What is a Gearbox?

What is a Gearbox?

Gearboxes are mechanical devices which transfer energy from the driving device (usually a motor) to the rest of the system. The gearbox is mounted to the motor shaft and, through the internal configuration of mated gears within the housing, produces an increased output torque and decreased output speed.

Advancements in technology and the evolution of gear production have given rise to more efficient and powerful gearboxes being developed and manufactured at lower costs. From simple fixed-axis gear systems, toothed gear systems have evolved into new and improved gear styles including helical, bevel, spur, worm and planetary gear systems, each available in differing configurations such as in-line, right-angle, and rotating-flange styles.

Advantages of a Gearbox

• Low noise level
• High efficiency
• High reduction ratios
• Increased output torque
• Decreased output speed
• Durable

Disadvantages of a Gearbox

• More costly than other drive systems
• Proper lubrication and maintenance required for smooth operation
• Poorly cut teeth may result in excessive vibration and noise during operation

Physical Properties of a Gearbox

Most gearboxes are constructed from steel, iron, aluminum and brass but can also be made with plastics such as polycarbonate or nylon. Other than the raw materials used, the orientation of the gear teeth play a major role in the overall efficiency, torque and speed of the system. Gearboxes with straight bevel gear teeth are typically used in low-speed applications, as they can be noisy and may have lower overall efficiency. Helical and spiral-bevel gearboxes are typically used in high-speed applications, as they are operate more quietly and with greater overall efficiency than straight gear teeth gearboxes.

How do Gearboxes Work?

All gearboxes operate on a similar principle: the direction in which the gears rotate depends on the input direction and orientation of the gears. For example, if the initial gear is rotating in a clockwise direction, the gear it engages will rotate counterclockwise. This continues down the line for multiple gears.

The combination of different gear sizes and the number of teeth on each gear plays a significant role in the output torque and speed of the shaft. High gear ratios allow for more output torque and lower speeds, while lower gear ratios allow for higher output speed and less output torque.

A planetary gearbox works in relatively the same way. A planetary gearbox system is constructed with three main components: a central sun gear, a planet carrier (carrying one or more planet gears) and an annulus (an outer ring). The central sun gear is orbited by planet gears (of the same size) mounted to the planet carrier. The planet gears are mated with the sun gear while the outer rings teeth mate with the planet gears.

There are several configurations for a gearbox system. Typical configurations consist of three components: the input, the output and one stationary component. For example, one possible configuration utilizes the sun gear as the input, the annulus as the output, with the planet carrier remaining stationary. In this configuration, the input shaft rotates the sun gear, the planet gears rotate on their own axes, simultaneously applying a torque to the rotating planet carrier that in turn applies torque to the output shaft (in this case, the annulus).

The rate at which the gears rotate (gear ratio) is determined by the number of teeth in each gear. So, adding a 3:1 gearbox to a motor with a speed of 300 RPM would result in an output speed of 100 RPM, with the output torque increasing inversely and proportionally. The torque (power output) is determined based on both the number of teeth and which component in the planetary system is stationary.

Where are Gearboxes Used?

Depending on the application requirements, different gearbox types may be more suitable than others. Gearboxes of all types are used in a variety of industries:

Industries Where Gearboxes Are Used (Partial List)

• Aerospace
• Agriculture
• Automotive
• Construction
• Food Processing
• Shipping Industry
• Medical
• Energy
• Mining

Common Types of Gearboxes

There are many types of gearboxes manufactured throughout the world. One of the main differences between individual gearboxes is their performance characteristics. Choosing from the various gearbox types is application dependent. Gearboxes are available in many sizes, ratios, efficiencies and backlash characteristics. All of these design factors will affect the performance and cost of the gearbox. There are several types of gearboxes:

Bevel Gearboxes

There are two types of bevel gearboxes which include either straight or spiral teeth gears. Straight bevel gears have straight and tapered teeth and are used in applications requiring slow speeds. Spiral bevel gears have curved and oblique teeth and are used in applications requiring high-performance, high speed applications. Bevel gears are typically constructed from cast iron, aluminum alloy or other steel materials but vary between manufacturers. Bevel gearboxes are mainly used in right angle applications with the shafts in a perpendicular arrangement.

Helical Gearboxes

Helical gears are cut at angles which allow for gradual contact between each of the helical gear teeth. This type of innovation provides for a smooth and quiet operation. Gearboxes using helical gears are applicable in high horsepower and efficient applications. Helical gears are typically constructed from cast iron, aluminum allow or iron material but may vary depending on the manufacturer. Helical gears are widely used in applications requiring efficiency and high horsepower.

Spur Gearboxes

Spur Gears Spur gears are made with straight teeth mounted on a parallel shaft. The noise level of spur gears is relatively high due to colliding teeth of the gears which make spur gear teeth prone to wear. Spur gears come in a range of sizes and gear ratios to meet applications requiring a certain speed or torque output.

Worm Gearboxes

Worm gears are able to withstand high shock loads, low in noise level and maintenance-free but are less efficient than other gear types. Worm gears can be used in right angle configuration. The worm gearbox configuration allows the worm to turn the gear with ease; however, the gear cannot turn the worm. The prevention of the gear to move the worm can be used as a braking system. When the worm gearbox is not active, it is held in a locked position. Worm gears are typically constructed of aluminum, stainless steel and cast iron. The material used varies depending on the manufacturer. Worm gears are used in heavy load applications requiring high speeds. These gearboxes can also be configured for right-angle applications.

Planetary Gearboxes

Planetary gearboxes are so named due to their resemblance to the solar system. The components of a planetary gearbox include a sun gear, ring gear and planetary gears. The sun gear is the central gear which is fixed in the center, ring gear (annulus ring) which is the outer ring with inward-facing teeth, and the planetary gears which rotate around the sun gears and mesh with both the sun and ring gear.

Gearmotors

A gearmotor is a combination of an electric motor and a mounted gearbox, integrated into a simple package. A gearmotor combination reduces complexity, saves time in matching components, and lowers costs in designs requiring high torque low speed output. Gearmotors can be manufactured to be integral or be combined as separate components. Gearmotors which have the motor and gear reducer sharing the same shaft is what is meant by integral.

Gearmotors are used in many applications and industries, even in everyday household appliances. Industrial applications include cranes, lifts, jacks and conveyor machines. Everyday household appliances gearmotors are used in are washing machines, mixers, clocks, hand tools like drills and dryers.

Gearmotors come in a wide selection of stepper gearmotors, brushless gearmotors, brush DC gearmotors and AC gearmotors integrated with either spur, planetary or worm gears.

How to Select the Appropriate Gearbox

When considering a gearbox, many factors need to be considered to meet specific application requirements:

Gear Ratio

Gear ratios are defined as the correlation between the number of teeth of two or more different gears. Typically, the number of teeth a gear has is proportional to its circumference. This means that the gear with a larger circumference will have more gear teeth. The relationship between the circumferences of the two gears can also give an accurate gear ratio. For example, if one gear has 36 teeth while another gear has 12 teeth, the gear ratio would be 3:1.

Output Torque

Output torque depends on the gear ratio used. To obtain a high output torque, a high gear ratio should be selected. This will lower the output shaft speed of the motor. Inversely, using a lower gear ratio will result in a smaller output torque value being delivered into the system, with a greater motor speed at the output shaft. This principle illustrates the inversely proportional relationship between torque and speed.

Speed (RPM)

Speed is inversely proportional to the gear ratio of the system. For example, the greater the number of teeth on the output gear, the greater the speed at the output shaft. Conversely, the more gear teeth at the output compared to the input, the lower the speed at the output shaft. In general, the output speed can be determined by dividing the input speed by the gear ratio. The higher the ratio the lower the output speed will be and vice versa.

Gear Arrangement

Gear arrangement offers various benefits over the traditional fixed axis gear system design. The unique combination of both power transmission efficiency and compact size allows for a lower loss in effectiveness. The more efficient the gear arrangement, (i.e. spur, helical, planetary and worm) the more energy will be transmitted and converted into torque, rather than energy lost as heat.

Another application factor to be taken into account is load distribution. Since the load being transmitted is shared among multiple planets, the torque capacity is increased. The higher number of planets in a gear system will increase the load ability and enhance torque density. Gear arrangements improve stability and rotational stiffness by creating a balanced system.

Fixed-Axis vs Planetary Gear System

In the figure above, the gear arrangement on the left is a traditional fixed axis gear system with a pinion driving a larger gear on an axis parallel to the shaft. On the right, is a planetary gear design system with a sun gear (pinion) surrounded by more than one gear (planet gears) and is encompassed in an outer ring gear. The two systems are similar in ratio and volume, but the planetary gear design has three times more torque density and three times the stiffness due to the increased number of gear contacts.

Planetary Gear System

Fixed Axis Gear System:

Volume = 1, Torque = 1, Stiffness = 1

Planetary Gear System:

Volume =1, Torque = 3, Stiffness = 3

Other gear arrangements as mentioned in the Types of Gearboxes segment of this guide are bevel, helical, cycloid, spur and worm.

Backlash

Backlash is the angle at which the output shaft of a gearbox can rotate without either the input shaft or the gap between the teeth of two adjacent gears moving. It is not necessary to consider backlash for applications which do not involve load reversals, however in precision applications with load reversals (robotics, automation, CNC machines, etc.) backlash is crucial for accuracy and positioning.

Cost of a Gearbox

The price of a gearbox varies and is typically determined by size, accuracy specifications, backlash, and gear ratio.

Gearbox Formulas

Motor Torque * Gear Ratio * Efficiency = Output Shaft Torque

Example:

Motor Torque = 175 oz-in

Gear Ratio = 5:1

Efficiency = 0.95

175 * 5 * 0.95 = 831.25

Output Shaft Torque = 831.25 oz-in

Input Shaft Speed (RPM) / Gear Ratio = Output Shaft Speed

Example:

Input Shaft Speed = 1500 RPM

Gear Ratio = 5:1

1500 / 5 = 300

Output Shaft Speed = 300 RPM

Gear Ratio = Teeth on First Gear : Teeth on Second Gear

Example:

First gear has 60 teeth

Second gear has 20 teeth

Gear Ratio 60:20 (reduces to 3:1)

Gear Ratio = 3:1

Glossary

Addendum:

the height of the gear tooth above the pitch circle diameter

Backlash:

the angle the output shaft of the gearbox can move without the input shaft moving

Base Circle:

an imaginary circle used in involute gearing to generate the involutes that form the tooth profiles

Bevel Gears:

used for right-angle applications. There are two types of bevel gears which are straight and spiral

Bore:

the diameter of the hole in a sprocket, gear, bushing, etc

Center distance:

distance between the axes of two meshed gears

Circular Thickness:

the thickness of the tooth on the pitch circle

Dedendum:

the depth of the tooth below the diameter of the pitch circle

Diametrical Pitch:

the teeth per inch of the diameter of the pitch circle

Differential Gear:

a bevel gear which allows two shafts to rotate at a different speed

Gear:

a wheel with teeth that meshes with another wheel with teeth to translate motion

Gear Center:

the center of the pitch circle

Gear Ratio:

the ratio between the numbers of teeth of meshing gears

Gear Train:

two or more gears meshed by their teeth. A gear train generates power speed through the meshed gears rotating

Helical Gear:

a gear with the gear teeth cut at angles

Line of Contact:

the line or curve along which two tooth surfaces are tangent to each other

Involute:

the curve which describes a line which is unwound from the circumference of the gear

Pinion:

a small cogwheel which fits into a larger gear or track

Pitch Circle:

the curve of intersection of a pitch surface of revolution and a plane of rotation

Pitch Diameter:

the diameter of the pitch circle

Pitch Radius:

the radius of the pitch circle

Planetary Gears:

a system that consists of three main components: the sun gear, ring gear, and two or more planet gears. The sun gear is the located in the center, the ring gear is the outermost gear, and the planet gears are the gears surrounding the sun gear inside the ring gear

Pressure Angle:

the angle between the line of action and the normal to the surface of the tooth

Spiral Bevel Gears:

shafts which are perpendicular to each other and are used in right-angle applications

Spur Gear:

connect parallel shafts which have involute teeth that are parallel to the shaft

Sun gear:

a gearwheel that rotates around its own axis and has other gears (planet gears) that rotate around it

Torsional Stiffness:

the measure of the amount of torque that a radial shaft can sustain during its rotation in a mechanical system

Working Depth:

the max depth a tooth of one gear extends into the tooth gear of mating gear

Worm Gear:

a gear with one or more teeth with screwed threads