How to Choose Spline Sleeve?

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 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.






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 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 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 psi for a 1" wide face. This level of contact stress is quite modest even for the Rc32 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

More specifically people will ask "How much of the blue should be showing?". The blue doesn't matter, it doesn't always start in the same spot so you should really be looking at how much spline is showing. The answer though depends on the type of spline you have. For Tom Wood's drive shafts there are mainly two types of spline; the S spline and the X spline. If you have the S spline, there should be about 1.5" of the smooth bar showing. For the X spline you'd measure from the dust cap to the center of the weld, this should be around 3 1/4".  As long as you are within a half an inch or so of these numbers, consider your shaft a good fit. If the amount of spline you have visible is much more or less than what is recommended, you may need to have your shaft lengthened or shortened.

Jundi are exported all over the world and different industries with quality first. Our belief is to provide our customers with more and better high value-added products. Let's create a better future together.

Below you can see what a shaft built with our X spline should look like when installed in the vehicle. The second photo is a side by side to give you an idea of how much spline would still be inside the slip yoke under normal operating conditions. 

For more information, please visit Spline Sleeve.

In the above photo, you can see the difference between the total length of the spline and the usable length. For safe and stable operation, the slip yoke and spline should maintain at least 2 inches of engagement at all times. That means that however long your spline is, you can subtract two inches from that to get your usable range of travel. You can see in the above photo that usable range is in red. The section shown in blue needs to remain inside the slip yoke at all times, even under full axle droop. 

Are you interested in learning more about Tube And Clamp? Contact us today to secure an expert consultation!