Causes and solutions for machine tool crawling failure

The moving parts of the machine tool feed system, when its running speed is low to a certain value (such as 0.5mm/min), it is not always a continuous uniform motion, but it is time to stop, flicker and slow, this phenomenon is called crawling. .

Crawling is a common and abnormal motion state of machine tools. It mainly occurs on the execution components of various drive systems of the machine tool (such as tool holder system, workbench, etc.), and generally occurs at low speeds. When the moving speed is low, the lubricating oil is compressed, the oil film is thinned, the oil wedge effect is lowered, part of the oil film is broken, and the friction surface resistance is changed. Usually, a slight degree of crawling has an imperceptible vibration, and a significant crawl is a large jump.

The creeping phenomenon in the feed motion destroys the uniformity of the motion of the system, which not only reduces the accuracy and surface quality of the workpiece, but also destroys the stability of the hydraulic system work, accelerates the wear of the machine guide rails, and even causes waste and accidents.

Cause of crawling

The main cause of creep is the variation of the friction factor with the speed of movement and the rigidity of the transmission system. In actual use of the machine tool, the creep phenomenon is mainly formed when the rigidity of the transmission system is insufficient, and the driving force and the load friction resistance fluctuate. The hydraulic system of the machine tool invades the air, the gap between the hydraulic components increases and the mechanical device itself may cause creeping failure.

Countermeasures

1. Method of crawling and eliminating air caused by air mixing into hydraulic system

The incorporation of air into the hydraulic system is the root cause of the creeping failure of the hydraulic system. When the air is mixed into the oil, part of it dissolves into the pressure oil, and the remaining bubbles form free bubbles floating in the pressure oil. Due to the great compressibility of the air, the fluctuation of the actuator load causes the oil pressure to pulsate, resulting in gas expansion. Or shrinkage causes a significant change in the amount of oil supplied to the actuator, thereby turning the "rigid" link into a "elastic" link, resulting in creep.

In order to avoid air intrusion into the hydraulic system, the nuts at the joints of the pipes should be tightened to prevent leakage. As far as possible, keep the oil inlet and outlet pipes far apart, ensure that the inlet and outlet oil lines do not interfere with each other, and separate the inlet and outlet oil pipes with partitions. Remove dirt adhering to the oil filter net or replace the filter with sufficient capacity to ensure smooth oil absorption. Add enough fluid and check frequently to keep the oil not below the oil mark line to prevent the system from taking in air. The oil with low viscosity is used to reduce the flow resistance, and to ensure that the oil absorption is smooth and sufficient, and to avoid the occurrence of cavitation and the gas in the oil seeps out.

2. Crawling and eliminating methods caused by the gap of hydraulic pump or valve parts The internal parts of the pump are worn out, and the gap is too large, causing the oil volume and pressure to be insufficient or severely fluctuating to cause creep. When the moving parts move at low speed, once dry friction or semi-dry friction occurs, the resistance increases. At this time, the hydraulic pump is required to increase the pressure. However, due to the large pump clearance, the oil leakage is serious, and the moving parts cannot be adapted to the change of the resistance, so that the speed of the moving parts is slowed down or stopped. . When the pressure rises to overcome the static friction force, the moving friction is smaller than the static friction force, causing the moving member to jump forward, the pressure is lowered, and the moving member speed is slowed down or even stopped. In this way, the loop is repeated to form a crawl.

In the hydraulic system of the machine tool, when the damping holes and the throttle openings of various control valves are blocked by dirt or the parts inside the valve are worn, the movement of the spool is inflexible, the pressure is fluctuated, and the thrust of the table is large and small, thus The speed of the workbench is fast and slow, and it can also cause crawling.

When the hydraulic pump gap is too large, repair or replace the hydraulic pump to reduce oil pump leakage. Use oil filter to prevent impurities from entering the hydraulic system, keep the oil clean, regularly clean the oil pool and replace the hydraulic oil, and strengthen the maintenance of the control valve to prevent hydraulic oil pollution. Ensure that the accuracy of the parts meets the technical requirements, check the matching clearance of each matching part, grind the valve hole (cylinder hole), and redo the slide valve (piston) so that the matching clearance is within the specified range.

3. Crawling and eliminating methods caused by changes in frictional resistance

The difference between the static and dynamic friction factors is the internal cause of the creep phenomenon. The smaller the difference between the static and dynamic frictional forces, the smaller the critical speed, and the easier it is to crawl.

The accuracy of the guide rail is not high, the number of points of the guide rail scraping is not enough and uneven, the surface of the guide rail is pulled, and the surface of the guide rail is rusted, which may cause local resistance change and poor contact of the guide rail, so that the oil film is not easily formed, resulting in dry friction or semi-dry friction, resulting in The frictional resistance of the components such as the work table or the knife box is unstable and causes creep. In this case, the rail must be repaired to meet the technical requirements.

In addition, due to manufacturing, installation and wear, the center line of the liquid claw cylinder is not parallel with the guide rail, the guide rail or the sliding clamp block (strip) is too tightly clamped, the seal has an excessive interference, and the piston rod Partial or full-length bending, scratching of the hole in the cylinder of the hydraulic cylinder, different axes of the piston and the piston rod, poor precision of the hole of the hydraulic cylinder, and excessive adjustment of the oil seal at both ends of the piston rod may also cause creep. When these defects produce uneven friction and the moving parts crawl, the accuracy of the cylinder parts should be checked one by one, and the production should be repaired.