Why Do We Use Belts?

It’s getting hot out, you turn on your AC, and everything works great for about an hour or two. Outside, your condenser reaches saturation, it’s cooling fan doesn’t budge, and soon the house is back to the sweltering mess it had been before. It’s unbearable, but there you are. Welcome to Summer, right?

The Belt Drive

There are a ton of applications where we need something to spin, whether it’s a fan, blower, or even the drum in a washing machine. We often use a belt to connect the blower and anything else back to a single motor. Eventually that belt gives out and needs to be replaced. This could be from simple wear and tear, rot, or even accidental damage, like running over a rope with a tractor, jamming up the mower deck blades, and then burning up the belt with friction.

Belts offer a number of advantages though. They’re cheap to manufacturer, flexible, and can reduce the total number of parts a given machine needs. A belt can power multiple output devices, synchronize those output devices, and it frees up the designers to create different designs with fewer parts or more compact layouts than could otherwise be achieved.

But WHY?!

You could put the blower right on the motor!

Belts also serve two other majorly attractive purposes in cutting down costs. A belt allows us to do gear reduction and it allows us to use some cheaper (but no less effective) approaches to safety. We’ve talked about the idea of gear reduction before, and it’s a pretty big deal.

The easiest way to think about this, is to think about a bike or a car. You come up to a steep hill, which is easier, getting up that hill in 1st gear or in 5th gear? From a dead stop. In first gear, every four or five revolutions of the pedals might turn the back wheel once. It takes less force to move that wheel a short distance when your legs (or the car’s motor) travels a greater distance. Gears are basically levers.

So, there’s a trade off: you can spend a little force over a lot of revolutions, or you can spend a ton of force in a few revolutions to get up that hill. The same thing applies to your blowers, fans, and everything else, with different design considerations about what the exact ratio is.

In some cases, we’ll pair an incredibly beefy motor, that might spin five times a minute, with a fan we want to spin a thousand times a minute. It is really hard to achieve high RPM’s and high RPM’s can cause wear on bearings, sometimes we want more torque than speed. The motor runs at low speed and we have a belt that goes from a large diameter around the motor to a very small diameter at the fan. A tiny movement of the motor is a massive movement at the fan.

Other times, the opposite is true. We know we’re going to be taking on a massive load, and for reasons of space, power, or cost, a beefy motor just isn’t feasible. A small motor, with the correct gear reduction will achieve the same work as a big motor. We convert a ludicrously high RPM down to a sane, but high-powered output.

There’s also a safety aspect. If you seize up a motor, it’ll start to take on more amps, trying to force its armature to move. Eventually the power will either make the armature move or melt something and short. To prevent that, there needs to be some fairly robust sensing and logic to detect that a motor has a problem and stop it before it burns up.

A belt on the other hand, doesn’t need any complex sensing. The motor will likely keep spinning, the belt will remain stationary. The motor will chew out the chunk of the belt interface to it, nothing moves, and at worst, the belt might make a little smoke from friction. There’s just about zero risk of fire or further damage. No chance of an electrical arc lighting up some debris. When it comes time to fix the issue, it’s a $10 belt instead of a $300 motor.

While we’re at it, enjoy this old video about gears. It’s nearly 100 years old, but look, we’re geeks and we love how it explains gear reduction.


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