A Centryco bellows for this linear guide offers a compression ratio (compressed length/extended length) as low as 6%. This reduces system cost because less rail and bellows material are needed. A metal clip secures the bellows to the carriage. Profiled internal supports hug the rail.

Machine tool builders increasingly install ball screws, precision linear guides, crossed roller-bearing linear ways, and servomotors into their equipment to gain high speeds and productivity. But these devices have little tolerance for chips, dirt, coolant, and other contaminants common in production processes. Ball and roller bearings, for example, can be easily damaged by the abrasive debris generated by high production machine tools.

One way to protect these motion devices, as well as reduce machinery break downs and maintenance requirements, is to install protective bellows and covers.

These devices are no longer available only in traditional sewn or glued versions. Today’s protective covers come in various styles, as well as sizes and materials. There are at least five types of square-cornered bellows, five types of round bellows, four types of roll-up style covers, and miscellaneous aprons, covers, and assemblies, Table 1. Much of the recent development has focused on bellows and roll-up style covers.

The best time to make a choice is during the initial design of a machine or motion device. Choosing afterwards may restrict the machine stroke. It may also lead to an improperly fitting cover, which often renders the cover useless. And at worst, a late choice could even lead to a redesign of the machine.

Rolling or folding

One style of protective cover, the rollup, is becoming popular because of its efficient use of space. In many applications, it lets engineers design smaller machines. Also, there have been changes in the design of its spring mechanism that eliminate earlier operation problems.

A roll-up cover looks like an industrial version of a window shade. These covers are available with or without a canister. With a canister, the cover material is already mounted on a tubular spring mechanism that rolls the fabric in and out. A bellows-style cover, on the other hand, compresses and extends folds or pleats of material. Because the dimensions of a roll-up cover are often smaller than the dimensions of a bellows with its folds compressed, engineers can often specify shorter lengths of linear guides, ball screws, shafts, and other components when using roll-up covers for the machine.

New spring mechanism designs prevent the snaking and twisting of the springs of earlier models. The designs also extend spring life to over 1 million movements at speeds ranging from 25 to 3,000 ips with accelerations to 1 g. Windup mechanisms may use two, three, or four of these internal springs to share the stress load.

From assembly to fit

Recent models of bellows and covers are more durable because of improved assembly techniques and materials, and better designs that ensure proper fit.

Round bellows can be sewn, welded, or heat molded. This is a sewn bellows, from Milwaukee Protective Covers.

The most familiar type of bellows, the sewn bellows frequently uses threads made of Kevlar to stitch fabric pieces together. Earlier thread materials broke down or deteriorated over time, as did many glues.

Through new assembly methods, bellows are now available with seams that are sonic or radio-frequency welded, or heat sealed. These methods eliminate joints and stitch holes, which can be entry points for oil, coolant, and other contaminants. Plus, these methods lower the cost to manufacture.

Rubber or neoprene coated fabrics were the most commonly used materials for protective covers. Now, however, manufacturers offer a range of fabric substrates and coatings. Coated polyesters and fiberglass typically offer improved resistance to contaminants, abrasion, chemicals, and heat, as well as more flexibility in motion. Polyurethane, for example, is a popular coating because it resists harsh machine coolants, oils, and many chemicals. It also weighs less and can be applied in thin coats, which lets manufacturers achieve smaller closed lengths.

Nomex is a fabric substrate popular for covers used in welding, laser cutting, and sparking applications. It handles temperatures up to 572 or 8428 F, depending on how long the heat source is in contact with the substrate. Coating it with a chemically treated, self-extinguishing PVC or polyurethane enhances its protective properties in these applications.

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