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Pages: 1
Dick Jacobson
scholastic sledgehammer


Reged: 12/22/06

Loc: Plymouth, Minnesota, USA
Easy Adjust Sliding Counterweight new
      #5630931 - 01/19/13 12:58 AM Attachment (63 downloads)

I recently built this sliding counterweight for a telescope that I'm currently rebuilding. Its purpose is to make it easy to keep the tube balanced when changing eyepieces. You slide the weight up when removing an eyepiece, then slide it back down while inserting another one. The counterweight is mounted just below the eyepiece along the tube, and takes about one second to adjust either way.

The weight is held in place by friction between the rod supporting it and the steel disk at upper left. To release the friction and lower the weight, you just squeeze against the disk with your thumb. To raise the weight, simply pull it up along the rod. There is a small spring between the weight and the disk to maintain friction when you release your thumb. The disk is loosely attached to the weight by a screw so it is free to move back and forth through a small angle.

The holes in the disk and weight are 1/64 inch larger than the rod. The principle of operation is that when a steel rod passes through a slightly oversize hole in another piece of steel, and rests at a small angle, friction will tightly jam it in place. I've used a similar device in an adjustable chair that supports at least 200 pounds.

I used a bandsaw and drill press to build this. If you don't have these tools available, you could build a simpler version using a short piece of steel or PVC pipe and a pair of end caps. For extra weight fill the pipe with fender washers, ball bearings, lead shot, pennies, or whatever. Any piece of steel about 1/8 to 1/4 inch thick with a slightly oversized hole should work well as a locking device. In this version I used a stainless steel rod, but an ordinary zinc-plated carbon steel rod also works.

The telescope isn't finished and I haven't used the counterweight yet while observing, but in testing it works like a charm!


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StarStuff1
Post Laureate
*****

Reged: 04/01/07

Loc: South of the Mason-Dixon Line
Re: Easy Adjust Sliding Counterweight new [Re: Dick Jacobson]
      #5631014 - 01/19/13 03:39 AM

Nifty idea!

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idp
sage


Reged: 08/21/09

Loc: New Haven, CT
Re: Easy Adjust Sliding Counterweight new [Re: StarStuff1]
      #5631144 - 01/19/13 08:23 AM

Love it! It only seems to work for small weights thought, right?

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Pinbout
Postmaster
*****

Reged: 02/22/10

Loc: nj
Re: Easy Adjust Sliding Counterweight new [Re: Dick Jacobson]
      #5631162 - 01/19/13 08:38 AM



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Dick Jacobson
scholastic sledgehammer


Reged: 12/22/06

Loc: Plymouth, Minnesota, USA
Re: Easy Adjust Sliding Counterweight new [Re: idp]
      #5631207 - 01/19/13 09:35 AM

Quote:

Love it! It only seems to work for small weights thought, right?



No, as I mentioned, I built a similar device for an adjustable chair that supports my 200 pounds. This chair used a 5/16" diameter zinc plated rod and a steel "mending plate" from a hardware store, about 3/16" thick. The counterweight pictured above weighs 5.5 pounds; it uses a 5/16" stainless steel rod and the center hole in the weights is 21/64".

At this point the only unanswered question that I have about its performance is whether it will cause a vibration problem. The rod vibrates a little when bumped but seems to settle down in less than a second. If the finished telescope vibrates, the counterweight could either aggravate or suppress vibration depending on whether there is resonance.


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Al8236
Pooh-Bah
*****

Reged: 08/18/10

Loc: 48.9173N 122.1390W
Re: Easy Adjust Sliding Counterweight new [Re: Dick Jacobson]
      #5631395 - 01/19/13 11:35 AM

Also if I'm not mistaken this will only work in an upright position, if inverted the weight would disingage itself would it not?

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Dick Jacobson
scholastic sledgehammer


Reged: 12/22/06

Loc: Plymouth, Minnesota, USA
Re: Easy Adjust Sliding Counterweight new [Re: Al8236]
      #5631512 - 01/19/13 12:51 PM

Quote:

Also if I'm not mistaken this will only work in an upright position, if inverted the weight would disingage itself would it not?



Yes, if the tube was upside-down the weight would slide down towards the eyepiece. It should work fine at any realistic tube angle.

The beauty of this device is that the friction of the clamp increases in proportion to the weight it supports. I would expect it to work for any realistic amount of counterweight. The "Denver chair" used by many astronomers works on the same principle. If properly constructed, it should not slip regardless of the weight on the seat.


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Pinbout
Postmaster
*****

Reged: 02/22/10

Loc: nj
Re: Easy Adjust Sliding Counterweight [Re: Dick Jacobson]
      #5631519 - 01/19/13 12:57 PM

It's basically the strategy used for one side of pipe clamps

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don clement
Vendor (Clement Focuser)


Reged: 02/02/11

Loc: Running Springs, California
Re: Easy Adjust Sliding Counterweight new [Re: Dick Jacobson]
      #5631641 - 01/19/13 02:05 PM

Dick,
That’s a really good idea!

I imagine that your quick change CW position would work well on a solid shaft with smaller counterweights. For longer shafts I prefer a hollow larger diameter shaft made of lightweight material such as aluminum so as to minimize the overall OTA weight and also keep vibration to a minimum. A long length of 5/16” diameter steel with a counterweight might tend to vibrate. What I have done in the past for mounting counterweights on a hollow lightweight aluminum shaft (e.g. truss member) was to use double tapered multiple split insert that compresses around hollow shaft like a collet. Like a collet the double tapered multiple split insert provides even clamping force distribution along a large area of the hollow shaft so there is minimal distortion to the hollow shaft. An example of this is shown below where two cast iron counterweights are sandwiched on either side of a double tapered split insert around a 3” diameter aluminum shaft. Quick change position similar to your idea could be made by spring loading the two counterweights and using a cam lever to separate the counterweights.

Don Clement



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Dick Jacobson
scholastic sledgehammer


Reged: 12/22/06

Loc: Plymouth, Minnesota, USA
Re: Easy Adjust Sliding Counterweight new [Re: don clement]
      #5632290 - 01/19/13 10:01 PM

Don, I can see where something like the mechanism you suggest would be helpful if I were using an aluminum tube for the counterweight shaft.

I chose 5/16" stainless rod because I guessed that this would be the minimum size that would be stiff enough. If vibration turns out to be a problem, the first thing I would try would be a larger diameter stainless shaft or possibly a stainless tube.

I think an aluminum tube or even a solid aluminum rod would not work well with my mechanism since it would be too easily damaged by the clamp. A different type of clamp that spreads the force more evenly, like you suggest, would be helpful though it seems more difficult to construct.


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don clement
Vendor (Clement Focuser)


Reged: 02/02/11

Loc: Running Springs, California
Re: Easy Adjust Sliding Counterweight new [Re: Dick Jacobson]
      #5632404 - 01/19/13 11:03 PM

Quote:

Don, I can see where something like the mechanism you suggest would be helpful if I were using an aluminum tube for the counterweight shaft.

I chose 5/16" stainless rod because I guessed that this would be the minimum size that would be stiff enough. If vibration turns out to be a problem, the first thing I would try would be a larger diameter stainless shaft or possibly a stainless tube.

I think an aluminum tube or even a solid aluminum rod would not work well with my mechanism since it would be too easily damaged by the clamp. A different type of clamp that spreads the force more evenly, like you suggest, would be helpful though it seems more difficult to construct.





I agree, my method is more difficult to construct (one needs a lathe) but does not damage soft aluminum tubing. I was thinking of using three skewers in place of the SHCSs shown, the kind that hold my wheel on my mountain bike to apply the pressure. Bike skewers are a cam-lock quick disconnect that are readily commercially available and work pretty well.

Don


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don clement
Vendor (Clement Focuser)


Reged: 02/02/11

Loc: Running Springs, California
Re: Easy Adjust Sliding Counterweight new [Re: don clement]
      #5632414 - 01/19/13 11:12 PM

Another lever cam-lock device that works well on my Austrian made REI Traverse Powerlock aluminum trekking poles applies pressure directly.
See: http://www.rei.com/product/847904/rei-traverse-powerlock-cork-trekking-poles-...,-chili-powder?preferredSku=8479040001&cm_mmc=cse_froogle-_-pla-_-product-_-8479040001&mr:referralID=cfd6d0c4-62b6-11e2-8751-001b2166c2c0


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