Tecnique Help Required Please.

[VeryFEDUP]

I am a total beginner with all things metal. My son's scout troop have been building a half scale Boulton Paul Defiant fighter. They have put so much effort and I don't want to ruin it for them! I have almost finished the propeller and I have roughly drawn the landing gear.
I think this design is better than what they had planned as the 30mm box is already in one piece. The forces will be straight up into the metal? What I can see being a problem is the wheels. They have to work as wheels as the thing needs to be pushed along.
All the weight (mostly wooden plus two smallish boys) will be trying to bend/snap the 10mm bolts. Please can you all cast your knowledgeable eyes over the drawings? Will the 30mm collar over the 25mm lower leg help or hinder the design?

I really need some advice please. Thanks in advance...

 

[nodeward]

can you not run another bit of box down the other side of the wheels so they are supported on either side?

 

[mike 109444]

Not sure of the ins and out of what you are doing but have in mind haw the wheel barrow wheel would have attached to a "wheel barrow"! Namely what size axle was that designed for. As a wheel barrow is designed to carry quite a bit of weight on just one wheel. If you use a bolt as the axle then make sure that the bolt has enough plain shank to act as the axle even to the point of the plain shank passing right through such that you have to fit washers to allow the nut (NYLOCK type) to secure the wheel/bolt. Reason if part of the threaded section is forming the axle it will be a weak point.
These might have what you need. Don't forget to allow for washers both sides of the bolt and any inbetween when getting your bolts.
http://www.screwfix.com/c/screws-nails-fixings/bzp-high-tensile-bolts/cat840456

 

[Rig Pig]

I think you would be better to find a tube that is a good fit in the wheel bearings, cut it fractionally longer than the bearing,coat it in grease, place a washer each side, push a long bolt up the middle and nip it up nice and tight.

 

[Wendelspanswick]

I think you would be better to find a tube that is a good fit in the wheel bearings, cut it fractionally longer than the bearing,coat it in grease, place a washer each side, push a long bolt up the middle and nip it up nice and tight.

But the bearings only take a 10mm bolt so by the time you tube it you are down to an 8mm bolt!

 

[Wendelspanswick]

I would be happier with an M12 or M16 bolt but you may get away with a good quality M10 bolt (not a set screw), either a 10.9 or a 12.9 grade.

 

[MrFluffy]

Your problem as you see it already is that the wheel axle is single shear whereas in the wheelbarrow, it was supported in double shear. Best would be as someone else said something just the other side for a short length to support it the other side then you don't have to worry about bending moments about the axle sleeve or diameter of the pivot bolt.
Having said that, I'm guessing you are stuck in single shear for some reason (aesthetics?) so you can improve things with a bush welded into the box, but your 30mm box collar is presumeably not going to be a tight fit to the spindle bolt? in which case, two bushes, drilled m10 on their id and welded in. It still wont fix the bending forces on the m10 bolt but you can improve that by careful selection. The point of maximum stress on a normal bolt is going to be at the root of the first thread, if you can arrange it so the threaded section starts inside the support bush and the unthreaded shank is a snug fit, you will be improving its fatigue resistance at no extra cost.

A m10 8.8 bolt will take 3 ton in shear so I think your issue is more the bending moment on the tube and your mechanical connection between the tube and axle bolt than the strength of the bolt itself...

 

[MrFluffy]

Another thing you could do, is run a tube between the two wheels and run a m10 bar threaded at each end for one long axle, then both wheels would be in double shear at no extra weight apart from the tube/m10 bar.

 

[Sidewinder]

Looking at the shear forces only.
The shear strength of a 12.9 bolt, circa 900N/mm sq proof load.
In the configuration you have there are 4 shear points, 2 per axle, ignoring the welded connections and anything further into the structure, as this takes the engineering assumption that they are fixed and thus "perfect".
Thus your axles will carry in true shear 900x4 N/mm sq that is 3600N, around 3.6t, per mm sq, (taking g as 10, it's less)!
A bolted joint is often considered as having 4 shear planes, so that would give you 8, but being conservative, we can use 4.
Work it out from the area of the bolt and the shear strength, that is the other way.
8mm dia bolts give you around 50mm sq cross sectional area.
This gives you a shear strength in one plane using a 12.9 grade bolt of circa 45kN, so giving a shear of 4.5t.
Take that out to 4 and you get 18t, if you actually have 3 or 4 shear plances per side, it goes up even higher.

Remember, though this is the absolute failure strength in shear from a perfect scenario.
Also no stress raisers.

I believe that it would be the bending moment that would get you rather than shear.

 

[KemppiFrog]

I spent some time around real a/craft. So, any estimates as to the actual weight of the whole 'airframe' + crew?

From your drawings, best idea IMHO is to get some thick walled tube with an i.d. of 10mm and weld it through the leg. This puts the bending load on the bolt as far out as possible, right up against the first bearing in the wheel. As said, it is the bending load that counts, so the strongest bolt you can find, like a 12.9, should do.
At least you don't have actual landing loads, especially those from a novice pilot.

A very interesting choice of project, given their service history. Care to elaborate?

 

[DuffyBB]

Not sure if you could get one long enough, but a brake caliper slide pin popped into my head...something like this maybe? There is different ones/styles, some have an internal thread at the end for securing bolts.

EDIT: Or maybe a cylinder head bolt, i'd assume they are a high grade steel?

 

[VeryFEDUP]

Thanks everyone. Like I said, I really don't have a metal background to rely on. It looks like they have gone ahead with an alternative design that relies on a much larger vertical simply coach bolted into the wooden wing spars. If this fails when tried I will have a go based on your advice.

The Defiant wasn't a total disaster. OK, it wasn't very good against enemy single seat fighters and was withdrawn from that role. It did a lot better as a night fighter going after raiding bombers. This was mainly down to the turret being able to fire upwards from below the bomber. I believe it was more successful than more regarded contemparies.

As to why, it is simply down to local interest. The factory was less than half a mile from this house. All the fields around here show crater damage.

Thanks again to everyone!

 

[Robotstar5]

As to why, it is simply down to local interest. The factory was less than half a mile from this house. All the fields around here show crater damage.

From crashing Defiants?