TRI has just completed the installation of a fluid drive brake. This system is similar to previous system that we have installed with the exception of larger calipers.
See our web site for more information about brakes and TRI Transmission and Bearing Corp.
Some of the first parameters needed when selecting the fluid drive are the power and speed requirements of the working fluid. These parameters will determine the size of the circuit diameter. TRI has created an online interactive application for this purpose. The Interactive Sizing Chart is free to use at the TRI Web Site
Here is a photo I snapped in the shop this morning. It is a TRI Transmission & Bearing pad that has been sent back for refurbishing. The surface is rough because the babbitt has been puddled. The next process will be machining the surface to the proper diameter.
The puddling process is a manual operation that takes some time. You can see the babbitt is laid down in rows. After machining the bore, porosity will be revealed. More babbitt will be laid down to fill the voids and the machining will be repeated.
by Turbo Research Inc.
There is only one purpose for having a brake on the output shaft of a fluid drive; to aid in the operation of the boiler feed pump by stopping the boiler feed pump shaft and to keep it from rotating when the pump is out of service.
There are certain boiler feed pumps which are designed and built in ways that will cause them to gall and consequently to seize if they rotate for more than a few seconds or a few minutes at low speed with no or low flow through them. Typically, these pumps have stainless steel components that can rub together. They are often the higher performance, higher pressure pumps, but this is not always the case.
There usually is sufficient experience with each boiler feed pump over the years to know if that pump has or has not seized when it was at low speed during those times when a) the pump was out of service and simultaneously, b) the brake is imperative.
For those boiler feed pumps which have not seized when they were operated at low speed for several hours, or for which the manufacturer indicates that no problem will arise by operating at low speed with low/no flow, the brake is not required, and there is no value in using it.
“Sliding Pressure” simply mans reducing boiler pressure in proportion to a reduction of MW generation. When “Power Wheeling” is fully effective, clearly those electrical generating units with the lowest cost will operate the most. Overall operating costs, efficiency, and/or “”eat rate”" as well as “turn down ratio” for each turbine-generator unit will be critical.
The “turn down ratio” is the ratio of maximum to minimum load. Those with high ratios are preferred because they can take a big swing from maximum load in the day to minimum load at night. Without coming off line.
“Sliding Pressure” for most sub-critical boilers is usually required to obtain the lowest minimum load a unit can achieve continuously and the maximum efficiency at that minimum load.
For fluid drive applications, a reduction of boiler pressure corresponds to a reduction of the output shaft speed/BFP shaft speed, resulting in increased heat, usually more than the amount for which the unit was designed, and often, high amplitude vibration.
TRI Transmission & Bearing Corp. has proven solutions in hand for fully evaluating and resolving these issues for all sizes and types of fluid drives, in BFP or other applications.
For certain units, during sliding pressure operation, new control valve opening patterns occur, leading to different “nozzle block” forces on the turbine rotors. TRI evaluates the changing rotor vibration conditions which result and makes recommendations, including the benefits provided by TRI Align-A-Pad ® bearings and other TRI products for reducing rotor vibration and bearing maintenance.
3 Case studies with solutions…read more
In recent years, TRI has encountered and resolved a number of problems with ring-oiled bearings for fans, motors, and pumps. Oiling rings damage the journal surfaces and a lack of good lubrication can lead to excessive wear and eventually complete bearing failure. Many cost effective improvements can be implemented for this class of bearings given the technology that is available today…read more
Alignment problems of ring oiled bearings can cause damage and oil leaks
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.
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
New TRI Extension Shaft and Steady Rest Bearing for Westinghouse Steam Turbine-Generators
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.
