Precision bearing heat treatment process

The rapid development of the times requires the production of equipment to become more and more refined. In order to ensure the production of equipment, bearings have played a very important role. Bearings are an important part of contemporary machinery and equipment. Its main function is to support the mechanical rotating body, reduce the friction coefficient during its movement, and ensure its rotation accuracy.

When it comes to refinement, the precision bearings in the bearing have to be mentioned. According to the ISO classification standards, precision bearings are divided into: P0, P6, P5, P4, P2. The grades increase sequentially, where P0 is ordinary precision, and the other grades are all Precision level. Of course, different classification standards, the same type of bearings, their classification methods are different, but the meaning is the same. The performance of the precision bearing requires the rotating body to have high runout accuracy, high-speed rotation and small friction and friction changes.

Before entering the topic, do you know how to distinguish between precision bearings and ordinary bearings? Next, let’s give you some answers in this regard.

The distinction between precision bearings and ordinary bearings:

1. The size requirements are different, and the dimensional deviation (inner diameter, outer diameter, ellipse, etc.) of products with high accuracy grades is smaller than the value required by products with low accuracy grades;

2. The required values ​​of rotation accuracy are different. The rotation accuracy of products with high accuracy grades (inner radial runout, outer radial runout, end face runout, etc.) is stricter than the required values ​​for products with low accuracy levels;

3. The required values ​​of surface shape and surface quality are different. The surface shape and surface quality (surface roughness of raceway or groove, circular deviation, groove deviation, etc.) of products with high precision grade are lower than the precision grade. The value of the product requirements must be strict;

4. The material properties of products with particularly high accuracy grades are more advantageous than those of products with general accuracy grades. Now that we can distinguish between precision bearings and ordinary bearings, let’s quickly get into the topic and take a look at the heat treatment process of precision bearings.

Heat treatment process of precision bearings

Preliminary heat treatment:

1. Normalizing: Normalizing, also known as normalization, is to heat the workpiece to Ac3 (Ac refers to the final temperature at which all free ferrite is transformed into austenite during heating, generally from 727*C to 912°C ) Or Acm (Acm is the critical temperature line for complete austenitization of hypereutectoid steel in actual heating) 30~50°C above, after holding for a period of time, take it out of the furnace or spray water, spray or blow to cool Metal heat treatment process. The expectation is to make the grain refinement and carbide distribution uniform. Parameters: The normalizing temperature of different components is slightly different.

2. Spheroidizing annealing: Its main purpose is to reduce hardness, improve machinability, and prepare for quenching in the future. This process is conducive to plastic processing and cutting, and can also improve mechanical toughness.

Final heat treatment:

1. Quenching: Quenching is to heat up the raw material to the austenitizing temperature, and keep it for a period of time as needed. Then it is quickly cooled to obtain a structure with higher hardness and strength than the original, so as to enhance the mechanical properties of the material and increase the durability of the material. Such as 45 steel, the original structure is ferrite + pearlite, martensite is obtained after quenching, and the strength and hardness are doubled.

2. Cold treatment: The retained austenite that has not been fully transformed during the quenching process continues to transform to martensite, which can not only improve the dimensional stability of the parts, but also slightly increase the hardness and rust resistance of the parts. But it will reduce the toughness of bearing steel. Sometimes, the principle that the martensite is larger than the volume can be used to save the scrapped products due to size reduction with cold treatment.

3. Tempering: parts must be tempered after quenching (or cold treatment). Tempering can reduce and stabilize the residual stress, stabilize the structure, avoid cracks and deformation, appropriately reduce the hardness and greatly improve the toughness, so that the parts can finally obtain the mechanical properties and dimensional stability of the comprehensive strength and toughness.

4. Additional tempering (stabilization treatment): The role of additional tempering is to eliminate grinding stress and further stabilize the organization, so as to improve the dimensional stability of the parts. Every step in the heat treatment of precision bearings is of the utmost importance. In order to ensure the precision of the bearings, every detail must be done without loss. The next thing I have to mention is the installation accuracy verification method of precision bearings.

Precision bearing installation accuracy verification method

After installing the angular contact ball bearing into the main shaft, the installation accuracy verification sequence is as follows (take an ordinary lathe with a shaft diameter of 60 to 100 mm as an example):

(1) Measure the size of the shaft and the bearing seat hole to determine the matching accuracy of the bearing. The matching requirements are as follows:

The inner ring and the shaft adopt an interference fit, and the interference is 0 ~ +4μm (0 under light load and high precision); the outer ring and the bearing seat hole adopt a clearance fit, and the clearance is 0 ~ + 6μm (but the bearing at the free end When using angular contact ball bearings, the gap can also be increased); the roundness error of the shaft and the seat hole surface is below 2μm, the end face parallelism of the spacer used in the bearing is below 2μm, and the runout of the inner end of the shaft shoulder facing the outer end face is less than 2μm; The runout of the bearing seat hole shoulder to the axis is less than 4μm: the runout of the inner end of the front cover of the injection shaft facing the axis is less than 4μm.

(2) Installation of the front bearing at the fixed end on the shaft

Thoroughly clean the bearing with clean cleaning kerosene. For grease lubrication, first pour an organic solvent containing 3% ~ 5% grease into the bearing for degreasing and cleaning, and then use a grease gun to fill the bearing with the specified grease (accounting for 10% ~ 15% of the bearing space volume); heat the bearing to increase the temperature by 20 ~ 30 °C, and install the bearing into the shaft end with a hydraulic machine; press the adapter sleeve on the shaft and press the bearing end face with appropriate pressure to make it shaft Orientation; Wrap the strap of the spring balance on the outer ring of the bearing, and use the method to measure the starting torque.
Whether the preload specified by the method check has a large change (even if the bearing is correct, the preload may also change due to the deformation of the fit or cage).

(3) Install the bearing shaft assembly into the seat hole

Heat the seat hole to raise the temperature by 20 ~ 30 °C, and use continuous gentle pressure to install the bearing shaft assembly into the seat hole; adjust the front cover so that the fastening amount of the front cover is 0.02 ~ 0.05 μm, and the outer end surface of the bearing seat is used as Benchmark, the head of the dial indicator is against the surface of the journal, and the shaft is rotated to measure its runout, and the error is required to be less than 10μm; the dial indicator is positioned on the shaft, the head of the indicator is against the inner surface of the rear seat hole, and the shaft is rotated To measure the coaxiality of the front and rear housing holes of the bearing housing.

(4) The free end bearing is selectively placed in a position that may offset the deviation, and installed at the rear support position of the bearing seat to offset the roundness deviation and coaxiality deviation between each other as much as possible.

Of course, even if everyone has mastered the installation accuracy verification method of precision bearings, for some high-precision imported bearings, the configuration requirements are also very high. Regarding the configuration of precision bearings, have you considered woolen cloth?

Precision bearing configuration problem

The traditional application field of precision bearings is machine tool spindles. According to different processes, machine tool spindles have different requirements. Generally speaking, lathe spindles are used to cut metals at lower speeds and larger cutting loads. This type of spindle usually transmits drive torque through pulleys or gears. This means that the load on the drive end of the spindle is also quite large.

Such applications do not require too much speed, and the more important parameters are stiffness and load-carrying capacity. A very common method is to install a row cylindrical roller bearing and a double row angular contact thrust ball bearing on the working end of the main shaft, and at the same time use a double row cylindrical roller bearing on the driving end of the main shaft. This configuration can ensure a long working life and rigidity, thereby producing high-quality workpieces. And, from the kinematics point of view, the bearing can work stably, because the two types of bearings (radial and axial) respectively carry the load applied to the main shaft (in fact, in order to prevent the angular contact thrust ball bearing from carrying the radial Load, foreign outer diameter has a special tolerance to ensure that it will never contact the bearing seat).

When designing these types of spindles (this usually applies when the load is heavy), a rule of thumb about where the bearing is located on the shaft is that the distance between the centers of the front and rear supports is 3-3.5 times the inner diameter of the bearing.

When higher speeds are required (ie, high-speed machining centers or internal grinding), different bearing solutions need to be found. Obviously, in these cases a little sacrifice is needed in terms of stiffness and load-bearing capacity. High-speed applications usually use direct drive spindles (so-called motorized spindles) that are directly connected to motors and/or couplings.

The above is all about the heat treatment of precision bearings. Different mechanical equipment has different requirements for precision bearings. Bearings, as an important part of contemporary mechanical equipment, only need to be better developed to make the operation of mechanical equipment smoother. The so-called details determine success or failure. Only paying attention to details can make production more efficient, and paying attention to details can make economic development better.