Common Problem! Overview of Excavator Track Roller Attributes and Analysis of Damage Causes

Common Problem! Overview of Excavator Track Roller Attributes and Analysis of Damage Causes

The supporting wheel of an excavator carries its own weight and operational load, and the attributes of the Track Roller are an important criterion for measuring its quality. This article analyzes the attributes, damage, and causes of the supporting wheel.

1. Attributes of Track Roller

(1) Structure

The structure of the Track Roller is shown in Figure 1. The outer cover 2 and inner cover 8 at both ends of the supporting wheel spindle 7 are fixed to the lower part of the excavator track frame. After fixing the outer cover 2 and inner cover 8, it can prevent the spindle 7 from axial movement and rotation. There are flanges on both sides of the wheel body 5, which can clamp the track chain to prevent the track from derailing and ensure that the excavator travels along the track.

There is a pair of floating sealing rings 4 and floating sealing rubber rings 3 respectively inside the outer cover 2 and the inner cover 8. After fixing the outer cover 2 and the inner cover 8, the floating sealing rubber rings 3 and 4 are pressed against each other. The contact surfaces between the two floating sealing rings 4 are smooth and hard, forming a sealing surface. When the wheel rotates, the two floating seal rings 4 rotate relative to each other, forming a floating seal. O-ring seal 9 is used for sealing between spindle 7 and outer cover 2, inner cover 8. Floating seal and O-ring seal 9 can prevent lubricating oil leakage inside the supporting wheel and prevent mud and water from entering the inside of the supporting wheel. The oil hole in plug 1 is used to add lubricating oil to the inside of the supporting wheel.

(2) Force condition

The Track Roller body of the excavator is subjected to the upward support force of the track chain, and the two ends of the main shaft bear the weight of the excavator, as shown in Figure 2.

The gravity of the excavator is transmitted to the main shaft 7 through the track frame, outer cover 2, and inner cover 8, and then transmitted to the shaft sleeve 6 and wheel body 5 through the main shaft 7, and finally transmitted to the chain rail and track plate through the wheel body 5 (see Figure 1). When excavators operate on uneven terrain, it is easy to cause the track shoes to tilt, resulting in the inclination of the chain track. When the excavator makes a turning motion, there will be axial displacement force between the main shaft and the wheel body. Due to the complex stress on the supporting wheel, its structure must be reasonable, and the main shaft, wheel body, and shaft sleeve need to have relatively high strength, toughness, wear resistance, and sealing performance.

(3) Technical requirements

The spindle material is 50Mn high-quality carbon structural steel, with a C content of 0.48%~0.56%, Si content of 0.17%~0.37%, Mn content of 0.7%~1.0%, S content less than 0.035%, P content less than 0.035%, Cr content less than 0.25%, Ni content less than 0.30%, and Cu content less than 0.25%. Among them, Mn is an important element in alloy steel, which can improve the forging and plasticity of steel, and ensure that the material has sufficient toughness and wear resistance. After heat treatment, 50Mn steel has high strength and hardness, excellent hardenability, deep hardening layer, fine pearlite structure, and good mechanical properties.

The spindle design standard requires a quenching depth T of 2-7mm and a quenching hardness of 50-62HRC, as shown in Figure 3.

The Track Roller of the wheel body is subjected to frequent impact loads on the track surface of the wheel body, which is prone to compression deformation, causing the track surface of the wheel body to extend into a conical shape. To improve the stress condition, we have extended the length of the wheel body track surface and designed the transition between the track surface and the wheel waist as a slope to enhance the support capacity of the track surface. The shaded area in Figure 4 represents the extended track surface and transition slope.

In order to improve the wear resistance of the supporting wheel body, the wheel body material is 40Mn2 alloy structural steel. The C content is 0.37%~0.44%, the Si content is 0.17%~0.37%, the Mn content is 1.4%~1.8%, the P content is less than 0.030%, and the S content is less than 0.030%.

The wheel body is formed by forging, and after forming, the rough wheel body is subjected to quenching and tempering treatment to achieve a hardness of 26~32HRC, in order to maintain high strength, plasticity, toughness, and comprehensive mechanical properties inside the wheel body. The surface of the wheel body is quenched to improve the hardness and wear resistance of the supporting wheel track surface. The quenched hardness is 50~58HRC, and the depth of the hardened layer is 6~12mm, so the hardness of the track surface is basically close to that of the chain rail joint (48~58HRC).

The size and surface roughness of the sealing O-ring groove should meet the design requirements, and the mating surface should be chamfered and cleaned of burrs and sharp edges.

Floating sealing rubber rings and O-ring seals should be made of high-quality nitrile rubber. The inner diameter, wire diameter, elasticity, hardness (Shore), and surface smoothness should meet the standards. Before assembly, they should be inspected and qualified according to the above items one by one before being put into assembly.

If the floating seal rubber ring is not wrapped around the groove of the floating seal ring, or is not in contact with the floating seal ring, it will cause uneven pressure from the floating seal rubber ring on the floating seal ring. Uneven pressure can cause increased local blade wear during the rotation of the floating seal ring, leading to oil leakage of the floating seal ring. If the edge of the O-ring seal is cut off, half of it is stuck outside the ring groove, or it is twisted when installing the inner and outer covers, it will cause a decrease in the sealing performance or service life of the O-ring seal. Sometimes, if the pressure during the airtight inspection does not reach a certain value, no leakage can be detected and no leakage will occur after oil injection. However, after the excavator has been working for a certain period of time, the friction temperature of the supporting wheel components and the pressure inside the cavity will rise to a certain value, and leakage will occur. If the floating oil seal continues to be used after a leak occurs, it will experience severe oil leakage.

The gap between the main shaft of the supporting wheel and the shaft sleeve on the wheel body is 0.2-0.4mm, and there should be no obstruction during rotation. The sealing performance is good and there is no oil leakage.

Lubrication should use an oiler to add gear oil to the Track Rollers, and if necessary, increase the viscosity of the lubricating oil appropriately. Before adding gear oil, the expired gear oil should be drained first, and then new gear oil should be added to the oil plug hole until the oil flows out. When adding lubricating oil, unscrew the shaft end plug, place the top of the oil injector nozzle head on the shoulder of the oil passage, and inject oil into the oil passage. When installing the oil plug, a torque wrench is required to control the tightening torque of the oil plug within the range of 157~255N • m.

2. Analysis of Damage Causes

This article takes a 20t excavator from a certain manufacturer as an example to analyze the reasons for the damage of its supporting wheels. We inspected the partially damaged supporting wheels of the excavator and found that the main faults were damage to the main shaft and wheel body, as well as oil leakage. The main shaft of the severely damaged supporting wheel was caught, causing the entire supporting wheel to be unable to rotate. We will now analyze it separately.

(1) Spindle damage

We used a spectrometer to detect the content of various elements in the damaged spindle of the Excavator Track Roller if the material was not qualified. The measured content of Mn element was 0.59%, which was significantly different from the standard content (0.7%~1.0%). This seriously affects the mechanical performance of the spindle. Unqualified heat treatment results in measured damage to the surface hardness and quenching depth of the spindle, as shown in Table 1. The test data shows that the surface quenching hardness and quenching layer depth of the spindle do not meet the design requirements, seriously affecting the strength, stiffness, and wear resistance of the spindle.

Serious wear was found through testing, with the main shaft bent and the mating parts of the main shaft diameter and shaft sleeve severely worn, as shown in Figure 5. After measurement, the clearance between the main shaft and the shaft sleeve has exceeded 0.3mm (this clearance should be less than 0.2mm), and the main shaft and shaft sleeve need to be updated.

(2) Wheel damage

Upon inspection of the damaged Track Rollers body, it was found that the track surface of the wheel body was deformed, cracked, and had fallen off. Figure 6 shows the extension of the track surface of the supporting wheel into a conical shape after being impacted.

The severe wear of the wheel body is mainly manifested in the track surface, guide flange, and shaft sleeve seat hole. This is because excavators often operate in sandy, muddy, and gravel fields, and a large amount of abrasive enters between the support wheel rail surface and the chain rail, forming strong abrasive wear. These sand and gravel also generate significant compressive stress on the track surface, flange, and track chain rail section, resulting in many microscopic compressive deformations and dry friction. These dry frictions include both rolling friction and sliding friction, resulting in severe wear of the wheel track surface and a decrease in wheel stiffness and strength. After severe wear of the flange, it is easy for the supporting wheel to derail.

Unqualified heat treatment, actual measurement shows that the supporting wheel is damaged and the surface of the wheel body is hard

The test results for the hardness and depth of the quenched layer on the wheel surface are shown in Table 2. The measured data indicates that the quenched hardness and depth of the quenched hardness layer on the wheel surface do not meet the design requirements.

Dissecting the wheel body with material issues revealed cracks inside, as shown in Figure 7. This indicates that the forging of the wheel blank is not qualified, and the forging process of the wheel needs to be improved to prevent coarse grains and abnormal structure.

(3) Oil leakage

The dimensions of the O-ring sealing groove of the Bulldozer Track Rollers D81 D8N D90 spindle (groove width, groove depth, O-ring roundness, etc.) are within the design tolerance range and meet the technical requirements. Detect that the wire diameter and inner diameter of the floating sealing rubber ring and O-ring exceed the tolerance, and there is damage on the surface. The reason for its damage is that the sealing ring material is not qualified, which is reflected in the expansion and deformation after contact with lubricating oil, loss of elasticity, early aging, and hardness (Shore) not meeting the design requirements, resulting in oil leakage due to poor sealing.

There are two reasons for the oil leakage of the supporting wheel: firstly, the Bulldozer Track Rollers is soaked in water for a long time during excavator operation, causing corrosion of the floating seal ring and O-ring groove; Secondly, construction in muddy water during winter can cause the floating seal ring to freeze and fail.