Cylindrical gears are fundamental components in power transmission systems. They are used across various machinery and industries for their efficiency in transferring motion and torque.
This blog post will examine different types, designs, and configurations of cylindrical gears. We will cover topics from spur and helical gears to gear profiles and axis configurations.
What is a Cylindrical Gear
Cylindrical gears are a type of toothed wheel used in mechanical power transmission systems to transfer rotational motion and force between parallel or intersecting shafts. The teeth of cylindrical gears are cut or formed on the cylindrical surface of a gear blank, parallel to the axis of rotation.
The basic structure of a cylindrical gear consists of a cylindrical body or rim with evenly spaced teeth projecting radially from its surface. The rim is typically mounted on a central hub with a bore for attaching the gear to a shaft. Key components include:
- Teeth: The projecting elements that mesh with teeth on a mating gear to transmit power. Tooth profiles are commonly involute but can also be cycloidal or other shapes.
- Pitch circle: An imaginary circle that contacts the pitch circles of mating gears at the point of mesh. Pitch diameter is a key sizing parameter.
- Addendum: The radial distance from the pitch circle to the top of a tooth.
- Dedendum: The radial distance from the pitch circle to the bottom of the space between teeth (tooth root).
Types of Cylindrical Gears
Spur Gears
Spur gears are the most common type of cylindrical gear, characterized by straight teeth parallel to the shaft axis. They are used to transmit motion between parallel shafts and can be further classified into two subtypes:
- External Spur Gears: These gears have teeth on the outer surface of the cylinder and are the most widely used type of spur gear.
- Internal Spur Gears: Also known as ring gears, internal spur gears have teeth cut on the inner surface of a cylinder. They are often used in planetary gear systems.
Helical Gears
Helical gears have teeth cut at an angle to the shaft axis, forming a helix shape. This design allows for smoother and quieter operation compared to spur gears. Helical gears are used to transmit motion between parallel or non-parallel shafts and can handle higher loads and speeds than spur gears.
Double-Helical (Herringbone) Gears
Double-helical gears, also known as herringbone gears, consist of two sets of helical teeth arranged in a V-shape. This configuration eliminates axial thrust loads, as the forces generated by each set of teeth cancel each other out. Double-helical gears are used in high-load applications and can transmit power between parallel shafts.
Internal Gears
Internal gears, as mentioned earlier, have teeth cut on the inner surface of a cylinder. They can be designed with straight (spur) or helical teeth and are often used in planetary gear systems or in combination with external gears to create compact gear trains.
Worm Gears
Worm gears consist of a worm (a screw-like gear) that meshes with a wheel (a helical gear). They are used to transmit motion between non-parallel, non-intersecting shafts, typically at a 90-degree angle. Worm gears provide high gear ratios and can transmit significant power in a compact space.
Rack and Pinion
A rack and pinion gear system consists of a linear gear (rack) that meshes with a circular gear (pinion). This configuration converts rotational motion into linear motion or vice versa. Rack and pinion gears are commonly used in steering systems, linear actuators, and CNC machines.
Cylindrical Gear Design
Gear Shape
The shape of cylindrical gear teeth is defined by their profile, which can be:
- Involute: The most common tooth profile, involute gears have a tooth shape based on the involute curve. This profile provides smooth operation, high efficiency, and reduced sensitivity to manufacturing and assembly errors.
- Trochoid: Trochoid gears have a tooth profile based on the trochoid curve, which is generated by a point on a circle rolling along a straight line. Trochoid gears are less common than involute gears but offer some advantages in specific applications.
- Cycloid: Cycloid gears have a tooth profile based on the cycloid curve, generated by a point on a circle rolling along another circle. Cycloid gears are rarely used in modern applications due to their complex manufacturing process and sensitivity to errors.
Module and Number of Teeth
The module of a gear is the ratio of the pitch diameter to the number of teeth. It represents the size of the gear teeth and is a critical parameter in gear design. The number of teeth on a gear, along with the module, determines the gear’s pitch diameter and overall size.
Pressure Angle
The pressure angle is the angle between the line of action (the line along which gear teeth apply force) and the perpendicular to the line of centers (the line connecting the gear shafts). Standard pressure angles for cylindrical gears are 14.5°, 20°, and 25°. Higher pressure angles result in stronger teeth but also increase the radial load on the shafts and bearings.
Gear Axes Configurations
Cylindrical gears can be used in various shaft configurations, depending on the application requirements:
Parallel Configurations
In parallel configurations, the axes of the gears are parallel to each other. Spur and helical gears are commonly used in parallel configurations to transmit motion between shafts.
Intersecting Configurations
Intersecting configurations involve gear axes that intersect at a certain angle, typically 90°. Bevel gears, which are not cylindrical gears, are used in intersecting configurations to transmit motion between shafts.
Non-parallel Configurations
Non-parallel configurations involve gear axes that are neither parallel nor intersecting. Worm gears and crossed helical gears are used in non-parallel configurations to transmit motion between shafts at various angles.
