servo system, it is in line with or close to the actual situation to treat the friction
resistance of the motion coupling parts in the cylinder as a constant.
It must be pointed out that if there is a part of liquid lubrication between the
sealing surfaces of cylinder piston, the friction force is usually expressed in the
following:
f ce friction force
ð
Þ¼f c Coulomb friction force
ð
Þ þ f e Viscous friction force
ð
Þ
f ce ¼ f c þ f e :
ð8:27Þ
or
f ce ¼ f c þ bv
where
b Viscous friction coefficient depending on the properties of lubricating fluids;
v Piston speed.
Frictional force
f /N
Residence time/s
Lubrication
condition
Poor l
Good
General
Fig. 8.17 Effect of residence
time
Frictional force
f /N
Piston speed /(m/s)
Fig. 8.18 Effect of piston
velocity
8.2 Structure and Characteristics of Actuators
21