Extension for the draw bar V's more weight.

[White Rabbit]

Hi Guys.

Could someone explain the physics of extending the counterweight shaft on an GEM as opposed to adding more weight. This is a stupid question but...


Is extending the shaft a way of cheating the weight restrictions of a mount or is it the same as adding more weight, I would imagine it is the same as adding more weight.

Technically speaking you have the same weight on the mount but increasing the distance increases the strain on the motor because the weight is at a greater distance. Do I have this right?

The reason I ask is that I have ordered an extension tube from oneofone here on the forum. I ordered it because this means I don’t have to carry as much weight around when setting up but just wanted to clarify.

Thanks
Sandy

[Dennis]

Hi Sandy

My (albeit simplistic) understanding is that a heavier weight on a short shaft near the axis will have less “flex” that a reduced weight on a longer shaft.

My analogy is that it’s a bit like trying to lift a heavy object with a fully extended arm versus lifting the same object with your arm close to your body.

I’m sure our Mech Eng’s and gifted mathematicians can provide a more comprehensive answer.

Cheers

Dennis

[Bolts_Tweed (Mark)]

Gday

I teach thiings similar to this this to undergrads daily however there are a myriad of variables in an astronomy GEM that seperate it from simplistic lever ideas taught in engineering statics. I might add that i am a geotechnical engineer (Dr Dirty) - about as far from a mechanical engineer as you can get I suppose and my comments are based on astro experience not professional experience.

Yes (simplistically) if there is a moment (force x distance) induced by a scope (that is perfectly balanced in the plane at 90 dergees to the dec axis - usually not!) then it can be visualised as being balanced by a moment at the counterweight end. This moment can be a F X D that is the same as the product of F X D at the other end so that the system is in equillibrium. as D increases F can decrease.

Going past simplicity - (and I look forward to comments from some mechanical guys) - this simplistic idea does not take into account things such as deflection (a function of the size of the moment and the size (shape and dimensions) and material properties of the counterweight bar). Even though the two moments may be balanced if the load is too high then you can induce deflections beyond the capacity the mount with obvous consequences.

The most obvious I think is that the gears and worms are not (I think) positioned at the balance point of the system. Thus as deflection increases the force the gear is jammed onto the worm is increased causing serious binding problems and subsequently causing motor burn out etc. (happens a lot).

A good mechanical engineer wil no doubt offer a more technical expansion on this and other problems but my comments would be that mounts are built the way they are built for a reason and load limits are given for a reason. Yes the designer will have built in a factor of safety allowing some slight overloading however as creatures that like pushing limits we tend to keep pushing (I am guilty of overloading a HEQ5 Pro too - solved it by getting a EQ6 Pro).

If however you are within the reccomended limits of the mount and you want to decrease the amount of counterweights carried (ie the fXd = f/2 x 2d ie the same moment) - it may work however there are probably other things that could be induced such as resonances etc that mech guys may comment on.

My 2c worth anyway

Bolts

[White Rabbit]

Thanks for that and what you say about the motor not being at the center of the system makes a lot of sense, however if the weight on both sides is balanced well then this should not be an issue. No?

Thanks
sandy

[White Rabbit]

Also could you go into a little more detail on what you mean when you use the word "moment"
"(a function of the size of the moment and the size)

I take it to mean that there is a moment where the balance is right however as the FXD ratio changes the weight is no longer evenly distributed across the system. ???

Thanks

[Bolts_Tweed (Mark)]

Gday Mate

Moment is term geek engineers use to measure or quantify bending - it is actually short for bending moment. If you had a force (point load) of say 10 kN acting at a distance of 0.5 metres then the quantification would be a bending moment of 10 x 0.5 = 5 kNm. In a situation like a counterweight bar maximum deflection occurs where the maximum moment is. Its a bit like having a roof beam cantilevering from a house loaded with heavy roofing say - the end of the beam away from the house is going to have the most deflection. So it is a unit of measurement as opposed to the normal use of the word like a moment is an instance or similar.

I dont know the set up on the mount but imagine that forces act vertically in the direction of gravity. Say the gear and worm was between the pivot point and the counterweight (but closer to the pivot point) and the distance from the pivot to the counterweight was say 0.5 m. So if say the counterweight bar is at 45 degrees the horizontal distance (ie D of the moment calc) from the pivot would be 0.5 * cos 45 or 0.7071 * 0.5 = 0.3535 m but if the mount rotates to 30 degrees it becomes 0.5 * cos 30 = 0.433 m. Therefore the lever arm increases as the mount rotates (ie the bending moment becomes larger). this will keep happening until the dec axis is horizontal where the lever arm would be 0.5 (cos 0 = 1) and this is where most bending will occur.

I assume the design of the mount would have some sort of mounted bearings that would limit this effect however, and what I am talking about is simple statics as I said - mechanically it may be all different (and I await being being shot by a mechanical engineer) but it is common sense to minimise all flexure especially in a photographic mount and flexure is usally a result of bending somewhere in the system.

Bolts

[Bolts_Tweed (Mark)]

Sry Sandy I just saw your previous question. Sry about the tomes but its hard to describe in minimal words. You prompted me to go and have alook at my HEQ5 and I am more confused now than ever. The counterweight bar is bassically independent but is connected to the housing of the dec axis. Forget what i said regarding motor binding however not regarding flxure.

I would have to pull it apart but it appears that the scope sits in the dovetail mounting plate which goes through some sort of bearing (I hope) in the dec housing and the dec gear must be on the inside of the housing behind the bearing. A mech guy would know the correct words but I hope that bearing is fixed (or the other end of the shaft is fixed)and not in a situation that allows any rotation out of the plane of the bearing - if so what i said before applys - the more weight I put on the saddle plate the more I would force the gear into (or out of?) the worm. It couldnt be a simple pivot through the bearing could it?

Anyway I am going to shut up and see if anyone knows the mechanics of whats is going on behind that bearing.

Bolts

[kon1966 (Kon)]

I have ordered a 15cm extension as well. To add to the confusion, I have just changed my configuration to a side by side setup on my HEQ5. C8 on one side and 80mm scope on the other, looks cool. I have balanced it on all axises as well but still need that little bit extra on the end of the shaft.(lol)

My question is should I move both scopes so they are sitting in the middle of the dovetail bar close together or move them to each end. If I have them apart then the balance point is closer to the C8 as it is heavier. Will that have any effect on my mount in terms of the discussion.

Regards
Kon

[Bolts_Tweed (Mark)]

Gday Kon - lol - this is one of the configurations (with cameras - guiding and imaging) that prompted me to go to the EQ6. With this load I found the HEQ5 always struggled and guiding was rubbish. However I wish you well - you might be luckier (especially with an extension).

If you are going to put this on the mount - I dont think it matters where you put the scopes on the twin dovetail saddle bar as long as it is balanced in 3 axis' counterweights, scope horizontal and scope vertical and all points in between (usually an iterative procedure is required) on both sides of the mount as a check.

Bolts

[bmitchell82 (Brendan)]

Ahhh Geek engineers its not a Geek engineer term! its a common engineering term!

All of your thoughts are just dandy and yes bolts your correct in thinking what you do Lot of weight close to the system is better to reduce flexure (bending moment) of the shaft. though when your like me im balancing close to 26kgs off the end of my mount. If i where to use just the same counter weight sizes ide need close to 6 5kg weights... try lugging that around

At present i have 2 x 5kg SW weights, 2 x 5kg dunbell weights + half a paver just to reach balance. hence why i have opted for a 200mm extension bar. I have to say looking at the materials used i belive that flexure will mean jack all, why do i say this? well its just a static weight on the end of the bar there is nothing depending on the lack of flexure aka guiding and the scope moves that slow that it will not matter.

The only time that you would worry about it is if you where really loading the thing up till it was close to the Yeilding point of the materials (but at a 20mm thick steel rod i don't think it will at 200GPa approx)

To give you an idea of what the EQ6 can handle "IF BALANCED CORRECTLY" will hold a 120mm Celestron refractor, 80mm SW Equinox and a 12" SW Dob along with all accociated photography equipment. Laugh if you will but it produces nice sharp images.

So what is the secret.!?
1. Balancing - very important
2. Ridgity in your scope mountings.. actually even more important than balancing....

The EQ Mounts are what i would consider the top of the range for beginners they are Tough, they are Sturdy and they are Resonably accurate in tracking.

If you balance the kit out perfectly that is put everything on before you balance (if your doing photography balance it initially with your photography kit) your motors will not experience a great deal of force... though be aware that a little bit of pressure is good to suppress back lash and give the motors something to push against.! (read up on the PHD guiding manuals!).

other than that Good luck remember your trying to get precision out of a non precision device! push it to its limits and learn!