There are many ways to manufacture bevel gears. Here we only introduce manufacturing bevel gears through machining because it is the most common, most widely used, and highest quality bevel gear production method.
1. Bevel gear blank manufacturing
1.1 Selection of bevel gear blank
Three primary forms of bevel gear blanks are steel round bars, forged blanks, and castings. According to the size and quality requirements of the gear, choose different blank forms.
- Forged blanks: When the mechanical properties of bevel gears are required to be high, forged gear blanks are usually used. Forging is divided into many types; closed-die-forging and open-die-forging are the most common applications in processing bevel gear blanks.
- Die casting: When the pitch circle diameter of the gear is more significant than 400mm, it isn’t easy to achieve this size with ordinary round steel diameter and forging. Castings are usually used as the blank of the bevel gear.
- Steel round bars: Steel round bar is used for bevel gears with relatively small sizes, simple structures, and low strength requirements. By cutting into desired gears, bevel gear cutting is performed directly after heat treatment.
1.2 Blank heat treatment
Before the bevel gear blank is subjected to gear cutting, initial heat treatment is required to eliminate residual stress, improve the material’s machinability, and enhance the comprehensive mechanical properties. The commonly used heat treatment methods are normalizing and tempering.
1.3 Blank processing
1.3.1 Turning (lathe)
Turning is one of the essential methods of mechanical processing. In the production process of bevel gears, turning is mainly used to process the surface and the end face of bevel gear blanks, remove most of the remaining margin of the blanks and make the size of the gear blanks more precise.
It can perform cutting, grooving, threading, drilling, reaming, and other work. Turning work is carried out by lathes. There are many types of lathes, among which CNC lathes are the most efficient and have the highest precision and cost.
1.3.2 Key points of processing blank
Datum plane
The primary task of gear blank processing is to ensure the accuracy of its datum plane (generally speaking, an end face and a hole for shaft gears, an end face, and an excircle), including dimensional shape accuracy and mutual position accuracy.
The accuracy of the datum plane of the gear blank will directly affect the machining accuracy of the gear, so in the process of producing bevel gears, the machining accuracy of the datum plane should be improved as much as possible on the premise of improving the economy.
Tip distance and face cone
The second task of rough gear machining is to ensure the crown distance and face cone correctness. Under different gear production standards, the limit deviation of the outer diameter, crown distance, cone angle, and the generatrix of the cone are recommended. Runout tolerance. (This article briefly introduces the production process of bevel gears and does not detail various standards and data.)
Generally speaking, the crown distance of gears only exists in theory and cannot be found on the actual blank, so it can only be controlled by technological means. In mass production, the adjustment of the multi-tool lathe is supplemented by the inspection of the gear blank; in small batch production, the wheel crown distance is generally controlled by controlling the outer diameter and using the front cone and rear cone templates.
2. Gear cutting
There are many ways to process the tooth profile of bevel gears. The most suitable processing method is selected according to a series of factors such as the structure, material, size, heat treatment, precision and production batch of the part.
2.1 Gear milling
Gear milling is a method of machining gear teeth on a multi-purpose milling machine using a form gear milling cutter. When the gear modulus m<8, it is generally milled with a disc milling cutter on a bedroom milling machine. When the bevel gear modulus m≥8, process it on a vertical milling machine with a finger milling cutter.
When milling bevel gears, the parts are installed on the dividing head of the milling machine, the modulus milling cutter makes the central movement of rotation, and the table makes linear feed movement. After machining a space between teeth, exit the tool, index according to the number of teeth Z, and then mill the next tooth. This way, milling is carried out tooth by tooth until all the spaces between the teeth are milled.
2.2 Dry cutting
The dry cut is a method of cutting bevel gears without the use of cutting fluid to protect the environment and reduce costs. It is the most efficient bevel gear-cutting method, so the requirements for bevel gear-cutting equipment are also higher.
Dry cutting is not simply to stop using cutting fluid but to ensure high efficiency, high product quality, high tool durability, and reliability of the cutting process while stopping using cutting fluid, which requires excellent performance tools, machines, and auxiliary facilities to replace the role of cutting fluid in traditional cutting to achieve real dry cutting.
2.3 Gear-shaping
Shaping is commonly used to produce straight bevel and internal gears. Gear shaping is processed according to the principle of a pair of straight bevel gears or internal gear meshing. The gear shaping cutter is equivalent to a gear with a cutting edge that grinds the front and rear angles on the teeth of the gear, and the gear blank is used as another gear when working is to use the cutting edge on the tool to cut.
During gear shaping, the movement between the gear shaping cutter and the workpiece has the following forms:
- Main movement.
The position of the workpiece remains unchanged, and the gear shaper cutter reciprocates up and down or back and forth.
- Tooth-splitting movement.
The meshing movement relationship of a pair of gear pairs is strictly maintained between the gear shaper cutter and the gear blank.
- Radial feed movement.
To make the gear shaper cutter gradually cut to complete teeth, each time the gear shaper cutter reciprocates, it should have a radial feed motion to the centre of the workpiece.
- Tooling relieving movement.
To avoid the wear and tear of the flank and the machined surface when the gear shaper cutter returns and retracts, the workpiece should leave the tool for tool movement. When the gear shaper cutter continues to cut, the workpiece returns to its original position.
The technological characteristics of gear shaping are as follows:
- High processing precision.
The manufacturing, sharpening, and inspection of gear shaping cutters are relatively simple, and it is easy to ensure manufacturing accuracy. However, the gear-splitting transmission chain of the gear-shaping machine is somewhat complicated, and the transmission error is relatively large compared with gear hobbing. The precision of gear shaping is higher than that of milling.
- The Ra value of the tooth surface roughness is small.
When shaping the gear, since the gear shaping cutter cuts continuously along the entire length of the tooth, the number of tangents of the tooth profile envelope formed is also more than that of the gear hobbing. Therefore, the gear processed by the gear shaping The surface roughness is better than hobbing and milling, and the surface roughness can reach 1.6um.
- Low production efficiency.
Since the gear shaper cutter makes a linear reciprocating motion during gear shaping, the increase in speed is limited, so the production efficiency is lower than that of gear hobbing.
2.4 General machining centre
The three bevel gear cutting methods mentioned above all use special bevel gear machine tools, but each machine tool has a limitation on the processing size. It is challenging to cut gears with special bevel gear equipment for some bevel gears with more complicated shapes. The general machining center can make up for this defect.
3. Rolling test
Rolling inspection uses a rolling inspection machine to mesh a pair of bevel gear pairs and rotate them for inspection. In this detection method, the detection content is mainly:
- Mounting distance
- Contact spots
- Backlash and variation
- Noise
In general, the contact state of the teeth of the bevel gear in the transmission will also change as the load changes. So when the load is light, 100% contact is not good. Moreover, even in the same contact state, there are insignificant deviations in the tooth top, tooth root, outer end, and inner end. Therefore, when measuring, the direction of the tooth line is from the outer end to the inside, and generally use, about 60% of the tooth line length as the gear contact center.
The contact area of the gear changes significantly according to the thickness of the paint applied to the tooth surface, so it is not very precise.
4. Deburring, chamfering & machining
4.1 Deburring
During the gear-cutting process of the bevel gear, burrs will be formed at the intersection of the faces of the parts. The presence of burrs may cause the entire mechanical system to fail to operate normally, reducing reliability and stability. When a machine with burrs moves or vibrates mechanically, the detached burrs will cause premature wear on the sliding surface of the device, excessive noise, and even the mechanism will be stuck, and the action will fail. Therefore, all burrs on the gear surface must be removed after tooth cutting.
There are probably the following methods for deburring:
- Manual trimming
Manual trimming is a relatively traditional method. Manual trimming is usually used in many small factories with incomplete equipment or burrs that are difficult to remove by industrial processes.
- Electrolysis deburring
Use electrolysis to remove burrs from the gear surface, also named ECD.
- Ultrasound
Utilizes the “cavitation phenomenon” generated by ultrasonic waves to remove burrs. This method is mainly aimed at some fine burrs. It is difficult to remove the burrs visible to the naked eye by ultrasonic.
- High-pressure water jet
Using water as the medium, use its immediate impact to remove burrs and simultaneously achieve the purpose of cleaning.
- Thermal explosion
Also known as electrothermal deburring, this is an advanced technology recognized by the world’s machinery manufacturing industry as the most suitable for deburring small workpieces in large quantities.
- Manipulator
This is a typical mechatronic device that comprehensively uses the research results of various disciplines, such as precision machinery, computer, automatic control, sensor, and artificial intelligence.
4.2 Chamfering
Chamfering is a processing method that cuts the corners of a part into a particular slope. Generally, the role of chamfering is divided into the following:
- Deburring.
- Reduce the stress concentration of parts and strengthen their strength.
- It plays the role of guiding and positioning during assembly.
4.3 Machining
The bevel gear must be machined with threads, keyways, holes, and other parts to assemble better and fix. According to different production methods and product details, the time nodes of these processing processes are not selected, but generally speaking, they will be performed before heat treatment. Because after the bevel gear is hardened, the surface hardness will increase, which will increase the difficulty of further processing. Therefore, many processing processes will be carried out before heat treatment.
5. Heat treatment
Bevel gear heat treatment refers to heating the bevel gear to close to or above its critical temperature, keeping it for a specific time, and then cooling it in a particular medium, which can be air, water or molten salt.
Gear heat treatment is a critical and complex element in gear manufacturing, affecting how each gear transmits power or motion to other components in the assembly. Heat treatment optimizes the performance and life of gears by altering their chemical, metallurgical and physical properties. These characteristics are determined by considering the geometry of the gear, power transmission requirements, duty cycle rates, stress at various points within the gear under load, mating part design and other operating conditions.
Gear heat treatment can improve the physical properties of the gear surface, such as hardness, thereby endowing it with wear resistance and preventing simple wear of the tooth surface and bearing surface. Heat treatment also improves the fatigue life of gears by creating subsurface compressive stresses to avoid pitting and deformation caused by high contact stress on gear teeth. These same compressive stresses prevent fatigue failure at the gear root due to cyclic tooth bending.
Generally, steel bevel gears adopt the following heat treatment processes:
- Surface hardening
Surface hardening is usually applied to bevel gears made of medium carbon steel and alloy steel, such as ASTM 1045 and ASTM 5140. After surface hardening, the tooth surface hardness is generally 40-55HRC. It is characterized by anti-fatigue pitting, high anti-glue ability and good wear resistance. Since the tooth’s centre is not hardened, the gear is still tough enough to withstand not particularly large impact loads.
- Carburizing and quenching
Carburizing and quenching are often used for bevel gears produced from low-carbon and low-carbon alloy steel, such as ASTM 1025 and ASTM 5120. After carburizing and quenching, the hardness of the tooth surface can reach 58-62HRC, and the tooth centre can still maintain high toughness. The gear has high bending strength, tooth surface contact strength, and good wear resistance. It is usually used in applications susceptible to impact loads—gear transmission. After the gear is carburized and quenched, the deformation is relatively large, so it is usually ground.
- Nitriding
Nitriding is a surface chemical heat treatment. After nitriding, no other heat treatment is required, and the tooth surface hardness can reach 700-900HV. Due to the high hardness, low process temperature and small deformation of the gear after nitriding treatment, it is suitable for internal gears and gears that are difficult to ground; it is often used for nitriding steel containing alloy elements such as chromium, copper, and lead.
- Conditioning
Quenching and tempering are generally used for medium and medium carbon alloy steel. The tooth surface hardness after quenching and tempering treatment is generally 220-280HBS. Because the hardness is not high, gear finishing can be done after quenching and tempering heat treatment.
- Normalizing
Normalizing can eliminate internal stress and refined grains and improve mechanical properties and cutting performance. Gears with low mechanical strength requirements can be normalized with medium carbon steel, and gears with large diameters can be normalized with cast steel.
6. Grinding
After heat treatment, the gear surface may be slightly deformed. In addition, ordinary machining makes achieving very high gear precision and smooth surface challenging. Therefore, bevel gears with high precision requirements must be ground and finished on the tooth surface.
In addition to grinding the tooth surface, the contact area where the bevel gear needs to be installed with other components, such as the contact area where the bearing and the sleeve are installed, also needs to be ground. Of course, the grinding of these positions is much simpler than that of the tooth surface.
For bevel gears that have not been ground, achieving AGMA class 11 accuracy is challenging, let alone higher accuracy. The precision of the ground bevel gear can reach at least AGMA class 11.
7. Inspection
Up to now, the production of the bevel gear has been completed. Before delivery, it must carry out a comprehensive inspection to verify whether it meets the production requirements and performance level. Bevel gears’ detection content and methods vary according to production methods, sizes, and applications. Here is the primary detection content:
7.1 Accuracy measurement
- Individual pitch deviation
- Adjacent tooth pitch deviation
- Cumulative pitch deviation
- Normal pitch deviation
- Tooth profile offset
- Cogging
- Helix deviation
7.2 Hardness testing
The hardness measurement of the bevel gear is divided into measuring the surface hardness and core hardness because the surface hardness and core hardness of the bevel gear will be different after heat treatment. Hence, they need to be measured separately. In addition, for gears after surface hardening, carburizing, and quenching, it is also necessary to measure the thickness of the hardened layer.
The range and hardness measurement methods are also other for bevel gears of different materials and heat treatment processes. At present, the primary forms of measuring gear hardness are:
- Rockwell hardness
- Brinell hardness
- Vickers hardness
- Shore Hardness Tester
- Portable Hardness Tester
7.3 Dimension measurement
The purpose of dimensional testing is to test whether the dimensions and tolerances of each part of the bevel gear meet the production requirements. If the dimensions and tolerances exceed the limited range, the gear cannot be installed and used commonly. Different gears have another appearance, so the inspection content is added. However, the appearance inspection of bevel gears should follow one principle: the size, tolerance and installation distance of each part of the gear marked in the production drawings should be strictly checked.
8. Rust protection and packaging
After the inspection is completed, if every inspection index of the bevel gear meets the requirements, this is something to be excited about. Still, our work is not entirely over because the gear may experience long transportation and multiple handling. It may also be stored in the warehouse for a long time, waiting to be installed and used, so it is easy to cause rust or be bruised during handling. Therefore, anti-rust treatment and proper packaging are essential. We usually apply anti-rust oil on the surface of the finished bevel gear for anti-rust treatment.
