A large torque screw mandrel surface strengthening method

Abstract

The application provides a large-torque screw mandrel surface strengthening method and belongs to the technical field of large-torque screw mandrel surface strengthening processes.The specific nitriding process and polishing and grinding method are used to process the surface of the screw mandrel, so that the surface roughness of the dangerous area is reduced, the microstructure of the surface of the screw mandrel is improved, the surface quality of the mandrel is improved, stress concentration is reduced, the fatigue limit of the mandrel is improved, the service life of the screw mandrel is improved, and the surface fatigue resistance of the screw mandrel is improved by about 20%.

Description

Technical Field

[0001] This invention belongs to the field of surface strengthening technology for high-torque screw mandrels, and relates to a method for preventing deformation of high-torque screw mandrels in rubber and plastic machinery during processing and heat treatment, reducing surface roughness, and improving surface hardness through gas soft nitriding. Background Technology

[0002] High-torque screw mandrels are widely used in various large-scale equipment manufacturing industries, including petrochemical and rubber industries, serving functions such as support, connection, torque transmission, and rotation. The mandrel is a core component in large-scale equipment manufacturing units, and its strength and service life directly affect the overall lifespan of the entire unit. Commonly used mandrel materials are high-strength alloy steel, which currently suffers from problems such as low surface hardness, poor surface roughness, and short lifespan. The most common failure mode of mandrels is fatigue fracture. Replacing the mandrel not only increases the operating cost of the unit but also causes incalculable production downtime losses for users due to the long replacement time.

[0003] Therefore, in order to improve the service life of the screw mandrel, this invention proposes a strengthening technology for the surface of involute spline mandrels with a screw length-to-diameter ratio of 20-27, a torque of 150,000-200,000 N·m, and a heat treatment state of quenching and tempering. By improving the surface quality of the mandrel, reducing stress concentration, and increasing the fatigue limit of the mandrel, the service life of the mandrel is improved. Summary of the Invention

[0004] To address the problems in existing technologies, through comparison of multiple sets of screw mandrel fracture failure analysis results, mandrel material fatigue experiments, and finite element stress-strain calculations, the ideal mandrel surface properties were determined to be: spline surface roughness within the range of Ra0.8-Ra1.6, spline surface hardness within the range of 500-900 HV, and mandrel straightness ≤0.2mm. High spline surface roughness will lead to stress concentration during operation; low mandrel surface hardness and low wear resistance will lead to localized fatigue wear; poor straightness will cause interference in the screw components. Under conventional mandrel manufacturing processes, involute splines are machined using hobbing, and the surface roughness can only reach Ra3.2, especially at the tooth root R angle, where surface roughness is difficult to reduce; the mandrel hardness can only reach 350-400 HV in the tempered state. The finished mandrel has a diameter within the range of 160-200mm, a length within the range of 8000-11000mm, and a straightness ≤0.2mm. Due to the constraints of mandrel length and spline structure, a 12-meter-long involute grinding machine and a 12-meter-deep deep-well gas nitriding and carbonizing furnace require special customization, which is difficult and expensive; due to the constraint of mandrel straightness <0.2mm, the deformation of the mandrel after nitriding is difficult to control; therefore, it is difficult to achieve mandrel surface photochemical treatment and hardening.

[0005] Based on this, this invention improves the mandrel machining process by adding rough milling of the splines and subsequent stress-relief annealing to the conventional process. This reduces residual stress during mandrel machining, thereby controlling the deformation of the mandrel surface after nitriding. A custom-designed 12-meter deep pit-type gas nitriding furnace is used to control furnace temperature and gas uniformity to achieve a mandrel surface hardness of 500-900 HV, while maintaining a straightness of ≤0.2mm. A special grinding and polishing process is designed for different parts of the mandrel splines, incorporating different tooling to achieve a surface roughness of Ra0.8-Ra1.6. Ultimately, this improves the fatigue life of the screw mandrel.

[0006] The technical solution adopted in this invention is:

[0007] A method for surface strengthening of a high-torque screw mandrel, wherein the screw has a length-to-diameter ratio of 20-27 and a torque of 150,000-200,000 N·m; the mandrel has a diameter of 160-250 mm and a length of 8,000-11,000 mm; the method includes the following steps:

[0008] Step 1: Complete the forming process of screw mandrel 1 according to the improved screw mandrel machining method.

[0009] Step 2: Transfer the screw mandrel 1 with the spline milled and place it on the mandrel indexing device 4. The spline teeth of the screw mandrel are displaced by the circumferential rotation of the mandrel indexing device 4.

[0010] Step 3: The manual or robotic clamping mechanism 8, the pen-type polisher 5, the pneumatic polisher 6, the felt wheel 7, and the floating polishing tool 9 perform a series of corresponding actions to achieve surface polishing of the screw mandrel.

[0011] Step 4: Place the surface-polished screw mandrel 1 into a deep-well nitriding furnace 2 for surface nitriding to achieve surface hardening of the screw mandrel.

[0012] The beneficial effects of this invention: This invention proposes a surface strengthening method for high-torque screw mandrels, employing a gas nitrocarburizing method. Compared to ordinary mandrel processing techniques, this method improves surface hardness while also possessing better toughness, lower crack sensitivity, better corrosion resistance, and smaller, controllable deformation. Gas nitrocarburizing creates compressive stress on the mandrel surface, and multiple comparative experiments have shown an improvement of approximately 20% in surface fatigue resistance. Attached Figure Description

[0013] Figure 1 This is a schematic diagram of the screw mandrel described in this invention.

[0014] Figure 2 This is a schematic diagram of the surface hardening method for the screw mandrel described in this invention.

[0015] Figure 3 This is a schematic diagram of the manual operation of surface polishing of the screw mandrel described in this invention.

[0016] Figure 4 This is a schematic diagram of the operation of the surface polishing robot for the screw mandrel described in this invention.

[0017] In the diagram: 1. Screw mandrel; 2. Deep-well nitriding furnace; 3. Sliding guide rail; 4. Mandrel indexing device; 5. Pen-type polisher; 6. Pneumatic polisher; 7. Felt wheel; 8. Robot clamping mechanism; 9. Floating polishing tool. Detailed Implementation

[0018] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0019] This invention proposes a surface strengthening method for screw mandrels. The workpiece to be processed has a length-to-diameter ratio of 20-27, a torque of 150,000-200,000 N·m, a screw diameter of 320-380 mm, and a screw length of 8,000-11,000 mm. The corresponding mandrel diameter is 160-250 mm, and the mandrel length is 8,000-11,000 mm. The heat treatment state is quenching and tempering, and the surface strengthening method involves involute spline screw mandrels, trapezoidal spline screw mandrels, or polygonal screw mandrels. Specific implementation schemes are as follows:

[0020] Step 1: Machin the screw mandrel forging to a smooth shaft and heat-treat it to a tempered state.

[0021] Step 2: Rough turning, machining the outer diameter of the screw mandrel, leaving an allowance of 8-14mm for the outer diameter.

[0022] Step 3, stress-relief annealing: In order to remove processing stress and reduce part deformation, heat to 500-600℃ in an electric heating furnace and hold for 4-6 hours.

[0023] Step 4: Semi-finish turning, machining the outer diameter of the screw mandrel, leaving a 3-6mm allowance for the outer diameter. Surface roughness Ra≤6.3, straightness≤0.2mm.

[0024] Step 5, stress-relief annealing: In order to remove processing stress and reduce part deformation, heat to 500-600℃ in an electric heating furnace and hold for 4-6 hours.

[0025] Step 6: Rough mill the splines. Rough mill the splines on both sides of the optical shaft surface according to the design requirements, with a cutting depth of 2-3mm.

[0026] Step 7, stress-relief annealing: In order to remove processing stress and reduce part deformation, heat to 500-600℃ in an electric heating furnace and hold for 4-5 hours.

[0027] Step 8: Finish mill the splines. Perform finish milling on both sides of the involute splines twice, with the first finish milling having a cutting depth of 1-2 mm.

[0028] Step 9: Polish the screw mandrel 1 until the surface roughness of the spline teeth, including the tooth root and tooth tip, reaches Ra0.4-1.6. The specific process is as follows:

[0029] Step 9.1: Prepare the pen-type polisher 5, pneumatic polisher 6, felt wheel 7, floating polishing tool 9, and robot clamping mechanism 8.

[0030] Step 9.2: Install the screw mandrel 1, adjust the position of the tip of the mandrel indexing device 4 along the sliding guide rail 3 to meet the length space of the screw mandrel 1, and then install the screw mandrel 1 onto the mandrel indexing device 4.

[0031] Step 9.3: Polishing the screw mandrel

[0032] Step 9.3.1: Use a pen-type polisher 5 to manually polish the root angle of the spline teeth to achieve a roughness Ra0.4-1.6.

[0033] Step 9.3.2: Manually use a pneumatic grinder 6 to grind the outer circle of the spline teeth and the tip angle of the spline teeth. Use sandpaper of different grits to grind in three stages from small to large to achieve a roughness Ra0.4-1.6 at the tip angle of the spline teeth.

[0034] Step 9.3.3: First, grind the shape of the felt wheel 7 according to the spline contour. Then, manually polish the spline contour. Take polishing paste of different grit sizes and complete the rough polishing in three stages. Grind intermittently and control the polishing temperature to avoid damage to the mandrel surface caused by excessive temperature.

[0035] Step 9.3.4: Input the spindle spline parameters through the robot control panel, teach the robot to grind, and use the floating grinding tool 9 in conjunction with the robot clamping mechanism 8 to perform fine grinding and polishing on the spline, requiring the surface roughness to reach the range of Ra0.4-1.6.

[0036] Step 10: Gas nitrocarburizing. Place the polished screw mandrel 1 into the deep well nitriding furnace 2. Set the CO2 gas percentage to 1%-10%, control the temperature at 500-600℃, and the time to 4-6 hours. The nitriding layer depth is 0.1-0.6mm, and the surface hardness is 500-1000HV.

Claims

1. A method of surface hardening a large torque screw mandrel, characterized by, The method includes the following steps: Step 1: Shape the screw mandrel; Step 2: Transfer the screw mandrel with the splines milled and place it on the mandrel indexing device. The spline teeth of the screw mandrel are repositioned by the circumferential rotation of the mandrel indexing device. Step 3: The manual or robotic clamping mechanism holds a pen-type polisher, a pneumatic polisher, a felt wheel, and a floating polishing tool to perform corresponding actions to achieve surface polishing of the screw mandrel; Step 4: Place the surface-polished screw mandrel in a deep-well nitriding furnace for surface nitriding to achieve surface hardening of the screw mandrel; the specific process of step 1 is as follows: Step 1.1: Machin the screw mandrel forging to a smooth shaft and heat-treat it to a quenched and tempered state; Step 1.2: Rough turning, machining the outer diameter of the mandrel, leaving an allowance of 8-14mm for the outer diameter; Step 1.3, stress-relief annealing, heat to 500-600℃ and hold for 4-6 hours; Step 1.4, semi-finish turning, machining the outer diameter of the screw mandrel, leaving a 3-6mm allowance for the outer diameter; roughness Ra≤6.3, straightness≤0.2mm; Step 1.5, stress-relief annealing, heat to 500-600℃ and hold for 4-6 hours; Step 1.6: Rough mill the splines. Rough mill the splines on both sides of the optical shaft surface according to the design requirements, with a cutting depth of 2-3mm. Step 1.7, stress-relief annealing, heat to 500-600℃ and hold for 4-5 hours; Step 1.8, finish mill the splines, finish milling the involute splines on both sides twice, with the first finish milling having a cutting depth of 1-2mm; In step 4, the screw mandrel is placed in a deep-well nitriding furnace, the percentage of CO2 gas is set to 1%-10%, the temperature is controlled at 500-600℃, the time is 4-6 hours, the penetration depth is 0.1-0.6mm, and the surface hardness is 500-1000HV. The surface strengthening method is applicable to workpieces with a length-to-diameter ratio of 20-27, a torque of 150,000-200,000 N∙m, a mandrel diameter of 160-250 mm, and a mandrel length of 8,000-11,000 mm. The heat treatment state is quenching and tempering, and the tooth profile is an involute spline screw mandrel, a trapezoidal spline screw mandrel, or a polygonal screw mandrel.

2. A method of surface hardening a large torque screw mandrel according to claim 1 wherein, In step 3, the screw mandrel is polished, and the surface roughness of the spline teeth, tooth root, and tooth tip reaches Ra0.4-1.6; the specific process is as follows: Step 3.1: Use a pen-type polisher to manually polish the root angle of the spline teeth to achieve a roughness Ra0.4-1.6; Step 3.2: Manually grind the outer circle of the spline teeth and the tip angle of the spline teeth using a pneumatic grinder. Use sandpaper of different grits to grind in three stages from small to large to achieve a roughness Ra0.4-1.6 at the tip angle of the spline teeth. Step 3.3: First, grind the shape of the felt wheel according to the spline contour. Then, manually polish the spline contour. Take polishing paste of different grit sizes and complete the rough polishing in three stages. Grind intermittently and control the polishing temperature to avoid damage to the mandrel surface due to excessive temperature. Step 3.4: Input the core shaft spline parameters through the robot control panel, the robot polishes the teaching, and uses the floating polishing tool to cooperate with the robot clamping mechanism to finely polish and polish the spline, and the surface roughness is required to reach the range of Ra0.4-1.6.

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