9.12.2020 RA worm housing rethink.

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Wednesday 9th overcast again. 

I need to look at my RA drive again. The worm bearing retention is so poor that I need a complete redesign. I am still basing the drive on the Beacon Hill channel profile off-cut for the worm. This channel section relies on shellac and a single 4mm grub screw for each bearing retention. 

The shellac gave way on the first mounting of the telescope. It took only a slight imbalance to push the complete worm literally out of its "housing". Releasing  the telescope into an uncontrolled swing to the low point. The grub screws have always locked up the bearings against free rotation as soon as the screws are tightened. 

The channel section is reduced to a mere illusion of strength and stiffness once the bearings are line bored. The wall thickness around the bearings is reduced to a complete nonsense level of fragility!

Even after I added 10mm plate at the drive pulley end for motor attachment there was no increase in channel strength. The 10mm plate was "drilled" with a hole saw to clear the large hub of the 32T drive pulley. So any potential for bearing retention was literally thrown away without a second thought. 

What was badly needed was a "trough" for the worm to sit in within its bearings. The sides of the trough would reinforce the [presently fragile] channel section ends against twisting under load. The ends of the trough should have positive bearing location and enough meat to avoid any flexure. 

I could bolt metal straps to each side of the present channel to make a trough. Though that wouldn't help bearing retention. While one bearing is now safely restrained against linear movement the worm shaft is not. It is literally sliding out of its new bearing! Dragging the drive side bearing out of its BH channel with it!

If I can add a drive side bearing stop on the motor plate then that would certainly help. How to do so while still allowing room for the 32T pulley hub? There is barely room enough on the foolishly short, worm shaft for the pulley hub. So I can't add any more [extra] thickness. I have ordered a range of M4 grub screws to allow me to reduce the hub diameter.  A thinner, motor mounting, end plate would really help. That would allow a smaller hole than the pulley hub to positively stop all bearing movement at the rim. 

I only used a 10mm motor plate because I had lots of this material. Its considerable thickness seemed to add strength. Though this was more of an illusion and quite a considerable nuisance. Reaching the grub screws in the pulley hub, to tighten them, meant an access hole had to be be drilled through the edge of the plate. Alignment of the hole with the grub screws was critical to using a long series hex key.  

I still want to enclose the worm channel somehow. Though none of my [scrap] box profiles is suitable.

The image shows how the separation pressure between the wheel and worm is applied radially and laterally to the very weak, worm housing. 

Made worse by the [heavily cutaway] motor housing box being subject to torque. Any extra stiffening material wants to be on the opposite side of the housing to the wormwheel. It needs to strongly resist the torsional loads on the entire motor housing and also the side pressure. Box sections are inherently weak where side loads are concerned. They want to become lozenge shaped.

A length of angle profile could be laid on top of the main 10mm main drive support plate. To extend downwards behind the motor box. It still needs packing between this angle profile and the worm housing. I have some very large rectangular box sections in aluminium up to 100x200mm. [4"x8"] Suitably sliced it could be made into an angle profile to reach the bottom of the motor box and worm housing.  

More later when I have checked my scrap metal stock for suitable, reinforcing materials. Hopefully, with an open mind to a potential reworking of the whole drive set-up. It is all too easy to suffer from designer inertia and to resist improvements as too much like hard work. Particularly when one has already built something quite complex. Albeit to an inadequate design.   

I have started making a 5mm thick, replacement, motor plate. None of my scrap material lends itself readily to worm housing reinforcement. It looks as if I may have to cut up some box section to obtain large section angle profile. I am trying to avoid making a completely new, worm, bearing housing with heavier material.

 

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