Gear Manufacturing Processes

In this tutorial, you will see the different parameters calculated when manufacturing gears and explore the various gear manufacturing processes. In short, you will learn about the common materials and processes involved in the manufacture of gears.

Contents:

1. Selection of Gear Materials
2. Designing Gear Blanks
3. Deciding Number of Teeth
4. Calculating the Face Width
5. Gear Forming Processes
6. Gear Generation Processes
7. Gear Finishing Processes
8. Why is Gear Lubrication Important?

Selection of Gear Materials

Gears are among the most common assembly components which are used to transfer power. Their design characteristics make them a suitable choice for applications that we see around us. However, the basis of good gear is how well it is designed and how strong the material is.

• The load capacity of any gear depends on the ultimate tensile or yield strength of the material. The gear material should have sufficient strength to resist the breaking of the tooth.
• For high-speed applications in power transmission, the sliding velocities are very high, and the material should have a low coefficient of friction to avoid scoring.
• Most Gears are made from cast iron, steel, bronze, or even phenolic resin. Large gears are manufactured by casting and are made of gray cast iron of Grades FG 200, FG 260, or FG 350. They are cheap and produce less noise and have good wear resistance.
• Due to thermal distortion and warping, the loads may get concentrated on the gear tooth. Alloy steels are consistent in this regard due to consistent thermal distortion.
• Case Hardened steel gears offer the best combination of a wear-resisting hard surface with a ductile and shock absorbing core combined.
• Bronze is often used for worm gears due to its inherent conformability and low coefficient of friction. Its corrosion resistance makes it useful in applications like water pumps.
• Non-metallic gears are made from materials like molded nylon, phenolic laminates like Bakelite and Celoron. They are used for light loads and when long lives are expected. They work with minimum vibrations and noise.
• For heavy-duty applications and planetary gears, alloy steels are recommended.

Designing Gear Blanks

Gears are manufactured from gear blanks which are derived from raw materials. Any gear requires five basic dimensions to be manufactured, and they are the Pitch Circle Diameter, the Addendum and the Dedendum, the Shaft Diameter, and the Width of the gear. Depending on the purpose and size of the gears, they are classified into small, medium, and large size gears.
The following figure shows the common dimensions required for gear manufacturing.

• Small size gears with root diameter near the required shaft diameter are made integral with the shaft. It reduced the amount of machining as there is no requirement for keyways. This reduces the cost and increases the rigidity of the shaft and the accuracy of contact. Integral manufacturing is only suitable for small gears. When the dedendum is significantly larger than the shaft diameter, it must be manufactured separately.
• Medium-sized gears are manufactured by machining of stock directly or forging the gears. The gear bars can be machine rolled or simply turned on lathes, depending on production volumes. Forging is preferred for large production sizes. Forged gears have lightweight and molded fiber orientation of the material, making them inherently stronger than machined gears.
• Large diameter gears have either a solid cast body or a rimmed or webbed body. They can be cast when the diameter size is very large, however, cast gears have a low torque transmission capability. Rimmed gears have a steel rim mounted on the central casing with hubs, arms or webs, and are usually forged using alloy steel.

Deciding Number of Teeth

The gear teeth are the most important aspect of any gear because, without them, power transmission is not possible at all. During gear design, it is required to decide the number of teeth on the pinion and gear. The module determines the size of the gears, but there is a limit to the number of teeth possible on a gear.

• As the number of gear teeth decreases, there will be interference between the standard tooth profile required for modification. The minimum number of teeth to avoid interference is given by the following equation
  z_min = \(\frac{2}{(sin \,⍺)^2}\)
  Where ⍺ is the pressure angle.
• In practice, a slight radius on the gear tip can further reduce the limit value. The following table lists the theoretical and practical value for the minimum number of teeth

  Pressure Angle	Theoretical Limit	Practical Limit
  14.5°	32	27
  20°	17	14
  25°	11	9

  Table 1. Minimum Teeth Count

• Gears often compensate for the hunting which may result in uneven tooth wear. When the meshing gears have a common divisor in their tooth count, the same pair of teeth engage periodically. To prevent this, an extra tooth called the hunting tooth is added which results in a more distributed wear.

Calculating the Face Width

The face width of gears is another property that determines the blank dimensions and is calculated from the module of the gear. The face width is where the tangential force is uniformly distributed, hence a proper width is important for a functional gear.

• If the face width is too large, there is a possibility of load concentration at one end due to misalignment, deformation, or warping of the tooth.
• Gears with a small face width have a poor capacity to absorb vibrations or resists shocks developed during usage and thus wear fast.
• The optimum range of the face width is ideally in the following range, 8m < b < 12m, where m is the module and b is the face width. In preliminary stages, the face width is assumed to be ten times the module of the gear, b = 10m.

Gear Forming Processes

Once the gear blanks are formed, there is a need for the final manufacture of the gears. Gear forming processes are among the most popular methods of gear manufacture. Forming processes include majorly two methods, form milling and broaching.

• Form milling involves a form cutter traveling axially along the length of the gear tooth. Once each tooth is cut, the cutter is slowly withdrawn and the blank is rotated before the cutter is employed to cut another tooth. The process is continued until all teeth are cut.
• Broaching involves forming teeth, especially for internal gears with the help of a special tool called a broach. The tool is placed in a pre-created groove and pulled. As the broach travels the length of the gear, it cuts the teeth on the gear. Broaching makes production in the gear industries exceptionally fast.

Gear Generation Processes

Gear generation involves tooth flanks obtained from the outline of the subsequent positions of the cutter which is in the form of the gear pair. One of the most famous and efficient processes is the gear hobbing process.

• Gear hobbing is a machining process where the gear teeth are generated by multiple cuts made from a helical tool. The hobbing process involves rotary motion where the blanks continuously rotate as the teeth are cut.
• The hobbing process uses a multipoint cutting tool called the gear hob which looks like a worm gear having several flutes shaped at proper angles to serve as cutting edges. The hob rotates at a sufficient speed as it is fed into the blank.
• The hobbing process is controlled with three major parameters – indexing movement, feed rate, and the angle between the axis of gear blank and the hobbing tool. Proper gear arrangement is used to maintain the speed ratio of gear blank and hob.
• Spur and helical gears are made using a hobbing process involving an axial feed. Here, the gear hob is fed to the blank along the face of the gear blank parallel to its axis.
• When the hob and the blank are set with their axis normal to one another, it is termed hobbing with radial feed. The rotating hob is fed against the blank in a radial direction or perpendicular to the axis of the blank. It is used to make worm gears.
• Hobbing with tangential feed is also used for cutting the teeth on a worm wheel. The hob is placed with its axis horizontal but at a perpendicular axis and then fed forward axially.
• Hobbing is a fast and continuous process with a lower production cycle. It is a versatile process with a wide variety of possibilities. Most importantly, several blanks can be mounted on the same arbor and processed simultaneously. However, gear hobbing cannot create internal gears.

Gear Finishing Processes

Surfaces of the teeth produced because of manufacturing processes are not accurate and need improvement. Dimensional inaccuracies generated become a source of noise and wear and can also result in backlash and play. To overcome these problems finishing operations are recommended.

• Gear shaving is the process of finishing by running the gear at high velocity in mesh with a gear shaving tool. The tool is a type of rack or pinion with hardened serrations which serve as the cutting edge.
• Roll finishing involves using two hardened rolling dies placed accurately with respect to the profile of the gear being finished. Pressure is exerted by both the dies on the gear. The dies are very strong and phase out by plastic deformation any deformities and burrs on the teeth.
• Gear grinding and lapping processes involve a reciprocating shaft in mesh with the gears. In grinding processes, the gears are fed against a grinding wheel which provides finish. In the lapping process, an abrasive paste is introduced on the teeth and lapped under load in mesh with a cast-iron lap.
• Gear honing is a superfinishing process where the hones are rubbed against the profile of the gear tooth.

Why is Gear Lubrication Important?

Proper lubrication for gears in service is essential to allow the gears to provide satisfactory performance and ensure the durability of the system. Lubrication reduces frictional wear and allows for a smooth contact between meshing teeth while preventing any impurities from affecting the system.

• Gears are lubricated with grease mineral oils or extreme pressure lubricants. Grease is used for manually operated gear mechanisms with intermitted low-speed applications.
• Medium velocity systems have a box enclosure around the system which is filled with mineral oil. This is the process of splash lubrication. Some cases see a jet spray of oil-directed towards the meshing teeth.
• For heavy-duty applications, extreme pressure lubricants are used. These lubricants are simply mineral oils with additives to enhance performance and are commonly used in automobile gearboxes.
• Gear lubrication systems are attached with oil seals and gaskets to prevent leakage, plugs for input points and drainage, and often an oil level indicator.