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Richard Berry Refractor Tripod

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#1 Jeff Morgan

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Posted 25 July 2009 - 03:04 PM

A few years ago I built a scaled down version of this mount for a Jaegers 6" RFT. It was a very robust tripod utilizing 1" sections of oak. I got to thinking about how much weight it could bear. Instead of the alt-az cradle I would place a Schaefer AT-9 equatorial head on top to carry a 8" f/12 refractor. Total weight for head, counterweights, and tube would be about 180 pounds.

Has any other CN member built this mount, and if so what does it carry? How well? Would it compare to a portable pier?

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#2 mathteacher

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Posted 25 July 2009 - 03:09 PM

Can't answer your questions, but I really admire your handiwork!

#3 rlalum

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Posted 25 July 2009 - 04:14 PM

Have you checked in the mounts forum?

#4 Scott Beith

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Posted 25 July 2009 - 04:24 PM

http://www.cloudynig...php?item_id=895

#5 Jeff Morgan

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Posted 25 July 2009 - 10:22 PM

Actually I made the original post in the Refractors Forum, but the Powers That Be moved it here. Hopefully, a few of the refractor folks stop by here from time to time.

#6 Scott Beith

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Posted 25 July 2009 - 10:53 PM

There is a pointer in Refractors

#7 mish

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Posted 26 July 2009 - 12:15 AM

I got to thinking about how much weight it could bear. Instead of the alt-az cradle I would place a Schaefer AT-9 equatorial head on top to carry a 8" f/12 refractor. Total weight for head, counterweights, and tube would be about 180 pounds.


To determine how much load the tripod could safely carry, you would want to examine all the various failure modes, and compute the stresses associated with those mechanisms. That would provide for a strength-based criterion for a safe load.

Hardwoods such as oak are pretty strong in this arena, and although their strengths are not tabulated as are softwoods (i.e., structural softwood lumber is regularly tabulated and tested in the AWPA's National Design Standard, and older versions of these tables can be found online), using a very strong softwood's strengths characteristics provides a good estimate for a clear (i.e., knot-free) grade of a strong hardwood such as oak or ash.

Typical allowable stresses for such species would be a few thousand psi, and hence if a tripod is well-detailed (more on that below), adequate strength to support a few hundred pounds of load is possible. But care must be taken to check all the various modes of failure, e.g., the tripod shown could fail in bearing at the bolts that attach the legs to the pier. In this region, the area resisting compression is reduced because of the bolt holes, and hence the net area (i.e., cross-sectional area minus the projected area of the bolt hole) must be used to compute allowable force from the usual formula of force = strength times area.

Determining the various failure modes requires a fair amount of judgment and experience. In the picture provided, I can see potential for a bearing failure at the bolt-leg junction, at the connectors that hold the platform to the base (that would likely be a compression failure perpendicular to the grain, and that failure mode involves a much lower strength for wood members), a gross overturning problem because the center of rigidity of the tripod may not coincide vertically with the center of mass of the OTA, etc., etc, etc.

Nature will cause failure to occur at the lowest load level, so we need to check them all, and that's why structural engineers used to working with timber are handy to have around for such considerations. Since the tripod will hold an expensive optical system, it's important to do the design right so that surprises don't occur out in the dark under a starlit sky.

But strength is commonly not the limiting criteria in the utility of a wood tripod... serviceability is often more important (i.e., excessive motion, etc.). It's very hard to create moment-resistant connections in wood construction, so connections are often not very rigid, and that can introduce a lot of subtle motions that compromise the use of high powers in a telescope.

In the picture shown, the tripod pier/leg connection is a likely source of such a serviceability problem, in that unless the bolts are very tight (and the tripod leg components are very square), some torsional motions can be expected to occur at this connection detail. For a photographer, those kinds of motions can generally be accommodated, but for an astrophotographer, they may be the weak link in the optical/mechanical system.

I've built similar tripods (typically using ash, as it's strong like oak, but less likely to splinter and easier to seal because of the lack of oak's open grain structure), and I've tended to incorporate steel members into the design to keep the strength up and the unwanted motions down.

Steel and aluminum may lack the esthetic appeal of wood, but each has a much better strength-to-weight ratio, and with either metal, it's easy to fabricate structural members so that both strength and serviceability criteria are readily satisfied. Don't get me wrong here -- I love wood (I do a lot of finish carpentry for friends, and teach our university's courses in timber structural design), but "the right material for the job" is the operative principle here.

Hope this helps...

And by the way, that's a beautiful piece of timber structure you got there. Nice work!

#8 Jeff Morgan

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Posted 28 July 2009 - 12:47 AM

Mish,

Thank you for your insightful post!

Typical allowable stresses for such species would be a few thousand psi, and hence if a tripod is well-detailed (more on that below), adequate strength to support a few hundred pounds of load is possible. But care must be taken to check all the various modes of failure, e.g., the tripod shown could fail in bearing at the bolts that attach the legs to the pier. In this region, the area resisting compression is reduced because of the bolt holes, and hence the net area (i.e., cross-sectional area minus the projected area of the bolt hole) must be used to compute allowable force from the usual formula of force = strength times area.

Determining the various failure modes requires a fair amount of judgment and experience. In the picture provided, I can see potential for a bearing failure at the bolt-leg junction, at the connectors that hold the platform to the base (that would likely be a compression failure perpendicular to the grain, and that failure mode involves a much lower strength for wood members), a gross overturning problem because the center of rigidity of the tripod may not coincide vertically with the center of mass of the OTA, etc., etc, etc.


In hindsight, all this makes sense. Since I was using a 6" f/5 refractor, I never pushed it hard - the Jaegers lens really was woeful above about 63x - but this was probably above the intent of the designers. The Berry plans call for a 1/4" carriage bolt at the hub/leg joint, with just a plain 5-1/2" hole drilled through the hub.

I certainly don't have the expertise to math this one out. I wonder if one were to reinforce the hub holes with flange bearings and/or sleeve bearings? Or increase the diameter of the carriage bolts? Given the depth of the required hole, the limiting factor would be the availability of extra-length drill bits.

On the spreader, again I would try re-enforcing the wood-wood joints with bronze bearings. Here the wood thickness is not that great, so it would be easy to increase bolt/bearing sizes.

I like your comments on Ash. I need more Baltic Birch this week, so I'll inquire. Any other species come to mind?

#9 mish

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Posted 28 July 2009 - 10:52 AM

I wonder if one were to reinforce the hub holes with flange bearings and/or sleeve bearings? Or increase the diameter of the carriage bolts? Given the depth of the required hole, the limiting factor would be the availability of extra-length drill bits.

On the spreader, again I would try re-enforcing the wood-wood joints with bronze bearings. Here the wood thickness is not that great, so it would be easy to increase bolt/bearing sizes.


Motions occurring from flexibility at the top of the tripod legs are probably due to compression forces perpendicular to the grain of the wood (as that grain runs along the length of each leg). Wood is very flexible in this mode of loading, because the individual fibers that make up the wood are easily crushed in a direction perpendicular to their length. So it's hard to detail such a wood connection to make it stiff enough so that mount motions don't compromise high-magnification views.

I would second your suggestion of adding metal to the mix, anywhere where cross-grain loadings occur. Substantial thrust washers at each connection in the bolt assemblies would lower the compressive stresses in the wood at the top of each leg to where you might be able to avoid inelastic deformations that would cause unwanted play in those connections. Since this detail is easy to implement, it's certainly worth a try.

As far as bigger bolts are concerned, steel is so strong compared to wood that increasing the cross-sectional area of the bolts is seldom required. And since increasing the bolt hole diameter decreases the stress in the steel while increasing the stress in the wood, that's probably not an optimal tradeoff. But the inclusion of sleeve bearings could be an excellent design choice, since one can specify these so that the bolt fits snugly into a metal cylinder, and that tight fit could help stiffen up the top of the tripod simply and permanently.

I would think that detailing the spreader assembly so that it forms a part of a frame would help make the tripod more stable. This could be handled with something as simple as having two connections (and hence the ability to develop a lever arm between them) to each tripod leg instead of just one. There's plenty of width in those beefy tripod legs in the picture, and although that kind of detail could make assembly efforts a bit more complicated, it would like result in a much less compliant tripod assembly (and a nice triangular platform for storing eyepieces, etc.)

In other words, if you could detail the pier/leg connections and the spreader assembly so that you've constructed a rigid tetrahedral shape when it is fully assembled, then you'll have a rock-solid mount that is light, relatively easy to fabricate, and pleasing to the eye as well.

As far as choice of timber is concerned, the harder the wood the better if a rigid mount is desired. Oak is a good choice, and it's widely available. Birch is a great wood for general finish work, but it is generally softer than oak, so it might require more detailing to keep unwanted play minimized in your design.

Hope this helps...

#10 Jeff Morgan

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Posted 28 July 2009 - 02:25 PM

Hope this helps...


Greatly!






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