Why Cushions Are Needed

When a hydraulic cylinder reaches end of stroke at speed, the moving mass must be decelerated. Without cushioning, this creates a severe shock that damages the cylinder, mounting, machine, and can cause dangerous seal failure.

Built-in cushions use a tapered spear or adjustable orifice to progressively restrict flow as the piston approaches the end cap, creating a controlled pressure rise that absorbs kinetic energy.

Energy Balance Equation

The kinetic energy of the moving mass must be absorbed by the cushion:

KE = 1/2 x m x v^2

This energy is absorbed by the work done against cushion pressure:

Work = F x d = P_cushion x A x S_cushion

Therefore: P_cushion = m x v^2 / (2 x A x S_cushion)

• m = Moving mass (piston + rod + load) in kg
• v = Impact velocity (m/s)
• A = Cushion area (piston area minus cushion spear)
• S_cushion = Cushion stroke length

Deceleration and G-Force

The deceleration can be calculated from the velocity and cushion stroke:

a = v^2 / (2 x S_cushion)

G-force = a / 9.81

• < 3 G: Smooth deceleration, minimal stress
• 3-10 G: Normal industrial cushion performance
• 10-25 G: High deceleration, verify component ratings
• > 25 G: Severe shock - increase cushion stroke or reduce speed

Pressure Limits

The cushion pressure must not exceed the cylinder's rated pressure:

• Standard cylinders: 160-250 bar (2300-3600 psi) max
• High pressure: 250-350 bar (3600-5000 psi)
• Safety margin: Keep peak cushion pressure below 80% of rating

If calculated pressure exceeds limits, increase cushion stroke, reduce mass, or reduce impact velocity.

Design Guidelines

• Cushion stroke typically 15-50mm (0.5-2 inches)
• Adjustable needle valve allows fine-tuning deceleration rate
• Cushion area is approximately 30-50% of piston area
• For very high speeds/masses, consider external shock absorbers