06-05-2020, 09:53 PM
I have been involved with Jowett Javelin cars for 50 years so, following on from the Seven, have given the topic of crank failure some thought!
At 60 mph, a mile a minute, Sevens are doing in excess of 4,000 rpm, or 4,000 revs per mile. So over whole life average say 5,000 revs per mile. 100,000 miles, which most non sport model original 2 brg cranks exceeded in stock engines, is 500 million revs. In tests of steel samples at constant loading a stress less than that giving a life of 10 million cycles usually ensures everlasting life. Presumably stresses seldom seriously exceed the fatigue limit. So 10 million or less of 500 million revs cause failure, 1/50th of use. The question is which? Is it the 50 mph full throttle, 50 mph overrun, 35+mph in 3rd, the 20 mph in top with advanced spark etc?
A small increase in stress hugely reduces life ie 10% may halve, and many have cr raised more than that. Rev related stress goes up with square of revs; so a 5% rev increase raises stress 10%.
(Jowett cars had cracked or broken cranks after Le Mans, 10 million revs including testing? If anyone here is interested I can very briefly recount the saga from and to reasonable reliability of the production Javelins with 5 variants of factory crank, including finally a nitrided one.)
At 60 mph, a mile a minute, Sevens are doing in excess of 4,000 rpm, or 4,000 revs per mile. So over whole life average say 5,000 revs per mile. 100,000 miles, which most non sport model original 2 brg cranks exceeded in stock engines, is 500 million revs. In tests of steel samples at constant loading a stress less than that giving a life of 10 million cycles usually ensures everlasting life. Presumably stresses seldom seriously exceed the fatigue limit. So 10 million or less of 500 million revs cause failure, 1/50th of use. The question is which? Is it the 50 mph full throttle, 50 mph overrun, 35+mph in 3rd, the 20 mph in top with advanced spark etc?
A small increase in stress hugely reduces life ie 10% may halve, and many have cr raised more than that. Rev related stress goes up with square of revs; so a 5% rev increase raises stress 10%.
(Jowett cars had cracked or broken cranks after Le Mans, 10 million revs including testing? If anyone here is interested I can very briefly recount the saga from and to reasonable reliability of the production Javelins with 5 variants of factory crank, including finally a nitrided one.)