Engineering Statics

E-13


A couple of magnitdue 50 ft-lb is applied to the drum. Determline the smallest force which must be exerted by the hydraulic cylinder if the drum is not to rotate when the applied couple is directed (a) clockwise, (b) counterclockwise. Note the the static coefficient and dynamic coefficients of friction are 0.40 and 0.30 respectively.

The FBD of the link is drawn.  In addition inches are 
converted to feet since the couple is given in ft-lb. 
Alternately, the couple could have been converted to in-lb.
The moment equation is written for the drum about the 
center of the drum D.  Remember that the frictional force 
on the arm is equal and opposite to the friction force on 
the arm.
Once the friction force is known, the normal force can be 
calculated by dividing by the coefficient of friction.
The FBD is redrawn.
The moment equation is written about point A on the link.