Shotpeening conrods

Road-race engines and ancillaries - general discussion
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ICI
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Shotpeening conrods

Post by ICI » Tue Feb 13, 2007 6:09 pm

Hi everyone

I did my Tipo 16V conrods lightened(701,8 grams), balanced and polished but not yet shotpeened.
Here in my city is a big factory, using similar treatment.
They mould cast iron machine parts and blast them with 1mm steel spheres through turbine.This is the finest medium they have in use.
The chief engineer there is concerned of possible twisting of small and big end axis, because of blasting too strong.
They never shotpeened conrods before and I need some guiding lines of the process like speed, distance, time for treating, etc.

Thanks in advance
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Guy Croft
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Post by Guy Croft » Tue Feb 13, 2007 6:42 pm

You'll need to buy certain published data to get the answer you want unfortunately, this is one place to start:

http://www.shotpeener.com/cgi-bin/search.cgi

I am not a fan of links but sometimes for copyright reasons it is unavoidable, there is a very good article here, I suggest you read most carefully, because there are some top firms under the microscope here who really have done it with conrods.

http://www.mtce.com.au/peenreport.htm

It's not my business - but - for the cost - I would not bother shot peening those rods, they are of such poor grade material, only heat treated En8 or so, unfortunately I discovered this by metallurgy test after my TC book went to print, having blown up an expensive NHRA TC engine in the process.

GC

sumplug
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Post by sumplug » Tue Feb 13, 2007 7:21 pm

I know from experiance, Tipo 16v conrods twist very very easily!!

Andy.

Uros Piperski
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Post by Uros Piperski » Tue Feb 13, 2007 7:25 pm

Andy, Guy, are the Delta and Dedra integrale 8v rods any better?

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Post by sumplug » Tue Feb 13, 2007 7:55 pm

8V Integrale rods are exceptionally strong and have a bolt only fixing compared to other 8v rods. Shot-peening and tuftriding after ligtening and polishing can increase the fatigue life by a big margin.

Andy.

ICI
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Post by ICI » Wed Feb 14, 2007 1:00 am

Is there any difference in material and strenght between Tipo 16V and Integrale 16V conrods ?
Actually my concern is are they will withstand to apr.300 bhp and 400 Nm torque?
Thanks again
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Guy Croft
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Post by Guy Croft » Wed Feb 14, 2007 10:04 am

The Integrale rods are OK up to a point but not exceptionally strong. That said it is high rpm and oil starvation that breaks rods. No high power/torque per-se. Of course a turbocharged engine can develop very high power at low-ish rpm, a 2 liter n/a engine needs high rpm. So a turbocharged engine could well survive almost forever on OE rods if the revs are kept down.

I lightened, polished and shot-peened a set of new OE 8v Integrale rods for an NHRA engine in 1996 to replace an older 131 set (again - post publication of my TC book unfortunately as I said above). That set suffered a one-off rod fracture and I had them metallurgically tested by a consulting metallurgist from Kent Heat Treatment. The metallurgy testing (including scanning at x 600 magnification) on both the broken and undamaged Integrale bolt-only rods and and the prior-used 131 2 liter nut/bolt rods identified the Integrale rods as being weaker than the 131 rods.

The report said firstly:

" Visual examination of the failed sample showed clear evidence of fatigue crack markings emanating from the top of the I beam section on the outside edge a little way down from the small end.."

GC note: Ferrous materials like rods only exhibit fatigue when exposed to cyclical stress in excess of endurance strength of the steel. If the endurance strength is not exceeded, they will never break in that section, though remember that there are several risk-prone regions in a rod, ours just happened to have a serious weakness below the small-end. My stress tables indicate that this state is reached at around 7800-8000 rpm, based on the Integral rod of hardness HRc 18, endurance strength appr 40% of UTS, 284MPa with a stress concentration factor of 2.5 - which is fairly realistic when you read what else the report had to say:

" .. there appears to have been and attempt to harden and temper the rods but only the old type of rod shows a correctly hardened and tempered structure which explains, at least in part, why it had better durability. The presence of a decarburized layer from the forging operation is another undesirable feature that gives a weaker zone on the surface which acts as a preferred site for fatigue cracks. Surface shot peening helps to reduce this effect but this is only partially effective."

Yes the later rods have bolt only fixing, but according to my calcs the rod bolts should have broken first based on a 1.5 stress factor. They didn't - and you can see now why the general I beam section has to carry such a high stress concentration factor in any realistic assessment of the rod strength. Having bolt-only isn't much help if the rod is let down by a catastrophic metallurgical weakness in the beam section, is it?

In summary - don't use production rods in race engines is my strong advice. There is a short article in GC V/W about rods too.

GC

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Post by Maki » Tue Feb 20, 2007 8:47 pm

I may sound funny, but what's the big deal in the shotpeening the rods anyway?
RST ZVH

Guy Croft
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Post by Guy Croft » Wed Feb 21, 2007 9:46 am

The reasoning behind shot peening is this:

Imagine a con rod being like the arch on a bridge. The upper section there is in tension and the lower is in compression. On a component subjected to tensile stresses - like a conrod which is subjected to a bending load to left and right during the cycle - there is an alternating tension/compression load on the outside of the I beam section - bit like the upper part of the bridge. That almost constant subjection to tensile load can cause fatigue cracks and failure in the surface of the material where the bending stress is highest. By shot-peening - firing shot (normally steel) at the surface - it is possible with controlled methods to induce a layer of compressive stress that prevents the surface layer from being subjected to a fluctuating tensile stress.

It can be used to overcome other stress-related problems too, like torsional stress in hollow section camshafts and bending stresses in gear teeth. It is certainly a proven and much used technique, especially in the aerospace industry. The air pressure and shot size, proximity of the blast gun, number of passes are wroked out using a simple device known as an 'Almen' gauge, a strip of material representative of that which you're going to work on. The strip will deform an amount according to the peening parameters. That's the easy bit. However, getting the level of deformation correct and thus the right kind of compressive layer requires a lot of 'science' and cyclical load testing to make it fully effective.

You can make most parts a 'bit' stronger by DIY shot-peening but industry standard peening - provided the substrate material is right for the applied loads, can actually prevent them from ever failing at all.

GC

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Post by ICI » Mon Mar 12, 2007 10:00 pm

The conrods already shotpeened
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Yugo_Turbo

Re: Shotpeening conrods

Post by Yugo_Turbo » Thu Jan 10, 2008 11:53 am

They look awesome, only.... that angle on I beam, where its meeting with small-end(sorry , my english is not that good)...it looks a bit sharp?
How much the shotpeening costed you?

Best regards from Serbia!

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Re: Shotpeening conrods

Post by ICI » Thu Jan 10, 2008 9:27 pm

About this edge, it looks sharp but it is nicely rounded. I just followed the shape of I-beam till the small end.
The shotpeening cost me nothing, unbelievable in these times :)

Regards,
Iliyan
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cos
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Re: Shotpeening conrods

Post by cos » Fri Jan 11, 2008 2:39 pm

I have a set of conrods from the 2.0 ltr 130 Abarth engine that have been shotpeened for sale (i'll add this to the for sale section as well- but it's relevant here) Originally were to be used on my engine project but will fit steel rods instead. Shotpeening costs around £30-35 per rod. I have my set for sale for £100 + postage if anyone is interested. Can email photos if you send me a pm.

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Re: Shotpeening conrods

Post by Sealord69 » Sun Apr 08, 2012 7:13 pm

I have a company who does shotpeening for me, but what about protection of the bushes and cheeks.

Is it enough to leave an old wrist pin and the old bearing in the rod, to protect the area? What about the cheeks, can they be shotpeened, too and grind the with 400 grade again later or should I protect them with Metall plates?

thanks Nils
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Re: Shotpeening conrods

Post by Guy Croft » Tue Apr 10, 2012 9:14 am

Nils, hi

re:

"Is it enough to leave an old wrist pin and the old bearing in the rod, to protect the area? What about the cheeks, can they be shotpeened, too and grind the with 400 grade again later or should I protect them with metal plates?"

No. If you rely on a pin to protect the bush there will be distortion of the outer region of the bush - one problem there is you may have problems trying to get the pin out! You need a bolted-in protector there and at the big end as well. You can shotpeen the cheeks but yes you need to restore the surface to a 'ground' finish (for oil retention) after and the way you suggest is OK.

G

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