Fault Analysis of Overheating in Excavator Hydraulic System

The heating phenomenon of the hydraulic system and its hazards

Hydraulic system overheating is a relatively common fault phenomenon in excavators and also a rather complex soft fault to analyze and handle. Under normal operating conditions, the oil temperature of the Komatsu PC200/400 excavator's hydraulic system should be below 60 ° C (the temperature of the oil pump is 5-10 ° C higher). If it exceeds this temperature significantly, it is considered as hydraulic system overheating. The fault characteristics are as follows: When the excavator is working cold, all movements are relatively normal. However, after about one hour of mechanical operation, as the temperature of the hydraulic oil rises, the various actuating mechanisms of the excavator become weak and move slowly, especially the digging force is insufficient and it is difficult to move and turn.

If the heating phenomenon in the hydraulic system is not dealt with in time, it will have an extremely adverse impact on the system:

(1) The viscosity of the oil decreases, leakage increases, and the system heats up, creating a vicious cycle.

(2) Accelerate the oxidation of the oil, forming a gel-like substance that blocks the small holes of the components, causing the hydraulic components to malfunction or get stuck, making them unable to work.

(3) Cause rubber seals and hoses to age and fail;

(4) It will accelerate the wear of the oil pump and hydraulic valve components, and even lead to their scrapping.

Analysis of Fault Cases of Overheating in the Hydraulic System of Excavators

For such faults, generally speaking, the first step should be to conduct an internal analysis from the outside of the hydraulic system. The main internal cause is the unreasonable system design. It is caused by factors such as unreasonable matching between components, overly narrow pipeline channels, excessive elbows, small bending radii, and insufficient tank capacity. Such issues should be fully considered during the design stage; otherwise, it will lead to inherent deficiencies in the excavator's hydraulic system, which will be difficult to overcome once the product is made.

2.1 Heating of the hydraulic system caused by external factors

For instance, the engine of the Komatsu PC200-5 excavator hydraulic pump is connected through the shock absorber damper in the shock absorber box. When dealing with the overheating fault of the hydraulic system of an excavator, it was found that the oil level in the shock absorber box was much higher than that at the observation oil plane screw (generally about 1.5L). During the rotation of the shock absorber damper, this excessive oil generated a large amount of heat and transferred it to the hydraulic pump, causing the system to overheat.

At this point, after draining the oil in the shock absorption box to the standard oil level, the fault can be eliminated. There are two reasons for the excessively high oil level in the shock absorber box. One is that the operator blindly adds oil. The second reason is that the oil seal at the end of the hydraulic pump shaft ages, causing the hydraulic oil to leak from there. In this case, the hydraulic pump should be disassembled and the oil seal replaced.

2.2 Poor heat dissipation performance of the radiator causes the oil temperature to be too high

The main forms of poor heat dissipation performance of radiators are: deformation or blockage of external heat dissipation fins, resulting in poor cooling effect; The cooling fan is insufficient in volume. The internal pipes of the hydraulic oil radiator are blocked. The first two can be judged not only intuitively but also from the fact that the temperature difference between the upper and lower tubes of the radiator does not change much. At this time, the heat sink should be cleaned and the fan belt tightened, etc. The determination of internal pipe blockage in the hydraulic oil radiator can be made by installing pressure gauges in the inlet and outlet oil channels of the radiator and checking the pressure difference between them. When the oil temperature is around 45 ° C and the pressure difference is below 0.12MPa, it is considered normal. If it is higher than 0.12MPa, it indicates that the oil pipe is severely blocked, and the upper and lower covers of the radiator should be removed to clear the pipe.

2.3 The malfunction of the one-way valve of the hydraulic return oil filter element causes the hydraulic oil to overheat

It can be known from the hydraulic system schematic diagram that the check valve of the hydraulic system's return oil filter element is connected in parallel with the hydraulic oil radiator at the outlet of the return oil filter element (refer to the PC200-5 hydraulic system schematic diagram). Its function is to automatically open when the pressure difference of the return oil radiator is above 0.185MPa, short-circuiting the radiator to form a return oil passage. In actual work, as this valve is installed at the bottom of the return oil filter element, it is difficult to inspect and maintain. In addition, some operators improperly select the quality of the hydraulic oil, fail to change the oil for a long time, and neglect it over the years, the oil is seriously contaminated, causing the valve to get stuck in the normally open position (and some even remove this valve without authorization). As a result, the return oil radiator fails to dissipate heat, which is bound to cause the oil temperature to be too high. Each time the hydraulic oil is changed, it is necessary to check whether this valve is stuck.

2.4 Improper selection of hydraulic oil grade or poor oil quality can cause the oil temperature to be too high

In recent years, there have been many faults in hydraulic systems where the oil temperature rose due to the selection of oil that did not meet the specified performance requirements or inferior oil. For instance, the misuse of oil with excessively high viscosity can cause excessive pressure loss in the liquid flow, which is converted into heat energy and leads to an excessively high temperature rise. The misuse of hydraulic oil with too low viscosity can also cause excessive leakage inside the working hydraulic pump and hydraulic components, generating heat. In addition, some low-quality oils have poor viscosity-temperature performance, are prone to emulsification and cavitation, and produce bubbles, which can generate local high temperatures under high pressure in the hydraulic oil and accelerate the wear of components.

2.5 System heating caused by improper adjustment of the pressure valves in the pump and hydraulic system

As the power source of the hydraulic system, the working condition of the hydraulic pump affects the degree of heat generation in the system. If the main pump of the PC200 excavator is a plunger pump, and the fit positions between the distribution plate and the cylinder, the sliding shoe, the swash plate and the plunger cylinder inside the pump wear significantly, it often causes the hydraulic pump to heat up quickly. This can be judged by observing the characteristics that the pump heats up quickly and makes noise. The repair method is to grind and trim the relevant sealing mating surfaces or replace the parts that cannot be repaired.

The pilot control pump of the PC200 excavator is a gear pump. Its function is to provide operation control pressure oil for the system and adjust the displacement of the main pump according to the load requirements. If the end faces of the gears inside the pump are worn significantly or the tooth clearance is large, the internal leakage will increase, which will cause the pump to heat up and affect the normal operation of the main pump.

Excessively high or low pressure in the relief valve can also cause the hydraulic system to heat up. For instance, if the system pressure is set too high, the hydraulic pump will operate at a pressure exceeding its rated value, leading to pump overload and an increase in oil temperature. Conversely, if the system pressure regulation is too low, the working mechanism will frequently experience the phenomenon of the relief valve opening to unload under normal load, causing the hydraulic system to overflow and heat up.

The set values of each pressure valve in the hydraulic system of Komatsu PC200-5 excavator are listed in the following table.

Remarks on standard values (Mpa) of test conditions for test parts

The oil temperature of the main relief valve is around 45 ° C, with two groups of 32.5(+0.8-1.1)

The high idle speed of the boom engine is 32.5(+0.8-1.1

The outlet of the bucket rod main pump is 32.5(+0.8-1.1)

The bucket is the same as 32.5(+0.8-1.1) above.

Rotation is the same as above 29.0(+1-0.5)

Walking is the same as above 32.5(+0.8-1.1)

Pilot control pump oil temperature around 45 ° C 3.2(+0.4-0.1)

TVC control valve operation valve stem neutral 2.0(+0.1-0.1)