Traditional In the traditional style of a babbitt-metal bearing, a
cast-iron pillow block is assembled as a loose fit around the shaft, with the shaft in its approximate final position. The inner face of the cast-iron pillow block is often drilled to form a
key to locate the bearing metal as it is cast into place. The shaft is coated with
soot as a
release agent, the ends of the bearing are packed with
clay to form a mold, and molten metal is poured into the cavity around the shaft, initially filling the lower half of the pillow block. The bearing is stripped, and the metal trimmed back to the top surface of the pillow block. Solidified babbitt metal is
soft enough to be cut with a
knife or sharp
chisel. A
steel shim is inserted to protect the face of the lower bearing and to space the cap of the pillow block away from the shaft. After resealing the ends with clay, more metal is then poured to fill the cap of the pillow block through the hole in the top of the pillow-block cap, which will eventually become a lubrication port. The two halves of the bearing are then split at the shim, the shim removed, the
oil holes cleared of metal, and oil ways cut into the surface of the new bearing. The shaft is smeared with
engineer's blue and rotated in the bearing. When the bearing is disassembled, the blue fills the hollows and is rubbed off the high spots, making them visible. The high spots are scraped down, and the process repeated, until a uniform and evenly distributed pattern of blue shows when the shaft is removed. The bearing is then cleaned and lubricated, and shimmed up so that the shaft is held firmly but not binding in the bearing. The bearing is then "run in" by being run heavily lubricated at low load and slow revolution, completing the process of exposing the hard bearing surface. After final adjustment of the shimming, a very reliable and high-
load-capability bearing results. Before the advent of low-cost
electric motors, power was distributed through factories from a central engine via
overhead shafts running in hundreds of Babbitt bearings. Often, leather, fabric or rubber belts would be used to transfer this rotating power to working machines. The expression a "run bearing" also derives from this style of bearing, since failure of lubrication will lead to
heat build-up due to
friction in the bearing, eventually leading to the bearing metal melting and running out of the pillow block.
Modern Until the mid-1950s, poured Babbitt bearings were common in automotive applications. The Babbitt was poured into the block or caps using a form. Tin-based Babbitts were used, as they could stand up to the impact loads found on the
connecting rods and
crankshaft. The poured Babbitt bearings were kept thin. The rods and caps would have shims that could be peeled off as the Babbitt wore down.
Ford was known to use two 0.002" on each cap and Babbitt that was 86%
tin, 7%
copper, and 7%
antimony (see the KRW catalogs for the
Model T). Steel shims were used, as the
brass shims used today tend to compress over time, contributing to shorter bearing life. The poured Babbitt bearings commonly get over 50,000 miles of use before needing replacement. Poured Babbitt bearings are also known to fail gracefully, allowing the car to be driven for extended periods of time. The failed bearing is not likely to damage the crankshaft. The crankshaft and connecting-rod big-end bearings in current
automobile engines are made of a replaceable steel shell, keyed to the bearing caps. The inner surface of the steel shell is plated with a coating of
bronze, which is in turn coated with a thin layer of Babbitt metal as the bearing surface. The process of laying down this layer of metal is known as Babbitting. ==Alternatives==