constant force mechanismsHi 1

[Yobbo]

L.S.,

I need to solve a problem to position a flat screen in three dimensions, where it is required that any given position must remain the same after ending the adjustment. My knowledge about mechanisms is only limited. I think I need to look for solutions in the field of (nearly) constant force springs. I know that constant force hangers are for example used to support pipes in an industrial environment. I would like to find some literature, where these kind of solutions are being explained. I would appreciate any hint to literature, articles or internet sites or to existing examples where constant force mechanisms have been applied.

With best regards,

Karel Postulart
The Netherlands


Karel Postulart, The Netherlands
Nuon Power Generation

 

[desertfox]

Something like this maybe:-

http://www.actuators-electric.co.uk/electric-linear-actuators/idm4-industrial-electric-actuator/

 

[drawoh]

Yobbo,

"Constant force" is a tricky concept. Do you exactly constant force, or approximately constant force?

It sounds to me like you need a counterbalance and some sort of friction device. If the friction is fairly low, you can move your screen around manually.

--
JHG

 

[Yobbo]

Hello Drawoh and Desertfox

Thank you for your responses. Constant force is an ideal situation. Small deviation from this concept is allowable, but the final mechanism should in all cases be able to bear the weight of the flat screen and not change the position of the item after being released. Apparently I forgot to mention the fact that the positioning of the screen should be done by hand. But if auxiliary power can simplify the situation it must definitely be considered as an option.

Karel Postulart

Karel Postulart, The Netherlands
Nuon Power Generation

 

[Timelord]

Yobbo,

I once designed a dental x-ray machine support arm which required the same move and holding capability you desire. We used a combination of pantagraphs with the springs inside. The spring anchors were located such that the angular force on the arm was matched by the spring force. This didn't require the use of a constant force spring, just very creative location of the anchors so the cam action matched the forces. We also had specific friction devices in the pivots which had high static friction and low dynamic friction to prevent creep. Settling time was our bugaboo because of the weight of the x-ray head and length of the arms.
If you do go the way of constant force springs, I have only seen one truly constant force spring design. It was made to unwind like a tape measure and had a constant retractile force. Next time you are at the dentist, look at the x-ray machine support arms, may give you some ideas.

Timelord

 

[Yobbo]

Hello Timelord,

Thank you for your reply. As a matter of fact I will visit the dentist next week for the semi yearly check. I will look at the X-Ray machine differently. The assumption of the constant force requirement seems to be too strict, reading about your solution. I will consider the solution you hinted on.

With best regards,

Karel Postulart

Karel Postulart, The Netherlands
Nuon Power Generation

 

[berkshire]

Something like this may be a starting point:

http://www.ebay.com/itm/Beseler-23C...430?pt=LH_DefaultDomain_0&hash=item519ec5686e

B.E.

You are judged not by what you know, but by what you can do.

 

[berkshire]

I forgot to mention , these are called neg'ator springs :

http://www.sdp-si.com/Gateway/D220-T167.htm

B.E.

You are judged not by what you know, but by what you can do.

 

[GregLocock]

I'm not convinced that constant force is what you wwant, in the springs themselves. As you move the screen around the forces in the arms change a lot. Anyway, how about the old Anglepoise designs?



Cheers

Greg Locock


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

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

 

[Tmoose]

For one or two axes if the screen CG coincided with a pivot there would be low tendency for it to rotate in response to gravity.

 

[dynatune]

Try contacting Chris Verheul in the Netherlands who is a famous expert on multi-body kinematics in this field. He will resolve your problem.

Cheers,

Paul

 

[tbuelna]

I would agree with Tmoose. A design that uses something like a ratchet or friction for locking, and that also maintains the screen mass CG close to the axes of rotation using balance counterweights, is probably a better option than using CF springs. Think about all the mechanical systems that use balance counterweights to allow precise control of positioning with minimal effort- elevators, telescopes, single-hung windows, etc.