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 endstroke 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.

Comparison of technologies
ACTUATOR	Bladder actuator	Rod and piston, pneumatic	Rod and piston, hydraulic	Screw	Stepping motor	Hydraulic motor
KEY STRENGTHS	Low installation cost, low part count, easy fit, uniform force, corrosion-immune, fast cycling, high force capability, contaminant-free	Fast cycling, low maintenance	High force capability	Positive displacement	Positive displacement, very high precision	High force capability
KEY TRADEOFFS	Limited precision, stroke; Force may decline near end of stroke	Point force	Point force High maintenance cost	High precision	High installation cost, precise fit needed	High part count, High installation cost, High maintenance cost
KEY APPLICATIONS	Conveyor braking, workholding, panel lifting, clamp actuation, sealing	Conveyor brakes, workholding	Conveyor brakes, workholding	Precise fit needed, Corrosion- sensitive	Parts positioning	Panel lifting, Clamp actuation
APPLICATIONS TO AVOID	Point-source applications	Precision positioning	Precision positioning, Dirty or corrosive areas	Dirty or corrosive areas	Dirty or corrosive areas	Cost-sensitive work
STROKE	1/16 to 2.5 in.	Unlimited	Unlimited	Unlimited	Unlimited	Unlimited
PRECISION	Poor	Good	Good to excellent	Excellent	Excellent	Fair to good
FUNCTIONALITY VS. SIZE	Excellent	Fair	Fair	Good	Excellent	Fair
MAINTENANCE COSTS	Zero	Low	Low	Low	Low	Moderate
UTILITY REQUIREMENTS	Air	Air	Hydraulic power supply	Electric or hydraulic	Electric	Electric, hydraulic power supply
COST	$	$$-$$$	$$$-$$$$	$$$	$$$$	$$-$$$$
Inflatable bladders have their limitations, but offer more than other technologies in certain design areas.

Thanks to Doc Ardrey. For more information, visit pawling.com or email the editor at eeitel@penton.com