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]

It would be really helpful if you could clean the part and then post the image.

Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

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

 

[triumph993]

Thanks Ron, I'll produce a better picture, in the meantime attached is a slightly cleaner photo of the PTO internal splines.

Thanks,
David

 

[hydtools]

Doesn't look like fretting. Looks like wear; overload or grit or poor material. The splines are almost gone.

Ted

 

[triumph993]

The manufacturer of the PTO says that the wear is caused by engine idle time.

The application is a hydraulically driven fan motor. The fan produces a variable load on the pump. The fan control is proportionally driven and the fan motor has built-in trim speed control.

There are 5 components, (1) the pto, (2) the pump, (3) the fan motor with integrated flow control, (4) a fan drive control assembly (FDCA), and (5) an electronic control module (ECM).

The ECM produces an electrical PWM signal which feeds the fan drive control assembly which then sends a proportional hydraulic signal to the fan motor flow control valve. This allows the ECM to gradually ramp up and ramp down fan speed and to arbitrarily set the fan speed.

At engine idle most of the pump pressure is routed around the fan motor - the pump is mostly freewheeling. The engine produces significant torsional vibration at idle which is fed thru the transmission and results in clattering of the PTO drive gears. I don't know how to quantify the cyclical and possibly reversing nature of the loads on the splines during idle.

The manufacturer is in favor of an oil bath lubrication system which draws and returns fluid from/to the transmission. I'm concerned over passing spline debris into the transmission.

 

[hydtools]

Might be worth using a smaller pump and then running the engine above idle. The current pump may be 'free-wheeling' but the engine is impulsing the drive and the spline. If most of the flow is being bypassed, the pump is producing too much flow.

Ted

 

[Tmoose]

If I was stuck with using this design for a while I'd assemble a set of new parts with Kluber's recommended grease/paste.
I expect the shaft that mates with that female spline in the hub is junk now, too, so will destroy a new hub in short order.

As other's said, rev it up.

More flywheel on the engine side might help smooth things out.

I's be thinking about a way to apply enough torsional preload to keep the spline from rattling too.

 

[triumph993]

hydtools and Tmoose, thanks for your help. Avoiding idle is a possibility. Thanks for the tip on Kluber spline grease. I still haven't had a chance to take new photos of the cleaned up worn splines but hopefully today or tomorrow.

Thanks,
David

 

[triumph993]

Here's a photo album of the cleaned up external spline.

https://picasaweb.google.com/dmb200...&authkey=Gv1sRgCNSloqW9qonCaQ&feat=directlink

Thanks,
David

 

[gearcutter]

The wear looks to me like it is as a result of many factors but, given the amount that there is, I don't think that fretting is one of them.
The width of the wear lip is uneven suggesting that there is an uneven load distribution.
I can't see any evidence of "torsional vibration"......this would create wear on both flanks not just the loaded flank.
A once off application of grease is the worst thing you could use in this environment.......you'd be better off assembling the parts dry.


Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

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

 

[gearcutter]

How is the helical pinion shaft supported?

Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

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

 

[triumph993]

Thanks Ron,

The pump external shaft uses DU bushing journals, one on either side of the pump gear, width: 1.125" x ID: 1.30".

The PTO helical shaft is supported on cone bearings with a lock ring. It's an SAE 'B' shaft. I don't know the bearing dimensions. The shaft is 1.25" OD, and the bearings are approximately 3.5" apart.

In my OP I had the wrong 'Face Width'. Here are the corrected numbers:

Diametral Pitch: P = 16
Number of Teeth: N = 13
Pitch Diameter: D = 0.8125 in
Face Width: F = 0.8335 in
Torque: T = 1550 in-lbs
Depth of Engagement: h = 0.05625 in
L/D = 1.026
Compressive Stress: Sc = 6260 psi

I'm not sure if I can risk a wet spline lubrication system. There's no filtering of the spline cavity output fluid before returns to the transmission.

David

 

[gearcutter]

It seems to me that, along with possible misalignment issue, the material and spline chosen are not adequate for the job.

Here is a link where you can check the numbers:

http://www.zakgear.com/External_Spline_IN.html

Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

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

 

[hydtools]

Both driving and driven splines are worn, I'd call them hammered.

A smaller pump would reduce the pump's torque reaction capacity and reduce the resistance to the engine impulses and should reduce the spline loading.

Ted

 

[triumph993]

Thanks Ted and Ron,

Ron, I used your link to check my numbers and they do checkout, but these are steady state conditions. There may be transient conditions which likely push the compressive stress well beyond my 6K psi steady state.

The pto is constant mesh, so the pump is always spinning. A common condition is having the fan turn on when the pump is spinning close to it's max RPM and delivering it's max GPH at low pressure.

At this point the ECM should turn the fan motor on gradually to avoid pressure spikes. In older equipment gradual fan turn on may not be effective, so there may be a pressure shock wave sent thru the system as the system tries to adjust to it's new equilibrium.

So I may be seeing metal fatigue do to hydraulic transients. I need to brush up on my fluid dynamics.

Ted, The fan motor requires 40 hp to move the CFM need to cool the heavy duty diesel engine. Unfortunately this is the smallest pump available that does the job at the PTO rpm's required. Thanks for the suggestion.

I think you guys are correct, the splines and/or material isn't adequate for the job. The stresses are likely higher than I thought.

David

 

[Strong]

"The engine produces significant torsional vibration at idle which is fed thru the transmission and results in clattering of the PTO drive gears. I don't know how to quantify the cyclical and possibly reversing nature of the loads on the splines during idle."

Torsional vibrations can be measured with battery-powered strain gage telemetry system or optical encoder and F-V (torsional) converter. Test the system with new components to identify torsional natural frequencies. Install a rotating mass, incease shaft stiffness, or add flexible (elastomer type) coupling to detune the system, as necessary. If you really want to get rid of this problem forever!

Walt

 

[hydtools]

Adjust the idle speed up and/or use a flexible coupling.

What are the flow and pressure requirements of the fan? Just curious.

Ted

 

[gearcutter]

It'd be interesting to see the numbers you used for your calcs.
Can you post a screen shot showing the entered values?


Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

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

 

[triumph993]

Hi Ron,

You're correct. After paying special attention to the correction factors in your spreadsheet (at zakgear.com) it appears that the compressive stress of the SAE 'B' shaft is on the edge.

I have the following factors:

Application Factor, Ka, 2.2: (driven by diesel engine, light shock)
Load Distribution Factor, Km, 1.5: (0.004" per inch of Face Width)
Life Factor Limited by Fatigue, Lf, 0.5: (number of torque cycles)

I tried to be reasonable conservative with these factors.

Also, I'm not yet sure of the material so I chose a reasonably conservative option: Surface Hardened Steel.

Thanks,
David

 

[triumph993]

Ted,

Attached is a chart showing pressure, rpm, and flow requirements.

Thanks,
David