CAM FOLLOWER 1

[kingcool]

hi guys ,
I am working on a project which was started last year, and my role is now to improve upon it. I have attached a file for your reference.

In the figure, A moment is applied on arm A to which a cam is attached. this rotation causes the arm B to rotate, Well the goal here is to provide variable resistance to the rotation of the arm A. So when theta and beta is zero , the spring on the follower does not resist the motion of the arm. but it either resists or assists for any other value of theta. lets also consider theta to be +/- 10 degrees and not a complete circle. and the cam and follower is attached by an extra component shown in green to hold the follower in place.

Now i have no idea how I SHOULD proceed with this. the problem we have with this is that at extreme angles the folo=lower is almost tangential to the cam, so any ideas on how i can overcome this? probably work on the form of the groove in the green part? thank you for helping me out.

And does kinematic study of this mechanism would be of any help to us? can i use it to determine the cam profile?

Thanks for your replies..

 

[desertfox]

Hi kingcool
Does the cam rotate about the arm its attached to? from your description it seems to me that the cam doesn't rotate but the green component pivots about the two pin joints at the cam and arm B.

 

[kingcool]

hi desertfox,
the cam is fixed to the arm A. so there is no relative motion between arm A and the cam...I have attached another file to understand the connection betwwen the cam and follower.

the green piece infact has a groove in it shown in red. and the green piece is fixed on the cam by a pin in blue. so when the cam and follower rotates the blue pin translates along the groove thus holding mecanism together.

one problem i noticed with this system is that high stresses are induced on the green component specially on the grooves and almost all our prototype pieces fail along those grooves. so maybe i may have to calculate and optimize the form of the groove too. I m not sure. thanks for guiding me.

 

[desertfox]

Hi again
Does the green link rotate about the blue pin in the red coloured groove as well as translation of the slot along the pin.
I'm not sure your mechanism will work but I need to fully understand it first.
Have you any of these mechanisms working in the field or just prototypes?
I presume there must be some proper layout drawing of this mechanism showing how the follower moves round the cam?

desertfox

 

[kingcool]

thanks desertfox for replying, again,
the green link is always axial to the follower axis so it just translates (for example.sort of like in a suspension system). in the figure the red groove is carved on the green link itself , i did it in red to just show it as a groove...so no the green link just translates and its rotating axis would be the same as that of arm B....if you imagine the system in 3d , the cam is a plate, and the green link has the form of a fork and has groove on both its arms. the blue pin could simply be a screw that holds the components together sort of like a sandwitch.
The concept has been in prototype phase still..I dont really have a layout drawing with me right now..sorry for that..

the prototype works as intended but not effective enough...we are not getting the resistance to the rotation of the arm A as intended..\\

 

[MikeHalloran]

Prototypes destroying themselves is kind of a hint that the mechanism needs a re-think or a re-start.

SOME kind of kinematic analysis is indicated, even if you have to do it by hand.






Mike Halloran
Pembroke Pines, FL, USA

 

[kingcool]

Thanks mike,
well previously the cam profile was determined by resolving the forces. well what do you exactly mean by kinematic analysis? sorry i am new to this and i need some guidance here. could you point me in the right direction so i can start with my calculations?
I mean how can I confidently determine the feasibility of such a mechanism? by proving what?

well should i get a relation between the two angles (like relative angular velocity) and see if at a given range(like for theta=10degrees to -40) the other angle beta is such that a good contact betweenn the cam and follower is maintained ?

THANKS FOR UR GUIDANCE..

 

[MikeHalloran]

I think you need to trace some arbitrary forces through the mechanism, to see where they get magnified and where they get reduced. Then add some friction coefficients, compute normal and tangential forces, and see if there are places where no real grease could possibly help. You could do it graphically on paper, or with Excel, or maybe some fancy parametric program that I don't know about.

I suspect that if you back up and start from a statement of the problem that the mechanism was intended to solve, you will end up with something very different. ... but for right now, just beat on what you've got, and try to figure out where the huge stresses are coming from.




Mike Halloran
Pembroke Pines, FL, USA

 

[hydtools]

Judging by the rough geometry, angles theta and beta can not be 0 at the same time. Is being 0 at the same time a requirement? Your description of wanting no spring force acting on the cam at theta=0 and beta=0 sounds like a requirement.

It looks like there is a force couple between the follower contact and the guide pin that may create side loading in the groove causing your failure.

I think there needs to be a clearer definition of the action desired and then do a kinematic analysis to verifiy output for given input. Then do a force analysis to verify loads are acceptable.

Ted

 

[kingcool]

Thanks mike and ted for ur insights,

Well I did a mistake in defining the angle theta. I have attached new file to define it well.
I have also made three diagrams to show three important positions of the mechanisms. I also simplified the cam profile to simplify the drawing.

The action desired from this mechanism is :
Zero spring force at theta = 0; and

In the first position, the spring resists the motion of arm A. In the second position, the spring applies no resistance to the arm A and in the third position the spring assists the motion of arm A.

So the cam profile was generated by force analysis (it is just the length of line in pink for every potion of rotation so that the above conditions are satisfied.) . So theoretically, the cam profile is generated

But now practically, to have an effective contact between the cam and the follower; a connecting piece(in older pic shown in green with a slot) has to be installed. Does it make sense in thinking that; as this component was not considered during our theoretical cam profile calculation; the mechanismm brakes at the groove of the green component? Probably in my calculation I have to consider :

1. The effect of friction between components; as it was not considered before.
2. To determin the cam profile ; consider the connecting piece too ? and try to calculate the profile of the groove so that maximum force transmission occurs between cam and follower?

Thanks,

 

[desertfox]

Hi kingcool

With your latest diagram it suggests that the slotted green link does rotate along with the cam follower arm or otherwise it would not work.
Now the slotted link needs to have a slot long enough to compress the spring by the right amount to achieve the force you require. If the slot is to short it won't reach the required force and possibly this is where your links are being damaged, because the pin hits the bottom of the slot and because the links are passing over centre they are trying to change physical length and thus putting a compressive force on both pin and slotted link.
Are any of the damaged parts bent.?

 

[hydtools]

Is the cam surface necessary? Use a variable length, sprung link like a gas spring connected to the pin/pivot shown as the cam center and to the grounded pivot. Eliminate the slot and pin yoke.

Ted

 

[MikeHalloran]

What you've drawn, as hydtools noted, is a toggle link mechanism.
It doesn't require a cam of any kind. ... but it may not meet your spec.
The mechanism is only quasi-stable at theta=0; the slightest disturbance will cause the spring to drive it toward either extreme.
If that is not what you want, then you probably do need a cam of some kind.




Mike Halloran
Pembroke Pines, FL, USA

 

[kingcool]

thanks desertfox, ted and mike ,
I taught about it; is my cam really necessary ?

A cam was choosen in this application , so that if at all i want to change the amount of resistance to the rotation of arm A; which is expected in this application ; i can just change the cam and i have the new resistance profile to the rotation.
Changing the spring would be rather expensive so a cam is used here.

I did new figures now showing even the slots on the connecting element. as desertfox noted, the slot size plays as important factor. But should I take this third connecting component with slot for determining my cam profile ? so along with the cam profile I could also determine the slot profile somehow; so that less stress is exerted on the slot but without compromising the transmissibility of effort from follower to cam?

And currently on the prototype, the brown surface on the slot (see figure) is not fixed on the cam; rather the connecting element with slot is held by just the pin/screw

 

[desertfox]

you need to make your last attachment a pdf I can't open it

 

[kingcool]

hi sry here it is

 

[zekeman]

So it looks like you need a toggle mechanism with some stable positions.A cam is a good solution.
If at the toggle position , you make the cam slightly concave you would have stable center stop and at both ends of the follower motion you would need mechanical stops. I don't think you can do much better,given the vague description of the requirement.