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Introduction:
In hydraulic systems, it is often important to regulate the movement speed of hydraulic cylinders or the hydropump. This can be achieved by using a hydropump with an adjustable displacement. However, this is very complex and expensive. A simple and cost-effective way to achieve this regulation is by using throttles and flow control valves. By introducing a restriction in the supply or return line of the cylinder or motor, the flow rate is throttled. This restricts the line, allowing the fluid to flow through it at a reduced flow rate.
Throttle:
With a throttle control, we create a restriction in the line. This can be compared to a faucet that allows water to flow; the further the faucet is opened or closed, the greater or smaller the water output is.
In the following diagram, the throttle is located in the supply line between the control valve and the cylinder. The arrow on the throttle indicates that it is variable: the throttle can be manually adjusted.
The pressure before the throttle is obtained from the hydropump. This is the maximum system pressure. After the throttle, there is a significantly lower pressure. The pressure energy is lost in the throttle and converted into heat. The flow rate that the throttle restricts is discharged to the return line by the relief valve. The throttle reduces the flow rate in both the outgoing and incoming strokes of the piston. This is not always desired. More on this later.
To understand the system pressures, here is an example of the pressure differences that a throttle can cause:
- pump output = 10 l/min;
- throttle is set to 8 l / min;
- flow rate discharged by the relief valve = 2 l/min.
A hydraulic cylinder can primarily be equipped with a flow-control throttle in three ways. The image below shows an electrically operated control valve and three cylinders. Each cylinder has a different throttle control:
- this cylinder has a variable throttle on the supply line. In both the incoming and outgoing movements, the fluid flow is throttled. The incoming and outgoing movements of the piston rod are at the same speed;
- the throttle with a spring-loaded check valve ensures that the outgoing movement of the piston is slowed. However, during the incoming movement, the fluid flow pushes the check valve open, allowing the fluid to flow back to the return line through both the throttle and the check valve. The outgoing stroke is therefore slower than the incoming stroke;
- with a throttle and check valve on the supply and return line, the piston speed can be most accurately adjusted: during the piston movement, the throttle actually determines the fluid flow to the cylinder. On the other side, the fluid can flow back to the reservoir without resistance.
Flow Control Valve:
In hydraulic systems where the movement speed of cylinders or hydraulic motors must remain constant despite a variable load, flow control valves are used. In the following diagram, we see, among other things, a series flow control valve and two pressure gauges.a0
The pressure that can be read from p1 depends on the relief valve, which returns a portion of the hydropump output to the return line. The pressure p2 is determined by the load on the piston: when it experiences more counterforce during the outgoing stroke, the pressure p2 increases.
The flow control valve is adjustable and allows the flow rate to the cylinder to remain constant, regardless of pressures p1 and p2.
The operation of the flow control valve is as follows: the pressure drop (94p) across the throttle in the flow control valve is constantly measured. As soon as there is a pressure change, the flow control valve adjusts the throttle size. The pressure control valve ensures that the pressure drop across the throttle, and thus the fluid flow, remains constant at all times.
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