Calculating a single-gear ratio

[bhuman]

Hi.

I am designing a model for a vehicle. The power source is solar energy, providing power to a single 12 volt motor of about 1W. Through tests I have calculated the stall torque and max RPM of this motor, and therefore a rated torque of 0.00154 Nm. At this load the motor should ideally spin at 6000 RPM (according to tests). It is therefore necessary to use a gear system to increase the torque at the wheels and reduce the RPM so that the model is able to start moving without a clutch, and maintain a desirable speed when moving. The model weighs 8Kg and has 4 wheels of radius 4.5cm. There is no target speed, we are just aiming for maximum efficiency.

My question is, how do you calculated a gear ratio for a single gear, so that the model will move efficiently and that the motor is loaded to its rated torque.

Any help is much appreciated!
Thanks

Ben

 

[drawoh]

bhuman,

Most DC motors have a linear speed torque curve at constant voltage. Torque goes from zero at no-load speed, straight up to the maximum torque at stall. Current is approximately proportional to torque. Your maximum power comes at halfway between stall and no-load speed. Your maximum efficiency comes at about 2/3 no-load speed.

This all is explained in detail on the websites of DC motor manufacturers like Micro Mo much better than I have done, above.

If you want to run at maximum efficiency at your standard velocity, you have a calculation that should be dead easy for anyone who has taken any sort of mechanical engineering course. If you are not an engineer, I recommend that you read up on gears. What you are trying to do is fairly simple.

JHG

 

[BrianPetersen]

What is not that simple, is knowing the relationship between speed and drag for your vehicle.

What force does it take to roll it? You need to measure this. If the vehicle needs to go uphill (at all), what's the design maximum slope that it needs to go up?

Your speeds are unlikely to be enough for aerodynamics to become an issue.

In the absence of any information whatsoever, and in the absence of any hill-climbing or acceleration needed, I've always found it advisable to assume a drag force for a wheeled apparatus of at least 2% and preferably 5% of whatever the weight of the vehicle is, to avoid people yelling at me afterwards for undersizing the motor. If there is a specified acceleration or hill climbing, add that on top of this.

It's all well and good to assume that wheels with rolling element bearings are frictionless and that you don't need acceleration or hill climbing, only for the vehicle to get hung up on a dust mite because there isn't enough "oomph" for the wheels to get over it.