Babbitt - What is Babbitt?

Modified on 2011/02/02 11:27 by BobHolcombe — Categorized as: Bearings, Restoration Topics

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History

Babbitt metal, also called white metal, is a soft, white non-ferrous alloy used to provide a bearing surface. It has properties that help reduce friction which make it a good material to use in a plain bearing.

Babbitt was first created by Isaac Babbitt. Born July 26, 1799 in Taunton, Massachusetts, Babbitt was well known as an inventor by trade. In 1839, he received a patent for a white metal alloy that showed excellent bearing properties. The original formula for Babbitt's bearing metal was 89.3% tin, 7.1% antimony and 3.6% copper and that formula is still marketed today by some manufacturers as ASTM B-23 Grade 2 Babbitt or as "Genuine Babbitt".

While Babbitt metal is soft and can be easily damaged when treated improperly, and seems at first sight an unlikely candidate for a bearing surface, its appearance is deceptive. The structure of the alloy is made up of small hard crystals dispersed in a matrix of softer alloy. As the bearing wears the harder crystal is exposed, with the matrix eroding somewhat to provide a path for the lubricant between the high spots that provide the actual bearing surface .{Reference:http://www.frymetals.com/products/babbitt.html?product=babbitt|Fry Metals Babbitt Reference Guide}

How Babbitt Works

The principle as to how Babbitt works is really quite simple. In order for any bearing to work, it must create a situation where there is a low coefficient of friction. The lower the amount of friction on the rotating shaft, the easier it turns.

In the case of Babbitt bearings, this low coefficient of friction is accomplished by two means. First, the bearing material itself (the Babbitt) has a relatively low coefficient of friction. This means, even without lubrication, Babbitt bearings will have much less friction than if a shaft was turning in other metals such as steel or cast iron.

However, with the addition of simple lubrication, Babbitt bearings can have amazingly low coefficients of friction - even lower than that of ball bearings.

In order to achieve this low level of friction, bearings must be properly lubricated. Under ideal conditions, the motion of the rotating shaft over the babbitt surface will draw in whatever oil may be present, which builds up a thin film according to the laws governing the flow of fluids.

As an example of how this works, when the shaft is still and not rotating, the entire load of the shaft is making contact with the bearing surface. Ideally, at this point, there will be oil in the bearing, which will be surrounding the shaft on either side of the contact point.

As the shaft begins to turn, oil from around the bearing will be pulled under the rotating shaft until there is a wedged-shaped film of oil completely around the shaft. If conditions are right and adequate lubrication is present, the shaft will actually be floating on a thin film of oil and will not come in contact with the bearing material, resulting in an extremely low coefficient of friction.

Fluid Film Lubrication

Fluid Film Lubrication



If there is not enough oil to create this film of oil, or if once formed it breaks down due to insufficient speed, poor oil viscosity, insufficient oil or excessive load, there is usually enough oil present to wet the surfaces and prevent high friction coefficients. In this situation, it is not uncommon for the bearing to get warmer than one would like but this will usually not cause great problems.{Reference:The Elements of Machine Design. By S. J. Berard and E. O. Waters. Published by D. Van Nostrand Company, Inc., New York., Second Edition, 1932. Pp. 166-168}



Types of Babbitt

The term "Babbitt" is commonly used to describe any kind of "white metal" bearing material no matter the actual composition. Babbitt metals can generally be broken into two major families: tin based or lead based. Tin based Babbitt is the best choice for high speed or hard to lubricate bearings while lead based works best with low speed or heavy load bearings.{Reference:http://www.alchemycastings.com/lead-products/babbitt.htm|Alchemy Castings, Inc., Babbitt Metal}

Selecting a Type of Babbitt

To determine which bearing material would be best for your application, the following are suggested guidelines: {Reference:http://www.alchemycastings.com/lead-products/babbitt.htm|Alchemy Castings, Inc., Babbitt Metal}

Babbitt Classification LIMITS
Surface Speeds
(# of Ft/min)		 LOAD
(Lbs/sq.in.)
MIN. MAX. MIN. MAX.
Tin-Based Babbitts 1,000 2,400 0 2,000
Lead Based Babbitts 100 1,000 0 500

Calculate Surface Speed

Surface speed of the shaft is defined as the number of feet traveled per minute by the shaft circumferentially. To calculate this value for your shaft, use the following formula:{Reference:http://www.alchemycastings.com/lead-products/babbitt.htm|Alchemy Castings, Inc., Babbitt Metal}

Surface Speed = Pi x D x RPM / 12

Where:
Pi = 3.1416
D = Diameter of the shaft in inches
RPM = Revolutions Per Minute

Example: Determine the surface speed of a 2-inch shaft going 1500 RPM.
3.1416 x 2 x 1500 /12 = 785 ft/min

Calculate Bearing Load

The Load the bearing is required to carry is 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 per square inch.{Reference:http://www.alchemycastings.com/lead-products/babbitt.htm|Alchemy Castings, Inc., Babbitt Metal} For most old wood working machines, the load on the bearings is not a major concern as the weight to surface area ratio is usually great enough to not be an issue. Load comes more into play when you are using extremely heavy shafting over long stretches, such as in line-shaft applications where there is a lot of weight hanging on a couple of small bearings. Even though load will probably not bear into the decision making on the type of babbitt on most woodworking machines, it is still a good idea to calculate this value to be sure.

Bearing Load = W / ID x L

Where:
W = Total weight in lbs carried by bearing (includes shafts, pulleys, cutter heads, etc.)
ID = Inside diameter of bearing in inches
L = Length of Bearing in inches

Example: Determine the load on a bearing of a 2 inch inside diameter bearing, 4 inches long and carrying a weight of 100 lbs.
100 / 2 x 4 = 12.5 lbs/sq in

Lead Based Babbitt

Generally, these compositions range from 10 to 15% antimony, up to 10% tin and the remaining amounts being lead. Compared to tin babbitts, lead-base materials are less costly and because they are softer than the tin based Babbitts, they have less tendency to score a shaft. However, lead based Babbitt does not hold up well on high speed shafts or under great loads. Lead based Babbitt will work fine for most light duty and low speed applications such as a band saw arbor. For high speed arbors over 1000 RPM, tin based Babbitt is preferred.{Reference:http://www.frymetals.com/products/babbitt.html?product=babbitt|Fry Metals Babbitt Reference Guide}

Tin Based Babbitt

These materials are composed of 80 to 90% tin, with about 3 to 8% copper and 4 to 14% antimony added. An increase in the copper or antimony increases hardness and tensile strength and decreases ductility. Increasing the percentage of these hardening alloys above this range decreases both cracking resistance and fatigue strength according to the paper.

Tin Based Babbitt is the best choice for high-speed applications (such as jointers and planers) and hard to lubricate bearings. In reality, probably overkill for band saw bearings but sure would not hurt to be safe than sorry. While the higher tin content does make this Babbitt more expensive, when you consider the cost of time and labor for pouring bearings, probably money well spent. Cost for tin based products is much higher than the lead based.{Reference:http://www.frymetals.com/products/babbitt.html?product=babbitt|Fry Metals Babbitt Reference Guide}

Recycling Babbitt

Many people who pour babbitt bearings are interested in reclaiming old babbitt from worn bearings and re-using the material on new bearings. Because there is rarely enough babbitt left in the old shells to complete a new pour, to save money, some people will be tempted to mix the old metal with new. This is not advisable.

Very seldom will somebody know the exact composition of their old babbitt, much less if it is even a tin based or lead based product. Inevitably, when somebody tries to mix old and new Babbitt, they end up mixing a lead alloy with tin-based babbitt. Mixing these two causes the low melting 63Sn/37Pb eutectic to be formed. This babbitt will selectively melt out of the lining and leave a “sponge” like bearing. The 63/37 melts at 361°F and the Grade 2 babbitt does not completely melt until 669°F. A premature bearing failure is the result.

Users should not mix unknown babbitts and all pots, mixers, and ladles should be free of lead. Even if the babbitt is a known tin-based babbitt, no more than 30% of a pour should be recycled alloy. A larger amount can cause excessive dross to be trapped in the casting.{Reference:http://www.frymetals.com/products/babbitt.html?product=babbitt|Fry Metals Babbitt Reference Guide}

ASTM B-23 Babbitt Specifications

The American Society for Testing and Materials (ASTM) was created in 1898 in order to standardize many materials used by engineers for various purposes. This assures that products manufactured by different companies adhere to the same standards in composition.

Among the many standards ASTM has, different grades of Babbitt are included. As such, many manufacturers of Babbitt will often provide a ASTM Grade for different brands of Babbitt, allowing consumers to compare one product to another knowing that its composition is similar.

While the number of grades detailed by ASTM has changed over the years with fewer grades being standard today than before 1959, ASTM still recognizes eight grades of Babbitt. Here are the current standards for Babbitt metal by ASTM.{Reference:http://www.astm.org/|ASTM International}

ASTM Babbitt Analysis Chart - ASTM B-23

  Chemical
Composition, % 		Tin Base Babbitt Grades
ALLOY NUMBER (GRADE)
  1 2 3 11
  UNS-L13910 UNS-L13890 UNS-L13840 UNS-L13870
Tin 90.0-92.0 88.0-90.0 83.0-85.0 86.0-89.0
Antimony 4.0-5.0 7.0-8.0 7.5-8.5 6.0-7.5
Lead 0.35 0.35 0.35 0.50
Copper 4.0-5.0 3.0-4.0 7.5-8.5 5.0-6.5
Iron 0.08 0.08 0.08 0.08
Arsenic 0.10 0.10 0.10 0.10
Bismuth 0.08 0.08 0.08 0.08
Zinc 0.005 0.005 0.005 0.005
Aluminum 0.005 0.005 0.005 0.005
Cadmium 0.05 0.05 0.05 0.05
Total named
elements, min 		99.80 99.80 99.80 99.80


  Chemical
Composition, % 		Lead Base Babbitt 
ALLOY NUMBER (GRADE)
  7 8 13 15
  UNS-L53585 UNS-L53565 UNS-L53346 UNS-L53620
Tin 9.3-10.7 4.5-5.5 5.5-6.5 0.8-1.2
Antimony 14.0-16.0 14.0-16.0 9.5-10.5 14.5-17.5
Lead remainder* remainder* remainder* remainder*
Copper 0.50 0.50 0.50 0.6
Iron 0.10 0.10 0.10 0.10
Arsenic 0.30-0.60 0.30-0.60 0.25 0.8-1.4
Bismuth 0.10 0.10 0.10 0.10
Zinc 0.005 0.005 0.005 0.005
Aluminum 0.005 0.005 0.005 0.005
Cadmium 0.05 0.05 0.05 0.05
Total named
elements, min 		       

All values not given as ranges are maximum unless shown otherwise.
Alloy Number 9 was discontinued and numbers 4,5,6,10,11,12,16, and 19 were discontinued in 1959.
A new number 11, similar to SAE Grade 11, was added in 1966.