Balancing calculation for GEM counterweights?

[michaelmorris]

My head hurts!  I've tried working out the problem below but I can't suss it out.

I've recently bought an EQ8 mount on to which I am hanging a whopping 33 kg of 3 x scopes, dovetails, dual mounting bar and cameras. The mount comes with 2 x 10 kg counterweights which at the moment I need to put centred around 25 cm along the 39 cm long dovetail bar to balance the system.  All the reading I've done suggests that putting these heavy counterweights so far out along the counterweight bar isn't the best of ideas and I would be be much better putting more weight closer to the pivot point of the system.

Unfortunately, Skywatcher only make the counterweights that fit the EQ8 dovetail in 10 kg size.  If I presently balance the system with 20 kg of counterweights centred 40 cm from the pivot point (26 cm along the bar which starts 15 cm from the pivot point), how would I work out were I should centre 3 x 10kg along the counterweight bar. (I don't want to buy an extra 10 kg counterweight only to find out it is too much weight).

Thanks

[Bizibilder]

All to do with moment about an axis - 20KG hung on a bar 1m long is equivalent to 40Kg hung on a bar 0.5m long.  i.e. Half the distance needs double the weight.  On either side of a pivot: m1 x d1 = m2 x d2  where m is the mass and d the distance.

In your case: 20 x 40 = 800   so for a mass of 30 you need a distance of 800/30 = 26cm (roughly) or a mass of 40 a distance of 20cm approx.

[michaelmorris]

Thanks Roger.

Damn, it is really a bit too tight for my liking.   Basically it would mean that all three counterweights would have to be right at the top of the bar with just 1 cm of leeway.  If I remove anything substantial from the mount I would have to take off one of the 10 kg counterweights.

I think my best bet might be to mount 2 x 10 kg counterweights nearer  to the top of the counterweight bar and buy a secondhand Skywatcher 5 kg counterweight.  I could ask my friend Chris to machine out the middle of this extra 5 kg weight (so that it will fit on the much larger diameter bar of the EQ8) this will reduce the weight to around 4 kg.  I could then use towards the 4 kg weight at the end of the bar to fine tune the balance.

[101nut]

We have close to 34Kg of Meade 14" SCT on an EQ8, so the centre of mass is about 9" off the saddle, and that needs 30Kg of c/w within 1/4" of the end of the c/w bar.

Perhaps the simple option of getting a standard Skywatcher 5Kg c/w drilled out to suit the EQ8 bar would provide a little more leeway?

AndyG

[Bizibilder]

If you are going the "boring to fit" route it may be worth investigating weightlifting weights?