General > General Technical Chat
Force multiplier
electrodacus:
--- Quote from: cbutlera on February 09, 2023, 11:48:43 pm ---
That video is a demonstration of something, but it is not an explanation of anything. Do you understand the difference between a demonstration and an explanation?
--- End quote ---
I do that is the reason I'm here.
The video/video's is not an explanation is a proof that my explanation is correct.
It shows the vehicle in case (a) being dragged. That means a large F1 force is applied so large that it can drag the vehicle in the direction F1 is pointing and there is no wheel rotation so basically it acts as a solid object.
Relation between F1 and F2 is that F2 = F1 except for the short duration when vehicle is accelerate from zero to a constant speed when F1 = F2 + (m*a)
I will love you try and find an alternative explanation that is consistent with what is observed in that video first 15 seconds with the last few seconds showing that vehicle is not faked and that wheels can rotate if the input wheels (the ones on the right) are allowed to slip.
Then there is the slow motion video that shows what happens when the front wheel is allowed to slip but in slow motion. And that again is explained fully including the energy storage and the trigher provided by the hysteresis between stick and slip.
The force needed to overcome static friction is larger than the force needed after wheels starts to slip
Fs - force needed to overcome static friction.
Fk - force needed to overcome kinetic friction.
This are the triggers allowing small and fast charge discharge cycles and why the mechanism looks smooth without slow motion video even tho there are many fast cycles of charge discharge and the only way any vehicle of this type can drive against the applied force.
Nominal Animal:
I've now also replicated the belt-driven mechanism, using Lego chains, as I happened to finally find where I stored my old Lego sets. I just need to simplify the model first.
Even electrodacus' own vehicle would work, if they just used a proper gear ratio. Their gearing is way too close to one (1:1). (That corresponds to \$\lambda = r/R\$, i.e. as shown in my math in my reply #97, when the movement is impossible. Simply put, electrodacus' picked exactly the worst possible gear ratio for their demonstration vehicle.)
As shown in the video linked to most recently by IanB, the driven wheel must turn multiple times for each turn of the driving wheel, and the wheels rotating in the same direction. (For soft rubbery tires, a ratio of 1:2 to 1:5 works well.)
The closer the ratio is to 1:1, the better traction you need. If you use a very small ratio, say 1:20 to 1:40 (i.e. for one turn of the driving wheels, the driven/belt wheel turns 20 to 40 times for wheels of same size), the movement is absolutely smooth and traction is not an issue.
electrodacus:
--- Quote from: Nominal Animal on February 10, 2023, 05:34:57 am ---I've now also replicated the belt-driven mechanism, using Lego chains, as I happened to finally find where I stored my old Lego sets. I just need to simplify the model first.
Even electrodacus' own vehicle would work, if they just used a proper gear ratio. Their gearing is way too close to one (1:1). (That corresponds to \$\lambda = r/R\$, i.e. as shown in my math in my reply #97, when the movement is impossible. Simply put, electrodacus' picked exactly the worst possible gear ratio for their demonstration vehicle.)
As shown in the video linked to most recently by IanB, the driven wheel must turn multiple times for each turn of the driving wheel, and the wheels rotating in the same direction. (For soft rubbery tires, a ratio of 1:2 to 1:5 works well.)
The closer the ratio is to 1:1, the better traction you need. If you use a very small ratio, say 1:20 to 1:40 (i.e. for one turn of the driving wheels, the driven/belt wheel turns 20 to 40 times for wheels of same size), the movement is absolutely smooth and traction is not an issue.
--- End quote ---
As far as I'm aware the convention for gear ratio is driver wheel divided by driven wheel https://www.sae.org/binaries/content/assets/cm/content/learn/education/motortoycar-samplelessonplan.pdf
In the case of example (a) and also my real world model the driver wheel was the one on the right (input wheel) and the driven wheel was the one on the left (output wheel) so the gear ratio is 2:1 so 2 rotation of the input wheel are needed for one full rotation of the output wheel.
This gear ratio of 2:1 is plenty and using a higher gear ratio will make no difference in the way the vehicle works. The amount of internal friction was fairly low for my model so even a 1.5:1 gear ratio will still have worked just fine.
I have also tested with a 3:1 gear ratio probably the same as the one in IanB video and even with that bad quality video and low frame rate I can see the same charge discharge cycles in his video is just not as evident as in my slowed down video.
In fact a 1:1 gear ratio and twisted belt will be the ideal combination that will allow no wheel slip and vehicle driving smoothly in the direction of the applied force.
Nominal Animal:
--- Quote from: electrodacus on February 10, 2023, 06:46:10 am ---As far as I'm aware the convention for gear ratio is driver wheel divided by driven wheel
--- End quote ---
Fine; I used the inverse.
--- Quote from: electrodacus on February 10, 2023, 06:46:10 am ---This gear ratio of 2:1 is plenty and using a higher gear ratio will make no difference in the way the vehicle works. The amount of internal friction was fairly low for my model so even a 1.5:1 gear ratio will still have worked just fine.
--- End quote ---
It's not about internal friction, it is about traction in the wheels. How about you actually try a 4:1 or higher gear ratio, instead of baseless assertions?
--- Quote from: electrodacus on February 10, 2023, 06:46:10 am ---even with that bad quality video and low frame rate I can see the same charge discharge cycles in his video is just not as evident as in my slowed down video.
--- End quote ---
And I can show you the exact kinematics of why there are no such discharge cycles when the vehicle does not flex. Yours flexes and behaves badly, because it has such a poor gear ratio, that's all.
If you start your examination from the contact point between the driver wheel and the belt, you have four forces at that point, with the net result a clockwise torque on the axle (assuming belt surface moves left). If we ignore friction losses in the gearbox, the gear ratio is also the torque ratio. If your driver wheel turns twice for each driven wheel turn, then the torque at the driver wheel axis is twice the torque on the driven wheel axis. (See e.g. here.)
The higher the ratio, not only is there more torque, but also the angular and thus linear velocity at which the driver wheel tries to move the vehicle is lower. That is, with higher gear ratios (using your definition), there is more torque available to move the vehicle forwards.
With lower gear ratios, there is less torque available, and the velocity needed is higher. When the gear ratio is insufficient, you will see jerkiness, because the wheels –– driven and/or driver –– will slip. With higher gear ratios (or with wheels with better traction, or with heavier vehicle), there is less and less of wheel slippage, with sufficiently high gear ratios and/or heavy vehicles, the motion is absolutely smooth.
If you change the gearbox so that the driven and driver wheels turn in opposite directions, you just change the sign of \$\lambda\$ as described in my reply #97, noting that \$\lambda\$ is the inverse of the gear ratio as electrodacus prefers it to be defined.
--- Quote from: electrodacus on February 10, 2023, 06:46:10 am ---In fact a 1:1 gear ratio and twisted belt will be the ideal combination that will allow no wheel slip and vehicle driving smoothly in the direction of the applied force.
--- End quote ---
So, you ignored my kinematics equations in reply #97, which shows that there are stable solutions for both same direction and opposite direction, whenever the gear ratio is outside the bad zone?
That with a straight belt, with treadmill surface moving left, the vehicle can move right at basically any speed except at the same speed that the treadmill surface moves left; but the vehicle can only move left faster than the treadmill surface?
That with a twisted belt, with treadmill surface moving left, the vehicle can move right only faster than the threadmill surface moves left; but left at basically any speed except at the same speed the treadmill surface is moving?
Everything I've told you and described to you is mathematically valid and verifiable in both theory and practice. What you have done, is made assertions and shown one video that fails to perform as I've described, which you have taken as proof that no vehicle can perform as I've described. Hell, I've even described exactly why your vehicle failed to perform, and instead of verifying it for yourself, you just assert that your vehicle is proof because you don't want to admit you're wrong here.
Are you sure you're not just playing word games and trolling here? Are you here just to try and convince others that you are right, or are you willing to admit you're wrong and learn?
Testability is a primary aspect in science, and even more so in Physics. You present one video of one non-performing device as proof, and ignore all examples of devices that are proven to perform. You even assert that you don't need to modify your device, because the modifications do not matter! In short, you are refusing to test your understanding. That is not science, it is religion.
pcprogrammer:
Hear Hear :clap:
My curiosity was triggered such that I decided to build my own vehicles to test the premises. Mind you it is just observational and not actual measurement to proof what is going on. I can't say much about the forces, because I do not have the equipment to measure them. And there in would lie the actual proof.
But to me my experiment gave enough proof to believe that what Nominal Animal states is correct.
The attached picture shows the 3 vehicles I build. All have the same base structure, but different size sprockets.
The first one has a 1:1 ratio based on 20 teeth sprockets at both ends.
The second one has a 2:1 or 1:2 ratio (depending on how you look at it) based on a 10 teeth sprocket and a 20 teeth sprocket.
The third one has a 3:1 or 1:3 ratio based on a 10 teeth sprocket and a 30 teeth sprocket.
Using manual force on the frame with both set of wheels on the table surface it is easy to move the 1:1 ratio based vehicle, but basically impossible to move the other ones without slipping wheels or sprockets on the axles.
Using a piece of paper like shown in the video of IanB it is impossible to move the vehicle with the 1:1 ratio without either the wheels on the table to slip or the ones on the paper. In this case I call it just dragging things along.
With the other two vehicles it shows the vehicle moving against the direction of the paper with different speeds between the wheels. This can easily be proven by the different distances traveled by the two axles. For this to work the wheels on the paper have the 10 teeth sprockets.
Reversing the vehicle such that the 20 teeth or the 30 teeth sprocket is on the paper, the movement reverses. The vehicle will start to move forward when the paper is dragged forward. There is again a difference in speed of the wheels, which is needed to make it work.
None of this shows any energy storage or slip stick hysteresis to me. With the bigger gear ratio the movement is very smooth.
All it needs is an initial force to overcome the friction and then movement starts. This force is supplied by my hand pushing or pulling the paper.
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