Gearbox -physics related question

Technical Motors
#19

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

#20

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

#21

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

#22

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

#23

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.

#24

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.

#25

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

#26

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.

#27

Power is the amount of work done per unit of time.

Work is the application of force over a distance.

In the case of a car engine, horsepower is not actually measured directly, instead it is calculated as the product of torque and angular velocity (rpm)

then you should be wondering about why it is that torque and work have the same units :slight_smile:

#28

Horsepower is a measure of power. Its just like a watt in and fact 1hp = 745.7 watts in mechanical terms. This is basically the amount of power that a horse can continuously put out.

But torque and work don’t have the same units. torque is Nm and work is the joule which equals 1 Nm… :smiley:

Alex

#29

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.

#30

The wording can be confusing…you don’t gain power by lightening parts. But you do change where the power goes, so instead of using that power to increase the rotational speed of the rotating parts, you are using it to accelerate the car forward.

We are saying the same thing, but it might be wise to be very careful with the wording, so that peope who don’t yet understand the concept, don’t get confused further…go back to the original post in the thread, and let’s see if we can explain it in a way that sporno can learn what he wanted to know. When we talk about “losing power” or “gaining power”, people normally understand it to mean that the power appears or disappears somehow, not that it is being stored as kinetic energy in some part of the drivetrain.

#31

so whats this mean in a nutshell , im kind of lost

#32

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.

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!

#33

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

#34

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.

#35

well knowing this, someone can dial in their bots a bit more

#36

I dont have Inventor, but has anyone taking the AM shifter gearboxes and seen how much you can remove from some of the gears. And then from this calculate the weight saving, any acceleration benefit and any weakness you creat in the gears by doing this?

#37

this past year my team cheesed our gear boxes , we cut off some weight , but we never calculated anything that this thread is about

#38

This past year 1726 left the chassis and drivetrain heavy, and the robot never fell over (unlike the previous year)