Today we will talk about several key points in mechanical design that we must master about bearings! Bearings are an important component in contemporary mechanical equipment, whose main function is to support the rotating body of the machinery, reduce the friction coefficient during its movement, and ensure its rotational accuracy.

According to the different friction properties of moving components, bearings can be divided into two categories: rolling bearings and sliding bearings. Rolling bearings have been standardized and serialized, but compared to sliding bearings, they have larger radial dimensions, vibration, and noise, and are also more expensive.

According to the direction of load bearing, bearings can be divided into radial bearings (bearing radial loads, also known as radial bearings), thrust bearings (bearing axial loads), and radial thrust bearings (bearing both radial and axial loads, also known as radial thrust bearings).

1. Sliding bearings

Sliding bearing: a bearing that operates under sliding friction. Sliding bearings work smoothly, reliably, and without noise. Under liquid lubrication conditions, the sliding surface is separated by lubricating oil without direct contact, which can greatly reduce friction loss and surface wear. The oil film also has a certain vibration absorption ability, but the starting friction resistance is relatively high.

▲ Sliding bearings

The part of the shaft supported by bearings is called the journal, and the parts that match the journal are called bearing shells. The anti friction material layer cast on the inner surface of the bearing shell to improve its friction properties is called the bearing liner. The materials of bearing shells and bearing liners are collectively referred to as sliding bearing materials. The application of sliding bearings is generally in low-speed and heavy-duty working conditions, or in operating parts where maintenance and lubrication are difficult.

Sliding bearings can be divided into two types based on their ability to withstand loads: radial (centripetal) sliding bearings and thrust (axial) sliding bearings.

1.1 Radial sliding bearings

Sliding bearings that withstand radial loads. The part of the shaft supported by bearings is called the journal, the parts that match the journal are called the bearing shells, and the parts that make the entire cylindrical bearing shells are called the shell parts of the shaft sleeve bearing shells. The upper half is called the bearing cover, and the lower half is called the bearing seat. The cover and seat are connected by bolts, and the joint surface of the two is located by a stop or a pin. Different thicknesses of washers can be placed to adjust the bearing clearance.

1.2 Thrust sliding bearings

A sliding bearing that withstands axial thrust and restricts axial movement of the shaft. The thrust bearings with two friction surfaces completely separated by a fluid film are divided into fluid dynamic pressure thrust bearings and fluid static pressure thrust bearings, suitable for high and medium speed operation. A thrust bearing with two friction surfaces that cannot be completely separated by a fluid film operates under boundary lubrication and is only suitable for low-speed operation.

2. Rolling bearings

Rolling bearings are precision mechanical components that convert the sliding friction between the running shaft and the shaft seat into rolling friction, thereby reducing friction losses. Rolling bearings generally consist of four parts: inner ring, outer ring, rolling element, and retainer.

The function of the inner ring is to match and rotate with the shaft; The function of the outer ring is to cooperate with the bearing seat and provide support; Rolling elements are evenly distributed between the inner and outer rings using a cage, and their shape, size, and quantity directly affect the performance and service life of rolling bearings; The cage can evenly distribute the rolling elements, prevent them from falling off, and guide them to rotate for lubrication.

2.1 Basic types of rolling bearings

2.2 Code for rolling bearings

GB/T272-93 specifies the composition and representation method of rolling bearing codes. The rolling bearing code consists of pre code, basic code, and post code, which represent the content and order of arrangement, as shown in the table below.

2.3 Type selection of rolling bearings

The magnitude, direction, and nature of the load on the bearing are the main criteria for selecting the type of bearing.

(1) Load size and nature: Ball bearings should be selected for light and medium loads; When subjected to heavy loads or impact loads, roller bearings should be selected.

(2) Load direction: For pure radial loads, deep groove ball bearings, cylindrical roller bearings, or needle roller bearings can be selected. When subjected to pure axial loads, thrust bearings can be selected. When there is both radial and axial load, if the axial load is not too large, deep groove ball bearings or angular contact ball bearings with smaller contact angles or tapered roller bearings can be selected; If the axial load is large, these two types of bearings with larger contact angles can be selected; If the axial load is large and the radial load is small, thrust angle contact bearings can be used, or a support structure combining radial bearings and thrust bearings can be used.

2.4 Calculation criteria for rolling bearings

When determining the size of bearings, necessary calculations should be made for the main failure modes of the bearings. For bearings in general operation, the main form of failure is fatigue pitting, and the service life should be calculated based on the basic rated dynamic load. For bearings that do not rotate, swing or have extremely low speeds (n ≤ 10 r/min), the main form of failure is plastic deformation, so the strength calculation should be based on the rated static load.

       3. Precautions for using bearings

      3.1 Lubrication of bearings

      Lubricating oil must be selected according to season and region according to regulations. Lubricating oil (grease) should be added regularly. The quantity and quality of lubricating oil in the oil bath or pressure lubrication system oil pool should be checked, supplemented, and replaced in a timely manner. The pressure lubrication system should provide sufficient oil supply. If the oil pressure is abnormal, it should be checked and dealt with in a timely manner.

     3.2 Working condition of bearings

     Bearing damage is mainly identified by abnormal working conditions. Unsmooth operation and abnormal operating noise may be caused by excessive wear of sliding bearings, alloy melting, alloy detachment, or wear of rolling surfaces of rolling bearings, resulting in excessive radial clearance. Heavy operation and abnormal temperature rise may be caused by sliding bearing alloy detachment, alloy scratches, poor contact between bearing shells and bearing seats, resulting in dry friction, etc; Or the rolling surface of the rolling bearing is damaged. If there are reasons such as metal peeling, cracking, erosion (i.e. high-temperature annealing with a purple black color), tight bearings, and poor lubrication, timely inspection and handling should be carried out.

    3.3 Check the integrity of bearings

    When conducting regular maintenance on the machine, the integrity of the bearings should be carefully checked. If the bearing is damaged or the clearance exceeds the allowable limit, it should be repaired again; The rolling bearing is damaged and loose, and should be replaced; The oil circuit of the lubrication system should be cleaned and unobstructed.