Failed Bandsaw Gearbox Shaft, Material Recommendation 3

[ornerynorsk]

Hello everyone,

Just had a main gearbox shaft failure (torsional bending fatigue?) in a production horizontal saw. It's been in service 19+ years (daily use) and is still in production by its original manufacturer. We may be in a situation where we have to machine our own replacement shaft just because of the time involved in getting a factory part.

The OEM part is a mystery steel at this point, induction hardened from both ends toward the center stopping right at a snap ring groove where the stresses concentrated to propagate the failure. I am proposing to machine our temporary replacement out of 4340, through harden, oil quench and temper to 42-46 RC, with a little radius in the snap ring groove and key seats. The gearbox is put together like a brick shothouse, massive bearings, worm and gear like new condition, oil bath, etc. I don't want to put an inherently flawed part back in and we may end up using our temp replacement long term. (we did discuss with the factory the perceived shortcoming of the original design, even though 19 years of hard use is pretty good)

Any pitfalls that anyone can think of concerning the potential next weakest link or my proposed material selection?

Thanks.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[arunmrao]

4340 is a safe selection in my opinion. It will serve you long .

I'm just one step away from being rich, all I need now is money.
( read somewhere on the internet)

 

[CoryPad]

A potential pitfall is environment assisted cracking, aka hydrogen embrittlement. You should consider the manufacturing processes and the application environment. Perhaps keep the hardness closer to 42 HRC. Sounds like you are starting from a good place.

 

[ornerynorsk]

Thanks for the speedy responses. The hydrogen embrittlement comment roused my interest. We've not tested for water contamination in the gearbox oil, but it came out rather creamy. We'll have a look at our maintenance interval, and a second, closer look at our bearings, as well.

Thanks again!

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[mrfailure]

I would suggest taking a little time and at least analyzing the material of the failed shaft before you machine your own. You can confirm the alloy, the hardness, and surface hardening by having a little metallurgical lab analysis done. That way, you'll have confidence in the material and heat treatment for the replacement.

 

[metengr]

Re;

Just had a main gearbox shaft failure (torsional bending fatigue?)

Most likely, rotating bending fatigue based on the fracture surface appearance. See if you can introduce a larger radius at the snap ring corners to reduce stress concentration in addition to the above suggestions.

 

[tbuelna]

From the photo provided, the shaft appears to be a fairly simple part to machine. Since the existing shaft lasted for 19+ years of service, it does not seem that there is a fundamental design issue with the component. I would doubt that the existing shaft was made from a material that was as high quality as 4340 alloy steel, so machining the replacement part from a piece of 4340 bar quenched & tempered to around Rc42 should provide service life well in excess of the original. And as suggested by others, you will want to pay attention to the machining details of the retaining ring groove, such as surface roughness and corner fillet radii.

Given the relatively simple design of the shaft and the long lead time/cost involved in procuring a replacement, it may indeed be cost effective to purchase a piece of heat treated 4340 bar and have your local machinist make the new shaft.

Good luck to you.
Terry

 

[ornerynorsk]

Thanks all for the helpful comments. We cut the new shaft in-house, placed in service, and it's performing as intended thus far. I'll let you know in 0 to ∞ years how things fare!

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[arunmrao]

Fast work, ornerynorsk, breakdown jobs know no mercy .

I'm just one step away from being rich, all I need now is money.
( read somewhere on the internet)

 

[TVP]

I love 0 to ∞ years!