Analysis of the Reasons for Excessively High Oil Temperature in Mechanical Hydraulic Transmission during Highway Construction

2 Operation and Usage

Due to the significant load variations of highway construction machinery, the oil temperature of its hydraulic transmission system is closely related to its operation and use. The oil in the hydraulic transmission system, as the medium of the torque converter, will generate a certain temperature rise during the process of transmitting torque. In order to keep the oil temperature within a certain range, a cooling system was designed to dissipate heat and lower the temperature. That is, the heat generated by the hydraulic transmission system at a certain time is balanced with the heat dissipated at the same time, and the oil temperature of the system is thus controlled within a certain range. If the load of the machinery does not change much, the oil temperature issue of its hydraulic transmission system during operation and use will not be so prominent. However, for highway construction machinery, the operation and use have a significant impact on the oil temperature of the hydraulic transmission system. The following takes a bulldozer as an example to illustrate the impact of operation and use on the temperature rise of the hydraulic transmission system.

The basic operation of a bulldozer consists of four processes: soil scooping, soil transportation, soil unloading and no-load return driving, forming a working cycle. The relationship between the oil temperature and time of the hydraulic transmission system in a normally operating working cycle.

The relationship curve between the system temperature and time of a bulldozer when the heat generated and dissipated by the hydraulic transmission system in one working cycle are balanced after it has been working normally for a period of time. A-b is the earth-moving condition, b-c is the earth-transporting condition, c-d is the earth-unloading condition, d-a 'is the no-load return condition, and A' -b 'is the earth-moving condition again... . During the soil-digging operation of a bulldozer, due to the significant resistance encountered by the blade, the mechanical movement is very slow or even stops. To make it easier for the blade to dig into the soil, the operator must increase the throttle and raise the engine speed. At this point, the rotational speed of the pump wheel of the hydraulic torque converter is synchronized with that of the engine, but the turbine rotates very slowly or even stops due to the resistance effect of the mechanical transmission system behind it. At this time, the torque output of the torque converter is the maximum, but the efficiency is the lowest, or even zero. Most or all of the power input by the engine is converted into heat during the torque transformation, causing the temperature of the oil in the torque converter to rise sharply. When the bulldozer is transporting soil, unloading soil and running back without load, the resistance it encounters decreases and the rotational speed of the torque converter turbine increases. Especially during the soil unloading condition, as the mechanical transmission after the torque converter is temporarily cut off, the turbine may synchronize with the pump wheel, thus generating less heat. For the entire hydraulic transmission system, the soil transportation, unloading and no-load return operation are heat dissipation processes compared to the soil shovel operation. If the working object of the bulldozer remains unchanged and the operator's operation method remains the same, the oil temperature of the hydraulic transmission system can be maintained within a certain range as shown in Figure 1.

It can be seen from this that for bulldozers, the proportion of the time spent on earth-moving to the total working cycle time has a decisive impact on the oil temperature of the hydraulic transmission system. If the time for shoveling soil can be shortened, that is, the working time of the torque converter under low efficiency, the oil temperature of the hydraulic transmission system will not increase significantly. Here is an operational issue. When the bulldozer is performing soil removal operations on hard soil or when the blade encounters obstacles such as tree roots or large rocks, the duration of the soil removal process becomes prolonged, and the turbine's rotational speed drops significantly. The oil temperature of the hydraulic transmission system rises rapidly. In terms of one working cycle, the heat generated is greater than the heat dissipated. If it continues to work, The oil temperature of the entire system will be too high.

Therefore, the operation and use have a significant impact on the oil temperature of the hydraulic transmission system. Here, there is a problem of reasonably arranging the operation objects according to the mechanical performance parameters, and also a problem of how to operate the machinery under relatively harsh working conditions. When a bulldozer is performing soil removal operations on a hard working surface and is forced to complete tasks it cannot handle, the oil temperature of the hydraulic transmission system will become too high, and even damage the hydraulic components may occur. At this point, the soil should be loosened in advance, or the time for shoveling soil should be reduced as much as possible during operation, or the rest time of the machinery should be increased, etc. Some people think that hydraulic transmission machinery does not have overload problems, which is a misunderstanding. Overloading can cause the oil temperature to rise excessively. Not only does a high oil temperature deteriorate the oil quality, but it also leads to carbon deposits, making the sealed oil drainage pipeline blocked and increasing the pressure at the pump wheel housing seal, resulting in the failure or complete scrapping of the pump wheel housing seal. Operators should master the above principles. In practical operation, by improving the operation methods and minimizing the resistance encountered by the machinery when shoveling soil, the oil temperature of the hydraulic transmission system can be prevented from being too high to a large extent.

The reasons of the hydraulic transmission system itself

The main reasons for excessively high oil temperature in a hydraulic transmission system due to its own components are: improper oil level of the hydraulic transmission oil, abnormal wear of machine parts, and slippage of the transmission gear brake pads, etc.

3.1 Improper oil level of hydraulic transmission oil

The working oil in the hydraulic transmission system is specified to have a certain quantity. However, only a very small part of the oil directly participates in the flow in the system, while the majority is stored in the oil pool. This oil not only replenishes the consumption during the flow but also regulates the oil volume. Therefore, under certain conditions, it is beneficial to have a larger reserve of working oil. However, if the oil level is too high and submerges various driving components, especially gears, it will increase frictional resistance and instead cause the oil temperature to rise. On highway construction machinery, the liquid level of the working oil is checked with an oil dipstick. The oil level should neither be too low nor too high.

3.2 Abnormal friction of machine parts causes the oil temperature to rise

The working oil of the hydraulic transmission system needs to lubricate and cool many components. Besides the heat generated by the normal fit (or meshing) friction of these transmission components, some transmission pairs may also experience jamming and burning due to processing and assembly reasons after repair, or some components may change their positioning for some reason after long-term use. Our company once experienced a fault where the torque converter of the D75S loader had a loose turbine bearing, causing direct friction between the pump wheel and the turbine. This kind of contact friction will generate local high temperatures, thereby affecting the temperature of the hydraulic transmission oil within the system.

3.3 The brake pads during gear shifting in the transmission slippage and overheat

The heat generated by the slippage of the brake pads during gear shifting in the transmission is another major cause of overheating of the oil temperature in the hydraulic transmission system. The brake pads of the transmission shift have an extremely short period of slippage at the moment they start to engage and start to separate, which is something that machinery cannot overcome. At this point, the hydraulic transmission oil plays a role in lubrication and cooling. In other words, the heat generated by the sliding friction needs to be transferred to the hydraulic transmission oil. If the sliding friction time is too long, the working temperature of the hydraulic transmission oil will rise rapidly. There are mainly three reasons for the long friction time of the brake pads during gear shifting in the transmission:

(1) The friction brake pads are severely worn, reducing the friction force. When the torque is large, they cannot achieve complete engagement.

(2) The pressure of the hydraulic cylinder for gear shifting fails to reach the specified value, resulting in weak clamping of the brake pads. The reasons for the decrease in the pressure of the working cylinder are the failure of the internal seal in the system, resulting in internal leakage, or severe wear of the hydraulic pump, insufficient oil supply pressure, or the pressure regulating valve being stuck or the spring failing.

(3) The transmission oil filter is clogged, the oil passage is not smooth, the flow is insufficient, and the pressure establishment is slow.