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c8 repair – opening the base part 3: the gears March 3, 2010

Posted by aquillam in Astronomy, MichiganAstro, telescope maintanance.
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This post is one in a series about C8 repair and maintenance.  You should see the introduction and disclaimers before trying anything here. This is part 3 (obviously.) You need part 1: opening the base before you can do the things on this page. That’s also where you’ll find the links to the other things you can do with the base open.

The Top Part of the Base

base_open6

Removing the ring that holds the gears in place

The gears will stay with the top part of the base when you take it apart.  This is also the part where the RA lock and fine adjustment are, but that’s another post. To get to the guts, remove the ring that holds the gears to the top. As I mentioned in part 1, I usually put the screws in masking tape and tape them to the ring so everything stays together.

With the ring off, the gears slip off the axis.  There are two gears separated by the RA dial and a spring spacer. Because of the spring, you really should not take the gears apart. There isn’t a good reason to take them apart, and the spring can be really hard to put back in place. It is important that the gears be greased so they don’t wear too much or bind.  It is easiest to grease the large gears, and they will transfer grease to the smaller gears on the motors and RA fine adjustment knob. Be careful not to get grease along the top and bottom of the gears – a greasy gear can stop the RA lock from working. The surface on the inside of the gears, next to the axis should also be greased so it can slip past the ring that holds the gears in place. The edge of the bottom axis  also needs to be greased (part of it is next to the RA lock so be careful where the grease goes.)

The gears shifted slightly aside to show the edge that needs to be greased

Lifting the gears off the axis. They are shifted slightly aside and show the edge that needs to be greased (dark with old grease)

If the ‘scope has never been opened, and it is stiff, it may be time to clean out that 30-year-old factory grease and apply some new stuff.  It needs to be a good, long-lasting grease that won’t get too sticky in cold temperatures. I use whatever the department machinist has on hand. However I can tell you that the lightweight white (probably lithium) grease he gave me about 3 years ago was not a good choice.  It is thin enough that it worked its way onto the RA lock in the two ‘scopes I used it on.

disassembling the gear assembly

Disassembling the gear assembly

In general, you shouldn’t have to take the gear assembly apart.  If you do, be warned: there is a spring that helps keep the setting circle stationary when the telescope rotates that may try to leap out at you.

Place the gear assembly on the table with the setting circle face down and remove the three screws that hold it together.

the pieces of the gear assebly

The four pieces in the gear assembly. Note the spring tucked in among the gears.

The grease may hold things together, so you may need to give the pieces a push to get them apart. Don’t take the screws out of the setting circle.  The hold the little nubs on that make it possible to move the circle around to set the RA correctly. There should be a total of 4 pieces.  The teeth and inner edge are all that needs to be greased.

replacing the spring

With the gears loosely held together with the screws, slip the spring into place.

Because of the spring, re-assembling this is slightly more than just reversing what you did to get it apart (in all probability, the spring came out on its own when you loosened the third screw!)  To get it back together, stack the two gears and the setting circle in order, and put the three screws in, but don’t tighten them all the way.  Slip the spring in between the setting circle and the top gear.  With most of the springs, it works best to put the spring on top of the top gear and fit one end in place, then feed the rest of it along until it slips into place. The spring in this ‘scope seems to be a bit shorter than usual, so it was easy to put back.

Once the spring is in place, tighten the screws down the rest of the way and you’re ready to re-assemble things.

From here, you may want to go on to:

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