Protecting linear motion devices

Oct. 1, 2000
To maintain the high performance levels required from precision linear devices, users need to protect them from the contaminants that occur throughout the production processes. One solution is the use of bellows, boots, and covers.

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.

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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Fit is a function of size and installation. Improperly fitted protective covers can snag on machine components, for example, unseating the covers. Poor tracking, as a cover extends and compresses, makes it vulnerable to rips and tears if it contacts lubrication lines, optical readers, proximity switches, and other machine components. Stiffeners are one way to prevent covers from dislodging from the linear power transmission mechanism.

Protective cover manufacturers are also turning to another solution to help to ensure proper fit. They have been working with linear component manufacturers to create standard sized bellows and covers, even for applications in hostile environments. These sizes fit standard-size rails, linear guides, ways, ballscrews, and other linear components. Users can order the protective covers off-the-shelf and have them in two to three days rather than the three to four weeks common with custom sizes.

If the protective cover fits properly, it will not restrict machine motion. It will also be unobtrusive, no more noticeable on a machine than any other component.

Specific application needs

Protective cover selection is not done until engineers consider potential restraints of the application. For example:
• Food handling applications require covers be FDA approved.
• In clean-rooms, covers should be certified for out-gassing and dust environments.
• Packaging applications need adhesive resistant covers.
• In painting environments, the materials and coatings of bellows and covers should resist paint solvents and not contaminate the paint.
• Precision and delicate inspection equipment need covers that easily flex.
• Covers for medical equipment may need UL certification.

Selection tips

The basic details engineers need to know before selecting a protective cover are:
• Diameter of rod, shaft, or screw.
• Pitch of screw (if applicable).
• Length and size of linear rail, ball screw or other linear component. Also model and manufacturer.
• Extended length.
• Stroke, which is extended length minus fold compression.
• Plane of operation.
• Working environment.

Usually, the most common errors involve determining the proper length. Not accounting for the needed closed-length of a cover is one such error. With bellows, this is the amount of space the folds take when compressed. For roll-up covers, it is the maximum diameter of the roll or size of the canister.

Another common error is giving a manufacturer the travel length when asked to provide the open length of a bellows or cover. Travel length is always less than open length for it doesn’t include the closed-length parameter. Giving the wrong information will result in a cover with too short a stroke.

In addition, failure to choose the right fabric and coating for the application will cause problems.

In vertical or frontal applications, allow some method for a bellows to grip and hold the machine. An internal stiffener is often used for this purpose.

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