Arise with all displacement machines as the sum of the leaked oil and compression losses. The pressurised chambers in the devices are sealed off from moving components by mating surfaces and rods. There are still leakage paths, through which the pressure fluid can flow.
The external leaked oil flow Q Le for pumps and motors flows out of the pressurised chambers into the housing; for cylinders it leaves via the piston rod seals. The internal leaked oil Q Li flows into the devices from the high-pressure section to the low-pressure section, for example in the case of pumps from the high-pressure section to the suction section. The total oil leaked depends on the pressure and rotational speed or velocity.
Compression losses are caused by the compressibility of the hydraulic fluid.
In the case of hydraulic fluids, the volumetric losses are described by the volumetric efficiency (cf. power losses).