# Stress Calculation on trailer legs

• simelliott

#### simelliott

Hi, I hope someone can help, I'm trying to design a trailer which uses adjustable legs on each corner as support legs while it is is use.

The legs are adjustable and have drilled holes for the position pins, and I am trying to work out what wall thickness of leg will be adequate to provide enough strength. The outside dimension of the leg is fixed, as is the pin diameter, only the wall thickness and hole spacings can be varied (see attached image).

I'm looking for a calculation on the stress, or assistance on how to work this out please? I'm sure there is an easy answer but I'm struggling...

The legs are 38mm (1 1/2") square/box section (hollow) 316SS if that is of any help... and the holes are 16mm diameter..

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• trailer leg.jpg
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## Answers and Replies

Thanks Danger,

Currently we've spec'ed 38mm hollow box, 1.6mm wall section SS316. The trailer weighs 2200kg total with a support leg on each of the 4 corners.

The holes on the support leg are currently at 34mm centres, with 5 holes if that makes a difference?

Thanks.

I just deleted the post that you responded to, because I realized that the number in your question was in reference to the material. I don't wear my glasses when working on the computer, so I misread it as ending in 55 rather than SS. My post, therefore, seemed to be pretty stupid.
As I mentioned in it, I can't help with calculations. I wish that I could, but I have a grade 9 math education. I've never dealt with anything in the weight class that you require; the heaviest stuff that I've worked with was 1/16" thickness mild steel.
From past experience, I suspect that your main concern should be directed toward the pin holes. The ones that the pins inhabit will be your weakest points, but all of them compromise the structural integrity.
Also, the weight distribution is more important than the total weight, so you might get away with weaker stuff on some legs. (I'm a symmetry freak, though, so that idea grits my gears.)

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I suppose you've deemed the 316SS as a requisite? Carbon steel would probably be better for this application, but if you're worried about corrosion or something, well, you'll just have to deal with that.

First off, it's a trailer, unless you are really worried about weight, sizing the legs up to avoid buckling and bending isn't going to do any harm, and won't cost you too much (well, maybe the SS will run you a few more bucks the next size up...). Not only does it have to support the dead load, you've got to factor in funny terrain and uneven loading, and any dynamic loads the trailer might experience while the legs are down.

Your big issue here is the wall thickness. The hole spacing is a concern, but only when you consider the wall spacing (i.e. if you've got thick walls, the holes won't pose a major concern). How adjustable do you need the legs? Is a screw method tenable (like the support leg on a trailer hitch)? The number of holes you've got shown there looks troubling for a 2.5 ton trailer.

I definitely don't like the 1.6mm wall thickness either for this application. I'm thinking more like at least 3mm (~1/8") to 6mm (~1/4"). Build this thing with some heft, with equipment like this, you don't want to skimp on important components, especially ones that you can beef up without costing yourself too much weight.

The real issue is that the trailer (we're modifying an existing trailer) already has 38 x 1.6mm box section SS legs, and we need to drill new holes half-way between the existing ones as they are now at the wrong height. I'm just wondering how to calculate whether or not this is going to have a big effect or not?

And if it is, we obviously have the option of building new legs to go in the existing slots.

SS316 is necessary because of the corrosion issues in it's application, so no other material can be considered.

There must be a calculation to work out stresses on a vertical box section (probably without without holes?) I would have thought?

I suspect that you'd be better off building new ones, but that is also an amateur opinion. One other thought that comes to me, though, is that any new holes that you drill should perhaps be at 90° to the originals. I'm not sure, but it seems to me that it would equalize the stresses better that way.

Hi Simelliott,

Just provide some more details so that it would be able to make it more clear. I got confused so unable to suggest you. Hope that next time it will be more specific.

simelliott: Using your 38 x 1.6 mm stainless steel SAE 316 square tube, the uniform axial stress, pin hole bearing stress, and buckling strength currently look OK.

But is there anything else in your system, other than these four legs, to resist horizontal force on your trailer? If not, it currently appears your 38 x 1.6 mm square tubes, with 16 mm pin holes on 34 mm centres, might withstand only up to 65 km/h wind gusts, assuming the leg extension length does not exceed 500 mm.

It is slightly unclear to me what your new position pin hole spacing will be. I currently assumed your new hole spacing will be 34 mm.

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SS316 is necessary because of the corrosion issues in it's application, so no other material can be considered.

Why? What's the trailer actually doing?

Also what are your shear pins/bolts (whatever is going through the hole made from)?

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There are quite a few other corrosion-resistant materials out there, some of them non-ferrous. Even within the family of austenitic steels you can consider 304L or 316L in the 1/2-hard 3/4-hard or full hard or extruded Cond. F grades as alternatives.

I can also see where Chris is going... those pins shouldn't cause galvanic corrosion either.