One driven power screw runs between two idler screws. The idlers are not driven or timed - fluid pressure on the power screw's threads turns them, and they float on a hydrodynamic film of the pumped fluid inside their bores. The meshing screws form the sealed cavities that carry fluid along the axis.

Because the idler screws ride on a film of the pumped liquid rather than on bearings. A fluid with inadequate lubricity or viscosity lets the idlers contact their bores, which wears them rapidly and permanently. This is the single most important selection constraint for the type.

No. Water has neither the viscosity nor the lubricity the idler screws need, so the pump wears quickly. Water and thin solvent duty needs a different pump family, and a triple-screw pump should not be specified for it however convenient the size and pressure rating appear.

The axial cavity principle delivers pulseless flow, there are no timing gears to generate gear noise, and the idlers are hydrodynamically supported rather than running on bearings. The combination makes them among the quietest positive-displacement pumps available, which is why elevators and machine tools use them.

Hydraulic power units, elevator hydraulics, machine tool systems, fuel injection supply, lubrication skids and marine lube oil service. The pattern is always the same: clean oil, high discharge pressure, and an environment where noise and pulsation would be objectionable to people working nearby or would disturb the machine the pump is feeding.

Considerably cheaper and simpler, with no timing gears and fewer precision components. The trade is capability: triple-screw needs clean lubricating fluid, while twin-screw handles gassy, abrasive, thin and variable fluids. Choose triple-screw whenever the fluid allows it, because it is the better value.

Almost always the fluid - inadequate lubricity, contamination with abrasives, or water ingress into the oil. Since the idlers depend on the fluid film, contamination attacks the pump directly. Effective filtration and keeping water out of the oil are what determine service life.