Fluid Film Bearings
February 6th, 2017
By Grant Slinger - Mechanical Engineer II
Fluid Film Bearings
Fluid film bearings are one of the most fundamental and critical components in rotating machinery. Fundamentally bearings constrain the motion of a rotating shaft and provide a means to transmit force from a rotating shaft to a stationary support with minimal friction. There are two main types of bearings used in industrial equipment; rolling element type bearings and fluid film bearings. Typically larger and higher speed and load equipment utilize fluid film bearings. In industry these are typically referred to as plain bearings, journal bearings, or sleeve bearings. From a predictive maintenance (PdM) standpoint bearings represent the ideal measurement location to diagnose machine faults as they are the main transmission point of forces from the rotating shaft to the supporting structure. Although most faults are first identified at the bearings, the source is not necessarily the bearing itself. Machine faults such as unbalance, misalignment, looseness, resonance, and process related issues can all be identified by monitoring the proper parameters on bearings in rotating equipment. But what is the best way to diagnose these faults and does the type of bearing matter when selecting a monitoring technology to use? For fluid film bearings the applicable monitoring techniques are not necessarily the same as with rolling element bearings. To understand why and determine the best monitoring technology for your application we will first look at the fundamentals of how fluid film bearings operate.
Fluid Film Fundamentals
In a fluid film bearing the shaft is supported by a thin layer of lubricating fluid, and the rotating and stationary parts are not in direct contact with one another. There are two main type of fluid film bearings used in industry; hydrostatic and hydrodynamic. Hydrostatic bearings are externally pressurized with a lubricant and do not rely on rotating of the shaft to develop a fluid film. Conversely, hydrodynamic bearings rely on the speed of the shaft to pressurize the fluid in the bearing and lift the shaft off the bearing. The following image shows how a spinning shaft in a fluid film bearing generates a pressure wedge which lifts the shaft off the bearing surface and prevents metal on metal contact. This separation between rotating and stationary surfaces allows for fluid film bearings to be designed with theoretically infinite life.
Figure 1: Lubricating fluid forms a pressurized wedge which lifts the shaft off the bearing surface (Source: Bently Nevada)
Hydrodynamic fluid film bearings are characterized by different lubrication regimes which occur on startup and shutdown. When a fluid film bearing first starts up at slow speeds there is insufficient pressure in the wedge to lift the shaft off the bearing surface and the two surfaces rub. Once the shaft reaches a sufficient speed the hydrodynamic lubrication generates enough pressure in the wedge to lift the shaft off the bearing and there is no contact between the stationary and rotating components. For this reason, the majority of wear in hydrodynamic bearings occur on startups and shutdowns.
Figure 2: Hydrodynamic bearing lubrication regimes (Source: Substech)
Fluid Film Bearings Failure Mode Analysis
The simplest way to identify the failure mode of a fluid film bearing is to examine the bearing surface and characterize how the material itself failed. Each failure mode can have several different root causes that lead to the eventual failure of the bearing surface. The most common fluid film failure modes observed can be categorized into the following four wear patterns; fatigue, wiping, scoring, and corrosion. Refer to Red Wolf Reliability’s post entitled Main Bearing Failure Modes for a more in depth failure analysis of fluid film bearings.
As a consulting company Red Wolf Reliability is often asked to analyze rotating equipment with fluid film bearings and provide assessments on the current health of the equipment as well as recommend preventative maintenance actions. Often this includes diagnosing fluid film bearing equipment which have little or no permanently installed monitoring systems. Typical monitoring technologies used in industry for these bearings include proximity probes, temperature sensors, and case mounted accelerometers. Through years of troubleshooting experience and numerous root cause failure analysis we have developed a thorough understanding of what works and what doesn’t when it comes to properly monitoring fluid film bearing equipment.
The industry standard for monitoring large-scale fluid film bearing equipment is the use of proximity probes. These sensors measure relative displacement of the shaft from the stationary bearing housing. The preferred sensors setup is two proximity probes oriented 90 degrees apart which allow for the dynamic measurement of the shaft center within the bearing housing. Plotting the dynamic output from two of these sensors generates what is called an orbit plot. Numerous different machine and bearing faults can be identified through shaft orbit analysis. This is the preferred sensor setup when analyzing fluid film bearings as it provides an accurate representation of the dynamic shaft motion in the bearing (shaft orbit plot) as well as the DC movement of the shaft in the bearing (shaft centerline plot). The downfalls to a proximity probe monitoring system is the bearing housing must be drilled and tapped for the probes and sensor signals must be slow roll compensated to account for shaft runout and surface imperfections. Analyzing data from a non slow roll compensated signal can lead to inaccurate orbit plots that do not represent the true motion of the shaft and can incorrectly show displacement amplitudes often in excess of the radial clearance of the bearing. The following image shows a typical sensor setup for proximity probes on a fluid film.
Figure 3: Proximity probe sensor setup (Source: National Instruments)
Direct bearing temperature measurement probes such as RTDs and thermocouples are also often installed in fluid film equipment as a monitoring technology. Each of these probes provide a relatively simple method of obtaining a measurement and interfacing with available measurement and logging systems. However, temperature measurements alone provide a minimal amount information to diagnose specific machine faults. It is seldom adequate to put simple alarm and shutdown limits on bearing temperature. The thermal lag within most bearings and sensors means that such information cannot be relied upon to provide a safe shutdown trigger. Often clients tell us that temperature sensors do in fact trigger an alarm but it is typically too late to save the equipment in the event of a catastrophic failure. A temperature probe is better than no monitoring at all but it is important to realize that they will only be effective in identifying gradual changes in temperature and typically will not provide sufficient warning on sudden faults.
Case Mounted Vibration
At Red Wolf Reliability, we often take routine case mounted accelerometer readings on fluid film bearing equipment. Typically, this is a last resort measure when there are no proximity probes or temperature sensors installed on the equipment. In every condition monitoring report we provide to clients there is a disclaimer stating that “Data collected on fluid film bearings using case mounted transducers is unreliable”. This isn’t just to cover ourselves in the event of a failure; we have routinely observed that case mounted readings on fluid film bearings are not a good representation of health of an asset. The transmission path for forces from the shaft to a case mounted transducer on the bearing cap is highly non-linear and typically generates a large amount of harmonics of shaft speed that are not a good representation of shaft motion within the bearing. This makes it difficult if not impossible to make accurate diagnoses of machine faults.
Red Wolf Reliability R&D
It’s clear that the best monitoring technology for fluid film bearings are properly configured X-Y proximity probes, but this is not always a feasible method for bearings that were not setup this way from the OEM. Pioneer Engineering’s Research and Development department has invested a significant amount of time and resources on developing a better way to monitor fluid film bearing equipment that lacks properly configured proximity probes. Pioneer engineering has developed a patent pending measurement technology that can be installed on fluid film bearings without the need to drill and tap the bearing for proximity probes. Initial research and beta test installs show that the data measured closely matches that of proximity probes and can provide a similar amount of protection at a fraction of the price without the need for costly sensor retrofits. If you are looking for a better way to monitor a fluid film bearing asset without having to retrofit a costly proximity probe measurement system, contact Red Wolf Reliability’s R&D department for more information.