What's new
What's new

Question on selecting steel for shaft

crossthread

Titanium
Joined
Aug 5, 2004
Location
Richmond,VA,USA
My son bought my grandson a go-cart for Christmas and he has bent the rear axle. After looking at it I told him it would be a pretty quick job to turn down the ends of a new shaft, thread it, cut the key ways etc. It is a 1" shaft about 4' long with three pillow blocks on it. Not having ever made a go-cart axle I would like some advice on what type of steel to use. What is in there does not seem very rigid. I have some 1" 12L14 but I am pretty sure this is not strong enough. What are your recommendations? Thanks very much.
 
Rigid (or stiffness) has nothing to do with strength - steel is steel as far as modulus of elasticity

Your 12L14 would be fine except its a rust magnet. 1018 would be my choice. I would get cold drawn and NOT machine the OD except where I had to
 
Thanks for the quick replies. So you are saying John that it doesn't really matter what steel I use because it is all going to be about as bendy? Hardness I certainly understand. I'm not really looking for hardness here. I thought of 4140 prehard ( I guess the same as 4140 ht or heat treated) but again I'm not so concerned with the "hardness" as the axle runs in pillow blocks and there really aren't any wear points. Are you implying that cold drawn would be less "bendy"? Finegrain, when you say tough are you referring to hardness or the bendy thing? I know enough about steel to know that some is very stiff while others may bend quite easily. I am obviously not a metallurgy wizard or I would know this. Thanks so much.
 
Stiffness and strength are not the same. Stiffness is the amount a material deflects under load BEFORE it takes a permanent set, or "bends". 12L14, 1018, A-36, 4140, and hardened drill rod will all deflect the SAME amount under the same load,as long as that load is not enough to permanently bend or break them. Of course the 4140 will flex a LOT farther (under a much heavier load), and still snap back straight when the load is removed, than the 1018/

4140 pre-hard is probably the best choice if available. Often 1045 cold-drawn shafting is sold, and would also be significantly stronger than 1018, but not as good as 4140.

If the shaft is flexing under load enough, f'rinstance, to misalign the sprockets, but comes back straight when the rider gets off, then the only solution is a a fatter (stiffer) shaft
 
Thanks for the quick replies. So you are saying John that it doesn't really matter what steel I use because it is all going to be about as bendy? Hardness I certainly understand. I'm not really looking for hardness here. I thought of 4140 prehard ( I guess the same as 4140 ht or heat treated) but again I'm not so concerned with the "hardness" as the axle runs in pillow blocks and there really aren't any wear points. Are you implying that cold drawn would be less "bendy"? Finegrain, when you say tough are you referring to hardness or the bendy thing? I know enough about steel to know that some is very stiff while others may bend quite easily. I am obviously not a metallurgy wizard or I would know this. Thanks so much.

There are two different measures of steel "strength" that have been mentioned:
1) Stiffness more or less defined as how much deflection a, in your case, 1" bar will deflect if you put a measured load on the center of it while it is suspended by blocks at each end. That modulus of elasticity is, counterintuitively, about the same for all of the common steel alloys you might use in your shop whether hardened or not.

Then there is yield strength. That is a measure of how much a bar can be bent before it no longer springs back to straight but takes on some bend. Surprisingly 12L14 bests 1018 on that. But hardened steel like the 4140 HT RC about 30 is better than most alloys that are unhardened. 4140 HT machines nicely as already stated. Annealed 4140 is gummy and is difficult (for me at least) to get a good finish. I think most of the 4140 on sale on eBay is annealed unless they say otherwise.

Please forgive me if these generalities are not strictly correct. I think that is the essence of it though.

Denis

Edit: Magneticanomoly was typing faster than me....







;
 
Sorry for the many posts. I just looked up modulus of elasticity so I wouldn't sound like a complete idiot and this is what it says briefly:

"An elastic modulus, or modulus of elasticity, is the mathematical description of an object or substance's tendency to be deformed elastically (i.e., non-permanently) when a force is applied to it. As such, a stiffer material will have a higher elastic modulus." Soooo what I am after is something that will not bend as easy. This shaft is absolutely deformed permanently so it has not been deformed elastically. All I know is that after pounding on steel for 65 years I know that some is very hard to bend and some is not. It does however raise an interesting question. Maybe I want something that is quite springy so it bends and does not yield. For now I would like to go for something that does not bend or spring much.
 
Finegrain, when you say tough are you referring to hardness or the bendy thing? I know enough about steel to know that some is very stiff while others may bend quite easily. I am obviously not a metallurgy wizard or I would know this. Thanks so much.

Rigidity, in the sense that you can bend a rod with a given force, is unrelated to hardness. But, the ability to recover (spring back) is correlated with hardness. Look at the yield strength -- that's when the material deforms and no longer "springs back". 4140HT has a significantly higher yield than mild steel.

Toughness is a more slippery thing. Yield and tensile don't tell the whole story when it comes to factors like how much a keyway will wallow out over time and 1000's of torque reversals, or the likelihood that a part will snap instead of absorbing the applied force. 1144SP, for example, looks great based on yield and tensile, but in practice is fairly brittle so it would be a poor choice for an axle. There are testing procedures that give values for toughness (Charpy, Izod, e.g.) (http://www.youtube.com/watch?v=tpGhqQvftAo). 4140HT scores very well on these tests.

Regards.

Mike
 
How did you know Cole???? Actually it comes from riding through the woods and clipping trees. The rear tires are proud of the frame by about 6" and my Grandson does not quite know the difference between a near miss and a collision. I didn't go into a whole lot of detail but the plot does thicken somewhat. This is actually the third time he has bent the axle since Christmas day. I straightened the axle twice in the lathe using the cross slide and an oxy/acetylene torch. This needless to say is a PIA. At this point I am just going to make a new axle out of something that hopefully will be "stiffer". I am also going to make some "bark busters" if you know what they are.
 
Force necessary to deform 1'' 1018 shaft roughly 20,000lbs.

Force necessary to deform 1'' 4140 ht shaft roughley 40,000lbs.

Primary suplier of energy grandsons body hitting harness.



Build some kick rails.
 
I take it this is not a competition go-kart required to use a spec axle? It's presumably driven by a chain pulling a gear, said gear bolted to a mount of some kind? (Usually keyed at least on a shifter kart)
 
OK. Remember that diameter is your friend - so if the bearing blocks allow, going to a larger diameter should help. And of course at some point the axle stops bending and you wreck the frame instead.... (:-)
 
...1144SP, for example, looks great based on yield and tensile, but in practice is fairly brittle so it would be a poor choice for an axle...Mike

It's only "brittle" in the sense that the band between yield and tensile is fairly narrow, but with a yield of 100,000 PSI a lot of other bars are gonna be pretzels long before that point.

I see his real problem as that 6 inch overhang which nobody has mentioned. He'd do better to put some support out at the ends.
 
It's only "brittle" in the sense that the band between yield and tensile is fairly narrow, but with a yield of 100,000 PSI a lot of other bars are gonna be pretzels long before that point.

I see his real problem as that 6 inch overhang which nobody has mentioned. He'd do better to put some support out at the ends.

This will more good than all the other solutions. Get that bearing right up next the wheel. The bending moment varies as the cube of the length.

Tom
 








 
Back
Top