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Ladder Cage Bar Challenge!

14 REPLIES 14
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Message 1 of 15
jeanchile
1220 Views, 14 Replies

Ladder Cage Bar Challenge!

Hello all,

 

I was wondering if you all could help me with a part I am trying to model? I have a ladder that needs a small cage and that has the cage bars extended down to the nearest guardrail per OSHA/MSHA requirements. This ladder is not our design, we are matching an existing design, or I would simplify a lot of things about this. The part I need modeled is shown in this picture:

CageExtensionBars-1.png

 

I have created a part that should show you what is needed. The lines in the sketches show the profiles of the part at the various locations.

CageExtensionBars-2.png

 

The particulars:

1.) This needs to be plasma cut from a flat steel plate, then bent to the angles needed.

2.) No twisting of the steel is allowed, only bending.

3.) The profile doesn't neet to have a uniform width (2") in between the upper and lower portions but something close would be preferable.

4.) I originally tried creating surfaces and thickening to get a shape but some of the bends need to be thickened inside and some of them outside and I couldn't get the results I needed.

 

Anyone care to take a crack at helping me with this?

Inventor Professional
14 REPLIES 14
Message 2 of 15
LT.Rusty
in reply to: jeanchile

Why wouldn't you just use 3 pieces of flat bar for each section of the cage? One vertical coming off the rail, one angled between the rail and the lower ring, and one between the lower and upper rings? Seems that would be a lot easier, although perhaps not as pretty ...

Rusty

EESignature

Message 3 of 15
graemev
in reply to: jeanchile

I'll give it a go, but is there some reason why sketch 7 isn't aligned with sketch 2?  It would be quite a bit easier a job - both modelling and manufacturing - if it was.

Message 4 of 15
jeanchile
in reply to: LT.Rusty

I am not opposed to doing that but the shop usually prefers to bend this from one piece of plate or bar. There is no welding on this project in the field because the equipment is in service. They said they prefer to bend it in the shop because of it warping while welding, then they have to grind it smooth and make it look pretty so nothing catches on it, and they have difficulty holding all the complex angles.

That being said, I would still have to get them all the details for the individual pieces. If you can get me an example of how you would do it from three pieces I would certainly consider telling them they'll have to do it that way. The first preference would be one piece of material though.
Inventor Professional
Message 5 of 15
jeanchile
in reply to: graemev

I agree it would be easier, and I am not opposed to seeing your example with the bars aligned in those sketches. We are trying to match existing ladders already installed on the platform (not our design) and we were given a drawing that shows them not aligned at the bottom parts (10" from outside edge to the middle bar, each side). I'm not sure if this is an MSHA, OSHA, or CAL-OSHA requirement but I can't see it in my documentation. This is also one of the areas I am willing to change if it means I can get the shop what they need. I would certainly appreciate your attempt or input on getting this modeled with them aligned as I don't think anyone at the site would ever even notice they were slightly further apart than the others.

Thanks everyone for the help!
Inventor Professional
Message 6 of 15
LT.Rusty
in reply to: jeanchile

If there's no field welding, then how are they planning to install the new cage parts?

 

Honestly, for something like this, I'd probably just grab three pieces of flat bar and field run everything.  Wouldn't be any less functional.  Weld it all to the outside of the existing cage, then there's no issues with cleaning up catchy spots.

 

OSHA doesn't care if it's pretty, they care if it meets their requirements. 

Rusty

EESignature

Message 7 of 15
jeanchile
in reply to: LT.Rusty

My apologies for being unclear Rusty. The entire ladder shown in the first picture is being fabricated at the steel shop and then delivered to the field to bolt into other steel framing we are providing. All of the field connections on this project are going to be field bolted (they can drill but they can't strike an arc).

Also, you are correct that OSHA doesn't care if it's pretty but they do mandate that the inside surfaces of the bars and guards are to be smooth and free of burrs. That plus the client is going to want it to look decent.
Inventor Professional
Message 8 of 15
graemev
in reply to: jeanchile

Here's the critical part.  You can attach the upper and lower straight segments to your shop's preferences.  This is done as a solids bend, so there may be slight variance from actual results, but it should be close enough for a field fit.

Message 9 of 15
jaybear
in reply to: graemev

See if this works for you

Message 10 of 15
swhite
in reply to: jeanchile

Take the line as you now have, add another sketch at the top or bottom may be best in this case, and sweep the profile similar to the attached. We do this all the time for our caged ladders. When you are done convert to a sheet metal part, set the sheet metal thickness and it will create your flat pattern for the shop.

 

Since your line is touchiing the frame, simply attach the rectangular shape to that line so it sweeps away from the frame, not into it. this can be done by projecting the end point of the line into the new skrtch for the metal shape. When setting your top sketch start your workplanes, click on the endpoint of the line, then the line itself, setting the WP perpindicular to the line. Start sketch on that WP.

 

And I seriously doubt the original has no twist, being that it is not aligned at all points towrds the center, but was probably added in the field to make it fit both ladder and handrail. At the most you will get a gap on one side and a small intrusion into the cage rail, unavoidable without twisting the part after it leaves the cagerails.

Steven White
Lee C. Moore, Inc.
www.lcm-wci.com
Inventor 2011
Intel Dual Xeon E31225 @ 3.1 GHz CPU
16 GB RAM
NVIDIA Quadro 600 GPU
Windows 7 - 64 Bit
Message 11 of 15
jaybear
in reply to: swhite

I like this version better than my first attempt.  I hope it works for you.

Message 12 of 15
jeanchile
in reply to: jaybear

Thank you all for your suggestions and help with this. I was able to play around with this some more this morning and the shop has preferred a method similar to jaybear's second example.

 

I checked the ones already installed in the field and none of them are twisted and they are laid out exactly like my sketches (the bottom verticals don't line up with the hoop bars). They are all bent plate material with no twist but the width is not uniform. It tapers from 2" to something slightly narrower. I'm not sure how they determined the angle of the plane they used for the "critical" in-between section and that's the part I'd like to figure out.

 

Thank you to all for helping me get something that works for the shop (especially jaybearSmiley Wink). I hope to play around with this some more to figure out how they did this before, if nothing more than to further my IV knowledge a bit.

Inventor Professional
Message 13 of 15
swhite
in reply to: jeanchile

That a twist or bend is required can be easily determined. Rotate your center strut with the circular array command to align with the position of your strut to your line at the top. You will find that the straight strut will not even come close to aligning where they show it to be. This requires either multiple bends to create a no twist situation or a twist itself. There is no other possibility besides a large gap between the top strut and rail where they bolted it together, or bending the strut to create a twist. Granted, if the radius is large enough, the gap created might be forced closed with enough pressure applied to the bolts, which is going to form its own twist if tightened down enough. I expect this was not done and that a gap still exists where the two meet at the top rail, or the twist applied from tightening the bolts is not redily apparent by eye, being that steel tends to flex back when pressure is released. Not that you need model such a twist, when they tighten the bolts it will do it all by itself.

Steven White
Lee C. Moore, Inc.
www.lcm-wci.com
Inventor 2011
Intel Dual Xeon E31225 @ 3.1 GHz CPU
16 GB RAM
NVIDIA Quadro 600 GPU
Windows 7 - 64 Bit
Message 14 of 15
jeanchile
in reply to: swhite


@SWhite wrote:

...This requires either multiple bends to create a no twist situation....



This is what has been done. They used a plate profile that was similar to what jaybear provided in his second example but theirs was not a uniform 2" width. It couldn't be because it was aligned as shown in the sketches in my first post. The middle part of this bar is sloped, skewed, and canted to achieve the travel in the three planes.

 

When I have more time I'm going to play with this to see how you would model something like that but it will only be for my education as they are already burning the solution I provided the shop (which doesn't match the field conditions).

Ladder Bar Pic.png

Inventor Professional
Message 15 of 15
swhite
in reply to: jeanchile

Can be done with loft in inventor, but it will not provide you with a form acceptable to the shop to make a pattern from, Sadly one cannot get flat patterns from a twisted shape. But as i said am sure when they tighten the bolts it will cause its own twisting. In the loft option there are two different types. One of those options will allow the shape to follow the line using it as the not only the guide, but the falcrum point the shape revolves about as it changes direction. This would require a starting steel shape and an ending steel shape, lofted ogether, with the line as the rail. Starting and end shapes exactly the same except for their orientation with regard to the line. The loft will then curve and twist as necessary to follow the line and reorient itself from the beginning shape to the end shape. It may also be necesary to put one ot more other shapes along the path to fine tune it, where obvious direction changes ocurr perhaps with additional guide rails as well. Something one cannot tell until one just does it 🙂

Steven White
Lee C. Moore, Inc.
www.lcm-wci.com
Inventor 2011
Intel Dual Xeon E31225 @ 3.1 GHz CPU
16 GB RAM
NVIDIA Quadro 600 GPU
Windows 7 - 64 Bit

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