Crank Broke . wtf .

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Matt Spencer

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O.K. want the failure pictures , preferably with close up of matching sides / break .
Can be morbid and explain what happened too .

Clearances in Bearings & big ends ( to spec , or outside ) of relevance also .

presumeing theyre torsional failures from pics Ive seen .
 
Most Norton crank breakages (all ?) are reported to be due to not correctly grinding the radius on the journals when the journals are reground undersize.
This is noted in several bulletins over the years....

Having problems with your Norton cranks, are you Matt ?
 
Only the one called Rohan , at the moment . :p :D more looking for overworked abused grenaded evidace of order of failure / distortion .
Wathe electrical problem the left or right rod thru the wireing loom . :( :shock: :?
 
I've had several crankshafts crack at the drive side mainshaft, right at the angle between mainshaft and counterweight cheek, but didn't take any pictures, just chucked them. Fortunately, there's a characteristic onset of vibration when they crack, and it's pretty recognizable. It starts as a vibration at redline, and then spreads to progressively lower rpm, and finally breaks. The vibration also increases in magnitude as it spreads down the rpm range. The last one I had do that took two laps at Willow Springs to go from a vibration that began at 7,000 rpm to one starting at 5,500 rpm, and that's when I shut it down. It was a factory short stroke crank, with the usual radius the race shop machined at the junction of mainshaft and cheek, as well as being shot peened, but it still cracked, and would have grenaded if I'd kept going. To be fair, it did have a lot of race miles on it. Most of the cracked crankshafts I had were discovered during teardowns. I always checked the crank and rods for cracks when I had the engines apart, but that was back before billet cranks and Carrillo rods. I did have one that cracked at the drive side rod journal, but we discovered that the cavity in the journal was off center a lot, and that the metal was thin at the location of the crack. And yes, the journal had the proper radius. I'm pretty sure that the mainshaft cracks are a result of fatigue from the mainshaft bending at the cheek as the crank speeds up and slows down with each power pulse. They always seemed to start in the same location on the shaft circumference. Just my humble opinion, though.

Like Rohan said, grinding the journals undersize without the radius is a guarantee of eventual failure, at least in a race engine. I never experienced that problem, but I've known a couple racers who have. Most automotive crankshaft grinders are reluctant to dress the large radius onto their expensive grinding wheels, because they just have to dress the wheel down to a smaller diameter afterwards to get a sharper radius for their automotive work. Those large grinding wheels aren't cheap, and the shops are reluctant to use them up any faster than they have to. Fortunately, there are at least a few shops around the country now that are willing and able to do the Norton cranks properly.

Ken
 
I rode home with a broken crank! Interference fit kept it together! :roll:
 
This is a souveneer of what Phillip Island circuit can do to your stock crank, as Ken described unusual vibration caused me to retire, the crack was not as gruesome as the pics depict as when we found the crack with the aid of spray on flaw finder I put it under the press for a better look at the extent of the crack. So no it was not a hand grenade event. This crank had often spun up to 8000 rpm when I was using the 2s cam, then I went to the 4s and kept the revs to 7500. It survived 12 meetings before the fateful Phillip Island event in 2005
Fred.



Crank Broke . wtf .


Crank Broke . wtf .
 
lcrken said:
Most automotive crankshaft grinders are reluctant to dress the large radius onto their expensive grinding wheels, because they just have to dress the wheel down to a smaller diameter afterwards to get a sharper radius for their automotive work. Those large grinding wheels aren't cheap, and the shops are reluctant to use them up any faster than they have to. Fortunately, there are at least a few shops around the country now that are willing and able to do the Norton cranks properly.

Ken
Click to expand...

This is because these shops have finally splashed out and brought another grinding wheel arbour to mount a radiused wheel on, which they keep for cranks with the relevant radius.
 
For what it is worth, I have experienced one failure near exactly as that depicted in the pictures above. Another two wer circumfrential and concentric to the filet radius on the drive side main shaft and one was a catastrophic cast iron fly wheel failure.

Not aware of any other modes.

On the pictures above I never could determine whether the crack started at the oil hole and propogated to the filet or at the filet and propogated to the oil hole. Have an opinion on this fredful? With the failure in the picture, I was able to get some distance on the bike before shutting it down. there was a marked change in vibration and a gradual loss of power.
 
In the late 1980's, I went with some friends for a tour around Scotland on my 920 Norton. On the way to the meeting place, I started to hear a rattle on acceleration only. I subsequently went on to do about 700 miles before getting home from the tour, engine still intact and rattling just a bit louder. When I stripped it I found that I had a crack across the drive side journal, through the oil feed hole.

I couldn't find a Commando crank back then and IIRC I put a Dominator crank in to that engine. It was never as good after that crank was fitted. It's a good job that I never knew what the problem was whilst I was riding the bike on the tour. I was pretty unhappy at "the noise" but to know the crank had cracked would have completely ruined my holiday.
 
Dances, On closer examination it looks like the crack propogated on the inside of the cavity where the one inch drill left a nice sharp edge, A radius on the cutting edge of the drill would have prevented this, heres another snap from the inside, Enjoy!

Crank Broke . wtf .
 
In the aircraft industry, they'd probably look at the fracture, and find either a material fault (inclusion in the metal) or a sharp edge from a machining mark to cause that.
Since its not a common fault.

Assuming there are no hacksaw marks in it !
 
Hobot,

I got off really lightly - the only other component destroyed was the alternator stator. The bottom of the bores caught a little bit of damage but just at the lead-in so easily dressed up.
I didn't get it inspected for a professional opinion, but I suspect there will have been chisel damage from previous attempts at getting the inner race off.

David
 
fredful said:
On closer examination it looks like the crack propogated on the inside of the cavity where the one inch drill left a nice sharp edge, A radius on the cutting edge of the drill would have prevented this
Click to expand...

Here's something the often controversial beltdriveman/BDM had to say on the subject of Norton crankshafts and stress-raisers on the Britbike forum.

http://www.britbike.com/forums/ubbthrea ... #Post66967

Just for Mr Jones and his rebuild it from the manual LETS JUST LOOK AT WHAT THE MANUAL DOES NOT TELL YOU ABOUT ONE BIT.....THE CRANK
With the crank in bits and nice and clean do you look up inside the D.S. hal to se if the drill some idiot at Nortons shoved up it to clean it out did not leave a perfect example of a stress raiser directly beneath the big ends outer 90 thou radius which is incorporated to REDUCE stress concentarions at this point of VERY HIGH stresses?? No point in telling the crank grinder to ensure they include the 90 thou radius if there is a stress raiser directly beneath it..is there Mr Jones. I had a 99 crank crack but not totally fail at this point decades ago in my ignorant youth. It took a few blows with a 14 lb hammer to get the cracks to go all the way round leaving that crank bit in two pieces... it knocked something awful at 7000 which was where the revs were normally in those days. HOWEVER when the olde so called 'classic' racing Atlas't had the crank break at the same point whilst leading a race at Oulton Park I thought 'this looks familier I wonder? and phoned my friendly Marconi Co Senior Metalurgist and asked him to take a look to determine the cause of failure. Within probably micreseconds of the internal post system delivering the bits he was phoning me asking' Who was the idiot who introduced that stress raiser in that position.....?' to which I replied 'What the ***is a stress raiser' to which he repiued something like 'Something every Engineering student is taught about for one day if they are lucky which they then totally forget in later life and in so doing kill innocent people'...like the one introduced into the Comet airliners that fell out of the sky not only killing LOTS of innocent people but also the British Aircraft industry damn nigh instantly stone dead giving the World market to Boeing and their 707s.... He also gave me a list of books to read which I did and in so doing learnt a tad about stress raisers so that when the Atlas appeared back from France one year after winning a Classic French GP and i noticed the broken base flange on the barrel it only took a nanosecond to scream 'what twat machined the base flange leaving that stress raisewr there?'.....the answer to which I was told is some pratt in the Plumstead D.O. As for the crank stress raiser on pointing it out to Mr Negus whilst visiting Shenstone decades ago he later told me he had had the crank drawings ammended so it was removed but that whilst they had initially thought it would cause a balance problem the test cranks being machined by Laystall Engineerings' Wolverhampton Crankshaft Division at that time balanced out OK but due to the very high machining costs non were to be produced for spares
Then of course I dont suppose many, because it aint in the manual, ever ensure that at least one of the two 'top' bolts are a good fit holding all together and then put the crank up between centres to ensure both big ends are in line...... Mr Lovell has just done that with my Mk2a crank BEFORE taking it apart and found there was an 8-10 thou difference. It isnt NOW because after removing the stress raiser,checking the crank half for cracks and reassembled the crank and correctly alighed it he reamed out, one at a time, the 4 accessable holes and fitted fitting bolts. In my young days I once found every set of crank bolts/studs in stock in a friends shop to ALL be well undersize and in reply to my enquiry of the designer whose name I wont mention for fear of upsetting a pratt or two he wrote back stating that at least one of the two top bolts opposite the centre locating dowel should be a fiting boly. He also stated more would of ben fiting holes but that Norton did not have the capacity for accure maching of 3 lumps so the holes all lined up. I still have the letter. I wonder how many peoples 500/600/650/750 motors vibrate because they bought incorrectly made sets of bolts/studs? A friends 650SS in its Manx rolling chassis goes to 8 with very litle vibration but he also selects the main bearings for ones that give virtually zero internal clearance with a cold motor so the crank doesnt bounce up and down due to excessive internal clearance with the motor hot.... But things like that aint in the manuals either. NOR funnily enough is how to remove the crank stress raiser that allows the alternator rotor lump to break off on later cranks...... One friend can sometimes be found repairing such cranks that have broken....
Click to expand...
 
Thanks for posting that, L.A.B. I hadn't run across that before (or, equally likely, I have, but have forgotten), and it's a great read.

Ken
 
In 2003 I was racing at Shannonville against a Yamaha XS750 that had me out horsepowered; in a bid to stay with with him I over-revved the Norton a bit- as I crossed the finish line I felt the motor tighten up and then it stalled on the cool down lap. When Herb stripped the motor down, this was what he found. If the race had been one lap longer, the crank would have undoubtedly flown apart. Looks like it started in the oil hole. Crank had a Steve Maney flywheel attached at the time.

Crank Broke . wtf .


Crank Broke . wtf .


Crank Broke . wtf .


Crank Broke . wtf .


Crank Broke . wtf .
 
yep, stock cheeks. The only mod was just the mounting holes bored out for larger bolts.
 
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