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What is a Cup Washer?

By Paul Scott
Updated: May 17, 2024

A cup washer is a light weight, dished fastener accessory used as an decorative, insulative, or supportive element for screws and bolts. This type of washer may be used as a surface mount or countersunk element. All cup washer types feature a bowl shaped central depression which allows for snag free bolt or screw head placement. A cup washer is usually made of light gauge steel, brass, leather, or plastic and available in a wide range of sizes and designs.

A cup washer is generally used to offer support, insulation, or a cosmetic finish to bolts or screws particularly those with countersunk heads. Cup washers are made of a variety of materials depending on the intended application. These include light weight metals such as steel or brass, insulating materials such as plastic and, in the case of faucet and pump washers, leather. Cup washers are either designed to be surface mounted or countersunk. They often include features such as decorative snap-on caps and electrical terminal cut-outs.

Surface mount cup washer designs resemble a dog's water bowl with raised sides and a dished center. This type of cup washer is well suited to applications which do not allow for screws to be countersunk. The washer then affords a good bearing surface for the screw and, as the screw head sits inside the washer, a flush, snag free finish. Surface mount washers are generally made of light gauge brass or steel and often feature a decorative cap which encloses the screw head.

Flush mounted cup washers are countersunk into the work surface and feature internal contours suitable for countersunk or flat headed bolts and screws. This type of cup washer often has a lip or flange built into its outer rim to offer better support for the screw. These washers may be made of a variety of materials including nylon although steel and brass are the most common. The flush mount cup washer is commonly used in timber constructions and affords a good bearing surface and snag free finish. This type of washer is also often supplied with a snap-on cap.

Insulating cup washers are used to electrically isolate metal screw and bolt heads. They are often used to isolate the screws used as stand-offs on printed circuit boards and the bolts which hold electrical components in place. These washers typically have a flat bottom and fairly high side walls to afford maximum protection against electrical shorts. Other cup washer designs in electrical and electronic applications are terminal cups. These washers feature a cut-out in the side wall which allows for tabbed termination terminals

Although leather cup washers are seldom used, they are frequently encountered as seals in various applications. These include faucet washers, pressurization pump seals on kerosene lamps, and small hand pumps or seals on piston-operated air rifles. This type of washer is particularly effective as a seal in environments which feature exposure to oily fluids. These washers are generally available as a flat disc which is attached to the operating element of the appliance and cup formed in situ.

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