Heating power analysis and cooling mode selection of reducer

Update:10 Sep 2020

During the normal operation of the reducer, due to the loss of efficiency, part of the input power will be converted into heat energy, which makes the internal temperature of the reducer rise. If the temperature is too high, the internal parts will be damaged. Therefore, it is necessary to meet the requirements of thermal power in the early selection and verification of reducer.
The thermal power of Pt continuous transmission does not exceed the actual power when the calculated oil temperature is + 90 ° C. PTH is the rated thermal power, PTH = f 1 × F2 × F3, F 1 = altitude coefficient; F2 = 1.07 for reducer with torque arm, F2 = 1.0 for foundation mounting reducer; F3 = 1.10 - pressure lubrication; F4 = 1.0 - splash and oil immersion lubrication.

It is mainly the heat dissipation area, thermal conductivity and efficiency of the reducer. The following will be from the design, lubrication, manufacturing and cooling device to analyze the measures to improve the thermal power.The gear structure parameters such as modulus, index circle diameter, tooth width, modification coefficient, transmission ratio, bearing structure and gear lubrication state all have certain influence on the efficiency of the reducer. Therefore, in order to improve the efficiency of reducer design, we should comprehensively consider the factors affecting the transmission efficiency, and take corresponding technical measures for different types of reducer.

Design of high speed light load reducer
The power loss of reducer under high speed and light load is mainly no-load loss. The following measures can be taken to reduce the loss and improve the bearing support conditions:
Light bearings of small diameter series are selected, and the bearings are lubricated with thin oil;
The gear tooth width and pitch circle diameter should be reduced as far as possible to reduce the pitch line speed;
The lubricating oil with low viscosity is used to reduce the loss of stirring oil;
Under the condition of sufficient lubrication, the oil supply of gearbox is reduced to reduce the power loss of lubricating oil acceleration.

Design of heavy load reducer
The power loss of low-speed heavy-duty reducer is mainly sliding friction loss, and the loss of each part of high-speed heavy-duty reducer accounts for a certain proportion
The gear with smaller tooth width and larger diameter should be used as far as possible to improve the lubrication condition and reduce the oil mixing loss;
When the bending strength is allowed, the gear with smaller modulus is selected to reduce the length of contact line and the sliding friction loss;
Proper modification and reasonable distribution of modification coefficient can reduce the relative sliding speed between tooth surfaces and reduce the sliding friction loss;
The use of lubricating oil with high viscosity is conducive to the formation of dynamic pressure oil film and the improvement of lubrication state.
If possible, oil mist lubrication can be used to reduce oil mixing loss.

Manufacturing analysis
The machining accuracy of gears has a great influence on the friction loss. According to different machining methods, many scholars have made experimental comparison and found that the friction loss of gears with high accuracy is small. However, there is still a lack of research on whether the main influencing factors are tooth profile error, pitch error or surface roughness.
The higher the gear surface roughness is, the greater the friction coefficient is, and the greater the friction power loss is. If the load is constant, when the roughness of the tooth surface is large, the tooth surface will contact directly and the gear will be in boundary lubrication, and the friction coefficient will be large. With the improvement of lubrication conditions, when the oil film thickness of the tooth surface is close to the surface roughness, the friction coefficient changes greatly in this range, but it is smaller than that of the boundary lubrication. When the surface roughness is very small and the two moving surfaces are completely separated by the lubricating oil film, the friction coefficient depends on the internal friction resistance of the fluid. With the increase of the sliding speed, the internal friction resistance of the fluid increases.
It can be seen that the tooth surface roughness can affect the lubrication state of gears, and it also determines the friction coefficient.
All kinds of machining methods, heat treatment and special surface treatment of gears will have a decisive impact on the surface morphology and microstructure of gears. This feature determines the formation of lubricating oil film thickness during gear operation, which has a significant impact on the friction coefficient between tooth surfaces, and thus determines the size of friction loss. Therefore, these two aspects are also effective factors affecting gear efficiency.

Analysis and comparison of various cooling methods
When Pt > PTH, it is necessary to add external cooling mode to cool down the reducer. The common cooling methods include fan, water cooling coil and external lubrication.
1. Fan cooling is simple and easy to operate. However, the defect of fan cooling method is that it is not suitable in some cases. For example, when cooling the gearbox in underground coal mine, the use of fan will seriously pollute the surrounding working environment and mechanical equipment, and it uses radiation heat transfer, so the heat loss can be limited.
2. The water cooling coil is a pipe which is installed at the bottom of the gearbox and connected with cooling water, which can effectively increase the thermal conductivity drop. The results show that the cooling effect is directly related to the flow rate and flow rate of cooling water. However, due to the limitation of the installation space in the reducer box, the coil length of the disk tube can not be very long, so its heat dissipation effect is limited. The advantages are simple structure, convenient installation, good heat dissipation effect and easy serialization.
3. External lubrication means that a water-cooled or air-cooled device is set outside the reducer to cool the lubricating oil in the reducer. The cooled lubricating oil is lubricated after returning to the reducer. This cooling is mainly used for cooling under high power or when the ambient temperature is high. The advantages are good cooling effect and long service life of lubricating oil. The disadvantage lies in the increase of control points and the increase of accident probability.
To sum up, the installation of heat sink is an effective way to increase the heat dissipation coefficient. In the selection of heat sink, economy and heat dissipation effect should be considered comprehensively.

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