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Friction
Resistance to relative motion (or impending motion) of two surfaces.
Section 2
F
Applied Force, F
Friction Force, Ff
The Friction Force is:
Tangent to surfaces in contact.
Opposite to the direction of motion (or impending motion).
Dependent on the contact force.
Ff
F
N
FW
The Friction Force is:
Dependent on the nature of the contacting surfaces.
Independent on relative velocity.
Independent on the contact area.
The Friction Force
Static coefficient of friction, s
Ff max = s N (impending motion)
Dynamic coefficient of friction, k
Ff = kN (motion)
Problem 2-1
The shear is used to cut electronic circuit-board. Determine the force exerted onto the board. Consider the friction between the blade and the vertical guide.
6 in
6 in
4 in
12 in
25 lb
s = 0.2
450
Problem 2-2
The material handling mechanism shown slides 8 lb packages along a counter. The coefficient of kinetic friction between the package and counter is 0.25. The collar weighs 1 lb, and the coefficient of kinetic friction between the collar and rod is 0.10. Determine the instantaneous torque required to operate this mechanism. The weight of the crank and connecting rod are negligible. Also, the mechanism operates at a low speed, so that inertial forces are also negligible..
Problem 2-2 (con’t)
20 in
8 in
3 in
3 in
4 in16 in
Problem 2-5
For the block brake shown, determine the maximum braking torque that can be provided.
18 in
8 in2 in
10 lbs
k = 0.3
8 in
Belt Friction
Belt/rope/cable wraped around a cylinder causes friction
Relative motion (or impending relative motion)
Belt Friction
For a flat belt/rope/cable
Tension ratio in belt is
T1 T2
NFf
FBD belt Ff
N
FBD Drum
eT
T
1
2
Problem 2-13
For the band brake shown, determine the maximum braking torque that can be provided
18 in
4 in2 in
10 lbs
k = 0.3
8 in
8 in
Problem 2-14A flat belt connects a motor sheave to a machine tool sheave. The maximum tension in the belt is 600 lb. Determine the maximum torque that the motor can provide. The static coefficient of friction of the belt on the sheaves is 0.4
2 in 16 in
300
Industrial V-belts
Common for power transmission.
3V 5V 8V
Selection based on speed and hp
Increased friction, as belt
wedges into sheave.
Tension ratio in belt is
V-Belt Friction
)2/sin(
1
2
eT
T
T1T2
2Nsin /2Ff
FBD V-belt
NN
Problem 2-15
A V-belt connects sheave A to a machine tool attached to sheave B. The maximum tension in the belt is 600 lb. Determine the maximum torque that the motor can provide. The V-belt has =320. The static coefficient of friction of the belt on the sheaves is 0.4
2 in 16 in
300
Problem 2-7
For the brake shown, the spring force is 50 lbs. Determine the braking torque, when a load, P, of 10 lbs is applied to the handle. The coefficient of kinetic friction between the pad and the drum is 0.4.
5 in
5 in
5 in
4 in4 in
10 in
P150
1 in1 in