Racing Clutch Match 3

[JS3]

Im trying to figure out plate load math, torque capacity formulas for a drag racing clutch. Crower pedal type used in the NHRA alcohol dragster/funny car classes. These are 3 disk, lever type with counter weight and static springs. Any clues? I'd like to build a spreadsheet to make tuning decisions.

 

[Fabrico]

It would seem that could only be done by carefully plotting of what works. Those massive multiplate clutches go from room temperature to almost melting in about 3 seconds. How do you factor that into spring pressure, torque, or lever position?

 

[JS3]

Fair point, massive amounts of heat, depending on how well the clutch is engaged on a particular run, would impact the coefficient of friction on the disks as they may glaze over etc. I'm keen on at least getting the math right for plate load. Ive seen various calcs for measuring coefficent of friction based on number of disks and surface area. I would have to also guess that measuring the plate load from the static springs is straight forward. Spring rate/in based on how made threads you have turned on the adjuster stands times number of springs etc. etc. Where I am hung up (because Im not a math for physics expert) is how the levers apply force which would increase with RPM. OF cource the leverage ratio on the levers and weight/washers attached to the end of the lever would influence the math. Id like to set up a spreadsheet where these are variables which can be changed. Any further input is appreciated. They type of clutch I am referring to can be found on the bottom of pg 181 of the Crower catalog....

http://www.crower.com/pdf/181-182.pdf

its the 10.7 Titanium Glide

 

[Greenlight]

I have written several Excel spreadsheet programs to do exactly what you want to do.

The clutch "clamp load" has two basic variables. One is the load imposed by the springs, often called the static load. This does not vary with RPM. The other is the centrifugal load or dynamic clamp load. This increases with RPM and can be changed by adding additional counterweights to the levers.

Calculating the static load is easy and is basically just as you described. If you know the spring rate, thread pitch, and number of turns, you can get the static load.

Getting the dynamic load is more difficult. You need to know EVERYTHING about the levers including, but not limited to:

The weight of the lever,
The center of mass of the lever
The dimensional relationship between the center of mass, the pivot hole, the load apply hole, and the counterweight hole. All of these must be known with the "plate height" at a defined position.

Then using F=ma and the priciples of leverage you can calculate the dynamic load. In the equation, the clamp force (F) is the dynamic clamp force, the mass (m) is the mass of the lever without any counterwieghts. The centripetal acceleration (a) is defined as the angular velocity squared (omega squared) multiplied by the distance of the center of mass as measured from the centerline of the clutch assembly (r).

This same equation is used for the added counterweight. The omega squared is the same, but the (r) will be a new value because the counterweight hole is at a different location from the center of mass.

My Excel programs use the engine dyno data and HP correction factor to calculate the required clamp load and the actual (as best as actual can be calculated) clamp load (static and dynamic) of the clutch for a particular day. The crew chief can then get the right setup right out of the trailer. Minor adjustments for track conditions must be made. It creates a chart that shows when slipping stops, engagement begins, and how much excess clamp load is applied at the higher RPMs.

The basic equation for clamp load on an axial type clutch is:

T = (F) x (mu) x (D+d)/2

where: T = torque
F = clamp load
mu = coeff. of static friction
D = OD of friction surface (clutch disc OD)
d = ID of friction surface

This equation is for one disc. Multiply is by the number of disc you have in your multidisc clutch.

There are other things that must be accounted for such as "effective area". The clutch disc has rivet holes, slots, etc. that reduce the "effective area".

I hope this helps and I didn't confuse you.

BTW, who's Alcohol Dragster/FC are you helping out?

I have an NHRA Competition Eliminator dragster.


Regards,

Greenlight

 

[JS3]

Thanks Green Light. I work with TAFC #7703. There are commercial programs available but I haven't found them useful for our application and variables are constrained to the defined database for stock levers and stock clutch discs etc. As we encounter different track conditions (eg starting line) I'd like to have a better handle on how I need to adjust counterweight in order to maintain plateload in lockup rpm range when adjusting static for launch conditions. Calculated HP in our situation is a little more challenging.

I've built many sophisticated financial spreadsheets in my real job but beyond addition/ subtraction of dollars and cents I'm lost. Would you be willing to share some of your work? I'm in the process of getting some lever data from Crower.

Look me up on insidetopalcohol.com under user J Shafer, send me a PM and we can get in touch. Thanks.