Reply to "Oops I'm doing it again....I lost my head..."

Steve,

You can borrow mine or I can send you a drawing.

The jack shaft center bearing is an over constraint, as all three jack shaft bearings can not line up perfectly. When three bearings are all on the same center line, at least one of the three bearings will be misaligned (especially when supporting structures expand and distort on a hot engine). Just like a four legged table must bend for all four legs to touch the ground (three points determine a plane and a fourth point is an over constraint), the jack shaft bends in order to accommodate the misalignment of one of the three bearings. The force required to bend the shaft to get it through all three bearings is less than the capacity of the bearings, so it doesn't wear out the bearings prematurely, but three bearings in a line is not accepted engineering practice.

Having said that, there is a potential (and possibly important) benefit in using the center bearing. The center bearing dramatically raises the "critical speed" of the jack shaft relative to the critical speed with a two bearing configuration. The critical speed of the shaft is the rpm at which the shaft experiences the first mode of vibrational resonance (harmonic). It is possible that when using only two bearings, the critical speed of the jack shaft may be within, or close to, the operating range of the engine. By using a third bearing in the center, the critical speed of the shaft is raised beyond the engine speed range such that the engine can not possibly spin the jack shaft fast enough to get into vibrational resonance.

Since the jack shaft bearings don't wear out prematurely, it may be worth using the center bearing even though it is somewhat unorthodox engineering.

Dick Chandler