Motion System Design

Pneumatic cylinders bounce back

Pneumatic cylinders perform an array of functions in electronics, automotive, and packaging industries. Their basic function is always the same — linear advancement of loads by attachment to a metal piston, pushed to and fro by columns of air. At some point in every application, however, a cylinder must slow down, stop, and change direction. Exactly how that happens determines how well the cylinder will perform in cycling applications.

Air throttle

When an uncushioned piston reaches one end of its stroke, it slams into the end cap, creating a hard metal-to-metal impact. The impact is often so loud that its exceeds OSHA standards for workplace noise. After the impact, the piston may bounce, during which time, the cylinder's motion is technically uncontrolled. The high amplitude, high-frequency impact can also damage the cylinder, as well as surrounding equipment.

Avoiding such problems and decelerating piston rods in a controlled manner requires external or internal cushioning. External cushioning employs a shock-absorbing mechanism outside the cylinder's body to absorb piston impact. The drawback is that it increases the footprint of the cylinder and adds weight and moving parts. Internal cushioning, on the other hand, operates within the cylinder footprint and tends to be simpler in function.

Here's how internal cushioning works: At end-of-stroke, a piston rod approaches the cylinder end and squeezes air out; a flow vent meters air for controlled velocity. Just before impact, a cushion spear or sleeve jumps into action, blocking the cushion seal and eliminating the exhaust path. The controlled volume quickly decreases, compressing the gas. Exhaust is then metered out even more slowly, through a cushion needle, completely decelerating the piston before it contacts the cylinder end.

The air cushion itself is adjustable, so the volume of air released can be metered during compression. A threaded needle screw piercing an orifice on the end cap provides the adjustment. Turning the screw further into the orifice decreases the amount of air that can escape in a given time. This diminished exhaust creates backpressure for an even more dramatically decelerated piston.

Physics makes it possible

The physics involved in a cushioned air cylinder is relatively straightforward. The laws of physics require that a negative force act on a piston to decelerate it. This occurs when air is squeezed or compressed in the end cap, and can be mathematically understood.

F - friction = ma

Two things actually prevent pistons from colliding with end caps. One is the deceleration of the piston with an auxiliary (cushion) system. The other is drag. Pistons (and loads attached to them) slow down quickly when — prior to hitting any air cushion — the actuator reaches equilibrium between the net driving and frictional force:

F - friction = ma = 0

Once the piston reaches the air-cushioned zone, it compresses the controlled volume of air, producing an elevated backpressure. In turn, this provides a negative force component to decelerate the load:

(-F) - friction = ma

Some assistance

So, how do we avoid piston and end cap contact, bouncing, and noise, while also maintaining reasonable cycle times? By extending the cushion seal and changing its attachment to the piston. Rubber seals that extend beyond the face of the piston assist in cylinder deceleration. These extended seals are usually made of nitrile-based rubber, press-fit into a machined groove on the piston. As a cylinder completes its stroke, the seal absorbs 80% of the energy, reducing pneumatic bounce, and effectively, noise. In this way, all the cushioning isn't done by the air cushion. Less time is spent draining air, so cycle times are maximized.

A cylinder that includes an energy-absorbing seal allows for a larger cushion orifice. With this, a piston can travel through the air cushion in one-fourth the time of a conventionally cushioned cylinder. Plus, extended piston seals can accelerate out of air cushions faster. One reason is that the larger cushion orifice doubles as a larger bleed orifice in the reverse stroke, letting air into the cylinder at a faster rate while exiting the air cushion at the other end. Another reason is the seal acts as a compressed spring, providing an initial force of 80 psi to push or accelerate the cylinder.

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Did you hear that?
PSI air sound pressure level Conventional 5 × 6 in. (dB) Extended 5 × 6 in. (dB) Conventional 2 × 6 in. (dB) Extended 2 × 6 in. (dB)
95 psi End 108 73 110 74
Side 112 84 110 81
50 psi End 108 73 113 74
Side 113 85 110 81
A cylinder with a conventional seal generates greater sound levels than an extended seal, regardless of size. In this test, a microphone is placed various distances from the cylinder. At five feet, cylinder sound levels are 9 dB less from the side and 13 dB less from the end. Total noise emitted depends on the structure to which the cylinder is attached. For example, mounting a thin flat plate of considerable area increases noise.
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