The change in the relative movement of parts sliding one on top of the other is not an even one, especially in the transition from the state of rest to that of movement. The degree of unevenness increases

• with the size of the coefficients of friction (static friction, breakaway torque)
• with the elasticity of one or both sliding faces
• with the magnitude of contact pressure
• with inconstant lubrication.
  
  This stick-slip effect can lead in mechanical seals to malfunctions extending to failure.
• Given axial movement of the shaft, a high slip-stick effect causes brief separation of the sliding faces and hence a surge of leakage..

• Inadequate lubrication and/or insufficient transfer of frictional heat from the mechanical seal results in a radial force of constantly alternating direction at the sliding face. This radial force can lead e.g. in bellows mechanical seals to deflections that can exert such a load on the bellows that it fractures.

The slip-stick effect can be reduced to the level of practical insignificance by selecting suitable material pairings (hardness, sliding characteristics), by lowering the contact pressures to the functional optimum, by using the right design for the job, and by providing for adequate lubrication and dissipation of the frictional heat.