Counterweight position question

[michaelmorris]

I wonder whether anyone can help me with a question I've been pondering for a while now.  Where to put my counterweight? (Keep it clean ).  As I see it there are a number of possible positions for where to place counterweights along the counterweight bar on a german eq mount.  

I've always worked on the basis of trying to put the counterweights at a similar distance from the centre of rotation of the RA axis as the centre of mass of the scopes/cameras on the other end.  Is this correct or would it be better to:

• Use less weight and put it as far along the counterweight bar as possible,
• Place the counterweights as near as possible to the centre of rotation (i.e. the top of the counterweight bar), or
• Evenly space the counterweights along the bar

Your thoughts/opinions welcomed.

 

 

[knobby]

Not at all scientific but more weight on shorter rod will probably dampen quicker and wobble less than less weight on longer rod.

It's a bit of trial and error I guess.

I personally went for less weight and longer rod as I carry the mount set up so it's lighter that way.

[libraryman]

Hi Michael, in my opinion the best way is to employ one heavy weight, as high up the bar as possible, then use one small weight low don to use as a balancing weight, this should reduce the moment and bar flex.

 

Ray

[andrew s]

I agree with Ray. Large weight close to the axis minimises the moment of inertia. This makes the load easier for the motors and control system to manage. 

Regards Andrew

[Zakalwe]

For the forces involved I don't think that it makes that much difference, especially on a mount like the EQ8 which has a very beefy CW bar. It does make sense to keep the weights close in though.

[RayD]

Can see where it's heading.  Agree with Zakalwe, at these loads and speeds neither inertia or kinetic energy is going to have any influence at all.  Do whichever is easier for you.

[andrew s]

40 minutes ago, RayD said:

just make sure your mount and bar can take the weight.

It is not about the weight as such if they can't take the static weight your in trouble. It is about the dynamic load seen by the drive system gears and motor. All gears and shafts deform and flex as the dynamic load changes and the motor must supply the torque. The dynamic loads are minimised by having the inertia of the system as low as possible i.e. counter weight close as possible to the mount.

Regards Andrew

[Zakalwe]

The mint in question is an EQ8. It uses a 435:1 gearing reduction so the torque multiplication is massive. 50Kg payload IIRC.

Itll be reet.

[andrew s]

11 minutes ago, Zakalwe said:

The mint in question is an EQ8. It uses a 435:1 gearing reduction so the torque multiplication is massive. 50Kg payload IIRC.

Itll be reet.

So will the the corresponding dynamic loads on the gear teeth. I am sure it will work either way...... but why not do it right.

Regards Andrew

[RayD]

11 minutes ago, andrew s said:

It is not about the weight as such if they can't take the static weight your in trouble. It is about the dynamic load seen by the drive system gears and motor. All gears and shafts deform and flex as the dynamic load changes and the motor must supply the torque. The dynamic loads are minimised by having the inertia of the system as low as possible i.e. counter weight close as possible to the mount.

Regards Andrew

Yep, as a Consultant Engineer I fully understand the concept of dynamic and static loads, and of course their influence on gear boxes and motors.  However, as far as the OP goes, the position of the counterweight(s) on his mount is going to have very little affect on torque, rotational inertia or acceleration at the loads and speeds we are dealing with, and certainly not any that are going to make an ounce of noticeable difference.

As always, I try to explain things in terms which anyone can understand, which is why I said it makes no difference so long as your mount can take the weight, because the reality is in this application it won't.  He will notice no difference at all with one weight at the end of the bar or two near the mount.  In fact, I see many very good imagers refer to deliberately running out of balance (East heavy), which completely flies in the face of perfecting dynamic loading?

[andrew s]

31 minutes ago, RayD said:

Yep, as a Consultant Engineer I fully understand the concept of dynamic and static loads, and of course their influence on gear boxes and motors.  However, as far as the OP goes, the position of the counterweight(s) on his mount is going to have very little affect on torque, rotational inertia or acceleration at the loads and speeds we are dealing with, and certainly not any that are going to make an ounce of noticeable difference.

As always, I try to explain things in terms which anyone can understand, which is why I said it makes no difference so long as your mount can take the weight, because the reality is in this application it won't.  He will notice no difference at all with one weight at the end of the bar or two near the mount.  In fact, I see many very good imagers refer to deliberately running out of balance (East heavy), which completely flies in the face of perfecting dynamic loading?

I was in no way questioning your qualifications sorry you took it like that.

However, balanced or unbalanced statically the setup with minimum moments of inertia will be easier for the system to control, for example, during guiding. 

As I said above either will work. 

When asked to offer an opinion I do so to the best of my ability and as simply as possible.  

Regards Andrew

[RayD]

Just now, andrew s said:

I was in no way questioning your qualifications sorry you took it like that.

However, balanced or unbalanced statically the setup with minimum moment inertia will be easier for the system to control, for example, during guiding. 

As I said above either will work. 

When asked to offer an opinion I do so to the best of my ability and as simply as possible.  

Regards Andrew

No apology needed at all Andrew and I didn't take anything adversely, and sorry from me if it came across that way  I only pointed out that I am a Consultant Engineer so that you knew I took on board and understood your argument.

Agreed re. minimising the moment of inertia generally, but the reality is inertia really isn't of any significance at all at sidereal rate speeds as we are dealing with such low rates of acceleration.  I only mentioned the out of balance condition as you noted the changes in dynamic load and the additional torque required by the motor to drive it.  Running out of balance, as many do, changes everything as it pushes the moment of inertia out again (OTA end).

If it means someone is having to buy another weight rather than simply using the extent of their weight bar, in reality it's simply wasted money.

I guess it's really down to do you apply something because good engineering practices and physics says the sums add up, or do you apply something because it really does or will make a difference.

It's an interesting subject for sure, as many balancing issues are.

[andrew s]

Ok I miss understood.

I agree for the typical amateur setup it will make little or no difference. I am in the " apply something because good engineering practices and physics says the sums add up " camp.

The best reference I know on this topic is the "Microcomputer Control of Telescopes by Mark Trueblood 1985". I accept he was looking at somewhat larger telescopes that the typical "small" refractor used commonly today. Of interest to me at least, is that Software Bisque still use a worm wheel cut into the bearing (to get them in the same plane to minimise shaft flexure) as recommended in the book.

Anyway this is too far from the OPs question.

Regards  Andrew