Difficulties and solutions for machining thin-walled bearings

 

 

 

Abstract:

As the name implies, thin-walled bearings have a smaller wall thickness. In engineering practice, parts with a diameter-to-wall thickness ratio greater than 15 are called thin-walled parts. The bearing industry usually quantifies the inner and outer ring wall thickness coefficient K (the ratio of the outer ring outer diameter to the outer groove diameter D/De, and the ratio of the inner ring inner groove diameter to the inner diameter di/d). When 1.04＜K≦1.14, it is called a thin-walled bearing.

 

 

 

Key words: Thin wall bearing, thin section bearing, bearing ring

 

 

As the name implies, thin-walled bearings have a smaller wall thickness. In engineering practice, parts with a diameter-to-wall thickness ratio greater than 15 are called thin-walled parts. The bearing industry usually quantifies the inner and outer ring wall thickness coefficient K (the ratio of the outer ring outer diameter to the outer groove diameter D/De, and the ratio of the inner ring inner groove diameter to the inner diameter di/d). When 1.04＜K≦1.14, it is called a thin-walled bearing.

 

Thin-walled bearings have the advantages of light weight, compact structure, and small moment of inertia. They are widely used in high-end product fields such as industrial robots, aircraft, space satellites, and medical treatment, and their market size is gradually increasing. However, the cross-sectional area of ​​thin-walled bearings is small, so their radial wall thickness is very small, the rigidity is poor, and large radial deformation is very easy to occur. Therefore, in order to prevent the rings from going out of tolerance in roundness and flatness, and from having defects such as ellipse, round edges, and end face warping and deformation during turning, heat treatment, grinding and other processes, special means are required to perform protective processing on the bearing rings.

 

♦ In the forging process, for thin-walled bearing rings with large size and small aspect ratio, two or more pieces are forged together to produce them. After the rough grinding process, the rings are separated by wire cutting to reduce the processing difficulty of the forging process, reduce the deformation of the rings and the end surface processing allowance, save raw materials and improve production efficiency.

 

♦ In the turning process, the main factors affecting the machining accuracy are clamping positioning, excessive cutting force, unreasonable fixture design, cutting thermal deformation and vibration during cutting. In order to reduce deformation caused by excessive machining stress, use a steel soft jaw clamping ring with a larger enveloping circle contact area and without quenching for rough turning, such as using a multi-point clamping chuck (12-point clamp or 24-point clamp); Change the positioning and clamping scheme (change radial clamping to end face positioning and clamping); adjust process parameters (high-speed cutting, small back cutting depth, larger main deflection angle of the tool, smaller tool tip arc radius, reasonable selection of cutting fluid, etc.). After rough turning, additional tempering is performed to eliminate stress. The end face is then soft ground and the ring is finish turned.

 

♦ During the heat treatment process, the internal structure of the ring undergoes phase change, mainly from austenite to martensite, the density decreases, the volume expands, and tissue stress is generated; in addition, the ring is rapidly quenched and cooled from the expanded state at high temperature (generally 830-845°C for thin-walled products), generating thermal stress. When these two internal stresses exceed the yield limit of the material, the material will undergo permanent plastic deformation. Usually, die quenching is used to control deformation. Usually die quenching is used to control deformation. When there is no condition for die quenching and the outer diameter deformation of the ring after quenching is too large and exceeds the process requirements, the method of comprehensive shaping and then tempering is used to correct it so that it is controlled within the range of process requirements.

 

♦ The main thing in grinding is: choosing appropriate grinding equipment, processing methods and grinding process parameters. For example, the method with reinforcement ring is adopted, and the “one to two” structure is used for processing; the method of multiple fine adjustments of the machine tool is adopted for grinding the outer diameter surface; the tempering stabilization treatment is added during the process, etc., to ensure that the grinding quality of the ring meets the process requirements.

 

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