Soft landings for pneumatic cylinders

Viscoelastic dampers are an alternative to air cushions. Viscoelastic dampers combine energy-absorbing materials with a special geometry. They install on cylinder pistons to cushion endofstroke impacts. The goal of pneumatics-system design is often to run actuators as fast as possible without introducing excessive shock loads to the system. At low speeds, a cylinder piston can usually hit the end cap without harm. But at higher loads and speeds, running without cushions can be noisy and prematurely damage the cylinder and machine. Designers generally use air cushions for end-of-stroke damping, but they now have another option. Air cushions are variable-orifice air dashpots. A spud on either side of the cylinder piston closes off the passage to the main port. This traps air in the cylinder end cap and bleeds it off through a much smaller passage controlled by a needle valve. The cylinder traps a fixed volume of air each cycle. But because air is highly compressible, the load, velocity, and pressure in the cylinder all affect air-cushion performance. For any given set of conditions there is generally only a small window of needle-valve adjustments that give proper cushioning. Closing the needle valve too far results in high initial reaction force. The load slows too early and the cylinder takes longer to complete the stroke. If the valve is open too far, the trapped air does not create sufficient back pressure. The load reaches the end of stroke too quickly and causes an impact between piston and end cap. In addition, once the needle valve is properly set, any change in weight, system pressure, or velocity affects cushion reaction, and means the valve must be readjusted. In spite of these limitations, adjustable cushions are one of the most common options on NFPA tie-rod air cylinders. Many machine specifications require that all air