engineering calculation needed

SteveB · 2005-12-14T23:11:33+00:00

I got some 2" x 2" x .250" steel square receiver stock today. Sheesh. $4 per foot.Anyway, I am making a figure 4 davit. The long leg will be 15 long. The short horizontal will be 3 foot.It will mount at the top and the bottom. I am considering putting in a standoff about half way up to work against the flexing.The winch weighs about 25#, and I intend to lift no more than 200# with this 800# capacity winch.The horizontal will be one foot down from the top with a diagonal flatbar going from the top out to the end of the horizontal to help transfer the load to the top of the upright.How much flex can I expect? Do I need the standoff in the middle? I will probably put one on anyhow just to be sure.Thanks in advance.Steve

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Emmo 20 years ago

My 19 year old son is a freshman Mech. Eng. student at the Univ of Texas at Austin. He had his Statics exam on Thursday, and I took him out to dinner last night, where I presented him with this problem. We discussed it for the whole meal, drawing with crayons on the back of the kid's menu.

After looking at his results, we agreed that we would redesign. I like Bruce's I-beam idea, myself, but what we decided was that the post should be fixed, and the davit pieces should swivel around the post with a collar, hinge, or other means. We also brought the winch in, right next to the post, and made it pull up a platform with a wheeled carriage riding on the post, something like a dumbwaiter. We didn't see the need to hang the winch so far out from the post.

We also came up with the gate idea so that the platform only would come up to the level of the deck rather than trying to go over the railing - this would make it a lot easier to unload as well. Of course, my son immediately saw the possibilities of making this platform a people lift, which I told him had already been vetoed by the professionals here...

Anyway, it was interesting to him, as the previously mentioned ME undergrad...

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Bill Schwab 20 years ago

Emmo,

Great test question for statics - not the deflection part of course, that's the next course. Your son might enjoy checking out the 4th edition of Hibbler's Engineering Mechanics: Statics, example 6-15. The "triangle" is curved, points downward instead of upward, and the dimensions are different, but it is otherwise the same problem, pinned of course. It's a one pager, including a lot of explanation.

Good luck to your son!

Bill

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SteveB 20 years ago

"Bruce L. Bergman" wrote

When looking at the situation, this is what I came up with. The rafters are there making the porch covering. It is just that they are notched on their ends where they sit on the beam, and at the wall where they tie in. So, they are equivalent to 2x4s. The intended site is about the strongest thing around to hook on to. Plus, I didn't want an industrial looking I beam thingus hanging over my porch swing.

Had the thought of cutting the railing, and will do that, but will am considering replacing current railing with mortise and tenon rough cut railings. The current is all square cut lumber. I may even do a fancy Indital panel on that end of the porch complete with swinging section. I would certainly like to lessen the height that I lift anything to, and that would cut it down by about three feet.

Believe me, if that beam was supported better, I would have just hitched onto it, and not had to go to the ground. Did a few lifts of things with the old rope and pulley trick, but it was hard to get in over the railing with the lifting point located where it is.

Other thing is that OTHER PEOPLE will be operating this from time to time. Were it me only, I could come up with something a little simpler. I don't want anyone who lacks experience from doing a header off the deck rassling with a load.

I thank all for the continued input. I am being able to really consider this before final fab and install.

We will be going up there for New Year's Eve if there isn't too much snow, and we can get in there with 4wd and chains. So, I will have a final look, beings as we probably can't get in until March without snowshoes. I'm too old and lazy for that.

Steve

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SteveB 20 years ago

Not 100% sure the roof joists could handle it, and wife would never allow me to hang anything that ugly over the porch swing. Only reason I get this is because it is way down to the side and doesn't interfere with the view.

myself, but what we decided was that the post should be

Thought of that, too, but I think for the purposes of swinging load into the deck, an axis that goes through the center of the vertical is best. Anything else increases the leverage and flex. Right?

We also brought the winch in, right next to the

Now, you're getting complicated.

We didn't see the need to hang the winch

Beat you to it, just didn't post it in the original post. I will probably do wrought iron on that end, probably the Indital "vineyard" stuff with metal leaves and nice decorations to make it look like foliage. Then airbrush in details.

Of course, my son immediately

That may still happen. Of course, I would never admit it. However, I do weigh #195, and that would be a good test load. (I have ridden a headache ball down from the top of a drilling derrick, but that was when the safety man was in the doghouse.)

Thanks for bouncing it off a fresh brain.

Steve

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