Electrohydraulic Servo Motors
One major advantage fluid has over electrical power is that the former is not bound by the physical limitation of the material. For example, the saturation limit of steel limits the performance of the electro magnet, and available force per unit area from the hydraulic system is something like ten times than that of a saturated magnet. Therefore, hydraulic units are used whenever a large force for a small volume or a small weight is required. This applies particularly to heavy machinery such as mining machines and mobile equipment but it is not as important as in machine tools where accuracy and stability are the prime requirements. One main disadvantage of hydraulic power is that they are operated by high pressure oil which means that an expert must operate them and the oil must be kept clean and leakage to outside environment must be avoided.