Machine Design

Compression Packings

Packings are seals that normally are packed around a shaft or rod and compressed to provide a sealing effect. A packing normally consists of many sections (or one long continuous strand wrapped several times to achieve a section effect); when compressed longitudinally, they expand radially to seal.

Normally, packings are not designed to produce a leak-free seal. Instead, they are tightened enough to produce a minimum but positive leakage. In many applications, the leakage is intended to lubricate the packing material; some manufacturers advise the packings be lubricated if this leakage falls below specified minimums or if it is of a nonlubricating liquid. The modest leakage reduces friction and wear.

Packing material may be supplied in many shapes, including impregnated yarn, continuous strands, or continuous strands of a compressed square-section material. Also, various types of interlocking packing shapes are provided; many of these shapes are proprietary developments of specific companies in the packing field. One very common style is the familiar V-packing, in which the seal-ring mating surfaces are designed to permit a void between each ring which acts as a lubrication reservoir.

Many materials are used in packings, ranging from solid metal to flax or cotton. The more common varieties include aluminum, copper, and lead foil, aluminum and copper braided wire, PTFE elastomers (tapes and solid sections), leather, flax, cotton, flexible graphite, plastics, and formerly asbestos. Materials are selected on the basis of service temperature of the application and the process fluid to be used. Packing manufacturers provide detailed information on compatibility and on temperature capabilities of the different packing materials.

An old rule of thumb states that optimum packing length is 1.5 times stem diameter, but like most rules of thumb, it has only limited validity.

TAGS: Hydraulics
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