Everything You Want to Know About Gear & Gearbox Manufacuturing

1. Introduction

Gears play a prominent role in mechanical power transmission. A gear or cogwheel is a rotating machine part having cut teeth, or cogs, which mesh with another toothed part to transmit torque.

Geared devices can change the speed, torque, and direction of a power source. Gears almost always produce a change in torque, creating a mechanical advantage, through their gear ratio, and thus may be considered a simple machine. The teeth on the two meshing gears all have the same shape. Two or more meshing gears, working in a sequence, are called a gearbox.

Gears of various type, size and material are widely used in several machines from simple wall clocks and wrist watches to simple machines to complex machines and manufacturing machine tools, to automobiles, aviation, defense, to very large gear boxes are used in large ships, cranes, wind turbines, Railways, aviation, space telescopes to construction machinery, for dam gate operations and heavy weight lifting machines and systems requiring positive and stepped drive.

2. Application

Gearbox is a mechanical system formed by mounting gears on a frame so the teeth of the gears engage. Gear teeth are designed to ensure the pitch circles of engaging gears roll on each other without slipping, providing a smooth transmission of rotation from one gear to the next. The speed ratio for a pair of meshing gears can be computed from ratio of the radii of the pitch circles and the ratio of the number of teeth on each gear. Friction and wear between two gears is dependent on the tooth profile. The most commonly used in modern times, is the involute profile.

Features of gears and gearbox include:

•  The ratio of the pitch circles of mating gears defines the speed ratio and the mechanical advantage of the gear set.
•  A planetary gear train provides high gear reduction in a compact package.
•  It is possible to design gear teeth for gears that are non-circular, yet still transmit torque smoothly.
•  The speed ratios of chain and belt drives are computed in the same way as gear ratios. There are several types of gears viz external gear with the teeth formed on the outer surface of a cylinder or cone and internal gear with the teeth on inner surface. Gears are generally specified by their type of tooth and blank shape e.g. spur, bevel, spiral etc., material, size or dimensions, geometry and special features, if any. The main types of gears can be classified as Spur, Helical, Worm Gears, and Bevel Gears etc.
•  Spur gears have straight-cut tooth aligned parallel to the axis of rotation. Each tooth has involute profile these gears are used on parallel shafts and no axial thrust is created.
•  Helical Gears are having tooth follow helix curve along the cylinder surface and therefore, leading edges of the teeth are not parallel but at an angle to the axis of rotation. The angled teeth engage more gradually than do spur gear teeth, causing them to run more smoothly and quietly. The helical gears invariably produce axial thrust load. The helical profile can also be used to mesh with shafts axis oriented in various angles and these gears are called "skew helix gears".
•  A bevel gear is shaped like a circular cone with most of its tip cut off. When two bevel gears mesh, their cone vertices are at the same point. Their shaft axes intersect at this point, forming an angle between the shafts. Spiral bevel gears are also manufactured as circular arc with non-constant tooth depth or circular arc with constant tooth depth. Spiral bevel gears have the same advantages and disadvantages relative to their straight-cut cousins as helical gears do to spur gears.
•  A worm gear is when having large helix angle, close to 90 degrees and fairly long in the axial direction. I.e., forms screw like shape. Worms and worm wheel form a special gear type, where worm resemble screws and meshes with a worm wheel, which envelopes the worm screw. A worm-and- wheel set is compact way to achieve a high torque, low speed gear ratio.
•  Special gears called sprockets are formed to engage with chains, viz bicycles and motorcycles. Even for belt drive pulleys with teeth are used in the timing belt drive, to synchronize the movement.

When gears are put in a frame or box, it is called Gear Box. There are several applications for witch, standard speed and power ratios are made available. The gear boxes have several purposes viz power/ torque amplifier or speed reducer where output gear has more teeth than the input gear. Conversely, if the output gear has fewer teeth than the input gear, then the gear train reduces the input torque and increases the speed of output shaft. All configurations and applications are possible for gears and gear box designs.

Gears are widely used in various mechanisms and devices to transmit power and motion positively without slip between parallel, intersecting axis and nonintersecting, non-parallel shafts, without change in the direction of rotation, with change in the direction of rotation, without change of speed of rotation, with change in speed at any desired ratio. Often some gearing system like rack – and – pinion is also used to transform rotary motion into linear motion and vice-versa.

The major applications are: Speed gear box, feed gear box and some other kinematic units of machine tools, Speed drives in all types of simple to complex manufacturing machinery, automobiles, Speed and / or feed drives of several metal forming machines, all most all industrial Machinery and appliances use gears and gear boxes. Large and heavy-duty gear boxes are used in mining, cement industries, sugar industries, cranes, conveyors etc. Precision equipment like clocks and watches and industrial robots and toys also use gears.

3. Industry and Status Sqo

Gears and gearboxes are widely used in various industries from heavy machine equipment like cement, construction, mining industries, power, fertilizer, machine tools, machine products, industrial machines and equipment in food processing, pharma, dairy, etc.

With its use in assembly lines, conveyor belts, bottling, packaging, and positioning of products, wind energy to fossil fuels power plants, the demand for precision and efficient gearboxes is expected to get a boost. The automated material handling sector will witness steady growth with new equipment installation, growing production facilities, extension of plants and construction of industries. The demand from all the industrial sectors in addition to auto sector will support the growth.

The presence of substitutes such as direct drive systems may hamper the market. These products eliminate the use of gears, thus reducing the weight and complexity, and increasing the efficiency as there is no energy loss. Use of direct drives through software is limited to precise speed control applications. But this shift from gear box is hard to affect the high power/ torque products as gear box has lower overall costs then the direct drive systems. The gear products are seeing huge jump in production with 3D modeling CNC machining, grinding etc. for design and manufacture precision and custom products at low costs.

Gears and gear boxes are products that are very specialized machined component. The manufacturers of industrial machinery and all most all end user segments have to normally source these products from special gears and gear boxes manufacturing units. Even the automobile gears are supplied by specialized manufacturing units.

Geared Motors – electric motors built in with gears are emerging as new trend in this sector mostly in range of 7.5 kW to 75 KW. The largest segment of the gear motors market is 7.5 KW to 15 KW range as it has large number of applications. Most applications are in material handling, food & beverage, and automotive sectors.

Gear motors are likely to grow in demand throughout the industry for production, transportation, and processing industries. In view of the rapid growth of industrial machinery and equipment sector, there is very good scope for a gear and gear box manufacturing unit with design, and manufacturing facilities and capabilities. The new unit specializing in specific product range and customer / product group in end users will have easy and successful access to the market. Choosing niche products like geared motors for supply to customers can be very helpful.

4. Raw Materials

Various grades of alloy steels are most commonly used because of their high strength-to-weight ratio and low cost. These are either cast or forged depending on end application. Gear production is done by machining of standard stock items like rods, billets. Numerous nonferrous alloys, cast irons, powder-metallurgy and even plastics are used in the manufacture of gears. The gear blanks are produced by die cast, investment casting, and powder metallurgy etc. processes. The project may select product mix and select gear blank process viz casting / forging to focus the end consumer segments.

5. Manufacturing Process

Manufacture of gears needs several processing operations in depending upon the material and type of the gears and quality desired. The stages generally are:

• preforming the blank without or with teeth
• Annealing of the blank, if required, as in case of forged or cast steels
• Preparation of the gear blank to the required dimensions by machining
• producing teeth or finishing the preformed teeth by machining
• Full or surface hardening of the machined gear (teeth), if required
• Finishing teeth, if required, by shaving, grinding etc.
• Inspection of the finished gears.

Gear blanks and even gears along with teeth requiring substantial to little machining or finishing are produced by various casting processes.

•  Sand mold casting: for large cast iron gears, low speed machinery and hand operated devices.
•  Shell mold casting: Small gears in batches are often produced by this process.
•  Centrifugal casting: The solid blanks or the outer rims (without teeth) of worm wheels made of cast iron, phosphor bronze or even steel are preferably performed by centrifugal casting. The performs are machined to form the gear blank of proper size. Then the teeth are developed by machining.
•  Metal mold casting: Medium size steel gears with limited accuracy and finish are often made in single or few pieces by metal mold casting. For general and precision use the cast preforms are properly machined.
•  Die casting: Large lot or mass production of small gears of low melting point alloys of Al, Zn, Cu, Mg etc. are done mainly by die casting. Such reasonably accurate gears are directly or after little further finishing are used under light load and moderate speeds, for example in instruments, camera, toys.
•  Investment casting: This near-net-shape method is used for producing small to medium size gears of exotic materials with high accuracy and surface finish hardly requiring further finishing. These relatively costly gears are generally used under heavy loads and stresses.

It is estimated that almost 80% of all gearing produced worldwide is produced by using gear blanks cast, forged, in near final shape.

Machining:

The most common form of gear machining is cutting metal by tools called hob. The hobbing cutters rotate and mesh with gear blank like a meshing gear thereby generating teeth profile on blank. Other processes like gear shaping, milling, and broaching also exist. For metal gears in the transmissions of cars and trucks, the teeth are heat treated to make them hard and more wear resistant while leaving the core soft and tough. For large gears that are prone to warp, a quench press is used.

Finishing Processes:

Gear-tooth shaving, grinding, honing and lapping is the finishing processes that provide tooth profile correction, accurate tolerances and surface finish. Gear honing machines produce teeth to reduce the surface roughness of the tooth profile. Gears are lapped on gear-lapping machines after they have undergone heat treatment.

Quality Control:

Overall gear geometry is inspected and verified using various methods such as coordinate-measuring machines, white light scanner or laser scanning. Metal composition is testes at blank stage. Other tests like teeth skin hardness etc. are done as per requirements. Important dimensional variations of gears result from variations in the combinations of the dimensions of the tools used to manufacture them. An important parameter for meshing qualities is backlash. Precision gears are inspected by a method where meshing gear vibrations are recorded showing variations with a high resolution as the gear was rotated.

6. Conclusion

A gear is a component part of gearboxes and is characterised by teeth around a circular or cylindrical surface, commonly known as a gear blank. This article provides a basic understanding of gear box manufacture, application, industry and manufacture process of gear box.

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