API 687 Chapter 0: Definitions (continued) – Hydrodynamic Bearings

Thank you for reading my posts and for your positive feedback!

While I think of the next topics and posts, it feels like an endless list. There is so much to say that I could easily fill books with all the details. I keep reminding myself to focus on the stuff that really matters in the world of equipment repairs; the essential practical things I learned in this business.

If I leave a detail that you think is important, feel free to write a comment or post on my Questions & Community blog page.

I had to say all this because the next term I will highlight from API 687 Chapter 0 is Hydrodynamic Bearings, and there is so much to cover behind this one term!

Hydrodynamic Bearings

At its core, hydrodynamic refers to the study of fluids in motion.

A Hydrodynamic Bearing is a type of fluid film bearing that supports the load using a thin film of lubricating fluid.

This film is generated by the relative motion between the bearing surfaces, which creates a self-generated pressure that supports the load of the rotating element.

In contrast, another type of fluid film bearing is the hydrostatic bearing, which relies on an external pump to supply enough oil pressure to lift the journal, regardless of the shaft motion.

A pump, a steam turbine, or a compressor rotor, when installed in a machine, experience two main types of forces: radial forces and axial forces.

Radial forces

The rotor weight creates a downward force; this force is supported by the radial bearings.

Radial bearings are also known as journal bearings. Technically the term journal refers to the part of the shaft that rotates within the bearing, particularly the surface that comes in direct contact with the bearing.

Radial and journal can be used interchangeably when referring to bearings that carry the radial forces. Radial bearings cradle the rotor and hold it in place, supporting its weight and any side-to-side (radial) forces that occur during operation.

Axial forces

As steam pushes through a turbine or as gasses are compressed within a compressor, the fluid acting on the rotor creates a force along the length of the rotor, pushing it in one direction.

This is the axial force.

A thrust bearing acts like a wall and prevents the rotor from moving back and forth.

There are many types of both radial and thrust bearings within the “fluid film” and “hydrodynamic bearing family”. We will not be able to cover them all in this installment, but perhaps in the future we can dive into all of them, as if we were exploring delicious pastries in a pastry shop.

Today we will focus on illustrating the important bits of a cylindrical hydrodynamic journal bearing, which I consider the cannoli of fluid film bearings.

When you image-search for fluid film bearing or hydrodynamic bearing, you often encounter an illustration like the one I have created below.

The important things for me to point out are the following:

1. The illustration is not to the correct proportions; the shaft is displayed smaller than in real life to illustrate the fact there is space between the journal and the bearing liner. The gap or “clearance” between the journal and the bearing liner is incredibly small, measured in thousandths of an inch. As a rule of thumb, we can calculate it as 1 to 1.5 thousandths of an inch per inch of journal diameter. So for a 6in diameter shaft, the gap will be 0.009”. That is about the thickness of two sheets of white paper! (A normal sheet of printer paper is about 0.004” thick)
   

2. The space between the shaft and the liner is flooded with oil. Remember, this is a hydrodynamic bearing, so it is self-pressurizing. One only needs to maintain enough flow of oil into the bearing to completely fill up the cavity and for there to be enough lubrication and removal of heat. In practice, the oil supply bearing pressure ranges between 12-20 psig.
   

3. Self-pressurizing means that due to the friction and viscosity within the oil, the shaft’s rotation creates an oil wedge, and this is what cushions the shaft. When a shaft spins, it lifts from the bottom of the bearing liner and floats on top of the pressure wedge. There is no contact metal-to-metal during operation.
   

4. There is no contact between journal and liner, but if there were, you don’t want two hard metal parts to touch. Therefore, the bearing is lined with a soft metal called Babbitt, an alloy made with tin or lead. (More on Babbitt, the metal and its inventor Mr. Babbitt some other time.) In the world of repairs, one often has to restore the surface of bearing liners in a process we call re-babbitting, where the worn-down/ damaged Babbitt is taken off and a new Babbitt applied either by welding or casting. By the same means, if there was damage on the journal (for instance, if it was badly scratched due to a failure in the oil system or rubbed badly), it may need to be resurfaced. We will also talk about this in great depth in the future.
   

5. You may notice the shaft is not perfectly centered within the bearing. This is because the pressure profile of the oil makes the journal “climb” up the wall of the bearing liner. The direction it climbs is against rotation and is often easy to confuse since it may seem counter intuitive. Remember there are strong hydrodynamic forces at play and there is no contact between journal and liner. It is the magic and physics of the oil wedge that cradle and position the shaft within the bearing.
   

6. Bearing liners are made of steel or copper. Copper is used when removing heat is important, and since it has better thermal conductivity than steel.
   

7. Academic illustrations will always display what I call the “mysterious pressure profile”. You can compare several from the internet, and the shape of the curve never seems to look the same. I am sure tribologists have calculated this; as a matter of fact, I am sure my favorite tribologist in the world, Dr. San Andres, must have explained it in detail in class. (Sorry I did not pay attention that day, Dr. San Andres. At the time, I did not really know how useful that knowledge was going to be!)

Anyways, this mysterious profile illustrates the fact that the oil wedge exerts a force on the journal by the means of that self-generating pressure.

Several times I’ve mentioned that there is no contact between metal parts, which may sound unbelievable, but it is true. This really thin film of oil provides the support and, in the context of rotor dynamics, provides all the stiffness and the damping to the “suspension” system that cradles the rotors inside the machines.

I really wish I had the attention span right now to go into Rotordynamics and the effect of bearing design on rotor stability, but this, like everything else, is very vast.

We’ve only just begun understanding the cannoli of fluid film bearings. Let’s save the eclair, beignets, baklavas, lemon bore, tilt pad, and squeeze film damper bearings for some other day.

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Bonus Term

Thousandths and Mils

Rotating equipment such as pumps, turbines, and compressors are designed and built with assemblies and parts that fit or interact with each other with high a level of precision.

When you enter any machine shop you will hear machinists, mechanics, and engineers call out precision measures and dimensions in thousandths-of-an-inch or mils.

One thousandth-of-an-inch or one mil is the same as 0.001 inches.

1 mil = 0.001 inches

Instead of writing or saying “zero point zero zero one” and to avoid committing errors, we will say “one thousandth” or “one mil”.

Based on the example of the thickness of a sheet of paper, you may hear me say, “Please hand me 12 mils worth of printer paper” and everyone will know I need 3 sheets.

On the example of a 6-inch diameter shaft with clearance of 0.009 in, I will describe that clearance as “9 mils” or “9 thousandths of an inch”

And please DO NOT confuse mil with millimeters; these are not the same thing.