Page History: Babbitt - What is Babbitt?
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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 "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.
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 whiel lead based works best with low speed or heavy load bearings.
Selecting a Type of Babbitt
To determine which bearing material would be best for your application, use the following are suggested guidelines:
|
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:
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.
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.
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.
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.
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.
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, here are the current standards for Babbitt metal by ASTM.
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.