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# Pulley set for a 150rpm motor (small rc car project)? Answered

hey,

I'm building a a car for a small competition in  my university (really small, like 3 teams)

The motor they are providing for the car is a small dc motor, 6v, 1.7A and 150 RPM (there's no info about the torque).

We have access to a 3D printer and wanted to print 2 pulleys to decrease the speed and gain strength (so the car can move), but I have no idea on what pulley size to use, what rpm should we aim? I know that without torque there's not much to say without testing, but any of you have at least a reference?

thanks!

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Thanks for all the replies. I was thinking about pulleys because I thought they were easier in general (gears have to match perfectly, right?) And we don't have experiece in 3D modeling so we have to find those pieces in old eletronics or in places like thingverse.

It is a very casual competition so we're gonna be happy to just make the car function, it may not win the top speed criteria or best time, but it'll be fun.

Th weight I don't know precisely. It's an arduino board + servo + dc motor. And we have yet to build the turning mechanism, gears, chassis, etc..

1kg, 1,5kg max I guess. I don't know how long the circuit is, but they said it will have lots of turns and a big straight (to measure top speed).

We're gona use wheels from a toy car, and most are around 2 inches in diameter

see my earlier post. Only you know enough to make decisions

I would aim to drive the motor direct to the rear wheels. Your wheel is 6.28 inch circumference

a mile is 63360 inches

So the wheel must turn 10682 turns to go 1 mile.

60 MPH is 1 mile per min

you can work out from this how fast you might go if you turn the wheel at 150 RPM.

And can you use Lego.

Some years ago my daughter outperformed (Won) her class in jet car straight distance....

We don't want to use lego as one of the teams already said they were going to use lego, so we want something new!

1. How fast do you want to go?

2. How big are the wheels?

3. How far do you need to travel?

How much will the car weigh?

Your gearing will be a compromise of these figures. A low torque motor will have problems getting a heavy car started from stop and will need a high gear ratio to start.

HOWEVER a dc motor that is outputting 150 RPM is likely geared and will have substantial torque.

if your wheels are 1 inch in diameter then you need 20165 RPM to go at 60MPH

Try working back, Power input is 10.2 Watts...

This kind of motor is about 70% efficient, so output power is 7.14 watts.

That converts to a torque of 0.455 nm about a half a Newton-meter.

If you know how to measure torque on small machines with a string and pulley, it will give you a better peak torque value.

Lego gears can be used in this project for easy propulsion !

Why pulley system and not gear system? Pulleys will bring more friction into the equation, could slip and take up more room then a set of gears.

Print a pulley system like it used on a drill press.
Several "disks" with decreasing diameter.
This way you can go (for example) from 1:1 all the way to 6:1 or lower - depending on the diameters you choose.
So you only need one belt lenght to connect and can test what ratio works best, after that just print a single pulley...