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Motion control on a budget

Pneumatic linear slides offer an economical way to precisely move heavy loads.

Doug Mills
Chief Engineer
Manufacturing Section
Fabco-Air Inc. Gainesville, Fla.

Pre-engineered pneumatic linear slides balance cylinder force, bearing capacity, and shaft strength to offer precise positioning despite overhung loads.

Pre-engineered pneumatic linear slides balance cylinder force, bearing capacity, and shaft strength to offer precise positioning despite overhung loads.

Pneumatic linear slides combine aircylinder power with an engineered guide to move loads along a precise path. They're used in everything from simple pressing operations to multiaxis robots. Today, equipment designers can choose from a wide array of linear slides from a host of manufacturers.

Fortunately, linear-slide design only involves four basic factors: force, load capacity, stroke, and operating speed. Armed with this data and slide manufacturers' specs, engineers can quickly determine air-cylinder size, bearing requirements, end stops, and related hardware they need. This saves considerable design time when zeroing in on the most-effective and economical unit for the job. Here's a closer look at each factor.

Force. Calculate a linear slide's output force F from:

F = PA
where P = available air pressure in psi and A = cylinder piston area, in.2, also called the power factor. From this, engineers can determine the linear slide's minimum cylinder bore. Note that slides often have different power factors for extend and retract strokes. That's because the piston rod reduces the working area on the retract side of the piston.

In part-pressing and assembly operations, force requirements often increase over time due to design or material changes, a product-line expansion, and other factors. So size a larger bore and use a regulator to lower supply pressure. Technicians can then increase pressure to raise the slide's output force as needed. Other options for boosting output force without increasing bore size include using tandem cylinders or multipower cylinders. The latter have more than one piston on the power stroke to generate higher forces from available shop air.

Equipment that lifts goods requires slides with output force at least twice the load. Underpowered slides that just barely lift the load operate poorly with slow, jerky, and uncontrolled motion.

Finally, many applications require relatively little force. In such instances, engineers often mistakenly ignore and oversize the cylinder. Select a slide with a bore that generates enough air volume to operate with smooth, controlled motion. Avoid excessively large bores that waste air and energy.

Load capacity. Slides must support the workload with the required precision over the entire range of motion. However, a linear slide that knocks boxes off a conveyor does not need the same degree of precision as one placing parts in an assembly jig. Because requirements vary widely, engineering specs indicate safe loading levels and predict toolbar deflection under different loads.

Most linear slides have two or more guiding shafts. Workpieces typically attach to the reciprocating toolbar and, in nonvertical configurations, generate an overhung load. Two factors ultimately determine load capacity: guideshaft strength and deflection resistance; and the linear bearing's load capacity. Although overhung workloads put undesirable loading on the bearing's leading edge, guideshaft deflection generally determines load rating.

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© 2014 Penton Media Inc.

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