How to analyse this chain and linkage system

[paulpatrick]

Hi All

I currently been asked to analyse the interaction of the forces of a chain and pulley system.
The thing is I am being asked to take into account friction slack etc in the analysis.
If the force applied at one end is 19.9kN in one instance from the chain end.
another instance 10kN is applied to the blue wishbone.
If I were to assume that the line of action in each item was in the same plain is this a valid assumption?
There is misalignment in the linkages as well but I am unsure as to how one could take this also into account.... its sort of agricultural in application.

Any advice i have done a really simplistic analysis where I break each area into a Free body diagrham etc and resolve starting from one end going to the other and vice versa when the application of force changes.

My problem is howfar into detail do I need to into the nitty gritty to include the friction interactions?

 

[GregLocock]

I'd include friction (I assume that is mostly in the pins) by doing a virtual work calculation. That is work out how much each pin rotates, per unit of motion at the application point of the force. You know the forces, you know mu, off you go. Might need to go round the loop a few times if there is a lot of friction.


As to how complex you make the model, well it depends. To do a down home job on the dynamics would be tricky, but possible, but I can't believe the effort would be justified.



Cheers

Greg Locock


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

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

 

[rb1957]

i think your pic shows enough to say that everything isn't in-plane, that there are (little) kick moments that need to be taken into account ... not the end of the world, but it is something you need to consider.

 

[GregLocock]

If you want to know the dynamics of the effect of slack then you need a non linear dynamic model of the chains, and you need to know the effective impedance on each end of the system, ie whatever is terminating it. This is quite a significant amount of work, to do it properly I am guessing a couple of weeks for someone experienced with dynamic analysis.

A series of tests would be quicker, more accurate and much cheaper.

Cheers

Greg Locock


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

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

 

[zekeman]

AS you show it, this is a non-problem.

Tell us how those free pins on the red connectors are mounted, if the socalled wishbone is mounted and if there are any masses connected.

Then you may have a real problem.

 

[paulpatrick]

Folks thanks for the advice so far.
I agree with earlier comments thats easier to just go out an look at the silly thing and get results but I cant.
Some one else in the company is doing that and I get asked to come up with the anwsers... I have asked for a video for future reference though so we shall see.. The pins are all mounted with sloppy clearance holes from what I can tell., the wish bone is mounted onto a sprung cylinder which operates a pair of levers.

 

[rb1957]

the red brkts work as three force bodies ...
if you know the load in the chain, then you can calc the load in the rod (moments about the pivot).

the far red brkt is easier to analyze, all the loads are in the same plane. the near red brkt has the cain load applied away from the rod plane, so there's a moment to be considered (probably reacted at the pivot, out-of-plane).

further questions about this are ok, but if you can't follow the bread crumbs then maybe you need professional help, or a text book.

 

[zekeman]

The key to this problem is the sloppy pin restraints.

Picture the upper chain and the blue link forming a plane .The forces are tension in the link and chain and the sloppy pin connection which allows the assumption that the pin force is a point force in that plane only.You satisfy the moment equation in that plane, which yields the link tension; the pin force is next obtained ( it might be needed later for friction considerations )from the equilibrium force equation.

The next link is done the same way, with the bottom chain and the link forming a plane, yielding the bottom chain force and its pin force.

Finally, as drawn. the wishbone pin must be rigidly connected to the link and have a linear force equal to the bottom chain force and have a moment equal to the couple formed by the chain force and the linear pin force, Fc*l where l is the perpendicular to the pin and the chain.Fc= chain force.
However if only a linear force is on the bottom pin then the force must be colinear with the chain.

The problem is symmetric, so you can do it given the bottom pin moment or force in one case and the top chain force in the other case.

 

[rb1957]

something to consider too is how much does the mechanism move (and change geometry) under load