Why is There a Belt in the Furnace?

Is it weird that we use belt drives in furnaces? Belts aren’t necessarily known as the most durable or flame-resistant thing in the world. Why would we put a belt next to a roaring inferno? It’s 2018, why don’t we just bolt the motor straight to the fan, blower, or whatever it is we’re driving?


Gear Reduction

Spinning blowers, fans, and other equipment requires a lot of mechanical power. At the same time, it can take more power to overcome the friction on a resting object, especially for heavier parts or equipment that’s not perfectly supported. There can be additional friction by such offset loads.

Providing enough force to move this equipment requires bigger and bigger motors. Or does it? Physics is full of tricks. We can use a concept called Gear Reduction so that a smaller motor can get the job done. Instead of outputting a lot of power at once, we output less power, but more revolutions of the motor. For every 2 rotations of the motor, the blower might only spin once. This reduction requires the motor to spin faster to achieve the same result as a bigger motor, but it won’t need as much torque to do that work.

The easiest way to see this concept in action is to look at a multi-speed bike. Your gears are different sizes to allow for different speeds. In low gear, it is incredibly easy to spin the wheels. If you work really hard, you can pedal incredibly fast, and maybe get some speed, but you’ll tire yourself out. In higher gears, you can spin the pedals once, and perhaps the rear wheel will spin one and a quarter times for every revolution you pedal. From a standstill, this takes a lot of work, but going downhill, it’s easy.

This is the key concept of gear reduction: we can manipulate how power is transferred from place to place. We can use gearing so that less power can do the same work as more. It saves on the hardware costs, allowing cheaper, smaller motors to get the job done, where otherwise high power, high torque equipment would be necessary.


Reducing the Gears

There is a problem with this however: gears are expensive. Think about it, gears have to carry all the speed and power being put into them. Once you get to spinning them with any appreciable speed, the centrifugal forces can make the gear start to shear apart. So, the gear has to be made of strong material to resist being shredded by fast spinning. It needs to be precisely made and balanced too, just like car tires, or it’ll wear out the shaft its on or destroy itself. Lastly, gears must be precise. They need to be lined up with sub-millimeter accuracy, otherwise, the mating of the gear surfaces can cause excessive wear and tear. This also means the gears have to be made of sturdy material to avoid chewing themselves up.

We need to replace the gears somehow. In low power applications, like kitchen appliances and kids toys, plastic is plenty good. These applications don’t transmit a lot of power and thanks to injection molding in mass production, they’re pretty safe from precision issues. Larger equipment however, especially metal equipment, face bigger challenges with that type of precision. And plastic would never hold up.

There is however, a key to gear reduction: it doesn’t actually need gears. The idea is just that the input and output sides of a system are different sizes. This variance in size is what matters most, that the distance traveled by one side is different from the other. We can use chains and belts instead of gears.

Generally speaking, belts will beat out chains in these systems. Chains usually require some sort of tensioner assembly. Chains are of a fixed length and they need some way to have slack during installation but not in operation. You could conceivably design a system so the entire gear assembly comes off to facilitate putting everything together all at once. Or just use a belt. Belts are stretchy, they can be stretched.

Belt drives can be configured so you put the belt on and just pull it into place. Belts can even take attributes from chains, like teeth. A belt can be made with keying so it can grip a toothed gear and more directly transmit power, without as much concern for slippage.


Cheaper & Easier

Ultimately, using belts in a furnace and HVAC system at large cuts down on hardware and maintenance costs, labor costs, and reduces the system complexity. Any system relying heavily on chains or gears would require lubrication in exchange for just slightly longer lifespans. The biggest drawback of the belt drive is the temperature limitations, but those aren’t as big a concern as they look like.

The majority of the furnace is in fact, not able to withstand high heat. We just keep the belts away from high temperature areas and everything is fine. Even then, modern manufacturing allows us to include metals and alloys in belt materials, making some resistant even to higher heat and temperatures.

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