Motion control on a budget

July 12, 2007
Pneumatic linear slides offer an economical way to precisely move heavy loads.

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

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.

For instance, a typical slide with linear ball bearings is rated for a 20-lb overhung load with 0.005-in. deflection. Yet the unit's four bearings may have a combined load rating of several hundred pounds. This bearing overcapacity ensures precision and long life despite unfavorable loading conditions.

Linear bearings, whether ball or sleeve, support the greatest loads when designs apply force over the entire length of the bearing. This is commonly known as a carriage load. Heavy loads, such as shuttling a toggle press or riveter between locations, are best handled with carriages. Carriageload slides with short strokes can carry several hundred pounds.

Cylinder bore and stroke also factor into load capacity. Obviously, a linear slide would be of no use if the cylinder couldn't produce enough thrust to move the load. By the same token, long-stroke slides with undersized guideshafts would not have the strength to be of any practical value. Pre-engineered linear slides account for these considerations and balance load capacity, available force, and structural integrity.

Stroke. Slides commonly come in 1.0-in.-stroke increments. Designers generally specify slightly longer strokes than applications require and add adjustable stops. These stops include clamp collars, and threaded bolts and stop nuts, and offer repeatable stopping accuracy to ±0.001 in.

Operating speed. Speed is an often-overlooked aspect of linear slides. Engineers sometimes find it difficult to get accurate speed information, yet ignoring speed factors can have disastrous results.

A safe speed range for pneumatic linear slides without external stops is generally 6 to 8 ips. A 12-in. stroke in 2 sec is approximately 6 ips — approximate because acceleration and deceleration time are not taken into account. On short strokes, ignoring acceleration/deceleration produces misleading results. A 1-in. stroke in 0.16 sec means an average speed of 6 ips but, in reality, final speed is much higher because a good portion of travel time involves acceleration.

High speeds create severe impact forces when loads abruptly stop at the end of stroke. Urethane bumpers mounted inside or outside the cylinder can cushion these forces if accuracy is not an issue. Adjustable stops with hydraulic shock absorbers or optional internal-cylinder air cushions provide more-precise, cushioned stops.

Speed is also related to the bearings. High speeds are best handled by linear ball bearings that permit travel velocity to 100 ips. However, avoid ball bearings with short-stroke, fast-reciprocating motions. Inertia of the ball circuit tends to make balls "skid" in their tracks when the direction suddenly reverses.

Options and specials
Not many years ago, preassembled air-powered linear slides were not available. Instead, slides were designed one at a time and fabricated from individual components such as air cylinders, couplers, bearings, and shafts; as well as custom-machined blocks, plates, and weldments. The results were expensive, but they performed as intended.

Packaged pneumatic-linear slides became available to meet the demands of machine designers and builders who didn't want to "reinvent the wheel" every time a system needed linear motion.

Packaged slides save time and money at every step in the design. For example, many suppliers offer free CAD files that drop directly into designs, saving numerous engineering hours. Offthe-shelf slides also reduce build time and costs. And proven, preengineered units tend to lower maintenance and machine-repair costs. They even simplify machine manuals and parts lists.

Designers can pick and choose many options to tailor standard slides for specific applications. Examples include different toolbar and toolplate styles, bearing types, adjustable stops, shock pads, air cushions, hydraulic shock absorbers and, importantly, sensors.

Sensors are the interface between linear slides and electronic controllers. Sensors include reed and Hall-effect switches actuated by a magnetic piston band on the slide's air cylinder. Many slides offer proximity switches triggered by a "target" mounted on the moving slide. Mechanical snap-action switches, air-pilot switches, and LVDT transducers may be available as well.

When off-the-shelf products simply won't do, some manufacturers offer customized slides. A "special" might involve modifications as simple as a custom sensor or nonstandard mounting holes and dowels. Such changes would be difficult to incorporate after the slide is assembled, so it is best left to the slide manufacturer to add such features. And sometimes, a completely new slide designed, tested, and manufactured by an expert can save machine builders time, money, and aggravation.

Fabco-Air Inc.,

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

Carriage loads on guideshaft-style linear slides can deflect orbend the shafts, especially on longstrokes. Adding an optionalcenter support member oneach guideshaft dramaticallyincreases load capacity. Attaching shaft supports at the guide's center reduces deflection to less than 0.005 in., even with heavy loads. Rather than needing a larger, more-expensive unit, this lets designers use smaller slides with the same results.

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