Soft actuators tackle hard jobs in adverse work environments

Paul Boughton
Air actuators shake off contamination that makes quick work of traditional cylinders. Bernd Stöter reports.

For more than 50 years, air actuators have played a key role in many pneumatically powered machines. Air actuators differ from metal pneumatic and hydraulic cylinders in that they have no piston rods and cylinder barrels. Instead, the simple and cost-effective design somewhat resembles a flexible rubber bellows. But construction is similar to that of a car tire, giving the devices toughness, pressure resistance, and dimensional stability.

A prime advantage is that air actuators eliminate the need for dynamic seals that are essential - and, at times, a headache - in traditional cylinders. This means there is no path for external contamination to enter the air actuator. It eliminates breakaway (stick-slip) friction that can plague cylinders in low-load or low-speed conditions, so the actuators respond immediately and uniformly even to small pressure variations. And because there are no seals to wear out, service life is often much longer than that of metal cylinders, especially in adverse environments.

Standard-elastomer construction handles temperatures from -30 to 160°F, though units for higher and lower temperatures are available.

Types of air actuators include single, double, and triple-convolution bellows, as well as rolling-lobe designs, in a wide range of sizes.

The ContiTech line, for instance, ranges from 2 to 40-in. in diameter. With a maximum working pressure of 115psi, the actuators generate compression forces or support loads to 67000lb. In addition, designs with reinforced internal materials handle pressures to 175psi.

Other advantages include:

Durability. Because the actuators have no seals or rubbing parts, the products are free from leakage even when exposed to dirt, dust, granular materials, or sediments - making them essentially maintenance free. They are also resistant by weathering, many chemicals, and have a long service life even when heavily loaded.

Lateral misalignment capacity. Due to inherent flexibility, air actuators operate reliably when misaligned up to 1inch. Compensating for slight misalignments also simplifies installation. And there is no need for precision guides which are sensitive to dust and other contamination.

Low height. Because air actuators do not have piston rods, this can permit machine designs more compact than with conventional pneumatic cylinders.

Multi media. Air actuators typically run on industrial compressed air. But they also work well with oil-free air, making them suited for special applications in the food and medical industries. They also operate with other gases, including nitrogen, and even low-pressure hydraulic media such as water and glycol.

Tilt motion. Air actuators can stroke through an arc, letting designers simplify articulated constructions. Maximum tilt angle varies with actuator type and kinematics but, for triple-convolution air springs, it's up to 30°.

Safety. Burst pressure is several times greater than the maximum working pressure.

Low cost. Up-front costs are considerably less than for conventional pneumatic cylinders of the same size. And long service life and low maintenance keep operating costs down, too.

Design considerations

When weighing air actuator versus other options, consider these factors:

Lateral guidance. Actuator stability can vary greatly and depends on the type, size, internal pressure, and height. Therefore, lateral guidance is advisable. In many applications, the overall machine design (lever arm, scissors construction) provides the necessary lateral guidance.

End stops. Air actuators must have external stops at the ends of stroke. They are not designed to withstand forces that arise when extended or compressed excessively without external stops. Actuators inflated without a load can suffer damage and risk injury.

Air consumption. An air actuator's outer diameter is somewhat larger than the effective diameter. Therefore, calculate air consumption based on volume curves in manufacturers' datasheets, not on the effective area and stroke.

Installation space. Working space must allow for changes in the actuator's diameter over the entire stroke. The bellows must not rub against machine parts or against itself. Take extra care that actuators do not chafe when operating in a tilted position.

Selection factors

To properly size an actuator, here are some additional design considerations.

Stroke length. Air actuators have a minimum (compressed state) and maximum (no end stop), design height. Maximum working stroke is the difference between the two.

Force. Unlike conventional pneumatic cylinders, an air actuator's effective diameter dw (and, thus, effective working area) is not the same as its outer diameter. The diameter can change with operating height. Therefore, exerted force decreases progressively with stroke, and this decrease varies according to the size and type of unit. Check the manufacturer's force/stroke data to determine force output and pressure requirements at maximum stroke.

Return force. Air actuators operate like single-acting cylinders in that they need a return force to retract the actuator to its compressed position. Actuator datasheets list this force. Convolution air actuators can function when completely depressurised. Sleeve type and rolling-lobe actuators require a minimum pressure to let the bellows roll down over the piston. Datasheets specify minimum pressure requirements.

Applications

Air actuators are used instead of pneumatic or hydraulic cylinders in a wide range of applications, including material-handling equipment, agricultural machinery, food processing, paper and textile equipment, punch and forming presses, and sawmill machinery.

They are especially suited for applications with adverse ambient conditions and high force requirements. Typical examples include low-temperature (-75°F) wood-processing applications or high-temperature glass and paper manufacturing with sustained temperatures of 240°F and peak temperatures reaching 265°F.

The ContiTech Group is an independent division of Continental AG, comprising seven specialised operative business units: Air Spring Systems (air suspension systems, rubber expansion joints, and air actuators); Benecke-Kaliko Group (surface materials); Conveyor Belt Group (conveyor belts and conveyor belt service materials); Elastomer Coatings (technical materials and diaphragm materials); Fluid Technology (hoses, elastomers, plastics, textile, steel, and aluminum); Power Transmission Group (drive belts, matched components, belt-drive systems, and aftermarket parts for vehicles, machinery, and systems); Vibration Control (moulded rubber parts, rubber-to-metal bonded parts, plastic products, and vibration-and noise-compensation systems).

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Dipl.-Ing Bernd Stöter is with ContiTech Air Spring, Hanover, Germany. www.contitech.de.

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