RcTek : Information & Resources for the Model Car Racer

How Gears & Pulleys Work

As gears form an important part of any R/C (Radio Control) model car, we have put together this article in which we explain how gears and pulleys work to help the radio controlled model car owner to better understand the issues involved.

Gears - Basic Principle of Operation

Gears are an important part of model cars, without them, they wouldn’t be able to transfer the rotation of the engine to the wheels (unless they used rubber bands, of course!).

They do more than transfer the rotation of the engine though, as they are responsible for matching the available torque/power of the engine to create a mixture of acceleration and speed to other parts of the car.

The basic operating principle of gears is shown when you Play the animation to the right and is a relatively straightforward concept to grasp.

Each gear is composed of a circular disc with teeth cut into it that engage with the teeth on another gear and make it rotate in the opposite direction.

You may Stop the animation if necessary.

Gear Ratio Reduction

With gears of the same diameter and number of teeth (as shown in the animation), one complete revolution of one gear generates one complete revolution of the other, which is conveniently expressed as a 1:1 (one to one) ratio.

However, by using gears with different numbers of teeth, the speed of rotation of one of the gears can be speeded up or slowed down.

This is demonstrated when you Play the animation above left, where the smaller (pinion) gear drives the larger (spur) gear.

The resulting speed reduction from this gear arrangement works because one tooth is in constant contact with another tooth on the other gear, and, as there are fewer on the smaller one, it has to do more than one rotation in order to make the larger gear rotate completely.

It may be easier to think of the gears simply as rotating circles, the larger diameter circle has a bigger circumference and so the smaller diameter gear, with its equally smaller circumference, has to travel further in order to keep up. The only way that the smaller gear can keep up is by travelling faster, as the teeth keep both circles synchronised. You may Stop the animation if necessary.

This difference in the amount of teeth is again specified as a ratio, so if the larger gear had 44 teeth and the smaller one had 22 then the ratio would be calculated by divivding 44 by 22, which gives a ratio of 2:1

There is another very important benefit from using a reduction ratio with the gearing though, namely the increase in Torque, which is dealt with below.

Torque versus Speed

There are very few circumstances (if any) in radio controlled model cars that a 1:1 gear ratio is used, everything from servos to gearboxes all use different gear ratio reductions because the motors and engines used are capable of very high speeds, but don’t produce sufficient torque to translate this into sufficiently powerful movement of the various parts of the model car.

This is overcome by the fact that when the speed of a shaft is halved using gearing the torque is doubled, this is because the same energy that it would haven taken to move the gear a certain distance is then used to move it half as far in the same amount of time.

As this speed/torque ratio is directly related to the gear ratio, model cars are able to have relatively small components that can be balanced between high torque and speed requirements.

Although this could seem a little confusing as some engines/motors produce more torque than others, it is the ability that changing gear ratios gives that allows these differences to be levelled out.

A better example would be the Servo, which you can buy with different torque and speed ratings. One servo that offers high torque could in fact use the same electric motor as another servo that offers higher speed. All that needs to be done by the manufacturer is adjust the ratio of the reduction gears.

What about Pulleys?

Pulleys are gears like any other as far as the ratios, torque and speed are concerned.

The only difference is that one pair rotates in the same direction as each other, which is shown when you Play and optionally Stop the animation to the left.

Pulleys have certain advantages over gear drives, which (when there’s time) will be expanded upon.

Pinion & Spur Gear Gap Setting

The gap between the pinion and spur gears on a radio controlled model car needs to be set correctly to allow the gears to work efficiently. Please see the Tips & Tricks section for further details.

The Shape of the Teeth

The shape, or more correctly profile, of the gear teeth is an important factor in the performance of any pair of gears.

Straight Tooth Profile

The animation on the left shows a simplified view of a pair of gears cut with a straight tooth profile of the same size rotating at the same speed.

If you Play the animation you will see that the teeth start off by contacting each other, but as the gear rotates they do not maintain contact. You can Stop the animation if necessary.

With this gear arrrangement only the tips maintain their relative position to each other and if you were to have a model car with these gears fitted, the gears would be constantly changing speed to try and contact each other.

The lack of constant contact between the teeth is due to the way that they move in relation to each other. This is a problem that is solved using what is called an involute gear profile and is described below.

Involute Tooth Profile

The animation on the left shows a simplified view of a pair of gears cut with a involute tooth profile of the same size rotating at the same speed.

If you Play the animation you will see that the teeth maintain their contact as the gear rotates. You can Stop the animation if necessary.

This is because the radius of the contact point between any two teeth is not constant.

If you study the animation you will see that although the tips are the first to contact, as they did in the straight tooth profile animation above, the shape of the teeth allows the contact point to remain constant.

What the involute profile in effect does is to simultaneously advance the speed of the front of the teeth and retard the speed of the rear of the teeth at the right point in the rotation of the gear.

Summary

Hopefully you will now have a better understanding of how the gears used in radio controlled model cars work and the positive benefits of having the right tooth profile and clearance between the gears can have.

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© 2001, 2009 by Darren Burnhill