Page 88 - Mechanical Engineer's Data Handbook

APPLIED MECHANICS 77
threads on nut and screw. The balls circulate continu- 2.4.7 Cocffjcient of friction for screw
ously. threads

Recirwlatiyl tube This ranges from 0.12 to 0.20 with an average value of
0.15. It is however much lower for the ball-bearing
thread.

2.5 Automobile mechanics

The resistance of a vehicle to motion is made up of overall efficiency of the drive, the power can be
‘rolling resistance’, ‘gradient force’ and ‘aerodynamic calculated. Additional power is required to accelerate
drag’. From the total resistance and a knowledge ofthe the vehicle. Braking torque is also dealt with.

2.5. I Rolling resistance For pneumatic tyres on dry road
Symbols used:
C, = coefficient of rolling resistance
m=mass of vehicle
v=speed (km h-I)
p = tyre pressure (bars)
F, = C,mg

cr C,

Asphalt or concrete, new 0.01 gravel, rolled, new 0.02
Asphalt or concrete, worn 0.02 gravel, loose, worn 0.04
Cobbles, small, new 0.01 soil, medium hard 0.08
Cobbles, large, worn 0.03 sand 0.1-0.3