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

According to the different frictional 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 bearing load, 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 bearing

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

▲ Sliding bearing

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

Sliding bearings can be divided into radial (centripetal) sliding bearings and thrust (axial) sliding bearings according to the direction in which they can withstand loads.

1.1 Radial sliding bearings

Sliding bearings that withstand radial loads. The part of the shaft supported by the bearings is called the journal, the parts that match the journal are called the bearing shells, and the parts that make up 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 pin, and different thicknesses of shims can be placed to adjust the bearing clearance.

1.2 Thrust sliding bearing

A sliding bearing that bears axial thrust and restricts axial movement of the shaft. Thrust bearings with two friction surfaces completely separated by a fluid film are divided into hydrodynamic thrust bearings and hydrostatic thrust bearings, suitable for high and medium speed operation. Thrust bearings with two friction surfaces that cannot be completely separated by a fluid film operate under boundary lubrication and are only suitable for low-speed operation.

2. Rolling bearings

Rolling bearing is a precision mechanical component that converts the sliding friction between the running shaft and the shaft seat into rolling friction, thereby reducing friction loss. Rolling bearings are generally composed of four parts: inner ring, outer ring, rolling element, and cage.

The function of the inner ring is to match and rotate with the shaft; The outer ring is used to cooperate with the bearing seat and play a supporting role; 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 lifespan 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 basis for selecting the type of bearing.

(1) Load size and nature: Ball bearings should be used for light and medium loads; When under heavy load or impact load, 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 under pure axial load, 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 or tapered roller bearings with smaller contact angles 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 angular contact bearings can be used, or a support structure combining radial and radial 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 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

Lubricants must be selected according to season and region according to regulations. Regular lubrication oil (grease) should be added. 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 have sufficient oil supply, and if the oil pressure is abnormal, it should be promptly checked and handled.

3.2 Working condition of bearings

Bearing damage is mainly identified by abnormal working conditions. Unsmooth operation and abnormal operation 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 result in dry friction caused by sliding bearing alloy detachment, alloy scratches, poor contact between bearing shells and bearing seats, etc; Or the rolling surface of the rolling bearing is damaged. Timely inspection and handling should be carried out for reasons such as metal peeling, cracking, erosion (i.e. high-temperature annealing, purple black color), bearing tightness, and poor lubrication.

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 bush is damaged or the clearance exceeds the allowable limit, it should be repaired again; The rolling bearing is damaged or loose and should be replaced; The oil circuit of the lubrication system should be cleaned and unobstructed.