Gearbox -physics related question

[eugenebrooks]

Originally Posted by sporno

And this isn't just for the gearboxes , its anything really , lets say a shooter for the AIM HIGH game. If the motor puts out so much power and , then you loose so much power in the drive train mechanism , if the motor didn't have to work as hard to move the wheel or drive train , it would spin faster , and shoot farther ....... right?

In Aim High, our shooter was directly driven by the large
CIM motor. The RC measured the speed of the shooting
wheel and adjusted the motor power to tightly regulate
the speed. The desired speed was a function of measured
range to the target goal. The considerations that you
mention, properly doing the physics, were a fundamental
part of the design process. To obtain a fast spin up,
the rotational inertia of the ball shooting
wheel needed to be low. To have the ball shooting wheel
store enough energy that it could shoot the ball and not
stall, the rotational inertia of the ball shooting wheel needed
to be high. When all was said and done, we found the
right compromise for the rotational inertia that provided
a fast spin up and enough energy storage to shoot balls
without losing too much speed.

I would encourage you to pursue your study of physics
so that you develop a proper understanding of these things.

Eugene

[MrForbes]

Originally Posted by sporno

Also in cars , people make much more power to the wheels , but getting a lighter flywheel.

Putting a lighter flywheel on an engine in a car can result in better acceleration, because more of the power made by the engine is being used to move the whole car, rather than just increase the rotational inertia of the flywheel.

However, the power to the rear wheels at a constant speed will not be any different.

You have an interesting idea, even if it's not likely to help robot performance, it at least shows you are thinking about new things! And asking questions like this will surely help you to better understand physics.

[sporno]

Thanks!

Well , if a car engine makes more power than the power to the rear wheels , DECREASING rotational inertia , and rotational mass decreases drive train power loss and makes the whole system more efficent


thats sorta what i was getting at with the gearboxes, wouldnt it be a bit more efficent and a bit more power gets to the wheels , i dont care if it is un noticable , its just a point , it could be a ball shooter from AIM HIGH

[Alex.Norton]

I wouldn't really make it much more efficient because the energy is still in the system it is just keeping the gear turning instead of the robot translating.

The energy will go towards making the robot move when you turn the motors off and let the robot come to a stop because this is when the robot will come to a stop.

In a car since the engine isn't connected to the ground and the fly wheel keeps turning when the car comes to stop this just adds inefficiency to the system but for a robot and an electric motor it doesn't have nearly as much effect on efficiency.

By the way, thanks for the question. I've been enjoying thinking about it.
Alex

[sporno]

this is an idea that sorta ticked my mind for about a year , the main thing that started my wondering was the 7 LB rear sprocket on my motor cycle. Now i knew i was gonna cut weight , but would it have more of an effect cutting it from the outside rather than closer to the center., so taling ot my physics teacher , we figured out off the normal drive train power loss i would get more power to the wheel VS, loosing it due to the motor having to exerct the force to get it moving and keep it moving

if that makes sense

[MrForbes]

Originally Posted by sporno

we figured out off the normal drive train power loss i would get more power to the wheel VS, loosing it due to the motor having to exerct the force to get it moving and keep it moving

what did you figure out? does it take more power to get it moving? what about to keep it moving?

[sporno]

Originally Posted by squirrel

what did you figure out? does it take more power to get it moving? what about to keep it moving?

ok like , if a car has a 50 lb flywheel , and you change it out to a 30 lb flywheel , you dont make any more horsepower , BUT more power gets to the wheels

same concept right?

is any of the stuff i have come up with remotely correct , cause thus far it seems really really really off

[114Klutz]

Nope - thats not exactly how it works.

Lets think of this in terms of a plumbing system. The motor generates power, think of it as a water supply.

Now, the goal here is to get the power output to the wheels - lets think of this as the faucet.

The flywheel would be analogous to a storage tank - if you turn on the water, it will first fill up the storage tank, and then after the tank is filled, it will come out of the faucet. Changing the rotational inertia of the flywheel(tank of water) doesn't make a difference in how much power goes to the wheels - all it does is store energy.

If you make the flywheel smaller - then it takes less time for the wheel to reach full speed - but it will make no difference in total power.

This explanation might be a little confusing, but I hope this is helpful.

Originally Posted by 114Klutz

If you make the flywheel smaller - then it takes less time for the wheel to reach full speed - but it will make no difference in total power.

This explanation might be a little confusing, but I hope this is helpful.

Actually, that's entirely spot on. Inside of a FIRST drive train, by making all gear and wheel components lighter, you'll gain acceleration. You won't pick up any extra torque or power.

On a car, it's somewhat different. The general rule is that for every powered pound you lose (brakes, axles, etc), you gain 1HP. For every 10 un-powered pound (seats, body, frame, etc), you gain 1HP. However, on a FIRST bot, we're not dealing with nearly enough power to get anywhere close to that kind of gain.

Good method of explaining, 114Klutz.

Making a drive system light is a good thing to shoot for. If you're really interested in lightning acceleration, do some research on the recent 254/968 drive system. Their entire gearbox weighed 1 pound. They were the fastest accelerating bot I've seen on the field.

[MrForbes]

Originally Posted by 114ManualLabor

Actually, that's entirely spot on. Inside of a FIRST drive train, by making all gear and wheel components lighter, you'll gain acceleration. You won't pick up any extra torque or power.

On a car, it's somewhat different.

actually it's exactly the same! you won't gain any power on a car by lightening components, you'll just gain acceleration.

Originally Posted by squirrel

actually it's exactly the same! you won't gain any power on a car by lightening components, you'll just gain acceleration.

Right. I had posted pre-breakfast (always a bad thing for a growing teen like me...).

I'm kind of confused at what Horsepower is actually measuring. Is it a force measurement? Acceleration over time? If someone could shed light on that, it would be pretty cool.

[Rickertsen2]

Originally Posted by squirrel

actually it's exactly the same! you won't gain any power on a car by lightening components, you'll just gain acceleration.

You DO gain horsepower & torque from the standpoint of the wheels from lightening rotating components but ONLY during acceleration. When a car/robot is accelerating it takes torque to accelerate the rotating assemblies. This is torque that could otherwise be available at the wheels. If the machine is spinning at a constant speed, the full power & torque of the motor is available at the wheels because nothing internally is being accelerated. F=ma You would be surprised at how much horsepower a car looses due to accelerating the wheels, diff, driveshaft, transmission, engine, water pump, alternator etc. The acceleration is essentially wasted as it doe not contribute to the forward motion of the vehicle. I have never done the math on a FIRST robot, but i can say for sure that the savings on a car are VERY significant. Rotating components must be accelerated twice, in rotation and in forward motion.

[MrForbes]

In a nutshell, what it means is that you're probably not going to gain the benefit you though you might, from lightening the rotating parts in a robot gearbox.

Originally Posted by sporno

If i cheesed a couple of the gears in the AM , or KOP gearboxes , decreasing rotational inertia , and rotational mass , would that have any effect aside from just cutting weight?


would it actually put more power to the ground , and create less drive train power loss?

It would put a little bit more power to the ground as you are bringing the robot up to speed--that is, it would help you win a "drag race" against another robot. But it will not give you any more pushing power, because once it is up to speed (or when it is standing still) there is no more gain from the lightening.

The idea might be useful for an arm, though, as it will take less power to raise an arm that has less inertia. So, when you are doing arm design, keep the weight at the outer end to a minimum!

[sporno]

The drag race is exactly what i was getting at , and the power power to the ground

lets say you were ramming another robot ( i know not legal)

but if you got up to speed quicker would that change the momentum that someone was hit with?


Was i sorta right about the first topic

[MrForbes]

Hmmmm.....I dont' know whether or not you would have more ramming energy with more speed and less flywheel inertia, or the other way around, or the same. Interesting question.

Let's say you have two robots, Robot A has normal unmodified gearboxes. Robot B has gearboxes with very lightweight gears in them, so they can "spin up" faster given the same motor torque as Robot A.

Robot B would win a drag race, that is it would accelerate more quickly. However, Robot A would win a "coasting" race, as that extra energy stored in the heavier rotating gears would keep it moving longer than Robot B, after you turn off power to the motors.