Shugart 800-8 media centering problem

[Jay Jaeger]

On 8/14/2015 11:56 PM, tony duell wrote:

> 
>> There is no play in the spindle at all.  Certainly not 1/2 track worth.
>> Can't wiggle it at all, no visible wobble, etc.
> 
> Not necessarily play. Anyway, half a track width is just over 10 thou
> and I am not sure I could notice that amount of runout by eye. Also
> I have learnt by bitter experience to check everything, otherwise you
> discover (far too late) that the thing that was 'obviously right and
> didn't need checking' was the problem
> 

Except that when the media is aligned to the spindle correctly, the
drive performs flawlessly, including exchange with another drive.  If
the spindle itself were wobbling so much as to cause the other symptoms,
I don't see how it could ever work right.

> 
>> The centering cone/clamp has just one continuous spring, all the way around.
> 
> Are you sure. The cone is in 2 parts. Normally there is a spring between them 
> inside the cone and a (stronger) spring between the cone and the clamp arm.
> 

On these drives, the centering cone (i.e., that which centers the media
on the spindle) is actually part of the spindle.  The part that comes
down onto it is a set of nylon fingers, all one piece, with a spring
surrounding them.  They call it the "Spindle Hub Clamp".  All this
latter part does is clamp the media to the spindle.  The SA-800 doc
makes it hard to see what is going on, but it is at

http://www.bitsavers.org/pdf/shugart/SA8xx/50576-4_SA800_Parts_May77.pdf

Page 98, parts 8 (spring), 9 (Hub Clamp assembly) and 11 the plate that
screws onto the clamp (part 10) and holds it all in place.  I swapped
that entire assembly.

Unfortunately, there is no detail on part 9.  It consists of:

A surrounding black plastic piece.
The nylon fingers and the flat part they are attached to (all one piece)
A black pastic washer
A screw, washer and nylock lock nut that holds it all together.

Then there is a spring above that, held in place by the hub clamp plate
(part 11).

As the clamp arm comes down, the nylon fingers splay apart over the
centering cone part of the spindle, and finally clamp the media down on
the flat part of spindle.

>> And, there is no point:  I did mention: I swapped those plastic
>> centering cones / clamps with the other drive, and it made no difference
>> to either drive.
> 
> What exactly did you swap? The cone and internal spring? The obvious spring
> on top of it and all the washers? The clamp arm itself (check for burrs round
> the hole where the cone spindle goes)? 

The entire assembly that sits over the hole in the clamping arm.  I did
indeed check the hole in the clamp arm when I aligned the entire clamp
assembly (transverse and up and down) to match the other drive.  When I
first got the drive, that was closer to the spindle that the other
drive, and not really well centered.

I did not swap the clamp arm itself (the part that attaches to the door,
and fits into holes via pins near the back of the drive.  If that is
itself warped (hard to see how, it is cast, and has lots of support
fillets so it isn't very bendable) or out of dimension somehow, then I
probably could not fix it anyway.

> 
> If the cone is alright then it pretty much has to be the spindle itself. Are you 
> sure the spindle bearings are correctly fitted with all the parts shown in the 
> service manual?
> 

Haven't taken the spindle apart.  (Page 6, parts 2 through 10).  Not
sure I would want to - if it is that worn somewhere, requiring changing
out its bearing or the like, then for me the case is probably hopeless.

> -tony
> 

My gut feeling about this remains that something interferes with the
media centering itself on the cone of the spindle before it gets clamped
down.

What I am really looking for is someone who had a drive with similar
behavior that can say "hey, I had that too, and it was ......".

JRJ