Sealing force f_D.

The illustration shows the axial equilibrium of forces in a mechanical seal.

The resultant sealing force F_D is expressed as:

F_D = F_F +F_H - F_Sp ± F_R

F_H = hydraulic loading force (load factor k)

F_H = spring force

F_Sp =force of the sealing gap

F_R = frictional force of the secondary seal
The frictional force of the secondary seal F_R can change its magnitude and direction with the mechanical seal's direction of rotation.

As a rule, F_R can be neglected.