Hydraulic motor

[1] One of the first rotary hydraulic motors to be developed was that constructed by William Armstrong for his Swing Bridge over the River Tyne.

The first simple fixed-stroke hydraulic motors had the disadvantage that they used the same volume of water whatever the load and so were wasteful at part-power.

[2] Unlike steam engines, as water is incompressible, they could not be throttled or their valve cut-off controlled.

Several types of "lip" designs are used, and the main objective is to provide a tight seal between the inside of the motor housing and the vane, and at the same time to minimize wear and metal-to-metal contact.

For lubrication, the gear motor uses a small amount of oil from the pressurized side of the gears, bleeds this through the (typically) hydrodynamic bearings, and vents the same oil either to the low pressure side of the gears, or through a dedicated drain port on the motor housing, which is usually connected to a line that vents the motor's case pressure to the system's reservoir.

Gear motors can be supplied as single or double-directional based on their usage, and they are preferred in either aluminum or cast iron bodies, depending on application conditions.

Several different designs exist, such as the Geroller (internal or external rollers) and Nichols motors.

This means that when an axial plunger motor (swept volume maximum 2 litres) is used, a gearbox is usually needed.

The crankshaft type (e.g. Staffa or SAI hydraulic motors) with a single cam and the pistons pushing inwards is basically an old design but is one which has extremely high starting torque characteristics.

Some motors have pistons attached to the cam using rods (much like in an internal combustion engine), while others employ floating "shoes", and even spherical contact telescopic cylinders like the Parker Denison Calzoni type.

This produces a very smooth output with high starting torque but they are often limited in the upper speed range.

Thus, for applications such as a crane or winch with suspended load, there is always a need for a brake or a locking device.