Gearboxes and gearheads, also called gear reducers or speed reducers, are power transmission devices that use a gear arrangement in an enclosed housing to transfer energy, increase torque and reduce speed from one device to another.

Gearboxes and gearheads consist of gears arranged in a housing with a method for attaching to an input drive (a motor or drive shaft) and an output component (usually a shaft). Input and output connection configurations for gearboxes and gearheads include solid shaft, hollow shaft and integral coupling.

The gears are mounted on shafts, which are supported by and rotate via rolling element bearings. The gearbox is a mechanical method of transferring energy from one device to another and is used to increase torque while reducing speed. Torque is the power generated through the bending or twisting of a solid material. This term is used interchangeably with transmission.

Located at the junction point of a power shaft, the gearbox is often used to create a right angle change in direction, as is seen in a rotary mower or a helicopter. Each unit is manufactured with a specific purpose in mind and the gear ratio used is designed to provide the level of force required. This ratio is fixed and cannot be changed once the box is constructed. The only possible modification after the fact is an adjustment that allows the shaft speed to increase, along with a corresponding reduction in torque.

In a situation where multiple gear speeds are needed, a transmission with multiple gears can be used to increase torque while slowing down the output speed. This design is commonly found in automobile transmissions. The same principle can be used to create an overdrive gear that increases output speed while decreasing torque.

Gearbox design animation Video credit: Faris Al-Ghalib via YouTube

 Components

• Gears
• Rolling element bearings (roller bearings and/or ball bearings)
• Shafts to connect the gears to the housing and also to connect the gearbox or gearhead to the drive device and driven member
• Housing or casing to contain gears, bearings and shafts.
  • Housing may have flanges for mounting a motor or to a machine or other assembly. Housing may have mounting holes for mounting gearbox to machine or other assembly.
  • It provides structural support for the shaft bearings. This in turn helps in gear loading.
  • It transfers the reaction of mechanical rotation (torque) to the other supporting structure of the gearbox or drive elements.
  • It prevents the lubricant from spreading and also prevents the unwanted particles from entering the gear assembly.
  • It is a safety provision and reduces the noise intensity.
  • It reduces the amount of heat that generates due to the internal friction.
  • Increases the visual qualities of the gearbox.

Materials

• Gear material is typically cast iron or steel.
• Shaft material is typically steel.
• Gearbox housing is usually cast iron, steel or aluminum. The housing may be cast or machined.

Purchasing a Gearbox

Before setting out to buy a gearbox, the most important thing is to be sure about the requirements of the gearbox. The best selection of a gearbox fulfills the buyer's output requirement. This kind of successful selection can be achieved by matching the requirements of the power transmission system with a particular range of gearboxes offered by manufacturers. It is advisable for buyers to have knowledge regarding the system and available hardware in the market.

A buyer may consider:

Table credit: Gears Hub

Types

Several different types and combinations of gearboxes and gearheads are available.

Gearing Arrangements

Bevel gear is a gear wheel that is meshed with another bevel gear in such a way so that the shafts can form an angle, which is less than 180°.

Helical gears join shafts that are parallel. The involute teeth of the helical gears are cut at an angle to the axis of the rotation. If there are two mating helical gears in the gearing arrangement of the gearbox, then they must have equal helix angle but opposite hands.

Spur gears, are straight-toothed gears with radial teeth that transmit power and motion between parallel axes.

Planetary gears have two or more small gears that are mounted either on the outside or inside of a large a large gear. Large gear ratios can be achieved with a planetary gear arrangement. Output rotation is in the same direction as the input rotation.

Cycloidal gears are used in pair form and they are arranged in such a way so that the angle formed by them should be equal to 180°. The reason behind the formation of 180° is to provide balance to the load and these gears are driven by many crank shafts. Multiple shafts are present for sharing the load and to enhance the twisting state firmness.

Worm gears are cylindrical gears with a spiral thread that drive mating worm wheels in high-reduction gearset applications. They operate on non-intersecting perpendicular axes.

Additional Arrangements

Harmonic gearing utilizes a nested gear within a circular spline, the inside one being flexible and containing two fewer teeth. Each turn of the inside moves the flexible gear anti-clockwise on the circular spline

Hypoid gearing is a right-angle, non-intersecting drive using a drive gear (resembling a worm) offset from the center of the driven gear. 

Shaft Alignment

In parallel in-line (simply, in-line) alignments, the input (driving) shaft and the output (driven) shaft can be connected with an imaginary axial line through the center of each shaft at 0° angle.

In parallel offset alignments, the input (driving) shaft and the output (driven) shaft are parallel. However, they reside in the gearbox at different heights above the horizon.

In right angle alignments, the input (driving) shaft and the output (driven) shaft are perpendicular to one another. This is most common with worm gear speed reducers.

Non-perpendicular angled shafts are uncommon. In this alignment, the input (driving) shaft and the output (driven) shaft are not perpendicular to one another, but they are angled. 

Specifications

It is particularly important to consider the gear ratio and torque need for each application.

Gear ratios are generally specified at X:1, where X is an integer. To ease the burden on our search engine, this ratio is entered as X/1, or X. Therefore a ratio of 5:1 is entered as 5. A 5:1 ratio means that a motor input of 1750 RPM is converted to 350 RPM.

Output torque is a measure of angular force that produces rotational motion. Torque is the real consideration for output specification. Horsepower output has little application relevance.

Input power is a reducer's input horsepower rating that represents the maximum prime mover size the reducer is designed to handle. 

Exceeding the maximum input speed can result in oil "churning," which is detrimental to the life of the speed reducer.

Features

Multiple output shafts can be driven by a single input shaft. The output shafts are usually parallel and in-line. However, some unique configurations exist that allow for offset shafts to be driven at different speeds. 

A reaction arm prevents to reducer housing from rotating when there is no base mounts or flanges. 

Applications

Gearboxes are used for many applications including machine tools, processing and other industrial equipment, conveyors, mixers, extruders, wind turbines, winches, cranes, pipelines, agricultural equipment and many other rotary motion power transmission applications that require changes to torque and speed requirements.

Resources

Wisegeek

Agro Engineers

Image credits:

Branham, W.C. Inc. | ATLANTA Drive Systems, Inc. | U.S. Tsubaki Power Transmission, LLC