Causes of Grinding Cracks in Bearings and Preventive Measures
The company mainly produces three core series of products: cylindrical roller bearings, self-aligning roller bearings, and thrust self-aligning roller bearings. The product can be adapted to multiple industrial fields such as metallurgical equipment, mining machinery, heavy machinery, engineering equipment, etc., and can meet the operational and load-bearing requirements of equipment under different working conditions.
SZ Bearings
Grinding is a common method of metal cutting in the machinery manufacturing industry and is also widely used in the roller bearing manufacturing sector. During the grinding process, bearing components that have undergone heat treatment and quenching may develop mesh-like cracking or fine cracks arranged in a relatively regular pattern, known as grinding cracks. These cracks not only affect the appearance of the bearing components but, more importantly, directly impact the quality of the roller bearing parts.
1. Characteristics of Grinding Cracks in Roller Bearings
Grinding cracks differ significantly from typical quenching cracks. They occur exclusively on the ground surface, are relatively shallow, and generally have consistent depths. Mild grinding cracks form as parallel lines perpendicular or nearly perpendicular to the grinding direction, appearing as regularly arranged strip-like cracks; this is the first type of crack. More severe cracks exhibit a tortoise-shell pattern (closed network), with a depth of approximately 0.03–0.15 mm; these cracks become clearly visible after acid etching, representing the second type of crack.
2. Causes of Grinding Cracks in Roller Bearings
The formation of grinding cracks in bearings is caused by grinding heat; during grinding, the surface temperature of the bearing can reach 800–1000°C or higher. The microstructure of quenched steel consists of martensite and a certain amount of retained austenite, which are in an expanded state (un-tempered). The expansion and contraction of martensite increase with rising carbon content in the steel, which is a particularly significant factor in the formation of grinding cracks on the bearing steel surface. During grinding, the residual austenite in quenched steel decomposes under the influence of grinding heat and gradually transforms into martensite. This newly formed martensite concentrates on the part’s surface, causing localized expansion of the bearing surface, increasing surface stress, and leading to stress concentration. Continued grinding accelerates the formation of surface grinding cracks; Furthermore, the newly formed martensite has a larger grain size, which also facilitates the formation of grinding cracks during the grinding process.
On the other hand, when parts are ground on a grinding machine, they are subjected to both compressive and tensile forces, which further promotes the formation of grinding cracks. If cooling is insufficient during grinding, the heat generated by the grinding process is sufficient to cause a thin layer of the ground surface to re-austenitize, followed by re-hardening into quenched martensite. This results in additional microstructural stresses in the surface layer. Combined with the rapid rise and fall in bearing surface temperature caused by the heat generated during grinding, the superposition of these microstructural and thermal stresses may lead to the formation of grinding cracks on the ground surface.
3. Measures to Prevent Grinding Cracks
From the above analysis, it is clear that the fundamental cause of grinding cracks lies in the fact that the martensite formed during quenching is in an expanded state and contains residual stresses. To reduce and eliminate these stresses, stress-relief tempering must be performed—that is, quenching followed by tempering—with a tempering time of at least 4 hours. As the tempering time increases, the likelihood of grinding cracks decreases. Additionally, rapid heating of bearings to approximately 100°C followed by rapid cooling can cause cracks. To prevent cold cracks, parts should be tempered at around 150–200°C. If the bearing is heated further to 300°C, the surface will contract again and cracks may form; to prevent this, the bearing should be tempered at around 300°C. It is worth noting that tempering the bearing at around 300°C reduces its hardness, so this method is not recommended. If grinding cracks still occur after a single tempering, a second tempering or artificial aging treatment can be performed; this method is highly effective.
Grinding cracks are caused by grinding heat, so reducing grinding heat is the key to resolving this issue. The wet grinding method is generally used; however, no matter how much coolant is applied, it cannot reach the grinding surface in time during the grinding process, and thus cannot reduce the grinding heat at the grinding point. The coolant can only provide instantaneous cooling to the grinding point on the grinding wheel and the workpiece after the grinding path has passed, while simultaneously having a quenching effect on the grinding point. Therefore, increasing the amount of coolant used is one of the primary measures to minimize grinding heat in the grinding zone. If dry grinding is used, a smaller grinding feed rate can help reduce grinding cracks. However, this method is not very effective, and it generates a lot of dust, which affects the working environment and is therefore not recommended.
Selecting a grinding wheel with softer hardness and coarser grit can reduce grinding heat. However, coarser particles affect the surface roughness of the workpiece; this method cannot be used for parts with high surface finish requirements, thus it is subject to certain limitations. Separating the process into rough and finish grinding—using a soft grinding wheel with coarser particles for rough grinding to facilitate aggressive grinding and improve efficiency, followed by a finer-grained grinding wheel for finish grinding with a smaller feed rate—is a relatively ideal approach. Using two separate machines for rough and finish grinding is the most ideal approach.
Selecting grinding wheel abrasives with good self-sharpening properties, promptly removing debris from the wheel surface, reducing the grinding feed rate, increasing the number of grinding passes, and lowering the table speed are also effective ways to reduce grinding cracks.
The rotational speed of the grinding wheel and the workpiece is another key influencing factor; excessive runout in the grinding wheel can cause the workpiece to shift.