1、 Cause analysis

There are several common oil leakage phenomena in reducers, including oil seal leakage, seal cover popping out, and air vent leakage.

The main reasons for oil leakage of the shaft oil seal are:

1) Improper assembly process of oil seal;

2) Improper processing technology of the shaft;

3) Insufficient hardness of the shaft;

4) Improper selection of shaft tolerances;

5) Foreign objects enter the lip of the oil seal.

The main reasons for the sealing cover popping out are:

1) Excessive internal pressure in the gearbox;

2) Improper selection of sealing cover hole tolerance.

The main reasons for oil leakage in ventilators are:

1) The structure of the ventilator is unreasonable;

2) Improper selection of ventilator position;

3) Refueling exceeds the prescribed upper limit too much.

2、 Solution

The reducer has a compact structure and various installation forms. Without special requirements, special sealing structures such as dry wells and labyrinth seals are generally not used. Therefore, the main direction to prevent oil leakage is to improve assembly technology, processing technology, and local structural optimization. Our company has achieved excellent results in this area over the years, summarized as follows.

1) Optimize the assembly process of oil seals

The oil leakage caused by improper oil seal assembly process is mainly due to damage to the oil seal lip during assembly, skewed oil seal, and dry friction of the lip during operation (note: attention should also be paid when installing oil seals during maintenance). In the process of assembling oil seals, if simple fixtures are used, or even if the oil seal is directly installed without fixtures, the lip of the oil seal is easily damaged, and the damage will gradually expand during future operation, causing oil leakage; At the same time, if the oil seal is installed askew, the lip of the oil seal will not work properly, resulting in lubricant leakage.

 

In Figure 1, the pressure sleeve 3 and the fixed sleeve 4 are assembled into a T-shaped assembly. The head of the fixed sleeve 4 is designed with a suitable cone shape, and the outer surface roughness is the same or lower than that of the oil seal gear. When the oil seal slides on the pressure sleeve 3, it protects the lip of the oil seal. The friction surfaces are quenched at high frequency to improve wear resistance;

The contact part between the pressure sleeve 3 and the oil seal is designed as a stopper, with a diameter slightly smaller than the outer diameter of the oil seal. The height of the stopper is the distance between the oil seal and the housing after assembly. Adopting a large chamfer and corner guard design to protect the main and auxiliary lips of the oil seal from damage. When assembling, first install the fixed sleeve 4 into the hollow shaft 5, and apply an appropriate amount of lubricating grease on the outer surface of the same fixed sleeve 4; Applying an appropriate amount of lubricating grease in the space formed by the main and auxiliary lips of the oil seal to be assembled can lubricate the lips; Gently push the oil seal through the fixing sleeve 4 to the vicinity of the box hole, and then use the pressing sleeve 3 to install the oil seal. Due to the certain coordination between the pressing sleeve 3, fixing sleeve 4, and hollow shaft 5, the installed oil seal will not tilt. And the consistency of the oil seal position using this type of assembly is very good.

2) Improvement of the shaft

The main causes of oil leakage due to shaft factors are improper machining processes, insufficient hardness of the shaft, and improper selection of shaft tolerances. The improper processing technology of the shaft is mainly manifested in the unreasonable surface roughness and processing method. If the surface roughness value is too high, it will cause early wear of the oil seal lip; The surface roughness value should not be too low, otherwise it is prone to oil leakage. The surface roughness of the general oil seal gear is 0.8-2.5 μ m. The machining methods for shafts include precision grinding, diamond sandpaper polishing, lathe precision machining, ultra precision machining, and rolling finishing. According to relevant literature, precision grinding is a suitable machining method. During grinding, the grinding wheel must feed radially, and the machining marks are discontinuous, but perpendicular to the axis. This state is suitable for the contact part of the oil seal lip. The hardness requirement for the contact part between the shaft and the oil seal lip is between 50-60HRC. If the hardness is too low, the shaft is prone to wear; The hardness is too high and the economy is poor. The tolerance of the shaft is generally selected between d9 and h9. If the tolerance is too small, the pressure between the shaft and the oil seal lip is too low, which can easily cause leakage; If the tolerance is too large, the pressure between the shaft and the oil seal lip will be high, and wear will intensify. The temperature of the oil seal lip will be high, and the oil seal is prone to early failure.

3) Prevent foreign objects from entering the lip of the oil seal

In Figure 2, single lip oil seals 5 and 7 are installed face to face. Maintain a small gap between the inner diameter of cover plate 4 and the outer diameter of shaft 6 to prevent the entry of large particles and debris. Regularly injecting grease from oil cup 1 can lubricate the main lip of oil seal 5 and prevent debris from entering between the inner hole of cover plate 4 and shaft 6.

In Figure 3, the gap between the inner hole of cover plate 3 and shaft 4 is very small to prevent the entry of large particles and debris. Oil seal 5 is a double lip oil seal used to prevent debris from entering between cover plate 3 and shaft 4, while oil seal 7 is a single lip oil seal. Lubricating grease is injected between the two oil seals to lubricate the lip and block debris.

 

Figure 4 shows the high-speed shaft seal structure. When the speed of gear 4 is above 1500r/min, the lubricating oil thrown out by gear 4 has a high temperature and contains foreign objects. It hits the lip of oil seal 3 with a large pressure, causing significant damage to the oil seal lip and even the oil seal gear on the shaft. After installing oil slinger 2, the lubricating oil from gear 4 cannot directly strike the lip of oil seal 3, eliminating damage to the lip of the oil seal. After using the oil slinger, the failure of the high-speed shaft oil seal no longer occurs, confirming the protective effect of the oil slinger 2 on the oil seal 3.

The structures in Figures 2 and 3 are mainly designed to prevent foreign objects from damaging the oil seal lip. Figure 4 is a structure for protecting the high-speed shaft oil seal, and there is also a situation where particles and impurities are prone to accumulate at the bottom of the reducer. Once these foreign objects enter the oil seal lip, they will accelerate the wear of the oil seal lip and shaft, causing leakage. For the particle impurities inside the reducer, the improved design is to install magnetic steel around the oil seal, which is fixed on the oil drain plug to adsorb the metal impurities inside the reducer.

4) Prevent the sealing cover from popping

The main reason for the popping of the sealing cover is the excessive internal pressure of the reducer and the improper selection of the sealing cover hole tolerance. The tolerance of the sealing cover hole can be selected according to the range proposed by the sealing cover manufacturer to meet the requirements. Based on experience, H8 is more suitable.

The internal pressure of the gearbox is the main reason for the sealing cover to pop out, which is mainly caused by the pressure generated by the height of the lubricating oil and the heating of the gearbox, as well as the pressure generated by the poor exhaust of the ventilator (reminder: enterprises should check the holes of the ventilator/breathing cap to keep them unobstructed in daily life). Although the reason for poor exhaust of the ventilator can be solved by improving the structure of the ventilator, improving the anti popping ability of the sealing cover is the most effective way to solve the problem of sealing cover popping.

 

Figure 5 shows a sealing cover anti pop structure, with a stop groove as shown in the enlarged partial view of the box processing. Generally, the h value is taken as 0-3-0.7mm, and the a value is taken as about 1mm. According to experimental testing, the sealing cover with a diameter of less than 70mm can withstand a pressure of more than 4 atmospheres, while the design test without a stop groove showed that the sealing cover popped out at 1.5 atmospheres, and 50% of the sealing cover popped out at 2 atmospheres (test sealing cover diameter of 80mm, test quantity of 20 sets).

5) Prevent oil leakage at the ventilator

The main reasons for oil leakage in ventilators are unreasonable ventilator structure and improper selection of ventilator position. If the position of the ventilator is not selected properly, continuous oil leakage will occur at the ventilator, as shown in Figure 6.

 

When gear 3 rotates, the lubricating oil thrown out directly reaches the position of the ventilator and forms a certain oil pressure, causing leakage in the ventilator. When designing, the ventilator should avoid these positions. If it is impossible to avoid them, a baffle structure can be installed at the ventilator position during the box design.

The reducer is lubricated according to the standard before leaving the factory. During operation, the gear meshing and bearing friction generate heat, and the internal temperature of the reducer will continue to rise. Due to the constant volume of the reducer, the expansion of air will cause the internal pressure of the reducer to continue to rise. If the ventilator is not opened, excessive internal pressure will prevent the sealing cover from popping out and the oil seal lip from turning outward, resulting in oil leakage. When the reducer is in operation, it should be isolated from the outside to prevent water vapor and dust from entering, which can damage the sealing performance of the oil seal and the meshing performance of the gears. So the four major series of reducers require the ventilator to be closed during normal operation and open when the pressure reaches a certain value.

In the gearbox industry, there are many structures of ventilators, and commonly used ventilators are shown in Figure 7-9.

 

Figure 7 is a representative of a constant flow ventilator, with vent holes facing downwards, on the side, and in a maze style. Because it is a constant flow type, it does not meet the requirements of the four major series of reducers for ventilators; The ventilator in Figure 8 is blocked with a valve core during transportation, and when in use, the valve core is pulled out to become a general ventilator. During operation, water vapor, dust, and other substances are prone to enter the reducer; The ventilator shown in Figure 9 requires a certain amount of pressure inside the gearbox to open during use. This type of ventilator basically meets the requirements, but its sealing is unreliable during transportation. Our company has developed a ventilator through years of research, as shown in Figure 10.

 

Under the dual sealing effect of dust washer 7 and sealing ring 4, lubricating oil will not leak during transportation; When the reducer is running, remove the dust washer 7. Under the sealing effect of the sealing ring 4, foreign objects such as water vapor and dust cannot enter the reducer. When the pressure inside the reducer is greater than the pressure of the spring 2, the valve core 3 moves upward and the airflow channel is opened. Due to the high internal pressure of the reducer, the airflow rushes outward, so foreign objects such as water vapor and dust cannot enter the reducer. When the internal pressure of the reducer is lower than the set value, the valve core 3 moves downward and the ventilator enters a sealed state.