What is the working principle and system composition of hydraulic transmission?

1 working principle of hydraulic transmission

The hydraulic transmission system of machine tool workbench is shown in Figure 4- 17, which consists of oil tank, oil filter, hydraulic pump, overflow valve, on-off valve, throttle valve, reversing valve, hydraulic cylinder, oil pipe and joints connecting these elements. Its working principle is as follows: after the hydraulic pump is driven by the motor, it sucks oil from the oil tank; The oil enters the hydraulic pump through the oil filter, and the oil in the pump cavity is from the low pressure at the inlet to the high pressure at the pump outlet. In the state shown in fig. 4- 17(a), it enters the left cavity of the hydraulic cylinder through the on-off valve, throttle valve and reversing valve, and pushes the piston to move the workbench to the right; At this time, the oil in the right cavity of the hydraulic cylinder is discharged back to the oil tank through the reversing valve and the oil return pipe 6.

Figure 4- 17 Hydraulic transmission system of machine tool workbench

1-workbench; 2- Hydraulic cylinder; 3 piston; 4- reversing handle; 5- reversing valve; 6, 8, 16- return pipe; 7— Throttle valve; 9— Start and stop handles; 10- open and close the valve; 1 1- pressure pipeline; 12- pressure branch pipe; 13- relief valve; 14-steel ball; 15-spring; 17- hydraulic pump; 18- oil filter; 19- fuel tank

If the handle of the reversing valve is changed to the state shown in Figure 4- 17(b), the oil in the pressure pipe will enter the right chamber of the hydraulic cylinder through the on-off valve, throttle valve and reversing valve, and push the piston to move the workbench to the left, and the oil in the left chamber of the hydraulic cylinder will be discharged back to the oil tank through the reversing valve and oil return pipe 6.

The moving speed of the workbench is regulated by the throttle valve. When the throttle valve is opened, the amount of oil entering the hydraulic cylinder increases (per unit time) and the moving speed of the workbench increases; On the contrary, when the throttle valve is closed, the amount of oil entering the hydraulic cylinder per unit time decreases and the moving speed of the workbench decreases. In order to overcome all kinds of resistance when moving the workbench, the hydraulic cylinder must generate enough thrust, which is generated by the oil pressure in the hydraulic cylinder. The greater the resistance to be overcome, the higher the oil pressure in the corresponding hydraulic cylinder; On the contrary, if the resistance is small, the pressure will be low. This phenomenon illustrates a basic principle of hydraulic transmission-pressure depends on load.

It should be noted that hydraulic transmission works by using the pressure energy of liquid, which is essentially different from hydraulic transmission that works by using the kinetic energy or potential energy of liquid in unsealed state.

The function of the safety valve is to adjust and stabilize the maximum working pressure of the system and overflow excess oil. When the workbench is working and feeding, the piston of the hydraulic cylinder (workbench) needs to overcome large load and slow motion. The pressure oil entering the hydraulic cylinder must have enough stable pressure to push the piston to drive the workbench to move. Adjust the spring force of the safety valve to balance it with the maximum load force of the hydraulic cylinder. When the system pressure rises to slightly greater than the spring force of the relief valve, the relief valve will open, and part of the oil output by the quantitative pump will overflow back to the oil tank through the oil return pipe 16. At this time, the system pressure no longer rises, and the workbench keeps stable low-speed movement (working feed). When the workbench quickly retreats, the overflow valve can't be opened because of the small load and low oil pressure, and all the flow of the pump enters the hydraulic cylinder, so the workbench moves quickly.

From the above example, we can see that the hydraulic pump converts the mechanical energy of the motor (or other prime movers) into the pressure energy of the liquid, and then the hydraulic cylinder (or hydraulic motor) converts the pressure energy of the liquid into mechanical energy to drive the load to move. The process of hydraulic transmission is the energy conversion process of mechanical energy, hydraulic energy and mechanical energy.

2 composition of hydraulic transmission system

As can be seen from the above example, the basic components of the hydraulic transmission system are:

(1) energy device-hydraulic pump. It converts the mechanical energy output by the power part (motor or other prime mover) into hydraulic energy and provides pressure oil for the system.

(2) Executive device-hydraulic press (hydraulic cylinder, hydraulic motor). Through it, hydraulic energy is converted into mechanical energy to push the load to do work.

(3) Control device-hydraulic valve (divided into flow, pressure and direction control valves). Through their control or adjustment, the pressure, flow rate and direction of liquid flow can be changed, thus changing the force (or torque), speed and direction of the actuator.

(4) Auxiliary devices-oil tank, pipeline, accumulator, oil filter, pipe joint, pressure gauge switch, etc. Through these components, the system is connected to realize various work cycles.

(5) Working medium-hydraulic oil. Most hydraulic oils use mineral oil, and systems use mineral oil to transmit energy or information.