Vex Robotics Design System's Inventor's Guide

Gears, continued

Idler Gears
Gears can be inserted between the driving and driven gears. These are called idler gears, and they have no effect on the robot’s gear ratio because their gear ratio contributions always cancel themselves out (because they are a driven gear relative to the fi rst gear, and a driving gear relative to the last gear—you would fi rst multiply by the number of teeth on the idler gear and then divide by the same number, which always cancels out).

However, idler gears do reverse the direction of spin. Normally, the driving gear and the driven gear would turn in opposite directions. Adding an idler gear would make them turn in the same direction. Adding a second idler gear makes them turn in opposite directions again.

Idler gears are typically used either to reverse the direction of spin between two gears, or to transmit force from one gear to another gear far away (by using multiple idler gears to physically bridge the gap).

Compound Gear Ratio
Compound gears are formed when you have more than one gear on the same axle. Compound gears are not to be confused with idler gears, as compound gears can affect the overall gear ratio of a system!

In the compound gear system, there are multiple gear pairs. Each pair has its own gear ratio, but the pairs are con-nected to each other by a shared axle.The resulting compound gear system still has a driving gear and a driven gear, and still has a gear ratio (now called a “compound gear ratio”).The compound gear ratio between the driven and driv-ing gears is then calculated by multiplying the gear ratios of each of the individual gear pairs.Compound gears allow confi gurations with gear ratios that would not normally be achievable with the components available. In the example above, a compound gear ratio of 1:25 was achieved using only 12 and 60-tooth gears. This would give your robot the ability to turn an axle 25 times faster than normal (though it would only turn with 1/25th of the force)!

Gear ratio with non-gear systems
The real nature of gear ratios is a little more complex than just counting teeth on gears. Gear ratio is actually defi ned as the number of rotations that the driving axle needs to make in order to turn the driven axle around once. When dealing with toothed gears, you can find the number of turns needed by counting teeth, as you have already seen above (see “Gear ratio”).

With other types of systems, you can still find the “gear ra-tio” by measuring the number of rotations on the driven and driving axles. Some of these other drive types include belt-and-pulley drives and chain-and-sprocket drives.

Belt or chain drives are often preferred over gears when the motor and the wheel are located far apart on the robot. However, both belts and chains introduce their own special maintenance and performance requirements into the system (chains require lubrication and tension, for instance), and you should carefully weigh their advantages against other design considerations.