Involute Spline Fretting (constant mesh pto)

[triumph993]

I have a hydraulic pump with a 7/8"-16/32DP straight involute external HT alloy steel spline shaft. The splines are experiencing contact fretting. (See spec's below and pic attached). The pump connects to internal splines of a constant mesh PTO, which in turn is bolted to a truck transmission and is powered by a heavy duty diesel engine. The spline is greased via a shaft centerline thru-hole which enables cleansing of the fretting debris.

Diametral Pitch: P = 16
Number of Teeth: N = 13
Pitch Diameter: D = 0.8125 in
Face Width: F = 1.31 in
Torque: T = 1550 in-lbs
Depth of Engagement: h = 0.05625 in
Compressive Stress: Sc = 3983 psi

In addition to the compressive stresses under load, there are torsional vibrations imposed by the drivetrain. Would you expect that the fretting is primarily caused by insufficient lubrication, alignment error, torsional windup, edge loading, shock loads, or something else? What are some options to limit contact fretting? My immediate options are a larger shaft, 1"-15T, and/or an oil bath lubrication system.

Thanks,
David

 

[gearcutter]

Based on the numbers you've come up with; I certainly wouldn't call this an ideal design.
I'd certainly like to see, at the very least, a 1.5x – 2.0x safety built in to the design if alignment cannot be guaranteed.
Most of the splines we work with in the mining industry generally operate at 10x – 15x safety factors.


Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

http://www.aussieweb.com.au/email.aspx?id=1194181

 

[triumph993]

It's puny! Again, thanks for your help Ron. Now I know why they don't last. I can move to the SAE BB shaft (1"-15T) and that will give me:

1) a 'compressive stress in teeth' margin-of-safety of 0.349 versus -0.129, and
2) a 'shear stress in teeth' margin-of-safety of 0.487 versus -0.032.

Better, but nothing to write home to mom about - certainly not good enough to pipe the effluent into my transmission, unfiltered.

David

 

[Tmoose]

How long are they lasting before looking like the pictures?