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

 

[motorsportsdesign]

"Why not just go with a rocker from a manufacture like Comp cams? "

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

 

[icrman]

I didn't know that. Thanks for the info. I agree with you about investment castings.
Anyway I also go with the 4340 camp. Not a good place for tool steel.

 

[motorsportsdesign]

"Not a good place for tool steel."

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

 

[icrman]

If I understand correctly it is a micro structure, and fatique resistance deal.
Just rough machine your parts, do the heat treat, and finish machine them. How high are you going? 160ksi?

 

[motorsportsdesign]

I would like to make them as hard as possible maybe 58RC if I use tool steel. Having made crankshafts from 4340 there are some problems that I experienced that I would need to avoid for the rocker arms.

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

 

[NickE]

First the dimensional change cna vary depending on what austenitize/quench/temper/NxTemper is used. S7 tempered to make ~58HRc is going to change size by~+0.025%. D2 is going to change size by~-0.02% And A2 ~+0.075%. (Again the preceeding data is from TimkenLatrobe data sheets.)

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?

 

[motorsportsdesign]

"I still dont see how you are going to reduce weight with higher strenght material though. Care to enlighten me anyone?"

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

 

[NickE]

Yes, I do understand that lower strengths means that thicker sections must be used to withstand the same stresses. Although strength aside the stiffness of a mild steel rocker and a tool steels rocker with the same geometry is going to be the same.

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!

 

[motorsportsdesign]

"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?"

I intend to compete with EPD with a similar but slightly improved design.

Jonathan T. Schmidt

http://www.schmidtmotorworks.com

 

[icrman]

I know we have some better materials these days. But
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.

 

[NickE]

icrman- As hardness goes up so does Ultimate tensile strength.

Excepting impact loading, which may occur, I know very little about rocker arm loading conditions, higher tensile strength indicates longer fatigue life.

 

[patprimmer]

FEA can be quite economical these days.

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

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.

 

[motorsportsdesign]

"FEA can be quite economical these days."
Yes, I used FEA to remove weight anywhere that I could.

Jonathan T. Schmidt

http://www.schmidtmotorworks.com

 

[unclesyd]

There is a tool steel that we used for services quite similar to a rocker arm that took a lot of impacts and was subject to potential wear.
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.

 

[motorsportsdesign]

Yes, I am also looking into S7, I have heard of D2 but i have never seen it or machined it that I know of. I will check it out tonight.

Jonathan T. Schmidt

http://www.schmidtmotorworks.com

 

[patprimmer]

Then you should be able to analyse the stress levels and the number of required cycles and using the steel suppliers data, you can estimate weight, deflection in use and number of cycles before cracks develop.

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.

 

[unclesyd]

One thing not generally mentioned about using tool steels for mechanical components is one particular aspect of the heat treatment.

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.

 

[Tmoose]

http://www.key-to-steel.com/Articles/Art137.htm

"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