Repair and improvement plan for cracked oil storage pan of crawler frame of excavator

  The oil pan of the crawler frame of a used excavator has two functions: one is to store grease to ensure good lubrication of the slewing motor and the slewing bearing meshing gear; the other is to serve as a mounting seat for the slewing joint. The installation status of the oil pan of a certain type of excavator is shown in Figure 1. This article analyzes the cracking of the oil pan of this type of excavator and proposes a repair and improvement plan.

1. Fault phenomenon

When a certain type of excavator was in operation for nearly 2000 hours, some grease appeared on the ground where it had been operating. After opening the bottom guard plate of the walking frame, it was found that cracks appeared on the bottom plate of the oil storage pan, and the grease penetrated from the cracks and dripped on the ground. After summarizing the faults, it was found that the cracks were divided into 4 categories: one is that the crack starts from the middle of the bottom plate of the oil storage pan, extends outward and then splits into 2 cracks; the second is that the crack starts from the middle of the bottom plate of the oil storage pan and extends outward; the third is that cracks occur at the edge of the oil drain hole and extend outward; the fourth is that the crack starts from a certain point inside the bottom plate of the oil storage pan and extends radially outward. Type 1 cracks are more common, and Type 1 and Type 2 cracks often appear at the same time, and Type 3 and Type 4 cracks often appear at the same time. Some excavators with heavier workloads have longer cracks, and some even penetrate the entire oil storage pan. The four types of crack forms are shown in Figure 2.

2. Analysis of cracking causes

(1) Unreasonable steel plate cutting

The original design of the oil pan bottom plate was to cut a whole piece of steel plate. In order to save materials during actual production, the oil pan bottom plate was welded from two parts, as shown in Figure 3.

The analysis shows that due to the reduced strength of the parent material in the heat-affected zone on both sides of the weld, welding defects occurred at two sharp points, resulting in the first and second type of cracks. In addition, the cutting edge of the oil pan bottom plate is relatively rough, especially at the oil drain hole where three threaded holes are drilled. Under the action of alternating loads, the oil pan bottom plate will produce third and fourth type of cracks. 

(2) Weak structure

Under normal circumstances, the oil pan with a bottom plate thickness of only 6mm can meet the strength requirements as a grease storage function. However, when it is used as a mounting seat for a swivel joint, the oil pan bottom plate deforms significantly. According to finite element force analysis, its displacement is as high as 1.19mm when only the gravity of the swivel joint is loaded.

(3) Stress concentration

During the excavation process, the force transmitted from the slewing bearing to the oil pan is the greatest at the moment when the bucket is digging. As the working posture of the working device changes, the oil pan is frequently subjected to tensile and compressive stresses. Finite element analysis shows that the stress concentration is most obvious at the tip of the bottom plate of the oil pan. When only the gravity of the slewing joint is loaded, the stress is as high as 130MPa.

(4) Poor working conditions

Through communication with customers, we learned that the working conditions of the faulty excavators are very poor. Most of the excavators have been working in mines for a long time, and many excavators are also equipped with breakers. The oil pan of the excavator equipped with a breaker is more severely cracked, and two types of cracks often appear at the same time.

3. Repair plan

The repair plan for the excavator with cracked oil pan is as follows: First, grind a 60° symmetrical groove along the crack, and the end of the groove should exceed the crack by 10mm; second, use an electric welder to weld and fill the groove; finally, according to the shape and position of the crack, weld a reinforcement plate at the corresponding position. After the faulty excavator with cracked oil storage pan was repaired according to the above plan, no similar faults occurred again.

4. Improvement plan

In order to prevent the oil pan of the newly excavator from cracking again, we decided to improve the structure of the oil pan. There are two improvement plans, and the specific implementation methods are as follows:

The first plan is to thicken the bottom plate to 8mm and weld two reinforcing ribs under the bottom plate to improve the strength and rigidity of the overall structure. At the same time, the corners are rounded to reduce stress concentration, as shown in Figure 4.

After finite element analysis, it is known that the maximum stress of the oil pan can be reduced to 67.91MPa and the maximum displacement is only 0.229mm by adopting plan 1. Although this plan has greatly improved the strength and rigidity, welding is required when repairing the oil pan, which is difficult to operate, and the weight of the oil pan is as high as 40kg.

The second plan is to connect the oil pan and the swivel joint.

The mounting seat is separated. The oil storage pan is stamped from 2.5mm thick steel plate and assembled on the slewing bearing seat. It is light and thin and has good rigidity. The slewing joint mounting seat is formed by bending 12mm thick steel plate and welded to the X-shaped plate of the track frame, as shown in Figure 5.

After finite element analysis, it is known that when the second scheme is adopted, the maximum stress of the oil storage pan is reduced to 54.01MPa, and the maximum displacement is only 0.044mm. This scheme has good maintainability. The oil storage pan can be replaced separately after being damaged. At the same time, the strength and rigidity of the slewing joint mounting seat can also be guaranteed. Therefore, it is decided to adopt the second scheme.

The second improvement scheme avoids the occurrence of cracking failure of the oil storage pan, and there is no need to worry about the oil storage pan being squeezed and deformed during transportation. At the same time, since there is no weld at the oil storage pan, the manufacturing accuracy of the slewing bearing mounting surface is easier to guarantee, which effectively reduces the manufacturing cost. After the products improved according to the second scheme were put into the market in batches, there was no quality feedback of oil storage pan cracking, which proved that this improvement scheme was successful. Source: China used Excavator https://www.lh-usedexcavator.com/