Babbitt (alloy)

Last updated
Pigs and bars of Grade #2 Babbitt Pigs and Bars of Babbitt.jpg
Pigs and bars of Grade #2 Babbitt

Babbitt metal or bearing metal is any of several alloys used for the bearing surface in a plain bearing.

Contents

The original Babbitt alloy was invented in 1839 by Isaac Babbitt [1] in Taunton, Massachusetts, United States. He disclosed one of his alloy recipes but kept others as trade secrets. [2] Other formulations were developed later. [3] Like other terms whose eponymous origin is long since deemphasized (such as diesel engine or eustachian tube ), the term babbitt metal is frequently styled in lowercase. [3] [4] It is preferred over the term "white metal", because the latter term may refer to various bearing alloys, lead- or tin-based alloys, or zinc die-casting metal.

Microstructure of babbitt BabbitB83.jpg
Microstructure of babbitt

Babbitt metal is most commonly used as a thin surface layer in a complex, multi-metal assembly, but its original use was as a cast-in-place bulk bearing material. Babbitt metal is characterized by its resistance to galling. Babbitt metal is soft and easily damaged, which suggests that it might be unsuitable for a bearing surface. However, its structure is made up of small hard crystals dispersed in a softer metal, which makes it, technically, a metal matrix composite. As the bearing wears, the softer metal erodes somewhat, creating paths for lubricant between the hard high spots that provide the actual bearing surface. When tin is used as the softer metal, friction causes the tin to melt and function as a lubricant, protecting the bearing from wear when other lubricants are absent.

Internal combustion engines use Babbitt metal which is primarily tin-based because it can withstand cyclic loading.

Traditional Babbitt bearings

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 are 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 such 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 Babbitt bearings

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, 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.

Example: modern Miba Babbitt thrust bearing Miba Thrust Bearing.jpg
Example: modern Miba Babbitt thrust bearing

In more modern practice, the crankshaft and connecting-rod big-end bearings in a modern automobile engine 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.

Alternative bearings

In many applications, rolling-element bearings, such as ball or roller bearings, have replaced Babbitt bearings. Though such bearings can offer a lower coefficient of friction than plain bearings, their key advantage is that they can operate reliably without a continuous pressurized supply of lubricant. Ball and roller bearings can also be used in configurations that are required to carry both radial and axial thrusts. However, rolling-element bearings lack the beneficial damping and shock-load capability provided by fluid-film bearings, such as the Babbitt.

Babbitt alloys

Industry names ASTM grade Sn Pb Cu Sb As Yield point, [lower-alpha 1] psi (MPa)Johnson's apparent
elastic limit, [lower-alpha 2] psi (MPa)
Melting point
°F (°C)
Proper pouring
temp. °F (°C)
20 °C (68 °F)100 °C (212 °F)20 °C (68 °F)100 °C (212 °F)
No. 1 [6] 190–92≤ 0.354–54–54400 (30.3) [5] 2650 (18.3) [5] 2450 (16.9) [5] 1050 (7.2) [5] 433 (223) [5] 825 (441) [5]
Nickel Genuine [6] 288–90≤ 0.353–47–86100 (42.0) [5] 3000 (20.6) [5] 3350 (23.1) [5] 1100 (7.6) [5] 466 (241) [5] 795 (424) [5]
Super Tough [6] 383–85≤ 0.357.5–8.57.5–8.56600 (45.5) [5] 3150 (21.7) [5] 5350 (36.9) [5] 1300 (9.0) [5] 464 (240) [5] 915 (491) [5]
Grade 4 [6] 474–769.3–10.72.5–3.511–13
Grade 11 [6] 1186–89≤ 0.355–6.56–7.5
Heavy Pressure [6] 79.3–10.772.5–76.514–160.3–0.63550 (24.5) [5] 1600 (11.0) [5] 2500 (17.2) [5] 1350 (9.3) [5] 464 (240) [5] 640 (338) [5]
Royal [6] 84.5–5.577.9–81.214–160.3–0.63400 (23.4) [5] 1750 (12.1) [5] 2650 (18.3) [5] 1200 (8.3) [5] 459 (237) [5] 645 (341) [5]
Grade 13 [6] 135.5–6.582.5–859.5–10.5≤ 0.25
Durite [6] 150.8–1.279.9–83.914.5–17.50.8–1.4
  1. The values of yield point were taken from stress-strain curves at deformation of 0.125% of gauge length. [5]
  2. Johnson's apparent elastic limit is taken as the unit stress at the point where the slope of the tangent to the curve is 2/3 times its slope in origin. [5]

The science of bearing Babbitt selection

The engineering of a bearing's Babbitt lining is usually completed during the design of the machine. In selecting the proper type of Babbitt for a particular job there are a number of factors to take into consideration, the most important of which are as follows:

  1. Surface speed of the shaft
  2. Load that the bearing is required to carry

There is no doubt that if a bearing is to be highly loaded in relation to its size, a high-tin alloy is desirable; whereas for much lower-speed work and less heavily loaded bearings, a lead-based Babbitt may be employed and is far more economical.

  1. Surface speed of the shaft (the number of feet traveled per minute by the shaft circumferentially):
    Formula: S = π × D × RPM / 12.
    Example: Determine the surface of a 2-inch-diameter shaft going 1,400 revolutions per minute (RPM):
    S = π × D × RPM / 12 = 3.1416 × 2 × 1,400 / 12 = 733.04 ft/min,
    where π = 3.1416, D = diameter of shaft in inches, S = surface speed of the shaft.
  2. Load bearing is required to carry (the weight which is being exerted through the combined weights of the shaft and any other direct weights on the shaft and measured in pounds-force per square inch):
    Formula: L = W / (I.D × L.O.B.).
    Example: Determine the load on a bearing of a 2-inch I.D. bearing, 5 inches long and carrying a weight of 3,100 lbf:
    W / (I.D x L.O.B.) = 3,100 / (2 × 5) = 310 lbf/in2,
    where W = total weight carried by bearing, I.D = inside diameter of bearing, L.O.B = length of bearing, L = load bearing required to carry.

Babbitt bearing selection

While not subject to precise calculations, the following considerations must also be taken into account:

For example, a bearing in continuous use in a harsh environment without regular maintenance will require different Babbitt and lubrication than a bearing in intermittent use in a clean, light duty environment. This so-called art is really the condensation of the experience of the technician and the experience of the bearing being rebuilt.

If the bearing has performed well in use over many years, the bearing needs simply to be rebuilt to its original specification and formulation. In this case the technician's greatest concerns are:

  1. Bearing shell surface preparation
  2. Bonding characteristics of the tinning compound and the Babbitt layer and,
  3. Load bearing surface preparation and finish

Eco-Babbitt

Eco-Babbitt is an alloy of 90% Sn, 7% Zn, 3% Cu that is not technically a Babbitt metal. See Solder alloys for more information on Eco-Babbitt. [7]

Related Research Articles

A lubricant is a substance that helps to reduce friction between surfaces in mutual contact, which ultimately reduces the heat generated when the surfaces move. It may also have the function of transmitting forces, transporting foreign particles, or heating or cooling the surfaces. The property of reducing friction is known as lubricity.

<span class="mw-page-title-main">Ball bearing</span> Type of rolling-element bearing

A ball bearing is a type of rolling-element bearing that uses balls to maintain the separation between the bearing races.

Fluid bearings are bearings in which the load is supported by a thin layer of rapidly moving pressurized liquid or gas between the bearing surfaces. Since there is no contact between the moving parts, there is no sliding friction, allowing fluid bearings to have lower friction, wear and vibration than many other types of bearings. Thus, it is possible for some fluid bearings to have near-zero wear if operated correctly.

<span class="mw-page-title-main">Bearing (mechanical)</span> Mechanism to constrain relative movement to the desired motion and reduce friction

A bearing is a machine element that constrains relative motion to only the desired motion and reduces friction between moving parts. The design of the bearing may, for example, provide for free linear movement of the moving part or for free rotation around a fixed axis; or, it may prevent a motion by controlling the vectors of normal forces that bear on the moving parts. Most bearings facilitate the desired motion by minimizing friction. Bearings are classified broadly according to the type of operation, the motions allowed, or the directions of the loads (forces) applied to the parts.

<span class="mw-page-title-main">Lubrication</span> The presence of a material to reduce friction between two surfaces.

Lubrication is the process or technique of using a lubricant to reduce friction and wear and tear in a contact between two surfaces. The study of lubrication is a discipline in the field of tribology.

The white metals are a series of often decorative bright metal alloys used as a base for plated silverware, ornaments or novelties, as well as any of several lead-based or tin-based alloys used for things like bearings, jewellery, miniature figures, fusible plugs, some medals and metal type. The term is also used in the antiques trade for an item suspected of being silver, but not hallmarked.

<span class="mw-page-title-main">Connecting rod</span> Piston engine component which connects the piston to the crankshaft

A connecting rod, also called a 'con rod', is the part of a piston engine which connects the piston to the crankshaft. Together with the crank, the connecting rod converts the reciprocating motion of the piston into the rotation of the crankshaft. The connecting rod is required to transmit the compressive and tensile forces from the piston. In its most common form, in an internal combustion engine, it allows pivoting on the piston end and rotation on the shaft end.

<span class="mw-page-title-main">Plain bearing</span> Simplest type of bearing, with no rolling elements

A plain bearing, or more commonly sliding contact bearing and slide bearing, is the simplest type of bearing, comprising just a bearing surface and no rolling elements. Therefore, the journal slides over the bearing surface. The simplest example of a plain bearing is a shaft rotating in a hole. A simple linear bearing can be a pair of flat surfaces designed to allow motion; e.g., a drawer and the slides it rests on or the ways on the bed of a lathe.

<span class="mw-page-title-main">Rolling-element bearing</span> Bearing which carries a load with rolling elements placed between two grooved rings

In mechanical engineering, a rolling-element bearing, also known as a rolling bearing, is a bearing which carries a load by placing rolling elements between two concentric, grooved rings called races. The relative motion of the races causes the rolling elements to roll with very little rolling resistance and with little sliding.

<span class="mw-page-title-main">Pillow block bearing</span> Bracket used to provide support to rotating shafts

A pillow block bearing is a pedestal used to support a rotating shaft with the help of compatible bearings and various accessories. The assembly consists of a mounting block which houses a bearing. The block is mounted to a foundation, and a shaft is inserted, allowing the inner part of the bearing/shaft to rotate. The inside of the bearing is typically 0.025 millimetres (0.001 in) larger diameter than the shaft to ensure a tight fit. Set screws, locking collars, or set collars are commonly used to secure the shaft. Housing material for a pillow block is typically made of cast iron or cast steel.

Zinc-aluminium (ZA) alloys are alloys whose main constituents are zinc and aluminium. Other alloying elements include magnesium and copper. This type of alloy was originally developed for gravity casting. Noranda, New Jersey Zinc Co. Ltd., St. Joe Mineral Co. and the International Lead Zinc Research Organization (ILZRO) were the main companies that pioneered the ZA alloys between the 1950s and the 1970s. They were designed to compete with bronze, cast iron and aluminium using sand and permanent mold casting methods. Distinguishing features of ZA alloys include high as-cast strength, excellent bearing properties, as well as low energy requirements.

Dry lubricants or solid lubricants are materials that, despite being in the solid phase, are able to reduce friction between two surfaces sliding against each other without the need for a liquid oil medium.

<span class="mw-page-title-main">Oil pump (internal combustion engine)</span> Internal combustion engine part that circulates engine oil under pressure

The oil pump is an internal combustion engine part that circulates engine oil under pressure to the rotating bearings, the sliding pistons and the camshaft of the engine. This lubricates the bearings, allows the use of higher-capacity fluid bearings and also assists in cooling the engine.

Brinelling is the permanent indentation of a hard surface. It is named after the Brinell scale of hardness, in which a small ball is pushed against a hard surface at a preset level of force, and the depth and diameter of the mark indicates the Brinell hardness of the surface. Brinelling is permanent plastic deformation of a surface, and usually occurs while two surfaces in contact are stationary and the material yield strength has been exceeded.

A thrust block, also known as a thrust box, is a specialised form of thrust bearing used in ships, to resist the thrust of the propeller shaft and transmit it to the hull.

Oilite is a brand of self-lubricating bearing that is made from metal alloys with pores that channel lubricants between the bearing itself and the shaft. It is manufactured from different types of material. Traditional Oilite is mostly made of copper with approximately 10% tin and up to 1% iron, while both Super Oilite and Super Oilite 16 are primarily made of iron with about 20% copper and, in the case of the latter, up to 1% graphite. Oilite is currently a registered trademark of Beemer Precision, Inc.

<span class="mw-page-title-main">Cast bullet</span> Made by allowing molten metal to solidify in a mold

A cast bullet is made by allowing molten metal to solidify in a mold. Most cast bullets are made of lead alloyed with tin and antimony; but zinc alloys have been used when lead is scarce, and may be used again in response to concerns about lead toxicity. Most commercial bullet manufacturers use swaging in preference to casting, but bullet casting remains popular with handloaders.

<span class="mw-page-title-main">Spiral groove bearing</span> Hydrodynamic bearings using spiral grooves to develop lubricant pressure

Spiral groove bearings are self-acting, or hydrodynamic bearings used to reduce friction and wear without the use of pressurized lubricants. They have this ability due to special patterns of grooves. Spiral groove bearings are self-acting because their own rotation builds up the pressure needed to separate the bearing surfaces. For this reason, they are also contactless bearings.

Self-lubricating chains, also referred to as lube-free chains, are commonly found in both roller chain and conveyor chain varieties, with specialty self-lubricating chains also available. These chains utilize a bush made of an oil-impregnated sintered metal or plastic to provide continuous lubrication to the chain during drive, eliminating the need for further lubrication.

A composite bearing is a bearing made from a combination of materials such as a resin reinforced with fibre and this may also include friction reducing lubricants and ingredients.

References

  1. Hellemans, Alexander; Bunch, Bryan (1988). The Timetables of Science. Simon & Schuster. p. 305. ISBN   0671621300.
  2. Isaac Babbitt, "Mode of making boxes for axles and gudgeons," Archived 2020-07-16 at the Wayback Machine U.S. patent no. 1,252 (issued: July 17, 1839). Babbitt did not patent his alloy, although he does state its formulation: "The inner parts of the boxes are to be lined with any of the harder kinds of composition known under the names of britannia metal or pewter, of which block tin is the basis. An excellent compound for this purpose I have prepared by taking about 50 parts of tin, five of antimony, and one of copper, but I do not intend to confine myself to this particular composition."
  3. 1 2 Oberg & Green 1996 , p. 2150.
  4. American Heritage Dictionary, 4th ed, headword babbitt metal.
  5. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 "The Art and Science of Babbitt Selection". Kapp Alloy & Wire, Inc. Archived from the original on 21 September 2013. Retrieved 26 October 2012.
  6. 1 2 3 4 5 6 7 8 9 "Babbitt Classifications". Kapp Alloy & Wire, Inc. Archived from the original on 21 September 2013. Retrieved 25 October 2012.
  7. Kapp Alloy & Wire, Inc. "Kapp Eco-Babbitt". Kapp Alloy & Wire, Inc. Retrieved 10 April 2013.

Bibliography