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Speed Changes in Motion Transmission Systems - Secondary 3
Concept sheet | Science and Technology
Table of contents
A motion transmission system transmits motion between two components without changing the type of motion. However, the rotational speed of the part initiating motion (driver component) may be different from the rotational speed of the part receiving the motion (driven component).
A speed change is a variation between the rotational speed of the driver component and the rotational speed of the driven component.
The speed change can be determined based on the motion transmission system or the torque involved.
Speed Changes: Round Gears
It is possible to determine whether the system is increasing or decreasing the rotational speed by analyzing the size or number of teeth on the driver and driven components.
The following table shows how to determine the speed change in systems with wheel gears and in systems with friction gears or pulleys.
| Gear trains and sprocket systems | Friction gear and pulley systems |
|---|---|
| To determine the speed change in these systems, the number of teeth on each wheel gear is used. The teeth on the gears found within the same system must be identical in size and shape. Therefore, a wheel with more teeth always has a larger diameter. | To determine the speed change in these systems, the diameter of each friction gear or pulley must be used. |
When the driver gear has more teeth than the driven gear, there is an increase in the rotational speed of the driven gear. |
When the driver gear has a larger diameter than the driven gear, there is an increase in the rotational speed of the driven gear. |
When the driver gear has fewer teeth than the driven gear, there is a decrease in the rotational speed of the driven gear. |
When the driver gear has a smaller diameter than the driven gear, there is an decrease in the rotational speed of the driven gear. |
When the driver gear has the same number of teeth as the driven gear, the rotational speed does not change. |
When the driver gear has the same diameter as the driven gear, the rotational speed does not change. |
Note: In this table and throughout this section of the concept sheet, the driver component in each system is on the left (green) and the driven component is on the right (blue).
In a motion transmission system with two gears of different sizes, the smaller gear always rotates faster, and the larger gear always rotates slower.
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If the motion is transmitted from a small gear to a large gear, the speed decreases.
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If the motion is transmitted from a large gear to a small gear, the speed increases.
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In this gear train, the driver wheel gear has 24 teeth, while the driven wheel gear has 16. Since the driven wheel gear has fewer teeth, it rotates faster than the driver wheel gear. This results in a speed increase.
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In this friction gear system, the driver gear has a diameter of |\text{3 cm},| while the driven gear has a diameter of |\text{6 cm}.| Since the driven gear has a larger diameter, it rotates slower than the driver gear. This results in a speed decrease.
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In this friction gear system, the driver gear and the driven gear have the same diameter |(\text{4 cm}).| Since they have the same diameter, their rotational speed is the same. There is no speed change.
Speed Changes: Worm and Worm Gear
The worm and worm gear system is irreversible: the worm is always the driver component, and it transmits rotational motion to the worm gear, the driven component. This system is mainly used in cases where the rotational speed must be greatly reduced during the transmission of rotation. For each full rotation of the worm, the worm gear only rotates by the distance equivalent to one tooth. The more teeth there are on the worm gear, the slower its speed.
The worm must complete one rotation for the worm gear to move by one tooth.
Since the worm gear has 16 teeth, the worm must rotate 16 times in order for the worm gear to complete one full rotation.
The worm turns faster than the worm gear, so there is a decrease in speed.
A worm and worm gear system