Failure analysis of engine crankshaft rolling bearing

The crankshaft of some small wheeled tractors is supported by two rolling bearings. The rolling bearing consists of an inner ring, an outer ring, a rolling body and a cage. The outside of the inner ring and the inside of the outer ring are raceways to facilitate the rolling of the rolling body. The inner ring is fitted with the shaft, and the outer ring is fitted with the bearing seat or the box. Usually the inner ring rotates with the axis, but the outer ring does not. The crankshaft spindle neck is fixed with the inner ring of the rolling bearing. The outer ring of the rear rolling bearing is fixed in the body bearing seat hole with the back bearing cover to prevent axial movement. The outer ring of the front bearing is directly mounted in the seat hole of the body, without axial fixation, so that the crankshaft is heated and expanded, and there is room for axial expansion forward.

Once the rolling bearing of engine crankshaft fails, it directly affects the technical state of engine. Therefore, attention should be paid to the failure of rolling bearing. Now, the common failure analysis of rolling bearing is as follows:

First, there is a gap between the movement of the bearing inner and outer rings and the wear of mating parts

1. The bearing outer ring moves. In order to disassemble and assemble easily, the outer ring of the rolling bearing and the seat hole adopt a transition fit. The maximum interference of the outer ring of the front bearing is 0.035mm and the maximum clearance is 0.013mm. The maximum interference of rear bearing outer ring 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 should be repaired.

2. Bearing inner ring walking. Interference fit is adopted between inner ring and journal of rolling bearing. The maximum and minimum interference of front bearing inner diameter are 0.055mm and 0.012mm respectively. The maximum inner diameter of the rear bearing is 0.046mm and the minimum is 0.003mm. From the above match, if the bearing has been disassembled and assembled for several times, there may be a gap between the bearing inner ring and the journal, then the journal must be repaired before it can be applied.

When the fit between the bearing and the shaft is loose, some maintenance workers use embossing or punching to make the surface of the shaft fur, in an attempt to obtain a tight fit surface. This not only destroys the geometry of the shaft surface, so that the bearing can not get the correct center position in the installation and occurs skew, and the lint part will eventually be flattened and become a loose fit. The correct maintenance method is to grind the journal first, and then brush plating or spraying the journal.

Two, rolling bearing cage wear, deformation or fracture

1. When assembling the inner and outer rings of rolling bearings, due to large interference, the bearing ball is stuck in the ring, and it is easy to crush the bearing cage during operation. Therefore, measurement should be carried out during assembly to ensure the standard interference quantity.

2. Lubrication of crankshaft bearings is splash type. When there is no lubricating oil in the bearing, especially when the load on the bearing is large, the bearing temperature rises. In general, the temperature of the bearing inner ring is about 10℃ higher than that of the outer ring. Because the thermal expansion of the inner and outer rings is different, the gap between the ball and the raceway disappears, and the cage is easy to be broken when the bearing is running. Therefore, when installing bearings, the raceway should be smeared with clean lubricating oil to prevent the oil from splashing and causing dry friction.

Three, rolling bearing raceway and rolling body surface spalling

1. Spalling caused by impact load and alternating stress. The rolling body of bearing and the raceway surfaces of inner and outer rings are subjected to periodic pulsating loads, resulting in periodic contact stresses. When the number of stress cycles reaches a certain value, fatigue spalling will occur on the working surface of the rolling body or inner and outer ring raceway. If the load of bearings is too large, this fatigue will be aggravated. In addition, bearing installation is not straight, shaft bending, raceway spalling phenomenon will also occur. The balance weight is set on the engine flywheel and belt wheel, and the rotational inertia moment generated is balanced with that generated by the rotating part of the engine. If the inertia torque generated is not balanced, the engine vibrates greatly after fire, increasing the alternating stress and stress cycles, resulting in premature spalling of raceway and rolling surface.

In order to avoid raceway and rolling body surface spalling, in use should avoid extra load and undue vibration, in the operation process to reasonable use of the throttle and speed, to prevent tractor bumps, to check the fixed body bolt often, to prevent loose.

2. The shape of raceway is distorted due to the external hard particles falling into the shell fitting surface or between the shaft and the ring fitting surface. This is where the greatest pressure is exerted by the roller, resulting in rapid wear and metal spalling on the raceway of the ring. Rolling bearing is a very precise component, very sensitive to foreign matter. In the maintenance process, clean wiper cloth that is not easy to fuzz should be used. Wiper cloth should never be replaced by old cotton silk, because the yarn head in old cotton silk is easy to mix with lubricating oil and enter the bearing, which is very harmful. Before assembly, the journal, seat hole, bearing inner and outer ring surface should be thoroughly cleaned to prevent dirt from entering the matching surface.

3. Damage caused by raceway damage during installation. If the installation, the added pressure is transmitted through the outer ring and the ball, it will inevitably cause pits in the ring raceway contact with the ball, these pits are the root of the damage, the increase is very fierce, only a short time to make the bearing spalling scrap. Therefore, special tools should be used when disassembling, and the stress part should be correct and even.

Four, bearing radial and axial clearance increase

The radial and axial clearance of bearings increases, except for dry friction and excessive friction and wear when starting. It is mainly caused by the friction between the rolling body and the raceway between the metal particles on the surface of the parts and the sand, dust and dirt in the lubricating oil.

Due to abrasive wear of bearings, the clearance between the ring and the rolling body increases significantly, and the raceway has formed a dull surface and uneven scratch marks. The prevention method is to replace the lubricating oil on time, the refueling hole should be closed, and the dirt should be prevented from entering.