Activating shields big and small

March 1, 2007
Large bellows protect ways, ballscrews, and cylinder rods against chips and dirt. They're manufactured to fit over parts, or to ride on top of machine

Large bellows protect ways, ballscrews, and cylinder rods against chips and dirt. They're manufactured to fit over parts, or to ride on top of machine ways. 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 for greater protection against adverse environments. This construction also provides the best open-to-closed length ratio.

Special bellows materials may be required for food or laboratory equipment, to resist certain chemicals, or where greater abrasion resistance is required. Sloped shapes can also shed chip loads, and internal or external tie strips maintain even expansion and prevent overextension. 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.

Telescoping steel covers are another option. They are often designed to support the weight of a person, and protect large moving machinery from even the most abrasive debris — excellent for machine tool and other rough applications. Some tips: Steel way covers should only be extended after they have been installed on the guideways to be protected. Otherwise they may jam and become damaged. Also, any extension brackets should be mounted flush with guide surfaces.

Too: Steel way covers should always be mounted on the machine in the compressed position, and the mounting surface must be perpendicular to the direction of cover travel. Finally, distortion of the covers during assembly must be prevented; bolting of the large box must not exert a load on the other boxes. If the large box must be extended, include the next box to properly support the large box for mounting.

Especially when used near lathes, way covers should be inspected at least once per month. These covers can be wiped clean; air hoses are not recommended for cleaning, as this forces dirt under wipers and into the cover.

If chips have penetrated wipers on their own, covers should be disassembled and cleaned thoroughly. Chips between the boxes can cause damage to the cover and particularly to the wiper lips. In fact, wipers and guide shoes are subject to wear and should be periodically replaced.

Combination covers are bellows that include 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.

Tiny but tough

Miniature bellows, just as their big cousins, deflect dirt and moisture — but in small precision systems. Their mechanism is different, but they also prevent contamination that compromises the operation of systems. One protective design for these applications is electrodeposited nickel bellows. They are flexible, but still provide hermetic sealing of designs such as surgical instruments, circuit breakers, and switches. This is especially useful against the autoclaving these units often see during service.

In one thumbtack-shaped design, control buttons can be welded to a metal bellows, in turn fastened to a handpiece body containing controls. When depressed, the bellows compresses, and electrical contacts close. When released, the spring contacts come apart, and the metal bellows, acting like a spring, returns to its original length.

How is the protective bellows so flexible? Electroplating allows thin walls and control of bellows thickness to 0.0003 in. for high sensitvity — useful in precision instrument applications. In fact, they allow relatively large deflections with the application of forces as small as four grams.

Liquid silicone rubber parts provide elasticity over a wide temperature range. A fast cure and cycle time makes it suitable for custom gaskets, bellows, and similar components for both under the hood and appliance designs. The material also has low compression, and resists damaging effects of sunlight and ozone for extended wear life on indoor and outdoor applications.

Some mold-design systems also include a cost-saving modular design — so designers need only purchase their part's mold cavity, and not the entire mold assembly.
Courtesy Rubber Industries, Inc.

For more information

Silicone rubber parts:
Rubber Industries, Inc.
Shakopee, Minn.

Roll covers and bellows:
A & A Mfg. Co., Inc.
New Berlin, Wis.

Miniature bellows:
Servometer Corp.
Cedar Grove, N.J.

Telescoping covers:
Hennig Inc.
Machesney Park, Ill.

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