Loading ...
Sorry, an error occurred while loading the content.
 

Re: tube flexure/idea/advices

Expand Messages
  • Don
    That really hertz. http://www.youtube.com/watch?v=j-zczJXSxnw
    Message 1 of 40 , Feb 1, 2011
      That really hertz. http://www.youtube.com/watch?v=j-zczJXSxnw

      --- In atm_free@yahoogroups.com, "art.bianconi" <artbianconi@...> wrote:
      >
      >
      > Don wrote: "IMO balsa would be stronger with better damping (lower Q)
      > than expandable Polyurethane foam."
      >
      > Hi Don!
      >
      > * 1. this is not a question of vibration so hystersis or damping is
      > not an issue. * 2. While end grain balsa is a common core material,
      > packing the tubes would not work without or without water. You could
      > simply pack the tubes with each sand and stiffen the tubes that way.But
      > that would weigh too much. * Balsa does not expand as much as foam
      > and in time will rot.
      >
      > A pre measured amount of Pour In Place is very effective in filling
      > closed voids. I've used it in stuctural applications often. It's viscous
      > but sufficiently fluid to run down the tubes with little effort. Within
      > ten seconds, the two-chemistry mix reacts and starts to generate gas in
      > closed cells which moves the medium evenly through out the system. Cap
      > the ends so it has no where to go and it will fill every void. It's
      > lighter than balsa.
      >
      > The only down side of Pour In Place is that it can't be hot wired. My
      > replacement cradle for the Schiefspiegler had to band sawed to shape
      > after being poured into a large card board box. Hot wiring is more fun
      > but what the Hell!
      >
      > Art
      >
      >
      >
      >
      >
      > >
      > > FYI: http://www.youtube.com/watch?v=p-GPFvsIRMM
      > >
      > > --- In atm_free@yahoogroups.com, "Don" donald_clement@ wrote:
      > > >
      > > >
      > > > Nature with millions of years of evolution has shown the way to
      > lightweight filled composite structures such as the beak of a Tucan.
      > http://tinyurl.com/4h233bx I was thinking that balsa wood would be a
      > good filler strong, fiberous, and lightweight. Dry balsa could be
      > inserted into the carbon composite shell then a small amount of water
      > vapor in the form of steam (I use a Shark SC650 steamer
      > http://tinyurl.com/4tv953n ) could cause the balsa to expand. The ends
      > of the struts would then be sealed. The British Mosquito is an excellent
      > example of composite using balsa.
      > http://www.historylearningsite.co.uk/mosquito.htm
      > > > IMO balsa would be stronger with better damping (lower Q) than
      > expandable Polyurethane foam.
      > > >
      > > > Don
      > > >
      > > > --- In atm_free@yahoogroups.com, Benoit Schillings
      > <benoit.schillings@> wrote:
      > > > >
      > > > > camera is about 2.5 pounds.
      > > > >
      > > > > Still trying to understand the tube filling solution. I always
      > expected the
      > > > > the diameter of a tube was the main stiffness factor, not the
      > thickness of
      > > > > the wall
      > > > >
      > > > > -- benoit
      > > > >
      > > > >
      > > > >
      > > > > On Mon, Jan 31, 2011 at 3:57 PM, art.bianconi <artbianconi@>
      > wrote:
      > > > >
      > > > > >
      > > > > >
      > > > > >
      > > > > > Glad you like them. That camera, is it at prime focus? How much
      > does
      > > > > > that weigh?
      > > > > >
      > > > > > Art
      > > > > >
      > > > > > --- In atm_free@yahoogroups.com <atm_free%40yahoogroups.com>,
      > Benoit
      > > > > > Schillings
      > > > > > <benoit.schillings@> wrote:
      > > > > > >
      > > > > > > these are both cool ideas Art.
      > > > > > >
      > > > > > > I just did a few test with the OTA horizontal by adding 20
      > pounds on
      > > > > > the top
      > > > > > > cage(!) and I can see the carbon tube having a bowing of about
      > 0.1
      > > > > > inch. so
      > > > > > > there it is, even carbon will flex ;-)
      > > > > > >
      > > > > > > If filling the tubes gains me a factor of 2, I would probably
      > be fine,
      > > > > > I
      > > > > > > will do a test on an old carboard tube first.
      > > > > > >
      > > > > > > I like the spreader idea, pretty easy to test also.
      > > > > > >
      > > > > > > -- benoit
      > > > > > >
      > > > > > > On Mon, Jan 31, 2011 at 6:42 AM, art.bianconi artbianconi@
      > wrote:
      > > > > > >
      > > > > > > >
      > > > > > > >
      > > > > > > > Hi Benoit!
      > > > > > > >
      > > > > > > > What is the weight of the upper cage?
      > > > > > > >
      > > > > > > > I agree with what some others have suggested: the problem is
      > not
      > > > > > with the
      > > > > > > > tubes so much as the geometry, Were it my scope, I'd rebuild
      > it as a
      > > > > > > > properly fixtured truss using six tubes in the classic
      > > > > > triangulation.
      > > > > > > >
      > > > > > > > You will need a larger base circle to prevent the tubes from
      > > > > > obstructing
      > > > > > > > the light path or you can increase the number of trusses
      > from three
      > > > > > to four
      > > > > > > > or more. Obviously, that complicates assembly but it
      > shortens the
      > > > > > chord and
      > > > > > > > avoids the chordal intrusion. You'll also be able to use
      > > > > > substantially
      > > > > > > > smaller diameter, and less costly pultrusions. I fixture my
      > tubes in
      > > > > > open
      > > > > > > > trusses using spherical rod ends. There are zero bending
      > loads, a
      > > > > > greater
      > > > > > > > tolerance for misalignment and a greater degree of
      > adjustment. And,
      > > > > > if you
      > > > > > > > use a left handed threaded rod end at one end and and a
      > ruight
      > > > > > handed thread
      > > > > > > > at the other, you can make adjustments without having to
      > remove the
      > > > > > poles.
      > > > > > > > Simple rotate each pole as needed and lock the jam nut.
      > > > > > > >
      > > > > > > > Assuming that you have properly identified the cause of the
      > problem,
      > > > > > there
      > > > > > > > are two possible methods for perhaps correcting it:
      > > > > > > >
      > > > > > > > - The one I'd try first is to fill the tubes completely with
      > > > > > "pour-In
      > > > > > > > place" foam. If you use a lightly higher quantity for a
      > given tube
      > > > > > volume,
      > > > > > > > the density of the foam will increase some with little if
      > any
      > > > > > noticeable
      > > > > > > > increase in weight. The stiffness of the tubes will increase
      > > > > > dramatically.
      > > > > > > > The use of core materials can be very advantageous. When
      > core
      > > > > > material is
      > > > > > > > used to double the thickness of a structure, the relative
      > stiffness
      > > > > > > > increases 7 times. The strength increases 3.5 times while
      > the weight
      > > > > > only
      > > > > > > > increases 1.03 times. When core material quadruples the
      > thickness,
      > > > > > the
      > > > > > > > relative stiffness increases an incredible 37 times, the
      > strength
      > > > > > increases
      > > > > > > > 9.2 times, but the weight only increases a mere 1.06 times.
      > Those
      > > > > > > > calculations are for planar layups but the same principle
      > applies to
      > > > > > cored
      > > > > > > > cylinders.
      > > > > > > > - The alternative is to use wired spreaders like the sport
      > tuna
      > > > > > > > fisherman use on their fishing poles for hauling heavy Blue
      > Fin
      > > > > > tuna. These
      > > > > > > > consist of a triangulated or quad post spreader half way up
      > the
      > > > > > pole. A
      > > > > > > > steel cable, equipped with a turn buckle, is attached to one
      > end of
      > > > > > the pole
      > > > > > > > stretched over the spreader and secured at the other end.
      > The wires
      > > > > > are then
      > > > > > > > tensioned and what you have is a dramatically strengthened
      > pole with
      > > > > > little
      > > > > > > > if any increase in weight. Using a riangulated system
      > suggets you
      > > > > > might be
      > > > > > > > able to place the wires outside the image field. You may not
      > have
      > > > > > exact 120
      > > > > > > > degree spacing but it will still work. String Truss scopes
      > use a
      > > > > > similar
      > > > > > > > principle so there's nothing new here. The thing to be
      > careful of
      > > > > > here is
      > > > > > > > that while composite structures are very strong in tension,
      > they are
      > > > > > not
      > > > > > > > quite as strong in conmpression so be careful how much
      > tension you
      > > > > > apply to
      > > > > > > > the wires.
      > > > > > > >
      > > > > > > > Frankly I am a little puzzled why you chose to use an open
      > pole
      > > > > > design in
      > > > > > > > the first place. Those are usually chosen for large aperture
      > scopes
      > > > > > that
      > > > > > > > must be disassembled and transported to darker skies.
      > > > > > > >
      > > > > > > > From the looks of the mount, that scope is not going
      > anywhere!
      > > > > > > >
      > > > > > > > Frankly, the suggestions I made are treatments of the
      > symptoms and
      > > > > > do not
      > > > > > > > address the underlying cause. As I suggested earlier: were
      > it my
      > > > > > scope, I'd
      > > > > > > > redo it as a six pole open truss or as a single tube scope.
      > > > > > > >
      > > > > > > > It's a lovely looking scope. It's obvious you have put a lot
      > of care
      > > > > > and
      > > > > > > > effort into it. Please don't stop now. Hope this helps some.
      > > > > > > >
      > > > > > > > Art
      > > > > > > >
      > > > > > > >
      > > > > > >
      > > > > >
      > > > > >
      > > > > >
      > > > >
      > > >
      > >
      >
    • Dave Smith
      Hey Art, The milled surface is unnecessary on a std Crayford. If you design in an adjustable tension mechanism against the shaft, it works great. I can load up
      Message 40 of 40 , Feb 27, 2011

        Hey Art,
        The milled surface is unnecessary on a std Crayford. If you design in an adjustable tension mechanism against the shaft, it works great. I can load up a 2x barlow and my heaviest ep on any of the 3 crayfords I've constructed without any slippage. I've added a dig camera to that for pics of the moon without issue.
        Just my personal experience.
        Dave


        Sent from Yahoo! Mail on Android



        From: art.bianconi <artbianconi@...>;
        To: <atm_free@yahoogroups.com>;
        Subject: [atm_free] Re: tube flexure/idea/advices
        Sent: Mon, Feb 28, 2011 1:17:56 AM

         


        Al, all the Crayfords I've seen have a flat milled into the draw tube
        to provide a surface for the axle. I suspect that the wall thickness
        selected for such an application is influenced more by the need to mill
        the flat and less by the strength needed.

        A lot depends on how much weight one expects the draw tube to carry
        without slipping.

        A 1.25' diameter, 20mm Erfle is a piece of cake compared to a digital
        camera.

        I'm going to try to find time to construct a second Crayford focuser.
        Again I'll use cored composite materials for all the body parts except
        the draw tube and Teflon guides.

        It will be limited to 1.25" EP's simply because the scope it will go on,
        a six inch f-4, is a cantelivered Ball & Socket open truss, needs a wide
        field EP but will not tolerate a heavy 2 inch focuser. Weight is the
        primary concern. Little else.

        Would you believe that the solid steel drive axle weighs more than the
        entire focuser?!

        The trick is to keep the weight under that of the Toilet Flush Valve
        Helical.

        Art

        --- In atm_free@yahoogroups.com, "trulyyours05" <algermaine@...> wrote:
        >
        > Exactly! A thin-walled tube has the very best strength to weight ratio
        against bending in all directions. The thinner the better, but then the
        walls need bracing to prevent buckling. They could also be braced by
        internal framing and bulkheads, like an aircraft fuselage.
        >
        > Al
        >
        > --- In atm_free@yahoogroups.com, "art.bianconi" artbianconi@ wrote:
        > >
        > >
        > > The foam core keeps the walls from buckling or deflecting.
        > >
        > > It's similar to a soda can of thin aluminum.
        > >
        > > When it's under pressure from within, you can stand on it. Pop the
        cap
        > > and you can crush it in your hand.
        > >
        > > Art
        > >
        > >
        > >
        > > --- In atm_free@yahoogroups.com, Benoit Schillings
        > > <benoit.schillings@> wrote:
        > > >
        > > > camera is about 2.5 pounds.
        > > >
        > > > Still trying to understand the tube filling solution. I always
        > > expected the
        > > > the diameter of a tube was the main stiffness factor, not the
        > > thickness of
        > > > the wall
        > > >
        > > > -- benoit
        > > >
        > > >
        > > >
        > > > On Mon, Jan 31, 2011 at 3:57 PM, art.bianconi artbianconi@ wrote:
        > > >
        > > > >
        > > > >
        > > > >
        > > > > Glad you like them. That camera, is it at prime focus? How much
        does
        > > > > that weigh?
        > > > >
        > > > > Art
        > > > >
        > > > > --- In atm_free@yahoogroups.com <atm_free%40yahoogroups.com>,
        Benoit
        > > > > Schillings
        > > > > benoit.schillings@ wrote:
        > > > > >
        > > > > > these are both cool ideas Art.
        > > > > >
        > > > > > I just did a few test with the OTA horizontal by adding 20
        pounds
        > > on
        > > > > the top
        > > > > > cage(!) and I can see the carbon tube having a bowing of about
        0.1
        > > > > inch. so
        > > > > > there it is, even carbon will flex ;-)
        > > > > >
        > > > > > If filling the tubes gains me a factor of 2, I would probably
        be
        > > fine,
        > > > > I
        > > > > > will do a test on an old carboard tube first.
        > > > > >
        > > > > > I like the spreader idea, pretty easy to test also.
        > > > > >
        > > > > > -- benoit
        > > > > >
        > > > > > On Mon, Jan 31, 2011 at 6:42 AM, art.bianconi artbianconi@
        wrote:
        > > > > >
        > > > > > >
        > > > > > >
        > > > > > > Hi Benoit!
        > > > > > >
        > > > > > > What is the weight of the upper cage?
        > > > > > >
        > > > > > > I agree with what some others have suggested: the problem is
        not
        > > > > with the
        > > > > > > tubes so much as the geometry, Were it my scope, I'd rebuild
        it
        > > as a
        > > > > > > properly fixtured truss using six tubes in the classic
        > > > > triangulation.
        > > > > > >
        > > > > > > You will need a larger base circle to prevent the tubes from
        > > > > obstructing
        > > > > > > the light path or you can increase the number of trusses
        from
        > > three
        > > > > to four
        > > > > > > or more. Obviously, that complicates assembly but it
        shortens
        > > the
        > > > > chord and
        > > > > > > avoids the chordal intrusion. You'll also be able to use
        > > > > substantially
        > > > > > > smaller diameter, and less costly pultrusions. I fixture my
        > > tubes in
        > > > > open
        > > > > > > trusses using spherical rod ends. There are zero bending
        loads,
        > > a
        > > > > greater
        > > > > > > tolerance for misalignment and a greater degree of
        adjustment.
        > > And,
        > > > > if you
        > > > > > > use a left handed threaded rod end at one end and and a
        ruight
        > > > > handed thread
        > > > > > > at the other, you can make adjustments without having to
        remove
        > > the
        > > > > poles.
        > > > > > > Simple rotate each pole as needed and lock the jam nut.
        > > > > > >
        > > > > > > Assuming that you have properly identified the cause of the
        > > problem,
        > > > > there
        > > > > > > are two possible methods for perhaps correcting it:
        > > > > > >
        > > > > > > - The one I'd try first is to fill the tubes completely with
        > > > > "pour-In
        > > > > > > place" foam. If you use a lightly higher quantity for a
        given
        > > tube
        > > > > volume,
        > > > > > > the density of the foam will increase some with little if
        any
        > > > > noticeable
        > > > > > > increase in weight. The stiffness of the tubes will increase
        > > > > dramatically.
        > > > > > > The use of core materials can be very advantageous. When
        core
        > > > > material is
        > > > > > > used to double the thickness of a structure, the relative
        > > stiffness
        > > > > > > increases 7 times. The strength increases 3.5 times while
        the
        > > weight
        > > > > only
        > > > > > > increases 1.03 times. When core material quadruples the
        > > thickness,
        > > > > the
        > > > > > > relative stiffness increases an incredible 37 times, the
        > > strength
        > > > > increases
        > > > > > > 9.2 times, but the weight only increases a mere 1.06 times.
        > > Those
        > > > > > > calculations are for planar layups but the same principle
        > > applies to
        > > > > cored
        > > > > > > cylinders.
        > > > > > > - The alternative is to use wired spreaders like the sport
        tuna
        > > > > > > fisherman use on their fishing poles for hauling heavy Blue
        Fin
        > > > > tuna. These
        > > > > > > consist of a triangulated or quad post spreader half way up
        the
        > > > > pole. A
        > > > > > > steel cable, equipped with a turn buckle, is attached to one
        end
        > > of
        > > > > the pole
        > > > > > > stretched over the spreader and secured at the other end.
        The
        > > wires
        > > > > are then
        > > > > > > tensioned and what you have is a dramatically strengthened
        pole
        > > with
        > > > > little
        > > > > > > if any increase in weight. Using a riangulated system
        suggets
        > > you
        > > > > might be
        > > > > > > able to place the wires outside the image field. You may not
        > > have
        > > > > exact 120
        > > > > > > degree spacing but it will still work. String Truss scopes
        use a
        > > > > similar
        > > > > > > principle so there's nothing new here. The thing to be
        careful
        > > of
        > > > > here is
        > > > > > > that while composite structures are very strong in tension,
        they
        > > are
        > > > > not
        > > > > > > quite as strong in conmpression so be careful how much
        tension
        > > you
        > > > > apply to
        > > > > > > the wires.
        > > > > > >
        > > > > > > Frankly I am a little puzzled why you chose to use an open
        pole
        > > > > design in
        > > > > > > the first place. Those are usually chosen for large aperture
        > > scopes
        > > > > that
        > > > > > > must be disassembled and transported to darker skies.
        > > > > > >
        > > > > > > From the looks of the mount, that scope is not going
        anywhere!
        > > > > > >
        > > > > > > Frankly, the suggestions I made are treatments of the
        symptoms
        > > and
        > > > > do not
        > > > > > > address the underlying cause. As I suggested earlier: were
        it my
        > > > > scope, I'd
        > > > > > > redo it as a six pole open truss or as a single tube scope.
        > > > > > >
        > > > > > > It's a lovely looking scope. It's obvious you have put a lot
        of
        > > care
        > > > > and
        > > > > > > effort into it. Please don't stop now. Hope this helps some.
        > > > > > >
        > > > > > > Art
        > > > > > >
        > > > > > >
        > > > > >
        > > > >
        > > > >
        > > > >
        > > >
        > >
        >


      Your message has been successfully submitted and would be delivered to recipients shortly.