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