engineering calculation needed

Bill,

Thank you for that thoughtful essay on the basis for structural mechanics. It was well written and I agree with the principles you outline. Although my structural mechanics may be a little rusty through lack of use, I did study it to second year university level, and other branches of mechanics to a higher level. I do consider myself to have a good understanding of Newton's laws, free body diagrams, etc. In your

500+ words you have still not explained precisely why my free body diagram is wrong. If it's wrong, I want to know why.

Chris

Reply to
Christopher Tidy
Loading thread data ...

Chris,

You left off the horizontal component of the tension in the "tension member". I've said that at least three times now. Given the idealized structure you created, it is simply where Newton takes us.

If you genuinely do not understand about the missing component, and are close to a university, you might seek a tutor to help you with some problems from a statics text. Find one for which you deviate from the solution so you will have questions from the beginning. Many graduate students are eager to pick up an hour's fee here or there, and won't care that you are not enrolled as a student.

Bill

Reply to
Bill Schwab

Whether or not this force is included depends on how you model the structure. If you look at my diagram of the idealised structure, the tension member, column and roller support are all connected by the same pin. In this case the roller support provides a reaction (transmitted through the pin) against the horizontal component of the tension, and this force is not seen by the column. The free body diagram is consistent with my model. You could draw a subtly different idealised structure in which the tension member is connected to the column, and the column to the roller support. In this case the horizontal component of the tension should be shown on the FBD. I chose to take the first approach as it simplifies the model and I do not believe it will have much effect on the answer.

Chris

Reply to
Christopher Tidy

Chris,

I don't know how else to say it. That is not correct. The tension member and the support both interact with the beam, and both must be represented by external forces when removed. We can go no further if you insist otherwise.

Work some sample problems in just about any statics book, and I promise you will soon encounter problems where you will have problems matching the answers, or you will suddenly realize what I am trying to tell you.

Bill

Reply to
Bill Schwab

Just one more question.

Well, maybe two.

When I DO get this thing running, I want to leave off the center standoff until I make one 200# lift, where I will measure the center deflection at midpoint. Just for my own curiosity, and I say it will be less than one inch. But I will surely do a slow lift, and measure it so that if it does deflect a LOT, it won't fail with the load too high.

If that goes good, I will install the center standoff, knowing about how much deflection it is stopping.

What other areas should I be concerned about watching/monitoring/testing?

I know the obvious, obviously, but anything that would give you guys answers to theories you are forming about this device?

Steve

Reply to
SteveB

That depends on whether the tension member is connected directly to the column. In my idealisation it isn't. The tension member is connected to the pin. The pin is also connected to the support and the column. It seems like you are disagreeing with my idealisation, which is fine, but the FBD is consistent with the idealisation. Perhaps someone else can offer an opinion on this?

Chris

Reply to
Christopher Tidy

I would want to give the upper support (where the davit is attached to the building) some careful thought. If the davit were to come loose from the building while lifting it could be quite impressive. If you plan to use fasteners such as coach screws or expanding bolts, see if you can find data giving the safe load for the fasteners. Choose them so that you have a good margin of safety. If you can't find data, build a quick prototype of the support from scrap materials and try to break it.

Chris

Reply to
Christopher Tidy

Chris,

Direct or indirect, it does not matter. If it is under tension, it is connected, and must be included.

You might try summing the forces you believe are acting on the beam in the horizontal direction. If you use the correct support reactions, you will be in for a surprise. The expected result is of course zero.

Bill

Reply to
Bill Schwab

"Christopher Tidy" wrote

I will have a saddle made out of plate. It will start with a piece 30" x

17". Rectangles will be cut from two corners so that when it is bent, it forms a channel saddle 5" wide for 30" long with two 6" legs that are 18" long. This will be bolted through three 2x10s on edge that make up the beam. The bottom 5" leg will stick out about 12" leaving about 18" of saddling to marry to the beam.

I am now considering making a truss to run from the beam back to the cabin so that when the load is swung, and the force is applied sideways to the beam, there is a stiffener there. The roof joists sit on that beam, but I think I would like another stiffener there at right angles that will have no compression at all. It can be made of decorative iron, probably Indital vineyard stock, which is solid, and 3/4" in diameter for the main components.

My wife thinks this is way too complicated, but I'm having fun, and when I do lift, I don't want any questions about the thing holding up.

Steve

Reply to
SteveB

It is neither direct or indirect. If you can visualise the type of ideal pin joint I'm talking about, the horizontal component of the tension is not applied to the column at all. It is transmitted around the column through the joint.

Chris

Reply to
Christopher Tidy

It sounds like you have thought this out pretty well. Build the saddle strong and everything will be fine. My suggestion was just a caution against using two tiny screws and rawlplugs, really. Common sense I know. I don't know if you'll need a truss for stiffening. Lean on the beams and see if you can flex them much!

Chris

Reply to
Christopher Tidy

Chris,

That is not at all what you drew. If you intend to transfer only a vertical force to the top of the beam, you are going to need a slot or horizontal roller on top of the beam, with the likely outcome being a statically indeterminate problem or something that leads to a contradiction because it is not adequately supported.

Bill

Reply to
Bill Schwab

Imagine the front forks of a bicycle. Now imagine two sets of forks, one slightly smaller than the other so that it fits snugly inside. Now make the column slightly narrower than the smaller set of forks and drill a hole through it. Attach one set of forks to the tension member, the other set to the roller support I described, then pass a pin through both pairs of forks and the column. This joint would behave as I explained, and does not in itself make the structure statically indeterminate. Instead it makes it easier to analyse.

Chris

Reply to
Christopher Tidy

Chris,

By its very complexity, what are describing is far more trouble than a conventional pinned frame, which BTW is not difficult to analyze. In fact, it can probably be done in less time than it would take to correctly draw your proposed system.

In the words of Professor Hoover, "take it to pieces". If I understand you correctly, the beam is not horizonally supported at the top, and is therefore not in equilibrium. Add another horizontal support to fix that, and it is no longer statically determinate.

Bill

Reply to
Bill Schwab

It is a conventional pinned frame. Furthermore the idealisation I proposed simplifies analysis.

I believe it was a correctly drawn idealisation. The two members and support all connect to the pin, not to each other.

It is connected to the roller support via the pin. Please see the diagram.

Goodness me! You're making me feel like I'm in court here. You make it your business to automatically disagree with everything I say. Fortunately I did think through what I did here, and can support all bar one of my decisions, although the odds are you'll disagree with that statement, too.

I really will give this a break now.

Chris

Reply to
Christopher Tidy

You mean C:\docs\davit_calc2.jpg? If so, isn't that the pin you said doesn't act on the beam? If that's the case, then we're back to the missing horizontal force.

Otherwise, you should draw the diagrams, write the equations, and either count the unknowns and equations, or check the moment balance on the beam.

Reply to
Bill Schwab

The pin does not transmit the horizontal component of the tension to the top end of the column, if you accept the idealised joint I proposed. This does not mean that the pin is unable to transmit any horizontal force to the column. It does provide a horizontal reaction which opposes the compression in the horizontal beam. This is a subtlety of the idealised joint, which you may like or dislike, but the FBD is consistent with the idealisation.

I call it quits.

Chris

Reply to
Christopher Tidy

Chris,

What you describe is almost certainly either unsupported, or statically indeterminate. Even if you have designed something that is determinate, it is by no means a simplification relative to the what was presented to you in your mechanics classes. If you think the pinned frame is complicated, you didn't learn the material.

Reply to
Bill Schwab

Either help the OP or give facts, a diagram, or a formula that presents your worries and concerns. Continually repeating that someone is wrong solves nothing, helps nothing, changes nothing.

Perhaps you did not notice that OP intends to try his original concept - win, lose, or draw. He will be watching for empirical evidence that he is underdesigned.

I would point out that I just loaded on my trailer two pieces of

2x2x1/4 wall twenty foot sticks that had been unloaded with a single choker at mid point. They both have a permanent 2-3" crown in them. Too good to toss, too bent to use without modification/adaptation. Could not be used on this job for their original intent.

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Keep the whole world singing . . . . DanG (remove the sevens) snipped-for-privacy@7cox.net

Reply to
DanG

Perhaps. If you have an engineering background, please review Chris'

12-17-05 12:39 pm post; he says everything is pinned. In that case the horizontal component of tension is indeed transmitted to the beam via the pin, and must be included in the diagram and equations, as I explained early on in this process.

Then go to 12-17-05 2:17 pm and note that he now says the force goes around the column/beam. The problem with the latter becomes clear when you look at an FBD of the beam; external moments to do balance. The horizontal member pushes the beam to the right; per Chris' design, the only other horizontal force is at the base, and it clearly needs to push to the left or the whole structure goes flying. Note that the beam is left wanting to rotate clockwise, which is a contradiction unless there is some other horizontal force at the top of the beam [*]. We are looking at two rigid bodies and the pin supporting the load, which gives seven equilibrium equations. Add that extra support and the system becomes underdetermined.

[*] the support reactions change, but hopefully you get the idea.

Even if there is a way to build a gizmo that can do all of this and be statically determinate, it will be far more complex than the pinned frame, and if built, would be more likely to fail than the frame.

I did notice; that's not the point. What if I were telling people hold endmills in a tapered chuck and than crank up the feed rate?

I'm sure he will.

Bill

Reply to
Bill Schwab

PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.