The importance of crack testing

[Tony Betts]

I have a lot of parts here I need to report the crack testing on for Geoff.

But I though I would bring this one to everyone's attention.

Sorting some parts out for a customer for spring beaulieu, I always crack test stub axles befor I sell them.

And this is why, it's only the start. I've seem them far far worse.

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[attachment=2717]

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Please excuse the mess on the bench, it's a result of to much work. And no time to tidy.

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This is the magnet I use, it's from a magnetic base to hold a gauge on the lathe.

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These are the crack test tins.

I do two light coats of the White for background.

Sit the item for crack test on an angle, and give it a good spray with the black. After the can is well shaken.

Tony.

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Well,

I've had a cup of tea.

Gone back out to get on with the beaulieu orders.

And here is the next stub axle, again ive seen worse. But we are now two tested, two cracked.

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[Tony Betts]

Third time lucky

[bob46320]

Do you find the magnetic spray better than the red dye penetrating stuff? When we use the magnetic, we had to pass a very low voltage and a few amps through the part to be tested. Clamped the bit to be tested between two lead jaws.
bob

[Colin Morgan]

Obviously crack checking is very useful to prevent critical bits being put back onto a car that might then fail sooner rather than later - and with Sevens we are fortunate in that there are generally plenty of parts around to test.  But I wonder how deep some of these cracks are - is the item scrap once a crack is found?  When rebuilding my rear axle, the diff carrier was checked - the old type from 1928 and earlier - and there was a tiny crack in it, so I swapped to the newer type rather than risk building it into a renovated rear axle.  However, if the crack were only a couple of mm deep or less, it could perhaps have been ground out, without the component seeing undue stresses in service?  This is perhaps a difficult call, as cracks usually occur at a stress concentration and a failure of this type tends to happen without warning - but I have kept the item in case one day it becomes worth the trouble for someone to attempt to repair it.  

Colin

[Chris KC]

I agree Colin. Just 'cos something's cracked doesn't mean it's scrap. A lot (most?) of us are driving around with cracked cylinder blocks! As long as you don't go crazy, grinding out or end-drilling a crack will reduce local stress tenfold or more. I think you have to wear different hats for different parts though - an axle housing seems unlikely to kill anyone. I'm not sure I'd advocate repairing a steering arm so long as new parts are available off the shelf. I certainly don't throw original bits away unless they are well and truly finished (even then I still keep them!)

[Colin Morgan]

Yes, the new manufacture steering arms have been engineered to improve fatigue life - it is good that they are available.  

Colin

[Tony Betts]

Do you find the magnetic spray better than the red dye penetrating stuff?  When we use the magnetic,  we had to pass a very low voltage and a few amps through the part to be tested. Clamped the bit to be tested between two lead jaws.
bob

Hi Bob,
Sorry for the slow reply, a bit busy.

I've used the dye pen in the past, but didn't get on with it very well. I'm sure if others use if often they would get better result.

The company that sells the cans, also sell the electric magnet machinery "thingy" to use with the tins. But these professional units can cost £2.5k. A bit pricy for home use.

You can see the results we get with a simple magnet, the cracks are quite obvious.

And yes I agree that just because something is cracked, does not mean it is scrap. BUT if it's a steering or brake part. It's your life and your risk if you use it.

For me if I know we're a part cracks, especially steering parts. I'll test them before I sell them. it's NOT a garrantee for the future use of the part. It shows customers the parts are in an acceptable condition when they buy them.    Think of it like an MOT ?

Tony.

[Stuart Giles]

Do you find the magnetic spray better than the red dye penetrating stuff?  When we use the magnetic,  we had to pass a very low voltage and a few amps through the part to be tested. Clamped the bit to be tested between two lead jaws.
bob

Hi Bob,
Sorry for the slow reply, a bit busy.

I've used the dye pen in the past, but didn't get on with it very well. I'm sure if others use if often they would get better result.

The company that sells the cans, also sell the electric magnet machinery "thingy" to use with the tins. But these professional units can cost £2.5k. A bit pricy for home use.

You can see the results we get with a simple magnet, the cracks are quite obvious.

And yes I agree that just because something is cracked, does not mean it is scrap. BUT if it's a steering or brake part. It's your life and your risk if you use it.

For me if I know we're a part cracks, especially steering parts. I'll test them before I sell them. it's NOT a garrantee for the future use of the part. It shows customers the parts are in an acceptable condition when they buy them.    Think of it like an MOT ?

Tony.

this thread has been an interesting read. I always use a penetrating  dye on important mechanical and/or "life or death" bits. This crank looked OK when tested with an Ardrox type dye and developer, but I decided to get it Magnafluxed to be absolutely sure whilst doing a fairly routine  "freshen up" on a motor.  



The lines indicate the (otherwise invisible) cracks in No. 4 big end. Bit of a b*gger that an apparently perfect crank is now a doorstop, but have at least avoided the carnage that would have been caused if that crank had let go at high revs.

BTW, one of the Magnaflux tests involves passing a current through the part, turning it into an electromagnet; cracks  with a 90 degree orientation are detected  by magnetising the part conventionally -all to do with the  direction of the magnetic flux lines in each test. I wonder how the aerosol magnetic test results would compare? certainly looks like something worth trying, I pay about £70 to get a crank Magnaflux  tested.

[Tony Betts]

hi stuart,

i notice the oilway is probably at 90 degrees.

it that a pheonix crank, or another modern crank.

thanks tony

[Colin Morgan]

I heard that mag particle tends to work better to find any cracks on used parts than dye pen because it can overcome grease or dirt lodged in a crack - does this sound right?  

Colin