob because the screws are free inside the barrel and can move out if needed. When I install a new gearbox I first install thecouplings onto both hex shafts and make that rigid. I then feel the rotation through the gearbox, spinning by hand and adjust to suit with my feeler gauges. I then bot down and then loosen the cooupling to ensure they slip nicely up and down both shafts.
Motor- 1750 rpm. Motro sheave-3"PCD, Gearbox-11"PCD, Input shaft gears-2.667"PD,12pitch,0.75" face width. Driven shafts gears-1.667"PD, 12 pitch, 0.75" face width.

Like I said earlier I am going to make one other gearbox taking into consideration all the advise given and install it and monitor it.

RE: Shaft steels

Bigger shafts isn't the answer.  Get rid of the stress concentrations and you'll have it.  It looks like your small shaft failures occured at the transition to the hex outside the gearbox.  If that's true, there's no reason you can't make a smooth transition from the large diameter down to the hex.  A smaller shaft without stress concentrations will have better fatigue life than a larger shaft with stress concentrations.

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