Page 178 - Tribology in Machine Design

164 Tribology in machine design

parameters .of helical seals are as follows: a = 10°-20 ; (3 = 4-6; y =0.5-0.8.
Taking mean values, eqn (4.192) becomes

5
Clearance is usually between 2.5 and 5 x 10 m. Face seals may also act on
the viscoseal principle. Then, spiral grooves are incorporated into the
diametral plane. Such grooves are often incorporated into the contacting
faces of seals made of elastomers in order to induce a self-pumping action.

4.15.5. Labyrinth seals
Labyrinth seals are based on positive, finite mechanical clearances which
are sufficiently large to preclude the possibility of contact between the parts
in relative motion. They may be used either in the radial or axial flow
configurations and are effective by reason of the generation of eddies within
the cavities. The spacing of the barriers between the cavities is usually about
twenty times the radial clearance. The most critical aspect of labyrinth seal
design is the provision for the thermal expansion of the equipment being
sealed. The adverse effects of inadvertent contact may be minimized by the
use of a relatively soft material, for example carbon, for one of the
components. Instances of failure of the barrier elements by fatigue are
usually due to aeroelastic instability which could be avoided by suitable
design. There are computer programmes available to design a labyrinth
seal.

4.15.6. Wear in mechanical seals
The sealing elements (the primary ring and the mating ring), of a nominally
contact type seal, usually operate in uni-directional sliding. Reciprocating
motion and various modes of oscillatory motion are common. Most often
those elements are in an impregnated carbon-graphite nose-piece sliding
against a harder material, such as ceramic, tungsten carbide or silicon
carbide as listed in Table 4.1. These materials are usually selected to be
chemically compatible with the lubricant or process fluid, as well as the
operating environment and the conditions of operation. All these factors
can contribute to the seal wear mechanisms that must be mitigated to
achieve wear control.
Adhesive wear is the dominant type of wear in a well-designed seal. Even
when there is a hydrodynamic lubricating film at the interface, solid
contacts occur during startup, shutdown, and operating perturbations; the
carbon-graphite nose-piece is usually considered the primary wearing part
and the mating surface wears to a lesser extent. Details of the adhesive wear
process, as such, were discussed in Chapter 2. In the special case of seals the
face loads are sufficiently low so that the mild adhesive wear process occurs.
The process is dominated by transfer films. The PV (product of specific
contact pressure and sliding velocity) criterion used in the design of seals is