Tired of breaking camshafts

[jdperform]

It seems once every two years we break a SBC camshaft right in front of number eight exhaust lobe. These are not 55 mm Cam journals. It is generally the 1.875 journals. I am thinking about getting the Cam Core shot peened before final grind. I have never seen a shot peened Cam core. It is impossible to magna flux a cam correctly with copper coating with an AC machine as they do not find sub surface discontinuities. These are medium aggressive lobes turning close to 9K. Has this been done? Or is there any other procedure to help prevent breaking of the camshaft? Circle track application.

[n2xlr8n]

There was a recent thread discussing this topic by a few knowledgeable individuals, iirc.

[Dan Timberlake]

Gear drive? Are you running a crank damper?

So the break is in the small space between #8 exhaust and #7 exhaust?
WAG I would not expect the highest stress to be there during a torsional party.

Same block?
Same cam manufacturer? What do they have to say?

Others will likely know if this points to an underlying valve train issue or something.

================

From the strength of materials side -

Got some pictures of the fracture surfaces? And the ( as-cast?) finish of OD on either side of the break?

Improving the surface finish and profile can do wonders , even before shotpeening a part free of "relevant surface indications."

[CamKing]

What's the barrel diameter of the core, where it's breaking?
What's the base circle diameter?

[Newold1]

What material billet cam core are you using?

What is your cam core supplier or manufacturer telling you?

What type of cam drive are you using?

This pretty much has to be unique to your part, spec. and set-up as I think this is pretty rare even at 9K rpms in a small block.

[CamKing]

This pretty much has to be unique to your part, spec. and set-up as I think this is pretty rare even at 9K rpms in a small block.

We were breaking them all the time in the early 90's, in the short stroke Dirt Latemodel engines(8,500-9,200rpm).
As soon as we went to 50mm journals, the problem went away.

[pamotorman]

you would think sprint car engines using the rear of the cam to drive the pump would have this problem

[MELWAY]

Do you have any pics of failed cam core?

A lot of the cheap cores have virtually No radius in critical areas

[hoffman900]

you would think sprint car engines using the rear of the cam to drive the pump would have this problem

Per Billy Godbold, the pumps act as a damper. He said on the Spintron, the valvetrain was mess until they realized they should probably hook up the oil pump and flow oil through it. Problem solved and now he jokingly wishes he could drive oil pumps from the cam on other applications.

[swampbuggy]

Interesting info. Bob, thanks--Mark H.

[englertracing]

you would think sprint car engines using the rear of the cam to drive the pump would have this problem

Per Billy Godbold, the pumps act as a damper. He said on the Spintron, the valvetrain was mess until they realized they should probably hook up the oil pump and flow oil through it. Problem solved and now he jokingly wishes he could drive oil pumps from the cam on other applications.

I had a conversation with someone about that, they said maybe the oil pump would do some dampening, but when you hit the scavenge stages with various slugs of air and liquid that it's a total mess

[hoffman900]

you would think sprint car engines using the rear of the cam to drive the pump would have this problem

Per Billy Godbold, the pumps act as a damper. He said on the Spintron, the valvetrain was mess until they realized they should probably hook up the oil pump and flow oil through it. Problem solved and now he jokingly wishes he could drive oil pumps from the cam on other applications.

I had a conversation with someone about that, they said maybe the oil pump would do some dampening, but when you hit the scavenge stages with various slugs of air and liquid that it's a total mess

Listen to the Power & Speed podcast with Billy. He discusses it towards the end.

[pamotorman]

i was referring to the external pump mounted outside the engine driven by the back end of the camshaft

[hoffman900]

i was referring to the external pump mounted outside the engine driven by the back end of the camshaft

I’m talking about that and the oil pump. They apparently also have the benefit of dampening velocity flucations in the camshaft. Just a happy accident really.

[340king]

In ME design class we learned that there were several failure modes that affect mechanical parts. The ones that we typically see in race engines are stress or rather too much stress. Then there is fatigue which might be culprit here, but it would depend on some things. Increasing surface hardness along with making it smoother might mitigate that issue. Another failure mode was chemical intrusion. I highly doubt that is something that could cause this. The last one was vibration. This one might be a component of one of the other issues, but I doubt it to be the main cause. The best bet is fatigue due to a repetitious or cyclical stress near the oil pump drive, assuming a rear distributor driven oil pump. If that is the case, then some notch root radius or surface imperfection might be the cause. The answer lies in the broken part and examining it.

If the broken cam has a fairly smooth clamshell looking finish to the separation area with a small ductile fracture looking area, then it is a fatigue crack propagation issue. The spring rate, open pressure, might be above the knee on the cam core exceeding the fatigue resistance stress level. Core diameter or core roughness or lobe to core radii or different hardening in the core might all be factors. Good luck.