It can be seen from the examples of oil-lubricated motors failing due to rolling bearings that most of the failures are caused by insufficient lubricating oil viscosity. The lower the viscosity of the lubricating oil, the worse the bearing capacity of the oil film. The oil film is easy to rupture. The metal materials on the relative moving surface of the rolling bearing have direct contact. The greater the friction, the more serious the wear, the life of the rolling bearing is greatly shortened, and the bearing burns and breakage accidents .
However, if the viscosity of the lubricating oil is too high, the frictional resistance will increase, and the heat generated by stirring the lubricating oil will also increase the energy consumption of the system. On the other hand, rolling bearings that work under special conditions such as high speed, high load, and high temperature may have special requirements for rust prevention, oxidation resistance, wear resistance, and improvement of lubricant adsorption capacity. Therefore, the choice of lubricating oil is mainly to determine the viscosity grade of the lubricating oil, as well as the type of additives used or other lubricating oils with constant additives.
The general selection principles of lubricants are as follows:
1. Working temperature:
Working temperature will affect the viscosity change and lubrication effect of lubricant. Therefore, when the operating temperature is low, a lubricant with a low viscosity must be used. When the working temperature is high, a lubricant with high viscosity or with appropriate additives must be used. When the ambient temperature is different, the viscosity of the selected lubricant must also be changed. For example, if you lubricate the same bearing, you must choose a lubricant with a viscosity lower than that of southern or summer in winter. If the working temperature changes frequently, a lubricant with good viscosity-temperature characteristics must be used. That is to say, in order to keep the oil film thickness constant within a certain range, the viscosity of the lubricant used with the increase or decrease of the operating temperature must be considered.
2. Movement speed:
The higher the speed, the greater the resistance to movement. In order to avoid excessive heat generation, a lubricant with a lower viscosity must be selected. On the contrary, in the case of low speed, in order to improve the bearing capacity, a lubricant with a high viscosity must be used.
3. Action characteristics:
Movements include shock, vibration, frequent loads, speed changes, and starting. Parking, frequent reversal, back-and-forth or intermittent movement is not good for the formation of oil film, so a high-viscosity lubricant must be used. Sometimes, in order to ensure reliable lubrication, we prefer grease or even solid lubricants.
4. Work load:
The higher the load on the rolling bearing, the higher the viscosity of the lubricant. The lubricant must have good meteor and extreme pressure properties to avoid protruding from the friction pair or direct contact between metals.
5. Structural features:
The smaller the radial clearance of the rolling bearing, the higher the machining accuracy of the friction surface, and the lower the viscosity of the lubricant.
6. Environmental conditions:
If the bearing is working under humid, corrosive gas, low temperature, dust and strong radiation conditions, the lubricant is prone to deteriorate into pollution. It is necessary to choose a lubricant with strong water, wear resistance, corrosion resistance, cold resistance and radiation resistance. In the case of water splash, emulsion spray, humid air, or severe dust and debris, it is generally recommended to use lubricating oil instead of grease.
7. Bearing accuracy:
When the friction surface of the bearing movement is rough, oil with a higher viscosity is usually used, and poor contact causes greater local pressure. When the accuracy of the movement friction surface is higher, a lubricant with a lower viscosity must be used to reduce unnecessary energy loss And temperature rise.
8. Bearing hardness:
When the friction surface hardness of the bearing is low, a lubricant with a high viscosity must be used, and there must be sufficient oil. Conversely, the viscosity of lubricating oil may decrease. In order to give full play to the role of lubricants, it is important to choose a lubrication method suitable for use conditions and a good lubricant. The grease injected into the bearing chamber when the motor is assembled must be clean. Lubricating grease is not clean and mixed with impurities, especially hard impurities, which can cause abnormal bearing wear and easily cause bearing failure. The amount of grease to be charged must be appropriate. Adding too little will cause intermittent oil shortage and heat generation, which will lead to bearing failure. Add too much, grease stirring heat will cause bearing failure, thereby crushing and contaminating the inside of the motor.
The motor grease charging must be injected according to the design amount and allocated to the bearing cavity space reasonably. The bearing cavity space is composed of three parts between the inside of the bearing housing and the outer bearing cover. The grease on the bearing cavity and the bearing lap surface must be dispersed as much as possible, and the filling space is determined by the speed. During the operation of the motor, the deterioration and loss of grease should be avoided to avoid bearing failure, and the special grease for the bearing should be replenished regularly in order to be designed to use the oil return pan. It is strictly forbidden to mix different grades of grease to prevent Greek deterioration and loss of lubrication performance, which may lead to bearing failure.
