Skip to main content

Failures in babbit bearings

 There are literally dozens of ways bearings can fail. Some of the more common include:

• Babbitt fatigue

• Babbitt wiping due to rotor to stator contact

• Babbitt flow due to high operating temperatures

• Foreign particle damage

• Varnish build-up

• Electrostatic discharge damage (frosting)

• Electromagnetic discharge damage (Spark tracks)

• Oil “burn” or additive plating due to high temperatures

• Loss of bond between babbitt and base metal

• Chemical attack

• Pivot wear in tilting pad bearings

• Unloaded pad flutter

• Cavitation damage

This is taken from a paper, Babbitted bearing health assessment" by John Whalen of John Crane, Thomas Hess of Rotating Machinery Group, Jim Allen of Nova Chemicals Corporation and Jack Craighton of Schneider Electric.

Babbitt fatigue

Babbitt fatigue is caused by dynamic loads on the babbitt surface. Typically in bearings of this type, the dynamic loads are caused by vibration and result in peak film pressure fluctuations. Cracks initiate on the babbitt surface and propagate radially towards the bond line. As the cracks get closer to the bond line the strength of the backing material reinforces the babbitt and causes the cracks to turn and spread circumferentially, meeting with other cracks and dislodging pieces of babbitt. A close-up of the babbitt surface shows portions of babbitt missing and a section showing only surface cracks.

Babbitt fatigue strength can be increased by utilizing thinner babbitt (to take advantage of the backing material strength) and by keeping the babbitt temperature low. Obviously for a running machine changing babbitt thickness is not an option, but taking steps to reduce temperature and vibration often times are available.

Edge Load Pivoted Shoe showing Babbitt Mechanical Fatigue

Loss of bond between babbitt and base metal

This is most common when babbitt is applied to copper alloy. Copper is used extensively to reduce bearing operating temperatures by allowing a significantly increased heat transfer coefficient. Most bearing companies use a copper alloy that has good stiffness and strength while still having good heat transfer properties.

Loss of bond with copper pads

It has been discovered that copper has a strong affinity for tin and this is magnified at elevated temperatures. When babbitt is applied to bearings the process includes a tinning operation so the tin adheres to the base metal and then the babbitt bonds to the tin. Should the tin diffuse into the copper the bond will become weaker and weaker, and brittle. Oddly enough this is oftentimes not discovered during normal NDT inspection (such as visual, PT or UT) since the bond is intact, it is just compromised and measures of bond strength are destructive. It has been discovered that applying a barrier layer prior to babbitting can eliminate this issue. A material must be selected that has good bond strength to copper and the tin will adhere to, while providing a barrier to tin migration into the copper. This failure mode is not possible to predict with conventional condition monitoring tools. As such the best way to address this issue is to verify the copper-backed bearings, new or repaired, have a proven barrier layer. If there is a question as to whether the barrier layer exists it is recommended that during the next outage this is addressed. Also while the primary author's company has seen dozens of bearings that have had this problem none of them have failed in service; it is theorized that the brittle bond is not challenged during normal operation.

Electrostatic discharge

As mentioned earlier it is known that rotors can build up a static charge and this charge will jump to the ground through the easiest path available. Preferably grounding brushes are utilized and these brushes take the charge off the rotor to the ground in a controlled way. If there are no grounding brushes or they are not working properly then this charge can go to the ground through the location where the rotor is closest to a grounded stationary.

Thrust pad with electrostatic discharge damage (baoduongcokhi.com)

Note that with a bearing like this the point of minimum film thickness is typically on the upper trailing edge as shown, Note that the babbitt has been spark eroded away starting in the minimum film thickness corner and working its way down the pad, as the pad shape changes. This damage can be avoided by ensuring there is adequate grounding.

Pivot wear

Since pads in tilting pad journal and thrust bearings actually tilt they can exhibit damage at the pivot interface(s) that can affect the performance of the bearing.

Pivot wear

This damage can be avoided with a lower stress pivot design and/or control of vibration levels.

Oil coking

From time to time oil analysis may indicate that the oil is oxidizing and/or an additive package content is diminishing. This could also be accompanied by slight rotor position moves and is usually present when there are elevated temperatures.


Varnish

As mentioned earlier the formation of varnish on lubricated surfaces has increased over the last 15 years or so as the more highly refined Group II oils have been utilized. At first glance, it may appear as though the damage is similar to coking but further analysis will show that varnish deposits can be found on any surface in contact with the oil, high temperatures and pressures are not required. Signs that you may be experiencing varnish deposits include rotor position changes (due to the deposits getting thicker) and temperature changes due to the insulating behavior of the deposits and the thickness of the deposits resulting in reduced clearances.


 

Comments

Popular posts from this blog

Maintenance 4.0 Implementation Handbook (pdf)

WHAT IS MAINTENANCE 4.0? Industry 4.0 is a name given to the current trend of automation and data exchange in industrial technologies. It includes the Industrial Internet of things (IIoT), wireless sensors, cloud computing, artificial intelligence (AI) and machine learning. Industry 4.0 is commonly referred to as the fourth industrial revolution. Maintenance 4.0 is a machine-assisted digital version of all the things we have been doing for the past forty years as humans to ensure our assets deliver value for our organization. Maintenance 4.0 includes a holistic view of sources of data, ways to connect, ways to collect, ways to analyze and recommended actions to take in order to ensure asset function (reliability) and value (asset management) are digitally assisted. For example, traditional Maintenance 1.0 includes sending highly-trained specialists to collect machinery vibration analysis readings on pumps, motors and gearboxes. Maintenance 4.0 includes a wireless vibration sensor conne

27 steps of the Gearbox Repair and rebuilding

 27 steps of the Gearbox Repair and rebuilding: Step 1 Cleaning exterior of Gearbox and identification. Step 2 Remove all bolts from the gearbox. Step 3 Disassembly for Gearbox preliminary evaluation of the condition and repair required Step 4 Mag inspect Gearbox. Step 5 check all Gears. Step 6 Customer communication of health of the Gearbox. Step 7 Parts to be repaired or, reverse engineered parts where needed required for Gearbox rebuild. Step 8 Failure analysis during complete disassembly and evaluation of the component wear and damage. Step 9 Cleaning all internal components and housing. Step 10 Check all bearings diameters in house. Step 11 Check all shafts Step 12 inspect all Gears. Step 13 Set up check line bore of the gearbox. Step 14 Repair and rebuild Gears back to O.E.M Step 15 Replacing all bearings seals and gaskets Step 16 Repair and rebuild all shafts again to O.E.M Step 17 Realigning all gears shafts and bearings back to O.E.M Step

Thermal growth: how to identify, quantify and deal with its effects on turbomachinery

Thermal growth, as used in the field of machinery alignment, is machine frame expansion resulting from heat generation. The generation of heat, of course, is caused by operational processes and forces. Materials subjected to temperature changes from heat generation will expand by precise amounts defined by their material properties. In turbomachinery, thermal growth results from the temperature differences occurring between the at-rest and running conditions. Generally speaking, the greater the temperature difference, the greater the thermal growth. The magnitude of the growth can be calculated from three variables: ∆ T (temperature difference) C   (coefficient of thermal expansion) L    (distance between shaft centerline and machine supports) When machinery begins to generate heat, the temperature difference between at-rest and running conditions will cause thermal expansion of the machine frame, thereby bringing about the movement of the shaft centerlines. This can produce changes in

John Crane's Type 28 Dry Gas Seals: How Does It Work?

How Does It Work? Highest Pressure Non-Contacting, Dry-Running Gas Seal Type 28 compressor dry-running gas seals have been the industry standard since the early 1980s for gas-handling turbomachinery. Supported by John Crane's patented design features, these seals are non-contacting in operation. During dynamic operation, the mating ring/seat and primary ring/face maintain a sealing gap of approximately 0.0002 in./5 microns, thereby eliminating wear. These seals eliminate seal oil contamination and reduce maintenance costs and downtime. John Crane's highly engineered Type 28 series gas seals incorporate patented spiral-groove technology, which provides the most efficient method for lifting and maintaining separation of seal faces during dynamic operation. Grooves on one side of the seal face direct gas inward toward a non-grooved portion of the face. The gas flowing across the face generates a pressure that maintains a minute gap between the faces, optimizing flui

Technical questions with answers on gas turbines

By NTS. What is a gas turbine? A gas turbine is an engine that converts the energy from a flow of gas into mechanical energy. How does a gas turbine work? Gas turbines work on the Brayton cycle, which involves compressing air, mixing it with fuel, and igniting the mixture to create a high-temperature, high-pressure gas. This gas expands through a turbine, which generates mechanical energy that can be used to power a variety of machines and equipment. What are the different types of gas turbines? There are three main types of gas turbines: aeroderivative , industrial, and heavy-duty. Aeroderivative gas turbines are used in aviation and small-scale power generation. Industrial gas turbines are used in power generation and other industrial applications. Heavy-duty gas turbines are typically used in large power plants. What are the main components of a gas turbine? The main components of a gas turbine include the compressor, combustion chamb