China YTO Group Corporation (hereinafter referred to as YTO) is one of the 156 key construction projects during the First Five-Year Plan period. As a large-scale Chinese machinery manufacturing enterprise with a history of nearly 60 years, it has a large number of equipment and a wide variety of models. However, with the continuous decline in the newness coefficient of the equipment, the technical condition of the equipment is also declining. How can the performance of the equipment be improved through partial structural modifications without changing the basic structure of the existing equipment, so as to meet the requirements of production and quality?
The following is a case study of repair work on equipment within YTO Group that has high production cycle requirements and is a bottleneck restricting the production of large wheel tractors.
1. Improvement of the transmission screw mechanism of the TQXL6511 horizontal planer boring machine.
(1) Overview of the original screw structure
The TQXL6511 horizontal planer boring machine, manufactured by Hanchuan Machine Tool Factory, is used on the large wheel tractor transmission housing machining line for back milling, drilling, and other processes. The machine's appearance and layout are similar to the TX68 boring machine manufactured by Kunming Machine Tool Factory on the same machining line. Both utilize a flexible spindle box on a column to perform drilling and milling operations on the workpiece. The structural difference lies in the TX68's standard 800mm×800mm square rotatable worktable, including X, Y, and B axes, suitable for machining smaller box-shaped parts; while the TQXL6511 has a 900mm×1800mm sliding worktable, moving only in the X direction, suitable for machining large box-shaped parts for large wheel tractors. Most parts, such as gears and shafts, are interchangeable, which facilitates spare parts supply.
With the continuous increase in the production capacity of large wheel tractors in recent years, metal cutting machine tools generally operate continuously at high intensity in three shifts 24 hours a day. Due to the vibration caused by the cutting force, the four internal hexagonal screws of the worktable screw drive nut support are often cut off, resulting in an extremely high rate of repeated failures.
Repairing the workbench requires first disassembling the box fixture. Because the 900mm x 1800mm workbench consists of two layers, the upper slide must be moved laterally by about 0.5m to expose the countersunk hole of the support before repair work can proceed. This results in a huge workload for repair and adjustment. Furthermore, the four screws of the nut support are located at the bottom of a 120mm x 120mm square countersunk hole, a narrow space that makes it difficult to reach with wrenches and other tools, severely limiting accessibility. Each repair is not only arduous but also requires two days, making it difficult to ensure production rhythm requirements are met.
(2) Discussion and formulation of improvement plans
In order to solve the repair problem and reduce the occurrence of recurring failures, and considering that this equipment is a special machine for processing transmission housings with a relatively fixed stroke range, we propose an improved solution that moves the difficult-to-maintain lead screw nut out from under the worktable and uses an external connection for transmission.
The original working stroke of the slide table is 1800mm, and the outer length of the nut bracket is about 200mm. After deducting the reserved 300mm of clearance stroke, it is expected that the working stroke of the slide table will be shortened to 1500mm after the improvement, while the length of the processed box is about 900mm, as shown in Figure 1, which can fully meet the requirements of box processing and adjustment.
Due to the change in height of the removed structure, the original square nuts were no longer suitable, and square nuts were also difficult to manufacture. Therefore, a new set of circular nuts with an insert structure was designed, as shown in Figure 2. During installation, the circular nuts are first inserted into the bracket holes and then secured with nuts. The modified triangular bracket is then fixed to one side of the slide. When the nuts are damaged, they can be easily removed for repair, greatly reducing the difficulty and workload of repairs and effectively saving repair time.
After modification, a manual idle test showed no lag. In actual trial operation, the equipment ran smoothly, the parts were of qualified quality, and the processing requirements were met, as shown in Figure 3.
2. Improvement of the clutch structure on the tractor chassis break-in test bench
(1) Analysis of the problem
The tractor chassis break-in test bench is a specialized piece of equipment designed to meet specific process requirements. It actively engages the chassis for gear shifting during break-in testing. During operation, a certain axial force is generated. Due to dimensional constraints and to simplify the structure, this equipment only uses the standard light-series deep groove ball bearing 61912, as shown in Figure 4. This results in frequent stress damage to the clutch bearing structure. On-site analysis indicated that the 61912 bearing has a low load-bearing coefficient. Consulting the relevant content on "Common Rolling Bearing Dimensions and Performance Parameters" in Volume II of the *Mechanical Design Handbook* (Chemical Industry Press), the basic rated dynamic load Cr of deep groove ball bearings (GB/T276—1994) is 16.4kN, making it prone to overheating and damage within a short time. However, increasing the bearing size would increase the installation diameter, while the housing support only has a single 60mm thin plate, making this impractical.
(2) Formulation of improvement plan
After repeated on-site deliberation and comparison, it was determined that only by increasing the bearing load factor could the repeated failures of the equipment be effectively reduced. The modification plan was finally determined to use a higher-grade bearing series type 6012, with a basic rated dynamic load Cr of 31.5kN, which is about twice the original load. The bearing caps on both sides were also modified. The caps were redesigned according to the selected bearings, and a bearing hole seat with the original bearing bottom hole φ60mm as the positioning reference was integrated on the cap. A small clearance was used for the fit, which facilitates disassembly and assembly.
Due to the increased bearing size, the axial dimensions of various components also changed significantly, necessitating the determination of the bearing positioning step length on the outer diameter of the splined shaft. To ensure production continuity and avoid on-site measurement during downtime, a transitional solution was devised: the outer diameter of the splined shaft was machined to match the bearing's inner diameter for later use. Two spacers were pre-machined for the bearing step, allowing for adjustment to a suitable position during actual installation. The spacers were then welded and fixed on the side not in contact with the inner ring. Based on the measured data (Figure 5), installation was successfully completed in one go. After a period of operation, this method effectively reduced the recurrence of bearing damage, and was subsequently applied to the break-in test benches of nine other tractor chassis.
In daily equipment maintenance, it is not only necessary to keep the equipment in good working order, but also to continuously innovate and improve the design and installation of the equipment to meet the requirements of production rhythm and product quality.
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