How Camera Jibs Remain at Any Angle Without Moving Counterbalances

[loozh]

TL;DR

Given that both ends of a camera jib are balanced, shouldn't a jib remain level / parallel to the ground instead of staying in any position?

Hi, I am trying to understand how camera jibs work.

They seem to be able to remain at whatever angle despite being balanced. The counter weight is static and does not move, yet the jib does not remain level nor does lift or drops. I have spoken to the operators and they mention they don't use resistance to hold it in place.

Jib length to the fulcrum on either side remains the same. no mechanism to length / shorten to compensate.

Should'nt a perfect balance lead to it being level?

 

[berkeman]

Welcome to PF.

Could you post some links to some more information about these mechanisms, so we don't all have to go looking for ourselves? Thank you.

 

[loozh]

Thank you for replying.

I dont have much info but a few video for reference on the setup and usage:




Hope this helps.

 

[hutchphd]

This works because the boom is rigid (from the cables) and the torque from the camera weight is exactly to counterbalanced by the the torque from the weights by adjustment at zero angle. Any change in angle shortens the moment arm of each in the exact same proportion, so the balance remains perfect.

 

[russ_watters]

Should'nt a perfect balance lead to it being level?

No, perfect balance is a neutral condition. Returning to level is an imbalanced condition. But there's a catch; where's the imbalance when it returns to level?

 

[jack action]

If you look at a window balanced by a counterweight through a pulley, what would be the "level" condition?

​

 

[loozh]

This works because the boom is rigid (from the cables) and the torque from the camera weight is exactly to counterbalanced by the the torque from the weights by adjustment at zero angle. Any change in angle shortens the moment arm of each in the exact same proportion, so the balance remains perfect.

Thanks Hutchphd,
moment arms are static. they do not lengthen or shorten while going up or down.
There is no spring mechanism from the camera head side to vary torque at different angles too.

The cables are meant to act as a tension cable to level the camera head. Its based on fig 1: the sum of angles for the purple is 180 degrees.

thus allowing fig 2 movements

Fig1:

Fig 2:

 

[loozh]

No, perfect balance is a neutral condition. Returning to level is an imbalanced condition. But there's a catch; where's the imbalance when it returns to level?

Given the case does it mean i would be able to balance this level scale so that it can stay in any position without changing the weight or distance? if so how?

 

[loozh]

If you look at a window balanced by a counterweight through a pulley, what would be the "level" condition?

​

It seems the pulley has springs to have increased torque depending on the length. A jib does not have spring mechanism, both heads are free. Given a scale lever, would it be possible to change the "level condition" to be flexible?

 

[loozh]

Hi,
i tried drawing a jib from memory. concepts are roughly similar across.
starting from left,
the yellow is a control plate attached to swivel plate (Green)
Its is connected via a tension cable / rod (Pastel blue) to the front to mantain level. ( see above response to hutchphd)

Red is the bar. Weights are attached to the blue circle via a rod where weights can be mounted on.
The bar has a T pin to prevent it from escaping.

Black is the fulcrum.

There is no spring mechanism in the jib. Perhaps i am missing some key concepts. appriate your love and patience. Thank you.

 

[hutchphd]

I fear you are. A child's teeter-toter, if balanced at horizontal, will also be balanced at any other angle so long as the children remain rigidly upright. there are no further requirements.
By the same token, any imbalance will likewise persist at all angles unless the geometry changes (e.g. one child leans back).
That's it. Not complicated.

 

[loozh]

Thank you hutchphd. That makes sense, perhaps i should try some hands on to fully understand the effect. Many thanks!

 

[russ_watters]

Given the case does it mean i would be able to balance this level scale so that it can stay in any position without changing the weight or distance? if so how?

View attachment 327113

In that example the weights are hanging below the bar. That's the answer to my question: it is imbalanced in the vertical axis (more weight below than above). Though if they are hanging they might be able to swing back and forth somewhat.

 

[russ_watters]

I fear you are. A child's teeter-toter, if balanced at horizontal, will also be balanced at any other angle so long as the children remain rigidly upright.

When you say "rigidly upright" do you mean they have to change their lean with respect to the teeter totter to remain vertical? If yes, I agree that's what's required to remain in balance. But if not (say, they are holding the handlebars so their geometry with respect to the bar is fixed), the balance isn't neutral. Moreover if the thickness of the teeter totter is significant and the fulcrum is small/sharp and below, the teeter totter itself is unstable. In both cases, offsetting weight above the fulcrum causes the teeter totter to want to flip over.

 

[hutchphd]

No I meant actively vertical. But the pivot on the classic teeter totter does lead to unstable equilibrium. Hadn't folded that into the camera support but I think the cables position make it a stable equilibrium

 

[Lnewqban]

TL;DR Summary: Given that both ends of a camera jib are balanced, shouldn't a jib remain level / parallel to the ground instead of staying in any position?

In order for the camera jib to remain level, or to return to level by itself, not only one, but two conditions are necessary:
- Both sides to be balanced.
- The vertical location of its center the mass is lower than the location of the pivot axis.

The second condition will always make the center of mass return to its lowest location, which is located on a vertical line that crosses the pivot.

In order for the camera jib to remain at any angle, not returning to level by itself, two conditions are necessary:
- Both sides to be balanced.
- The vertical location of its center the mass exactly coincides with the location of the pivot axis.

 

[Lnewqban]

Note a friction disk in the video below, which, besides the fine balancing, helps stop any small movement induced by a less than perfect spatial coincidence of the net center the mass and the axis of the pivot.