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Equipment Discussions >> ATM, Optics and DIY Forum

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TxStars
Pooh-Bah
*****

Reged: 10/01/05

Loc: Lost In Space
Re: car valve springs in cell : ) new [Re: MessiToM]
      #5468755 - 10/13/12 06:45 PM

I see no problem with some grade 8 bolts & all metal parts to prevent wear.

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Jeff Porter
super member
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Reged: 09/03/10

Loc: Utah
Re: car valve springs in cell : ) new [Re: TxStars]
      #5469472 - 10/14/12 09:41 AM

I like it. You could then add an old Mercedes hood ornament as a finder.

-Jeff p


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MessiToM
scholastic sledgehammer
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Reged: 12/21/09

Loc: Huntingdon PA
Re: car valve springs in cell : ) new [Re: Jeff Porter]
      #5469804 - 10/14/12 01:25 PM

lol

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bremms
Carpal Tunnel
*****

Reged: 08/31/12

Loc: SC
Re: car valve springs in cell : ) new [Re: kfrederick]
      #5469850 - 10/14/12 01:52 PM

I still have some inner valve springs from a Fiat X1/9 they would work well with a fine thread bolt. It wouldn't take that much to compress them if you oil the threads and use a nice knob.

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careysub
Carpal Tunnel
*****

Reged: 02/18/11

Loc: Rancho Cucamonga, CA
Re: car valve springs in cell : ) new [Re: JasonBurry]
      #5470425 - 10/14/12 07:58 PM

Quote:

A friend has Fiat valve springs on his 13.5" cell. Works well, but stiff to adjust.

I'd rather have too strong a spring installed than too weak.

J




Hmm... how strong does a spring to be so that is not too weak?

Lets say a mirror+cell weighs 30 lb and is supported by 3 springs. Pointed at the zenith, 10 lb of weight compresses each spring, then we want the collimation bolts to pull the spring down further, to create enough force to hold it securely against the bolt. I'm guessing that another 10 lb of force, the equivalent of 1 G holding it against the bolt would be more than adequate. If you treat that as the top of your collimation adjustment range, you will want to pull it down further, a quarter-inch maybe (and have a total 1/2 inch adjustment range)?

So a spring that has 1/2 inch of travel left with a 20 lb load would be about right, wouldn't it? Say a rate of 20 lb/inch and 1.5 inches of working length, or a 40 lb/in rate and a 1 inch working length.

Why would a spring need to be stiffer than that (adjusting for the mirror weight)?


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bremms
Carpal Tunnel
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Reged: 08/31/12

Loc: SC
Re: car valve springs in cell : ) new [Re: careysub]
      #5470444 - 10/14/12 08:08 PM

The cell shifting is the problem. I use push pull screws on the periphery for my cells. I would want at least 10 times the preload on the springs compared to the weight on the spring. springs are just not a positive way to locate a mirror. I have one PP cell that has locating goves for the push of the push pull screw. The unit came from an industrial laser.

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careysub
Carpal Tunnel
*****

Reged: 02/18/11

Loc: Rancho Cucamonga, CA
Re: car valve springs in cell : ) new [Re: bremms]
      #5471168 - 10/15/12 09:50 AM

Quote:

The cell shifting is the problem. I use push pull screws on the periphery for my cells. I would want at least 10 times the preload on the springs compared to the weight on the spring. springs are just not a positive way to locate a mirror. I have one PP cell that has locating goves for the push of the push pull screw. The unit came from an industrial laser.




That explains some of the comments I have seen over time that has puzzled me about collimation springs. That is - some cell designs are using the springs to hold the cell in position by gross pressure when the scope is tilted, in addition to their role in holding the cell against the collimation bolt stop.

If the cell is kept in position by having it "travel" vertically in the mirror box by some sort of guide rail, then the demand put on the spring is reduced to simply holding the mirror cell against the collimation bolt stop.

For example a spring mounted on a guide rod with the cell sliding up and down on this for collimation through a sleeve bearing, or simply a hole drilled in the cell, with the collimation bolt a separate component (a linear bearing would be over-engineering I think).

Other possibilities are slots cut in the cell which ride on a flat rails created by aluminum angle pieces, or a "Dobsonian bearing" approach where there are teflon bearing surfaces on the inside front and back of the mirror box with the cell having sledge extensions that slide on these perhaps similar to a simple vertical drawer slide.

These all fit under the umbrella of "kinematic design" (which mechanical engineer Benach on this forum turned me on to), where the position of the mechanism is completely deterministic. That is, the degrees of freedom are locked down, so that it can be in only one state at any one time, and is also not over- constrained creating an "elastic design" that warps in unpredictable ways. Although jiggling the mirror box can make the cell bounce against the collimation stop it would immediately settle into exactly the same position without affecting collimation or allowing the mirror to shift (it would take pretty severe jiggling to overcome a one-gee-plus load against the stop though - you wouldn't see this at all during an observing session).

Poster "Pinbout" on this thread commented about the problems of trying to combine multiple roles with one structural element - the Kriege and Berry design having the teeter bar support also being the collimation bolt, and adding spring loading on top of that. Separating these roles probably makes for more predictably behaving designs, a few more parts perhaps, but in some sense still a "simpler" design.


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Pinbout
Postmaster
*****

Reged: 02/22/10

Loc: nj
Re: car valve springs in cell : ) new [Re: careysub]
      #5471250 - 10/15/12 10:42 AM

Quote:

That explains some of the comments I have seen over time that has puzzled me about collimation springs. That is - some cell designs are using the springs to hold the cell in position by gross pressure when the scope is tilted, in addition to their role in holding the cell against the collimation bolt stop.






in my 8in stellafane dob, I have a common cell where I have springs on the bolts. I only have three support pads, not floating.

I have to seriously torque those bolts to compress the springs enough so that I won't get a cell shift. and it does start to inflict pain...

and I test it everytime running the tube thru alt axis from horizon to zenith and the laser collimation will not change.

if I don't compress the &(*$%^ out of the springs I will see the laser collimator light move as I rotate the scope.



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JasonBurry
sage


Reged: 04/27/12

Loc: Cape Spencer, NB, Canada
Re: car valve springs in cell : ) new [Re: Pinbout]
      #5471283 - 10/15/12 10:59 AM

Agreed, Danny. My 8" cell is similar to yours, basically a plate with 3 collimation bolts, and 3 fixed support pads. The springs surround the col. bolts and the spring/bolt tension is all that holds the cell in position. My springs need about 20# of preload before col. stays stable.

J


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