Unintuitive wheel jamming 3

[johnlo]

Hi all,

I have a simple assembly in which a wheel with a rod turns a bracket that slides over a block. The wheel turns on its own axis.

Strangely, the wheel can get stuck even when the bracket can freely slide along the block and its lateral position w.r.t. to the block is not constrained. Here's a video demonstrating the issue.

Essentially, the problem is that, at the current position where the red crank hits the end of the slot in the bracket, the wheel can no longer turn anti-clockwise. I find it counter-intuitive because the bracket should be able to slide along the block. If the bracket is manually shifted such that the rod no longer hits the end of the slot, the wheel turns.

Does anyone know why this is happening?

Any help appreciated.

Thanks

UPDATE: With the longitudinal planes of the green part and yellow bracket parallel, the system still binds up. Here's the video.

 

[IRstuff]

I don't know what the exact problem is, but it seems to me that you have way more degrees of freedom in the mechanism that is necessary.

TTFN
I can do absolutely anything. I'm an expert!
faq731-376 forum1529

 

[chicopee]

I have seen the video and I have read your problem, however, I don't have a grasp of the motions that you intend to have with the slider and bracket.
As the wheel turns clockwise, is it to make a complete revolution, ie, clockwise (CW) on its rotating axis? I also see the slider flip flopping inside the bracket, so is that a desired feature? Are both slider and bracket to remain horizontal at all time?

Also, semantics is everything in communicating ideas, next time use the word counterclockwise (CCW) instead of anti-clockwise.

 

[zeusfaber]

"anti-clockwise" is fine for me on this side of the Pond.

A.

 

[LittleInch]

1) This is not a "simple assembly", but really quite complex, with different axes and multi components.
2) I can only assume this is a simulation and hence much will depend on the weights, friction factors and torque being used.
3) Your lock up position would seem to have only a small amount of force left to slide the outer sleeve.
4) The torsional aspects of this system may be playing a part.
5) The key issue would seem to be the additional friction caused by the rod hitting the end of the sleeve. Try extending the slot and see if it still happens

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[3DDave]

The problem is in the software evaluation of the model. If you build a real model it will likely fail to move smoothly for other reasons.

 

[desertfox]

Hi johnlo
Well I'm not quite sure how the bracket just floats in mid air, however I think the reason for the jam is one of friction.
If the block slides it must do so under its own weight or more precisely a component force of its own weight, for example place a small block of wood on a table and unless pushed by a force the block won't move, now if you place the block on an angled surface and keep increasing the angle of the surface the block eventually slides under its own weight but notice that any angle below a certain level the block stays motionless because it cannot overcome the friction between the surfaces.
I think this is what's happening in your case because the angle to the horizontal of the block when it gets jammed is quite shallow however when you move the block this changes the mechanism geometry and allows movement once again.

I think if you want better answers we need more of an explanation as to what the movents are meant to achieve and how the blocks supported.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[IRstuff]

One obvious thing to do is to eliminate the angular and linear degrees of freedom that exist between the two rectangular pieces. Have you considered making them ball bearing sliders?

TTFN
I can do absolutely anything. I'm an expert!
faq731-376 forum1529

 

[GregLocock]

What friction values are you using for each interface? Have you drawn a free body diagram for the slider at lockup?

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[chicopee]

zeusfaber, except that I am on the other side of the pond.

 

[gruntguru]

The lock-ups seen in the video don't have an obvious cause however the mechanism does have a couple of potential freeze points eg any time the line drawn through the two pins is at 90 degrees to the slot/brackets.

je suis charlie

 

[dhengr]

Johnlo:
There seem to be two fairly short yellow, square inner tubes, one within the other. I don’t understand what they are and how they work or what they do. They are on a common fixed, rotating axis (axle/shaft) in the x-y directions, rotating about the z axis through this shaft. The two sq. tubes can’t move w.r.t. each other, they only rotate about their one sided shaft. This shaft arrangement will cause a torsional loading on these sq. tubes, along with a longitudinal frictional force on one face of the sq. tubes. In your second sketch, that frictional force will be along the top face of the outer sq. tube, on the inside of the top flg. of the channel.

Then there is a blue channel shape which fits over the yellow sq. tubes, t&b, and on one side of them. The blue channel rotates with the yellow sq. tubes and slides axially on them to allow this rotation. The blue tube is driven by a wheel and crank which can rotate on its own (fixed in x-y directions through its shaft) central axle, at the center of the wheel’s circle. The wheel can be rotated counter clockwise or clockwise on its axle. The red crank on the wheel drives the wheel, and is also the driving force for the movement of the blue channel, but the wheel primarily provides a circular motion path for the red crank, and the blue channel follows this. But, this red crank also loads the blue channel torsionally as well as axially, through a long slot in the web of the channel, and channels or not very stiff torsionally, not very strong torsionally. As GregLocock suggested draw some FBD’s with forces, sizes, thicknesses, dimensions, etc. for the condition shown in your second sketch, which is one of the times which I would guess the system binds up. Post this and we’ll critique the FBD’s and your further descriptions of what’s really going on. I think I have an idea why it is binding up at that point, but I’m not giving that away until we see your further sketches, FBD’s and explanations. Include some discussion about how this whole thing is supported and how the parts are kept aligned while sliding and rotating. What is the shaft on the yellow sq. tubes driving, and at what torques, etc?

I think some of you guys should get rid of your CAD, FEA, Kinematics software which allow you to pretend you understand what you are doing, as long as (and if) you can get them to run. Do some sketching, crack a couple of your old college text books, or buy a couple you can crack, try doing some real, basic, fundamental engineering. Learn to study, understand and refine your mechanisms, by sketching them at various orientations, studying forces, binding points or orientations, etc, etc. You might do some of this sketching with CAD and the kinematics programs, because they certainly are faster and more accurate, since engineers don’t seem to know how to draw any longer. But, you have to do the thinking and the studying, the programs don’t do that for you, if you really want to start understanding what you are doing and what’s happening.

 

[RolMec]

imo you need to involve someone who is fluid in planar mechanism design & analysis. There's kinematc (motional) instability points in your mechanism leading to "wedging" in certain positions.
In your example, the word "intuition" may be not applicable, due to the several degrees of freedom (e.g. extensive play between the hollow sections, impact of graviational force) you have here a quite complex mechanical issue.


RSVP

 

[johnlo]

Thank you all for the pointers.