5-ton ajustable work table leg design and alignment 1

[GPCNC]

Hi guys,
I'm currently working on an ajustable welding platform that is 100" long, 80" wide, that needs to adjust in height from 12" to 72" above ground, and must be able to support a load of 10,000 lbs. This platform is used to weld dump truck bodies and the welders need to be able to work directly underneath it. The table must be pretty dead level throughout its height range. My first goal is for the lift table to be safe, and my second goal is for the lift to have the less osbtructive structure possible.

The first viable solution I have found is a scissor lift, although I would not be powering it the usual way. I would use two mechanical screw jacks (each 5t), one located at both ends of the table, centered between the scissors, mechanically synched together and driven by a single motor. The jack housings would be hidden below grade in a concrete pit. I have come up with this solution to avoid the huge hydraulic cylinders & hydraulic power unit that takes floor space, and especially to have the jacks working in direct vertical with the load. In short, the scissor mechanism would take care of side loads and moments applied to the platform and the two jacks would take care of the vertical load. More importantly, the jacks are self locking and can be equipped with a redundant nut that serves as fail safe unit according to EN1570-1:2014 Safety Requirements for Lifting Tables, making the design safe with the proper interlocks. My only problem with this solution is that the scissors make it difficult to walk underneath the table. See the attached .dwg.

My ideal setup would be a four-legged table driven by four screw jacks. The only problem I'm having is finding an adequate design for the four telescopic legs. I thought at first to go with four legs sliding in four leg supports (square tube inside another square tube, like a crane telescopic boom), and to have the jacks attached as close to the legs as possible but I'm afraid that this setup will bind, being a bit sloppy. I have also thought about linear slides but I can only imagine the difficulties of installing four vertical linear slides in perfect parallelism in both cases.

Anyhow, I'm wondering if I'm not just thinking too much about this 4-legged table. Do you think it wouldn't be that bad to install and align four telescopic legs, being a square tube inside a square tube or a tighter linear slide? Does anybody have any experience with a similar machine?

Thanks!
GPC

 

[Nescius]

I'd buy an existing commercial product already designed and engineered to lift a large, heavy object to facilitate work underneath...an automotive hoist. Go to the local muffler or tire shop and observe.

 

[GPCNC]

Nescius,
I've taken a look at many commercial garage lifts, 2-post, 4-post, scissor lift, etc., however, one thing I forgot to mention is that we assemble the dump truck bodies upside-down on the table, so the table has to fit inside the truck body, hence the 80" width (smaller than our narrowest dump) and 100" length (smaller than our 9 footer). I haven't found anything suitable on the commercial side.

 

[Nescius]

I'm picturing you assembling the bottom of the dump bed on the table, then raising it up to install or build the sides of the bed. Is this close?

 

[GPCNC]

We have two different kinds of body, one that is assembled upright with the welding table set at 12-24" height, and another kind that is assembled upside-down at a height of ±72". See CAD drawing in attachment.

Thanks for the help Nescius.

 

[MikeHalloran]

Unistrut used to make corner-welded square tubing called TeleSpar that was intended to slide, one size inside the next, with or without punched holes every inch. Very nice for building adjustable structures.



Mike Halloran
Pembroke Pines, FL, USA

 

[Nescius]

So, the table never moves while loaded? Then, perhaps an "adjustable structure", as Mike Halloran terms it, would work just as well as a "powered lift"?

That may depend on how you remove the assembled dump bed from the table. Overhead crane, or must it be lowered down by the welding table itself?

 

[GPCNC]

Sorry guys, I'm not providing enough detail here . The table does need to move during assembly to provide a nice ergonomic position for the welders. For the upright bodies, they will work at 12-24" most of the time and then raise the table up to 48-60" to work under the body. For the upside-down dumps, they will be working outside the body at around 48-60" height depending on the exact body type and then raise the table up to 72" so that they can crawl underneath and tack weld on the inside. It's something they would do several times a day so we need to adjust the height while the body is installed on the table. Once welding is done, we pick up the body using the shop crane.

Fast, easy height adjustment is something that the welders are looking for.

 

[SwinnyGG]

I would simply use your 4-screw-jack arrangement, with legs of steel tubing of an appropriate size. You could use UHMW lining, fastened to the outer or inner tube, to lessen the friction between the leg components, but remember that if your actual live load is 10,000 lb, you're going to need at least double that in rated capacity, which means 4x 5-ton jacks. Whatever binding there is between telescoping members will be insignificant compared to the force of the mechanism being raised or lowered.

 

[BUGGAR]

Can you just make a large frame to support a lifting apparatus and do it all from above?

 

[desertfox]

Hi GPCNC
any chance you can reload the attachments as pdf's?

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[dhengr]

GPCNC:
I second Desertfox’s request for PDF’s of your sketches/drawings. In particular, I would like to see some drawings of a couple of the different dump box bodies you are dealing with. There are several different ways to skin this cat, and there would probably be some advantage to keeping the welders below 4' off the floor, as much as possible. OSHA gets all excited about fall protection over about 4', and this complicates almost everything.

If I could, I would sub-assemble the dump box floors, sides, fronts and dump gates on heavy flat tables so I could fixture the various parts and gag parts down during the this initial assembly. These tables can be at floor level and most of the welding will be downhand, or it’s just a matter of flipping the sub-assemblies. Then I would do the final assembly of the floor structure, sides and front in a rotating fixture. This fixture clamps the sub-assemblies together and squares them up. Its two end halves clamp to the two ends of the dump box, and one end is powered, while the other just rotates and follows. Looking at an end view of the dump box, its C.G. should be about centered btwn. the sides and about 12-18" above the floor plate, and this is approx. the longitudinal axis about which you want to rotate the dump box for final assembly and welding. The two end halves of this rotating fixture can also raise and lower the entire load in unison for rotating and positioning for welding. Then several sets of scaffolding at this position allow almost all of the final welding to be done downhand too. Just lifting the dump box to several different heights only solves half the welding problems, in my mind. You must be doing a fair amount of welding in the vertical and overhead positions. And, this is not a particularly good situation for this type of equipment.

 

[GPCNC]

Hi guys, thank you so much for the help. Here is a PDF showing both body types assembly setups.

On the bottom portion of the drawing is how we assemble the upside-down bodies. The floor, sidewalls, tailgate and head board are all preassembled upstream in the shop, and this welding table is for fitting-up these parts together to be fully welded at the next weld station. The welders currently first lay the floor down on the jig (which provides a dead level support for the floor - jig is fixed as of now), and then they start adding the sidewalls, headboard, tailgate, and other smaller stuff. The upside-down, flat and level floor is very important from the start as this garantees that the whole dump will be square. During assembly, they need the body to be as low as possible to the floor, the limiting factor being the headboard which is usually 60" above the dump floor, though they do would like to raise the body to weld the lower parts. Once the dump box is assembled, the welders need to raise the body by about 22" to crawl underneath it and finish tackwelding inside. Once done, they can pick the body up with the shop crane, send it to the next welding station, and start anew.

On the top portion of the drawing is how we assemble the upright bodies. These bodies cannot are assembled with the floor upside-down as it is not flat. The body frame (34" wide) is first put on the welding table, and the other parts (sidewalls, headboard, etc) are added in. In the lowest position, our guys have access to most parts and only require a 30" movable step platform to reach the top of the body for welding gate hinges. At some point during assembly, we would like to be able to raise the table by from 12" to 48" to be able to weld the lower parts while standing instead of kneeling down.

Buggar - the cranes are super busy in our shop so we need to avoid using for body assembly. Sometimes the guys need to wait 10-15 minutes to get access.

Dhengr - as I said just above, we do pre-assemble the various parts on flat welding tables. We have thought about making a jig that clamps and locates everything together but since we have many different kinds and lengths of body (we change setups every 3-4 days almost), it's quite a challenge to design such a jig. But, by pre-assembling everything, there's not much welding required in height - the only welding required is welding the tailgate hinges and headboard, it's minimial I would say. By reaching the positions shown on the attached drawing, we have determined that our welders would work in ergonomic positions.

jgKRI - I like the idea of a square beam within a beam with plastic sliding surfaces - I think you are right about the binding force being insignificant compared to the actuator force. I guess it would be a matter of manufacturing square legs and really taking care of alignment during installation. I could have a laser guy to come in to check for alignment. I will try to sketch up a design.

 

[BUGGAR]

Thanks for the drawing. I'm on a car building forum where guys build something like this to build their frames. Most guys use a leveling mechanism to fine tune the frame before final welding. Most of these tables are built for rigidity and likely have the strength to lift your bodies. They use screw jacks in a mechanism that keeps them "out of the way".
I'll see if I can get you the info.

 

[kjoiner]

How about a sprialift. They are compact and very strong. I think they are typically used in theater stage applications but the web site shows some industrial applications.

http://pacospiralift.com/

Kyle

 

[Jboggs]

Or one of these two:

http://tsubakimoto.com/power-transmission/linear-actuator/zip-chain-actuator/zca/

http://www.serapid.com/en/industrial-equipment/industrial-lifts/industrial-lifting-systems

You could bury the horizontal section in a trough in the concrete floor to prevent the tripping hazard.

 

[GPCNC]

Hey kjoiner! Thanks for suggesting the Spiralift. Coincidentally, I called them last week to inquire about their lift system! I've known them for a while since I sent my CV to them when I got out of school back in 2008 and was living in Montreal. Anyhow, very cool products, but they are quite expensive, about 3x the price of an equivalent screw jack. Also, I would be worried about damaging the Spiralift because of metal dust, weld spatter etc.

Jboggs, looks like what you propose is a direct competitor to the Spiralift. Both companies seem to find a lot of business in the staging business. These must be pretty expensive as well.

I'm off work until Monday. I'll upload a few screenshots of the telescopic leg / machine jack design when I'm back in the office. Thanks!

 

[Tmoose]

How about a 15" long piece of pipe and one of these -

http://www.basspro.com/Bass-Pro-Shops-Fish-Scale/product/13327/

 

[kjoiner]

I can imagine weld spatter, metal chips etc. would not be good for the spiralift. I finally got to see one in the real world when I toured a concert hall near Atlanta. They had some under the stage. Keep us updated on what you finally choose for your table. It's always good to see ideas. That's what I like about this forum.

Kyle