Each piston has a valve within it and another valve behind it. Fluid comes in on the wobble plate side (on the bottom left in this drawing) and exits under pressure in the back (on the right here).

The pistons are pushed against the wobble plate with large springs. A pair of smaller springs force the valves (small metal balls) closed. The spring inside the piston is fairly weak, since only suction is used to force it open.

This type of pump can develop incredible pressure -- 10,000 P.S.I. or more. It is commonly used for low-volume applications. Hand-operated wobble pumps were used as emergency fuel pumps on some early aircraft.

Compare this pump, also known as a "wabble" plate pump, to the radial piston pump, swash plate pump, and bent axis pump.

(This may be a low-resolution and jerky animation of a pump which is smoothly animated in the CD-ROM version of this glossary.)