Working principle of double acting vane pump and some problems needing attention
(2) Working principle of double acting vane pump and some problems needing attention
① Working principle diagram e shows the working principle of double acting vane pump. The inner surface of stator 1 looks like an ellipse, which is composed of two large arcs with radius r, two small arcs with radius R and four transition curves connecting large and small arcs. Rotor 2 and stator 1 are concentric. The rotor is provided with evenly distributed radial sliding grooves, and the rectangular blade 3 is installed in the sliding groove of the rotor and can be flexibly retracted. The rotor, blade and stator are sandwiched between the front and rear valve plates. The rectangular blade divides the space between the two port plates and the rotor and stator into a sealed working chamber with the same number of blades along the circumference. Because the radial distance between the rotor and stator changes along the circumference in the transition region, these seal cavities will expand and shrink periodically during the rotor rotation. The four port windows on the oil distribution plate are respectively connected with the oil suction and pressure windows. When the rotor rotates clockwise as shown in Fig. e, the blade is close to the inner surface of the stator under the action of centrifugal force and pressure oil passing through the bottom of the blade slot. When the blade moves from the small arc area to the large arc area on the inner surface of the stator, the volume of the seal working cavity increases gradually, and the oil is absorbed through the oil suction windows in the upper left corner and the lower right corner of the valve plate; when the blade moves from the large arc area to the small arc area, the volume of the seal working cavity decreases gradually, and the oil is pressed through the oil pressure windows in the upper left corner and the upper right corner of the valve plate. A section of oil sealing area between the oil suction area and the oil pressure area separates the oil suction area and the oil pressure area. Every revolution of the rotor, each blade slides back and forth in the groove twice, and each sealing chamber completes the action of oil suction and pressure twice, so it is called double acting vane pump. Based on the same principle, vane pumps with three or more action times can be made, but they are rare.
② Some problems should be paid attention to
a. Different from single acting vane pump, double acting vane pump can only be used as quantitative pump because the rotor center and stator center of double acting vane pump are concentric.
b. Because the two oil suction and pressure zones of the double acting vane pump are radially symmetrical, the radial hydraulic pressure acting on the rotor, transmission shaft and bearing is balanced, so the double acting vane pump is also called unloading pump. Therefore, it is conducive to improve the working pressure of the pump, and has a long service life.
c. The stator inner surface curve and its angle distribution of double acting vane pump have great influence on the flow uniformity, suction performance and service life of the pump. It is known from the above that the inner surface of the stator is composed of two large arcs with radius r, two small arcs with radius R and four transitional curves connecting the large and small arcs.
In order to ensure smooth operation and uniform output flow, the ideal transition curve has the following characteristics: the blade should not be "void" (separated from the inner surface of the stator) when sliding in the slot, so as to avoid impact, noise and reduce the service life of the pump; when the pump rotates to the junction of the transition curve and the arc or sliding along the transition curve, the radial speed and acceleration of the blade expansion change In order to reduce the impact, noise and wear, it is necessary to make the structure uniform without sudden change and hard impact. The transition curves include modified Archimedes spiral, sine acceleration curve, equal acceleration equal deceleration curve and higher order curve. In general, double acting vane pump uses equal acceleration and equal deceleration curve with better comprehensive performance as the transition curve, while some high-performance pumps use high-order curve as the transition curve.
Generally, the radius of small arc is r = R0 + (0.5-1) mm (R0 is the radius of rotor). The displacement of the pump can be increased by increasing the difference (R-R) between the large arc radius and the small arc radius, but it is restricted by the strength of the blade and rotor and the condition that the blade does not fall off. The analysis and calculation show that when the radial motion of the blade changes according to the law of equal acceleration and equal deceleration, it is allowed to choose a larger R / R value, so a larger (R-R) value can be obtained, so the inner surface transition curve of the stator mostly adopts the curve of equal acceleration and equal deceleration.
In order to ensure the sealing between the oil suction chamber and the oil pressure chamber and avoid oil trapping, the angle distribution of the inner surface curve of the stator should meet certain conditions. Figure f shows the relative position relationship between the valve plate and the stator curve. It can be seen that if the number of blades of the pump is Z, in order to ensure the sealing between the oil suction chamber and the oil pressure chamber, the sealing angles α 1 and α 2 between the oil suction and discharge chambers of large arc section and small arc section should meet the conditions of α 1 ≥ 2 π / Z and α 2 ≥ 2 π / Z. In order to avoid the phenomenon of trapped oil, the volume of the cavity between the two oil sealing blades should be kept unchanged when it moves within the angle range of α 1 and α 2 (at this time, the cavity is not connected with the high and low pressure cavities), that is, to ensure that the Center angles β L and β 2 corresponding to the large arc section and small arc section meet the requirements of β L ≥ A1 and β 2 ≥ α 2.
d. Pressure shock and vibration reduction if the rotor of the pump rotates clockwise, when the working cavity between two adjacent blades enters the large arc area from the oil suction area, the pressure in the oil cavity is kept at the lowest. When the working chamber starts to connect with the oil drainage area, the high-pressure oil flows into the sealing chamber and compresses the oil in it, so the pressure rises sharply. This process will cause pressure shock and noise. The common way to solve this problem is to set up triangular damping groove (Fig. f), so that the high-pressure oil and low-pressure oil are gradually connected. When the high-pressure oil enters the sealing chamber, it is throttled and damped, so as to slow down the pressure shock phenomenon and play a role in vibration elimination.
e. Blade tilting forward When the blade slides along the stator curve in the pressure oil area, the normal contact force FN on the inner surface of the stator can be decomposed into the component force FP and the transverse component force ft along the direction of the blade groove. Because the extended part of the blade is a cantilever structure, the transverse component force will produce greater friction at the contact between the blade and the slot side wall The smaller the angle between the normal direction and the blade direction, the smaller the transverse component ft = fnsin α is, the more favorable for the blade to slide freely in its groove, and reduce the friction, so as to reduce the wear between the blade and the groove. Therefore, the blade groove is not set in radial direction, but in forward direction at an angle of θ (usually θ = 10-14 °), so that α < φ, that is, α = φ - θ [Fig. g (a)]. Otherwise, the pressure angle a = φ will be larger and FT will be larger. But this conclusion is not applicable to the oil suction area [Fig. g (b)]. On the one hand, in the oil suction area, the blade groove inclines forward, but makes the pressure angle α increase, and becomes α = φ + θ, which worsens the stress condition of the blade; on the other hand, when the blade slides along the stator curve, its top is actually affected by not only the reaction force of the inner surface of the stator, but also the friction force FF opposite to the sliding direction The resultant force F is the basis for calculating the harmful transverse component ft, so it is bound to come to the wrong conclusion that the smaller the pressure angle is, the better. The new point of view is that it is more reasonable to take θ = 0 degree. At present, the blade groove of some double acting vane pumps abroad is opened radially, so the problem of blade setting angle is still worthy of further discussion.