Babbit bearings Failure Modes

Failures in babbit bearings:

1. Low speed “wiping” and fatigue;

A slight low-speed wiping coupled with at-speed fatigue damage?

The bottom portion of this bearing shows an even wipe (or polish) pattern from low speed operation (operating in the boundary lubrication regime). We also see fatigue damage; however, this is presenting itself only on the right side of the bearing – indicating that the bearing became misaligned once up to speed.

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.

2. 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.

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.

****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.?

3.Varnish:

The varnish is not isolated to the hot spot of the thrust pads but can be seen on the retainer, links and even the back side of the thrust pads.

This damage can be avoided by being aware of varnish potential as predicted by oil analysis and taking steps to mitigate the phenomena. 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.

Even worse side effects could be found where the oil is used for other purposes in the machine (such as servo valves, etc). In these cases, the tacky varnish coated surfaces can result in “sticky” operation of the components in question.

4.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.?

the presence of this “coke” is typically located in the area of the bearing exposed to high film pressure coupled with high operating temperatures; also note that the 75-75 location does seem to represent the most distressed portion of the pad.

This damage mechanism can be avoided with careful selection of additive packages, monitoring of oil analysis and control of operating temperature.

5.Electrostatic discharge:

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

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.

Tilting Pad Journal (TPJ) bearing outer shell damage can be caused by dynamic load being transmitted across this interface (typically brinnelling damage) and by relative motion of the pad to the shell (typically fretting). Either or both of these wear mechanisms can act to wear the pivot interface which can manifest itself as an “opening” up of the bearing clearance, possible resulting in increased vibration.

One way to detect the possibility of this happening is by monitoring the shaft centerline position for journal bearings and axial position for thrust. If moves are detected and there is no tin found in the oil then one may conclude there is a possibility of pivot wear. For journal bearings this could also be accompanied with increased vibration (due to loss of stiffness and damping with the increased clearance).?

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

Reference:

(John K. Whalen is the Chief Engineer - Bearings for John Crane Engineered Bearings. James “Jim” Allen is a Senior Rotating Equipment Specialist in the Reliability Team for Nova Chemicals, in Red Deer, Alberta. Thomas Hess works in the Rotating Machinery Group for DuPont located in Wilmington, Delaware. John Craighton works for Schneider Electric as a Technical Consultant.)

https://www.turbomachinerymag.com/view/failures-in-babbit-bearings