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 bladder-type 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 high-pressure 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?
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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.
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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.
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.
Bladders also offer significant savings in installed cost and part count. For example, in high-speed web printing presses, small inflatable bladder actuators operate clamps that hold flexible mats in place on the transfer roll, which spins at a few hundred rpm. The bladders themselves weigh two thirds less than traditional electric or magnetic clamp operators, and exert uniform opening force over the entire clamp length. The newer bladder design also cuts assembly cost to one third, and part count to one sixth.
Bladders are also suitable when shock is unacceptable. In web-type glass cutting machines, machine builders continually rework brake designs for gentleness and reliability. Mechanical brakes, especially older models, can jerk belts to a stop, and are the main source of excessive scrap rates and service calls for the machine as a whole. Inflatable bladders introduce little shock to systems, and serve as soft brakes to hold webs in place as needed.
Bladders can also fit in tight spaces — as in our press-clamp application, for example. Inflatable actuators are often retrofitted or integrated into existing design spaces that would never accommodate a cylinder or stepping motor.
Basic cross-sections
The most common bladder is the footed actuator. Molded-in feet facilitate mounting, while the bulb portion expands upon inflation to effect the desired motion. This is the cross section used in most conveyor stops and part positioners. The top contact surface can also have a gripping cross section: This cross section also helps ensure positive contact in applications involving grit and dirt. The ridge tops stay clean and the grit drops through the grooves between them.
In typical part-positioning applications, footed inflatable actuators provide uniform lifting force for entire plates; springs then return the system to the default position. The same basic arrangement serves in applications in processing and material handling equipment. Low-profile cross sections are usually used in hoop-type actuators, where they can be snap fit into a retention groove.
There are exceptions: One is the straight actuator used in press clamp openers. Here, the actuator fits into an existing space, held in place by the wedging action between two members. Retention is usually achieved by installing the actuators into some rigid channel. Adhesives may be adequate for less demanding applications. For the footed types, standard mating channels or metal clamps are available.
Convoluted cross sections offer both greater stroke and higher end-stroke force than the other types. Their applications include load locks in processing equipment and tray positioners in hospital sterilizers.
Channel-style sections are designed to fit into standard aluminum channel stock dimensions. These bladders provide for a large contact area, which translates to higher forces. Heavy-duty cross sections can also deliver higher forces, but with shorter strokes. The sides elongate during pressurization while the flat top surface remains unchanged. This kind of bladder actuator is usually installed in a rigid channel, which holds it in place and ensures that all internal pressure is converted to an upward force.
Materials
Most bladder cross sections are available in eight principal elastomers. All of them can be reinforced with fabric for extra strength, more wear resistance, and shape retention. Reinforcement eliminates some of the physical limitations of silicone, fluorosilicone, fluorocarbon rubber, and nitrile.
Note that many of these materials are rated for food grade or nuclear applications, since they are contaminant-free and introduce no lubricants or chemical protectants. This — coupled with the fact that bladders are air powered and don't require oil — means they're particularly well suited for clean applications.
Thanks to Doc Ardrey. For more information, visit pawling.com or e-mail the editor at [email protected].