Side Project Questions: What Steel?

I'm currently working on a side project to get my own Longboard trucks built. For those not familiar with skating, here's a basic rundown on what the device needs to do:

1. When a rider leans the board, the force turns the Hanger, which is suspended between Polyurethane Bushings to allow it to return to center easily.
2. Hangers have conventionally been made out of cast aluminum. These ones don't handle speeds past 60mph very well, and have a tendency to bend during slides.
3. Conventional axles are made of undersized non precision ground steel. The exact ally I don't know.


So here's what I'm hoping people can help me work out: What kind of steel should I go with for my axles? I'm guessing some sort of heat treating is going to be needed, but if that can be avoided it would save cost. A bit of math that I have yet to learn might help out here...

With my current Hanger design, the axle mounts to the hanger with a 5/16 nc thread, 3/4" in depth. Let's assume the hanger is invincible for the sake of simplicity. The axle extends out about 2.5 inches, with the load spread across the 2 inches closest to the hanger. We've got a 200lb rider (safety gear is heavy) going 60mph down a hill, and the rider attempts to do a 90 degree turn at that speed. We're dealing with some fairly large forces...

Anyone who really knows their steel and could give me some advice would be totally bribed with cookies if I ever met you irl.

[Brandon Holley]

While I'm not inclined to recommend a specific steel, there are a couple thoughts I had:

1. These are going to be used on the road/street. If corrosion is going to be a possible problem, you may want to think stainless.

2. There are a HUGE range of heat treated steels, but generally what you are doing is hardening the steel. The harder the steel, the more brittle. Be careful if going for a heat treated variety. In an application like a longboard, I would imagine you would want the axle in a failure to bend, not shatter and break. (The point here being heat treated does not necessarily mean "better" or "stronger")

Those were just a couple quick thoughts I had.

-Brando

[martin417]

Originally Posted by CraigHickman

We've got a 200lb rider (safety gear is heavy) going 60mph down a hill, and the rider attempts to do a 90 degree turn at that speed....

OK, I see what your problem is, and I have a solution that I guarantee will work. Go to your local psychiatric hospital, and have yourself committed to the suicide prevention ward. With a few years of intensive therapy, and probably lots of drugs, maybe a few electroshock treatments, I am confidant that they can expunge your death-wish. Otherwise, why not fling yourself in front of a train, it would be faster.

[Dick Linn]

I'm not sure if a "shoulder bolt" AKA "stripper bolt" would work for you or not. The threaded portion is smaller than the ground portion of the body, so it may concentrate stress if it isn't designed correctly. Unbrako and Holo-Krome both make quality fasteners.

For experimentation sake, you might see if you can use a stud made from "stressproof" steel or 4140 chrome-moly alloy. ENCO has some Gibraltar cut-thread studs on page 764, and maybe the outside diameter would fit your bearings. You could at least try something like that and do a lot of jumping up and down on it before daring to do any road testing. Cantilevering a decent load a couple inches out on a 5/16" bolt may be pushing it. Maybe you could put one of your trucks on a handtruck-like arrangement and tow it behind a vehicle with a 300 pound sandbag on it. See how it holds up.

http://www.use-enco.com/CGI/INPDFF?PMPAGE=764

Originally Posted by martin417

OK, I see what your problem is, and I have a solution that I guarantee will work. Go to your local psychiatric hospital, and have yourself committed to the suicide prevention ward. With a few years of intensive therapy, and probably lots of drugs, maybe a few electroshock treatments, I am confidant that they can expunge your death-wish. Otherwise, why not fling yourself in front of a train, it would be faster.

Yeah, it's an intense sport. We love it though... It's the rush of flying over the pavement, ripping out pendulum slides to control speed, and drifting around corners.

Originally Posted by Brandon Holley

While I'm not inclined to recommend a specific steel, there are a couple thoughts I had:

1. These are going to be used on the road/street. If corrosion is going to be a possible problem, you may want to think stainless.

2. There are a HUGE range of heat treated steels, but generally what you are doing is hardening the steel. The harder the steel, the more brittle. Be careful if going for a heat treated variety. In an application like a longboard, I would imagine you would want the axle in a failure to bend, not shatter and break. (The point here being heat treated does not necessarily mean "better" or "stronger")

Those were just a couple quick thoughts I had.

-Brando

Hm. Good call on the stainless. I'll add that to my list of qualifications.

As for the hardened part, the board won't be dealing with sudden impact, just intense load for a sustained amount of time, with the load changing based on turns or straight lines.

[Dick Linn]

Stainless? Brittle heat-treated steel? Take it from someone who knows that he doesn't know, it's not as simple as all that.

[DonRotolo]

OK, let's dispel a myth here: Heat-treated steel is NOT STRONGER, it is HARDER. What you want is a very "Tough" steel (that is a real term used to describe steels) not a very "hard" steel. Strong (Tensile strength) isn't the ultimate goal, you want a high yield strength with a relatively low elongation (like less than 50% or so).

While a 1045 or 4340 alloy will be way strong, even annealed, these alloys may be hard to find or expensive. 1040 is nearly as good and a bit easier to work/machine. Make it thick enough and even 1010 won't break.

Case-hardening can give you high surface hardness while maintaining the tough interior body, but it still fractures more easily.

Ultimately you want a steel that is tough without heat treatment, other than annealing or perhaps normalizing (depending on the steel), both of which can be done at home easily enough (heat it up hot, and let it cool. Done). If rust is a problem, Rustoleum paint will fix that easily enough.

[purduephotog]

Originally Posted by CraigHickman

As for the hardened part, the board won't be dealing with sudden impact, just intense load for a sustained amount of time, with the load changing based on turns or straight lines.

*grin*

What do you call every little impact on the road then? Every bump is a 'sudden impact'. Those shock and vibe tables only go up and down 2 or 3 mm and they can generate tremendous forces.

Every little bump is basically 2x your weight being deflected by the cantilevered axle at whatever the change is.

You're definitely going to need a qualified metallurgist to figure your numbers... and then add in their safety factor.

[Brandon Holley]

Originally Posted by Dick Linn

Stainless? Brittle heat-treated steel? Take it from someone who knows that he doesn't know, it's not as simple as all that.

I wasnt getting in to specifics. There are a lot of variables to consider for Craigs application. If this is going to be a shaft used on a road surface that may be moist, damp, etc...stainless would be something I would consider...as I stated above in my original post I am not going to recommend a specific grade. I was simply recommending he look into stainless grades.

And heat treating makes steel harder...thats a fact, I didn't say it wouldnt work for his application, I was just throwing out ideas for him to consider. If you read the point that I stated in parentheses, I was simply stating "hardened" steel does NOT make it the "better" steel.

Take it from someone who knows what he's talking about...I was simply giving him things to consider.

[Kevin Sevcik]

I haven't run any numbers on this yet, but I think it's entirely possible you'll want a harder steel, rather than a tougher steel. Here's the difference between the two:

Tough = more energy required to ultimately fail the material. For identical tensile specimens, a tougher steel will require more total energy to ultimately break the specimen. Typically, this is because it stretches and deforms a LOT, so you exert a reasonable force over a long distance. Tough steel is good for things like car crumple zones, buildings, and other things where you'd rather the member yield and deform while absorbing energy than suddenly break.

Hard = more force required to ultimately fail the material. This typically implies less toughness, because while the material requires more force to break it, it doesn't stretch near as much. Think of the difference between breaking off a pice of cold, hard taffy versus warm stretchy taffy. High hardness is good in situations where any yielding/bending will be just as catastrophic as a clean fracture. Typically, this is in shafts and suchlike. Additional benefits to high hardness are a higher fatigue limit. This means that the steel will better resist repeated low level stresses on it. Such as from bumpy pavement at 60mph, or a slightly misaligned rotating shaft.

So.... Long story short, I think you need to decide if you'd rather have an axle bend somewhat while you're going downhill at 60mph, or break off completely. Also, if you have or can find pictures of the remains of a board that HAS failed, it can be remarkably useful in determining what it was that caused the axles to fail.

[Brandon Holley]

Originally Posted by Kevin Sevcik

So.... Long story short, I think you need to decide if you'd rather have an axle bend somewhat while you're going downhill at 60mph, or break off completely. Also, if you have or can find pictures of the remains of a board that HAS failed, it can be remarkably useful in determining what it was that caused the axles to fail.

My hesitation for a hard steel is the fact you just mentioned. If i were moving 60 miles an hour, i'd prefer for the axle to bend rather than just snap off...but again, this is at the discretion of Craig

[Dick Linn]

A properly heat-treated steel can be stronger in both tensile and yield strength. The hardness and "brittleness" are factors to control, and different alloys respond quite differently.

For example, I ran across this a while back during a discussion of heat treat requirements for 4140 steel over at Practical Machinist. I found the information on embrittlement effects interesting, especially the fact that the notch toughness decreased in the midpoint tempering ranges (figure 2).

http://www.nhml.com/resources/2007/4...ment-of-steels

Some of you may find this discussion on bolt strength of value:
http://www.rockcrawler.com/techrepor...ners/index.asp

As far as sourcing metal, you can get 1144 Stressproof, 4130, 4140, or 4340 fairly inexpensively at places like www.onlinemetals.com. I've seen some Grade 8 bolts specified with AISI 4037 or 4042.