Steel for rocker arms machined from plate
Steel for rocker arms machined from plate
(OP)
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





RE: Steel for rocker arms machined from plate
Mike Halloran
Pembroke Pines, FL, USA
RE: Steel for rocker arms machined from plate
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
RE: Steel for rocker arms machined from plate
RE: Steel for rocker arms machined from plate
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.
RE: Steel for rocker arms machined from plate
RE: Steel for rocker arms machined from plate
I doubt that the rocker arms will get as hot as 300F
Jonathan T. Schmidt
http://www.schmidtmotorworks.com
RE: Steel for rocker arms machined from plate
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
RE: Steel for rocker arms machined from plate
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.
RE: Steel for rocker arms machined from plate
RE: Steel for rocker arms machined from plate
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...
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))
RE: Steel for rocker arms machined from plate
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
RE: Steel for rocker arms machined from plate
Jonathan T. Schmidt
http://www.schmidtmotorworks.com
RE: Steel for rocker arms machined from plate
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.
RE: Steel for rocker arms machined from plate
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.
RE: Steel for rocker arms machined from plate
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...
RE: Steel for rocker arms machined from plate
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.
RE: Steel for rocker arms machined from plate
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
RE: Steel for rocker arms machined from plate
RE: Steel for rocker arms machined from plate
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.
RE: Steel for rocker arms machined from plate
Those rockers are considered junk for serious competition engines. They are investment castings so they are not nearly as strong as a piece the same size made from bar or plate.
Jonathan T. Schmidt
http://www.schmidtmotorworks.com
RE: Steel for rocker arms machined from plate
Anyway I also go with the 4340 camp. Not a good place for tool steel.
RE: Steel for rocker arms machined from plate
Why do you say it's not a good place for tool steel?
I will have to remachine the critical features after heat treating if I use 4340.
Jonathan T. Schmidt
http://www.schmidtmotorworks.com
RE: Steel for rocker arms machined from plate
Just rough machine your parts, do the heat treat, and finish machine them. How high are you going? 160ksi?
RE: Steel for rocker arms machined from plate
The threads can't be tapped practically after heat treat if I go above 40RC. If I tap them before heatreat the threads are rusty not smooth enough for a rocker arm.
My experience in air hardening heat treat has been much better in this regard.
Jonathan T. Schmidt
http://www.schmidtmotorworks.com
RE: Steel for rocker arms machined from plate
Second: YOu might be able to find a heat treater who can process your parts in an inert atmosphere start to finish. This will give a much cleaner final part with little scale.
I still dont see how you are going to reduce weight with higher strenght material though. Care to enlighten me anyone?
RE: Steel for rocker arms machined from plate
Because the cross section required to accomplish the same strength is less when higher strength material is used. This seems rather obvious to me, is there something more to your question that I am amissing?
For example, the design of an mild steel rocker will be much more bulky than a tool steel design because mild steel is not as strong as tool steel.
Imagine something basic like a bolt, an mild steel bolt would have to be much larger than a tool steel bolt to acomplish the same strength.
Jonathan T. Schmidt
http://www.schmidtmotorworks.com
RE: Steel for rocker arms machined from plate
It appears to me that at some point the section size is going to be too small to retain the stiffness required to perform the function. Admittedly geometry can be used to overcome this, but at some point there is no way to reduce weight any further without going to a stiffer material.
It seems that EPD's design is lighter than the aluminium ones and is a highly optimized design, are they that expensive that manufacturing your own is of any benefit?
good luck.
Nick
I love materials science!
RE: Steel for rocker arms machined from plate
I intend to compete with EPD with a similar but slightly improved design.
Jonathan T. Schmidt
http://www.schmidtmotorworks.com
RE: Steel for rocker arms machined from plate
from the information I have studied. The harder the steel is, the lower the resistance to fatique. And a rocker arm is something that you want to have a fairly high fatique resistance on. And especially if you are planning on a thin cross section. That means you will have higher stresses. You relize most of the stress is carried on the surface of the structure, and this is the reason shot peening improves fatique resistance.
It sounds like it is something you are going to have to experiment on. I think your on the right track. There isn't much available as far as a "good" rocker arm.
RE: Steel for rocker arms machined from plate
Excepting impact loading, which may occur, I know very little about rocker arm loading conditions, higher tensile strength indicates longer fatigue life.
RE: Steel for rocker arms machined from plate
You could test several designs in several materials that way, and remove any material that does not contribute to strength, and also eliminate hard and soft spots that cause stress concentrations.
Regards
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RE: Steel for rocker arms machined from plate
Yes, I used FEA to remove weight anywhere that I could.
Jonathan T. Schmidt
http://www.schmidtmotorworks.com
RE: Steel for rocker arms machined from plate
The material was S-7 and in particular Bearcat made by Bethlehem Steel. I'll try to dig up my information on it. In it's day it had the highest impact properties of any tool steel available. In fact the standard CHARPY impact specimen wouldn't break under maximum load.
This material was used in all pneumatic tools for years, both as the anvil and hammer along with a few other parts.
We also use D2 in all our gear pumps where the teeth were highly loaded and the material being pump was fairly abrasive. The beauty of D2 was very predictable with size changes when heat treating small sections.
RE: Steel for rocker arms machined from plate
Jonathan T. Schmidt
http://www.schmidtmotorworks.com
RE: Steel for rocker arms machined from plate
Regards
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
RE: Steel for rocker arms machined from plate
Look at the heat treating curves for each steel and if the material has a secondary hardening temperature use the material at or either slightly on either side of the secondary hardening peak.
To assure maximum wear resistance of a tool steel you will need to get the hardness above Rc 58.5.
You can use as cryogenic treatment as part of the heat treating process if possible. This will ensure maximum stability with a minimum number of tempers. On some tool steels and under certain conditions we have also seen greatly improved wear properties along with the increased stability.
RE: Steel for rocker arms machined from plate
"The results indicate that below a tensile strength of about 200,000 psi (~1400 MPa) the fatigue limits of quenched and tempered low-alloy steels of different chemical composition are about equivalent when the steels are tempered to the same tensile strength. This generalization holds for fatigue properties determined in the longitudinal direction of wrought products. However, tests have shown that the fatigue limit in the transverse direction of steel forcing may be only 60 to 70 percent of the longitudinal fatigue limit. "
http://www.shotpeening.org/ICSP/icsp-4-29.pdf
http://www.metalimprovement.com/images/fat1.gif
The endurance limit is not a true property of a material, since other significant influences such as surface finish cannot be entirely eliminated. However, a test values (Se') obtained from polished specimens provide a baseline to which other factors can be applied. Influences that can affect the endurance limit include:
Surface Finish
Temperature
Stress Concentration
Notch Sensitivity
Size
Environment
Reliability