The actuators of hydraulic equipment often need to move (hydraulic cylinder) or rotate (hydraulic motor) at a very low speed (for example, a few millimeters or even less than 1mm per minute). At this time, there will often be obvious uneven speeds, intermittent movement and stopping, fast and slow, jumping and stopping. This phenomenon is called creeping, that is, the problem of low-speed stability. Creeping is very harmful. For example, for machine tool hydraulic equipment, it will damage the surface quality (roughness) and processing accuracy of the workpiece, reduce the service life of machine tools and cutting tools, and even produce waste and accidents, which must be eliminated.
The reasons for the creeping failure are:
① When the friction surface is in the boundary friction state, there are changes in the dynamic and static friction coefficients (the difference between the dynamic and static friction coefficients) and the dynamic friction coefficient decreases with the increase in speed.
② The rigidity of the transmission system is insufficient (such as air mixed in the oil).
③ The movement speed is too low, and the mass of the moving parts is large. The lowest speed without the creeping phenomenon is called the critical speed of movement stability.
The ways to eliminate the creeping phenomenon are:
① Reduce the difference between the dynamic and static friction coefficients: such as using hydrostatic guides and unloading guides, using friction-reducing materials for guides, using rolling friction instead of sliding friction, and using guide oil to lubricate guides.
② Increase the rigidity K of the transmission mechanism (hydraulic, mechanical): such as increasing the rigidity of the piston rod and the hydraulic cylinder seat, preventing air from entering the hydraulic system to reduce the rigidity changes caused by the compressibility of the oil, etc.
③ Take measures to reduce its critical speed and reduce the mass of moving parts.
Specific reasons for creeping.
Even if it is creeping, its fault phenomena are different: there are regular creeping and irregular creeping; some creeping is irregular and has large amplitude; some creeping occurs only at extremely low speeds. The reasons for the creeping of these different phenomena have different focuses. Some are mainly mechanical reasons, some are mainly hydraulic reasons, some are mainly caused by air entering the oil, and some are mainly caused by poor lubrication. The maintenance and operation personnel of hydraulic equipment must constantly summarize and summarize, quickly find out the reasons for the creeping, and eliminate them; the specific reasons for the creeping are summarized as follows.
1. The difference between static and dynamic friction factors is large:
① The guide rail has poor accuracy.
② There are rust spots on the guide rail surface.
③ The guide rail pressure plate inlay is adjusted too tight.
④ The guide rail is not scraped well, the number of points is not enough, and the points are uneven.
⑤ The oil groove opened on the guide rail is not good, the depth is too shallow, it has been worn away during operation, and the oil groove opened is uneven.
⑥ New hydraulic equipment, the guide rail has not been run-in.
⑦ The axis of the hydraulic cylinder is not parallel to the guide rail.
⑧ Local rust (local segment creep) and strain in the hydraulic cylinder body hole.
⑨ Poor precision of the hydraulic cylinder body hole, piston rod and piston.
⑩ Poor assembly and installation accuracy of the hydraulic cylinder, poor coaxiality of the piston, piston rod, cylinder body hole and cylinder head hole.
⑴ The hydraulic cylinder piston or cylinder head seal is too tight, blocked or too loose.
⑵ The downtime is too long, and the moisture in the oil (especially the grinder coolant) causes rust in some parts.
⑶ The hydrostatic guide rail throttle is blocked and the guide rail is cut off from oil.
2. Air enters the hydraulic system and the volume modulus decreases.
⑪ The hydraulic pump inhales air: ① The oil level in the oil tank is lower than the specified value of the oil mark, and the oil suction filter or the oil suction pipe is exposed on the oil surface. ② The return oil pipe and the oil suction pipe in the oil tank are too close, and there is no partition between the two (or no bubble breaking net is installed). The foam generated by the return oil stirring has no time to float up and is sucked into the pump. ③ The pipe joint exposed between the oil surface and the oil inlet of the oil pump is not well sealed or the pipe joint is loose due to vibration, or the oil pipe is cracked, sucking in air. ④ Air is taken in due to the damage of the pump shaft oil seal and the seal between the pump body and the pump cover. ⑤ The oil suction pipe is too thin and too long, the oil suction filter is blocked by dirt, or the capacity of the oil filter is originally too small during design, resulting in increased oil suction resistance. ⑥ The oil deteriorates and deteriorates. Due to water emulsification, the bubble breaking performance deteriorates, the bubbles are dispersed inside the oil layer or float on the oil surface in the form of mesh bubbles, and the pump is sucked into the system when it is working.
⑫ Air backflows from the oil return pipe: ① The oil return pipe is exposed above the oil surface for a long time during operation. ② In the oil return line without a back pressure valve, the cylinder is sometimes under negative pressure. ③ The cylinder head is not well sealed, sometimes air is taken in, sometimes oil is leaking.
3. Reasons related to hydraulic components and hydraulic systems: The pressure of the pressure valve is unstable, the damping hole is sometimes blocked and sometimes unblocked, the pressure swing is large, or the adjusted working pressure is too low. ‚ The flow of the throttle valve is unstable, and it is used at a flow rate exceeding the minimum stable flow of the valve. ƒ The output flow of the hydraulic pump pulsates greatly and the oil supply is uneven. ④ The piston rod of the hydraulic cylinder is connected to the workbench without a ball pair, especially the long hydraulic cylinder creeps due to the force. ⑤ The hydraulic cylinder has large internal and external leakage, causing the pressure in the cylinder to pulsate. ⑥ The lubricating oil stabilizer fails, resulting in unstable guide rail lubrication and occasional interruption. ⑦ The lubrication pressure is too low and the workbench is too heavy. ⑧ The pipeline resonates. ⑨The hydraulic system adopts the inlet throttling mode and has no back pressure or back pressure adjustment mechanism, or although there is a back pressure adjustment mechanism, the back pressure is adjusted too low, which is most likely to cause creeping in a certain low-speed area.
4. Reasons for hydraulic oil: The oil brand is not selected correctly, it is too thin or too thick. ‚The viscosity changes greatly due to the influence of oil temperature.
5. Other reasons:
poor rigidity of the cylinder piston rod and cylinder support;
Sealing reasons;
Poor dynamic balance of the motor, uneven motor speed and unstable current, etc.