Pneumatic cylinder end caps have to withstand shock loads at the extremes of piston travel. These loads arise from the kinetic energy of the moving parts such as piston cup rings of the cylinder and load.
When the piston of a normal cylinder is about to complete its stroke, the piston runs into a cylinder’s ends. If inertia is high enough at this point, the cylinder may experience a shock that could be damaging. The end of travel shock loads can be reduced with fixed cushions or valves built into the end caps.
As per the cushion arrangement, pneumatic cylinders are classified into the following three types.
Non cushioned cylinder
Fixed cushion cylinder
Adjustable cushion cylinder
Pneumatic cylinders are capable of very high speed and considerable shock forces can be developed at the end of the stroke. They are suitable for full stroke working at low speed. Higher speeds are achievable with external cushions or internal cushions. Either fixed cushions or air cushions can be provided in the cushion cylinders.
As a rule, cushions are applied to cylinders whose piston speeds exceed 01 m/s(20 ft/min).
Fixed cushion cylinder:
When the piston is moving near the end of the stoke, the cushion set acts to reduce the speed of the moving piston and make the moving smooth to avoid a strike.
If the cylinder is long, that is, if the stroke length is more and the speed is high, then naturally, the piston impact on end covers is considerably high.
The continuous hammering of the piston on these cover plates causes the loosening of ties rods.
Smaller cylinders generally have fixed cushioning I.e rubber buffers, to absorb the shock and prevent internal damage to the cylinder.
Adjustable cushion cylinder:
Adjustable cushioning slows down the cylinder when the piston movement is just entering either head or cap before reaching the end of its stroke. The idea is to avoid a hard shock when the piston hists these elements.
The below figure shows a double-acting cylinder with an adjustable cushion and its arrangement. This cushion consists of a throttle valve and a sleeve attached to the piston.
On large cylinders, the impact effect can be absorbed by an air cushion that decelerates the piston over the last portion of the stroke. This cushion arrangement before allowing it to bleed off more slowly through an adjustable needle valve traps some of the exhausting air near the end of the stroke.
How to adjust cushioning?
As the piston approaches the end of its travel, the cushion sleeve blocks the normal exit for the air and forces it to pass through the throttle valve which restricts the flow, progressively retarding the piston movement. The air cushions are generally designed to function over the final 2 cm of the piston stroke.
The cushioning can be adjusted by controlling the throttle valve. Slowly loosen the throttle screw, continue to loosen the throttle screw anyway. Continue to loosen the screw until the piston impact on the cushion is only just audible. If the noise level of the piston impact increases again then tighten the throttle screw until the noise level decreases again.
Advantages of adjustable cushioning:
# Smooth running with little vibrations, so less noise.
# Reduce shock waves in the hydraulic circuits.
Disadvantages of adjustable cushioning
# Cushioning procedure involves increased cost and complexity.
Article by PSS Bapu Rao