Minimum servo torque to hold 170g load in gripper

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Homework Statement


I have this problem. I'm trying to calculate the minimal servo torque for the gripper to hold the part in place.
http://img52.imageshack.us/img52/8784/96199865.jpg

Mass of load is 170grams. u friction between load and walls is 0.5.
the diameter of wheel on which the pegs that pull the grippers is 2cm.

Homework Equations


Force to squeeze * Friction between load and gripper walls > Mass of cube
Torque=Force * radius

The Attempt at a Solution


F(friction)=0.17kg x 0.5=0.085N
F(G)=0.17kg x G = 1.666N
F=F(f) + F(g)= 1.751N

T=1.751N*0.01m=0.01751Nm

This is probably wrong...

Any ideas?
 
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duzO said:
F(f) = (0.17 kg)*0.5 = 0.085 N

duzO: This is not the correct formula for friction force. What is the general formula for friction force? Keep trying. Your formula for weight, F(g), is correct. Don't you want the summation of vertical forces on the gripped object to equal zero? Keep developing your formulas. You are getting close.
 
nvn said:
duzO: This is not the correct formula for friction force. What is the general formula for friction force? Keep trying. Your formula for weight, F(g), is correct. Don't you want the summation of vertical forces on the gripped object to equal zero? Keep developing your formulas. You are getting close.

https://www.physicsforums.com/mgc_gloss/latex_images/mgc_gloss_extexpl_39-4.png ?

so itll be F=umg . friction force=friction x normal force

so you are saying that mg=umg+F ?
1.66=0.833+F;
F=0.827N ?
 
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W acts downward. 2*Fs acts upward. Perform a summation of vertical forces, and set it equal to zero.
duzO said:
|Fs| ≤ mus*N

Very good. Now what is N? Write an expression for N. N comes from the servo, right? Try again.
 
nvn said:
W acts downward. 2*Fs acts upward. Perform a summation of vertical forces, and set it equal to zero.


Very good. Now what is N? Write an expression for N. N comes from the servo, right? Try again.

mg + 2(umN) = 0 ?

N=Torque/radius?

so Torque=gR/2u = 0.098Nm?
 
m*g should be negative, not positive. Fs = mu*N, not u*m*N. N = T/diameter, not T/radius. m = mass, mu = static coefficient of friction, N = normal force, Fs = frictional force. Try again.