Effect of counter weight mass vs rod length.

[Adam J]

I have been thinking..

I have read left right and centre that if you can get away with a single light counter weight on a longer rod then you are better off than using a heavy weight / multiple weights on a shorter rod.

In my personal opinion this is a load a rubbish as surely when the mount is balanced the factor effecting guide accuracy / maximum load is moment of inertia / torque driven and nothing to do with total counter weight mass.

As such the torque required / moment of inertia increases as a fixed weight is moved out from the pivot point, but you get exactly the same torque / moment of inertia with a higher mass on a shorter arm as you do when you place a smaller mass on a longer arm so long as its balanced with the scope. 

The end point here is that the motors in you mount have to work just as hard in either case and so the only way of making them work less hard is to balance better or to reduce the weight of the scope so that you can move the counter weight inwards. 

Someone please correct me if I am missing something, but I have seen someone saying this so many times its starting to bug me. 

 

Adam

 

 

[John]

It might be more to do with keeping the weight close to the axis of the mount rather than having it on the end of a long counterweight bar ?. Having a weight some way from the central mass of the mount on the end of a rod can lead to more vibrations I think ?

 

[DRT]

7 minutes ago, John said:

It might be more to do with keeping the weight close to the axis of the mount rather than having it on the end of a long counterweight bar ?. Having a weight some way from the central mass of the mount on the end of a rod can lead to more vibrations I think ?

 

That would be my assumption, John. As Adam suggests I think the most critical factor is balance so that the mount doesn't experience uneven stress, but having a weight sticking out on a long pole is bound to lead to more opportunity for vibration in the same way that long OTAs vibrate much more than short ones.

 

[triton1]

The consensus on here seems to keep the weight as near to the axis as you can.A heavier weight nearer is better than a lighter weight further.I certainly would never add the extension bar to my neq6.

[Wikiastro]

The moment of a heavy weight on a short rod is the same as a lighter one on a long bar.  BUT, I would guess a longer bar, with a weight at the end of it would flex more and hence cause more vibration.  Try it and see!

[Adam J]

I have never heard anyone say to go short with a bigger weight due to vibration so that is a new one for me. My complaint was people saying the opposite under the incorrect assumption that a smaller counter wright further from the pivot point is superior. Glad you agree that the moment of inertia and hence stress on the mount is identical though.

[andrew s]

12 minutes ago, Wikiastro said:

The moment of a heavy weight on a short rod is the same as a lighter one on a long bar

Yes but the moment of inertia is distance^2 * mass. So a lighter weight further from the axis requires more torque from the motors when accelerating / decelerating the scope.

I would always go for a greater weight closer to the axis.

Regards Andrew

[Wikiastro]

Good point - It's harder to start something moving on the end of a long bar compared to a short bar.

21 minutes ago, Adam J said:

I have never heard anyone say to go short with a bigger weight due to vibration so that is a new one for me.

I'm only making a hypothesis - all metal has some flex in it, and a longer bar with a weight on the end would flex more than a short one, then spring back causing vibrations to last longer.  It'd need testing to prove it.

Both seem to suggest a short bar with a heavier weight is better.

[DRT]

29 minutes ago, Adam J said:

I have never heard anyone say to go short with a bigger weight due to vibration so that is a new one for me. 

Think about the extremes of this - a short bar with a weight that is the same as your OTA plus bits that is exactly the same distance from the centre of the mount versus a 4, 5 or 6m bar sticking out the other side from the OTA but producing the same balance through the weight of the bar without the need of a weight. Intuitively the first option just seems the way to go  

[AlistairW]

Interesting as i do agree with the shorter length camp. But by increasing the mass there must be an overall greater downward force simply due to it being a bigger weight. Is there not an effect from this ?

[DRT]

2 hours ago, AlistairW said:

Interesting as i do agree with the shorter length camp. But by increasing the mass there must be an overall greater downward force simply due to it being a bigger weight. Is there not an effect from this ?

No. The force excerpted on the fulcrum (in this case being the mount) is a function of the mass of the weight multiplied by the length of the lever (the bar). 

 

[Alien 13]

A lighter weight at a distance is useful for a widefield portable rig like a SA if you have to carry it up a hill but otherwise big and close to the pivot is best. The amount of vibration you need if imaging at the pixel scale its tiny so it all helps.

Alan

[AlistairW]

6 hours ago, DRT said:

No. The force excerpted on the fulcrum (in this case being the mount) is a function of the mass of the weight multiplied by the length of the lever (the bar). 

 

I agree, but my thinking was that the overall weight of the system will be more with the extra weight. There must be a limit to the number of weights before the mount struggles

[andrew s]

8 hours ago, DRT said:

No. The force excerpted on the fulcrum (in this case being the mount) is a function of the mass of the weight multiplied by the length of the lever (the bar).

What you define is the moment of the forces not the force. Yes the moments must be equal if the axis is in balance but also the forces (proportional to mass) must also balance if you don't want the axis to accelerate off down the road.

Bigger weight close in results in greater downward force through the mount.

Regards Andrew

[ollypenrice]

Trust Andrew. The maths has it. Although it may be counter intuitive since the point of balance is the same in both cases, the torque against the motor is greater with lighter weight-longer distance and so is any flexure. (This question has come up before and I couldn't remember the formula.)

Not to metion blundering into a long counterweight arm while looking for the thing you just had in your hand a minute ago. 

Olly

[andrew s]

18 minutes ago, ollypenrice said:

Trust Andrew

Mad fool

Regards Andrew

[Peter Drew]

The extra weight bearing down on the complete system can improve the stability, look at all the recommendations for hanging an extra weight on the tripod.  

[andrew s]

11 minutes ago, Peter Drew said:

The extra weight bearing down on the complete system can improve the stability, look at all the recommendations for hanging an extra weight on the tripod.  

I agree but the keyword here is "can". The stability gain from hanging the weight from the tripod is due to lowering the center of gravity of the system in addition to the additional inertial mass. 

With a heavy counter weight on a typical Newtonian it can raise the center of gravity compared to a lighter weight further out along the Dec shaft. 

In principle I agree that more mass is better but just watch out were the C of G is especially with portable systems.

Regards Andrew

[Peter Drew]

Agreed. 

[RichLD]

Whilst I agree that the way forward is shorter bar/more weight for all the reasons mentioned above, I can't help but worry about the potential for slippage of the counterweight bar clamping mechanism when not fully extended, especially on mounts like the HEQ5.

For this reason and for ease of portability I tend to go with the counterweight bar fully extended with less weight - this has never caused issues but then I image at very forgiving pixel scales ?

[Stub Mandrel]

There won't be one correct answer for every mount.

Taken to extremes a huge weight close to the pivot might balance, but be enough to completely seize up the bearings. A tiny weight on a long rod might balance, but vibrate enough to cause issues.

The rods are long so the counterweight can be small compared to the OTA. a very heavy weight on a short rod with a heavy OT could easily overload the mount and degrade performance.

My personal take is that its better to move a heavy weight inwards rather than fit a larger weight, when changing to a smaller scope, but don't add loads of extra weight to balance a larger scope.