Created on:2021-10-28 13:10

Working principle of hydraulic transmission

Working principle of hydraulic transmission

1. Transmission principle of hydraulic jack

Hydraulic jack is a common tool in the machinery industry. This small tool is often used to lift heavy objects. The following takes it as an example to briefly describe the working principle of hydraulic transmission. Figure a shows the working principle of the hydraulic jack. There are two hydraulic cylinders 1 and 6, which are respectively equipped with pistons. The piston and the cylinder block maintain a good matching relationship. Not only can the piston slide in the cylinder, but also reliable sealing can be realized between the mating surfaces. When the lever is lifted upward, the piston of hydraulic cylinder 1 moves upward, the volume of the lower chamber of hydraulic cylinder 1 increases to form a local vacuum, 2 is closed, and the oil in oil tank 4 enters the lower chamber of hydraulic cylinder 1 through the oil suction pipe jacking check valve 3 under the action of atmospheric pressure to complete an oil suction action. When the lever is pressed down, the piston of hydraulic cylinder l moves down, the volume of the lower chamber of hydraulic cylinder 1 decreases, the oil is squeezed and the pressure increases, close the check valve 3, the pressure oil in the lower chamber of hydraulic cylinder 1 pushes open the check valve 2, and the oil enters the lower chamber of hydraulic cylinder 6 through the oil drain pipe to push the large piston up to push the lifting object. In this way, the weight can be raised continuously by constantly pulling the lever up and down to achieve the purpose of lifting. If the lever stops acting, the oil pressure in the lower chamber of hydraulic cylinder 6 will close the check valve 2, and the piston of hydraulic cylinder 6 together with the weight will be self-locking and stop at the lifting position. If stop valve 5 is opened, the lower chamber of hydraulic cylinder 6 is connected to the oil tank, and the piston of hydraulic cylinder 6 will move downward under the action of self weight and quickly return to the original position. If the areas of hydraulic cylinders 1 and 6 are A1 and A2 respectively, the pressure P1 on the unit area of hydraulic cylinder 1 = f / A1 and the pressure P2 on the unit area of hydraulic cylinder 6 = w / A2. According to Pascal's law of fluid mechanics, "the pressure value at a certain point in the equilibrium liquid can be transmitted equivalently to all points in the closed liquid"

p1 = p2 = F/A1 = W/A2 (1-1)

According to the working principle of the hydraulic jack, the hydraulic cylinder 1 and the check valves 2 and 3 complete the oil suction and drainage together, and convert the mechanical energy of the lever into the pressure energy output of the oil. Hydraulic cylinder 6 converts the pressure energy of oil into mechanical energy output and lifts heavy objects. With the load force, the liquid pressure is generated. Therefore, in terms of both load and liquid pressure, load is primary and pressure is secondary. Hydraulic transmission device is essentially an energy conversion device. Here, hydraulic cylinder 6 and hydraulic cylinder 1 constitute the simplest hydraulic transmission system to realize the transmission of force and motion.

From the working process of hydraulic jack, the working principle of hydraulic transmission can be summarized as follows:

(1) Hydraulic transmission uses liquid (hydraulic oil) as the working medium to transmit motion and power.

(2) After two energy conversion, hydraulic transmission converts mechanical energy into liquid pressure energy for easy transportation, and then converts liquid pressure energy into mechanical energy for external work.

(3) Hydraulic transmission relies on the change of sealing volume (or sealing system) to transfer energy.

The working principle of hydraulic system transmission is applied to cranes, bulldozers, truck cranes, injection molding machines of construction machinery, sliding platforms of modular machine tools in machine tool industry, clamping of workpieces of CNC lathes, loosening and broaching of spindle of machining center, etc.

2. Hydraulic transmission system of machine tool workbench

Figure B shows the structural principle of the hydraulic system of the machine tool workbench. It is composed of oil tank 1, oil filter 2, hydraulic pump 3, overflow valves 4, 5 and 6, handle 7, hydraulic cylinder 8, workbench 9, oil pipes and joints connecting these components. The hydraulic cylinder 8 is fixed on the bed, and the piston and piston rod drive the worktable 9 to move back and forth. The hydraulic pump is driven by a motor (not shown in the figure), sucks oil from oil tank 1 through oil filter 2 and sends it to a closed system.

BC.jpg

If the change-over valve handle 7 is pushed to the right to make the valve core in the position shown in Figure B (b), the pressure oil from the pump output → throttle valve 5 → change-over valve 6 → left chamber of hydraulic cylinder 8 will push the piston and workbench 9 to the right. At this time, the oil in the right chamber of hydraulic cylinder 8 → directional valve 6 → oil tank 1.

If the change-over valve handle 7 is pushed to the left to make the valve core in the position shown in Figure B (c), the pressure oil from the pump output → throttle valve 5 → change-over valve 6 → right chamber of hydraulic cylinder 8 will push the piston and workbench 9 to the left. At this time, the oil in the left chamber of hydraulic cylinder 8 → directional valve 6 → oil tank 1.

If the valve core of directional valve 6 is in the middle position as shown in Figure B (a), the pressure oil output by the pump → throttle valve 5 → directional valve 6 is closed. At this time, the pressure oil output by the pump → overflow valve 4 → oil tank. Since the two chambers of the hydraulic cylinder are closed by the reversing valve 6, the piston stops, and the workbench 9 stops moving.

The speed at which the table moves can be adjusted by the opening size of throttle valve 5. When the valve port of throttle valve 5 increases, the amount of oil entering the hydraulic cylinder increases and the moving speed of the worktable increases; On the contrary, when the valve port of small flow valve 5 is closed, the amount of oil entering the hydraulic cylinder decreases and the moving speed of the worktable decreases.

Turn the adjusting screw of overflow valve 4 to adjust the preload of the spring. The greater the preload of the spring, the higher the oil pressure in the sealing system, and the greater the maximum load that can be overcome when the workbench moves; The smaller the preload of the spring, the smaller the maximum working pressure it can get and the smaller the maximum load it can overcome. In addition, in general, the oil delivered m by the hydraulic pump is more than the oil required by the hydraulic cylinder, and the excess oil must be discharged back to the oil tank in time through the overflow valve 4. Therefore, the overflow valve 4 plays the role of pressure regulation and overflow in the hydraulic system.

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