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    A detailed explanation of the selection and application of ball bearings in electric tool motors

    2020-11-12 14:55:29 11

    Abstract

    The selection and application of ball bearings in electric tool motors are the basic skills required by engineering and technical personnel in the design, manufacture, testing, use, and maintenance of the motor.

    With the maturity of the electric tool motor and its control technology, the application becomes more extensive, and the amount and scope are also greatly expanded. In the course of use, people encountered various problems, these problems mainly include electrical problems and mechanical problems. Among mechanical problems, bearing problems account for the main part. If you carefully analyze these bearing problems, you will find that very few of them are the quality of the bearing itself. According to statistics from relevant manufacturers, bearing quality problems account for no more than 5% of mechanical failures. The remaining 95% are application problems of motor bearings. As an electrical engineer, the application of bearings is an important issue that inevitably needs to be faced and resolved.

    Bearings are the key components of mechanical action actuators. At the same time, many design factors of bearings as standard parts have become standard options. The bearing application technology is to realize the correct selection of the bearing that has been designed, check calculation, process use, operation and maintenance, operation monitoring, and analysis after bearing failure.

    Key Factors: Accuracy class, speed, rated dynamic load, life

    Reference Standard: GB/T 271-2017 ,  GB/T6391-2010 ,  ISO 281 2007


    1. Foreword

    Refer to the type of ball bearing (GB/T 271-2017 ). Deep groove ball bearings are the most frequently used bearing type in electric tool motors, and a small amount of thrust ball bearings. The life cycle of the motor starts with the selection and focuses on the failure analysis of the bearing. The correct arrangement of the rolling bearing in the motor structure is the most critical step for the bearing to fully play its role. Usually, the bearing in the motor structure will be arranging intertwined with the selection of the bearing, repeated iterations, and mutual verification, Finally, it becomes the starting point for the bearing to be put into use, and it is the origin of drawing design for all bearing problems that may occur later.


    2. Rolling bearing arrangement in electric tool motor

    2.1 Bearings in the motor structure and their functions

    Common power tool structures include motor rotor (shaft, rotor core, winding), stator (stator core, stator winding, junction box, end cover, bearing cover, etc.), and connectors (bearing, seal, carbon brush, etc.), etc. Several major components.

    Electric power tools

    Among all the structural components of the motor, some bear axial and radial loads but do not have relative internal movement; some of them will move internally but do not bear axial and radial loads. Only the bearing bears the shaft and radial load while moving internally (the relative movement of the inner ring, outer ring, and rolling elements.) Therefore, the bearing itself is a sensitive part of the motor structure. This also determines the importance of bearing arrangements in industrial motors is more prominent.

    2.2 The basic steps of the arrangement of rolling bearings in the motor

    The arrangement of rolling bearings in a power tool motor refers to the process of how engineers place different types of bearings into the system in the shaft system when designing the structure of the power tool motor. To achieve the correct motor bearing arrangement, you need:

    Step 1: understand the working conditions of rolling bearings in the tool. These include:

    • Horizontal motor or vertical motor: There are different types of electric tools, such as an electric drill, electric saw, electric hammer, electric hammer, etc. Confirm that the installation form of the motor is vertical and horizontal. For the bearing, the load direction will be different. For horizontal motors, gravity is relatively a radial load, while for vertical motors, gravity is relatively an axial load. This will largely affect the choice of bearing type and the arrangement of the bearing in the motor.

    • The required speed of the motor: The speed requirements of the motor will affect the bearing size and type, as well as the way configured in the motor.

    • Bearing dynamic load calculation: According to the motor speed, rated power/torque, and other parameters, refer to (GB/T6391-2010/ISO 281 2007) to calculate the dynamic load of the ball bearing, and select the appropriate ball bearing size, accuracy class, etc.

    Step 2: Other requirements: such axial movement requirements, vibration, noise, dustproof, differences in chassis materials, motor tilt, etc.

    In short, before embarking on the design and selection of electric tool motor bearings, it is necessary to have a comprehensive understanding of the actual working conditions of the motor to ensure reasonable and reliable selection.

    Step 3: Determine the bearing type. According to the first two steps, consider the bearing load and the selected fixed end and floating end shaft system structure, and then select the appropriate bearing type for the fixed end and the floating end according to the bearing load characteristics.


    3.1 Double deep groove ball bearing structure

    The double deep groove ball bearing structure is the most common shafting structure in industrial motors, and its main shafting support structure consists of two deep groove ball bearings. Two deep groove ball bearings are supported together.

    As shown below:

    ball bearings in electric tool motors

    The shaft extension end bearing in the figure is a locating end bearing, and the non-shaft extension end bearing is a floating end bearing. Bearings at both ends bear the radial load on the shafting, while the positioning end bearing (located at the shaft extension end in this structure) bears the axial load of the shafting.

    Generally, the motor bearing arrangement of this structure is suitable for occasions where the axial and radial load of the motor is not large. The most common is the structure of a micro motor with a coupling connected to a load.

    Mainly used in electric drills, electric circular saws, electric hammers, electric pick tools;

    4. Summary

    This article introduces the basic concepts, method steps, and typical arrangements of bearings in motors. Bearing arrangement is a comprehensive knowledge point application, requiring engineers to master the basic performance of bearings as a basis. At the same time, it can be used dynamically according to actual working conditions. After the bearing is configured, it is necessary to perform various check calculations on the bearing according to the load condition, and then determine the size, and then determine the tolerance fit in the drawing. Coupled with lubrication selection, oil circuit design, etc., can it be regarded as the completion of the drawing work of bearing selection.

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