×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

constant force mechanismsHi

constant force mechanismsHi

constant force mechanismsHi

(OP)
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

RE: constant force mechanismsHi

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

RE: constant force mechanismsHi

(OP)
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

RE: constant force mechanismsHi

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

RE: constant force mechanismsHi

(OP)
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

RE: constant force mechanismsHi

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: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: constant force mechanismsHi

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.

RE: constant force mechanismsHi

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

RE: constant force mechanismsHi

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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources