Machine Design

Give Switches a Boot

Ken Schwinn
APM Hexseal Corp.
Englewood, N.J.

Toggle switches and pushbuttons that mount to operator panels can see all kinds of hostile conditions. This is particularly true for control panels on military gear or off-road vehicles. When saltwater, dirt, oil, or wash downs with a hose is part of the operating environment, panel switches need some sort of protection to survive.

It’s often best to protect such switches with a separate seal or boot, and to seal the panel cutout around the switch. There are other methods of sealing switches, but boots provide several advantages that competing techniques don’t.

For one thing, it frequently costs less to add an external boot or seal to a conventional (and inexpensive) switch, rather than use an integrally sealed (and more expensive) switch module. Damage to the switch boot, moreover, is highly visible. One problem with alternative methods is that trouble doesn’t become evident until the switch fails from contamination.

At least one manufacturer’s external switch boot seals the panel cutout with an integral perimeter rib. It protects behind-the-panel components and circuitry from harm caused by leakage or blow-by. This feature makes the boots widely accepted for both internally sealed and unsealed switches, because they prevent leakage around the switch bezel and through the panel cutout.

Finally, switch boots are reuseable. When a switch wears out, the new replacement can often use the old boot.

The process of selecting a boot involves answering a few simple questions about conditions the operator panel will likely see. Designers will need to know what kind of switch needs protection: toggle, pushbutton, rotary, rocker, or something else. They also need the size of the threaded bushing the switch uses (15⁄32-32, 3⁄8-24, 3⁄8-27, 1⁄4-40, or something else). Important as well is the height of the switch actuator’s tip above the threaded switch bushing, and the maximum diameter of the toggle or pushbutton actuator, or the rotary shaft on a rotary switch. For rocker switches, the critical dimensions are those of the switch bezel and actuator, and the rocker height above the plane of the switch bezel surface.

The choice of seal material may depend on the nature of the hostile environment; protection against jet fuel, cleaning solvents, or caustic chemicals could require specialized formulations that would be overkill for handling more mundane dirt, grease and oil. Expected temperature range is another consideration, as is whether or not the switch seal must be Mil-Qualified/QPL-listed.

The toggle switch’s detent torque can impact boot or seal selection. One kind of boot might be appropriate for plastic toggle switches with light detent force, but not for metal switches with detent force that is typically much heavier.

© 2010 Penton Media, Inc.

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