Several points about bearings in mechanical design2019-7-6
Today we talked about mechanical design, we must master a few key points of the bearing! Bearing is an important component in contemporary mechanical equipment. Its main function is to support the mechanical rotating body, reduce the friction coefficient during its movement, and ensure its rotation accuracy.
According to the friction properties of moving components, bearings can be divided into two categories: rolling bearings and sliding bearings. Among them, the rolling bearing has been standardized and serialized, but its radial size, vibration and noise are larger and the price is higher than that of the sliding bearing.
According to the direction of the load, the bearings can be divided into radial bearings (bearing radial loads, also known as radial bearings), thrust bearings (bearing axial loads) and radial thrust bearings (at the same time bearing radial and axial loads, Called radial thrust bearing).
1, sliding bearing
Sliding bearing: A bearing that works under sliding friction. The sliding bearing works smoothly, reliably and without noise. Under liquid lubrication conditions, the sliding surface is separated by the lubricating oil without direct contact, and the friction loss and surface wear can be greatly reduced. The oil film also has a certain vibration absorbing capability, but the starting friction resistance is large.
▲ sliding bearing
The part of the shaft that is supported by the bearing is called the journal, and the part that matches the journal is called the bearing. The layer of friction reducing material cast on its inner surface in order to improve the frictional properties of the surface of the bearing pad is referred to as a bearing lining. The material of the bearing bush and bearing bushing is collectively referred to as a plain bearing material. Sliding bearing applications are generally used under low-speed and heavy-duty conditions, or where it is difficult to maintain and fill lubricants.
Sliding bearings can be divided into radial (central) sliding bearings and thrust (axial) sliding bearings according to the direction in which they can bear the load.
1.1 Radial plain bearings
Sliding bearing with radial load. The part of the shaft supported by the bearing is called the journal. The part that matches the journal is called the bearing bush. The part that is made into the full cylindrical shape is called the part of the shaft set bearing bush. The upper part is called the bearing cover, and the lower part is called the bearing cover. The half is called the bearing housing. The cover and the seat are coupled by a stud, and the joint faces of the two are positioned by a stop or a pin, and spacers of different thicknesses can be placed to adjust the bearing clearance.
1.2 thrust sliding bearing
A plain bearing that receives axial thrust and limits axial movement of the shaft. The thrust bearings whose two friction surfaces are completely separated by the fluid film are divided into hydrodynamic thrust bearing and hydrostatic thrust bearing, which are suitable for high and medium speed operation. A thrust bearing whose two friction surfaces cannot be completely separated by a fluid film works under boundary lubrication and is only suitable for low speed operation.
2, rolling bearings
A rolling bearing is a precision mechanical component that changes the sliding friction between the running shaft and the shaft seat into rolling friction, thereby reducing friction loss. Rolling bearings generally consist of an inner ring, an outer ring, a rolling element and a cage.
The inner ring is matched with the shaft and rotates together with the shaft; the outer ring acts to cooperate with the bearing seat to support the rolling; the rolling body distributes the rolling body evenly between the inner ring and the outer ring by means of the cage. The shape size and quantity directly affect the service performance and life of the rolling bearing; the cage can evenly distribute the rolling elements, prevent the rolling elements from falling off, and guide the rolling elements to rotate to lubricate.
2.1 Basic types of rolling bearings
2.2 Code of rolling bearing
GB/T272-93 specifies the composition and representation of the rolling bearing code. The rolling bearing code consists of a pre-code, a basic code and a post-code, which represent the content and the order of the arrangement, as shown in the table below.
2.3 Type selection of rolling bearings
The size, 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 load and medium load; roller bearings should be used for heavy load or impact load.
(2) Load direction: For pure radial load, deep groove ball bearings, cylindrical roller bearings or needle bearings can be used. Thrust bearings are available for pure axial loads. When there is radial load and axial load, if the axial load is not too large, deep groove ball bearings or angular contact ball bearings with small contact angle and tapered roller bearings can be selected; if the axial load is large Two types of bearings with large contact angles can be selected; if the axial load is large and the radial load is small, the thrust angular contact bearing can be selected, and the support structure of the radial bearing and the thrust bearing can also be used.
2.4 Calculation criteria for rolling bearings
When determining the bearing size, the necessary calculations should be made for the main failure mode of the bearing. For the normally operated bearings, the main failure mode is fatigue pitting, and the life calculation should be carried out according to the basic dynamic load rating. For bearings that do not rotate, swing or have very low speed (n ≤ 10 r / min), the main failure mode is plastic deformation, so the strength should be calculated according to the rated static load.
3, the precautions for the use of bearings
3.1 Lubrication of bearings
Lubricating oil must be selected according to the season and region. Lubricating oil (fat) should be added regularly. The quantity and quality of the oil in the oil bath or pressure lubrication system oil pool should be checked, supplemented and replaced in time. The pressure lubrication system should be fully supplied with oil. If the oil pressure is abnormal, it should be inspected and treated in time.
3.2 Bearing working condition
Bearing damage is mainly identified by the abnormal working conditions. Unstable operation and abnormal running noise may be caused by excessive wear of the sliding bearing, melting of the alloy, loss of the alloy, or wear of the rolling surface of the rolling bearing, resulting in excessive radial clearance. Abnormal operation and abnormal temperature rise may result in dry friction of the sliding bearing alloy, alloy scratch, poor contact between the bearing bush and the bearing housing, or damage to the rolling surface of the rolling bearing. If the metal is peeled off, cracked, ablated (ie, high temperature annealing, the color is purple-black), the bearing is too tight, and the lubrication is poor, etc., it should be checked and treated in time.
3.3 Check the integrity of the bearing
When performing regular maintenance on the machine, carefully check the integrity of the bearing. If the bearing bush is damaged or the clearance exceeds the allowable limit, it should be re-fitted; if the rolling bearing is damaged, the loose ring should be replaced; the oil circuit of the lubrication system should be cleaned.