SKF developed the world’s first hub bearing units and the first custom-designed engine oil seals. Today, SKF offers a complete portfolio of products and services to help reduce CO2 emissions from single bearings and seals to sensor-bearings and complete powertrain solutions. With over 45,000 employees and 140 locations in more than 30 countries, SKF relies on their strong global presence to maintain profitable growth without compromising their commitment to the environment and society.

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Kevin Huberty of Sargent & Lundy provided a comprehensive review of SMR technology, including the various models, ongoing projects, government regulations, and major vendors within the industry.

EthosEnergy’s turbine workshop and Performance Center work hand-in-hand to maintain operators’ turbine equipment and provide remote operations and monitoring.

EthosEnergy’s turbine workshop and Performance Center work hand-in-hand to maintain operators’ turbine equipment and provide remote operations and monitoring.

For decades SKF has been a trusted OE supplier to the world's top car manufacturers around the globe. In fact, SKF has a strong global network of research and development (R&D) centers and laboratories, as well as established collaborations with major universities and research institutes. Through more than 10,000 distributors and dealers within the industry, their spare parts for cars and trucks reach their destination at workshops and retailers all over the world.

As the orbit and shaft centerline displays are in displacement units, they can be presented together in the same graph (Figure 3). The shaft centerline portion of the graph shows shaft lift from the supporting oil wedge at increased speed, and a position above bottom-dead center (normal for a tilt-pad bearing). The orbit at full speed is fairly circular and at an acceptable magnitude. Together, these observations indicate good alignment and balance, conclusions that can be further validated by bearing metal temperatures. Presentation of shaft orbit and centerline plots together in an overlay graph is convenient, and also enables easy correlation of findings derived from these two plots. An external preload due to coupling misalignment, for example, will constrain shaft response in the plane of the preload force, yielding a much flatter orbit than normal. Such a preload can also force the shaft to run in a different position within the bearing clearance than under normal conditions. The shaft centerline plot shown in Figure 4 shows position changes at the exhaust end bearing of a large gas turbine rotor during startup and load-up. The machine exhibited normal centerline position up to about 3,000 rpm. For the remainder of the startup to 3,600 rpm and as load was added to the machine, it showed movement down and to the right, reaching the bearing clearance boundary. Also shown in the same graph is an overlay of a highly preloaded figure-eight shaped orbit. The orientation and tilt of the orbit is parallel to the bearing wall. Taken together, the orbit and shaft centerline data confirm presence of preloading and a heavily loaded bearing. Severe misalignment was suspected, but exact root cause was yet to be determined.

An expanded view of the figure-eight orbit (Figure 5) also shows the time-base waveform signals from the X-Y probe pair used to draw the orbit. In the Y probe signal, two vibration peaks can be re-seen between successive rotational timing marks indicating 2X running speed vibration; another symptom of misalignment. Based on these indications, plant operations personnel were advised to shut down the unit. Subsequent inspection revealed large cracks in structural elements supporting the exhaust-end radial bearing large enough to compromise the supports. The thermal distortion induced misalignment, resulting in the excessive preload.

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For vibration analysis of large rotating machinery, shaft orbit and centerline analysis are useful diagnostic techniques for the specialist’s toolbox. Especially when used together, they can yield diagnostic information for evaluating machinery condition and pinpointing the root cause of malfunctions.

Some change in shaft centerline position with increased machine speed is normal and expected. This is because a radial fluid-film bearing provides hydrodynamic support of the rotor on the oil wedge which develops underneath the shaft as it rotates. The supporting oil wedge generally delivers more lift to the shaft at higher rotational speeds, causing the gap to change between the shaft and the observing proximity probes.

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For 110+ years, SKF has been reducing friction, making things run faster, longer, cleaner, and more safely, contributing to their vision of reliable rotation. With over 20,000 part variations, SKF promises and delivers wide market coverage, enabling customers to find the exact part they need for their vehicle’s make and model. SKF components reduce emissions, increase fuel efficiency, and stand apart in a rapidly changing market. The SKF catalog of parts extends from bearings, seals, and universal joints to drivetrain and transmission parts.

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The shaft centerline display is most useful when centerline position data points are referenced to the bearing clearance boundary. This reference can easily be established on a horizontal machine by capturing DC gap reference voltages from the X and Y probes with the machine at rest, preferably following a shutdown to minimize hysteresis in the data from thermal effects.

The DC gap voltages captured at rest can then be used to reference the plot starting point at the bottom of the bearing clearance boundary. The resulting graph displays shaft centerline position changes relative to the starting point.

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When changes in DC voltages from the two probes are converted to displacement units and plotted together in X-Y format, the resulting two-dimensional graph is called the shaft centerline. The most common application of the shaft centerline graph is to display changes in shaft centerline position within the bearing, such as might be experienced during a startup or shutdown, or which may occur at constant speed over time given changes in process load.

Then, consider the same rotating shaft with the introduction of increased unbalance force at higher speeds. The shaft will now begin to exhibit dynamic response as a result of the unbalance force. Because unbalance is an outward force that is also rotating at the rpm of the machine, shaft deflection will tend to follow a circular path at a frequency equal to running speed (Figure 2).

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Most shaft orbits are slightly elliptical due to the net effect of steady state side loads, or preloads, acting on the shaft. Examples include reaction from discharge flow on a pump, partial arc admission of steam into a turbine, and gravity on a horizontal machine. A significant aspect of orbit analysis, therefore, is differentiating between effects of normal preloads and significant deviations from the expected shape and form that are the result of machinery malfunctions.

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The shaft orbit presentation is constructed by plotting the AC vibration signals from each probe pair together in X-Y format. The resulting graph in displacement units represents the dynamic deflection of the vibrating shaft within the bearing. One way to conceptualize how a shaft orbit develops is to first consider a shaft rotating at low speeds with little dynamic motion.

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Did you know that SKF is an acronym for the Swedish name Aktiebolaget Svenska Kullagerfabriken. The brand was founded in Gothenburg, Sweden in 1907. Learn more about SKF here.

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While orbits are derived from the AC portion of the signals captured from an X-Y probe configuration, shaft centerline plots are derived from the DC portion. The DC signal from each probe is proportional to the average gap between the probe tip and the shaft surface.

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SKF, an acronym for the Swedish name Aktiebolaget Svenska Kullagerfabriken, was founded in 1907 in Gothenburg, Sweden. Eight years later, the first factory in the U.S. had been built. Branch offices began to open around the world from Rio de Janeiro to Uruguay to Norway. By 2007, SKF had celebrated their 100th anniversary with production in 3 new factories in Asia, along with the creation of a new energy-efficient bearing product line that delivered at least 30% less friction. And today, they’re committed as ever to producing parts with zero-impact on the environment.

Shaft orbit and centerline analysis apply to equipment such as turbines, generators, motors, pumps, compressors and fans with fluid-film radial bearings. By design, radial bearings allow for a nominal amount of shaft movement within the bearing clearance.

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Kevin Huberty of Sargent & Lundy provided a comprehensive review of SMR technology, including the various models, ongoing projects, government regulations, and major vendors within the industry.

Shaft vibration measurements relative to the bearing are made possible by pairs of radial- mounted, non-contact proximity transducers installed in an X-Y configuration 90 degrees apart at each bearing location (Figure 1). Because the frequency range of proximity transducers starts at DC (0 Hz), the information from each X-Y probe pair enables two-dimensional views of shaft dynamic motion and average position within the bearing clearance.

SKF has delivered friction management technologies to a variety of industries, including the automotive aftermarket for more than 110 years. Drawing on their experience as an OE supplier to car and truck manufacturers around the globe, SKF also supplies premium, affordable aftermarket parts for cars and trucks. SKF’s core expertise focuses on engine, drivetrain, and wheel-end components. Their mission is to offer top-notch products and services to their customers from technicians to drivers.

At low speeds, the resulting orbit presentation will not be much more than a dot on the graph, assuming that the shaft is straight and there is little noise in the signal from scratches or other surface irregularities.