[R-390] Friction in KC Change Chain

[Curt Nixon]

Thanks So Much Randy:

That link was exactly what I needed.  It answered a couple of questions 
on the Nitrogen purge process..

Also, it got me to search through all 255 pages of the PTO archive for 
references to non-Cosmos version.

So..here is where I got to:

Dis-assembled the PTO down to pulling the slug and leadscrew assembly.  
Cleaned all that with the end bearings and thrust bearing/Belleville 
washer, etc.  and evaluated the drag in the assembly. 

I found the front end shaft seal O-ring to be brittle and cracked and 
was offering very little detectable drag.  After cleaning and relubing 
and assembling the leadscrew and slug and its preload nut, I found that 
the majority of the drag I could feel was not the O-ring but the screw 
preload nut.  I re-assembled with the minimum preload availkable--ie run 
the nut down all the way to the slug and then back it out only far 
enough to align the first available hole pattern---less than 1/3 of a 
turn.  This was 1/3 less than the original preload amount but did not 
reduce the drag significantly.  I tried several types of lubricant on 
the screw.  Phil Wood teflon, pure teflon grease, light oil, etc.  The 
pure teflon grease seemed the best but was still not real free.

I left the O ring out of the re-assembly

I am in process of building an adapter to use in quantifying the torque 
to turn using a special small dial torque meter..more to follow on this.

Also, as I mentioned earlier, the "roller" follower for the 
linearization stack did not rotate.  After studying the construction, I 
am convinced that it is supposed to rotate.  It has a steel roller over 
a plastic bushing over a staked-in pin.  I forced it to turn and tried 
to free it up but the whole pin turns in the bracket so I cave up.  Even 
tried heat cycling it with penetrating oil..no go.  I suspect the thing 
is either over staked, or the plastic bushing has been affected by heat, 
or it was just bad from scratch.  When opened up, the stack had grease 
along the contact area of the follower and no wear is noticeable, but I 
still think it should turn. 

I re-assemble it all with clean grease there and left it frozen in 
place.  I will investigate later when I get ahold of another Collins PTO 
to experiment with.  I have sourced some miniature roller bearing cam 
followers that will replace the staked-in unit with a real ball bearing 
roller.

I will also experiment later with putting washers under the screw 
pre-load nut spring leafs to reduce the preload and drag more than 
simple backing it off 1/3 of a turn. (reducing the spring tension on the 
pre-load nut)

No particular problems encountered in the process..thanks to having 
pieced together numerous tid bits from the archive.

One problem I anticipate, and others will need to be aware, the holes in 
the forward and rear mounting brackets (the green screw brackets) are 
much larger than the screws to allow for significant movement of the PTO 
body upon re-assembly into the radio.  Alignment of the Oldham coupling 
can be tedious but is critical to the free operation of the assembly. 

I left the heater wires disconnected inside the enclosure as I believe 
the advice about the ovens not being needed.  I will be monioring the 
overall stability later and may choose to re-connect them and run all 
the ovens for FMT events.  Also decided to put off the nitrogen purge 
until I have a chance to verify the PTO linearity and operation.  I have 
some ideas regarding the stack adjustment process that I would like to 
try later with a second PTO.  It should be possible to correlate the 
amount of stack offset to frequency error at a series of points and 
create a "template" of sorts for adjusting the entire stack at once. 

So thanks to all for the inputs so far.  I will be finishing up the 
gearbox cleaning this week and should have my cap order by then. 

73

Curt
KU8L
www.curtsworkshop.blogspot.com



Randy and Sherry Guttery wrote:
> On 2/27/2011 9:55 AM, Curt Nixon wrote:
>   
>> Thanks Drew:
>>
>> The seal O-ring is EXACTLY what it feels like.  I mentioned that to
>> another tech off list and said it was smooth but felt like an shaft
>> sealed with an O-ring.  I have dry nitrogen here purge with but being
>> there is no valve, it must have been assembled in a nitrogen chamber
>> which I DON'T have :).
>>     
> of course you do...  with a bit of imagination... Here's a 
> small blurb on nitrogen "purging" and/or charging...
>
> http://www.comcents.com/radios/PTO.html
>
> Since there is quite a bit on repairing, linearizing, etc. 
> I didn't get into any of that - but there wasn't much on 
> nitrogen - so here 'tis....
>
> HTHs...
>
>