Reply To: Bearing clearances

[Jim Chase]

Tony, in the paper I have been referring to Sparrow cut down the surface area of his bearings in half and improved their life at 4500 rpm. There is more than enough surface area at higher speed, maybe not so much at idle speed. At speed the large bearing surface area just adds a lot of heat and friction.

Blade and fork designs allow the cylinders of each bank of a V engine to be in the same line instead of offset by the width of the connecting rod. On a V-8 such as early Lincolns, this would have prevented a rocking couple slight imbalance unless counterweights were added to the crankshaft. It is theoretically a more elegant design, albeit less practical for an auto engine.

There are two ways to do the fork and blade, I am not familiar with how Lincoln did it. The first is to have two narrower bearings for the forks and a wider bearing for the blade. The bearings and crankpin can end up having the same total area as if they were offset with two con rods bearings of equal width. The narrower bearings actually tend to have fewer problems than wide bearings. The Rolls Royce Merlin used on Spitfires and Mustangs used this approach, and I imagine was what Lincoln did.

The Allison V-12 used on P-38, P-39, P-40 and the last P-82 Twin Mustangs had a single bearing for both. The bearing was fixed to the forked rods in the conventional way, while the blade oscillated on the outer diameter of the bearing bushing similar to how the articulated rods attach to the master rod of a radial engine. This arrangement allowed for a smaller total bearing area needed and a reduction in weight. Some air race engine rebuilders retrofitted Allison rods into Merlins because they were stronger.

Jim