For convenience, boundary friction and mixed friction are collectively referred to as non-liquid friction hereinafter. The sliding bearing is in the ideal state of liquid friction. Because the frictional resistance at this time is small, the friction factor is close to the equivalent friction factor of the rolling bearing. In addition, the bearing oil film can completely eliminate wear and tear, and at the same time has the advantages of good vibration absorption and buffering. However, when the sliding bearing is in the state of boundary friction, if the boundary oil film is destroyed, the frictional wear (firstly abrasive wear) that occurs directly between the metals will increase sharply. At the same time, the local temperature caused by the friction between the metals increases sharply, and the increase in temperature causes the metal surface to expand rapidly, and the expansion of the metal surface in turn aggravates the friction between the metals. When the local high temperature generated by friction reaches the melting point of the material, adhesion between the main shaft and the bearing occurs, that is, shaft holding.

The plain ground plain bearing material here is tin bronze. When the shaft is slightly locked, there are black burn marks on the surface of the main shaft. The sliding bearing is a copper-based alloy material, which is relatively soft. In addition to black burn marks, pits will form on the burn surface. In this case, the cause of the shaft holding should be analyzed first, the fault should be eliminated, the surface of the main shaft and the sliding bearing should be repaired and then scraped and assembled again. The main shaft and sliding bearings can still be used. Serious shaft holding will cause the spindle to crack, in which case the spindle should be replaced

Analysis of the Causes of the Sliding Bearing Holding the Shaft

(1) There are impurities, dirt and poor oil cleanliness in the lubricating oil and sliding bearing oil pool. When the oil ring rotates with the main shaft, the oil supply system starts to work. Impurities and dirt enter between the main shaft and the sliding bearing together with the lubricating oil, and the gap between the main shaft and the sliding bearing is generally 0.03-0.04mm. When impurities and dirt ruffle the main shaft or the sliding bearing, non-liquid friction is very likely to occur, resulting in shaft holding.

(2) The grinding head lubrication system is faulty. Plain ground plain bearings are lubricated with oil rings. This lubrication method has the advantages of simple, automatic, and low oil consumption. There are 16 5mm holes evenly distributed on the circumference of the oil ring. When the oil ring rotates with the main shaft, the small hole on the oil ring splashes the lubricating oil in the oil pool, the lubricating oil splashes on the main shaft, and the spiral oil groove on the main shaft sends the lubricating oil between the main shaft and the sliding bearing. When the grinding head lubrication system does not supply oil or the oil supply is unstable, the shaft holding phenomenon will occur.

After the shaft is stuck, the reasons can be found from several aspects:

①Whether there is oil leakage in the grinding head.

② Whether the oil pool is short of oil, and whether the oil level is lower than the indicator line on the oil mark. One of the flat grinding lubricating oils is prepared with kerosene, which is volatile. When the grinder is in normal use, the operator should always pay attention to the oil mark. When the lubricating oil in the grinding head is lower than the indicator line on the oil mark, the lubricating oil should be replenished in time.

③ Grinding machine manufacturers must check the ovality of the inner hole of the oil ring (ovality ≤ 0.05mm) and whether the 16 5mm holes on the oil ring are evenly distributed before assembling the grinding head. The oil ring is sleeved on the main shaft, the inner circle of the oil ring is closely attached to the outer circle of the main shaft, and the lower end of the oil ring is placed under the oil surface. When the main shaft rotates, the oil ring is driven to rotate synchronously by static friction. When the ellipse of the inner hole of the oil ring is too large, the contact surface between the main shaft and the oil ring is suddenly large or small, or even not in contact, so the friction force is suddenly large and small, or even no friction, resulting in the oil ring rotating fast and slow , the oil supply is instantaneously normal and instantaneously abnormal and the shaft is held.

(3) The lubricating oil of the grinding head sliding bearing does not meet the specified requirements. There are two types of lubricating oil for flat grinding head: one is N7/GB3141 mechanical oil, and the other is 1 part of N23 engine oil and 9 parts of kerosene. Some users buy a new flat grinder without reading the instruction manual, and directly inject other types of mechanical oil, which will cause the shaft to stick.

(4) The radial clearance between the main shaft and the sliding bearing is too small, the lubricating oil is not easy to input, and the temperature rise between the main shaft and the sliding bearing is too high, resulting in the shaft holding.

(5) The sliding bearing scraping method is incorrect, and the contact point does not meet the requirements. The sliding bearing scraping method is incorrect, the contact points are uneven or the contact points are too few, which will make the bearing oil film unstable and the oil film stiffness is poor.

(6) The coaxiality between the rear bearing seat and the grinding head body and the verticality of the end face, as well as the deviation of the parallelism between the outer spacer and the inner spacer between the two rolling bearings is too large. When this happens, the runout of the front end of the spindle, that is, the taper surface of the spindle in contact with the grinding wheel chuck is relatively large. When the spindle is rotating, the runout and position of the spindle taper surface are extremely unstable, ranging from large to small. In fact, this is a superficial phenomenon, which reflects that the spindle axis is drifting and unstable, resulting in unstable bearing oil film and poor oil film stiffness. If the grinding wheel is ground, it is very easy to hold the shaft.

(7) The vertical feed is too large and the cutting is overloaded. At this time, the rotational speed of the grinding wheel decreases, the stiffness of the bearing oil film decreases, and non-liquid friction is formed between the main shaft and the sliding bearing, which eventually leads to the shaft holding. There are many factors for the flat grinding head to hold the shaft. When looking for the cause of the shaft, it must not be less than the rest, and the fault should be eliminated from multiple perspectives and in all directions.

Repair of the sliding bearing after holding the shaft and the measures to prevent the holding of the shaft

(1) It is used as a radial ball bearing 210, and the outer spacer and the inner spacer between the D grades (when two 46210, D grade angular contact ball bearings are used, no spacer is required). The axial force determines the height difference between the inner and outer spacers, and the parallelism of the two ends of the inner and outer spacers is ≤0.003ram.

(2) Install the rear bearing seat, rolling bearing, bearing outer spacer and inner spacer on the main shaft, and then put the above assembly into the grinding head body.

(3) Place the grinding head body vertically (the taper of the spindle faces upwards) without sliding bearings, and measure the radial runout of the spindle ≤0.1mm. Put the dial indicator seat on the end of the main shaft, the needle points to the inner diameter of the front bearing seat, and measure the coaxiality between the main shaft and the front bearing seat ≤0.02mm. If the measured value is out of tolerance, the inner and outer spacers of the bearing should be re-adjusted, and the rear end face of the grinding head body should be ground until the requirements are met.

(4) The main shaft and the sliding bearing are equipped with scrapers. Attention should be paid to: ① The red powder or red ink oil applied should not be too thin. It should be even when applied. ②The radial force of 10kg when scraping the sliding bearing with a scraper, adjust the gap between the main shaft and the sliding bearing seat to 0.02-0.025mm.

(7) The sliding bearing is injected with lubricating oil as required, and it should be filtered with silk cloth during injection.

(8) The grinding head body is placed horizontally, and the idle run is 4 hours, so that the main shaft and the sliding bearing can be fully run-in, and the temperature rise of the bearing is measured to be ≤30 °C.

(9) Drain all the lubricating oil in the grinding head. Refill with lubricating oil as required. When filling oil, filter with silk cloth. It is necessary to spot check the cleanliness of the lubricating oil. The impurities and dirt should not exceed 200mg according to the gravimetric test.

(10) Adjust the gap between the main shaft and the sliding bearing seat to 0.03~0.04mm, and start the test run. To measure the radial runout of the spindle taper and the axial movement of the spindle, the old machine tools should be ≤0.01ram, and the new machine tools should be ≤0.005mm