Re: Clutch slipping due to adjustable spring force.

THE QUESTION: 
What i would like to know is:
I have a simple two plate clutch, and rotational force is being applied 
to it.
I want it to slip at a point that i can predetermine using a spring that 
applies force to one of the plates (the other plate is fixed in place and 
cannot move [lateral,longitudinal or rotational]) Ignoring wear and tear, 
could spring force and slipping point be plotted on a straight line graph?



Hello Rik,
Yes, spring force and the slipping point should be a fairly straight line 
relationship.  
In a linear system (let’s use a car skidding on a road for example), the 
force required to slide the car with all four wheels locked is equal to 
the ‘normal’ force times the coefficient of friction (COF) between the 
tires and the road.  The normal force in this case (assuming a level 
road) is the force acting perpendicular to the road surface, or just the 
weight of the car.  So, assume a 2500 pound car, and a COF of .6.  The 
sliding force would be 2500 x .6 = 1500 pounds.  
Notice something interesting- the contact area does not enter into this 
equation!  You could have big, wide tires or small, thin tires and 
theoretically the sliding force would not change.  
Now, let’s consider the clutch in your example:  The principle is the 
same:  If you look at an infinitely small area  (call it A1) of the 
contact surface somewhere between the two clutch plates, the sliding 
force (F1) will be equal to the force acting on A1  x the COF between the 
plate materials.  Here, the clutch example gets a bit more complicated:  
The sliding force in our A1 contributes to the overall breakaway torque 
of the clutch as follows:  The torque contribution of A1 is equal to F1 x 
R1, where R1 equals the radial distance from the rotational axis to the 
center of A1.  You can see that “tiny areas” close to the axis will 
contribute smaller breakaway resistances than areas near the outer 
diameter of the plates.
It involves some calculus to develop an exact formula for the breakaway 
torque of a clutch, based on contact force, plate diameters, etc., but 
fortunately there is a simplified formula that gives good approximate 
values.  The formula uses a value called the “Mean Effective Radius” (Re) 
to help calculate the breakaway torque.  Re is a radial distance from the 
axis where all of our “tiny areas” can be assumed to act through.  In 
other words, it is the effective average distance of all the contact 
surfaces of the plates from the axis.

Re is determined as follows:  Re =  Squ. Root (Ro2 – Ri2) /2 + 
Ri2)        
where Ro is the outer radius of the contacts and Ri is the inner radius 
of the contacts.
The breakaway torque of the clutch can then be calculated from the 
following:
T =  P x Re x COF / 12
Where T = Torque, P = total contact force (in pounds).
As you can see, “T” is linearly related to the contact force “P”, so you 
should be able to control the force of the springs in your example to 
determine and control a breakaway torque in your system.

I hope that this helps you in your project.  I wish you the best results!

Jay Shapiro