Steel for Skateboard Truck Axles

[ZealousLongboards]

So after searching for many long hours and constantly being directed back to eng-tips.com I decided to join as I always seem to have many questions. I make downhill racing trucks for skateboarding and we are looking for a new steel for our axles. The axles are 8mm in diameter, 10" long and they are pressed into a 6061 aluminum shaft. We are currently using 17-4 stainless steel heat treated to a h900 condition.
We have been looking at some maraging steels and other steels but I was hoping to get some good advice from someone who knows what they are talking about. So we are looking for a metal that has:
High strength at temperatures under 120 F.
High elongation % (bending it better than breaking)
Medium to High corrosion resistance (they are constantly used in rainy and even salty conditions).
And the lower the cost the nicer
Any help would be greatly appreciated. Thanks very much.

 

[unclesyd]

I use a lot of the Nitronic 60 and Nitronic 50 for hardware on boats. Probably more Nitronic 60 of the maybe a problem with with galling. We do use a lot of17/4 PH for fishing reel parts with the exception that we use it in condition H1125. Both Nitronic 60 and Nitronic 50 would be good candidates to try if you can workout the pricing.

 

[KENAT]

Just to check, if you're long boarding there won't be any grinding/40-40's or ollying etc going on?

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[ZealousLongboards]

Ollying not as much as a skateboard, but they do go through some very heavy abuse. Early grabs of stairs and hitting cracks going sideways at 50+ seems to take their toll on the axles. We are also considering going with Carpenter Custom 465.

 

[Maui]

I suggest taking a look at 416R stainless. It would be a good replacement for 17-4 PH in this application, and should be less expensive.

Maui

http://www.EngineeringMetallurgy.com

 

[swall]

How elaborate is your processing of the axles? Are you machining them or are you just cutting stock to length and assembling them to the aluminum shaft?

 

[ZealousLongboards]

Pretty basic. We just add about .5" of 5/16" 24 threading to each side and then press the axles in through the aluminum.

 

[EdStainless]

If you need more strength and similar toughness to `7-4PH you don't have too many options.
A 250ksi maraging steel would work, but they are tough to work with. I would not use a 300ksi maraging since these have lower fatigue strength.
You could look at 17-7PH in RH950 condition. I would avoid the CH condition as it has very low ductility.

You need to look at your design carefully. If you can increase diameter slightly it would help a lot. You also need to make sure that you avoid sharp corners.

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[KENAT]

Ed, if it's a typical skateboard type set up then the diameter at the point of most loading is pretty much fixed by the size of bearings/wheels. Assuming stock bearings and wheels there's probably not much the op can do about it.

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[EdStainless]

These are not straight stock, if he can beef up the center section and use a bigger radius at the reduction on the ends he might help out the situation.

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[BMKR]

Wouldn't anything in the PH family be right up your alley with this one maybe 13-8Mo would give similar properties. I am assuming it's a cost issue.

 

[ZealousLongboards]

We have thought about bulking it up in the middle section but our design wont allow it. The bending is coming from the ends and not in the middle at all. We have been looking at Carpenter Custom 465 and so far that seems to be our best option as far as adding strength. Cost isn't a real big issue just as long as its not WAY more than 17-4.

 

[BMKR]

I tend to stay far far away from the pricing side of things, as far as aerospace is concerned (you wouldn't believe how cheap everyone is).

 

[EdStainless]

Custom 455 is fairly available, 465 is a pain.
Are these failing from straight overload or from fatigue?

Right no you are at 170ksi min Yield and 190ksi min UTS
going to 17-7PH in the RH900 will get you to 190ksi min Yield and 210ksi min UTS.
A 250 maraging will get you to 250ksi Yield and 260ksi UTS.
Or a 300M will be about 290ksi Yield and 300ksi UTS.

For this service you may really need to think about how to get more diameter. I know that means incompatible hardware but it may be the only way to get enough strength.

If it was me I would use a 250 or 300 maraging (depending on availability) and see how it goes. That is a good step up in strength.

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[Maui]

Zeleous, when you say that the axles are bending at the ends where the support points are located, do you mean that

1.) They are elastically deflecting by an unacceptable amount while under load? Or,

2.) Are they permanently deforming so that when the load is removed (i.e. the rider steps off the board) the axles don't spring back to their original undeformed condition?

If the first scenario is what you are describing, then increasing the tensile or yield strength of the alloy won't make any difference. Either you need a steel with a higher elastic modulus, or you need to increase the physical diameter of the axles, or both. You can read FAQ faq330-1441 to understand why this is the case. If #2 above is what you are describing, then Ed's suggestions are certainly good advice.

Maui

http://www.EngineeringMetallurgy.com

 

[ZealousLongboards]

Thanks everybody for your help so far. We have been looking into a lot of stuff. Maraging steel would be the ideal choice so far if it wasn't for the price. Maui, the axles are permanently deforming. We haven't had any problems with anyone noticing them moving while riding.
Maraging 300: $27/ft, Ultimate TS: 2169 MPa Yield TS: 2135 MPa Elongation: 7.70 %
13-8mo rh950: $11.27/ft, Ultimate TS: 1620 MPa Yield TS: 1480 MPa Elongation: 12.0 %
Custom 465 h900: $6.98/ft, Ultimate TS: 1779MPa Yield TS: 1703MPa Elongation: 14.0%
The custom 465 comes in a slightly bigger size but I know a place that can precision grind it down for about $5-6/ft. Both the maraging and 13-8 come in 8mm. Another cost would be to heat treat the 465 and 13-8. Not sure if I can just keep the maraging steel how it is.

 

[TVP]

FYI, maraging steels and 300M are not stainless, and therefore would require some type of coating in order to resist corrosion.

 

[EdStainless]

The aging treatment on the 300M is easy, but the PH stainless ht is easy also.

Since you can get 465 in nearly the correct size that sounds like the winner.
It should come to you annealed. You would machine and then heat treat at 900F for 4 hours, and oil quench. Not a lot of trouble. You can do it in air if you don't mind the heat tint.

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