[Bigbob]

I am in the process of building a trailer. For the hitch I am building something similar to this. A 3 axis hitch.


It looks like this one uses either 5/8" or 3/4" bolts. I have 1" pins on mine. Where my question lies is in the 3rd pivot point at the rear of the hitch. (the right side in the picture). I am planning on a grade 8-1" bolt here. It will have bushings on one side and be secured on the other side. It will also be longer so the load will be spread out a little more.

Is a 1" bolt going to be strong enough? My thoughts are many small trailer axles are 1,000 lb 1" affairs and at the axle more shock/strain is applied than what would be at the hitch area. Before I start beating and welding I wanted to hear your thoughts.

[el_magico]

You can use a heavy hex cap bolt, 1"-8tpi. The heavy hex is has a hex head that is 1/8" bigger than normal, I like using them for big applications cause, the more hex head area the better. The ones I am talking about are ASTM193 hex. The tensile strength of a 3" bolt is 125,000psi for a 5" 125,000psi.

Regular hex grade 8 bolts tensile strength of a 3" bolt is 150,000psi and for a 5" is 125,000psi.

The answer to your question is YES grade 8 bolts will be plenty strong for this application.

[Bigbob]

Originally Posted by el_magico

You can use a heavy hex cap bolt, 1"-8tpi. The heavy hex is has a hex head that is 1/8" bigger than normal, I like using them for big applications cause, the more hex head area the better. The ones I am talking about are ASTM193 hex. The tensile strength of a 3" bolt is 125,000psi for a 5" 125,000psi.

Regular hex grade 8 bolts tensile strength of a 3" bolt is 150,000psi and for a 5" is 125,000psi.

The answer to your question is YES grade 8 bolts will be plenty strong for this application.

Thanks. The bolt I am using is a grade 8. No welding will be done on the bolt of course. The non-threaded portion of the bolt will be bushed and greased. My bushings actually fit a 1" bolt perfect. I figured I'd need to go with hard shaft material.

Another thing I look at in my assumptions is, all 2" receiver hitches are held in with a 5/8" pin. 9,000 lb trailer, 5/8" pin. So as far as linear strength goes I know I am good. It's just the jarring and shock that worries me laterally on the 1" bolt. The trailer will seldom ever go past 500-700lbs and stay on the lighter side off road.

[JeeperDon]

Why is this setup better than a conventional ball hitch, or even a pintle? You can but regular hitch ball sockets at Tractor Supply and other places.

[Bigbob]

Originally Posted by JeeperDon

Why is this setup better than a conventional ball hitch, or even a pintle? You can but regular hitch ball sockets at Tractor Supply and other places.

A regular ball hitch is simple, but has a limited range of motion. A pintle hitch is much better than a ball hitch, but they are very expensive and in my experience rattle annoyingly. The 3 axis hitch has a much better range of motion than both. Most pintle hitches are only 2 axis and they depend on the slop to allow motion. And the hitch I am building is much cheaper being I am doing it myself which also serves as entertainment to me.

[timatoe]

I think the 1" would be fine for the small trailer, I use 1" grade 8 through my tow bar for flat towing my YJ.

[mrblaine]

Originally Posted by Bigbob

Is a 1" bolt going to be strong enough? My thoughts are many small trailer axles are 1,000 lb 1" affairs and at the axle more shock/strain is applied than what would be at the hitch area. Before I start beating and welding I wanted to hear your thoughts.

Is a 1" bolt strong enough? Let me put it in a perspective to illustrate.

If you took a 1" grade 8 bolt, clamped two plates together and then used them to lift up a group of Jeep Wranglers, you would be able to hoist 12 of them in the air before the joint slipped if you torqued it to the recommended 900 lb ft of torque.

It would take another 4 Wranglers added to the string to reach the proof load rating of the bolt and some additional number before the bolt failed.

How much did you say this trailer weighed?

Purely from an engineering standpoint, you could do what you want with a 1/2" bolt in grade 8. The way that hitch is designed, the only forces that bolt will see are tongue weight and accelleration forces when you take off from a stop.

[5-90]

The three-axis hitch will allow for greater articulation off-road. The typical ball hitch is good for (usually) 15-20* of RPY (Roll, Pitch, Yaw) displacement before it binds. A pintle hook and lunette ring do rather better - allowing for some 30-35* RY and about 15-20* of P displacement (due to the ring binding against the pintle hook,) but the hook/ring combination can get rather noisy once you're off flat land.

The three-axis hitch will allow for a great deal of RPY displacement (limited primarily by design and construction) without trouble, and does so without excessive relative motion - combining the advantages of a ball/socket hitch (typically used) and the articulation advantages of a pintle hook/lunette ring (but rather moreso. It's not outlandish to think of 90* RP and 35-40* of Y in a three-axis setup.)

Some advice:

1) Where possible, you're better off using pins than screws for pivot points. They're going to be strained rather more than simple attachment screws (due to intermittent loading, shear loading, and relative motion of the parts you just can't get rid of,) and the threads of the screw can provide a starting point for stress cracks. Yes, screw threads are rolled - but a straight section is still better than a rolled thread any day, when considering stress.

2) If you have to use screws, try to stress the threads as little as possible. Longer screws are typically threaded for 2-1/2 the nominal dimeter (i.e. a 1"-8x4" screw should have 2-1/2" of thread on it,) and you can use this factoid to your advantage. The unthreaded shank is called the "grip" of the screw, and it is this section you want loaded in shear (not the threads.) Therefore, figure your length needed before you get to a threaded part, and select your screws accordingly. You can always cut off (and dress the cut end!) the threaded bit you don't need, as long as you A) make sure to seal the cut end, and B) have at least one full thread protruding past the nut when torqued. (A 60* chamfer is ideal, and preserves the profile of the thread.)

An SAE8 J429 screw is H&T to a 120ksi proof load, 130ksi yield load, and a 150ksi tensile load - minimum. What do these mean?

The "proof load" is the load that the screw has to be able to hold indefinitely without permanent deformation.
The "Yield load" is the loading point at which the elastic limit of the material is exceeded, and permanent deformation sets in. Note that the screw must still not rupture!
The "tensile load" is the minimum load the screw must be able to maintain before it ruptures. Tensile stress is a "pulling" stress end-to-end on the item at question ("shear" is a "cross-cutting" load, and a "compression" load is pushing inwards end-to-end. Tension and compression are typically measured by sample, and shear is taken as 75% of tensile strength in general.)
ksi = 1,000 pounds per square inch.

So, let's go back to our 1"-8 screw. With 8tpi, we have a minor diameter of 3/4"". That gives a minor tensile stress area of (3/8)*(3/8)*3.14159, or .441876 sq. in. (roughly.)

This gives a proof loading of 120,000*.441876 = 53,025.1 psi, or just over 50ksi.
Proof loading in sheer will be 75% of that, or 39,768.8 psi (just under 40ksi.)
This does not account for the natural "stress riser" of the thread - which serves as a concentration point for local stresses. A good design engineer avoids stress risers insofar as possible (a "stress riser" is any localised change in section, any sudden change in profile, any surface defect/stratch/gouge/whatever, and the like. Any change in section should be as gradual as possible, and sharp corners are very much to be avoided. Weldments aren't filleted because it looks good - they're filleted to reduce stress concentrations at the weldment...)

The grip, not being threaded, will be just about the full nominal diameter of the screw. So, we'll have .5*.5*3.14159, or .785398 sq. in.

Tensile proof load will be 120ksi * .785398 = 94,247.7ksi (just over 94ksi)
Shear proof load will then be .75*94,247.7 = 70,685.8 (just over 70.6ksi.)

Deformation (without rupture!) loads for the threaded section will be:
Tension - 53,025.1 psi
Shear - 39,768.8 psi

And for the grip:
Tension - 102,102psi
Shear - 76,576.3psi

Ultimate failure loads for the threaded section will be at least:
Tension - 57,443.9 psi
Shear - 43,082.9 psi

And for the grip:
Tension - 117,810psi
Shear - 88,357.3psi

Yeah - the 1" screws are going to be massive overkill.

To simplify things for you, consider the full weight of the trailer and anticipated load. Add 20% of that for overage and "fudge factor." You now have what will probably be the most weight you plan to have on the thing.

Because trailer hardware is subject to repeated shock loading in shear, you should now add 50%-100% of that load - that's your design load. Even with that sort of overage, you should check the pivot and attaching hardware, say annually, with regular use.

Now, start working out your hardware. SAE5 J429 hardware is H&T to 85ksi proof, 92ksi yield, 120ksi ultimate failure in tension (for sizes up to and including 1" nominal diameter.) SAE8 J429 will be H&T to 120ksi proof, 130ksi yield, 150ksi UTS.

Figuring the grip diameter is easy - it's the nominal diameter of the screw. The "minor diameter" of the threaded section may be readily determined by taking the thread pitch as a fraction and subtracting it from the nominal diameter (a 1"-8 screw has a thread pitch of 8tpi, and therefore loses 1/8" in diameter per side. Minor diameter is therefore 3/4".) You can see from the information above that there is a rather large disparity in strength between the threaded section and the unthreaded section, and this should be accounted for in design (if it's done properly, a screw is never loaded in shear on the threads. And, it's always better to modify a screw by "cutting short and painting" than it is to "cut another thread or two onto it." As Carroll Smith once said - "Any machinist worth what you pay him can cut a clean thread, but a cut thread is a crime against Nature. This has nothing to do with the machinist.")

Whew!

Oh - and for under the head of the Yaw screw (the one closest to the trailer, in the pic,) I'd check around for some Torrington bearings that can make that turning a bit easier. Either that, or a close-fit Oilite bushing can be made (and should be made for the other joins...) in a "split top hat" shape - giving you a two-part bushing that can have the flange between the steel bits that pivot. It will also help with longevity. The heads of the R & P screws should not move relative to the surface underneath them, but the Y screw should have a bushing flange under it - or a Torrington bearing - to allow for free movement. You may have to design that join with a bit of "slop" under the head to allow it to move freely - you don't need much (as little as you can get away with, a few thousandths when torqued should serve,) but you will want some.