Skip to main content

UNDERSTANDING HYDRODYNAMIC BEARINGS

AN OVERVIEW OF HYDRODYNAMIC BEARINGS, THEIR DEFINITION, THEORY OF OPERATION AND TROUBLESHOOTING TIPS.

BY AMR HATEM RASHED.

Hydrodynamic bearings (also known as fluid film bearings) are often deployed as journal bearings. As such, they come in several types, including radial tilting pad bearings, thrust bearings for axial displacement and journal sleeve bearings.

Journal sleeve bearings are typically used in low-speed and low-friction applications while radial tilting pad bearings are used for high-speed applications due to the high amplitude of vibrations.

Thrust bearings, on the other hand, are generally employed for axial displacement in high-speed applications as they contain tilting pads that support high-thrust loading of rotors.


Pivoted shoe journal bearing


Pivoted shoe thrust bearing

Frictionless support

The journal bearing has several functions. It acts as frictionless support for the rotor while it is rotating. It cools down the rotor by transferring the heat energy generated from the process gas to the rotor and then to the oil by convection, or from the process steam in the case of steam turbines. Another role is to dampen high-amplitude vibrations by means of pivoted tilting pads and an oil stream entering the bearing, which creates an oil film between the rotor and the bearing stationary pads.

The bearing itself consists of the journal region of the rotor and the bearing housing, containing the internal chamber of bearing and pivoted tilting pads. There is also a channel for oil entrance, an oil outlet and a thrust collar in the case of thrust bearings.

The bearing undergoes a hydrodynamic wedge effect as the rotor spins eccentrically inside the bearing housing at its normal operating speed. The rotor exerts a force on the oil enclosed between the rotor and the pads according to Newton’s second law and in accordance with oil’s incompressibility property.


 Hydrodynamic principle


Hydrodynamic thrust bearing illustration

The oil exerts a reaction force equal in magnitude on the rotor resulting in raising it upwards. This action is called the hydrodynamic or wedge effect. In other words, the heavy rotating rotor is supported by the hydrodynamic effect (viscous force).

The main parameter controlling hydrodynamic bearings is called “load carrying capacity” which determines the size of the bearing. This depends on oil pressure, temperature, flow rate, viscosity, oil film thickness and rotating speed. The load-carrying capacity (LCC) is directly proportional to all of those parameters except for oil temperature, which is inversely proportional.

When oil pressure increases, it can generate vortices and eddies inside the bearing housing which can cause high-amplitude vibrations at a frequency equal to 0.45X of the rotating speed (oil whirling). When oil pressure decreases, it can lower the thickness of the oil film which can lead to a drop in the vibration damping ability so high-amplitude vibrations can occur.

Another point to note is that when oil temperature increases, it causes a decrease in oil viscosity which in turn leads to low oil film thickness, low vibration damping and an increased oil flow rate (the viscous effect will decrease).

However, when the opposite occurs (oil temperature decreases), there is greater vibration damping ability and a higher temperature difference between the rotor and the oil. This equates to better cooling which is a desirable condition for machine operation.

Other operational points:

·     Lowering the rate of oil flow decreases the thickness of the oil film as well as oil pressure. However, poor cooling of the rotor also occurs.

·     An increase in oil viscosity improves vibration damping and cooling.

·     Rotational speed is directly proportional to load-carrying capacity.

·     An increase in oil film thickness can cause vortices and eddies, but if the thickness decreases there will be low damping and a low heat transfer rate for rotor cooling.

Oil and vibration

Oil is controlled by means of the oil system. The oil temperature is controlled by oil coolers to maintain the temperature within a constant accepted range. Oil pressure control valves maintain constant pressure. An orifice maintains a constant oil flow rate during operation.

Vibrations are associated with rotational speed. 1X is a synchronous harmonic frequency equal to the speed of rotation. Imbalance can be due to mechanical factors (noted by rising vibration as rotor speed increases) or during operation because of fouling of process gas inside the impellers.

2X is super synchronous, superharmonic that can be generated by misalignment or mechanical looseness. This can best be recognized by side frequencies associated with vibration frequency peaks.

Angular misalignment is indicated by an axial reading and a high 1X frequency compared with 2X frequency.

Parallel misalignment, or offset, on the other hand, is indicated by a radial (vertical) reading and a high 2X frequency compared to the 1X frequency.

Finally, oil whip instability may occur if the machine is operated at or above 2X the rotor’s critical frequency; oil whirl becomes oil whip instability as the shaft speed passes through 2X of the critical frequency.

Author

Amr Hatem Rashed is a Senior Turbomachinery Engineer at Abu-Qir Fertilizers Company in Alexandria, Egypt. He has a Master degree in mechanical engineering.

Labels:

Comments

Post a Comment

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
Post a Comment
[Read more...]

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
Post a Comment
[Read more...]

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
Post a Comment
[Read more...]

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
Post a Comment
[Read more...]

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
Post a Comment
[Read more...]