Principle and analysis of lubricating oil channel layout in electric drive axle reducer

With the increasingly accelerated process of automobile electrification, the electrification of commercial light trucks has ushered in a market explosion. The industry generally adopts parallel shaft electric drive axles in which the motor is directly connected to the reducer. The development of reducer assemblies as core components presents a competitive situation of flourishing. There are compact three-stage reduction products with complex processes, two-stage reduction products with good versatility, aluminum alloy shell reducers and even two-speed reducers. The lubrication conditions of different bearings in each reducer at high and low speeds are very different.

Considering that if there is insufficient lubricating oil when the reducer is moving forward and backward at low speed, even a very short period of high temperature will cause irreversible damage to the bearings. Therefore, it is particularly important to explain the layout principle and analysis practice of the lubricating oil channels in the reducer when moving forward and backward at low speed.

Reducer use conditions and lubrication methods

The lubrication inside the reducer must first consider the normal operating temperature of the friction pairs such as bearings, gears, oil seals and gear oil, and the second consideration is to consider the reasonable lubrication method that adapts to different production processes.

For rubber oil seals, the operating temperature range of nitrile rubber (NBR) is generally required to be -40 to +120°C, and it can stably work within 100°C and work at 120°C for a short time. For fluororubber (FKM), it can stably work at -30 to +200°C and work at 230°C for a short time. The heat treatment stable temperature of general bearings is 150°C, so the local temperature in the reducer should be strictly limited to exceed 120°C, and the temperature is only allowed to approach 150°C under extreme conditions of long-term high speed and high torque. Generally, it is necessary to select the appropriate brand and grade of gear oil according to the ambient temperature of the area where the vehicle is used. For commercial vehicle reducers, it is recommended to use gear oil of quality grade GL-4 or GL-5. SAE 80W/90 gear oil can be used in areas in southern my country where the winter temperature is not lower than -20°C, SAE 75W/90 gear oil can be used in areas in southern my country where the winter temperature is not lower than -35°C, and SAE 75W/90 gear oil can be used in hot areas in southern my country where the maximum summer temperature reaches 40°C [1]. Combined with the high-speed performance test conditions in QC/T 1022-2015 “Technical Specifications for Reducer Assemblies for Pure Electric Passenger Vehicles” which require a test oil temperature of 90 to 110°C [2], it can be concluded that the normal operating temperature range of commercial vehicle reducers is -30 to +120°C.

Common lubrication methods include active lubrication and passive lubrication, and both lubrication methods can also be used at the same time.

Active lubrication, also known as forced lubrication, is mainly composed of an oil pump, filter, special oil channel (oil pipe), oil nozzle and oil cooler. The pressure of the oil pump presses the oil into the oil channel, through the filter, special oil channel (oil pipe) and oil nozzle, and then into each friction pair to lubricate and cool each component. Active lubrication is common in high-power gearboxes with high performance requirements. It has the disadvantages of high cost, complex structure and limited space layout, which will not be discussed here.

Passive lubrication is also called splash lubrication (see Figure 1). It does not require additional oil circuit design. The oil can be splashed by the rotation of the gears and then lubricated and cooled by a properly designed oil channel or guide plate. It is commonly found in medium and low power, low cost, and easy-to-disassemble reducers.

Figure 1 Passive lubrication

When passive lubrication is applied to a four-axis three-stage reduction or a three-axis two-stage reduction structure, it is usually done by using large plate teeth that can be immersed in gear oil to splash oil to supply oil to bearings that are arranged at a higher or farther position. This passive lubrication sets the following prerequisites:

1) The gear responsible for oil throwing needs to be immersed in oil, and the immersion depth is 1 to 3 times the tooth height (to reduce oil stirring loss, the lower the liquid level, the better).

2) The oil level should be able to immerse as many of the differential bearings as possible and even the bottom 1 to 2 balls of the upper-level bearing (to reduce the difficulty of oil channel design and process production).

3) Normally, at medium and high speeds, the gears can drive the gear oil to splash along the tangential direction of the gear operation at a relatively high linear speed, and the oil will be blocked by the inner wall of the housing and the gear shaft and then overflow. Therefore, there is generally no risk of insufficient splashing oil at medium and high speeds.

Therefore, the structure in which the oil passage and the oil baffle are integrally cast has many advantages, such as low cost, fewer parts, good processability and high versatility.

Reducer oil channel layout and lubrication principle

The assembly of a passively lubricated reducer is shown in Figure 2. Combined with this figure and the cast housing and housing cover, the oil channel arrangement and lubrication principle are described in detail as follows.

Figure 2 Passive lubrication reducer

(1) The housing of a reducer is shown in Figure 3, and the casting has the following features:

Figure 3 Reducer housing

1) A transverse strip oil baffle is set on the right bearing hole of the first shaft and the right bearing hole of the second shaft, and a square oil groove is set at the bottom of the oil baffle to communicate with the right bearing hole of the first shaft. Therefore, the gear oil splashed from the first-stage passive gear and the second-stage passive gear can be collected and discharged to both sides. Part of the overflowed gear oil flows to the right side into the right bearing hole of the first shaft, and part flows to the left side and falls to the oil baffle of the shell cover and collects.

2) A long oil groove is set between the right bearing hole of the second shaft and the right bearing hole of the differential, so that a large amount of oil at the right bearing hole of the second shaft can be introduced into the right bearing of the differential, and the lubrication of the differential bearing can be effectively supplemented. On the other side, because the gear oil splashed by the first-stage passive gear can flow directly to the left bearing of the differential, there is no hidden danger of insufficient lubricating oil.

3) A square oil groove is set between the right bearing hole of the first shaft and the right bearing hole of the second shaft, which is conducive to allowing the gear oil at the right bearing hole of the first shaft at a high point to naturally flow into the right bearing hole of the second shaft for lubrication.

4) A square oil groove is hollowed out directly above the right bearing hole of the first shaft and the right bearing hole of the second shaft. Relying on the outer walls of the bearing holes of the first and second shafts, the gear oil flowing down from the shell wall can be naturally collected and introduced into the right bearing hole of the second shaft through the oil groove, thereby forming a second stable oil flow, which effectively supplements the lubrication of the right bearing hole of the second shaft.

(2) A reducer housing cover is shown in Figure 4, and the casting has the following features:

Figure 4 Reducer housing cover

1) An L-shaped oil baffle is set on the upper side of the left bearing hole of the first shaft, and a square oil groove is provided at the bottom of the L-shaped oil baffle, which is interconnected with the left bearing holes of the first and second shafts. Therefore, the gear oil splashed from the first-stage passive gear can be collected and introduced into the left bearings of the first and second shafts.

2) The L-shaped oil baffle is located lower than the transverse oil baffle of the reducer housing and can receive the gear oil thrown out by the secondary passive gear, and guide the gear oil into the left bearing holes of the first and second shafts.

3) A square oil groove is set between the left bearing hole of the first shaft and the left bearing hole of the second shaft, which is conducive to allowing the gear oil at the left bearing hole of the first shaft at a high point to naturally flow into the left bearing hole of the second shaft to increase its lubricating oil amount.

According to the above structure and principle, when moving forward, especially at low speed, the secondary passive gear acts as a power source to splash the gear oil to form an oil source; when reversing at low speed or when the reducer is placed on the opposite side and moving forward at low speed, the primary passive gear acts as a power source to splash the gear oil to form an oil source; the smaller the gap between the oil baffles and the gears at each level, the more conducive it is to collect the gear oil, and 5mm is recommended. It is achieved that the gear oil splashed by the gears can be collected under any working condition and guided to each bearing to form a stable and sufficient lubricating oil flow, providing a stable lubricating oil flow for the differential bearing on one side that is covered by the gear and lacks gear oil.

Lubrication simulation analysis and experimental verification

XFLOW fluid simulation software was used to simulate the

Lubrication simulation of the electric drive axle assembly of a light truck under forward and reverse rotation.