Which lubrication form uses high pressure air or water to create a bearing cushion in the simplest sense?

• A. Hydrostatic
• B. Hydrodynamic
• C. Elastohydrodynamic
• D. Solid-film

Answer: (A)

Hydrostatic lubrication uses an external source to push lubricant, such as high-pressure air or water, into the bearing gap to create a film that supports the load. Because the film is supplied by an external pump or compressor, a cushion forms even when the surfaces aren’t moving or are moving slowly, making the bearing load-bearing ability largely independent of speed. This external pressure keeps the surfaces separated and reduces wear.

The other lubrication forms rely on different mechanisms. Hydrodynamic lubrication builds the film from the motion and geometry of the parts; as parts slide or rotate, viscous forces in the lubricant generate pressure that separates the surfaces, but the film strength depends on speed and motion. Elastohydrodynamic adds very high pressures that deform the contacting surfaces elastically, altering the film thickness and pressure distribution. Solid-film lubrication uses a thin solid layer (like graphite or MoS2) instead of a liquid film, so there isn’t a pressurized liquid cushion formed between surfaces.