Help deciding on clutch size...

[christianroth]

We are currently in the process of designing a new lockup clutch in an automatic transmission, and we are undecided on the size of the actual clutch material size. I need some help computing when clutch area becomes to large and actualy hurts holding force.

Say we have 50 psi behind a steel plate of approximatley 50sq inches. On the other side of the steel plate, resides the actual clutch material in a circular fashion totaling an area of 10 sq inches. The clucth is pressed against the front cover of the torque converter. This clutch area is what we are testing, and how it effects the holding force. For this example the static coefficent of friction is 0.75. We will disregard the effect of rotational force that the fluid applies. Also for this example the radius of the start of the clutch is 4" and the outer edge ends at and radius of 5".

So now that we have 2500 lbs pressing the clutch together how does one compute if a larger clutch area is better evening though the effective force being applied is less.

 

[christianroth]

I guess the main jist of the post can be answer by the question, Is static holding force dependant on area?.

 

[GregLocock]

In theory, no. In practice it depends on an impossibly large number of factors - if the mating surface is wet you may find improved grip with reduced area, for instance.

The main consideration is wear, I would have thought, since the LU clutch needs to outlast the trans.
Cheers

Greg Locock