"Crawling" is an abnormal motion state that often occurs in hydraulic transmission. Slight "crawling" causes weak vibration of moving parts, and significant "crawling" causes large-distance runout of moving parts. The harm of "crawling" phenomenon is great. Eliminating "crawling" phenomenon is very important to improve the stability of hydraulic system and improve the machining accuracy of machine tool.
(1) "Creeping" caused by poor driving rigidity“
After the air enters the oil, part of it dissolves in the pressure oil (hydraulic oil), and part of it forms bubbles and floats in the pressure oil. Because the air has compressibility, the hydraulic oil has obvious elasticity, resulting in poor driving rigidity, and the reasons for "crawling air mixing into the hydraulic system" are as follows:
① Between reciprocating parts, a certain fit clearance is required, and air is easy to mix in from these clearances.
② The hydraulic pipe joint is loose or the seal is not tight, so air enters the system.
③ The accuracy of hydraulic components is poor and the performance of seals is poor, resulting in leakage.
④ The oil suction pipe is improperly set to suck in air, or local vacuum is formed due to blockage by dirt.
⑤ Insufficient oil in the oil tank or insufficient insertion depth of the oil suction pipe causes air suction.
⑥ The local pressure in the hydraulic system of Shengang excavator is lower than the separation pressure of air, which separates the air dissolved in oil.
⑦ The system design is unreasonable. When the machine stops working, the left and right cavities of the hydraulic cylinder communicate with each other and pass through the oil return circuit. The oil flows back to the oil tank under the action of in place energy, forming a local vacuum in the system, and air enters the system from various channels.
For the above reasons, the following measures are taken:
① When manufacturing and repairing parts, the tolerance requirements shall be strictly met, and the fit clearance shall be ensured during assembly.
② Fasten all pipe connections to prevent leakage.
③ The connecting screws at each joint surface shall be tightened evenly, and the sealing gasket shall be uniform. Multi layer paper gasket is not allowed.
④ The oil inlet and outlet pipes in the oil tank shall be kept at a certain distance, and diaphragms can also be added to separate them.
⑤ The oil filter with sufficient capacity shall be used to remove the dirt attached to the oil filter.
⑥ The oil tank shall have enough oil to make it not lower than the indication line of the oil standard.
⑦ In order to ensure that all parts of the system can often be filled with oil, a check valve shall be installed at the pump outlet, and a back pressure valve shall be set on the oil return road.
⑧ Improve the hydraulic system, try to prevent local vacuum in the system, and set necessary vent plugs or vent valves.
(2) "Crawling" caused by large clearance of hydraulic components
① "Crawling" caused by the low-speed movement of moving parts. When the moving parts move at low speed, once dry friction occurs, the resistance increases. At this time, the hydraulic pump is required to increase the pressure, but due to the large clearance of the hydraulic pump and serious oil leakage, it can not adapt to the pressure change of the actuator due to the change of resistance "Crawling" at this time, repair or replace the parts in the hydraulic pump to ensure the clearance required for assembly, so as to reduce the leakage of the hydraulic pump.
② "Crawling" caused by the failure of the regulating valve. The damping hole and orifice of various regulating valves are blocked by dirt, and the valve core moves inflexibly, resulting in large pressure fluctuation and "crawling" caused by large thrust or small flow. The oil shall be kept clean frequently, cleaned and replaced regularly, and the maintenance of components shall be strengthened to prevent hydraulic oil pollution.
③ "Crawling" caused by component wear due to the wear of valve parts, the fit clearance increases, and some high-pressure oil and low-pressure oil communicate, resulting in insufficient pressure. In addition, the fit clearance between the piston of the hydraulic cylinder and the inner hole of the cylinder increases due to wear, resulting in internal leakage, which reduces the pressure difference between the two chambers of the hydraulic cylinder and the thrust, resulting in "crawling" due to the change of friction at low speed The specific measures are to carefully check the fit clearance, grind or rework the components to ensure the fit clearance, and replace the damaged seals.
(3) "Crawling" caused by the change of friction resistance“
This phenomenon often occurs in the hydraulic cylinder, mainly because: the center line of the hydraulic cylinder is not parallel to the piston rod, the piston rod is bent locally or in full length, the inner circle of the cylinder barrel is roughened and scratched, the piston and piston rod have different shafts, the cylinder barrel accuracy can not meet the technical requirements, the oil seals at both ends of the piston rod are adjusted too tightly, etc. the measures taken are to check the accuracy and damage of the hydraulic cylinder item by item and repair it. The installation accuracy of hydraulic cylinder shall meet the technical requirements.
