Stiffness Defined

Oil-film stiffness is the change in supporting force per unit change in film thickness. High stiffness means the workpiece position barely moves even as cutting forces vary—essential for precision.

How Membrane Feedback Works

In a membrane-feedback restrictor, a thin diaphragm separates the supply side from the cavity side. When load increases and the film thins, cavity pressure rises; the diaphragm deflects and progressively opens the supply orifice, pushing even more flow and pressure into the cavity. This continuous, proportional compensation yields the steepest force-vs-displacement curve—i.e., the highest stiffness.

Measured Performance

Well-designed hydrostatic systems reach oil-film stiffness above 500 N/μm, several times that of equivalent rolling bearings. The membrane type is the key enabler of that performance.

Design Implication

Because stiffness is localized at each cavity, engineers size the number and area of cavities to match the expected load map of the spindle or rotary table. Membrane feedback makes that design predictable and repeatable.