Adjustable air cushion
To control, or meter, the volume of air being released during the compression phase at end of stroke, an adjustable cushion is used (see illustration on page 46). This typically consists of a threaded needle screw that adjusts into an orifice in the cylinder end cap. By adjusting the screw further into the orifice, the amount of air that can escape in a given time is reduced. Slowing the exhaust of air creates back-pressure, slowing the piston as it enters the end cap. The slower the air exhausts, the more dramatic the deceleration.
The objective of cylinder deceleration is:
- to avoid potentially damaging metal-to-metal contact
- to reduce vibration, and
- to reduce noise.
With the adjustable cushion, all three objectives can be accomplished ... to a point.
A designer may be inclined to say, "Just increase the time it takes to slow down the piston to the extent that we avoid hard contact, thus eliminating bounce, and bring in the piston so slowly we don't hear any contact." While that will work, how long does this take? Taking longer to decelerate and stop the piston-translates to a drastically increased cycle time, which will result in reduced efficiency and profits.
Redefining the cushion
What is the best way to avoid hard contact (that results in wear, bouncing, and noise), while at the same time making sure cycle times are reasonable? By redesigning the cushion seal and its attachment to the piston, deceleration problems can not only be remedied, but the redesigned cushion seal can provide additional benefits to cylinder operation.
Cushion seals that extend beyond the face of the piston have proven very effective in cylinder deceleration and at the same time increase cylinder performance. When the piston nears end of stroke, the rubber seal contacts the cylinder cap rather than the metal face of the piston. Hard metal-to-metal contact is avoided. This eliminates the wear caused by metal-to-metal contact, extending the life of the cylinder.
What about vibration caused by bounce? These extended seals, usually comprised of nitrile-based rubber pressed-fit into a machined grove on the piston, absorb 80% of the energy as the cylinder completes its stroke. This reduces the pneumatic bounce, a large amplitude, high frequency, end-of-stroke impact vibration that is destructive to the cylinder and to the surrounding equipment. Eliminating it extends the life expectancy of equipment.
Higher cycle rates
In addition to reducing noise caused by vibration, there is further benefit: with the seal absorbing 80% of the energy, the desired cushion orifice can be much larger — allowing the cylinder to travel through the cushion in 1 /4 the time of a conventionally cushioned cylinder. Plus, the extended type seal piston accelerates out of cushion at a higher rate.
This is accomplished in two ways. First, the cushion orifice can be much larger for the desired end of stroke performance, and this larger orifice provides a larger bleed orifice to allow air into the cylinder at a faster rate while exiting the air cushion. Second, the seal acts as a compressed spring and provides an initial force equivalent to 80 psi to push or accelerate the cylinder into motion.
This reduces in-cushion time by approximately 75%.
This article is adapted from the white paper, Improving Air Cylinder Deceleration, published by Norgren, Littleton, Colo. Click here to download a copy.