Cushions - Air Cylinder

Cushions; no, not the kind you'll find gracing your couch or favorite easy chair, but cylinder cushions.

Air cylinders often have a mechanical cushioning system that is used to reduce the internal impact of the cylinder piston as it reaches and hits the end caps of the air cylinder during normal use.

Over time, and depending on the air cylinder cycle speed, this repeated impact of piston to end cap will cause damage.

End Cap Impact

One common result is a fracture of the rod thread that screws into the piston center. A second is the stress cracking and ultimate failure of the piston itself.

I am a strong proponent of using the rod end tooling outside the cylinder to stop the piston travel inside the cylinder before the piston can reach or impact on either end cap. I believe that this will increase the life of your cylinder substantially in high cycle speed applications.

There are many methods of reducing impact, including external shock absorbers or internal bumpers. These options will be covered elsewhere.

Air cylinder cushions have a variety of designs, depending on the cylinder manufacturer. The graphic below is not to try to show how all cylinder cushions are designed and built, rather, to provide information to help us understand the concept.

In the graphic, you can see this piston has been machined with cushion spears on both sides of the main piston.

The piston is depicted as moving towards the blind end or rear end cap of the air cylinder."

Cushion Spear

The cushion spear (the wide part of the piston rod adjacent to the piston) will enter the cushion receptacle (female hole in the rear end cap) before the piston itself can reach and hit the end cap.

There are seals around the hole in the cushion receptacle, and these seals trap the air that's in the cushion receptacle as the cushion spear enters the orifice. This prevents the trapped air from exhausting out through the cylinder exhaust port, along with the rest of the air on that side of the piston.

There is a small air passageway that leads out of the cushion receptacle. This small airway allows a controlled exhaust of the air that has been trapped by the cushion spear.

This trapped air is slowly (relatively speaking) vented to atmosphere through a cushion exhaust outlet. Some of the cushion exhaust vents are fixed size, meaning the air cylinder piston is decelerated the same way every time, or they can also be adjustable (more expense, usually) allowing the operator to vary the deceleration.

Cushion Vent Screw

All cylinder cushions trap air and vent it to atmosphere through an integral air passageway.

Not all cylinder cushions are adjustable. If they are adjustable, usually the adjusting screw is "captive" and cannot be screwed fully out. The adjustable kind of cushion vent screw is a needle valve with which an operator can increase or decrease the speed of the air venting from the cushion receptacle.

Downside

The downside of air cushions in cylinders is that the cushions will slow the cycling of the cylinder. If the application requires high speed cylinder cycling, cushioning the piston in this manner will not provide satisfactory results. Another method of preventing damage to the cylinder end cap will be required.