Hello and welcome to the latest revolution in our series on rotor dynamics and bearing analysis. This month, we’ll be looking at the importance and procedure of modeling the bearings and structural supports in a rotortrain. If you haven’t had a look at the other entries in this series, you can find them here: Series Preface
- What is Rotor Dynamics? And Where is it Found?
- Why is Rotor Dynamics so Important?
- What API Standards Govern Rotor Dynamics Analysis?
- Basic Definitions and Fundamental Concepts of Rotating Equipment Vibrations
- The Purposes and Objectives of Rotor Dynamics Analyses
- The Importance of Accurately Modeling a Rotor-Bearing System
So let’s get started, one of the first things a rotor bearing system needs aside from a rotor, is, well, bearings! But what are bearings? I’m glad you asked!
Bearings are mechanical components used to restrict the motion of the machine and support the load while protecting other elements by reducing friction between moving parts. In fact, you might even say it bears the loads (axial and/or radial) caused by a rotor.
Bearings come in different materials, shapes and styles depending on their application, and can be found in everything from turbomachinery to reciprocating engines to things like hard drives and even fidget spinners. But what are the bearings commonly encountered in turbomachinery, and what effects can they have on the machines they are used in?
- – Plain (also known as journal bearings)
- – Fixed and Tilting bearings
- – Elliptical bearings, (also known as lemon bearings)
- – Thrust bearings
- – Gas foil bearings
- – Fluid bearings
- – Rolling element bearings
Different bearings can influence the rotor’s behavior differently. More specifically, the damping and stiffness characteristics of the bearing can influence the location and the severity of the rotor’s critical speed and amplification factor which, as we have seen, are critical to understanding the safe operation of a rotor-bearing assembly.
So, having established that these are the types of bearings commonly found in turbomachinery rotor-bearing systems, what qualities need to be examined when designing bearings and determining applications?
Design considerations (depends on the type of bearing) include but are not limited to:
- – Minimum film thickness
- – Maximum film pressure
- – Maximum bearing temperature
- – Lubricant flow rate
- – Power loss
With regards to rotor dynamics, two bearing characteristics are important that need to be considered: bearing stiffness and damping characteristics.
These two characteristics and the design considerations I mentioned above can be considered and integrated into a rotor bearing system through AxSTREAM Bearing™ and imported into SoftInWay’s own AxSTREAM RotorDynamics™.
There are typically a total of 8 different bearing coefficients that need to be considered in, say, a journal bearing for example. 4 of the coefficients are stiffness coefficients, and 4 of the coefficients are damping coefficients.
Having covered all of that, let’s move on to structural supports and their effects. So, what are structural supports in the context of turbomachines? Typically, they can include the bearing pedestals, skids, steel foundations, and frame supports. What do all of these have in common?
They’re elements of the machine that secure it in place, and with regards to the bearing pedestals, act as the main contact points between the rotor train and the rest of the machine that is not
By now you may be asking, why would this be so important if we have already got the bearings covered, especially if some of these don’t even move? Well, good on you for asking because:
rotating during operation.
- – Neglecting supports results in errors:
– predicted 1st critical speed: 14 – 21%*
– 2nd critical speed: 40 – 88% error*
*(Nicholas and Barrett, API 684)
I probably don’t have to say it, but I will anyways, those are massive and unacceptable errors! In fact, bearing support resonance and/or lack of bearing support stiffness is one of the major sources of high, systemic, and expensive vibration problems. Think about it, in a car one of the major causes of excess vibration and movement is from a failed motor mount, which connects the engine to the body of the car. When these parts are worn or broken, your car will vibrate and resonate significantly more than it had previously. Now, scale it up to a massive gas turbine that could weigh several thousand pounds, and is spinning at thousands of revolutions a minute; it’s worth considering what is supporting this massive machine, and if the supports will enable it to have the maximum lifetime possible.
According to API 684, if the bearing support stiffness is 3.5 times greater than the bearing stiffness, the effect of the bearing support will be minimal and might be neglected. Otherwise, the bearing support and other structures need to be accounted for, for a design to be API compliant. So how does one simulate support structures in their rotor train model?
A popular method to simulate support structures and their stiffnesses is to utilize springs in series, as seen below (c):
By using the spring in series method and formula, you can simulate a wide variety of different support structures, however keep in mind it is not the silver bullet as there are limitations to using this method with certain structures like flexible skids, vertical machines, and frame foundations.
Another reliable method is modal reduction, which enables you to account for the dynamic influence of structural supports while also getting extremely accurate results.
By utilizing the above methods in a reliable and powerful program like AxSTREAM RotorDynamics and AxSTREAM Bearing, modeling even the most complex rotor train, bearings and support structures can be made easy, while maintaining a high degree of accuracy. Remember, accurate models lead to accurate results; so don’t put garbage in, or you won’t be happy with what comes out!
Coming up in next month’s rotation…
Next month, we’re going to cover the actual procedure for performing a rotor dynamics analysis, starting with lateral and then moving on to torsional analysis. This will probably take more than one entry to cover, but by the end, you’ll have all the basic knowledge of rotor dynamics there is to know!
If you want to learn more about the importance of rotor dynamics, or about the tools our engineers and thousands of others around the world rely on for their turbomachinery designs, reach out to us at email@example.com
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