Cushioning of some sort normally is required to decelerate a cylinder's piston before it strikes the end cap. Reducing the piston velocity as it approaches the end cap lowers the stresses on cylinder components and reduces vibration transmitted to the machine structure.
End-of-stroke impact can be dealt with in three ways: by simple impact cushioning, pneumatic cushioning, or by installing shock absorbers. This discussion deals with pneumatic cushioning and is intended to serve as an instruction on how to optimize cushioning for a given mass.
The concept of ideal cushioning
Ideal pneumatic cushioning occurs when all kinetic energy is dissipated to decelerate the piston to exactly zero velocity when it reaches the end of its travel. Any contact between the piston and end cap would be negligible, so the piston would not rebound off the end cap. Ideal pneumatic cushioning produces minimal noise from end cover contact, and minimum piston deceleration time. Thus, properly adjusted pneumatic cushioning can improve the work environment and increase machine throughput in rapid-cycle applications.
Knowing operating pressure, cylinder characteristics, and the specified load mass, the first step is to ensure that the piston velocity is within that specified in cushioning charts in the manufacturer's catalog. The best results in adjusting piston velocity are obtained by installing throttling non-return valves directly in the cylinder end ports. This permits free inlet flow while allowing the outlet pressure to be adjusted simply by altering the area of the exhaust port with an adjusting screw. Directional-control valves with integral restrictors may be used as an alternative.
A critical aid in achieving ideal conditioning is an electronic instrument to measure piston velocity. Such an instrument also allows measuring the time for all sequences in a cylinder cycle.
Understanding the dynamics
Although the cushioning adjustment from the factory may prevent the piston from striking the end cap on its first stroke, the cushioning is far from ideal. Furthermore, the factory adjustment may also provide too much damping, making it difficult to achieve rapid cycling.
Figure 1 illustrates what can occur when the adjusting screw is opened, with all other parameters being constant. Starting at point 1, which represents the initial factory setting, opening the adjusting screw a turn at a time moves the setting to the left. During the first two to three adjustments, the deceleration time becomes progressively shorter, but the end impact becomes correspondingly higher. The normal reaction at this point is to stop, and return the adjusting screw toward the original setting to reduce the increasing shock.
However, ideal cushioning is achieved by continuing to open the screw an additional one to two turns. At some point, the end impact reaches its minimum, point 5 in Figure 1. Continuing to open the adjusting screw will cause end impact to suddenly increase substantially without a significant reduction in deceleration time. So at the ideal pneumatic cushioning point, not only is end shock minimized, but deceleration time is also reduced, which translates to shorter cycle times. A 20% to 40% reduction in total cycle time is not unusual when ideal cushioning is achieved.
Effects of pressure variations
So far, all parameters have been assumed to be constant, with changes made only to the cushioning setting. In reality, though, operating pressure often fluctuates, especially in large circuits having dozens of actuators and end effectors. The effects of pressure variations demonstrate the importance of providing consistent pressure control. This is best accomplished by specifying properly sized flow passages to minimize pressure drop and strategically placing pressure regulators where ever they are needed.
Operating pressure can have a dramatic effect on end-of-stroke shock, Figure 2. As shown, ideal pneumatic cushioning for a given mass occurs at 6.3 bar. But end impact shock increases sharply if pressure decreases. At 5 bar, the cylinder is subjected to a shock equivalent to 30 to 40 times the mass it was intended to cushion. Apart from loud noise, severe vibration, and a longer cycle time, this shortens the life of the cylinder appreciably. Unfortunately, higher pressure produces the same result.
Over and underdamping
Ideal cushioning cannot be achieved with an overdamped cylinder. This is because the effects of damping will become progressively worse, regardless of the adjustment. Any of three actions may be taken to solve this problem.
Increase piston velocity — Piston velocity can be raised by increasing the area of the cylinder outlet port, adjusting throttling non-return valves, or altering the restrictors in the outlet ports of directional control valves. Sufficient kinetic energy for ideal cushioning can be developed by this means.
Reduce operating pressure — This may be accomplished simply by installing a pressure regulator in the feed line to the cylinder.
Increase the moving mass — A higher kinetic energy can be achieved by increasing the moving mass, although, in practice, this can be difficult to achieve.
If severe shock occurs regardless of the cushioning adjustment setting, the cylinder is underdamped. In this case, the possible courses of action are the opposite of those described for overdamped conditions. To correct underdamping:
• reduce the piston velocity,
• increase operating pressure,
• reduce the mass, or
• incorporate external shock absorbers into the cylinder assembly.