Archive for October, 2010

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Tin-Copper Bondline Embrittlement and Debonding Phenomena

October 28, 2010

A simple literature review, or search on the Internet for “intermetallic compounds” would lead one to believe that this represents the state of the art in material science.  While the study of many new compounds is on the leading edge of technology, the intermetallic nature of copper and tin in an alloy form has been studied for more than 5000 years.  Without the aid of high tech tools, the Chinese developed the beta bronze alloy form of tin and copper some 1400 years ago.  This was the first metal that could be intentionally heat treated to provide a wide range of mechanical properties.  In more recent research, much attention has been paid to the formation of Cu6Sn5 and Cu3Sn intermetallic compound layers, and their effect on solder joints in electronic assembly.  Unfortunately, little, or no attention has been paid to the identical reaction that occurs when bonding a tin based Babbitt to a copper alloy backing material typical of many fluid film bearings used in industry today.

My first direct exposure to the resultant phenomenon of the formation of these compounds came about 10 years ago.  During the dis-assembly of a high speed gas compressor, the thrust pads were removed from the unit for inspection.  In this particular bearing, the pads were designed with ASTM-B23 Grade 2 Babbitt bonded to a copper alloy containing approximately 2% chrome for increased mechanical strength.  In this application, the high sliding velocity present in the oil lubricated thrust bearing would have yielded unacceptably high bearing temperatures if conventional steel backing material had been used.  The copper alloy backing material was used due to its high thermal conductivity to provide improved bearing performance.  In this instance, following successful dimensional checks, and ultrasonic inspection of the Babbitt bond, the pads were returned to the compressor deck to be re-installed in the machine.  During the installation process, one of the pads was inadvertently dropped from a height of about three inches on to a steel workbench.  As a result of this minor impact, the Babbitt completely separated from the copper alloy backing material.  This was indeed somewhat disturbing that the Babbitt could fall off of an otherwise acceptable part that was ready for installation in a very expensive machine that operates in excess of 10,000 RPM.

Read more…

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Common Problems of Extension Shafts, Oil Pumps and Steady Rest Bearings

October 26, 2010

The extension shafts of Westinghouse large steam turbine-generators are used to drive main oil pumps, support a thrust bearing, hold a mechanical overspeed bolt, and/or to power a fluid drive and boiler feed pump. The design and maintenance of these extension shafts have been the sources of many problems over the years. TRI Transmission & Bearing Corp. has designed and installed proven solutions to these problems…read more

A steady rest bearing

New TRI Extension Shaft and Steady Rest Bearing for Westinghouse Steam Turbine-Generators

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Babbitt

October 21, 2010

Babbitt was first created by Isaac Babbitt in the early 19th century.  Babbitt is a relatively soft metal that is used principally as a liner for oil-lubricated sliding bearings, both journal bearings and thrust bearings.   There are two characteristics that make Babbitt an extremely useful material for sliding bearing applications.  The first is “lubricity”, or the ability to slide surface to surface without galling, generally in the presence of a lubrication such as a mineral oil.  The second is embedability, that is, the characteristic whereby hard particulate matter in lube oil embeds in the layer of Babbitt and thereby minimizing scratching or other damage to the surface of the journal or thrust runner.

There are many combinations of constituents that are used in manufacturing Babbitt.  The first distinction is whether the Babbitt is “tin-based” or “lead-based”.  When tin was difficult to obtain during WW 2, some equipment manufacturers added portions of lead to the Babbitt used in bearings in order to stretch the supply of tin. Many of these bearings failed because lead made the tin-lead Babbitt brittle. This experience was a good justification for using only tin-based Babbitt for rotating equipment where ductility and endurance are always important.

Two widely used Babbitt compositions are known as ASTM B23 Grades 2 and 3.  There are other grades, however, for rotating machinery, Grades 2 and 3 are very common.  Grade 3 has higher strength than Grade 2, but Grade 2 is easier for Mechanical Technicians to use in the refurbishment of existing bearings, and for this reason, it is more commonly used. Other compositions may be used.  TRI typically uses a proprietary Babbitt material that has a unique combination of constituents and special methods of manufacture, with the result of significantly higher strength at higher temperatures.

One of the most important issues that affects the success of the performance of a bearing is the attachment of the Babbitt layer to the backing of the bearing. Carbon Steel is an excellent backing material.  The surface of the steel can be machined and then tinned, avoiding the use of mechanical Babbitt anchors, or dovetails.  Suitable tinning compounds are readily available in the commercial market.

Babbitt thickness is also an important factor in the ability of oil-lubricated babbitted bearings to take abusive pounding.  Thinner layers can survive higher levels of pounding forces, yet thin layers cannot permit large particulate matter to embed without damage to journal or runner surfaces.  Consequently, a compromise is required, usually in the range between 0.030 inches to 0.125 inches, depending upon the application and cleanliness of the lube oil.

Copper based backing materials that are not properly coated before tinning can be expected to develop a brittle coating at the boundary between the tin-based Babbitt and the copper-based backing material.  This phenomenon is called “copper-tin embrittlement and debonding”, and was discovered several years ago by another well-known Babbitt bearing manufacturer. This phenomenon can cause Babbitt layers of bearings with copper-based backing materials to fall off in the storeroom even if they have never been used.  They can also fall off in service, which obviously can damage a machine.  With proper surface preparation and coating, a copper-based backing can have a tin-based Babbitt layer attached, and this bearing can be expected to have excellent performance and service life.

For more information about Babbitt, Babbitted Bearings and Babbitted Bearing Repair, please contact an engineer at TRI Transmission & Bearing Corp.

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Lube Oil Systems

October 19, 2010

Many bearing failures are due to poorly construction lube oil systems. TRI has built or redesigned many lube oil systems. Read more…

damaged bearing caused by faulty lube oil system

A damaged bearing caused by a faulty lube oil system

 

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Fluid Drive Over Heating and Vibration Issues

October 12, 2010

Two common problems found in existing fluid drives (by other manufacturers of course) are over heating and vibration damage. TRI has a Tech Note that explains the history of fluid drives, the historic application for them in the US power industry and the problems that arose after power plants went moved away from providing base load power. Read more…

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Scraped Babbitt Bearings Conform to Journal

October 11, 2010

A machine journal bearing creates a proper oil wedgeA Babbitt bearing bore that has been “scraped” will conform to the shaft journal it supports. A scraped bearing will not create a converging oil wedge which is necessary for proper vibration and temperature control. TRI highly recommends bearings that are machined to tight tolerances. For better performance, an elliptical bore will produce a secondary wedge film above the journal that will help to retrain the vibratory motion of the journal.

Journal bearing with a machine elliptical bore

TRI repairs babbitted bearings. We can centrifugally cast Babbitt or puddle the bore. We have many years of experience in machining to tight tolerances.

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Turbomachiney Show

October 8, 2010

I have just returned from the Turbomachinery Symposium. I would like to thank everyone for stopping by the booth. If you wanted to stop by for an informal conversation but didn’t get a chance, we will be at the Power Gen show in December. I hope to see you then.