Soft landings for pneumatic cylinders

Mike Ferkany
Engineering Manager
ACE Controls
Farmington, Mich.

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
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 cylinders have cushions at both ends. For a wide variety of applications, air cushions provide an adequate means of deceleration. When conditions fall outside the capabilities of traditional air cushions, or where operating parameters frequently change (for example, an unloaded and loaded shuttle, uneven air pressure, or uncontrolled velocity), external shock absorbers are an excellent alternative.

VISCOELASTIC DAMPERS
Recently, a new deceleration product, called the NuCushion, covers performance ranges that traditional air cushions cannot handle. Like air cushions, the decelerators mount inside air cylinders and maintain standard NFPA mounting dimensions. But unlike air cushions, they require no adjustments to handle a variety of loading conditions for a given cylinder size.

The key to the product's development lies in advances in engineered elastomers and software that dynamically models the effects of impact on various damper geometries. Compared with air cushions, the proprietary viscoelastic material provides more-uniform reaction forces to decelerate and stop the cylinder load at the end of stroke, as well as uniformly distribute the impact load. The sidewall profile is engineered to give a rapid rise in deceleration force followed by a gradual decline. This provides more-linear deceleration than does an air cushion.

The result is more-robust cushioning at a lower cost than standard air cushions. It works in all applications suitable for standard air cushions and has the added benefit of automatically handling changes in weight, velocity, and system pressure without adjustment. Peak reaction forces are generally less than those in air cushions, and they require no maintenance during a lifetime of at least 3 million cycles.

Every product has limitations and NuCushion is no exception. First, the device is currently available only for 3.25 and 4-in. NFPA cylinders, although additional sizes will be available in the near future. And unlike standard air cushions, at the end of stroke the viscoelastic material pushes back against the load with a force of approximately 50 psi multiplied by the piston area, referred to as the “complete stroke pressure.” In many horizontal applications this has no effect because the units operate above this pressure threshold. However, two points must be considered. First, the operating pressure minus the complete stroke pressure determines net force remaining at the end of stroke. And second, if air exhausts from the cylinder and pressure falls below 50 psi, the cushion will force the rod to retract a short distance from full stroke.

For vertical applications, the net weight-holding capacity must account for complete stroke pressure at both ends of the stroke (rod end and cap end). Standard air cushions may offer an advantage when holding a weight up against a positive stop. However, NuCushions offer a distinct advantage when lowering a weight.