A gearbox primarily affects torque by changing the relationship between speed and torque between its input and output shafts, fundamentally altering torque transmission.
The Core Effect: Speed vs. Torque
Based on the principle of power transmission, a gearbox works by using different-sized gears meshed together. The key effect on torque, as highlighted in the reference, is:
A gear's transmission torque changes as it increases or decreases speed.
Specifically, when a gearbox reduces the speed at the output compared to the input, it increases the torque at the output. Conversely, when it increases the speed at the output, it reduces the torque.
Think of it this way: For a given amount of power (which is the product of speed and torque), if the gearbox reduces the speed, the torque must increase proportionally to maintain the power (minus any small efficiency losses).
The reference further explains:
Generally, by reducing the speed, a small torque at the input side is transmitted as a larger torque at the output side.
This is the most common use of gearboxes in many applications, such as vehicles or industrial machinery – reducing engine speed to multiply torque for increased force.
How Gear Ratio Determines Torque Change
The extent to which a gearbox changes torque is determined by its gear ratio. The gear ratio is calculated based on the relative sizes of the gears involved.
The reference states:
The calculation of torque depends on the number of teeth.
While also mentioning pitch circle diameters, the number of teeth is a common way to understand the gear ratio.
Here's a simplified look:
- Input Gear (Pinion): Smaller gear, faster speed, lower torque.
- Output Gear (Driven): Larger gear, slower speed, higher torque.
The ratio of the number of teeth on the output gear to the number of teeth on the input gear gives you the speed reduction ratio. This same ratio (ignoring minor inefficiencies) also represents the torque multiplication ratio.
For example, if the output gear has twice as many teeth as the input gear, the output speed will be half the input speed, and the output torque will be roughly double the input torque.
Illustrating Speed vs. Torque Relationship
A simple table can help visualize this inverse relationship for a speed-reducing gearbox:
| Metric | Input (e.g., from Engine) | Output (e.g., to Wheels) | Effect |
|---|---|---|---|
| Speed | High | Low | Reduced |
| Torque | Low | High | Increased |
This is characteristic of lower gears in a car transmission, providing high torque to get the vehicle moving from a stop or climb hills.
Practical Applications
The ability of a gearbox to modify torque is crucial in many systems:
- Vehicles: Allowing engines to operate at optimal speeds while providing necessary torque for acceleration and pulling power.
- Industrial Machinery: Providing the high torque needed to drive heavy conveyor belts, mixers, or presses.
- Robotics: Delivering precise torque for controlled movements.
- Wind Turbines: Increasing the low-speed torque from the rotor blades to the high speed required by the generator.
In summary, a gearbox acts as a mechanical torque converter. By changing the rotational speed between shafts using gears of different sizes (determined by the number of teeth or pitch diameters), it can effectively increase torque (while decreasing speed) or decrease torque (while increasing speed), adapting mechanical power to suit different needs.
