Crawling - The actuating components of hydraulic equipment often need to move (hydraulic cylinders) or rotate (hydraulic motors) at very low speeds (such as a few millimeters or even less than 1mm per minute). At this point, there is often a noticeable unevenness in speed, with intermittent movements and stops, one fast and one slow, and one jump and one stop. This phenomenon is called crawling, which refers to the problem of low-speed stability.
Crawling poses great harm. For example, for hydraulic equipment such as machine tools, it can damage the surface quality (roughness) and machining accuracy of the workpiece, reduce the service life of the machine tool and cutting tools, and even produce waste products and accidents, which must be eliminated.
The reason for crawling failure is that: ① When the friction surface is in a boundary friction state, there is a change in the dynamic and static friction coefficients (difference in dynamic and static friction coefficients) and a phenomenon where the dynamic friction coefficient decreases with an increase in speed. ② The stiffness of the transmission system is insufficient (such as air mixed in the oil). ③ The movement speed is too low, while the mass of the moving parts is relatively large. The minimum speed at which crawling does not occur is called the critical speed for motion stability.
The ways to eliminate crawling phenomenon include: ① reducing the difference between dynamic and static friction coefficients, such as using static pressure guide rails and unloading guide rails, using anti friction materials for guide rails, replacing sliding friction with rolling friction, and lubricating guide rails with guide oil. ② Improve the stiffness K of transmission mechanisms (hydraulic, mechanical), such as increasing the stiffness of piston rods and hydraulic cylinder seats to prevent air from entering the hydraulic system and reduce stiffness changes caused by oil compressibility. ③ Take measures to reduce its critical speed and decrease the mass of moving parts.
Specific reasons for crawling:
The fault phenomena of crawling are different: there are both regular crawling and irregular crawling; Some crawls irregularly and with large amplitudes; Some crawls only occur at extremely low speeds. The reasons for these different crawling phenomena have different focuses, some are mainly mechanical reasons, some are mainly hydraulic reasons, some are mainly due to air entering the oil, and some are mainly due to poor lubrication. The maintenance and operation personnel of hydraulic equipment must constantly summarize and generalize, quickly identify the causes of crawling, and eliminate them; The specific reasons for crawling are summarized as follows.
1. The difference in static and dynamic friction coefficients is significant: ① Poor precision of the guide rail. ② There are rust spots on the guide rail surface. ③ The guide rail pressure plate strip is adjusted too tightly. ④ The guide rail scraping is not good, the points are not enough, and the points are uneven. ⑤ The oil groove on the guide rail is not good, the depth is too shallow, it has been worn off during operation, and the oil groove 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 Localized corrosion (crawling in some sections) and strains in the cylinder body holes of the hydraulic cylinder. ⑨ Hydraulic cylinder body holes, piston rods, and piston accuracy are poor. ⑩ The assembly and installation accuracy of hydraulic cylinders is poor, and the coaxiality of pistons, piston rods, cylinder body holes, and cylinder head holes is poor.
⑴ Hydraulic cylinder piston or cylinder head seal is too tight, blocked or too loose.
⑵ Prolonged shutdown time and moisture in the oil (especially in the grinding machine coolant) have caused some parts to rust.
The static pressure guide rail throttle is blocked, and the guide rail is cut off from oil.
2. When air enters the hydraulic system, the volume modulus decreases
Hydraulic pump sucks in air: ① The oil level in the tank is lower than the specified oil level, and the oil suction filter or suction pipe is exposed on the oil level. ② The oil return pipe in the oil tank is too close to the oil suction pipe, and no partition board is installed between them (or no bubble breaking net is installed), so the foam generated by the oil return mixing is sucked into the pump before it can float up. ③ The pipe joint exposed between the oil surface and the oil pump inlet is poorly sealed, or the pipe joint becomes loose due to vibration, or the oil pipe cracks and sucks in air. ④ Air intake is caused by damage to the pump shaft oil seal and the seal between the pump body and pump cover. ⑤ The oil suction pipe is too thin and long, causing the oil suction filter to be blocked by dirt or the capacity of the filter to be selected too small during design, resulting in increased oil suction resistance. ⑥ The oil deteriorates and deteriorates due to water emulsification, resulting in decreased bubble breaking performance. Bubbles are dispersed inside the oil layer or float on the oil surface in a mesh like pattern, and are sucked into the system during pump operation.
Air backflow from the return pipe: ① The return pipe may be exposed above the oil surface for a long time during operation. ② On the return oil line without a back pressure valve, and sometimes there is negative pressure inside the cylinder. ③ The sealing of the cylinder head is poor, sometimes causing intake and sometimes oil leakage.
3. Reasons related to hydraulic components and hydraulic systems: ① Unstable pressure in pressure valves, intermittent blockage and opening of damping holes, large pressure oscillation, or low working pressure adjustment. ② The flow rate of the throttle valve is unstable and is used beyond the minimum stable flow rate of the valve. ③ The output flow pulsation of the hydraulic pump is large, and the oil supply is uneven. ④ The piston rod of the hydraulic cylinder is connected to the non ball joint of the worktable, especially when the long hydraulic cylinder crawls due to force. ⑤ The large leakage inside and outside the hydraulic cylinder causes pressure pulsation changes inside the cylinder. ⑥ The malfunction of the lubricating oil stabilizer leads to unstable lubrication of the guide rail and occasional interruption of flow. ⑦ The lubrication pressure is too low and the workbench is too heavy. ⑧ Resonance occurs in the pipeline. ⑨ The hydraulic system adopts imported throttling method and does not have back pressure or back pressure adjustment mechanism, or although there is a back pressure adjustment mechanism, the back pressure adjustment is too low, which is most likely to cause crawling in a certain low-speed zone.
4. Reasons for hydraulic oil: incorrect selection of oil grade, too thin or too thick. The oil temperature has a significant impact on viscosity.
5. Other reasons: poor rigidity of oil cylinder piston rod and oil cylinder support; Reasons for sealing; Poor motor dynamic balance, uneven motor speed, and unstable current.
Methods to eliminate crawling:
Based on the reasons for crawling mentioned above, elimination methods can be taken one by one, with the main measures being:
① Strictly control geometric deviations, dimensional tolerances, and fit clearances when manufacturing and repairing parts.
② Scrape the guide rail, remove rust and burrs, ensure that the contact area between the two guide rail surfaces is ≥ 75%, adjust the insert strip, and ensure smooth lubrication of the oil tank.
③ Based on the flat guide rail surface, scrape the installation surface of the oil cylinder to ensure that the parallelism is less than 0.1mm along the entire length; adjust the side busbar of the oil cylinder piston rod based on the V-shaped guide rail to ensure that the parallelism between the two is within 0.1mm. The piston rod and worktable are connected by a ball pair.
④ The coaxiality requirement between the oil cylinder piston and the water pipe is ≤ 0.04/1000. All seals should be installed in the sealing groove, and there should be no unequal compression on all sides. If necessary, the outer circle can be used as a reference to grind the bottom diameter of the sealing groove. When sealing assembly, it is not allowed to be too tight or too loose.
⑤ To prevent air from being sucked into the system through the pump and backflow into the system through the return pipe, measures should be taken one by one based on the reasons for air intake mentioned above.
⑥ Eliminate faults related to hydraulic components and hydraulic systems. For example, the system can be replaced with a return throttle system or an inlet throttle system that can adjust the back pressure automatically.
⑦ Use appropriate lubricating oil for the guide rail, and if necessary, use guide rail oil because it contains polar additives that increase its oiliness, allowing oil molecules to tightly adhere to the guide rail surface. After the movement stops, the oil film will not be squeezed out, ensuring fluid lubrication and minimizing the difference in dynamic and static friction coefficients.
⑧ Enhance the stiffness of various mechanical transmission components; Exclude crawling caused by sealing issues.
⑨ Add dimethyl silicone oil anti foaming agent to the oil to break the foam.
⑩ Pay attention to the cleanliness of turbulence and hydraulic systems.
2026 August 4th Week VAFEM Product Recommendation
VAFEM Hub units are designed to allow rear wheels, especially in passenger cars, to turn on an axle even under heavy loads.
Features:
magnetic, electric, and mechanic ABS
with and without ABS
flange only
Steel Material: GCr15/52100/100Cr6
Steel Type: Forged
Ball Grade: G10
Grease: Shell
2026-08-28