Hefei Shuoyuan Engineering Machinery Parts Co., Ltd.

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A New Method for Testing the Leakage Defects of the Rear Axle Box of Bulldozer

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The gearbox, central transmission and final transmission installed on the rear axle box of the bulldozer not only bears the power transmission function of the bulldozer, but also acts as a stress carrier for ripper loosening and bulldozer traction.

Fault phenomenon

When an SD42-type high-power bulldozer was assembled and went offline, during the no-load test, the operator discovered that oil leakage occurred at the bolt holes at the rear panel of the bridge with the ripper.

2. Reason analysis

(1) Structure and welding process

The SD42 bulldozer rear axle box is mainly composed of a lower leg assembly 1, an upper panel 2, a right shell 3, a bottom panel 4, a rear panel 5, a partition member 6, a left shell 7, and a front shell 8.

The material of the rear axle box is both cast steel and plate (Q345, Q460C). The structure of the rear axle box is complex, and there are many welds inside and outside, which require strict requirements on the weld seam. The welding process flow is as follows: primary assembly→first welding→secondary assembly→secondary welding→robot welding→welding. The oil leakage at the above position of the rear axle box may be due to a leakage defect in the weld between the left shell 3, the bottom panel 4 and the rear panel 5.

(2) Weld inspection

In order to ensure the quality of the weld in the rear axle box, ultrasonic flaw detection and kerosene leakage test methods are currently used to inspect the weld leakage defects.

When kerosene is used for leakage inspection, it is necessary to brush lime water or chalk aqueous solution on the outer surface of all welds. After the painted lime water or chalk aqueous solution is air-dried, the kerosene is painted on the other side of the weld seam. When the weld has a penetrating leaking defect, kerosene can penetrate and show signs on the surface of the welds painted with lime water or chalk.

The following problems exist with this inspection method:

First, the detection time of each rear axle box is too long (approximately 1.5h). Second, if there is a penetrating permeation defect in the weld, the kerosene will gradually spread out after penetration and form fuzzy marks, making it difficult to accurately determine the defect location; The third is to lift the workpiece multiple times, the inspection efficiency is low, and there are hidden dangers.

3. Improvement method

(1) New ideas for air pressure inspection

When two plates are combined by means of welding, the contact surface of the two plates must have certain internal voids due to roughness and the like. If a device is used to inject a gas with a certain pressure into the gap surrounded by this weld, then if there is a leak defect in the weld, gas must escape. The use of pressurized gas to inspect welds for the presence of leakage defects is quick and accurate and can be a new method of weld inspection.

(2) Feasibility analysis

The plate of the rear axle box of the bulldozer is directly formed by blanking. The roughness of the incision is generally 25 to 50 μm, and a gap of 2 to 3 mm must be reserved between the plates. The welding seams are welded on both sides. The feasibility of air pressure test has the following 3 points:

One is that the seams of the plate after welding of the rear axle box group are surrounded by welds, forming a closed weld seam surrounding space, and it is feasible to adopt a pneumatic test for this purpose.

Second, although the structure of the rear axle box is complex, the weld seam is completely continuous and the test gas can flow in the inner space of the weld seam. Therefore, soap water can be applied to the outside of the weld seam to check if the gas leaks. The longer the weld, the higher the inspection efficiency.

The third is to use clean air with certain pressure, which is simple and easy.

This method is called "inside weld tightness test" and provides a new method for the inspection of welds in the rear axle box.

(3) Inspection and repair process

Equipped with a simple charging device, pressure regulator (with pressure gauge) and soapy water, and connected to the factory's internal compressed air piping, a rapid inspection of the weld quality of the rear axle box can be achieved, as shown in Figure 2. The inspection and repair process is as follows:

First, an M12 tapped hole is added to the rear axle box as a charging interface. Or make one or pressure process block welded at the pressure filling interface, and communicate with the internal space of the continuous weld through the pressure process block.

Next, the rear axle box is placed on a special pressure bearing support frame, one end of the gas connection pipe is screwed to the pressure connection interface, and the other end is connected to the internal compressed air pipe of the factory.

Again, the compressed air in the compressed air pipeline is adjusted to 0.3Mpa through the pressure regulator valve through 0.6 to 0.8Mp a, which leads into the weld of the rear axle box.

Then, apply soapy water to the inner and outer welds, check for bubbles in the inner and outer welds during filling, and mark the air bubbles.

Finally, the welding seam at the location of the bubble is repaired and repaired and recharged to check if there is any leakage defect in the weld, until the quality of the weld is fully qualified.

4. Improve the effect

The method of “inspection of internal tightness of welds” has changed the current status of kerosene leakage test with low efficiency, high absent detection rate, and difficulty in clearing kerosene pollution, and expanded the scope of application of airtightness test methods. The inspection method is simple, intuitive, efficient, and low in missed inspection rate. It can inspect welds in complex structures. The non-destructive testing of welds can fully and accurately reflect the state of weld leakage, and the experience of the inspectors is not high. At present, this method has been applied to other parts of our company that need to perform weld leakage defect inspection.

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