Bearings are a critical component of rotating machinery and equipment. For condition monitoring purposes, they provide the ideal measurement location for diagnosing machine faults on rotating equipment. They take the brunt of both radial and axial loads on the shaft in order to maintain smooth operation. Faults such as misalignment, unbalance, resonance, and process-related issues can be identified by measuring bearing parameters.
There are two main types of bearings used in industrial equipment:
- Journal bearings
- Rolling element bearings
Having a basic understanding of the different characteristics, functionalities, and typical applications for the various types of bearings is important to be able to accurately diagnose machine problems and perform the required maintenance.
Let’s take a deeper dive into each.
Journal bearings (also known as plain or sleeve bearings) allow a shaft to rotate freely within a supporting sleeve or shell. The “journal” is simply the part of the shaft that fits in the bearing. The shaft in a journal bearing is supported by a thin film or layer of liquid – usually oil – preventing the rotating and stationary parts from coming in contact with one another.
There are two types of journal bearings: hydrostatic and hydrodynamic. Unlike hydrostatic bearings that are externally pressurized with a lubricant, hydrodynamic bearings rely on the speed of the rotating shaft to pressurize the fluid in the bearing, creating a region of high pressure (known as a pressure wedge) that lifts the shaft off of the bearing. As the load on the shaft varies, so too does the area of high pressure, making journal bearings quite tolerant of overloads and shock loads.
The animation below depicts how the rotating shaft in a journal bearing creates a pressure wedge that lifts the shaft off of the bearing surface and prevents metal on metal contact.
Most critical turbomachinery in a facility are equipped with fluid film bearings. This machinery includes steam and gas turbines, compressors, pumps, motors, gearboxes, fans, generators, etc.
Advantages of Journal Bearings
Some of the primary advantages of journal bearings include:
- Lower cost.
- Unlimited fatigue life due to lack of contact between rotating and stationary parts.
- Able to withstand overloads and shock loads – this is due to the fact that as the load on the shaft varies, so too does the area of high pressure.
- Provides more damping than rolling element bearings due to the lubricant present. The thicker and more viscous the lubricant, the higher the damping properties and ability to absorb vibration and impact loads.
- Reduced noise.
- Bearing fatigue is usually visible at an early stage, allowing for better diagnostics of failure modes and earlier corrective action.
- Lubricant removes heat generated in the bearing and helps to flush debris from the load area.
Disadvantages of Journal Bearings
Some disadvantages of Journal Bearings include:
- Surface damage can occur at start-up and touch down since there is no supportive oil film when the shaft is at rest.
- Potential for oil whirl and oil whip which, if left uncorrected can be catastrophic and can destroy the bearing and rotor very quickly.
- Susceptible to particulate contamination.
- Higher friction (HP loss) than rolling element bearings.
Generally, a rolling-element bearing consists of an inner raceway that is mounted on a shaft and an outer raceway that is carried by the housing or casing. In between the inner and outer raceways are the rolling elements and a cage. The cage is required to maintain the spacing between rolling elements, preventing them from coming in contact with each other. During rotation, the motion of one raceway relative to the other occurs. Rolling-element bearings support the load via the contact that exists between the rolling elements and the raceways.
The most common classification of rolling-element bearings is based on the geometry of the rolling element itself (ball bearing, spherical roller bearings, cylindrical bearings, etc.) with ball bearings being the most common type. Ball bearings are described as having point contact meaning each ball touches the race in a tiny patch or point. Roller bearings are said to have a line of contact (rather than a point), allowing them to have greater capacity and shock resistance. However, because of this contact, rolling-element bearings do not have infinite lives. They are designed so that, statistically speaking, a certain number are expected to fail after a set number of revolutions has accumulated, thereby defining the “useful life” of the bearing.
Advantages of Rolling-Element Bearings
Some advantages of rolling-element bearings include:
- General low starting friction.
- Can withstand lubricant starvation.
- Can support radial and axial loads.
- No low temperature starting issue.
- No need to re-lubricate – lubricant well-sealed inside bearing.
Disadvantages of Rolling-Element Bearings
Some disadvantages of rolling-element bearings include:
- More noise during operation (especially at high speeds).
- Higher initial cost.
- Low resistance to shock loading.
- Wear during standstill (pitting).
- Design of bearing housing is complicated.
At IVC, we have almost 40 years of experience monitoring all types of rotating machinery and their bearing components. We understand the vibration signatures inherent in bearings beginning to fail and can alert our customers before catastrophic failure occurs. We can help you too – contact us.