Controlled leak tightness
Dynamic Seals Commonly Used in Mechanical Industry Dynamic seals for reciprocating motion

For a component to move through a wall, there must be a clearance between them. The greater the clearance, the greater the leakage. However, with certain liquids, especially if the viscosity is high enough, leakage will be low and perfectly acceptable if the liquid remains inside the system, as in a hydraulic directional valve spool, for example. This simple type of sealing is also common in transmission hydraulics.

Flow through an annular leak in laminar regime gives a leak volume flow rate Q _v equal to :