Motion System Design

Air it out

When motion applications require high speed and linear movement, you may want to consider using pneumatic actuators. Pneumatic actuators use air pressure and flow to create high force and velocity. They are used in a variety of industries, such as packaging, food processing, and medical design.

What pneumatic actuator attributes are most tightly linked to speed, and how do they affect it?

Frank/Festo: Airflow into the actuator and friction are critical parameters in achieving speed. How fast you can put air into the actuator relates directly to how fast you can accelerate the piston. Friction is the force that works against the motion of the piston. It can be found between the piston seal and cylinder and also on the bearing seal and rod itself.

Phil/Bosch: Bore size and air pressure affect speed, as does the volume of air to be exhausted (assuming a double-acting cylinder). As for pressure, the net force applied to the cylinder piston is a function of its cross-sectional area and the differential pressure across it.

Walt/PHD: Air pressure and flow have the greatest effect when it comes to obtaining speed from pneumatic actuators. The valve, port size, fittings, and tubing must be properly sized to obtain the required speed. If you cannot obtain the flow and pressure required, you will not obtain the desired speed. Another factor is the distance from the valve to the actuator. This is most relevant when reaction time is combined with a requirement for actuator speed. Reaction time can be greatly reduced by applying a valve directly to the actuator, eliminating the pneumatic tubing and the volume of air within.

Gary/Beswick: To maximize speed, you'll want to minimize the internal volume of the actuator. You also want to make sure the actuator has large ports and well designed internal airways to minimize pressure drop as the fluid enters. Naturally, internal friction should be kept as low as possible; rolling diaphragm designs are best in this regard. It's also important that actuation shafts and rods have a small diameter as well as support from low-friction bushings or bearings. The mass of the moving components should be minimized as well.

What are some of the limiting factors associated with speed, and how do you overcome them?

Phil/Bosch: Rate of exhaust is the primary limiting factor. Quick release valves installed into the cylinder port can exhaust directly to atmosphere for minimum exhaust times. For higher speeds, two 3/2 valves with independent solenoid control may be used. In this case, pressurized cylinder chambers can be exhausted before the cylinder is required to move, creating maximum differential pressure across the piston. Tube inside diameter and length also limit speed. The longer the tube, the greater the pressure drop and more restrictive the air path.

Frank/Festo: How air is ported into the cylinder can make a big difference on speed. Porting the cylinder axially rather than perpendicular to piston motion greatly improves airflow into the cylinder. Cylinder barrel surface finish and lubrication also impact speed. The surface inside the barrel should let the grease spread without being swept to the end of the cylinder. Too smooth a surface is as bad as too rough a surface.

Walt/PHD: One of the biggest limiting factors with pneumatic actuators is decelerating the load or mass at the end of travel. It's one thing to accelerate a load using air, and another to decelerate that load in a short distance or time. This can be overcome with built-in deceleration; adjustable pneumatic cushions, hydraulic shock absorbers, and deceleration circuits that control backpressure at the end of travel.

What can system designers do to help pneumatic actuators attain maximum speed?

Phil/Bosch: Optimized speed and adequate cushioning at end of stroke requires different pressures to extend and retract the cylinder. For example, a cylinder retracted with 25 psi and extended with 90 psi will only have to exhaust 25 psi when the cylinder extends. Higher differential pressure across the piston corresponds to faster breakaway and higher steady state speed. Higher speeds can also be achieved using two valves. A second valve that reverses the pressure applied to the directional control valve can optimize speed in both directions, provided there is sufficient time for the reversed pressures to reach their nominal values before the cylinder is cycled again.

Walt/PHD: Many pneumatic actuators are designed for power and motion, but not necessarily for carrying a load. Precautions should be taken to ensure that the load is handled through the use of bearings or guides. For this reason, many suppliers provide actuators with bearings, creating devices such as linear slides, grippers, escapements, and rotary actuators. Each of these are equipped with varying types of bearing systems with recommended load and speed requirements.

Gary/Beswick: Designers should consider the number of fittings, tubing lengths, and valves. These components should be sized to minimize pressure losses upon actuation. Fittings should have the largest orifice possible for maximum flow. Thread sizes on fittings should be large enough to allow a passageway of the same inner diameter of the tubing being used. Rigid metallic tubing should be used over flexible tubing. Quick relief fittings can be used advantageously to exhaust air close to the actuator. Other components of interest include mufflers and valves. Mufflers should not have a significant pressure drop. Valves should have a fast response to the control signal as well as high Cv values. They should also be placed as close to the actuator port as possible (mounting on the port itself is ideal). Supply pressures should be as high as possible. Supply airlines should also be examined to eliminate pressure starvation of the control system.

Frank/Festo: High piston speed translates to high kinetic energy. To prevent this from causing problems, you have to slow the piston before it impacts the cylinder end caps, causing premature failure. Without considering this in your design it's possible that the piston and rod could detach. Another consideration is heat build-up. You have to consider how this could affect elastomeric seals used inside the cylinder or any thread locking compound used throughout.

For more information, contact the editor at [email protected]

Meet the experts

Phil O'Neill
Bosch Rexroth Corp.
Hoffman Estates, Ill.
(859) 281-3426

Gary Treadwell
Beswick Engineering Co.
Greenland, N.H.
(603) 433-1188

Frank Langro
Festo Corp.
Hauppauge, N.Y.
(631) 404-3213

Walt Hessler
PHD Inc.
Fort Wayne, Ind.
(260) 479-2212

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