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Worm Gears
These gears are the type of gears that consist of a threaded cylindrical gear (worm) and a toothed wheel (worm
wheel). They are used to transmit motion and torque between perpendicular shafts, with the worm driving the
worm wheel. Here are some key details about worm gears:
• Worm Structure: The worm gear has a cylindrical shape with a helical thread wrapped around it. This helical
thread is called the worm. The worm usually has one tooth, called the “thread,” that engages with the teeth of
the worm wheel.
• Worm Wheel: The worm wheel is a toothed wheel that meshes with the worm. It has a set of teeth around its
circumference, which engage with the single tooth of the worm. The number of teeth on the worm wheel is
usually greater than the number of threads on the worm.
• Shaft Orientation: Worm gears are used when the driving and driven shafts are perpendicular to each other.
The worm is mounted on the driving shaft, which is parallel to the worm wheel’s axis, and the worm wheel is
mounted on the driven shaft.
• Gear Ratio: The gear ratio of a worm gear system is determined by the number of teeth on the worm wheel and
the number of threads on the worm. The gear ratio is given by the equation: Gear Ratio = Number of Teeth on
the Worm Wheel / Number of Threads on the Worm. The gear ratio typically has a high reduction ratio, making
worm gears suitable for applications requiring high torque and low-speed output.
• Efficiency: Worm gears provide high torque transmission but have lower efficiency compared to other types of
gears due to the sliding motion between the worm and the worm wheel. The sliding action generates more friction,
resulting in energy loss and heat generation. Lubrication is essential to minimise friction and improve efficiency.
• Self-locking: One significant advantage of worm gears is their self-locking capability. When the worm gear
system is at rest, the friction between the worm and the worm wheel prevents the driven shaft from back-driving
the driving shaft. This self-locking feature makes worm gears ideal for applications where holding the load in
place is required.
Example: Let’s consider an example of a robotic arm that needs to lift and hold a heavy load. A worm gear system
can be utilised in this scenario. The worm, mounted on the driving shaft, engages with the worm wheel mounted
on the driven shaft. The worm wheel has a larger number of teeth compared to the worm’s single thread.
The gear ratio of the worm gear system determines the torque and speed of the robotic arm. For example, if the
worm has one thread and the worm wheel has 40 teeth, the gear ratio would be 40:1. This means that for every
revolution of the worm, the worm wheel rotates 1/40th of a revolution, resulting in a high torque output.
The self-locking feature of the worm gear prevents the robotic arm from back-driving and provides stability to hold
the load in place even when the driving shaft is not actively applying torque. This self-locking characteristic is essential
for applications that require holding the position of the load, such as robotic grippers or robotic lifting mechanisms.
Components of Robots as a System 71

