FAQ on Machines

Bearings and Bushings

Based on posts by Michael Barthelemy, Karen Bergstralh, Walter Bjorneby Rick Boatright, Walt Boyes, Jody Dorsett, Joe Ennis, Eric Flint, Thomas Hassan, John Leggett, Karl-Johan Noren, Pam (Pogo) Pogianni, and Charles Prael, with additional information from Laura Runkle.

In all of the following, it's important to remember that it's possible to make any precision part with a handfile, emery, and precision gauges. It just takes forever, requires training, and makes no economic sense when dedicated machines can be operated by people with only a few weeks of training.

A machine shop contains lathes, mills, drills - all nice spinning things. Without bearings, spinning and turning parts will not turn smoothly and quickly; they will wobble. You don't want wobble in a machine tool doing precision work. Bearings are absolutely necessary.

Which kind of bearing? When most people think of bearings, they automatically think of ball bearings, or maybe roller bearings -fast, smooth, round, and not possible to make to any economy of scale given 1880 to 1900 technology and measurement. The machine tools made in the USE will not use ball bearings for years. They will use the same bearings used back in the 1880s.

If they won't use ball bearings or roller bearings, what will they use? It depends on the speed of the item. Low-speed equipment will use bronze bushings, preferably made of oilite bronze. Higher-speed equipment will use Babbitted bearings. These are stationary sleeves for the rotating shaft which can be cast and machined precisely, and which are lubricated with oil. Because the stationary bearing is made of a very soft alloy, it won't scratch the crankshaft, and it will conform very well to the crankshaft. The downside is that Babbitt metal has a very low melting point. The tools can't be run at the extremely high speeds that modern tools use; they would get too hot. (Modern shops using Babbitted bearings often have temperature sensors that cut power when the bearing gets above 205 F.) Although Babbitted bearings can't take extremely high speeds, they can certainly take up to 4000 rpm if properly lubricated.

Isaac Babbitt invented the soft-metal stationary bearing in 1839. His bearing metal (also known as soft metal or white metal) was an alloy of tin, antimony, and copper. Several different alloys exist that are called "Babbitt" today. They are either tin (>80% tin) or lead (>80% lead) Babbitt alloys. In general, the tin alloys are higher quality, but the lead alloys are much less expensive. Which alloy is chosen will not only depend on which is cheaper, but how often you want to redo the bearing, whether the bearing will be water-cooled, and what type of oil will be used for lubrication.

Job shops today that work in repair and reconditioning of older machinery have a lot of familiarity with Babbitted bearings - the shops in Grantville would certainly know about this. The big problem will be getting a source of the metal. Or maybe not.

Before we get further into metals, it's important to remember that we're discussing modern alloys. Seventeenth century Europeans did not know how to smelt zinc, for example. Sphalerite, the main ore of zinc, was considered a nuisance mineral. Bronzes and brasses with zinc content were made either by mixing in the ores, or by using ores with zinc impurities. In any case, either the individual metals used in the following alloys are well known by the seventeenth century, or their ores are well known. All of the metals below were mentioned in ore form by Georgius Agricola in his 1556 book, De Re Metallica. Most of them were smelted in the Erzgebirge and the Harz.

Modern pewters are 92% tin, 6-7% antimony, and 1-2% copper. Pewters in the seventeenth century used lead with or instead of antimony, but antimony was well known in the seventeenth century. It was mined in Bohemia, and ores also exist in Sweden. Stibnite (antimony sulphide ore) is also available in small amounts in the Erzgebirge and the Harz, mixed in with lead, zinc, and silver ores. If necessary, it will be possible to purchase tin, copper, antimony, and lead, and create alloys. It will be much simpler to specify alloys and have them shipped to wherever the machining is done. Impurities that are usually found in the ores of tin, copper, lead and antimony are each other, silver, zinc, and arsenic. As the ores are smelted, most of the zinc will be vaporized. Arsenic is an acceptable impurity in the proportions it occurs in Stibnite. Although silver would be rewarding to extract, it's not easy to do so. (Electrolysis is the simplest method to get silver out of lead and tin, and gold out of copper. Cupellation? Let's not go there, right now.)

Bronze bushings work on the same logic as Babbitted Bearings. The best bushings are oilite bronze. Just like Babbitt metals, there are many different compositions of bronze. Most of them contain anywhere from 60-90% copper, with the remainder as tin, plus possible antimony, zinc, lead, or other metals. Some bronzes contain small amounts of phosphorus for strengthening. Copper does not like lead (or antimony). Only about 2% lead can be put in a mostly copper alloy. If the full 2% of lead is put into the bronze this makes the bronze a bearing material as it now has a low coefficient of friction. (Henley's 1928 workshop handbook gives a bearing bronze for machining as 3% lead, 14% tin, and the rest as copper.)

If bronze granules are pressed together so there are gaps between the pressed pellets, this can be dipped in normal 30 weight motor oil and the oil will soak in and never all come out again. These oilite bronzes are excellent for light duty on low speed shafts where low friction and no maintenance is required - their friction coefficient is smaller than that of ball or roller bearings.

It would be a good idea for both the first batches of Babbitt metal and bushing bronze to be analyzed by the spectrometer while it is still possible. This means that work with the metals must begin before 1635.

For more information,

Web Sites

Books

The 'every engineer and machine shop has one or both' standards: These are the machinist's and engineers equivalent of the CRC Chemistry and Physics Handbook (rubber bible.)

Marks' Standard Handbook for Mechanical Engineers
by Eugene A. Avallone (Editor), Theodore, III Baumeister (Editor) (Hardcover)
Price: $150.00 Find in a Library

Machinery's Handbook
by Erik Oberg (Editor), et al (Hardcover)
Price: $85.00 Find in a Library

Probably at least one copy per machine shop:

Standard Handbook of Machine Design
by Joseph E. Shigley (Editor), et al (Hardcover)
Price: $125.00 Find in a Library

Modern Machine Shop's Handbook for the Metalworking Industries
by Woodrow W. Chapman (Editor) (Hardcover)
Price: $55.00 Find in a Library