Blog 18: Gear Basics: Design Characteristics

Introduction:

A gear is a rotating circular machine part having evenly spaced teeth cut around cylindrical or conical surfaces. or, in the case of a cogwheel or gearwheel, inserted teeth (called cogs), which mesh with another toothed part to transmit torque. Gears are used to transmit motion or change speed or direction in a machine. Different speeds can be obtained by using a different combination of gears. It can be used to raise the torque, adjust the speed, change the direction of movement, etc.
Two or more gears that are doing one work behind another are called transmission. An advantage of gears is that the teeth of a gear prevent slippage. By interlocking a pair of these elements, they are used to transfer rotation and forces from the driveshaft to the driven shaft. There are many other power transmission devices like belt drive, chain drive, rope drive, etc. but the main advantage of the gear system has almost negligible or no slippage between driving and driven member. 

Gears are operated in mated pairs. The gear or toothed component that is attached to a machine shaft or base component is called as driving gear (i.e., the gear that provides the initial rotational input) and the gear that rotates along with its shaft component is called the driven gear (i.e., the gear or toothed component which is impacted by the driving gear and exhibits the final output). Depending on the design and construction of the gear pair, the transference of motion between the driving shaft and the driven shaft can result in a change in the direction of rotation or movement. Additionally, if the gears are not of equal sizes, the machine or system experiences a mechanical advantage which allows for a change in the output speed and torque

Gear Design Characteristics

Gears are available in a variety of designs, constructions, and configurations to suit a wide range of industries and applications. These various characteristics allow gears to be classified and categorized in several different ways, which include:

• Gear shape
• Gear tooth design and construction
• Gear axes configuration

Gear Shape

Most types of gears are circular that is, the gear teeth are arranged around a cylindrical gear body with a circular face, but some non-circular gears are also available. These gears can feature elliptical, triangular, and square-shaped faces.

Devices and systems which use circular gears experience constancy in the gear ratios both for rotary speed and torque. The endurance of the gear ratio means that for the same input the device or system consistently provides the same output speed and torque.

On the other hand, devices and systems which employ non-circular gears experience variable speed and torque ratios. Variable speed and torque enable non-circular gears to fulfill special or irregular motion requirements, such as alternatingly increasing and decreasing output speed, multi-speed, and reversing motion. Additionally, linear gears, such as gear racks, can convert the rotational motion of the driving gear into the translational motion of the driven gear.

Gear Tooth Design

Gear teeth are also referred to as cogs, hence why gear is also called the somewhat archaic term of a cogwheel. While in the previous section, gears were categorized based on the overall shape of the gear body, this section describes characteristics relating to their tooth (i.e., cog) design and construction. There are several common design and construction options available for gear teeth, including:

• Teeth structure
• Teeth placement
• Tooth profile

Gear Teeth Structure

Depending on the gear structure and size, gear teeth are either cut directly into the gear blank or inserted as separate shaped components into the gear blank. In most applications, once a gear succumbs to fatigue, it is to be replaced entirely. However, the advantage of employing gears with separate tooth components is the ability to individually replace the teeth as each becomes fatigued rather than replacing the whole gear component. This capability helps to reduce the overall cost of gear replacement over time as individual cogs are available at a lower cost compared to that of a complete gear. Additionally, it allows specialized, custom, or otherwise difficult-to-find gear bodies to be retained and preserved.

Gear Teeth Placement

Gear teeth are cut or inserted on the outer or inner surface of the gear body. In external gears, the teeth are placed on the outer surface of the gear body, pointing outward from the gear center. On the other hand, in internal gears, the teeth are placed on the inner surface of the gear body, pointing inward towards the gear center. In mated pairs, the placement of the gear teeth on each of the gear bodies largely determines the motion of the driven gear.

When both gears in a mated pair are of the external type, the driving gear, and driven gear or move in opposite directions. If an application requires the input and output to rotate or move in the same direction, an idler gear is typically employed to change the direction of rotation of the driven gear.

If one of the mated gear pairs is an internal gear and the other is an external gear, both the driving gear and driven gear rotate in the same direction. This type of gear pair configuration removes the need for an idler gear in applications that require the same direction of rotation in the driving and driven gear. Additionally, configurations which employ an internal-external gear pair are suitable for limited- or restricted-space applications as the gears and their shaft or base components can be positioned closer together than is possible with a comparable external-only gear pair.

Gear Tooth Profile

The tooth profile of a gear refers to the cross-sectional shape of the gear’s teeth and influences a variety of the gear’s performance characteristics, including the speed ratio and experienced friction. While there are a large number of tooth profiles available for the design and construction of gears, there are three main types of tooth profiles employed

• Involute
• Trochoid
• cycloid.

Involute gear teeth follow a shape designated by the involute curve of a circle, which is a locus formed by the endpoint of an imaginary line tangent to the base circle as the line rolls along the circle’s circumference. Throughout the industry, the majority of gears are produced employ the involute tooth profile both because of its ease of manufacturing and its smoothness of operation. Compared to some of the other profiles, the involute profile consists of fewer curves, making the manufacturing of involute gear teeth simpler and, consequently, the manufacturing equipment necessary cheaper, which reduces the overall cost of production. The advantage of involute gear teeth lies in their constancy of pressure angle throughout gear engagement and the ability to tolerate variation in the spacing of gear centers without impact on the constancy of the gear ratio for torque and speed. The constancy of pressure angle allows involute gears to run smoother than gears with other tooth profiles and the tolerance of variation allows for greater flexibility within the gear’s design specifications.

Unlike an involute curve where the line rolls along the circumference of a circle, a trochoid curve is a locus formed by a point at a fixed distance (a) from the center of a circle with a given radius (r) as the circle rolls along a straight line. Trochoids are a general category of curves that include cycloids.

If a<r, then the curve formed is known as a curtate cycloid

if a=r, then the curve formed is a cycloid

if a>r, then the curve formed is a prolate cycloid

Compared to the involute gear tooth profile, these profiles are rarely employed for gear design and construction except for use in specialized applications. For example, trochoidal gears are often employed in pumps and cycloidal gears in pressure blowers and clocks. Despite their limited applications, the trochoidal and cycloidal profiles offer a few advantages over the involute profile, including greater tooth durability and elimination of interference.

Gear Axes Configuration

The axes configuration of a gear refers to the orientation of the axes along which the gear shafts lay and around which the gears rotate in relation to each other. There are three principal axes configurations employed by gears:

• Parallel
• Intersecting
• Non-parallel, non-intersecting
• Parallel Gear Configurations

As indicated by the name, parallel configurations involve gears connected to rotating shafts on parallel axes within the same plane. The rotation of the driving shaft (and the driving gear) is in the opposite direction to that of the driven shaft (and driven gear), and the efficiency of power and motion transmission is typically high. Some of the types of gears that employ parallel configurations include spur gears, helical gears, internal gears, and some variants of rack and pinion gears.

Intersecting Gear Configurations

In intersecting configurations, the gear shafts are on intersecting axes within the same plane. Like the parallel configuration, this configuration generally has high transmission efficiencies. Bevel gears—including miter, straight bevel, and spiral bevel gears—are among the group of gears that employ intersecting configurations. Typical applications for intersecting gear pairs include changing the direction of motion within power transmission systems.

Advantages Of Gear

• Gear drives provide an extensive range of speed and torque for the same input power, with more accurate timing than a chain system does, and less friction loss, and noise.
• Gear is a positive drive; hence large velocity ratio can be obtained with minimum space.
• Gears are mechanically strong, so higher loads can be lifted.
• Gears are used for transmission of large H.F.