Spline coupler material selection 2

[724napier]

I am working on re-designing a spline coupler that failed. I do not have a lot of the details of the coupler that failed, as we had no involvement with the original design. Pictures of the failed coupler are attached. It is believed that it was made from 4340 and heat treated to RC 32. Looking at the coupler, it appears that the spline was manufactured rather poorly and I believe the profile might not have met the required standard. The coupler shows clear signs of overheating.

This coupler mates an engine to a splined male shaft. The male shaft is made from 15-5 PH H925 stainless. The torque transmitted by the engine is approximately 350 ft-lbs, but spins at speeds up to 20K RPM. There is another shaft currently being used in place of the adapter that works without any problems. It is made from nitralloy 135M and the internal spline teeth have been nitrided. The shaft is almost identical to the spline coupler, but measures roughly 18 inches in length.

My questions are:
The hardness of the male shaft made from 15-5 stainless is around RC 42. The original plan was to make the adapter out of nitralloy 135M, since the working shaft is made from that alloy. However, with the teeth being nitrided and the surface hardness being much greater than that of the male shaft, would that pose any problems? In general, shouldn't the hardness of both the male and female splines be similar? So if one is case hardened, shouldn't the other be hardened?

Would it not be better to choose a material with similar strength while closely matching the hardness of the 15-5?

Any help is appreciated.

 

[CoryPad]

In general, the splined components should have similar hardness/strength. A GM best practice is to have a maximum hardness difference of 10 HRC.

 

[724napier]

CoryPad, does that include localized hardness such as spline and gear teeth? Do you have any recommendations on materials that would work well with 15-5 PH H925 stainless?

 

[CoryPad]

It applies specifically to splines and gear teeth. I do not know any of the details of your design, so I cannot advise you. In general, a steel (stainless or not) should work for load transfer and durability. A stainless pair may present problems regarding galling.

 

[tbuelna]

724napier-

From the pictures it definitely appears the coupler's internal spline tooth flanks experienced severe abrasive wear and the coupler body experienced elevated temperatures. However, it's hard to determine which occurred first- the overheating or the structural failure of the spline tooth flanks. The operating environment you describe that this coupling is subject to sounds quite severe, especially the speeds and torques. If there is even a small amount of sliding at the spline contacts with marginal lubrication/cooling, then the resulting friction could easily cause create excessive heat buildup in the coupler body.

If you would be kind enough to provide more details regarding the spline specifications, lubrication provisions, interface dimensions/tolerances at each end, etc, then I'd be happy to provide recommendations on potential resolutions for the issue. With spline couplings, factors like lubrication and tooth geometry can be just as critical as materials and heat treating.

Hope that helps.
Terry

 

[724napier]

Terry,

I don't have all the information, but this is what I do have.

The spline profile follows the ANSI B92.1 standard:
Class 5 tolerance
Fillet side fit
12/24 pitch
30 deg pressure angle
18 teeth
1.5" pitch diameter

The mating male splined shaft follows the same spline standard (except it's for the external spline) and is made from 15-5 PH H925 stainless.

There was an aerospace grease used with the coupler, I do not have the specifics on it. However, I can probably specify a grease/lubricant to be used for the re-designed coupler. It has no provisions for lubrication as it's not part of a gearbox, so it used externally.

Thanks for the help

 

[tbuelna]

724napier,

Assuming your shaft and coupling have good alignment, then the 350 ft-lb torque would only result in a simple flank contact stress (P/A) of around 3733 psi for a 1" wide face. This level of contact stress is quite modest even for the Rc32 4340 material the coupler was constructed from.

Since your coupling operates at up to 20K rpm, one thing I would suggest is to use a major diameter fit type spline instead of the side fit spline you currently are using. At high rotational speeds like 20K rpm, the radial play in a side fit spline will allow the shaft to "shuffle around". I would also recommend that you crown the external teeth of the major diameter fit shaft spline to allow some misalignment capability. Your choice of Nitralloy with nitride hardened teeth for a crowned major diameter fit shaft spline seems good to me.

The other modification I would suggest is to provide some sort of seal (like an o-ring) at each end of the spline joint to keep the grease in place. With the coupling spinning at speeds up to 20K rpm, without some type of seal to keep it in place, the grease will simply be flung out of the spline coupling.

Hope that helps.
Terry

 

[724napier]

Terry,

The male shaft of the mating component is not being fabricated by us. It is an existing assembly, so I cannot crown the teeth. Although, I wish I could. I thought the fillet side fit is capable of holding more load than a flat root side, or major diameter fit. I don't fully understand when to use a major diameter fit over a fillet root side fit.

Thanks for the help.

-Chris

 

[mfgenggear]

724napier

because the parts are spinning at a high RPM rate. it will wear prematurely. having an interface that will retain alignment will help immensely.
altho I am a big fan of Terry. I would recommend a coupling that is self aligning. diameter interface with o ring groove like mentioned earlier to prevent
the grease from spinning out.

 

[tbuelna]

Providing proper radial constraints at each end of a fairly long, slender quill shaft spinning at high rpm is also important in regards to shaft dynamics. The first critical speed of an 18" long solid steel shaft having a body diameter around 1.3" (the ext spline root dia minus .05") and having good radial constraints at each end (ie. bearings) is probably not a whole lot higher than 20krpm. But if there is radial play in the shaft end constraints, like you would have with a side fit type of spline joint, then the first critical speed of the shaft system may likely even end up below your 20krpm operating limit.

As a rule of thumb, you try to maintain a .50 margin between the max shaft operating rpm and the calculated first critical speed of the shaft system. In this regard, it is very beneficial to use major diameter fit splines for quill shafts since they provide the best radial position control of the shaft ends. But since you don't have the option of manufacturing a new shaft, then you might consider reworking the existing shaft and coupling to add some sort of feature at each end like a bushing, that will provide more precise radial positioning of the shaft.