Valves of type DRE(M)E are pilot-operated pressure reducing valves. They are used for operating pressure reduction.
These valves basically consist of a pilot control valve (1) with proportional solenoid (2), a main valve (3) with main spool insert (4), as well as an optional check valve (5). On the proportional solenoid, there is moreover a housing (23) with the control electronics. Supply and command value voltage are applied at the connector (24). At the factory, the command value pressure characteristic curve is adjusted with little manufacturing tolerance.
The pressure in channel A is set in a command value-dependent form via the proportional solenoid (2).
In rest position – no pressure in channel B –, the spring (17) holds the main spool (4) in its initial position. The connection from channel B to A is closed. A start-up jump is thereby prevented.
The pressure in channel A acts on the surface (7) of the main spool via the bore (6). The pilot oil is taken from channel B and flows through the bore (8) to the constant-current control (9) that keeps the pilot flow constant independent of the pressure drop between channel A and B. From
the constant-current control (9), the pilot flow flows in den spring chamber (10), through the bores (11) and (12) via the valve seat (13) into channel Y (14, 15, 16) and from there to the return flow. The pressure required in channel A is preset at the related amplifier. The proportional solenoid moves the valve poppet (20) in the direction of the valve seat (13) and limits the pressure in the spring chamber (10) to the set value. If the pressure in channel A is lower than the preset command value, the higher pressure in the spring chamber (10) moves the main spool to the right. The connection from B to A is opened. If the set pressure in A is reached, the forces at the main spool are balanced – the main spool is in control position.
Pressure in channel A • Spool face (7) = pressure in spring chamber (10) • Spool face – spring force (17)
If in a standing hydraulic fluid column (e. g. cylinder piston at stop), the pressure in A is to be reduced, (e. g.) a lower command value is preset at the control electronics and in this way a lower pressure is preselected which is present immediately in the spring chamber (10). The higher pressure in A which acts on the surface (7) of the main spool pushes the main spool to the stop against the plug screw (18). The connection A to B is blocked and A to Y is open. The force of the spring (17) now counteracts the hydraulic force acting on the surface (7) of the main spool. In this main spool position, the hydraulic fluid can flow from channel A via the control edge (19) to Y into the return flow.
When the pressure in A has dropped to the pressure in the spring chamber (10) plus ∆p from spring (17), the main spool closes the large control bores in the socket at the control edge A to Y.
The residual pressure differential of approx. 10 bar to the new command value pressure in A is now only unloaded via the fine control bore (21). In this way, a favorable transient response without pressure undershoot is achieved. For the free flow back from channel A to B, a check valve (5) can optionally be installed. Part of this flow from channel A simultaneously flows into the return flow via the open control edge (19) of the main spool A to Y.
For hydraulic protection against an inadmissibly high electric control current at the proportional solenoid, which imperatively results in excessive pressures in port A, you can optionally install a spring-loaded pressure relief valve as maximum pressure limitation (22). The maximum pressure limitation is pre-set based on the relevant pressure rating (see table).