Control of the flow rate of a hydraulic system by flow resistances in general with the objective of controlling the velocity of a cylinder or motor. For this purpose flow valves, manually controlled directional valves, proportional or high response proportional valves are used (Figure D 46).
In a hydraulic system with constant pumps the flow rate not required Q V (Q V = Q 1 – Q N) is fed back to the tank (Figure D 47). The power converted into heat at the throttling point on the 3-way flow control valve is proportional to the product of Q v and the load pressure p L:
![6dd2c1b763696e71c3be6497a7b2a70304ea8aef drosselsteuerung_01.gif](/fileadmin/smc/files/6dd2c1b763696e71c3be6497a7b2a70304ea8aef.gif)
With all other methods (that is also with continuously adjustable valves) Q v must be reduced via the pressure-limiting valve, i.e. under full nominal pressure. In addition there is also the loss on relieving the useful flow Q N from the nominal pressure to the load pressure (Figure D 48):
If, despite these relatively high losses, control with throttle is used relatively frequently, this situation is because it is relatively cheap (constant pumps) and because it has a very high dynamic performance (up to a few hundred Hz for control with throttle using servo valves).
Alternative: Pump control
![6a7a8b637a3452896f7b1d87f6dac6e621e778c4 0001_drosselsteuerung](/fileadmin/smc/files/6a7a8b637a3452896f7b1d87f6dac6e621e778c4.gif)
Figure D 46: Overview of control with throttle
![0778f5d966bd51078a18ee58da5bb63c2ad2d08e 0002_drosselsteuerung](/fileadmin/smc/files/0778f5d966bd51078a18ee58da5bb63c2ad2d08e.gif)
Figure D 47: Energy balance for control with throttle a) 2-way flow control valve, b) 3-way flow control valve
![c4d060ff95411a3081a73e9f354d5f79eb3e7d87 0003_drosselsteuerung](/fileadmin/smc/files/c4d060ff95411a3081a73e9f354d5f79eb3e7d87.gif)
Figure D 48: Q-p diagram with power loss on throttling using 2-way flow control valve