Gearbox -physics related question

[sporno]

So Ive been doing alot of thinking, and a bit of research about decreasing rotational mass and decreasing rotational inertia.

For example ( example guys , this is all theory)

if F=MA

and lets say F is 10HP ( horsepower , which is derived from torque)
and M= 2 lbs ( lets say a gear , i know heavy , but my motorcycle if any of you guys remember my thread on the bike had a 7 lb rear sprocket)

And lets say that A is 2,000 RPM ( yes i know its not actually RPM )

ok so if the mass decreases , but the force stays the same wouldn't the RPM go up ... in theory

ok now on to the decreasing rotational inertia.

So on a gear , or any kind of rotating object , there is more torque on the outer part of the gear , as in a friction drive , the closer to the center the more HP , the closer to the outside the more torque.

but if the mass closer to the outside was decreased , rather than the inside closer to the center , wouldn't it take less initial power to move the object?

This is all just stuff Ive been thinking about.

Now how this relates to Gearboxes

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?

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

So please tell me your thoughts and dont flame this too much

[techtiger1]

Sporno what your saying is all right for the most part. However relating to the AndyMark/FIRST application gearboxes the mass is so light that even lighting a gear by say a lb which is tough to do in a well designed gearbox, won't make a huge diffrence. Also when considering F=M*A and initial power you have to look at what your motors can do in comparison to the force they can put out and the object they are moving. Run some numbers on that theory with the CIMS and tell me if your still worried about them spinning anything less then 2lbs. Lighting the gear does make a diffrence by putting less load on the motor but in this application it is only a slight diffrence and generally a bigger worry is weight more so then power. My advice would be to test it if you really think it's worht the time and effort.

-Drew

[thefro526]

In my understanding (in no way am I an expert) When you decrease the mass of an object and the force stays the same said object takes less time to spin up to speed and i think because less force is need to spin said object it is transfered to the drive or what ever.

when I built a vex shooter a while back I had 3 motors using a 1:49 gear ratio which had theoretical rpm of 4900. I hit no where near that. But originally i used normal 2.75 inch wheels with rubber on them. They took super long to spin up to speed, but when i replaced thouse wheels with omnis they took much less to spin up to speed and also may have spun a bit faster.

hope this helps,
Dustin

[sporno]

yes , i totally understand that the decreasing the weight is more important , im just trying to figure out, is is there a better way to cut the weight.

Also if rotational mass of the gear , sprocket , some round object , is decreased its less stress on the motor to get it initially moving , so it takes less rotational inertia to move it , so wouldn't that increase " throttle response " too?

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?

this is more of my theory

im in physics 1

[techtiger1]

Yes your right in theory, the lighter the rotating object is the less energy is used to spin it (in this case) making the whole system more efficient. What I am asking you to do now is to put the principle in practice I think you'll find that in a application for FIRST where components are generally light (not a steel vs. alumnium flywheel) you will see only a slight diffrence in proformance but a difference none the less. Good ideas really everything you've said has a viable point.

[Madison]

I'm going to caution you to, as you start to think more about how these different concepts are related, be sure that you're using appropriate units to describe each of the elements present in your equations. If an equation involves a force, use newtons (or lbf.), etc. You've got an awful jumble of crossed wires to start with here, describing force in terms of power or torque and acceleration with velocity.

You'll help us to understand your questions better and you'll understand our explanations more because we'll all be speaking the same language, so to speak.

[techtiger1]

Thank you Madison ! :] yes proper notation is always important you beat me to that one.

[sporno]

i was throwing it out there in theory, next time ill try and clear it up , i think it seems simpler using the terms i threw out there rather than lots of numbers and different units .. for theoretical stuff atleast , its all an idea

[Alex.Norton]

If you had two identicle systems (wieght, support...) and two wheels that have the same weight but wheel A has less rotational inertia than wheel B, then no, they would spin up to the same speed. However wheel A will take less time to get up to speed than wheel B. However, if you got both wheels up to speed turned off the power, then tried to shoot a ball with the momentum of the wheel (say in 2006), wheel A would shoot fewer balls than wheel B.

In reallity wheel A or gear A would probably also wiegh less, so would put less strain of the system and it might be better balanced which would also reduce stress and so on. So in a real system like a shooter the rotational inertian of the wheel would have a huge effect on the efficiency of the system.

However in a drive train the gears shouldn't be big enough to make a difference. So I will calculate the time it would take the first big gear in the am planetary to spin up. This gear spins at 1540 RPM or 161.26 Radian/sec

m = ~.31kg
w (omega) = 161.26 Radians/sec
I = (mr^2)/2 = 1.562 E-4
T (average) = ~3.95 Nm

in this cas Torque (T) = Moment of Inertia (I) * angular acceleration (a)
so a (alpha) = T/I = 25280 Radians/sec^2

angular velocity (w) = angular acceleration (a) * time (t)
so t = w/a = .00636 seconds

A 120 pound robot (I know its actaully more like 133 lbs) with a reduction of 1:12.75 (AM single speed) to a four inch wheel on the other hand will take:

m = 54.54 kg
F (average) = ~138.04 N
a = F/m = 2.531m/sec^2
vFinal = 2.295 m/s
t = v/a = .907 seconds

So the gear wil take .00636 seconds to spin up and the robot will take .907seconds to accelerate (under absolutely idea situations). There are some assumptions in this. If anyone thinks these assumptions are too extreme, sees a mistake in my math or wants to know where I got a number please PM me.

So since the time of acceleration of the bot is huge compared to the gear it really doesn't make a difference. However you can see that the gear would slow the robot down a little.

In the case of a car there is a much bigger difference because your talking about a 50 lbs fly wheel not a half pound gear. So your theory is correct but I would always sugjest doing to math cause you'll see if you actaully need to think about implimenting it. To do this you should calculate the acceleration of the robot with two different gears (one lightened, one not) and see if there is a difference.

hope that helps
Alex

[Qbranch]

Bzuhhh... Just making sure... In your conceptual post you're ok...

But as stated above (and its so important i'll restate it) F is a force. Hp is a rate of energy conversion. 2000RPM is a speed, while 2000RPM/s is an acceleration. 2 lbs is a force, while 0.0622slugs or 0.9072kg is a mass.

And, to reaffirm, yeah the rotational inertia of the AM gearbox is negligable... as well as the rest of your drivetrain (unless you have some gigantic flywheel your storing energy in). Just worry about the mass of your robot.

-q

[Alex.Norton]

Also, I hadn't thought enough about your theory above, and no it is not actually true. Lightening the gear would not speed up the output.

The limiting factor in the speed of the system is the max RPM of the motor which is a result of motor design. The CIM won't spin any faster than ~5200 RPM or so even if there is no pinion gear or drive train or anything.

When I calculated the time above I was using your equation F = m*a. This relates to a force, a mass, and an acceleration. acceleration is only applicable to speed when introduce time. The reason that the motor won't spin the gear faster is because as the output RPM of the motor increases the torque decreases to 0 at the max RPM of the motor.

Again, hope it helps
Alex

[techtiger1]

Alex is correct. Thats why I said in theory that this idea works but there is a limit too it.

[Kevin Sevcik]

As noted, the rotational inertia of most all gearboxes in FIRST are negligible. And if you lightened the gearbox with everything else the same, your max speed would remain the same, though the robot would accelerate slightly faster. The best way to think about it is as a division of kinetic energy. Assuming you have a flywheel attached such that it spins faster the faster your robot moves, the flywheel would store some amount of rotational kinetic energy for every Joule of translational kinetic energy stored by your robot moving forward. With a standard gearbox, it's probably something like .01 J of rotational energy for every 1 J of translational. With a big flywheel, you'd be storing more in the flywheel, and a comparatively smaller amount of every J from your motor would be going towards actually moving the robot forward.

To correct one error I've sen in this thread. The top speed of your robot does not depend on the max RPM of the motor. It depends on the friction in your drivetrain. The friction represents a torque that must be subtracted from the torque developed by the motor before you start firguring the acceleration of the robot. So at some point, the torque put out by the motor equals the friction torque, and your robot stops accelerating. Since the motor is putting out mre than zero torque, this isn't going to be the no-load speed.

[sporno]

ok thanks for all the help

once again this is all just theory, im just trying to find out if what im saying is even ballpark

and this isnt just in relation to the gearboxes , but any rotating object on the bot.

Also , if the drive train is more efficent , less power is lost , and more gets to the wheels , thus some power is picked up .... right

[Alex.Norton]

As noted, the rotational inertia of most all gearboxes in FIRST are negligible. And if you lightened the gearbox with everything Since the motor is putting out more than zero torque, this isn't going to be the no-load speed.

This all depends on how you define your motor. Technically no motor has a no load speed since the motor has to spin itself which takes some power input. Since the CIM has a resistance and you have to put power in which means that your producing power somehow. I think you could define the whole drive train as a motor, maybe a linear motor, and then say that the top speed of the robot is the free speed.

A lot of solving a problem in physics is just choosing the correct frame of reference and it might be easier to define the drive train as the motor and just measure its speed. I'm not sure what the use of this is but hey...

Alex

[MrForbes]

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]

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?

[eugenebrooks]

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

[sporno]

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.

[CraigHickman]

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]

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.

[CraigHickman]

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.

[MrForbes]

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 :)

[Alex.Norton]

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 N*m and work is the joule which equals 1 N*m........... :D

Alex

[Rickertsen2]

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]

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.

[sporno]

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

[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.



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.

[sporno]

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

[chris31]

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?

[sporno]

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

[MrForbes]

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