At present, the hydraulic clamping system is widely used in the machine tool industry. It is recognized by users for its large clamping force, convenient operation, and easy automation with electrical appliances. However, in order to maintain the constant pressure of the system, the general tightening system keeps the pump and the motor running for a long time, which not only wastes precious energy, increases the heat of the system, but also greatly reduces the life of the hydraulic components. As a hydraulic accessory, accumulators are more and more widely used in machine tool hydraulic systems with the improvement of manufacturing technology. For this reason, in the hydraulic design of the machine tool, the accumulator used in the hydraulic clamping system has received good results.
(1) The accumulator is used in the working process of the hydraulic clamping system
As shown in Figure 6-1, when clamping work is required, segment 1 starts, hydraulic pump 2 starts to work, part of the pressure oil enters the clamping cylinder 10, and the other part enters the accumulator 8 for storage. As the pressure increases, when the upper limit of the system use pressure is reached, the pressure relay 6 sends a signal to stop the motor and the hydraulic pump through electrical control. At this time, the pressure of the system is maintained by the accumulator. As the system leaks, when the pressure drops to the lower limit of the pressure relay, the pressure relay sends out a signal to start the motor and pump to supplement the oil pressure. The time from the stop of the hydraulic pump to the restart depends on the number of clamping cylinders in the system and the leakage of the system. According to experience, if the leakage control of the system is better, the interval can reach 2 to 3 hours. This greatly reduces the running time of the motor and the pump, reduces the heat of the system, and increases the life of each component.
(2) Selection of types and parameters of accumulators
Accumulators can be divided into three categories: gravity-loaded, spring-loaded and gas-loaded. Gravity accumulators use the potential energy of the heavy hammer to store energy. The pressure generated by this accumulator depends on the weight of the hammer and the size of the plunger area. Spring-loaded accumulators use spring compression energy to store energy. The production of this kind of accumulator depends on the stiffness and compression of the spring. The working principle of the gas-loaded accumulator is based on Boyle’s law.
When in use, the accumulator is first filled with air or nitrogen at a predetermined pressure, and then under the action of the hydraulic pump pressure, the oil enters the accumulator through the oil hole, compresses the air bag, and the pressure of the air cavity and the oil cavity are always equal. So that the airbag is in a floating equilibrium state. When the system needs oil, the oil is discharged under the action of gas pressure. When the system discharges excess high-pressure liquid, it is also stored in the accumulator by compressing the gas in it.
The total volume of the accumulator is the inflation volume. It is very important to correctly select the total volume of the accumulator. The capacity is too small to meet the working requirements of the system. If the capacity is too large, the weight and size of the system will increase.
(3) The benefits of adding an accumulator to the hydraulic clamping system
Adding the accumulator will bring the following benefits to the system;
① It can partially absorb the pulsation of the hydraulic pump and reduce noise;
②Reduce the hydraulic shock caused by the end of the cylinder or the direction valve reversing;
③It can compensate the hydraulic pressure fluctuation caused when the oil temperature changes;
④ When the pump or motor fails, the clamping work can still be completed for a long time.