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

Modern highway construction machinery widely uses hydraulic transmission systems, that is, a hydraulic torque converter is set at the power output end of the engine, and then the power is output to the power shift gearbox. Within a certain range, this hydraulic transmission system can automatically change torque in response to variations in external loads, and can stabilize the engine's operation near the rated working conditions, thereby enhancing the power performance and economy of highway construction machinery.

During the operation of the hydraulic torque converter, it is normal for the system oil temperature to increase to a certain extent due to frictional heat generation. Generally speaking, the outlet oil temperature of the hydraulic torque converter should be below 120℃. If it exceeds this temperature, the oil temperature of the system will be too high. Take the Caterpillar 980C loader as an example. When the oil temperature of the hydraulic transmission system reaches 120℃, the oil temperature indicator light starts to flash. When the oil temperature reaches 139℃, the red warning light will come on. At this point, the operator must stop the operation and look for the cause.

If the oil temperature in the hydraulic transmission system is too high, on the one hand, it will dilute the oil concentration, reduce its viscosity, and deteriorate the lubrication performance of the system, leading to accelerated wear of other transmission components. On the other hand, it reduces the volumetric efficiency of the system and prevents the system pressure from rising. Furthermore, when the oil itself is exposed to high temperatures for a long time, it is prone to deterioration and aging, and the sealing parts related to it will also age faster due to high temperatures, losing their sealing function. Therefore, excessively high oil temperatures in hydraulic transmission systems pose a significant threat to highway construction machinery and should be avoided at all costs.

Based on the author's several years of practice in using and managing highway construction machinery at construction sites, this paper analyzes the reasons for the excessively high oil temperature in the hydraulic transmission system of highway construction machinery, including the selection of hydraulic transmission oil, the operation and use of the machinery, and certain reasons of the hydraulic transmission system itself, and discusses the measures and countermeasures that should be taken.

1. Selection of hydraulic transmission oil

Improper selection of hydraulic transmission oil is one of the main reasons for the excessively high oil temperature in the hydraulic transmission system. The power of modern highway construction machinery is getting larger and larger, and the on-site working conditions are harsh. The working load changes frequently and greatly. The oil temperature of the hydraulic transmission system generally varies from 70℃ to 140℃, and sometimes even reaches 150℃. Therefore, the requirements for hydraulic transmission oil are also getting higher and higher.

Hydraulic transmission oil, also known as automatic transmission fluid, is called ATF oil abroad. Its function is to serve as the medium for the transmission of dynamic fluid energy in the torque converter, the medium for the transmission of static fluid energy in the hydraulic control circuit and servo mechanism, the lubricant for the load tooth surfaces of shift bearings and gears, the medium for the transmission of sliding and friction energy in wet clutches, and the heat transfer medium in liquid cooling or air cooling systems.

To meet the requirements of different working conditions, it is required that the viscosity of the working oil changes very little with temperature. At low temperatures, the viscosity should not be too high to maintain high transmission efficiency and improve low-temperature starting performance. Generally, the kinematic viscosity υ50 of the working oil for hydraulic transmission is 55 to 54mm ² / s. Viscosity-temperature property: Generally, the ratio of υ50 to υ100 should be as small as possible, usually less than 4.5(for example, for 8 # hydraulic transmission oil, it should be less than 3.60). If expressed in terms of viscosity index, the viscosity index should be greater than 170(it has reached 210 abroad). A high viscosity index means that the viscosity changes less with temperature.

The allowable working temperature of the hydraulic transmission system during operation can reach 70℃ to 110℃. If it works for a long time in the low-efficiency zone, it can even reach around 120℃. Therefore, the flash point of the hydraulic transmission working oil should be greater than 180℃ and must not be lower than 155℃. The freezing point of working oil varies by region and season, and is generally above -20℃. If water-soluble acids or bases are found in the working oil, it will have a strong corrosive effect on metals. At this time, the working oil must be replaced. In addition, the critical load value, oxidation stability, demulsibility, anti-foaming property, moisture content and the presence of mechanical impurities, etc., should all be guaranteed in accordance with relevant performance indicators.