PDA

View Full Version : Mechanical design question

Lew Hartswick
03-08-2010, 11:28 PM
One of the students at school is building a trailer (for a lawn tractor)
and is using the dwgs. form a "project book" but the wheels he has
are "real" highway rated type and the axel is 1" coldrolled. The
instructor would like to see the wheels out under the edges of the bed
but that is going to make about 8" of axel between the support and
the inside of the wheel. This is going to be a realy heavy duty trailer
body and I'm concerned about that much "overhang" of the wheel.
The wheels are, I think, 8" with about 15" D tires.
So what sort of loading / impact (like from a drop into a hole while
moving along with a good load in the body) would a 1" steel bar be
capable of with about a foot from the support to the center of the
tire?
How I think of the problem is force at a 1 ft distance to excede the
elastic limit of the steel rod. But no idea of where to check that sort
of info. ( Hey I'm an electronics engineer not an ME) :-)
Thanks for any insight.
...Lew...

rebel54
03-08-2010, 11:54 PM
The one inch diameter shaft is way to small for any load. For that type of load I would use some heavy wall pipe for the axle not a solid rod.

darryl
03-08-2010, 11:59 PM
The bar will bend. After some time you'll see the wheels leaning inwards at the top. I don't know about the design specs, but I've seen many manufactured trailers with bent axles. Of course people overload them all the time, but what can you expect-

Maybe a way out is to add a piece of rebar to the bottom of the axle- something like you see on some trucks- goes under the diff housing and angles up to the axle at either side. In this case you'd need some spacers to kind of bend the rebar over to give the same sort of help to the axle so it won't bend as easily.

Evan
03-09-2010, 12:56 AM
Use a multiplier of at least 4.5 times for the G loading that will result from pothole impacts. This is the same load factor that is used for ultimate failure loading on aircraft and the forces involved are very similar.

Given that as a starting point a calculation of stress and deflection give a deflection of 0.2" for a 1 foot x 1 inch cold rolled round bar of 1018 steel under a 500 lb impact load 1 foot from a cantilever support. The bending stress at the support point is 122ksi. That exceeds the elastic limit of mild steel by about double when carting a 100 lb load if we assume that on some occasions all the load will be carried by one tire and the maximum G force load is imposed.

If we assume a 1" round axle of 4140 steel heat treated and oil quenched and then tempered at 1000 F it will have an elastic limit of 143 KSI. That is enough to support the 100 lb load with some safety margin to allow for repetitive strain hardening and eventual failure. Steel is considered to not undergo accumulated strain failure as long as the load does not regularly exceed 50% of the elastic limit.

The bottom line is that even using a high strength alloy for such a poor design the allowable load with a reasonable safety margin is very small. If you use a tubing stub axle with a 2 inch section everything changes dramatically.

macona
03-09-2010, 02:53 AM
Dont even mess with building an axle. Thats a bad idea all around.

Axle and spring kits are pretty cheap. And there are the torsion type axles as well. They are even simpler.

Evan
03-09-2010, 03:37 AM
There is no reason not to build your own axle IF you know what you are doing. It is no more or less important than other parts of the trailer that would present a serious hazard if failed such as the tongue and the hitch.

For instance, if 2" OD DOM tubing with a .25 wall is used the numbers are suddenly very acceptable. With a 2000 lb load on the same 1 foot cantilever the bending stress drops to 44 KSI with a deflection of only .071" which allows carrying a 400 lb load on that axle with a 5 times safety margin and a 50% safety margin over the plastic yield of mild steel to allow for repeated stress.

Black_Moons
03-09-2010, 05:36 AM
Evan does great with the math, but even without the math that seems really scary from what I know of steel strength.. its just not very strong, ever see a long 1" rod bend under its own weight? its kinda sad to see really.
1" axle is like, what you'd use on a heavy gocart, assuming little overhang from the axle supports :)

Im pertty sure all the trailers axles iv seen have been at least 2" diamiter, if not closer to 3" for big ones. (likey hollow mind you)

While you don't need to buy a prebuilt trailer axle/suspension/etc, you would do very well to start off with dimensions copyed from one

vpt
03-09-2010, 08:41 AM
All the trailers I have seen built at school used at least a 1.5" and bigger square tubing with at least a 1/8" wall thickness. The bearing part of the axle was bought and welded in.