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The size of the piston rod can be as important as the cylinder size and air pressure. A common problem is overloading the rod, which often happens when pairing a heavy load with a long cylinder rod. During horizontal motion the load will hang off the end of the rod, which may cause the rod to bend when fully extended. When lifting a heavy vertical load, the piston rod may even buckle if it’s too small.

Cylinders are designed primarily to push or pull a load, so supporting heavy side loads requires extra planning. Keeping the cylinder thrust as close as possible to the centerline of the piston rod should be factored into the design.

Another consideration is the rod length. Strokes of about 24 in. and longer can compromise a long, skinny rod. In this case, the best solution is to choose a bigger cylinder. Some manufacturers also offer cylinders with oversized piston rods, which can be more economical in some cases.

Also consider cylinders with guide rods in difficult loading situations. With rods and blocks mounted parallel to the piston rod, guided cylinders prevent the piston from rotating and increase load-carrying capacity, thanks to the added support of the guide rods and additional bearings. This is important when a system is subjected to large side loads or requires highly accurate controlled linear motion.

Finally, rodless cylinders in which the load sits on an external carriage that slides along the tube can also be used for applications with long strokes, heavy loads, or high moment loads. These cylinders come in a number of different configurations and their compact size makes them a good fit in tight spaces.

Getting the valves right

Valves control the switching and routing of air in a pneumatic system. Aside from controlling the flow of compressed air, valves also direct the flow of the exhausted air. Many types of valves are used in pneumatic systems, with the specific application dictating the best choice.

Regardless of the type of valve, however, one common mistake in designing a pneumatic system is to properly spec the cylinder but undersize the valve. Properly matching the valve and cylinder is imperative, since the cylinder won’t move as intended if the valve is too small. With higher speeds, airflow must be increased to move the load quicker, and that often means a valve with higher flow capacity.

Pneumatic cylinder design factors, Fig. 2

Most valves come with a flow coefficient (Cv) rating. In essence, the bigger the Cv, the more air flows through the valve. The valve rating should typically be selected for a five-psi pressure drop at the required flow rate to drive the cylinder at the desired speed.

Flow-control valves work well at controlling cylinder speed. These can either be adjustable restrictors on the control-valve exhaust ports or special valves mounted on or near the cylinder. Cylinder-mounted flow controls have a one-way bypass built in to allow free flow in one direction and restricted flow in the other. For the best results, install these valves to give free flow into the cylinder and restricted flow out.

Improving cycle time

Position switches and sensors can improve overall pneumatic performance. In a system with multiple pneumatic actuators operating in sequence, position sensors that indicate the piston location in each cylinder will promote shorter, more-reliable cycle rates.

Simple switches such as reed, Hall-effect, and magneto-resistive switches are all commonly used as position sensors on pneumatic actuators. Regardless of the type, they all detect the piston position as the cylinder approaches the end of the stroke.

Without sensors, unnecessary stops (time cushions) must be built into the timing of the system. This is the result of air-supply variations and other factors. In a plant, airflow might be slightly less during the afternoon as compared to the morning. This means in the afternoon it might take a fraction of a second longer to complete the stroke, which could disrupt the timing of other steps in an operational sequence. To adjust for these and other variations, low-cost sensors should be included in the design. Adding such sensors will result in shorter cycle times, smoother operation, and higher operating efficiency.

Pneumatic systems remain popular due to their low entry and maintenance costs. While they’re relatively simple, weight, force, speed, and other requirements must be considered at the design level to ensure proper operation. Given the many different types of cylinders, valves, and sensors, taking the time to determine the right components in a pneumatic system –and how they interact – will result in better performance in both the short and long term.

Considerations for effective cylinder performance

Account for each of these factors when designing pneumatic cylinders into machines, robots, and any other piece of equipment.

Load: A force at least 25% greater than the load is typically necessary to make up for system pressure losses.

Force factor: The force factor is simply the area of the cylinder piston. Force factor times air pressure equals the force produced by the cylinder.

Speed: Higher speeds require a greater force margin to overcome increased system pressure losses.

Sequencing: Adding sensors can shortens cycle times by eliminating time delays.

Other components: Available pressure at the cylinder can be affected by system compressor, filters, regulators, control valves, and all connecting piping. Right-sizing these components helps ensure the best performance from a cylinder.

Pat Phillips is product manager, AutomationDirect, Cumming, Ga. For more information on AutomationDirect and pneumatic systems, visit www.automationdirect.com.

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