Hydraulic & pneumatic cylinders including shocks have a term called cushioning. Cushioning is required to decelerate a cylinder’s piston stroke before it hits its lowest point and strikes the end cap. Excess cushioning increases the time it takes for the piston to reach the end of its stroke, and not enough cushioning increases the impact shock at the end of stroke.
Lowering the piston velocity as it reaches the end cap eases cylinder component stress and reduces vibration transferred to the machine. Nearly all cylinders have a setting for specific loads and operating pressure cushioning limits. Other attributes used to improve cushioning include higher fatigue-resistant material and supplementary coatings applied to the bushing and pins surfaces to improve life.
End-of-piston stroke impact can be dealt with in three ways:
- Simple mechanical member cushioning (using bushings and pins)
- Pneumatic cushioning, or
- Adding shock absorbers
Impact cushioning requires components that have highly-durable construction and manufactured to stringent design specifications. Pneumatic cushioning is provided by valve or porting control features to manage spike pressures. And lastly, spring damping, introduced on the cylinder end as a separate device to control stroke contact and shock.
Ultimate cushioning occurs when all kinetic energy is dissipated and bushing/pin component friction is controlled to decelerate the piston to exactly zero velocity. At this point, contact between the piston and end cap would be insignificant, so the piston would not rebound off the end cap. Proper calibrated cushioning also produces minimum piston deceleration time.
A critical aid in achieving ideal cushioning performance is selecting a manufacturer familiar with fatigue resistant bearing grade steels for the cushion bushings and piston pin members, along with suitable manufacturing capability in making precise parts efficiently.