Repair of Broken Output Shaft of Precision Planetary Reducer

Due to the large carbon equivalent of No. 45 steel, the hardening tendency of the heat-affected zone is relatively large during welding, and cold cracks are prone to occur, and the welding amount of the shaft is large, so there is inevitably a large welding deformation and welding stress. In order to reduce welding deformation and improve weld quality, preheating measures before welding are taken, and corresponding heat treatment is carried out on parts before machining to slow down the cooling rate, facilitate the diffusion and escape of hydrogen, prevent cold cracks, and eliminate welding stress. After heat treatment, turn the welded part according to the requirements to meet the size requirements of the shaft. 

Preparation before welding (1) Selection of equipment and materials In view of the construction site conditions, manual arc welding is adopted for the shaft. Welding equipment adopts BX1-330 AC arc welding machine. The power supply adopts DC power supply, and the opposite pole is connected (the electrode is connected to the positive pole). In order to improve the crack resistance and strength of the weld, the welding material is welded with low-hydrogen type J507 (E5015) basic electrode. In an incubator at 150 ℃, take it out and put it in a drying cylinder when in use. Ⅰ and Ⅱ layer welds use electrodes with a diameter of 3.2 mm to reduce welding stress. In the future, welding rods with a diameter of 4 mm will be used to improve work efficiency.

 (2)  Design of welding groove and gap Before welding, the fractured part of the shaft is processed and leveled. At the same time, in order to reduce the welding stress, certain measures are taken in the design of the welding groove. Firstly, a U-shaped groove was designed according to the fracture situation and the diameter of the part. The upper width of the groove is 20 mm, and the depth is 37 mm. In order to make the shape of the groove consistent, after the end surface of the fractured part is processed flat, the groove is processed with a forming turning tool. In order to avoid stress concentration, all parts of the groove should be rounded to reduce the proportion of the base metal melted into the weld metal and prevent welding cracks. The welding gap also has an important impact on the quality of the weld. If the gap is too small, defects such as incomplete penetration or excessive weld accumulation may occur, and if the gap is too large, it is easy to burn through. The shaft welding gap is selected as 2 mm. 

(3) Cleaning before welding Before welding, the rust, oil stain, moisture and oxide skin on the groove and blunt edge surface of the shaft welding part must be removed. First use flame baking to remove water and oil, and then grind with manual abrasive cloth or grinder Carry out rust removal treatment. The oxide skin can be directly removed by grinding with a grinder, and for some difficult-to-clean oil stains, it can be cleaned with acetone or ethanol. 

2. Welding When welding, in order to prevent welding deformation, a positioning block should be added at the groove to ensure that the gap between the welds is equal. In order to avoid welding deformation of the shaft, the location of the positioning spot welding is selected on both sides of the positioning block. Due to the general weldability of No. 45 steel, acetylene flames were used to preheat the parts of about 50-80 mm on both sides of the weld to 150-200 °C before welding to reduce the cooling rate of the entire welding area and prevent cracks. When welding the first and second layer welds, try to use small current, low voltage, and slow welding speed, that is, small energy welding method, and the welding layer should be thinner (not more than 2 mm), but it must be penetrated, and short arc welding should not swing . When welding, the welding should be carried out evenly with the rotation of the shaft. After each layer of welds is welded, the appearance of the welds should be visually inspected. If there are pores, slag inclusions and cracks, etc., carbon arc gouging should be used to remove weld defects. Welding can be continued after welding. When the welding reaches the height of the positioning block, when the cooling temperature of the weld is not lower than 200 ℃, remove the positioning block and continue welding. The height of surfacing welding should exceed the diameter of the original shaft, leaving enough machining allowance. The process parameters of each weld are shown in the attached table.

 3. Post-welding treatment and machining After welding, cover with asbestos quilt for heat preservation to reduce the degree of hardening and avoid embrittlement. In order to eliminate the welding stress, the weld area can be heated to 350-550 ℃ with an acetylene flame, buried in hot sand or lime and slowly cooled until it cools to room temperature. The appearance of the welding place is required to be neat and beautiful, the weld seam is uniform, and there are no pores, slag inclusions, incomplete fusion, splashes and undercuts. After checking that the appearance is qualified, re-turn the outer circle and end face of the output shaft to the required size. The above repair method is simple and practical, and the repair cost is low. It has been verified that the repaired output shaft can fully meet the requirements of the reducer in terms of strength and precision.