Anti-breaking worm for RV reducer and surface strengthening method thereof

Abstract

This invention relates to a fracture-resistant worm gear for RV reducers and its surface strengthening method. The outer diameter of the overtravel zone is equal to the root circle diameter of the helical tooth. A large fillet is formed between the helical tooth and the overtravel zone. The end of the overtravel zone and the journal form a flared section with gradually increasing radial dimensions. The outer surface of the flared section is a concave arc surface formed by rotating an elliptical arc. Both the outer surface of the flared section and the outer surface of the overtravel zone are rolled to form a reinforced outer surface layer. This reinforced outer surface layer includes a mechanically plastically deformed layer and a residual compressive stress layer beneath the plastically deformed layer. The fact that the outer diameter of the overtravel zone is equal to the root circle diameter of the helical tooth completely eliminates the traditional right-angle step and removes a major stress concentration source. The large fillet design significantly reduces the stress concentration factor and greatly inhibits fatigue cracking. The flared section forms a streamlined, expanded-diameter transition zone, improving the worm gear's resistance to bending fatigue. Controllable plastic deformation forms a reinforced outer surface layer, increasing surface hardness and strength.

Description

Technical Field

[0001] This invention belongs to the field of speed reducer transmission, and in particular relates to a fracture-resistant worm gear for RV speed reducers and its surface strengthening method. Background Technology

[0002] Worm gear drives, as an important form of mechanical transmission, are widely used in key areas such as speed reducers, lifting equipment, and heavy machinery due to their advantages such as compact structure, large transmission ratio, and smooth operation. The two ends of the worm are supported by bearings for rotation, and the middle part of the worm is machined with helical teeth that mesh with the worm wheel. One end of the worm serves as the input end, connected to the input motor to transmit power.

[0003] However, the structure of the worm gear causes an overtravel zone at both ends of the helical teeth when machining the helical teeth in the middle part of the worm. The outer diameter of the overtravel zone is generally smaller than the root circle diameter of the helical teeth, which creates a geometric step between the overtravel zone and the root circle, making it a location where stress easily concentrates. The price difference between different specifications of speed reducers is significant. The center distance of high-torque speed reducers is larger than that of low-torque speed reducers. Increasing the center distance by one standard gear increases the price of the speed reducer by 35%-80%. Therefore, when selecting a speed reducer, people often choose a speed reducer one standard gear lower due to price considerations. In actual working conditions, speed reducers experience both long-term low-load overload and temporary high-load overload during operation. Once a lower gear is selected, the speed reducer often experiences the combined effects of long-term low-load overload and temporary high-load impact. Especially during use, due to the complexity of the working conditions, the speed reducer may also experience overload, easily causing fatigue fracture failure of the worm gear, seriously affecting the reliability and safety of the equipment.

[0004] The main fracture sites of worm gears are the overtravel zone and the geometric steps, where they are subjected to complex alternating bending and torsional stresses. Furthermore, severe stress concentration exists at the geometric steps, causing cumulative fatigue damage to the worm gear, ultimately leading to low-stress, high-cycle fatigue fracture at the smallest cross-section. Worm gear fracture can cause a series of serious hazards. First, the transmission chain is instantly interrupted, causing an emergency shutdown and production losses. Second, metal fragments from the fracture can fly into the gearbox, easily jamming the meshing pairs or damaging bearings, seals, and even the gearbox housing, leading to costly secondary damage. More seriously, in safety-sensitive applications, sudden transmission failure can directly cause major safety accidents such as equipment loss of control or falls.

[0005] Therefore, there is an urgent need for a fracture-resistant worm gear, which can improve the strength of worm gears of the same model and specifications, thereby adapting to complex working conditions and avoiding worm gear breakage and the resulting damage. Summary of the Invention

[0006] The purpose of this invention is to provide a fracture-resistant worm gear for RV reducers and its surface strengthening method. This method employs a large-radius transition between the worm gear helical tooth root and the overtravel zone to avoid stress concentration caused by geometric steps. Simultaneously, it increases the diameter of the overtravel zone, improving its strength. Furthermore, an elliptical arc-shaped flared section is used between the overtravel zone and the journal shaft to shorten the axial length of the overtravel zone and increase the radial dimension between the journal shaft and the overtravel zone, thereby improving the strength of this part of the worm gear. Further, a mechanical plastic deformation layer is generated on the surface of the overtravel zone and the flared section through rolling, and a residual compressive stress layer is generated below the plastic deformation layer. This residual compressive stress layer is used to offset the tensile stress that leads to fracture, improving the worm gear's fracture resistance.

[0007] The specific technical solution of the present invention is as follows: a fracture-resistant worm gear for an RV reducer, comprising journals at both ends and a helical tooth in the middle. An overtravel zone is provided between the journals and the two ends of the helical tooth. The outer diameter of the overtravel zone is equal to the root circle diameter of the helical tooth. A large fillet is provided between the helical tooth and the overtravel zone, with a radius of 0.4m-0.6m. The large fillet surrounds the root portion of the first and last rings of the helical tooth in a 360° manner. Between the end of the overtravel zone and the journal, there is a flared section with a gradually increasing radial dimension. The outer surface of the flared section is a concave arc surface formed by rotating an elliptical arc. The starting part of the flared section is axial and tangentially connected to the end of the overtravel zone. Both the outer surface of the flared section and the outer surface of the overtravel zone are rolled to form a reinforced outer surface layer, which includes a mechanical plastic deformation layer and a residual compressive stress layer below the plastic deformation layer.

[0008] The outer diameter of the overtravel zone is equal to the root circle diameter of the helical tooth, completely eliminating the right-angle step formed by the journal diameter being smaller than the root circle diameter in traditional structures. This removes a major source of stress concentration at its source. The radius of the large fillet is controlled between 0.4m and 0.6m and extends 360° around the root of the helical tooth, forming a continuous and smooth spatial transition surface along the helical direction. This allows the force flow lines to expand smoothly, significantly reducing the stress concentration factor and greatly suppressing fatigue cracks at the root of the helical tooth. The overtravel zone and the journal are designed as a streamlined flared section, with the starting point axial and tangentially connected to the end of the overtravel zone. The outer surface of the flared section is a concave shape formed by rotating an elliptical arc. The curved surface creates a streamlined, widened transition zone between the overtravel zone and the journal, ensuring a continuous and gradual rate of change in cross-section from the overtravel zone to the thickened journal. This avoids abrupt changes in stiffness, further optimizes stress distribution, and particularly improves the worm gear's resistance to bending fatigue. Through controllable plastic deformation, a reinforced outer layer is formed on the outer surfaces of the flared section and the overtravel zone by rolling, including a mechanical plastic deformation layer and an underlying residual compressive stress layer. The plastic deformation layer increases surface hardness and strength, while the residual compressive stress layer actively counteracts the tensile stress generated by the working load, thereby effectively improving the fatigue limit of the part. The overall design requires no additional accessories or significant changes to external dimensions, making it particularly suitable for RV reducers that are sensitive to space and weight.

[0009] Preferably, the length of the overtravel zone is greater than or equal to 3πm, and the starting point for calculating the length of the overtravel zone is the ingress or egress end of the helical tooth.

[0010] Preferably, the major semi-axis of the ellipse is a, and the minor semi-axis is b, where the major semi-axis a corresponds to the radial direction of the worm, and the minor semi-axis b corresponds to the axial direction of the worm, and 1.5 ≤ a / b ≤ 4.0. The axial length of the flared section is the distance between the end position of the overtravel zone and the end face of the shoulder at the journal. The end of the flared section is radial, and the elliptical arc corresponds to an elliptical parameter angle of 0-60° to 0-90°.

[0011] Preferably, the surface roughness Ra of the reinforced outer layer is ≤0.4μm, the depth of the plastic deformation layer is 0.02-0.3mm, and the depth of the residual compressive stress layer is 0.05-0.5mm.

[0012] Preferably, the tooth root curved surface within one lead at both ends of the helical tooth and the starting part of the overtravel zone are a shot peening strengthening layer, the shot peening strengthening layer has a residual compressive stress layer with a depth of 0.05-0.3mm, and there is a 1-5mm overlap between the shot peening strengthening layer and the strengthening outer surface layer formed by rolling.

[0013] A surface strengthening method for an anti-fracture worm gear for an RV reducer includes shot peening the tooth root surface within the first lead at both ends of the helical tooth to form a shot peening strengthening layer, and performing contour rolling treatment on the overtravel area and the horn section to form a strengthened outer surface layer, wherein the strengthening outer surface layer is rolled at least twice.

[0014] Surface strengthening includes shot peening and profile rolling. Shot peening targets the root surface of the helical teeth within the first lead at both ends, using high-speed shot impact to significantly improve the surface microhardness and strength, while eliminating residual tensile stress from machining and introducing residual compressive stress in this area, thereby improving the surface stress state. Profile rolling targets the overtravel area and flared section of regular rotating surfaces, allowing for greater and more controllable plastic deformation, resulting in a deeper reinforced layer and better surface finish. In the two rolling processes, the first uses greater pressure to achieve sufficient plastic deformation and a deep residual stress field, while the second uses less pressure for finishing rolling to reduce surface roughness, seal microscopic defects, and stabilize the reinforced layer. This ensures that the entire stress-critical area from the helical tooth root to the journal receives a residual compressive stress layer with sufficient depth and strength, without any weak reinforced areas.

[0015] Preferably, the shot diameter used for shot peening is 0.2-0.6 mm, and the shot peening intensity is 0.2 mmA to 0.4 mmA. The shot peening path fully covers the root surface of the helical teeth within the first lead at both ends, and the shot peening area extends axially into the overtravel zone by 1-5 mm. The shot peening area and the rolling zone form an overlapping and continuous reinforced layer, ensuring that the entire hazardous area is under beneficial pressure protection and preventing pressure faulting. Preferably, the contour rolling process employs cylindrical rollers and elliptical arc conjugate rollers. The axial length of the cylindrical rollers is equal to the axial length of the overtravel zone. The working surface of the elliptical arc conjugate rollers refers to a surface whose generatrix shape is designed or derived based on the principle of conjugation with the target elliptical arc surface of the trumpet segment. During the rolling process, this roller can achieve continuous, fully contacting line or surface with the trumpet segment surface. Using elliptical arc conjugate rollers for rolling the trumpet segment ensures that the rolling pressure always follows the normal direction of the contact point, thereby generating maximum and most effective plastic deformation, guaranteeing optimal pressure distribution. Simultaneously, the fully contacting rollers during rolling avoids scratches or new stress concentration sources caused by point contact or localized line contact.

[0016] Preferably, the working surface of the cylindrical roller is a mirror-polished surface with a surface roughness Ra value ≤ 0.1µm; the working surface of the elliptical arc conjugate roller is a mirror-polished surface with a surface roughness Ra value not greater than 0.1µm, and the axial working width is equal to the axial projection length of the trumpet segment.

[0017] Preferably, the cylindrical roller and the elliptical arc conjugate roller are integrally formed; or the cylindrical roller and the elliptical arc conjugate roller are manufactured separately and installed on the same roller shaft in a detachable manner.

[0018] The beneficial effects of the present invention are: 1. The outer diameter of the overtravel zone is equal to the root circle diameter of the helical tooth, which completely eliminates the traditional right-angle step and removes a major stress concentration source; 2. The large rounded corner design creates a continuous and smooth spatial transition surface along the spiral direction, which allows the force flow lines to expand smoothly, significantly reducing the stress concentration factor and greatly suppressing the occurrence of fatigue cracks at the root of the spiral teeth. 3. The overtravel zone and the journal are designed as a streamlined horn section. The starting part is axial and tangentially connected to the end of the overtravel zone. The outer surface of the horn section is a concave arc surface formed by rotating an elliptical arc, thus forming a streamlined diameter expansion transition zone between the overtravel zone and the journal. This makes the cross-sectional change rate from the overtravel zone to the thickened journal continuous and gentle, avoiding abrupt changes in stiffness, further optimizing stress distribution, and especially improving the worm gear's resistance to bending fatigue. 4. Through controllable plastic deformation, a reinforced surface layer containing a mechanical plastic deformation layer and a residual compressive stress layer is formed by rolling on the surface of the trumpet section and the overtravel area. The plastic deformation layer improves the surface hardness and strength, while the residual compressive stress layer can actively offset the tensile stress generated by the working load, thereby effectively improving the fatigue limit of the part. 5. The overall design requires no additional accessories or significant changes to the external dimensions, making it particularly suitable for RV reducers that are sensitive to space and weight. 6. By utilizing high-speed shot impact, the surface microhardness and strength are significantly improved, while residual tensile stress during machining is eliminated and residual compressive stress is introduced into the area, thereby improving the stress state of the surface. 7. Contouring rolling is used for the overtravel zone and trumpet section of regular rotating surfaces, which can apply larger and more controllable plastic deformation to obtain a deeper reinforcement layer and better surface finish. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of a structure of the present invention; Figure 2 This is a schematic diagram of the roller structure of the present invention; In the diagram: 1. Input end, 2. Output end, 3. Horn section, 4. Overtravel zone, 5. Shot peening reinforcement layer, 6. Helical tooth, 7. Elliptical arc conjugate roller, 8. Working surface, 9. Cylindrical roller. Detailed Implementation

[0020] The present invention will be further described below through specific embodiments and in conjunction with the accompanying drawings. Example

[0021] like Figure 1 As shown, a fracture-resistant worm gear for an RV reducer includes an input end 1, an output end 2, and a central helical tooth 6. Both the input and output ends are provided with journals for mounting bearings. An overtravel zone 4 and a flared section 3 are provided between the journal and the two ends of the helical tooth. The overtravel zone is located outside the two ends of the helical tooth, and the flared section is located between the end of the overtravel zone and the journal. The overtravel zone is cylindrical, and its outer diameter is equal to the root circle diameter of the helical tooth. The radial dimension of the flared section gradually increases, and its outer surface is a concave arc surface formed by rotating an elliptical arc. The starting part of the flared section is axial and tangentially connected to the end of the overtravel zone. A large fillet is formed between the helical tooth and the overtravel zone, with a radius of 0.4m-0.6m. The large fillet encircles the root of the first and last rings of the helical tooth in a 360° arc. In this embodiment, the radius of the large fillet is 0.45m, where m is the design module of the worm gear.

[0022] The overtravel zone length is greater than or equal to 3πm, where m is the design module of the worm gear. The starting point for calculating the overtravel zone length is the ingress or egress tip of the helical tooth. The major semi-axis of the ellipse on the outer surface of the horn section is a, and the minor semi-axis is b. The major semi-axis a is along the radial direction of the worm gear, and the minor semi-axis b is along the axial direction of the worm gear, and 1.5 ≤ a / b ≤ 4.0. The axial length of the horn section is the distance between the end of the overtravel zone and the end face of the shoulder at the journal. The elliptical arc corresponds to an elliptical parameter angle of 0-60° to 0-90°. In this embodiment, the end of the horn section is radial, and the elliptical parameter angle is 0-90°.

[0023] Both the outer surface of the trumpet section and the surface of the overtravel zone are rolled to form a reinforced surface layer, which includes a mechanically plastically deformed layer and a residual compressive stress layer beneath the plastically deformed layer. The surface roughness Ra of the reinforced surface layer is ≤0.4μm, the depth of the plastically deformed layer is 0.02-0.3mm, and the depth of the residual compressive stress layer is 0.05-0.5mm.

[0024] Furthermore, a shot peening strengthening layer 5 is formed between the tooth root curved surface within one lead at both ends of the helical tooth and the starting part of the overtravel zone. The shot peening strengthening layer has a residual compressive stress layer with a depth of 0.05-0.3mm, and the residual compressive stress layer ranges from -200MPa to -800MPa. There is a 1-5mm overlap between the shot peening strengthening layer and the strengthening outer surface layer formed by rolling.

[0025] A surface strengthening method for an anti-fracture worm gear for an RV reducer includes shot peening the tooth root surface within the first lead at both ends of the helical tooth to form a shot peening strengthening layer, and performing contour rolling treatment on the overtravel area and the horn section to form a strengthened outer surface layer, wherein the strengthening outer surface layer is rolled at least twice, and the rolling pressure of the first rolling is greater than the rolling pressure of the second rolling.

[0026] The diameter of the shot used for shot peening is 0.2-0.6 mm, and the shot peening intensity is 0.2 mmA to 0.4 mmA. The shot peening path fully covers the tooth root surface within the first lead at both ends of the helical tooth, and the shot peening area extends 1-5 mm axially into the overtravel zone.

[0027] like Figure 2As shown, the contour rolling process employs a cylindrical roller 9 and an elliptical arc conjugate roller 7. The axial length of the cylindrical roller is equal to the axial length of the overtravel zone. The working surface 8 of the elliptical arc conjugate roller refers to a surface whose generatrix shape is designed or derived based on the principle of conjugation with the target elliptical arc surface of the trumpet segment. The working surface of the cylindrical roller is a mirror-polished surface with a surface roughness Ra value ≤ 0.1µm; the working surface of the elliptical arc conjugate roller is a mirror-polished surface with a surface roughness Ra value not greater than 0.1µm, and its axial working width is equal to the axial projection length of the trumpet segment. The cylindrical roller and the elliptical arc conjugate roller are separate structures. The cylindrical roller and the elliptical arc conjugate roller respectively roll the overtravel zone and the trumpet segment. The roller axis of the cylindrical roller and the roller axis of the elliptical arc conjugate roller are parallel to the axis of the worm gear.

[0028] Furthermore, the cylindrical roller and the elliptical arc conjugate roller are integrally formed, or the cylindrical roller and the elliptical arc conjugate roller are manufactured separately and installed on the same roller shaft in a detachable manner.

[0029] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Any simple modifications, alterations, and equivalent transformations made to the above embodiments based on the technical essence of the present invention shall still fall within the protection scope of the present invention.

Claims

1. A fracture-resistant worm gear for an RV reducer, comprising journals at both ends and a helical tooth (6) in the middle, wherein an overtravel zone (4) is provided between the journals and the two ends of the helical tooth, characterized in that, The outer diameter of the overtravel zone is equal to the root circle diameter of the helical tooth. There is a large fillet between the helical tooth and the overtravel zone. The radius of the large fillet is 0.4m-0.6m. The large fillet surrounds the root part of the first and last rings of the helical tooth in a 360° manner. Between the end of the overtravel zone and the journal, there is a trumpet section (3) with a gradually increasing radial dimension. The outer surface of the trumpet section is a concave arc surface formed by rotating an elliptical arc. The starting part of the trumpet section is axial and tangentially connected to the end of the overtravel zone. The outer surface of the trumpet section and the outer surface of the overtravel zone are both rolled to form a reinforced outer surface layer. The reinforced outer surface layer includes a mechanical plastic deformation layer and a residual compressive stress layer below the plastic deformation layer.

2. The anti-breakage worm gear for an RV reducer according to claim 1, characterized in that, The length of the overtravel zone is greater than or equal to 3πm, and the starting point for calculating the length of the overtravel zone is the ingress or egress tip of the helical tooth.

3. The anti-breakage worm gear for an RV reducer according to claim 1, characterized in that, The major semi-axis of the ellipse is a, and the minor semi-axis is b, where the major semi-axis a corresponds to the radial direction of the worm, and the minor semi-axis b corresponds to the axial direction of the worm, and 1.5 ≤ a / b ≤ 4.

0. The axial length of the flared section is the distance between the end position of the overtravel zone and the end face of the shoulder at the journal. The end of the flared section is radial, and the elliptical arc corresponds to an elliptical parameter angle of 0-60° to 0-90°.

4. A fracture-resistant worm gear for an RV reducer according to claim 1, 2, or 3, characterized in that, The surface roughness Ra of the reinforced outer layer is ≤0.4μm, the depth of the plastic deformation layer is 0.02-0.3mm, and the depth of the residual compressive stress layer is 0.05-0.5mm.

5. The anti-breakage worm gear for an RV reducer according to claim 1, characterized in that, The tooth root curved surface within one lead at both ends of the helical tooth and the starting part of the overtravel zone are a shot peening strengthening layer (5). The shot peening strengthening layer has a residual compressive stress layer with a depth of 0.05-0.3mm. The shot peening strengthening layer and the strengthening outer surface layer formed by rolling have an overlap of 1-5mm.

6. A surface strengthening method for an anti-fracture worm gear used in RV reducers, characterized in that, This includes shot peening the tooth root surface within the first lead at both ends of the helical tooth to form a shot peening reinforcement layer, and contour rolling the overtravel area and the trumpet section to form a reinforced outer surface layer, wherein the reinforced outer surface layer is rolled at least twice.

7. The surface strengthening method according to claim 6, characterized in that, The diameter of the shot used for shot peening is 0.2-0.6 mm, and the shot peening intensity is 0.2 mmA to 0.4 mmA. The shot peening path fully covers the tooth root surface within the first lead at both ends of the helical tooth, and the shot peening area extends 1-5 mm axially into the overtravel zone.

8. The surface strengthening method according to claim 6, characterized in that, The contour rolling process uses cylindrical rollers (9) and elliptical arc conjugate rollers (7). The axial length of the cylindrical roller is equal to the axial length of the overtravel zone. The working surface (8) of the elliptical arc conjugate roller refers to the surface whose generatrix shape is designed or derived based on the principle of conjugation with the target elliptical arc surface of the trumpet segment.

9. The surface strengthening method according to claim 8, characterized in that, The working surface of the cylindrical roller is a mirror-polished surface with a surface roughness Ra value ≤ 0.1µm; the working surface of the elliptical arc conjugate roller is a mirror-polished surface with a surface roughness Ra value not greater than 0.1µm, and the axial working width is equal to the axial projection length of the trumpet segment.

10. The surface strengthening method according to claim 6, 7, 8, or 9, characterized in that, The cylindrical roller and the elliptical arc conjugate roller are integrally formed; or the cylindrical roller and the elliptical arc conjugate roller are manufactured separately and installed on the same roller shaft in a detachable manner.

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