A distinction is drawn between A-gaps and V-gaps in radial direction, and between parallel gaps and wedge gaps in tangential direction.

Parallel gaps: After finish machining the sliding faces at right angles to the axis of rotation, they are technically flat and produce a parallel sealing gap in both radial and circumferential directions when fitted. Sliding faces are deformed in the operating state by the effects of pressure and temperature. The type and magnitude of the mechanical and thermal deformations and combinations thereof are conditional on

• the design of the components and the load factor k
• the mechanical and thermal characteristics of the face materials
• the magnitude and direction of action of the pressure to be sealed
• the temperature in the sliding face zone and the temperature differential between it and the adjacent parts and media.

 A-gap, V-gap

 

A-gaps, V-gaps: If deformation in the sliding faces cause them to make contact at their outer diameter D, the sliding faces form an A-gap; if contact is made at their inner diameter d, they form a V-gap. In either case the result is a change in the factor describing the distribution of pressure across the sealing width (k₁). A seal with an A-gap is liable to open from internal pressurization, and a seal with a V-gap is liable to open from external pressurization (separation of sliding faces). If pressure is applied in the opposite direction, there is greater wear at the points of contact D and d. Formation and reversion of an A-gap or V-gap are favoured by large fluctuations of pressure and/or temperature; this is reflected outside the mechanical seal in heavily fluctuating leakage rates.

Wedge gap: Deformations in circumferential direction lead to alternating gap widths. This effect is desirable because it promotes the formation of a lubrication film (hydrodynamic mechanical seals, circle segment grooves). Serious deformations are undesirable, however, because they increase the leakage rate and give rise to repetitive contact friction. Sealing gap deformations arise in every mechanical seal loaded by pressure and/or temperature. Correct selection of the mechanical seal keeps the deformations within tolerable and sometimes desirable limits.