Steel for rocker arms machined from plate 1

[motorsportsdesign]

I am planning the construction of some rocker arms machined from steel plate. I am not a steel expert so the best guess I have is to use H11 or H13. Any better suggestions? Material price should be under $20/lb to keep it practical and something reasonably machinable with carbide.

Jonathan T. Schmidt

http://www.schmidtmotorworks.com

 

[MikeHalloran]

You are aware that tool steel requires heat treating (in this case an elaborate cycle) to develop its best properties, right?



Mike Halloran
Pembroke Pines, FL, USA

 

[motorsportsdesign]

"You are aware that tool steel requires heat treating (in this case an elaborate cycle) to develop its best properties, right?"

Yes, I've made thousands of parts from H13 for molds and dies. I am just interested to know if there is something better at up to about 4 times the price. I am looking to save weight on there parts so if there is a stringer material, I want to consider it.

Thanks

Jonathan T. Schmidt

http://www.schmidtmotorworks.com

 

[ornerynorsk]

Why not use 4140 at about $1 per pound?

 

[NickE]

I agree with Norsk, the H11 and H13 tool steels have specific composition and heat treat for high red hardness. If your rocker arms are red hot you could be in trouble. I would depending on section thickness use either 4140 or 4340 with a quench and temper. You could easily get strength of ~200ksi, if you need more strength you could try 300M and get ~280ksi.

I think that with the section size you are going to need to get the required stiffness, you will likely have almost the same weight with any steel alloy.

 

[Tmoose]

I think if I was making rocker arms for either valve trains of suspension the design would be driven first by stiffness, then by (fatigue) strength. Machining them from billet starts off on the wrong foot by scarificing grain optimization, but has the advantage of putting material right where it is needed. A surface process like shot peening does wonders for endurance/fatgue too. I wonder if the requirement for extreme heat treat could be designed out?

 

[motorsportsdesign]

" If your rocker arms are red hot you could be in trouble."

I doubt that the rocker arms will get as hot as 300F

Jonathan T. Schmidt

http://www.schmidtmotorworks.com

 

[motorsportsdesign]

" I would depending on section thickness use either 4140 or 4340 with a quench and temper. You could easily get strength of ~200ksi, if you need more strength you could try 300M and get ~280ksi. "

I have a friend that makes billet crankshafts from these materials. He suggested that I look for something stronger.

I would gladly pay 5X the price of 4340 to get 10% more strength.

Jonathan T. Schmidt

http://www.schmidtmotorworks.com

 

[gearHTr]

More questions than answers: Is there any concern for vibration dampening that cast iron provides?
Can you use a S-grade or O-1 tool steel?
Regardless of material, machining from bar instead of plate would give better strength from grain flow.

 

[swall]

Motorsportdesign--keep in mind that in alloy steels, all of your realized strength comes from heat treating. Max hardness depends on carbon content. Tensile strength equates to max hardness with a 90% martensitic structure. Both H12 and H13 tool steels have .35 nominal carbon content, so you can't get as high a strength level as you can with 4140 or 4340. The main thing that all of the extra alloy does in tool steels is to help form 90% martensite at the center of the heaviest section. So, unless you are making huge rocker arms for a marine diesel, 4340 is all you need. If you have unlimited funds, consider Aermet 100 which is a maraging steel and has excellent fatigue properties.

 

[NickE]

Grain direction is'nt going to be the limiting factor here. The limiting factor is stiffness. No matter what steel alloy you use you are going to be stiffness limited in your design.

This stiffness limiting requires that you use a specific geometry that uses teh 200GPa of modulus the most effectively. All steel alloys have roughly the same modulus. This modulus does not change significantly due to heattreatment or alloy composition. Going to higher strength materials is not going to reduce weight.

Try running a MatWeb search using modulus as the property to search by, get all metals with a modulus >40,000ksi. You'll notice that all thats left are beryllium, osmium, molyebdenum, iridium, rhenium, rhodium, and tungsten are in this grouping. These alloys are much more (>5x) expensive to purchase, not to mention working, machining etc...

"I would gladly pay 5X the price of 4340 to get 10% more strength."

Again, you are going to have a really hard time finding anything with tensile strength greater than the ultra-high strength alloy steels. (300M & 4340) Elgiloy at ~$160/lbs comes to mind. There are others, but again costs for the alloy itself can get extreme quickly, not to mention heattreating etc...

(I do know of a part that was manufactured by WireEDM in 3 dimensions from S7 tool steel. Then the part was hardened to HRc 59. I'm not sure how much they cost to make, we were trying to have it quoted and all we got was RTQ (refuse to quote))

 

[motorsportsdesign]

Thanks for the input.

I will exmain the suggested materials.

I would prefer not to use an oil hardening steel as I think it may be possible to heat treat an air hardening steel piece with no need for after heat treat machining.

This rocker arm is used in similar application as the ones made by Jesel

http://www.jesel.com

Jonathan T. Schmidt

http://www.schmidtmotorworks.com

 

[motorsportsdesign]

I have used S7 before for some mold parts before. Standard practice in molds is to use S7 where the part may be required to flex some. Does anyone know the characteristics of S7?

Jonathan T. Schmidt

http://www.schmidtmotorworks.com

 

[NickE]

S7 can have really high strengths and still retain some impact toughness. I have been searching for standard tensile data on tool steels. Using a hardness to tensile strength conversion chart on my cube wall gives ~340ksi UTS for 59HRc. That coupled with a charpy V-notch toughness of >12.5ft-lbs (I'm not exactly sure about that number though it just doesnt seem right, it is however from a Timken-Latrobe data sheet. Especially considering that Crucible's sheet gives a value of 125ft-lbs for the same test at HRC57.)

I think that one problem you may encounter when investigating tool steels for non-tool parts/components is the lack of valid data regarding these uses. It'd be nice to know the yield strength of HRc50 A2 for example.

good luck.

 

[unclesyd]

You can usually find all the physical data you need on tool steels if you look in the older mills technical data books , like Bethlehem, Vanadium Alloy, etc. The data is still around but getting the information has become a pain in rear.
You would normally look at the trade names of the different steels, like Bethlehem's S7 is called Bearcat. Bearcat has the highest impact strength of any of the older tool steels.

I'm trying to get my old books on tool steels returned.

 

[Rob45]

In examining the Jesel rocker arms, I see they're made of aluminum, suggesting that for example ultimate tensile strength of the material isn't the major design criterion.

It would seem that if you wanted to approximate the Jesel rocker arm shape in steel, you could use just about any steel.

Unless I'm missing something here...

 

[NickE]

"In examining the Jesel rocker arms, I see they're made of aluminum, suggesting that for example ultimate tensile strength of the material isn't the major design criterion."

Also this suggests that I was also wrong in stating that the design is limited by stiffness. Obviously stiffness and strength are important, however it seems from reviewing the site that the geometry creates the required stiffness, and that 2024 is strong enough also.

The way I would try to re-design is by optimizing the geometry for the stiffer steel. Then you coudl adjust shape a bit to modify the actual strength. Then you could use thsi to do a material selection. Although I doubt this will reduce the weight below that of the aluminium rockers.

 

[motorsportsdesign]

The life of the aluminum rockers is very short. Some people are making stell rockers at about the same weight that are lasting at least ten times longer that the aluminum rockers.

The reason that I need the strongest material that is practically affordable and machinable is to allow a reduced weight design.

Here is an example of a steel rocker.

http://www.epdracing.com/rockerphoto.htm

Jonathan T. Schmidt

http://www.schmidtmotorworks.com

 

[NickE]

Ahhh that makes more sense, then why didn't you say that you were trying to improve fatigue performance. That is definately governed by UTS. I think that 300M is going to be a good choice. Also you might want to try A2/D2/S7 or other air hardening toolsteel. Just remember: dimensions change on heattreat. A final grind is definately going to be important for tight dimensional tolerances.

 

[icrman]

Why not just go with a rocker from a manufacture like Comp cams? They are supposed to be lighter than the Al ones.
Remember the harder the alloy steel the greater the notch sesitivity. Meaning if there is a small imperfection like an inclusion or stress riser of some sort, its gona break.