The engine crankshaft of some small wheeled tractors is supported by two rolling bearings. A rolling bearing consists of an inner ring, an outer ring, rolling elements, and a cage. There are raceways on the outside of the inner ring and the inside of the outer ring to facilitate the rolling of the rolling elements. The inner ring is matched with the shaft, and the outer ring is matched with the bearing seat or housing. Usually, the inner ring rotates with the shaft, while the outer ring does not rotate. The front and rear main shaft necks of the crankshaft are fixed to the inner rings of the rolling bearings. The outer ring of the rear rolling bearing is fixed in the bearing seat hole of the machine body with a rear bearing cover to prevent axial movement. The outer ring of the front bearing is directly installed in the seat hole of the body without axial fixation, so that there is axial expansion and contraction space forward when the crankshaft is heated and expands.
Once the rolling bearings of the engine crankshaft fail, it directly affects the technical status of the engine. Therefore, the failure of rolling bearings should be taken seriously. The common faults of rolling bearings are analyzed as follows:
1、 There is a gap between the movement of the inner and outer rings of the bearing and the wear of the mating parts
1. The outer ring of the bearing moves. For the convenience of disassembly and assembly, the outer ring of the rolling bearing adopts a transitional fit with the seat hole. The maximum interference fit of the outer ring of the front bearing is 0.035mm, and the maximum clearance is 0.013mm. The maximum interference fit of the outer ring of the rear bearing is 0.016mm, and the maximum clearance is 0.06mm. During assembly, the seat hole should be measured and its interference or clearance should not exceed the above range, otherwise the seat hole needs to be repaired.
2. The inner ring of the bearing moves. The inner ring and journal of the rolling bearing adopt interference fit. The maximum interference fit of the inner diameter of the front bearing is 0.055mm, and the minimum interference fit is 0.012mm. The maximum interference fit of the inner diameter of the rear bearing is 0.046mm, and the minimum interference fit is 0.003mm. From the above cooperation, it can be seen that if the bearing has undergone several disassembly and assembly, there may be a gap between the inner ring of the bearing and the shaft neck, and the shaft neck must be repaired before it can be applied.
When the fit between the bearing and the shaft becomes loose, some maintenance workers use embossing or punch punching methods to make the surface of the shaft fuzzy, attempting to obtain a tight fit surface. This not only destroys the geometric shape of the shaft surface, causing the bearing to deviate from the correct center position during installation, but also the fuzzy part will eventually be flattened and become a loose fit. The correct maintenance method is to first grind and repair the shaft neck, and then brush or spray coat the shaft neck.
2、 Wear, deformation or fracture of rolling bearing cage
When assembling the inner and outer rings of rolling bearings, due to the large interference fit, the bearing balls are stuck in the race, which can easily cause damage to the bearing cage during operation. So measurements should be taken during assembly to ensure the standard interference fit.
2. The lubrication of crankshaft bearings is splash type. When there is a lack or absence of lubricating oil in the bearing, especially when the load acting on the bearing is high, the bearing temperature increases. In general, the temperature of the inner ring of a bearing is about 10 ℃ higher than that of the outer ring. Due to the different thermal expansion of the inner and outer rings, the gap between the ball and the raceway disappears, and the bearing is prone to crushing the cage during operation. So when installing bearings, clean lubricating oil should be applied to the raceway to prevent dry friction caused by oil splashing.
3、 Rolling bearing raceway and rolling element surface peeling off
1. Peeling caused by impact loads and alternating stresses. The rolling elements and the inner and outer raceway surfaces of the bearing are subjected to periodic pulsating loads, resulting in contact stresses that vary periodically. When the number of stress cycles reaches a certain value, fatigue spalling occurs on the working surfaces of the rolling elements or inner and outer raceway. If the load on the bearing is too high, it will exacerbate this fatigue. In addition, improper installation of bearings and bending of shafts can also cause peeling of the raceway. Balance weights are installed on the engine flywheel and pulley to balance the rotational inertia moment generated with the rotational inertia moment generated by the rotating parts of the engine. If the generated inertia moment is not balanced, the engine will vibrate greatly after ignition, increasing the alternating stress and stress cycle times, causing premature peeling of the raceway and rolling element surfaces.
In order to avoid peeling off the surface of the raceway and rolling elements, additional loads and unnecessary vibrations should be avoided during use. The throttle and speed should be used reasonably during operation to prevent the tractor from shaking. The fixing bolts of the machine body should be checked regularly to prevent loosening.
2. Due to external hard particles falling onto the mating surface of the outer shell or between the shaft and the sleeve, the shape of the raceway is distorted. The rolling element exerts the maximum pressure here, resulting in rapid wear and metal peeling of the ring raceway. Rolling bearings are highly precise components that are highly sensitive to foreign objects. During the maintenance process, a clean and lint free wiping cloth should be used. The wiping cloth should never be replaced with old cotton threads, as the yarn ends in the old cotton threads are prone to getting mixed into the lubricating oil and entering the bearings, which is very harmful. Before assembly, thoroughly clean the surface of the shaft neck, seat hole, and inner and outer rings of the bearing to prevent dirt from entering the mating surface.
3. Damage caused by damage to the raceway during installation. If the pressure applied during installation is transmitted through the outer ring and steel ball, it is inevitable that pits will be formed at the part where the ring raceway contacts the steel ball. These pits are the root cause of damage and increase rapidly, causing the bearing to peel off and be scrapped in a short time. So when disassembling, special tools should be used, and the force bearing parts should be correct and uniform.
4、 Increase in radial and axial clearance of bearings
The radial and axial clearances of the bearings increase, except for dry friction and excessive friction wear during startup. Mainly due to the friction between metal particles on the surface of the parts and sand, dust, and dirt in the lubricating oil on the rolling elements and raceways.
Due to abrasive wear, the clearance between the bearing ring and the rolling element significantly increases, resulting in a dull surface and uneven scratch marks on the raceway. The prevention method is to replace the lubricating oil on time, seal the oil filling hole, and strictly prevent dirt from entering.