tight latitude screws on EQ6Pro

[bmitchell82 (Brendan)]

Originally Posted by bojan

When you un-fasten the screws and then leave your mount lay on the side, are the screws tight then?

Mine were not - they were actually reasonably loose, as it should be expected of tapping properly done.
Then, what I saw as a potential problem was the fact that adjustment of the screw that was pressured by the mount under load (back screw) was changed with tightening of the opposite one, which should not happen and what I saw as a clear indication that the screws are not perpendicular to the surface of the post, so they were stressed radially, with the potential of damaging the threads by load weight (and even bending the screws under load.. remember, there is potentially 50 kp of load (telescope + counterweight) on one side of the the 200mm lever, held by that screw only 50mm away from pivot point.. which means more than 200kp load on post and screw thread.. a lot, especially if you have additional lateral stress.
So I decided to stiffen the whole thing a bit by adding the nuts. I simply like to have details done properly, no matter how simple or complicated they are

Yes they are still tight, im very handy with a spanner, and i know that they are too tight to be normal. Considering that we all brought our mounts within the last few months ide say they are the same batch. Im not too fussed as at most im only going to have to give it a few turns or a bit of a tweak here or there. and if i notice that the thred is being damaged its not a great deal of worry, as you can either tap it out again into a bigger thred (even a coarse thread though this might make fine adjustment harder) or resleave it with something like a SS thread.

What unit of measurement are you using bojan, kP is Kilo pascal, unit of pressure of fluids. Do you mean Newtons (N) (Force) or Newton meters (N.m)(Moment Force). and further more how did you measure the force, as you would have had to of known the angle of the bolt in relation to the perpendicular thus giving a moment force at the point of interation with the mount head. Damage could be caused to the thread but if you have loaded it up enough to do so then quite likely you have exceeded the manufactures load limits as it would be fracturing the cast alloy mount head as goes for the bolt.

I thought the reason why you would change it is the dickky little handle they give you bends like butter and has no screw/hex head to do your tweaking. and generally they give you a tougher material aka SS or Steel (stainless doesn't rust and looks all shiny)

In any rate relating kP to kN that would indicate this bolt would have in excess of 500kgs on it. and if it was just N would be 5kgs. I want to know as i might have missed something and have a chance to learn something new, but my knowledge i thought to be reasonably sound in this area as i am a student of civil engineering with my focus on Structural and lifting engineering in particular.

[bojan]

:-) I was using kiloponds, sorry :-)
Started with kg, and was lazy to change the numbers. That is the reason for confusion.
The force I estimated as a possible maximum... you have 25kg of telescope, 25 kg of counterweight and all that 200mm away - that is the rough distance of DEC axis from pivot point in the base of the mount. It reality it will be lower because the load will not be that much and second, the latitude angle is 37° so we have reduction of momentum here by factor of 0.8 (cos (37°)). However, I saw some people using EQ6 as alt-az mount, so it is a possibility.
The distance of where screw is in contact with the post inside mount is ~50mm. So the max net force to a screw is 3.2x the load [ (200mm x 0.8)/50 ]. That makes ~1400N on the single screw. Or, 142kg.

[bmitchell82 (Brendan)]

sorry bojan, your rational still doesn't make sense, as if you have the telescope balanced it doesn't matter how much you put on it because the net moment force is 0 this classified as a static system if the mount can stay in a particular spot without moving then there is enough forces to cancel it out and make the sum of the net for = 0 N.

If manufactured correctly the bottom weight bearing bolt (what gives us our dec of 30 something degrees) should be perpendicular to the surface when the mount is at 0 degrees. if it is not then the measurement would have to be made. Problem with it staying perpendicular is that the actuall thread would have to pivot to mate the face at 90deg. In all fairness you could estimate the angle of deviation but that doesn't give you a "factor" amount. it will give you a definate force F or N.m.

Force is just the mass * gravity
N.m is Newtons * Meters

when you split things into right angle triangles all you are doing is giving a Force in the X and the Y direction as its extremely complex to be able to deal with a force in one direction.

So theres only one factor that you need to consider here, and that is in the Positive Y direction and if the bolt is capable of accepting such stress.

Also remember that any calculations that you are doing must be in standard SI units eg, Seconds Meters Kilograms, or you will have erronus results

first draw a FBD free body diagram making sure to split each area up to work out the net force transfered to each element, ending up with a foce amount placed on to the bolt in question. a quick split of the x and y components will see you with the amount of N applied in the Y direction and the of moment of inertia needed by the bolt itself to counteract this

[bojan]

Sorry for possible mistakes, I think I've made a few minor ones... But we are thinking in the same directions I would say.

So, lets go this way... and I will try to be more careful and exact with my calculations from now on :-)

A cross-section drawing of the mount is attached, and it illustrates my direction of thinking....
The mount on the sketch is set to 45° latitude, but I wanted to explore the worst case, which is 50kg of load (telescope + counterweight) and as used on equator.

If we assume the mount is used on equator, we have momentum (pivot point is latitude bolt) on the load side expressed as M1=F*D1*sin(alpha).
The angle alpha between D1 and F can vary from 75° (if we are on equator) to ~-15° (near pole.. then the load is on the other side of the pivot point, so I used "-" here).

This momentum is held in balance by M2= F2*D2. The angle between D2 and F2 is close to 90° for the purpose of estimating the axial force on screw, however the radial component is also present and significant since the screws are designed for axial component primarily and they can not hold much radial (side) force (they bend, or they damage the thread in the case of excessive force and if material is not thick enough) .
You must excuse my English here, I went to grammar school in Europe (non-English) and it was long time ago so the terminology I use here may not be quite right, but I am trying my best to make myself as clear as I can :-)

If we assume the worst case, that is the use of this mount on equator, D1 is at roughly 75° (discussed above) so we have load momentum M1= 98.1Nm*sin(75°) = 94.75Nm.

From the above, we have:
F2 = M1/D2 = 94.75Nm/.05m = 1895N


For our latitudes (~37°) this force will be lower, since the angle between D1 and F1 is ~20°, so F2 is around 650N [F2 = 1895N * sin(20°) = 648.128N] but still significant.

There is no force on screws for latitudes around 75° (if my estimate of 15° angle between D1 and F1 is correct).

I hope there are no more mistakes here :-)

[bmitchell82 (Brendan)]

Kinda close but not really, tonight if i get a bit of time ill sit down draw some drawings and do a bit of measurements and calcs then you can see how it should work.

Ultimately you should have 3 diffrent Free body diagrams, using the worst case like you said and in our case would be at lower dec like myself which im close to 30 degrees.

[Shawn]

I have found this too yo be an issue, particularly at 17S, Now Im looking at the casting thickness and wondering why iy cant be drilled out to a reaonable sized fine thread, OK this doesnt change the sums, but a greater contact area would be better equiped to deal with it...TCW