Section 2
Friction
 Resistance to relative motion (or impending
motion) of two surfaces.
F
Friction Force, Ff
Applied Force, F
The Friction Force is:
W
F
F
Ff
N
 Tangent to surfaces in contact.
 Opposite to the direction of motion (or
impending motion).
 Dependent on the contact force.
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, ms
Ff max = ms N
(impending motion)
 Dynamic coefficient of friction, mk
Ff = mkN
(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.
25 lb
12 in
4 in
6 in
450
ms = 0.2
6 in
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
3 in
3 in
8 in
16 in
4 in
Problem 2-5
For the block brake shown, determine the
maximum braking torque that can be
provided.
w
8 in
10 lbs
mk =
0.3
2 in
8 in
18 in
Belt Friction
 Belt/rope/cable wraped around a cylinder
causes friction
w
Relative motion
(or impending relative motion)
Belt Friction
 For a flat belt/rope/cable
FBD
belt
Ff
T1
FBD
Drum
N
w
N
T2
 Tension ratio in belt is
T2
 e m
T1
Ff
Problem 2-13
For the band brake shown, determine the
maximum braking torque that can be
provided
w
8 in
mk = 0.3
10 lbs
2 in
4 in
8 in
18 in
Problem 2-14
A 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
300
2 in
16 in
Industrial V-belts
 Common for power transmission.
3V
5V
8V
 Selection based on speed and hp
V-Belt Friction
 Increased friction, as belt
wedges into sheave.
FBD
V-belt
T1

N
N
/2
Ff
2Nsin /2
T2
 Tension ratio in belt is
T2
e
T1
m
sin(  / 2 )
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
300
2 in
16 in
Problem 2-7
For the brake shown, the
P
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
150
5 in
w
5 in
10 in
1 in
4 in
4 in
1 in