Hoist Davit is Complete!

Careful, there, what you need to worry about (big time) is buckling on the main vertical component. Might want to weld on a piece of flat bar on edge to stiffen it or something. As is, it can suddenly fail and if you're under it you can get hurt, almost got my leg broke one time when a similar thing buckled on me ..

GWE

Thanks, Grant. I have been standing back and looking at this, and I EXPECT bowing in the middle. How much, of course is going to depend on the load. Since I don't anticipate loading it that much, I think I can live with the bowing. HOWEVER, knowing myself, I will continue to increase the load. Welding a flatbar strengthener along the edge is a great idea, it's just that if I do it, I won't be able to measure the ACTUAL deflection. But then, to accurately assess this, I would almost have to load until failure occurs, then backtrack. I believe I will put a 1/4" x 2" FB stiffener for about six feet, centered, and call it a day.

I don't want to do this again if it fails.

Steve

This is on the vertical member, mind you, NOT the horizontal one.

Machinery's Handbook has tables of max loading on columns. Worth checking, the library has one if you don't, don't need the latest.

GWE

Yep, that would be on the vertical. The horizontal is much shorter, and has a diagonal brace under it at 21" out. I have a 'Thomas Glover Pocket Ref, and will see if I can find out anything. BUT, it would be different, as it connects into the vertical, and the vertical bow is the critical part.

Again, as I say, I doubt the thing will ever have 100# on it, but I just like being triple sure when something can come down and do brain damage.

Steve

Your water will be 168 lbs plus container weight.

You might want consider adding a section of 1 X 2 channel to the vertical column (web side out), opposite side of the horizontal boom.

Or a jackstay with turnbuckle. If it flexes more than you like.

Pete Keillor

Steve You did not say what kind of lifting device you plan to use. If you plan to use a rope and one pulley at the top to lift 21 gallons of water, it will need to be good for over 350 lb. If you have water at 168 lb. on one side of the rope, you will need to put more than 168lb.+ 10% for pulley loss on the other side of the rope to get it to move. So you will have more than 352.8 lb. on everything from the pulley up. For 50# of groceries it will be 105 lb. Don

The whole thing will be rotating on pins to swing it in to the balcony. GWE made the suggestion of a flatbar stiffener, which I think I will do.

Steve

Sorry I forgot that important fact. I will be using a generic HF 450# capacity electric hoist that can be used with a straight line, or with the line reeved through a traveling hook, and deadmanned back to the hoist body.

Steve

With 150 lb at 3 feet from CL, your moment is 450 in-lbf. That's equivalent to a 3 lb load on a cantilevered 150 inch beam, neglecting the weight of the beam itself -- which doesn't apply here with a vertical column. Too lazy to get out Machinery's Handbook, but that doesn't seem like a lot. Note that if the beam does deflect much then the moment increases so it's an unstable situation.

Could you translate that to a clueless newbie version?

My greatest concern is the top mounting plate. I made a saddle for a 3 ply

2x6 beam that is 18" long. The insertion hole for the pin will be 2" out from the end of the beam, giving the saddle 16" of contact with the beam. There will be nine 1/4" x 2" lag bolts. The beam it is attached to is only supported by an 8x8 post, and that post sits directly onto the top deck, not running through to the ground or anything solid.

The bottom pin will go into a receiver that I made out of 2x2 square tube, and welded 8 pieces of rebar onto to go into the concrete form I will make around the base. I think the base will be substantial, and far steadier than the top. The base should have no tendency to kick out, as it will go down subgrade. The top is the weak point.

Again, as I said, this is a convenience grocery hoist, and probably will lift no more than 50# at a time. Mostly in the 25# range.

But it sure is going to be spooky using it the first time with a test load.

Steve

Oops, brainfart, feet vs inches. Moment is 450 ft-lbf or 5400 in-lbf. Still doesn't seem too bad.

I'd place a tension member opposite the load, as perhaps a strip of 2" x 3/16" band from top to bottom with maybe a 3" or 4" bridge in the middle. Take a look at how engine hoists are built, e.g. the horizontal beam in Harbor Freight item number 35915

This won't reduce the moment, but it will concentrate it to a small region near the base. I'll do the moments of inertia trick later (after dark) but my sense is that 2 x 2 x 1/4 square tube should be able to handle this with max stress of well under 15 KSI in the steel. The tension band(s) won't change this unless it (they) have some spread at the base, but it will markedly reduce deflection in the length of the column. That's good because deflection will increase moment in your situation.

An even better solution would be to have a plate embedded in the footing to which the tension band(s) could attach. My biggest concern would then be with the footing itself. It had better go down a fair ways so it won't tip.

Steve,

Don't centre the bar on the 150" post. Start the reinforcing bar on the base plate and run it half way up the post. Weld securely to the base plate also.

Personally I'd use two bars and weld them to the back corners, at 45 degrees to the back side of the column.

Maximum unsupported length is 8.33 ft as per my CISC handbook.

Wolfgang

No, there is a pivot pin at top and bottom ends of the vertical so I can pick up a load, lift it up, and swing it in and land it on the upper porch. Same thing about taking things down off the porch. Pick it up, swing it out, let it down. If this were rigid, bracing and bowing would be a simple thing to fix.

Steve

I am now thoroughly confused. I envisioned a 150" high upside-down L, like a gallows. The stuff about pins at top and bottom, saddle, concrete base and 8x8 post has me baffled.

>

Steve,

Do you mean that your 2" x 2" post is restrained, from bending, also at the TOP??? Ie. the post has a swivel (slewing bearing) at the top AND bottom to permit slewing of the jib?

In which case you should have nothing to worry about.

Just to make sure we understand you correctly, can you post a sketch in the appropriate place?

Wolfgang

"Don Foreman" wrote

Follow me:

Three pieces: one 150" long one 48" long one 21" long, each end cut on the diagonal

Assembled as you say, a gallows

Two pins welded along the center axis top and bottom of the 150" piece

The "gallows" sill sit into a receiver hole in a metal receiver that is encased in in a concrete base.

The top pin will go into a short extension that comes off a beam end that sticks out past the 8x8 vertical roof support at the corner of the deck.

Because it is at the corner of the top deck, it can rotate about 270 degrees.

A hoist is placed at the end of the gallows arm.

When I get it up, I shall post it to the mailbox.

I'm just mounting a swinging gallows arm on two pins.

Steve

You pretty much got it. On the corner of my deck, there is an 8x8 support post. It runs from the deck up to the beam that supports the rafter ends. BUT, it runs about 18" past the vertical support column.

So, the only thing that is keeping the top beam stable is really the rafters.

When I finish, I will post photos to the mailbox, or if you like, e mail me at snipped-for-privacy@cox.net, and I can send you a picture.

Steve

Got it, thanks! The column is restrained from lateral movement by vertical pins at top and bottom.

Cranking the formulae from Machinery's Handbook thru MathCAD, using a

150 lb load hanging from a 3 foot horizontal beam at the top, it looks like the peak bending stress in the column is about 2000 PSI -- minimal for steel. Peak lateral deflection in the column looks like about 0.294 inches at a distance of about 92" down from the top. This isn't completely accurate, is a bit pessimistic, because I treated the load moment as a single lateral 900-lb load 6" down from the top pin to produce the moment. If I got me sums right, looks like you should be in good shape. I figure a 1/2" dia top pin will see shear stress of about 4000 PSI, again very modest for steel.