One way roller outer race design analysis help

[lfw618]

I'm a recent grad and relatively new to the automotive industry. I've been given an outer race for a one way roller clutch as one of my first projects. I'm hoping someone here may have some insight that may help me. One of my primary concerns for failure of the race is from the hoop stress from the rollers. I've researched a bit and found the equations for hoop stress at the loads and between the loads from Timoshenko's Strength of Materials. I'm having some difficulty determining how to apply these to get useful numbers that will best reflect the real-world stresses.

The calculations for hoop stress are derived from a cross section of the race. Since the race I am designing needs to have an oil collector groove, (kinda similar to the 4T80E race), the cross sectional area will not be a regular geometry. Additionally the rollers will only directly apply pressure against maybe 2/3 of the width of the race. Should the cross section I am using to determine the stress, include just the area that would be on the directly on the path of the roller, or the full area of the cross section of the race. The numbers depend a lot on the neutral axis, so it is hard for me to determine if it will be more accurate to analyze the complete area at the cross section, or just the area directly under the load from the rollers? Or determine the neutral axis just from the cross sectional area directly on the roller path, and then apply that to the entire cross section?

Apologies if this is unclear. If anyone has any insight that could help it would be very much appreciated.

Thank you

 

[MikeHalloran]

The hoop stress would be computed using the full cross sectional area.
The sprags also apply compressive stress that would be Hertzian in nature, so distributed near the contact area only.
... and that's about as far as I can go, based on zero experience actually analyzing real sprag clutches.

You need to chat up an old-timer at your workplace.
Bring coffee.
Ask questions.
Listen, hard.
Take notes, or record the conversation, or make a delayed but contemporaneous record of what you can recall before you are distracted by other things.



Mike Halloran
Pembroke Pines, FL, USA

 

[enginesrus]

One other detail that I think is important is what material and what physical properties does the case that the clutch is going to fit into have, since it also adds strength to the race. Also what is the fit going to be? I think most roller clutch races are similar to a roller bearing and use a case hardening. Just some ideas.

 

[tbuelna]

Getting the design of overrunning clutches (roller/sprag/etc) right is no simple matter. Typically, there are other considerations such as PV at the overrunning interface, Hertzian contact stress levels at the race surfaces, load distribution between the roller/sprag complement, and load distribution along the individual roller/sprag element face widths due to skewing, torsional wind-up, or inner/outer race angular misalignment.

The rotorcraft OEMs have done lots of work with the design of high-performance roller/sprag clutches. Take a look at this helicopter freewheel design guide published by Sikorsky in 1977.

http://www.dtic.mil/dtic/tr/fulltext/u2/a047559.pdf

 

[romke]

assuming that we are talking about a failed outer ring that calls for redesigning, can you tell us some more about the type of failure, where damage occurs (outer ring, inner ring, rollers), whether discolourization is present etc?

 

[lfw618]

Thanks everyone for the responses, I appreciate the help, it's definitely given me help in going in the right direction. Romke, I will check again on this to make sure, but I believe its mostly on the inner surface, and failure is starting right at the oil holes usually.

I am not sure as far as discoloration goes, what would that signify to you?

 

[romke]

Local changes in color (darkening) could indicate that the lubricant overheated, due to insufficient supply and/or lack of lubrication when needed.

 

[lfw618]

it's a slightly different color along the roller path, but I'm not sure it's quite what you're talking about, except maybe on the edges of the roller path. It's hard to tell what could be just regular wear and would could be darkening due to overheated lubricant. I am not quite sure how to fix this if this were the problem, the oil holes are already probably the weakest part of the design.