WM2D

[Gregory]

Oh, yeah... Sorry I see now, not my best explanation...

So, there is the big circle pinned to the background.
The smaller circle (piston) also pinned to a stable point / anchored object.
This means both can rotate.

There is a slot on the big circle in which the piston rod moves up & down in radial direction.


Now, what I want is to rotate the big wheel with a motor for example,

But I want the small circle to stay pinned in rest and not rotate around with the big wheel,
Instead of this rotation normally happens, I would like the attachment point of the piston-rod on the small circle to go around as the big wheel rotates.

Got it?
Is there any way to do that?

I know that's crazy... :) Sorry for that... I would need it for a special experiment.

Thanks already for your time!

[Tinhead]

HELP

LOL, I'm still not with you Gregory :) Anyone else out there got it? If the attachment point moves around the circle ... that's like the circle is rotating ... isn't it? Otherwise I could just use a rod from the center to the attachment point ? Maybe create 3-4 fake screenshots to explain the movement you imagine?

Cheers,
Rainer

[cyberdust]

Hi Cyberdust,

use the "rotational Damper' instead of the normal joint. Forget about the air resistance, the way it is calculated it doesn't help a lot.

Cheers,
Rainer

That was really helpful. Is it possible to make that pendulum stop at a maximum position? I wonder how comes and this wheel spins without stopping. It happens all the time to me. There must be a bug in wm2d.

[Gregory]

Ok Rainer, no problem. I give it a try from another viewpoint.

Ok, here is a screenshoot of a wm2d pulley, you taught me how to use.

As the circle rotates, the attachment point of the rope stays in position in the coordinate system, but the same time it goes around on the circumference of the circle, right?

What I would like is the reverse of this action:
The pulley stays motionless, and the attachment point (with the rope) go around on its circumference.

In the case of my earlier piston example,
I want to attach a rod (constant length), and make the attachment point go around while the circle stays motionless.

Check my earlier post & screenshot.

I can't do that, and I suppose it's not possible. Well, perhaps it has no use too... So, I am sorry to bothered you with this...

P.S. Drink a beer for my health, and laugh on it a big! ;-))

[Bessler007]

I am beginning to wonder if working model is useful beyond gaining an understanding of physics at a very fundamental level. There are times when I don't think wm2d is practical even at that level; at times the only way to get an answer is to accurately build the simulation. The forces of reality are too complex for it to describe.

This sort of build isn't to see if you have a working wheel. What I'm speaking of is a build used to analyze a brief moment in time. wm2d draws some absurd results that might not be trustworthy.

Attached is a screen shot where in 3 brief seconds the system cog is at a point above where it began. The model doesn't begin 'wound' up.

[Bessler007]

This is a shot of a model starting with a loaded spring. It seems obvious the system cog would move up. The previous simulation managed to 'wind' itself.

[Deven]

A couple of quick questions;

I'm having some trouble getting the system center of balance to appear on my screen. I click the icon under view, and I still cannot see it. Any ideas how to fix this?

Also, is there a way to increase the internal limit when running a simulation?

[Fletcher]

Click on each object & go to the Window menu > Appearance > Pattern > No - this makes objects around the axle [that you have selected] transparent - then you should be able to see the system CoM icon when you 'run' the sim - use your own accuracy settings to increase accuracy of iterations or decrease accuracy to increase the number of frames calculated

[graham]

I'm a little confused at the results of a simple sim .

I'm trying to measure the kinetic energy of two rotating weights mounted on a wheel 8' in diam and rotating at 60 rpm.

A 2lb weight is 4' from the axis and a smaller 1lb weight is 8' from the axis. The wheel is balanced.

The motor spins the wheel at 60 rpm and I wanted to check the kinetic energy of the two weights. My thinking was that the smaller weight travelling at twice the speed would show twice the kinetic energy of the larger one .

WM2D is telling me that the K E of both weights is zero while rotating.
How can that be right ???

Graham

[Fletcher]

Graham .. use >Measure >Kinetic Energy <TOTAL>

Don't use the <ALL> in this instance - click on top left hand arrow [output] to change presentaion of results to see a blue line graph & you will see what you were expecting - keep cycling the arrow & you will see the numbers as well.

[graham]

Fletcher, I'm just about to eat my supper (fish & veggies), my wife is nagging :-)

I'll try what you suggested in a bit.
Thanks a lot for your help

I'll be back.

Graham

[graham]

Hi Fletcher, I did as you suggested and got the KE values for each weight thankyou.

However the results were not as expected.
The larger 2lb weight showed a reading of .088 KE
The smaller 1lb weight showed a reading of .370 KE

The smaller 1LB weight travelling at twice the speed has close to FOUR times the KE of the of the 2lb weight moving at only half that speed.

I had expected it to be only double the KE of the slower moving 2LB weight since it weighs only half as much.

Knowing that doubling the velocity of a mass increases it's KE fourfold then surely doubling the speed of a mass only 1/2 the wt should only result in an increase of TWICE the KE .

Is my math all screwed up Fletcher ???

Graham

[jim_mich]

Graham, you have found another BUG in wm2d. After doing some testing I find that wm2d seems to calculate the KE correctly when a weight is pinned at the center or at one of the four edges, but it calculates KE wrong when pinned at any random location. The error is in the rotational part of the KE and it is consistent depending on how the pin joint is defined. Maybe you have noticed that when you pin an object at the center or at an edge then the pin joint dimensions are in parenthesis. It seems that these dimension get calculated correctly but real number coordinates don't.

As a side note the KE values will not be an exact 2:1 ratio unless the size and shape of the object are the same. This is because the size and shape determine radius of gyration. And radius of gyration determines the KE.

Now I wonder if this affects how a model behaves or if it is just a measurement display problem?

Another note: The KE values are often too small and end up being displayed as zero. You can edit the formulas so that the values get multiplied by some large number. I had to multiply each of the Trans and Rot formula by 100000 to be able to see the kinetic energy values.

[graham]

Thankyou Jim. I learn something new every day.
WM2D gets even trickier the more you use it.

I find it interesting that a pair of weights of different values can achieve a balanced state yet have unbalanced kinetic energy values while rotating dependant upon their mass and distance from the axis.

I'm looking for that "Window of opportunity"

Graham

[Gregory]

Hey Wm2d masters!

Can you help me?
In few of my designs I prefer to use one or more overruning clutches. I made some clutch models earlier, and they work fine. The problem is that they increase the simulation time greatly, and in a more complicated design they could make the program thinking all-day-long, which is sad thing.

I would like to simulate / imitate an overruning clutch by using active when dialogues, formulas, etc.
But I would like the results to be correct & virtually the same as with the clutch models. I have a hard time figuring this out, any idea is appreciated!

The problem with this is the way the clutch works. The inner cogwheel revolving in one direction can turn the outer wheel, and the outer wheel can turn the inner wheel in the opposite direction. So one can only turn the other in one direction, and only at times when the actual driver part would rotate faster than the driven.

Simple... But I would prefer that with two circle and formulas, or anything without messing around with all the small components & collision in the sim.

I attached one of my clutch models.