Spherical bearings are sliding bearings, consisting of an outer spherical inner ring and an inner spherical outer ring. Generally used for low-speed and heavy-load swing motion, tilt motion and rotation motion. Compared with rolling bearings, they have simple structure, small size and large load-bearing capacity. They are widely used in engineering machinery, heavy-duty vehicles, agricultural machinery, aerospace, mining and metallurgy, printing, textiles, railways, ships and other fields. In recent years, the application fields of spherical bearings have been continuously extended, and they are also widely used in buildings and bridges.

The materials used for spherical bearings are Gcr15 and Gcr15SiMn high carbon chromium bearing steel. The production of spherical bearings generally goes through forging, annealing, turning, heat treatment (quenching, tempering), grinding and other processes. Grinding is a subsequent process. Quenched high-carbon chromium bearing steel is prone to grinding cracks during the grinding process, which often occurs in daily production and quality processing. The shape of the grinding cracks is special and only occurs on the grinding surface. It is obviously different from the quenching cracks in macroscopic observation. The depth of the grinding cracks is shallow, and its depth is roughly 0.05~0.20mm. The cracks are more obvious after etching with acid, as shown in Figures 1 and 2. Lighter grinding cracks are distributed perpendicular or nearly perpendicular to the grinding direction and are called type I cracks. The more serious cracks are tortoise shell-shaped and are called type II cracks, which are customarily called cracks. During the grinding process, once grinding cracks occur, it often results in a batch of scrap products and serious losses; some grinding cracks are very small and difficult to be found. Once they flow into the hands of customers, they will cause serious consequences and are fatal defects. Therefore, it is of great significance to study the causes of grinding cracks and take preventive measures to avoid their occurrence.

1. The generation mechanism of grinding cracks in spherical bearing rings

After the spherical bearing is heat-treated and quenched, it is then ground. During grinding, the cutting speed of the grinding wheel is very high, and the sand grains and the surface of the workpiece are heated due to severe friction. If the coolant supply is insufficient, the heat generated by grinding cannot be diffused quickly, and the temperature If it decreases rapidly, the instantaneous temperature in the grinding area can be as high as 800 to 1000 degrees. Such a high temperature will inevitably cause the surface temperature of the workpiece to be too high. The instantaneous temperature of the workpiece surface can easily cause burns on the workpiece surface.

In addition, the structure of high-carbon chromium alloy (bearing steel material) will inevitably austenitize again at high temperatures, and transform into martensite with a larger specific volume during the cooling process. This situation is similar to that of steel during quenching. The same heating and cooling process will inevitably produce thermal stress and tissue stress. When the stress is greater than the tensile strength limit of the steel, grinding cracks will occur. Because the grinding layer is very thin and the instantaneous grinding area is small. The cracks formed are shallow. Generally, the depth of grinding cracks is 0.05~0.2mm. When etched with acid, the cracks are clearly visible. Therefore, high grinding heat is the root cause of grinding cracks.

2. Analyze the causes of grinding cracks

According to the mechanism of grinding cracks, the grinding heat is high, the grinding heat cannot diffuse quickly, and the tensile strength of the material is low. Focusing on these three points, combined with processing experience, we analyze the incoming materials and grinding processing to find the causes of grinding cracks.

2.1 Quality of incoming materials (rings to be ground)

Before the spherical bearing rings enter the grinding process, they must undergo raw material preparation, forging, annealing, turning, and heat treatment (quenching and tempering). Once these processes are not controlled well, the tensile strength of the material will inevitably be affected.

① The steel itself has poor composition uniformity, which affects the tensile strength of the material;

② If the forging stop temperature is too high and the cooling is too slow, the structure will be overheated to form a coarse and closed secondary carbide network. The carbide network will be brittle, resulting in low tensile strength of the ferrule;

③ Poor control of the dimensions of turning, and parts with large ovals and tapers will intensify the grinding heat during grinding and cause the temperature to rise instantly;

④ If the quenching and tempering structure level is too high, the thicker the martensite needles will be, the toughness of the material will decrease. High quenching heating temperature and long holding time will lead to greater deformation of the heat-treated product, which may also indirectly produce grinding cracks. wait.

In addition, due to insufficient aging and continuous fatigue operation, the stress is not released in time, which affects the tensile strength of the material.

2.2 Grinding processing

① The feed is unstable and the amount of feed at one time is too large.

If the feed speed is too fast and unstable, the temperature of the grinding area will rise instantly, causing grinding cracks. According to experience, manual feed semi-automatic internal cylindrical grinders, swing spherical grinders, vertical surface grinders, and invisible grinders are most likely to produce grinding cracks, accounting for 50% of grinding cracks.

②Improper selection of grinding wheel.

The grinding wheel hardness is too high. The self-detachability is poor, and the passivated abrasive grains are not easy to fall off, which increases the friction between the grinding wheel and the working surface. If the grinding wheel grade number is too small, the chip space between the abrasive grains is small, and the grinding wheel chips are easily blocked, which affects the cutting performance. Both of these situations will cause serious heating during the grinding process.

③The pressure of the machine tool is unstable and the beat is large, which aggravates the grinding heat.

④The cutting fluid is not cooled in place, and the grinding heat cannot diffuse quickly, causing the temperature of the grinding area to rise instantly.

⑤ The tooling and support blocks used are too hard and too sharp, which will scratch the surface of the ferrule and cause serious cracks. This is also one of the causes of grinding cracks. This situation is easily missed when grinding cracks. Not long ago, a quality accident occurred in our company. When grinding the GEZ101XS/K(S).01 spherical surface, the support frame scratched the outer diameter surface of the ferrule, causing outer diameter grinding cracks.

⑥Irregular grinding, such as the workpiece speed is too slow, is not conducive to the rapid diffusion of grinding heat. This situation is more prominent in large-scale snare processing.

In addition, if the diamond pen is blunt and the grinding wheel is not trimmed in time, it will also affect the cutting performance and cause serious heating during the grinding process.

3. Measures taken

Based on the causes of grinding cracks, the following countermeasures should be taken to avoid their occurrence.

① In terms of raw materials: Check the quality of raw materials. Raw materials with uneven composition, excessive inclusions, microscopic cracks, and poor carbide morphology and distribution must be strictly prohibited from being put into production.

② When forging, strictly follow the heating and cooling specifications, control the forging heating temperature and the cooling rate after forging, and prevent overheating, overburning and the generation of network carbides during forging.

③ The annealing process must be strictly controlled to avoid the generation of overheated or underheated annealing structures and ensure that the workpiece has good metallographic structure and condition before quenching.

④ Turning processing must ensure the size and accuracy of processing.

⑤Heat treatment:

First, the fundamental reason for grinding cracks is that the martensitic structure of the quenched part is in an expansion state, and there is stress. To reduce and eliminate this stress, stress relief and tempering should be performed, that is, tempering should be performed immediately after quenching. deal with.

Second, grinding cracks are caused when the workpiece is rapidly heated to about 100°C and then cooled rapidly. Therefore, in order to prevent such grinding cracks, the workpiece should be tempered at around 150 to 200°C. The second type of grinding crack is caused by the surface shrinking again when the workpiece continues to heat up to 300°C during grinding.

Therefore, in order to prevent this second type of grinding crack, the workpiece should be tempered at about 300°C, and the tempering time must be more than 4 hours. It should be noted that the hardness of the workpiece will decrease when tempered at 300°C, so it is sometimes not suitable to use it.

Third, sometimes grinding cracks may still occur after one tempering. In this case, secondary tempering or artificial aging can be performed. This method is very effective.

⑥Grinding processing:

First, if conditions permit, the process of using cars instead of grinding can be considered. Especially large-type spherical plain bearings, such as radial spherical plain bearings with inner diameter d>φ300 and outer diameter D> 430mm; or thrust spherical plain bearings and angular contact spherical bearings with large thickness differences.

Second, rationally select the grinding feed amount and grind multiple times to reduce the generation of grinding heat.

Third, adopt good cooling measures, such as increasing the grinding fluid flow rate, increasing the injection pressure, improving the nozzle structure, spray cooling, etc., to ensure that the grinding fluid can enter the grinding contact area to the maximum extent and take away most of the grinding fluid. Cut the heat. Increase the temperature of the grinding fluid appropriately to reduce the generation of thermal stress.

Fourth, rationally select the grinding wheel hardness and particle size, and perform timely dressing of the working surface to reduce grinding heat and grinding stress. On the premise that the quality can meet the requirements, a grinding wheel with softer hardness and coarser grain size can be used for grinding.

Fifth, the workpiece that has just been heat treated must wait for the workpiece to cool naturally (cooled to normal temperature) before grinding. If time permits, it is best to let the workpiece age naturally for 1 to 2 months, and then grind it after the stress is eliminated, which will also achieve good results.

Sixth, during the grinding process of large-sized ferrules, you can consider adding a mutual tempering process to eliminate the residual stress generated by the grinding of the ferrules.

Seventh, avoid using tooling and support blocks that are too sharp and hard, which may scratch the surface of the ferrule and cause cracks.

4. Conclusion

In short, as long as we strengthen control in terms of raw material quality, forging, turning, heat treatment, and grinding processing; when grinding cracks appear, there must be grinding burns on the surface. During the grinding process, by strengthening the cooling of the workpiece, Pickling inspection can effectively prevent the occurrence of grinding cracks.

RLB Spherical Plain Bearing

RLB Spherical Plain Bearings are high-quality bearings designed for use in numerous applications in the industrial and automotive sectors. These bearings are designed to handle both radial and axial loads, making them a versatile solution for a wide range of applications. 

RLB Spherical Plain Bearings are made from premium materials, including high-quality chrome steel and a variety of synthetic materials, to ensure long-lasting, reliable performance. The bearings are precision engineered and manufactured to the highest quality standards, ensuring they meet the specific requirements of each application.

The bearings are designed to offer exceptional durability and resistance to wear, even in harsh operating conditions. They are designed to operate smoothly and efficiently, minimizing friction and reducing energy consumption. This allows for improved equipment performance and an extended service life for both the bearings and the equipment they are used in.

RLB Spherical Plain Bearings feature a unique design that allows for easy installation and maintenance. The spherical shape of the bearing's inner and outer ring surfaces allows for self-alignment, ensuring a reliable fit and efficient operation. The bearings are also designed to be easy to disassemble, making it easy to perform routine maintenance and repairs.