Motion control on a budget
Pneumatic linear slides offer an economical way to precisely move heavy loads.
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
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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