Page 225 - The Tribology Handbook

BIZ Capstans and drums

Capstans and drums are employed for rope drives. The former are generally friction drives whilst the latter are usually
direct drives with the rope attached to the drum. The roles, however, may be reversed. Friction drives control the rope
motion by developing traction between the driving sheave and the rope and might be preferred to direct drives for reasons of
economy (smaller drive sheave), safety (slippage possible) or necessity (e.g. endless haulage).
105

104

T = TENSION N
'- /.I = COEFFICIENT OF FRICTION

h' 103 e = ANGLE OF LAP-RADIANS
(FULL AND PART LAPS)
H = POWER W
H'= TENSION DIFFERENCE N
V = PERIPHERAL VELOCITY m/s
1 02 w= WEIGHT/UNIT LENGTH OF ROPE kg/n
1 DENOTES 'TIGHT' SIDE
2 DENOTES 'SLACK'SIDE
c DENOTES CENTRIFUGAL

10 05
10
T1/T2 =e@ (AT ROPE SLIP)

l-!=(T1-T2)V=H'VW
102

-?- 103 THE CONSIDERATION OF THE ROPE
X CENTRIFUGALTENSION (Tc = wV2)
WILL NOT AFFECT THE CALCULATION
OF DRIVE POWER. HOWEVER THE
'TIGHT' AND 'SLACK'SIDE TENSIONS
WILL BE INCREASED BY AN AMOUNT
SUCH THAT:
104
'TIGHT' SIDE TENSION = Ti + Tc
'SLACK'SIDE TENSION = 7-2 + Tc

105
FRICTION DRIVES

The figure shows the determination of the approximate performance of friction drives at the rope slip condition. In contrast
to the belt and pulley situation the required 'tight' or 'slack' side tension is usually known. Capstans are widely used with
vegetable, animal or man-made fibre ropes, but more rigorous conditions demand wire rope.
B12.1