22-05-2021, 12:39 AM
(This post was last modified: 22-05-2021, 02:45 AM by Bob Culver.)
Having got this far some further may interest a few.
The fatigue limit is the stress below which steel "never" fails. Well designed car engines only fleetingly operate above the fatigue limit, and as several million cycles just above are required for failure, no problems occur in normal lifetime.
More disturbing, with components more occasionally operated just above the fatigue limit, as original Seven cranks obviously do, a very small increase in stress dramatically shortens the life of millions of cycles at that stress. So Sevens with raised CR and/ or operated at high rpm have reduced crank life. It is no coincidence that the few owners who claim very long life of original cranks have unmodified engines. By the same token, raising the fatigue limit slightly, can very dramatically increase life for the same usage.
Increasing the tensile strength of the steel say 10% may typically increase the fatigue limit by about 4%. So limited scope for improveement over the original, or one ht steel compared another. But nitriding a suitable steel may increase the fatigue limit 20% and considerably more. Hence the success of nitrided replacement cranks.
(Editted to fix obvious error)
The fatigue limit is the stress below which steel "never" fails. Well designed car engines only fleetingly operate above the fatigue limit, and as several million cycles just above are required for failure, no problems occur in normal lifetime.
More disturbing, with components more occasionally operated just above the fatigue limit, as original Seven cranks obviously do, a very small increase in stress dramatically shortens the life of millions of cycles at that stress. So Sevens with raised CR and/ or operated at high rpm have reduced crank life. It is no coincidence that the few owners who claim very long life of original cranks have unmodified engines. By the same token, raising the fatigue limit slightly, can very dramatically increase life for the same usage.
Increasing the tensile strength of the steel say 10% may typically increase the fatigue limit by about 4%. So limited scope for improveement over the original, or one ht steel compared another. But nitriding a suitable steel may increase the fatigue limit 20% and considerably more. Hence the success of nitrided replacement cranks.
(Editted to fix obvious error)