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Counter-weights : Weights vs length


vernmid
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Hi All,

I have always balanced my 130PDS on an HEQ5 using a single counter-weight almost at the end of the extended shaft. It also balances well with 2 weights on a sorter shaft. 

I understand that 2 weights increases mount loading but does either method effect guiding. I recall something about turning moments etc from Sk00L but it's hazy!!

Thanks

Vern

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2 weights on a shorter bar present exactly the same loading to the mount as one weight on a longer bar assuming both balance the scope. 
 

That’s how Newton’s laws work :). The only practical issue is a longer bar has more chance of getting caught on tripod legs, tangling wires etc. 

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32 minutes ago, dannybgoode said:

2 weights on a shorter bar present exactly the same loading to the mount as one weight on a longer bar assuming both balance the scope. 

Both options will balance the scope but 2 weights weigh more and so will load the mount more.

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Hello Vern,

Andrew is right.

If you have two weights, then go with that option. The shorter the distance between the weights and the mount the better.

Depending on your seeing conditions, you might not notice much difference in guiding, but rest assured you are being kinder to the motors and if you upgrade the scope to something bigger, the basic principle will help a lot...

Gordon.

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On 07/02/2020 at 12:10, Phillips6549 said:

Both options will balance the scope but 2 weights weigh more and so will load the mount more.

Not so. The counterweight bar acts as a lever and the counterweight itself is the force that provides that leverage to balance the telescope. Therefore the forces acting on the bearings inside the mount are the same. It is not the weight that matters, but the forces.

In addition a weight closer in to the centre can be "spun" faster with the same effort that it takes to move a smaller weight further out. Hence having your counterweights closer in means the mount will respond faster to changes in the driving motors.

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18 minutes ago, pete_l said:

Not so. The counterweight bar acts as a lever and the counterweight itself is the force that provides that leverage to balance the telescope. Therefore the forces acting on the bearings inside the mount are the same. It is not the weight that matters, but the forces.

In addition a weight closer in to the centre can be "spun" faster with the same effort that it takes to move a smaller weight further out. Hence having your counterweights closer in means the mount will respond faster to changes in the driving motors.

I think different things are being confused here. Putting it in simple terms.

For the mount to balance the moments tending to turn it one way must balance those turning it the other way. The moment is the mass x distance of mass from the fulcrum. So twice the weight half the distance for the same moment.

Static force on the mount is proportional to the total mass twice the mass twice the force.

Dynamic load depends on the moment of inertia which is the mass x distance^2 so as in the second paragraph in the quote the load on the motors accelerating the mount are reduced by increasing the mass of the counterweights.

Regards Andrew 

 

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Looking this up on Wikipedia 

https://en.wikipedia.org/wiki/Moment_of_inertia

The "moment of inertia" is proportional to the square of the distance the mass (weights) are from the point of rotation, so the motors will have to work harder the higher the moment of inertia is.

So the nearer the mass is to the centre the better.  It's Physics -  I think😀

 

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2 hours ago, pmlogg said:

My understanding was that there were vibrational advantages to having a heavier weight closer than a lighter weight further out.

I was part of a couple of previous discussions on this subject. The bottom line was when the laws of physics are applied, having the weights as close to the mount as possible is preferable but in the real world you'd never discern the difference. 

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Andrew can be relied on for the physics and experimenters will confirm that more weight on a shorter arm is the better way to go. There is no doubt about this in my view. How much difference it makes will depend on how close your system is to its limits of performance. My own system is not under any significant stress from this issue but anyone wanting to refine their autoguiding should pay attention to the physics and use more weight on a shorter arm.

Olly

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I also vote for the laws of physics, who would not unless you are a current US president, so a shorter bar with more weights is the way to go, not the least because it reduces your chance of stumble into it. So whatever you do never buy a bar extension.

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