Properties of Metals

Can you incorporate a spring loaded chain tensioner to absorb the momentary overloads?

What about a spring loaded clutch? I have one on a piece of equipment that is designed to prevent damage if the front of the equipment can not move while the operator tries to move it anyway.

Another way is to paint a stripe on the shaft in a straight line. As the shaft starts to twist, the paint will show the twist. Then you will know when to change the shaft.

Three choices, plus the harder steel option. Harder steels tend to be more brittle though. Good luck.

Hello tjohns2067

In addition to bob c suggestions above, you could try machining a replacement shaft from axle grade steel, which will absorb shock loading better.

The spring-loaded clutch is really the first and best option.

A combination of both the above, should see no more of that problem.

Kind Regards....

I am not sure what your budget limitations are, and what is the size of this shaft, but it looks pretty obvious that you need to go to a better steel, such as 4140 for example.

To my earlier answer I would add that changing the shaft material with a more hardenable steel will most probably prevent torsion-induced plastic deformation since the outer region of the shaft would be stronger. The question of what steel to select depends on the overall size and on the torque applied. The idea is that usually a higher yield point also indicates a higher shear stress, so you need to strengthen bith the surface and the core to prevent the described failure.

How disruptive is a shaft failure to your operation? How long does it take to replace a shaft? How long do the current shafts last? How long is the shaft, and what is on it.

Heavy duty trucks (10,000 lbs gvw and up) have rear axle shafts that have splines on one end, and a mushroom like flange on the other, with a series of holes around it. these are as strong as cheap whiskey. Once you set up for this type of design, shaft changes could be done in 10 minutes.

Several issues here, .

frequent overloads... WHy?How often? Why? How much over design limits? what are design limits? what is shaft dimension (and actual grade and heattreatment) and how much is overload force?

Without these FACTS you will only get well meaning guesses for prescriptions.

Perhaps 4140 as has been suggested may be able to accomodate your conditions, but without the above facts, that could be a well meaning but insufficient prescription. Could also make the next failure even more catastrophic. Maybe its better that the shaft bends than...???

Why is your management tolerant of frequent overloads? Stronger shaft may in fact be necessary bandaid, but "stopping the beatings" may eliminate the need for said bandaids...

Finiding and eliminating rootcauses is always more rewarding than just fixing the obvious thing that broke. my challenge to you and your management, suppose magically 4140q&t, 4340q&t or properly heat treated 8650 (for hard surface and good core toughness) were used as replacement, and fully up to transmitting the overload power to the rest of your works. Now, on the nth overload, what breaks this time and how expensive is it not just in replacement cost, but in the new amount of downtime as a result of your "make it harder once its failed under overload conditions" arms race? Who gets killed?

Stopping the abuse should be major emphasis. Getting your wish of "a shaft that don't bend in abuse" may give you "wisher's remorse" down the road.

Given shaft dimensions and forces involved, one of us here may be able to give you some ideas on a grade switch.

The clutch ideas are excellent if you have room to install. How many will you need to install (how many of these shafts are there on this transfer line? that could have aill be able to similarly fail?)

What exactly do you mean by "low carbon"- i understand it to mean under 0.25wt%, A cold heading guy might take it to mean less than 0.08 wt% , and a tooling guy might mean less than 0.85wt%

FYI, We used truckloads of 1050 or 1144 for power take offs for agri business in the 50mm range as cold drawn, with out heat treatment. 4140 without suitable heat treatment is "not better" than 1045 steel also with out heat treatment. Alloy steel without heat treatment is money wasted.

milo "no prescription til we get the patient's vital signs"

Milo, I didn't want to get into a discussion without the details either. My reply re 4140 was a typical simple solution for this sort of problems and of course I meant treated 4140; it was mentioned in the question that there are space limitations, and I assumed they don't want to redesign. The question was what to do within the existing application and space requirements. I guess when he said 'low carbon' he meant un-heat treatable, i.e below ca. 0.25. I wouldn't be surprised if the steel used was actually something between 1010--1016. However, you are right of course, a much better solution would be to redesign the whole thing, but in my view this has been ruled out in the initial question.

Hi sensei2004. I have done/continue to do this for a living and I have learned that the folks on the problem side often have quite different ideas about things than I do. I have much job experience in this area, and remember full well when a tier two supplier sent me failures of 4140 (unheat treated) after they twisted apart. "your salesman said 4140 is better than 1045." Did he mention they needed to be heat treated?- "uhh I don't remember, but we paid more for it so we thought it was better steel." As delivered put into into half shafts...(no i won't name any names!)

or the 'moderns' that make lawn and garden equipment that put an extra 500 pound on the hitch of their garden tractor, put clutch in with a bar, then rev engine to fullest rpm and pull out the clutch bar into highest gear to see what breaks. Then these du******s wanted to strengthen that until the next thing broke. they went fromshearing the key on the gear, to fracturing the gear, to overtorquing/splitting the shaft. (No , no engineers or designers were involved in this priceless little study...)

Then wanted to enter a claim for the steel to boot. And by the way, went to get the cheapest steel they could from a minimill that had inclusions (internal cleanliness issues) the size of pencil erasers but loved the price... My company probably had $2000 in the lab work, because we had to prove they weren't actually routine service failures, when we showed the forces involved, the company came clean. they were pursuing the claim through a local machine shop, my customer... at the time.

They were trying to get "heat treated Aircraft Quality 4140 performance" out of one step above rebar inclusion ridden plain carbon steel mini mill bars... Weren't even aluminum grain refined.

These experiences make me quick to ask for more information and slow to prescribe...

when they follow our advice ... but only with half the problem described and half the answer understood, often very bad things happen.

Hardenability as you pointed out could come into play, but typically though hardening isn't the best way to get toughness in shaft(s). So I shied away from an answer until we get more data.

I remain concerned that this shaft that failed is but one in a long series of these puppies, and so toughening this baby means the failure will go somewhere else...so then they upgrade all the shafts, and now they trash a gearbox...

They need to eliminate over loading. THAT's ENGINEERING. Engineering as medic for out of control process often ends up with engineer making depositions and on the witness stand.

Your conjecture of 1010-1016 is a good one, I would suspect 1018 if this is here in the states, thats the low carbon vanilla grade for 'special bar quality' as opposed to structural. Who knows the design intent may have been to let the shafts twist rather than blow the gear box/ speed reducer???

best regards.

milo

This was a scenario out of a text book for a class. I was looking for the same answer. I like how commited you guys are to your trade.

I didn't post this, I just stumbled onto it by web search. Sorry though...