I've had some recent scares with the worm gear coming completely
out of mesh twice without warning.
The scope has tracked beautifully for hours then suddenly done an out
of mesh nosedive!
I've luckily been in the dome both times to catch it and the second
time last week I heard the worm scrape across the mountaintops of
the worm gear hob. A truly heartbreaking moment.
I thought I had definitely chewed out the hob surface both times.
I was at a loss to explain why this had happened because the worm
carriage is completely bolted down solidly and cannot drift out of mesh.
Secondly, the RA shaft cannot drift upwards and out of mesh in it's
bearings either. There had to be something in the system that was the
cause and I reckon I have found it.
Firstly I pulled it all down and was relieved to find no obvious damage to
the hob surface. What I did find though was the worm carriage had some
slight play in the brass bush on the drive end.
Not enough to cause a complete loss of mesh though so there had to
be something else.
What I found next was the worm when bolted down firmly had a
tendency to pull sideways to one edge of the pre-cut groove
shape of the worm gear.
I've attached a rough paint pic to illustrate this.
Even though the pre-groove should seat the worm snugly, if I let
it bolt down firmly to slightly one side as seen in the right side pic
of the pic, then only a substandard mesh has been achieved.
It felt like proper mesh but when combined with the play in the worm
bush it must have been enough to come completely out of mesh at
the point on the worm gear that was slightly lower. I know I have some
ever so slight non-concentricity of the worm gear.
Anyway...PIC 2 is a shot looking straight down the axis of the worm.
Even though it' slightly oblique an not a good focus, you can see
a slight misalignment of the worm in the pre-cut groove.
All bolted back together now and tracking fine. No obvious play in
the 360 dgerees of travel. (checked with a battery drill driving the
worm direct drive).
Hopefully back in business.
Steve