Exclusion Seals

Nov. 15, 2002
Exclusion seals, wipers, and scrapers clean the surface of a retracting rod, removing abrasive particles such as dirt, mud, and ice.

Exclusion seals, wipers, and scrapers clean the surface of a retracting rod, removing abrasive particles such as dirt, mud, and ice. This protects the seal and extends service life. The importance of wipers cannot be overemphasized. Often a seal fails prematurely because the wiper wears out long before the primary seal. In such cases failure is not due to misapplication of the seal, but to the wiper not protecting the seal from abrasive wear.

Conventional rod wipers are usually produced from a material like high-quality polyurethane. They are designed to snap directly into the glands on common cylinders.

Typical properties for wiper materials are: hardness, 85-95 Shore A; tensile strength, 6,000 psi; elongation, 400%. They can typically be used in a temperature range from -40 to 210°F, although some manufacturers predict reduced life if the rods are exposed to steam and water over 190°F.

Sealing wipers are made with a sealing lip at the bottom and a rod wiper at the top, so the wiper can perform as a pressure-energized lip seal inside the cylinder at the same time it wipes contaminants from the outside. The two most common types are a double-lip U-seal and a double-lip V-seal. Both are one-piece snap-in replacement seals that provide both sealing and wiping action.

Scrapers are exclusion seals that have a metallic lip or hard plastic scraping element to scrape heavy or tenacious materials from reciprocating shafts. The hard lip is kept sharpened by the honing action of the shaft. Wipers are sometimes used behind scrapers to catch any fine particles or fluids that pass the scraper.

There are two types of scrapers, conical scrapers and scraper rings. The conical scraper has a sharp knife edge that digs under the foreign matter and lifts it from the shaft. The scraper ring has a flat top, 1/32 to 1/16 in. wide. Although this produces somewhat more resistance to scraping than a knife edge, it has the advantage of extra strength.

The material of the scraping element must be tough enough to withstand the impact and abrasion of materials adhering tightly to the shaft. It must be resistant to corrosion, have low friction against the shaft material, and little tendency to score the shaft. Good spring characteristics are needed for conical scrapers.

Metals such as copper-silicon, aluminum, manganese bronze, beryllium-copper, and brass are most commonly used for scraping elements. For corrosion resistance, they may have cadmium or other platings.

Boots are used to keep undesired material from entering the seals of hydraulic cylinders and other fluid-power devices. Motion of the rod relative to the cylinder is absorbed by the flexing of the boot so there is no frictional contact between stationary and moving elements as with other seals.

Although many shapes of boots are commercially available and made to accommodate such devices as universal joints, shift levers, and hinges, specialized boots are designed particularly for hydraulic and pneumatic cylinders. They may be made of leather, plastics, elastomers, or occasionally of thin formed metal, although neoprene-coated nylon fabric is widely used for strength and resistance to abrasion. Leather tends to crack at low temperatures, but other materials can often be used over a range of -45 to 220°F. Special boot materials are available for high and low-temperature operation.

Boots are available to cover rod sizes from 0.5 to 10 in. diameter; because the boots are normally made to order, nearly any length is available. Typically, the minimum compressed length for a boot is 0.25 in. for the smallest rods; for some of the larger rods, minimum compressed length can be over 1 in.

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