Ancillary equipment basically includes those components or subsystems that are not directly involved in the major functions of the circuit. In many applications accumulators provide a supplementary flow source that can be used to meet high transient flow demands, compensation for leakage and absorption for pulsation and shock situations. These employ a volume of pressurised gas, usually nitro­gen, which can be used to displace a fixed volume of hydraulic fluid as and when required.

All circuits require the fluid to be filtered, as contaminant particles in the fluid are the biggest cause of unreliability and failure of components and systems. These particles enter the system from the environment and are also generated by the wear process such as occurs in pumps and motors.

Inefficiencies in pumps, motors and actuators generate heat, which is absorbed by the fluid. It is necessary to be able to estimate the rate of such heat generation in order to install a cooler for the fluid if necessary. The fluid reservoir needs to be designed so as to enable absorbed air to be released and minimise the possibility of contaminant particles re-entering the system.