Where flow resistors (orifice, throttle) are used to control the movement of hydraulic motors and cylinders (control with throttle), this is based on the principle of connecting pressure dividers (similar to electrical voltage dividers). A flow rate Q A and a pressure p A will apply between two hydraulic resistances connected in series, based on the two resistance values and the load at the output. These circuit arrangements are referred to as hydraulic half bridges by way of analogy with the electrical half bridge. In order to control each displacement chamber of an actuator, a single half bridge is required. Within the field of hydraulics, there are now five different types of half bridges. The first four are summarised in (Figure H 14). The fifth half bridge is like a spare fuse for half bridge D: the annular area of a differential cylinder is subjected to the full pump pressure.
Figure H 14: Hydraulic half bridges A-D