Hay Wagon Bed Design (pseudo-engineered)

This message is a reply to an archived post by Bill VA on March 23, 2015 at 09:28:08.
The original subject was "Hay Wagon Bed Design".



Well it is my turn and I've decided to go the engineered route to size the main beams. The design bending strength formula you need is (WxL)/(8xZ) to determine the material/dimensions.
W is the load you want to carry
L is the span in inches
Z is the section modulus of the lumber cross section

For example, I am going to use spruce 2x8's which have a section modulus of 13.1 cubic inches. I want to match the capacity of my 3-ton chassis using a 10 ft (120 inch) span. This gives me a design bending strength of 6870 psi.

2x8 #2 spruce has a bending strength of 1050 psi (neglecting adjustment factors) so I'm going to need a total of at least 6 boards.

Reworking the formula for 6 boards (3 per beam) gives me a final capacity of 5502 lb between the front and rear axles.

Then I'm just going to run decking perpendicular to the beams and skirt around with 2x4 to keep them even.
 

Well that is nice for the new materials. For how long does the spruce maintain that RBM out in the weather? Are you speccing glue between the boards so that there is no opening for moisture to get between? No amount of nailing, screwing, or bolting will truly seal the boards together to keep moisture out.
 
Aluminum or tin roof flashing over the top of the stringers has let me use the same stringers for the third decking on a hayrack, the frame is
older than me.

Paul
 
I believe the calculation is a bit more complicated than that because a significant part of the beam and load are cantilevered past the rear axle and (to a lessor extent) in front of the front axle. It wouldn't surprise me if the greatest shear is at the real axle. But your design of 6 2x8s seems plenty strong so long as the end joints of the boards are away from the axles and mid-spans.
 
You are overthinking the whole build. Go look at some bale racks in your area and copy the features you like from them. As said, your design will soon rot.
Loren
 
2 -4x8's will carry much more than you can haul on your 5 ton gear and still not be over loaded. I would use White Oak for the main timbers,if cost was not an item. Then use 2x6or 8's for the floor. Oil it down with used oil and keep it inside will last for longer than you will need it.
 
I think you are going to want to look at the area momet of inertia. The formula is I = (b * h^3)/12 (b=base, h=height). The higher the I, the stronger the beam.

As you can see, the height drives everything.

What I found is once you set the stringer height tall enough that the stringers with 2x4's perpendicular clear the wheels, they will be plenty strong to hold the weight.

BTW - I used saw mill/rough cut - full cut lumber.
 
Well, I don't know anything about all those figures and calculations. For my "hay ride wagon" (8x15 +/- , I went to the local Amish sawmill and got 2 stringers that actually measured 3x10 and used 2x8 / 2x6 for cross boards , again actual measurement.1x3 across top on ends and another vertically along ends to tie/box ends together. All dry white ash. Top quallity screws for all. Tar paper over stringers before deck. All slopped down with linseed oil and covered with tarp when not used. Pretty sure it will outlast me. Also, if needed , would haul all the hay a normal person could load on it. And safely. Only thing missing are the calculations / blueprints etc. Guess I'll have to roll the dice on that.
 
All responses have been excellent. I will reply to each here:

Bruce, I am a machine designer, so wood construction is a bit out of my expertise, but similar calculations for strength.

showcrop, I like your idea of using the glue as a seal. I'll make sure to go around the perimeter.

paul, I do have some flashing laying around but no longer lengths. I might go with tar paper

TomH, I haven't really thought about how much I will overhang the axles, but you are correct, there is a more involved formula. Shear usually happens parallel to grain unless you were referring to joints.

Adirondack, You are correct. Reverse engineering something that already is proven in the field is better than any theoretical calculation. I just gotta start somewhere.

caterpillar guy, Your 4x8 white oak will be in the same ballpark for strength since the oak is a little stronger. Dimensions (mine are 4.5 x 7.25) are similar. I do have some used engine oil that I haven't recycled yet. Thanks for the tip.

Bill, You are correct about the moment of inertia. I am lazy and use the section modulus (I/y) since it already takes into account the distance to the neutral axis.

RandyB, I will use the tar paper idea and oil (used oil) but I think I'll be looking for a tarp.

Thank-you all!

I was originally going to just design for a small square bale evenly distributed load and then I remembered an old engineer who said, "If you are going to design a flat screwdriver, make sure you design it strong enough to be used as a prying tool." I am already getting comments about hay rides from my kids and parade floats from my neighbor. I don't even want to think about uneven and dynamic loading.

I got the chassis for 200 bucks and don't want to spend more than a few hundred on lumber. I already have the 2x8 kicking around. I just gotta get my hay to the yard.
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