Mounting update: Whoops [again!!]


Sunday: I was just lifting the mounting off a reinforced, B&D workbench with the chain hoist. When the steel strap I hook over the ceiling joists snapped without warning. A loud crash and the mounting toppled sideways off the bench to lodge horizontally on a stack of timber off-cuts. So now I had to reach underneath the precariously balanced mounting to extract the hoist hook from the lifting strop. Which I had wrapped around the PA assembly for the lift.

Having arranged another strop above the mounting I could gently lift it clear with the hoist. That's the second time the mounting has exceeded the limits of supposedly strong, supporting devices! I have now added a thick plate of aluminium to the top of an old plastic beer crate to support the mounting rather nearer the floor.

The mounting was becoming covered in condensation, from my breath, as I removed the Dec wormwheel in the unheated workshop. I needed to turn the brass, spacing collar down a little to center the worm on its wheel. On a proper wormwheel the "teeth" are cut as indentations with a radius equal to the worm.

It is obvious that there is only one possible position where the worm fits intimately in the slanting curves cut into the wormwheel rim. A smooth cylinder could be made to rotate in a matching, curved pulley rim but there would be no drive. By adding a helix to both the worm and wheel rim the worm can drive the wheel, albeit slowly. Another advantage of closely matching teeth is that the worm locks the wheel against all free rotation. Any unwanted freedom is called backlash. This can occur when the drive is reversed during guiding. Or if there is any imbalance in the instrument on the mounting.

Here is an excellent series of explicit instructions for making a wormwheel on a mini lathe. This limits wormwheel diameters to only [about] 7" maximum diameter.  The same as the Fullerscopes MkIV. Those with larger lathes can obviously make larger wheels.  One can save a lot of money making your own but it does require an awful lot of time and effort to do it correctly:

Step by step Telescope worm wheels and gears on a mini-lathe

And another website offering advice on DIY wormwheels:

Making Wormgears

The image shows the latest arrangement of the Dec stepper motor drive cables.

I decided the short motor cable was far more fragile than the long, sleeved cable connected to the AWR Microstep Drive box.

The initial cable was also rather long. So I folded the motor cable and zip tied it to the edge of the 10mm thick, motor support plate.

Then I P-clipped the heavier connecting cable to act as a strain relief device. This much sturdier cable goes through the large hole in the mounting's fork face plate and thence off to the AWR electronics. AWR were very generous with these connecting cables to allow for a very tall pier for the refractor and/or some distance to the drive electronics.

It is important that no load is placed on the initial, motor cable as the mounting moves the telescope around the sky. So I have added a slack loop to allow for 180° of Declination axis rotation. Not to mention catering for similar PA movements.

I used to park the folded refractor horizontally with the wights up on the north side of the pier. This was only to reduce wind loading on the temporary pier. I doubt I shall have to resort to such "gymnastics" with the telescope housed in its dome.

I don't have a name [yet] for the black, 9 pin, latching plug and socket. The AWR guide mentions a rectangular 'Quickmate' system, but I haven't found any images online to confirm this. I thought I might be able to shorten the first motor cable myself, to tidy things up, if I can source a spare socket. This will save me purchasing a complete, but shorter motor cable from AWR. Though there could be some soldering or crimping of the wires to the pins and sleeves involved. I haven't discovered if the plugs and sockets can be disassembled. Or even re-wired after use without destroying anything. Only six of the 9 pins available are actively used.

Click on any image for an enlargement.


No comments: