Bob Paugh and Ken Sterry

Pawling Engineered Products
Pawling Corp.
Pawling, N.Y.

In the broad arena of motion control, bladder-type inflatable actuators fill a limited niche: Short stroke, wide area, high or constant-force motion. That said, when bladders are appropriate, they provide motion more cost effectively than any other system. After all, what could be simpler for affecting motion than inflating a rubber pillow or length of flat tubing? That's why inflatable bladdertype actuators are becoming a mainstay as soft brakes on belt conveyors, workholders, and parts positioners, as well as expanding mandrels, clamps, and bonding bladders.

Improved elastomeric materials and increasingly diverse cross sections have broadened the niche for bladder-type actuators. Today's inflatable actuators can deliver longer strokes (to 2.5 in.) and exert force over the entire stroke more uniformly than ever. For example, bladder actuators open and close highpressure control valves in large filtration equipment, where maintenance access is very limited. No metal, lubricant, or moving parts makes for a reliable actuator mechanism. Since just a bit of inflation pressure translates into very high forces, operation is dependable even under extreme process pressures. In fact, bladder actuators operate under pressures that would deform valve gates.

Because inflatable bladder actuators are not off-the-shelf items, involving an experienced vendor early makes for better designs. The main factors to define at the outset are load, required stroke, and duty cycle, available envelope, and ambient conditions. Keep in mind that the standard types are a starting point only: Inflatable actuators can be molded to almost any cross section, to fit into most envelopes.

Two challenges are getting higher force toward the end of the stroke and faster cycling. How to address these issues?

• For more force at the end of the stroke: One option is to specify a longer stroke than that actually needed, so the actuator never has to operate at the end of its stroke. Another option is to mount the actuator in a snug rigid channel, so that all expansion is upward, not outward. Too, convoluted cross sections and reinforced materials partially address inadequate end-of-stroke force.
• For faster cycling: Specify multiple inflation and deflation points. Also: More conformable elastomers react almost instantly to pressurization, so they should be specified if this is a design objective. s

Common applications

Inflatable bladder-type actuators are incapable of precision motion, so they're best for applications with a very forgiving force-displacement characteristic, not needing the highest positional precision for proper functioning. Their force control, on the other hand, is highly accurate. They distribute force uniformly, even over wide areas.

A principal application for inflatable actuators is in automatic door seals for environmental chambers, processing vessels, sterilizers, and flood barriers. The door slides into

place along a track, then the bladder actuator, mounted to the side and shaped to match the door outline, presses it laterally to seat all seals. Thus, with a single pressurization, a single, inexpensive actuator exerts a surprisingly high, very uniform force all around the door.