Throttle valve use and maintenance

- Jul 17, 2019-


1,What is the flow control principle?

Throttle orifices are usually available in three basic forms: thin-walled orifices, elongated orifices, and thick-walled orifices, but regardless of the form of the orifice, the flow rate q through the orifice and its differential pressure Δp The relationship can be expressed by the formula (2-63)q=KAΔpm. The flow characteristics of the three orifices are shown in Figure 3-61.

3-61

Figure 3-61 Throttle valve characteristic curve

1) The effect of differential pressure on flow. When the pressure difference Δp between the two ends of the throttle valve changes, the flow rate through it changes. In the three types of throttle ports, the flow through the thin-walled small holes is minimized by the pressure difference change.

2) The effect of temperature on flow. The oil temperature affects the viscosity of the oil. For the elongated hole, when the oil temperature changes, the flow rate will also change. For the thin wall, the viscosity of the small hole has little effect on the flow rate, so the flow rate is basically unchanged when the oil temperature changes.

3) Blockage of the throttle. The throttle of the throttle valve may be partially blocked due to impurities in the oil or colloidal or asphalt precipitated due to oxidation of the oil, which changes the flow area of the original orifice and changes the flow rate. Especially when the opening is small, the effect is more prominent, and in severe cases, it will be completely blocked and the flow will be broken. Therefore, the anti-clogging performance of the orifice is also an important factor affecting the flow stability, especially affecting the minimum steady flow of the flow valve. Generally, the larger the flow passage area of the orifice, the shorter the throttle passage and the larger the hydraulic diameter, the less likely it is to block, and of course the cleanliness of the oil also affects the blockage. The minimum steady flow rate of a typical flow control valve is 0.05 L/min.


2,What are the 2 outlet forms?

In order to ensure stable flow, the form of the orifice is preferably thin-walled. Figure 3-62 shows several common throttle forms. Figure 3-62 (a) shows the needle valve type orifice, which has long passage, large wet circumference, easy to block, and the flow rate is greatly affected by oil temperature. It is generally used in occasions where performance is not high; 62(b) shows the eccentric groove type orifice, which has the same performance as the needle valve type orifice, but is easy to manufacture. The disadvantage is that the radial force on the spool is unbalanced, and it is more laborious to rotate the spool. For low pressure, large flow and low flow stability requirements; Figure 5-29 (c) shows the axial triangular trough throttle, its structure is simple, the hydraulic diameter is medium, can get smaller Stable flow, and the adjustment range is large, but the throttle channel has a certain length. The oil temperature change has a certain influence on the flow rate. It is widely used now. Figure 3-62(d) shows the circumferential slit type orifice. A slot of unequal width is opened along the circumference of the spool, and the size of the opening can be changed by rotating the spool.The valve port is made into a thin blade shape, the passage is short, the hydraulic diameter is large, and it is not easy to block. The oil temperature change has little influence on the flow rate, so its performance is close to the thin wall small hole, which is suitable for low pressure and small flow occasion; Figure 3-62(e) Shown as an axial slot type orifice, the thin-walled valve port is machined on the bushing of the valve hole, and the valve core can be moved axially to change the opening size. The performance is shown in Figure 3-62(d). The throttle is similar. In order to ensure stable flow, the form of the orifice is preferably thin-walled.

3-62


Figure 3-62 Structure of a typical throttle


3,How to play the role of throttling components?

In the hydraulic transmission system, the throttling element and the relief valve are connected in parallel to the outlet of the liquid pump to form a constant pressure oil source, so that the pressure at the pump outlet is constant. As shown in Figure 3-63 (a), at this time, the throttle valve and the relief valve correspond to two parallel liquid resistances, the hydraulic pump output flow rate qp is constant, and the flow rate q1 flowing through the throttle valve into the hydraulic cylinder and flowing through The flow rate Δq of the relief valve is determined by the relative magnitude of the flow resistance of the throttle valve and the relief valve. If the liquid resistance of the throttle valve is greater than the liquid resistance of the relief valve, then q1 < Δq; otherwise, q1 > Δq.

3-63

Figure 3-63 The role of the throttling component

The throttle valve is an element that can adjust the flow rate over a wide range to change the liquid resistance. Therefore, by adjusting the liquid resistance of the throttle valve, the flow rate into the hydraulic cylinder can be changed to adjust the movement speed of the hydraulic cylinder; however, if there is only a throttle valve in the circuit and there is no overflow valve connected in parallel, as shown in FIG. As shown in Fig. 63(b), the throttle valve does not function to regulate the flow rate. The hydraulic oil output from the hydraulic pump is all entered into the hydraulic cylinder through the throttle valve. Changing the throttle orifice size simply changes the pressure drop across the throttle. The orifice is small, the flow rate is fast; the orifice is large, the flow rate is slow, and the total flow rate is constant, so the movement speed of the hydraulic cylinder does not change. Therefore, it is conditional that the throttling element is used to regulate the flow rate, that is, a link for receiving the pressure signal of the throttle element (a relief valve or a constant pressure variable pump connected in parallel) is required. Through this link, the flow change of the throttling element is compensated.


4,What are the requirements for hydraulic control systems for flow control valves?

The main requirements of the hydraulic drive system for flow control valves are:

1) Large flow adjustment range, and the flow adjustment should be uniform.

2) When the pressure difference between the front and the rear of the valve changes, the flow change through the valve is small to ensure the stability of the load movement.

3) The change in oil temperature has little effect on the flow through the valve.

4) The pressure loss when the liquid flow passes through the fully open valve is small.

5) When the valve port is closed, the leakage of the valve is small.


5,How does the normal throttle work?

3-64

Figure 3-64 shows the structure and graphical symbols of a common throttle valve. The throttle passage of such a throttle valve is axially triangular. The pressure oil flows from the inlet port P1 into the orifice a and the triangular groove at the left end of the spool 1 into the tunnel b, and then flows out from the outlet port P2. The adjustment handle 3 can be moved axially by the push rod 2 to change the flow cross-sectional area of the throttle to adjust the flow rate. The spool is always attached to the push rod under the action of the spring, and the inlet and outlet ports of the throttle valve are interchangeable.