and p 2e when the piston speed is stable, the friction f s at starting and the friction f e at
piston motion can be obtained, respectively, by the following:
f s ¼ p 1s À p 2s
ð
Þ A f À W
f e ¼ p 1e À p 2e
ð
Þ A f À W
'
ð8:26Þ
where
A f Effective area of cylinder piston;
W Total weight including piston and piston rod.
For the same lubrication condition of the cylinder coupling parts, the experimental results obtained by changing the residence time, i.e., the relationship
between starting friction and residence time, are shown in Fig. 8.17. The variation
of frictional force f v caused by different piston speeds is described in Fig. 8.18.
From the above experimental results (Figs. 8.17 and 8.18), it can be seen that the
length of piston residence time in cylinder and the difference of piston speed have
little effect on the friction force. Therefore, in the analysis and design of pneumatic
Fig. 8.15 Cylinder piston seal construction
Pressure
1
2
Time t
Displacement z
Fig. 8.16 Pressure diagram
inside cylinder
20
8 Pneumatic Actuators, Driving Elements and Accessories