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Machine Design

Give cylinder-rod contaminants the boot

Stitched bellows protect equipment from dirt, chips, and coolant in moderately severe environments.

Hydraulic cylinders are constantly exposed to contaminants ranging from dust and abrasive grit to metal chips and weld spatter. Boots or bellows can protect finished rod surfaces from wear-causing elements to reduce downtime and lengthen cylinder life. A variety of designs are available in any quantity with little or no tooling charge. This lets you optimize the type of protection for your application, rather than making do with off-the-shelf components.

More than one style of cover or type of material may work in an application, so the best option may not be obvious. First, consider the operating environment. Solid contaminants such as machining chips, abrasive grinding particles, or grit can cause mechanical abrasion and wear. Fluid contaminants such as coolants, washdown water, rain, or snow require more complete sealing.

Second, look at the equipment on which the covers will be mounted. Make certain space is available for covers to fully retract and extend, and no interference points lie along the travel path. Also consider the source and volume of contaminants, and ambient and operating temperatures.

Common types of bellows include stitched, vulcanized, injection molded, and an "expanded" type. Standard stitched construction provides sufficient protection against most contaminants in moderately severe applications. The bellows have rings of elastomer-coated fabric sewn along alternate inside and outside diameters. For added protection, seams can be coated after stitching.

For more demanding conditions, vulcanized covers offer complete sealing. The covers are similar to stitched bellows, but the seams are sealed to exclude fine particles and moisture. They are also preferred where aesthetics are important. Compared with stitched construction, vulcanized covers have a lower open-to-closed-length ratio because the seams are slightly wider. This becomes critical when space is limited in retracted positions.

Injection-molded bellows offer complete sealing but present several disadvantages. For instance, mold costs are usually too high for retrofit applications. And even if stock-molded bellows are available, the material is unsupported and can tear easily. Also, the rounded convolution shapes usually provide low open-to-closed-length ratios.

Bellows-type covers expanded from a tube of fabric-reinforced elastomer are a good alternative to injection-molded bellows. They offer the same tight-sealing advantages but provide additional strength and can be made with little or no tooling charges. The bellows are cured or vulcanized after forming and feature a variety of end mountings.

Manufacturers can add many design modifications or special features to meet specific conditions. For example, cylinder-rod bellows may not exhaust internal air quickly enough for high-speed cylinders. One or more screen breathers may work if the particles are not too fine. Finer contamination may require a breather hose to a clean location or an internally drilled breather hole to vent through the machine.

Other options include tie strips or tape to ensure uniform extension, and longitudinal zippers for retrofit installation. Most convoluted covers can be held in place by collars and hose clamps or flanges and backing plates.

This information supplied by A & A Manufacturing Co. Inc., New Berlin, Wis..

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