Roller Bearing Cranks

This crankshaft has roller bearings on the throw journals, not just the mains.  Most did.

I love the beatuy of roller cranks: they're triumphs of precision manufacturing.   Wouldn't want to own one--not that any manufacturer would be crazy enough to put one in a car for sale to the general public.  The maintenance intervals and costs were "for racing only."  But in their day they had very practical value.

They were important for racing in the '20's and '30's because babbit bearing technology and tolerances could not stand up to the pounding of high loads and r.p.m.'s.  But you could "do" a roller crank with tighter tolerances and much lower friction losses.  Thin wall shell bearings solved the durability problem, although their friction losses are higher.  They took over from rollers in car racing engines in the 1950's.  Rollers are still common in small, high powered, racing engines.

Because the piston rods of a roller crank are a single piece, with the bearing press-fitted into them, the crankshaft itself must be made from multiple precision-machined segments and assembled around each throw journal.   The Hirth joints themselves are in the crank's main journals, surrounded by (more) roller-bearings.

Except for highly specialized applications, the roller bearing crankshaft has gone the way of the precision-movement mechanical watch, and for the same reasons.  It was complicated and expensive, both to produce and assemble.  Advanced materials and electronics provide better solutions to the same problems (loads and revs).  So it is an almost-forgotten achievement of the mechanical age.  But making one was an achievement maybe even more impressive than imagining it.

Illustration of a Hirth Joint.  If you think about the dimensions and tolerances in an automotive crankshaft, and the
loads put on the joint in operation, you can see how precisely the joint must be machined and how carefully the
crankshaft must be assembled.

Thing of beauty: a Bugatti straight-8, 5 main-bearing roller crank.