Keeping contaminants out

Aug. 1, 2006
It's not always clear-cut which protective bellows or cover is most appropriate. Construction and material choices abound, and several combinations may

It's not always clear-cut which protective bellows or cover is most appropriate. Construction and material choices abound, and several combinations may work for one application. Among environmental variables to consider are contaminant type, source, and volume; machine operating speed and frequency; space restrictions — both in cross-section along travel path and for retracted length — and ambient and operating temperatures. The most suitable cover options must also be determined, including:

  • Its shape and construction
  • Supports for the cover along travel paths, including the maximum unsupported span
  • The degree of sealing needed, including venting if necessary
  • Aesthetics

Some preliminary engineering is usually necessary. Data sheets that can help in gathering parameters; these can include general operating conditions — for example, chip size, type and load, lubricants, temperatures, travel rates and cycles — as well as travel limits and space available for the cover when installed. Any drawings or dimensioned sketches of the machine parts to be protected are helpful; they should include cross-sectional and longitudinal views, and indicate any areas of interference — for example, from digital readouts, limit switches, or brackets. 3D models are the most useful for complicated applications.

Once the decision is narrowed down, bellows and other protective covers require little if any special tooling to meet most requirements.

Several designs

Bellows-type protectors are generally used to protect ways and cylinder rods against light to moderate volumes of chips, dirt, dust or coolants. They're cylindrically enclosed to fit over cylinder rods, or manufactured in open shapes to ride on top of machine ways. Way covers that span entire areas protect leadscrews located between or adjacent to machine ways.

SUITABILITY OF BELLOWSType of cover Available for plain machine ways and linear rails Available for screws, rods, shafts Open to closed ratio Resistance to sharp chips Ability to withstand chip loads Sealing against liquids Stitched bellows X X Excellent Good Good Good Sealed bellows • Elastomer X X Good Good Good Excellent • Thermoplastic X X Good Good Good Excellent • PTFE X X Fair Good Good Excellent Formed bellows • Elastomer X Good Good Good Excellent L • Thermoplastic X X Good Good Good Excellent Folded bellows X Good Good Good Excellent Stiffener supported bellows • Sewn folded X X Good Good Good Good • Heat sealed X X Good Fair Fair Excellent • Metal clad X Good Good Good Good Coated fabric roll-up shade X Good Good Good Fair Metal roll-up shade X Good Excellent Excellent Fair Multi-axis shield Good Good Telescopic metal cover X X Fair Excellent Excellent Fair

The most common construction is an accordion-type design manufactured from flat pieces of material, joined at alternating inside and outside edges to form convolutions. Typically, elastomer-coated fabrics are used. Stitched designs are sewn together, and are often coated with a liquid elastomer for greater sealing and protection against adverse environments. This construction usually provides the best open-to-closed length ratio, and there usually are no tooling costs.

In sealed designs, seams are thermally bonded together through a variety of processes. Because the seams are integrally consistent, this construction provides better sealing against fine machining and grinding particles. But wider sealed seams on this design do mean slightly lower open-to-closed length ratios. Material choices range from elastomers (such as DuPont's Hypalon) to thermoplastic polyurethane, and even fluoropolymers such as PTFE — which is now made in forms suitable for harsher environments.

Folded-construction designs are another bellows option. Generally they're formed from multiple layers of elastomer-coated fabric with embedded stiffeners bonded in place. Folded covers are limited to flat-sided shapes and can have lower open-to-closed ratios than the other constructions mentioned. They do, however, provide full sealing and a neat appearance. The folded type covers are found in applications such as optical and measuring equipment, electrical cable bus and HVAC ducts, and include laser beams for laser cutting machines.

Stiffener-supported covers (for example, sewn-folded varieties) are used for applications where better travel characteristics and shape retention are needed. They are made using a sheet of elastomer-coated fabric with a PVC stiffener stitched into each convolution. This makes the bellows more rigid and helps retain its shape; this design also retracts more completely for a given travel.

Heat-sealed bellows can also include stiffener-supported sections. Molecular bonding between a single sheet of thermoplastic-coated fabric and stiffener inserts eliminates stitch holes, seams, and breaks. Heat-sealed bellows are crisp, uniform, and consistent, sealing and fatigue life are greater than with that of sewn bellows, and unlimited open lengths can be achieved, with exceptional open/closed ratios.

Expanded mold-type covers are generally manufactured in round or oblong cross sections, although some variations are possible. Those that are expanded from a tube of elastomer-coated fabric provide excellent sealing comparable to injection-molded bellows, with greater strength but without expensive mold charges. Therefore, small quantities are practical.

Other design considerations

Special bellows materials may be required for food or laboratory equipment, to resist certain chemicals, or where greater abrasion resistance is required. Additional design features include sloped shapes that may be needed to shed chip loads, and internal or external tie strips to maintain even expansion and prevent overextension. End mountings for bellows come in the form of flanges with backup plates or collars, which are typically secured with hose clamps.

Internal or external supports eliminate problems associated with high travel speeds, or environments that require special materials and cover shapes. In the case of fast applications, bellows often require two things: Nylon guides or rollers and (if sealed bellows are used) breathers to exhaust the air inside. These breathers can be fine-screened vents or tubes that lead from the end of the bellows to an outside environment if necessary.

Guides, stiffeners, and supports can be added to bellows to prevent sagging on wide unsupported spans or under heavy chip loads, and prevent covers from being damaged from the inside by the equipment it protects. Low-friction guides can be added to stiffeners to extend cycle life, and round covers can be furnished with internal guides that ride on lead or ballscrews. Combination covers may include optional metal flaps or cladding over a stiffener-supported cover to protect against sharp or hot chips. These flaps are hinged at one edge, and overlap each other like shingles.

Other cover types

Rollup protectors resemble heavy-duty window shades and are generally used to protect ways or other horizontal surfaces from falling contaminants. They may be as simple as an elastomer-coated fabric shade on a spring-loaded roller, or as complex as a sheet of flexible stainless steel reinforced with aluminum extrusions and mounted on an air motor-powered takeup roller. In the latter, the air motor keeps the cover under constant tension to prevent sagging. Reinforcements can be strong enough to allow workers to walk on the covers, providing access to areas of a machine that would otherwise be difficult to reach. This also means they can serve as pit covers, shielding actuators or other equipment below floor level.

Roll-up cover combinations can also be used in multi-axis configurations. In this arrangement, the covers are mounted in large, vertically oriented frames to act as a shield and protect entire machine faces, letting only operating components protrude. For example, two roll-up covers might protect the face of a machine as its spindle moves horizontally, while two others protect the machine face as the spindle moves vertically.

Telescoping metal protectors are available as way protectors with either flat or sloped top surfaces, as well as in cylindrical shapes for protecting devices such as ballscrews. They're needed when heavy chip loads or other environmental concerns pose a significant threat to working parts — and where open-to-closed ratios are not critical. They can be made of stainless or carbon steel and the boxlike sections move on nylon riders or rollers to reduce friction. Typically, they are supported and guided by the ways they cover. The leading edge of each section incorporates an elastomeric wiper that cleans the adjacent section.

Round telescoping covers are used for applications such as protecting ballscrews. One type resembles the familiar Boy Scout drinking cup, while another is a helix made of spring steel that expands along its centerline. With either type, chips and fine grit can cause jamming, so they should not be used where this might be a problem.

For more information, call (800) 298-2066 or email the editor at [email protected].

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