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Cushions. No, not the kind you'll find gracing your couch or favorite easy chair, but cylinder cushions; 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 cycles.
Over time, and depending on the air cylinder cycle speed, this repeated impact of piston to end cap will cause damage.
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.
Those that have read the complete air-actuator section of www. ABOUT-air-compressors.com know that we are strong proponents of using the rod end tooling outside the cylinder to stop the piston travel inside the cylinder, before the piston can impact on either end cap. 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.
On this page we cover air cylinder cushions.
There are a host of mechanical designs for cushions, developed by the various cylinder manufacturers.
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The graphic 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.
The piston is depicted as moving towards the "blind end" or "rear end" cap.
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 at high speed, with the potential for piston and/or rod damage.
There are seals around the hole in the cushion receptacle, and these seals trap the air that's in the cushion receptacle. This prevents this air from exhausting through the cylinder exhaust port along with the rest of the air on that side of the piston.
There is a small air passageway in the cushion receptacle. This small airway allows a controlled exhaust of the air that is trapped.
The trapped air is slowly vented to atmosphere through a (usually) adjustable cushion screw.
The cushion screw is a needle valve with which an operator can increase or decrease the speed of the air venting from the cushion receptacle, and in so doing, create back pressure inside the cushion receptacle for a certain period of time. This dampens or cushions the piston travel.
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.
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.
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