Scissors Lift

"Doug" wrote in news:RtEQb.3413$ snipped-for-privacy@nnrp1.uunet.ca:

I suggest oil impregnated bronze, I also suggest you weld in a steel sheath first to press the bronze bushing in. It will not have enough support with just the rectangular tubing. You can purchase the bushings from McMaster-Carr cheaper than you can make them.

On Sat, 24 Jan 2004 19:55:10 -0500, "Doug" brought forth from the murky depths:

Why not go with ball or roller bearings vs. bushings?

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I thought roller bearings wouldn't be neccesary because these pivot points would be moving slowly and were only there to distribute the load evenly on the bolt so it doesn't wear grooves in it.

I would make the bushings out of what ever you have laying around, even steel, but put grease zerks on them so you can lube them up well. I would also go with a larger shaft for a pivot, maybe 1" CRS For more bearing area. Myself, I would weld a steel stug on the tbe where the pivot is to be, drill and ream a hole a couple of thousands over 1", grease it good, and never worry about it again! Realize this bushing and shaft are pretty much stationary, it will rotate very little in its life! We are not talking about a high RPM shaft! Greg

I agree, but bronze bushings are probably overkill in this application! Lubed steel to steel will run for ever in this case! Greg

Have to agree again completely with Roy on this. And the larger you can make them, the better for keeping the motion aligned, and less wear on everything of course. And in fact, the bigger, the less accuracy you'll need. A nice few chunks of say drill rod in 1" won't be a whole lot more than a standard nut and bolt at CTC either.

Whatcha gonna do with it anyway? For the sculptures?

Take care.

Brian Lawson, Bothwell, Ontario.

ps... stopped at the little general store downtown at 2 and K.B. Road on the way home for the first time yesterday. Cute place.

Hi Brian,

Thought I might hear from you on this, The table is for whatever welding fabricating etc I'm up to. It started when I bought some scrap from Trudell Salvage in Tilbury. It was a 48 x 51" piece of grating welded up as a workplatorm probablt in ront o a machine. With its frame it weighed about

200 pounds and I bought it for $40. Seemed like a good way to get a flat surface when welding stuff up plus it perfect for clamping stuff down.

The little store in Kent Bridge is run by Sue and she's got one o my plant trellis's in there or sale. Nice people nice store.

Doug

It really depend on how much weight you are planning to put on this table, and whether you are going to adjust it with the weight on it. I would probably use small pipe for the bushings. I think 1/4 inch pipe actually has a id just under 3/8 inch. So I would use that and run a 3/8 drill thru it after it is welded in place and then use 3/8 bolts. Put some grease on the bolts before inserting and remove bolt and regrease every ten or fifteen years.

Dan

On Sun, 25 Jan 2004 09:38:31 -0600, Roy J brought forth from the murky depths:

Ah, gotcha. I hadn't thought sintered bronze would handle higher loads but hadn't looked into it. After hurting my back (late 1985, but am back to 85% again), I've been out of the biz for too long.

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In general, they won't--ball bearings will almost always have a higher load rating. At least when comparing standard bearing bronze to the most common bearing series (62xx, for example) in the size being considered. The max allowable load on bronze bushings is 4000-5000 psi for SAE660 and

2000-3000 psi for Oilite.

Ned Simmons

Yes, but ball or roller bearings want to turn and continually refresh the point of contact. Bushings don't care, usually. And in this application, I doubt that the pivot point will ever see much over a 30 degree rotation. And for this use, a bearing will have a significant "wall thickness" relative to a thin-walled bushing. And it will take at least two bearings per pivot point to keep alignment. And the application here as a welding table will see slag and dirt at the bearing points for sure, an so will require some bearing protection, whereas the bushing will clear itself just with a squirt of lube.

I'm still voting for bushings.

Take care.

Brian Lawson, Bothwell, Ontario.

If we're voting, I wouldn't even bother with bronze bushings. As long as they're kept greased, a steel pin in a steel bushing would be fine for this application. For maximum load capacity at low speeds, a piece of Thomson shaft running in a drill bushing is hard to beat.

As far as using rolling bearings in oscillating service, it's not an ideal situation, but it's not as big a problem as you might think from reading the manufacturers literature. I've seen, and designed, cam operated machinery that runs 24-7 at 100+ ppm for years on end with ordinary ball bearings in the pivot points of the cam follower arms. In many cases only a few degrees of total motion.

Ned Simmons

I'd drill holes through the structural tubing, insert mechanical tubing of the appropriate size, and weld in place to form the bearing housings. Then I'd press in oil impregnated bronze bushings. Ream in place to size and concentricity at final assembly.

The tubing is going to distort when it is welded in place, and the bushing is going to distort when pressed in place, so you need to ream them in place to wind up with round on-size holes *that line up* with each other in the two facing pieces through which the shaft (bolt) will pass. A long hand reamer will be the most suitable tool to achieve that within the limits of what this application requires.

Note, if you wish to fit zerks so the joints can be lubricated, the bushing needs a hole drilled in it aligned with the zerk, and a groove turned around the outside of the bushing centered on that hole. That way, if the bushing ever turns in the housing, grease can still make its way from the zerk, around the groove, and into the hole in the bushing to provide lubrication. This is important because you want a *light* press fit for the bushing to avoid distorting it too much, and it is likely to turn a bit over a period of time in service.

Gary

Well I'll probably not have much weight on it but the table top probably weghs 150 pounds. O course I have to engineer this or more weight then I need now because you never know what will happen in the future. I started building one pair o legs already and temporarily used !/2 pipe as bushings with 5/8 bolts as the pins but there's way too much play for the finished product. I'm afraid it would bind. I will be adjusting it while its loaded and in the future it might be handy for loding heavy objects into a truck or who knows what the future will bring.

Damn it! Quit taking credit for my idea!! I suggested steel pin, steel bushing already!;-) Greg

Doug,

All the guys are right that recommend using greased steel bushings for this application. The pivot points on my Bobcat Skid Steer loader are all greased steel bushings and they see a lot more load and cycling than your table will. This would be the simplest most economical approach. Anything more is overkill.

No,no. I was voting for your idea, not taking credit for it.

Ned Simmons

Thanks everyone for all the great ideas. I'm sure I'll be posting more questions when I finish fabricating the legs.