Shaft steels

[agpowder]

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?

 

[Artisi]

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.

 

[carnage1]

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.

 

[minerk]

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.

 

[agpowder]

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.

 

[minerk]

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.