by Frederick Lea, Jr.
“Shaft alignment is likely the most misunderstood facet of rotating installation and maintenance. Most shaft alignment today is done by laser alignment tools. But, the laser alignment tools are a “black box”. The problem with a black box is that you only get outputs with no knowledge of what is going on inside the box. All laser shaft alignment tools are based on the Reverse Dial Indicator principles of alignment. This blog will take you through the basic principles of shaft alignment so you will understand if the “black box” is lying to you.”
Shaft alignment, next to proper lubrication, is the single most important parameter in the successful maintenance and operation of any type of rotating equipment. Misalignment may be responsible for more machinery failures than any other disorder.
Shaft alignment on initial installation is critical. Many equipment issues such as high vibration, coupling failures, and bearing and seal failures that arise later could have easily been prevented by proper alignment during the initial installation. Alignment should also be checked periodically during the service life of the equipment, as there are elements that will cause it to change.
There are many misunderstandings about the concept of shaft alignment. Unless there is a stringently enforced alignment record-keeping system, there is no way to determine the true alignment condition of the equipment in a plant. Usually, when asked about the alignment condition of a problem piece of equipment, there are two answers given by owners; “Perfect” and “Close”, followed by the explanation, “We use flexible couplings.” The response “Perfect” means they do not understand shaft alignment because shaft alignment is rarely, if ever, “Perfect.” “Close” means they have no idea, and “We use flexible couplings” means you are really in trouble. Alignment condition is based on numbers, +/- the shim adjustment required for “Perfect” alignment.
A good alignment program means putting forth a diligent effort to keep track of the alignment status of every piece of equipment in your plant and recording that status in the equipment work order file.
Most people do not realize the economic impact that simply implementing a good alignment program can have. There have been instances where the entire maintenance budget of a facility was reduced by 25 to 30 percent with the implementation of very stringent alignment and lubrication programs.
In order to discuss the concept of alignment objectively, there are some universal terms that need to be defined and understood. For the purposes of this article, they are as follows:
- Plane A surface of such a nature that a straight line joining any two of its points lies entirely on that surface. A sheet of paper is a good analogy of a plane, except that a plane continues infinitely in all directions and has no thickness.
- Shaft Centerline – An infinitesimal line passing through the geometric center of a machinery The location of this line is normally indicated by the machined centers on each end of the shaft.
- Runout – The amount by which any given segment of a rotating element oscillates about its shaft centerline. Runout is measured on the D. of the element normally using a dial indicator while rotating the element.
- Indicator Sag – The amount by which the weight of a dial indicator bracket and its assembly will cause an indicator to misread when rotated from the 12:00 position to the 6:00 position. This difference is measured in mils (thousandths of an inch).
- Soft Foot – A condition where one or more of the mounting feet of a piece of equipment, or the mounting pads of that equipment, do not lie in the same plane, thus causing stress on the bearing housing when that foot is tightened.
- Resultant Pipe Strain – Stresses imparted to the driver or driven unit of an equipment train by improper piping fit-up to the nozzles, or casing of the equipment. Excessive pipe strain will result in bearing housing distortion.
- Thermal Growth or Shrink – The distance that a shaft centerline will move as the temperature of the unit changes from ambient to operating conditions. The unit frame will have a temperature gradient from the mounting foot to the shaft centerline that will determine, along with the coefficient of expansion of the frame material, the amount of movement the shaft will experience.
- Stationary Unit – The rotating equipment piece in an equipment train to be aligned that is considered immovable for whatever reason. Normally this is the driven unit because of large bore process piping, etc., attached to the equipment. If there is a gearbox in the train, it will always be the stationary unit.
- Movable Unit – The equipment piece(s) in the equipment train to be aligned that will be adjusted during the alignment process. In a motor driven train, this is normally the electric motor because it is the easiest to move. In multi-unit trains it could be any of the pieces. Once it is aligned to the previous piece, it becomes the stationary unit.
- Driver Unit – The unit that provides the power input to the equipment train. In a multi-compressor train, the compressor could be both a driven unit and a driver unit.
- Driven Unit – The unit(s) that power is imparted to by either a driver, a gearbox, expander. etc.
- Coupling Gap– The distance between the coupling hub faces of the driver unit and the driven unit when both shafts are in the normal running position.
- Impossible Alignment Conditions – A physical condition affecting the adjustment of the movable unit that prevents it from being brought into alignment with the driven unit.
Before we can discuss alignment, we must first understand the function of the shaft coupling in the drive train. The features of the various types of shaft couplings must also be understood.
In order to transmit energy from the driver unit to the driven unit, the two shafts must be connected in some manner. This connection is accomplished by what is referred to as the “Shaft Coupling”.
There are probably as many different types of shaft couplings as there are imaginations to dream them up. Every conceivable configuration from a piece of rubber hose with two worm gear clamps to solid coupling devices have been used, depending on the service conditions and size of the units.
In order to keep the importance of the coupling in the proper perspective, we must first understand the primary function of the shaft coupling. The function of the shaft coupling is “to transmit torque from the driver unit to the driven unit”; nothing else.
The shaft coupling component is one of the most misunderstood equipment applications in all of industry. The selection of a shaft coupling many times has nothing to do with its intended use. A coupling is chosen because it is easy to work on, will take a lot of misalignment, does not require much maintenance, is cheap, etc.; not for the purpose or application it was intended.
There are a number of philosophies on coupling applications. Traditionally, the higher speed, higher horsepower applications utilized a lubricated, gear-type coupling with a forged gear hub and a gear-driven shroud. These couplings were normally force lubricated by the same lube oil console that lubricates the rest of the machine.
The trend in industry today, however, is to use non-lubricated couplings in order to avoid the problem of choosing the proper lubricant, having to maintain a lube schedule or a lube system, etc., for extended coupling life. Examples of this type of coupling in a normal low horsepower application would be gear type, disc pack, elastomer element, or grid type, to name a few.
Some of the higher speed, higher horsepower applications call for a special coupling such as a diaphragm or double diaphragm for non-lube applications.
The important point to remember is to choose the proper coupling for your application. Once the coupling has been selected, be sure to familiarize yourself with the do’s and don’ts of your particular coupling and be sure that it is installed properly. For alignment considerations, always select a spacer coupling. The increased distance between the coupling hubs increases the accuracy of the process.
A secondary function of the shaft coupling is to provide some flexibility between the driver and driven units. If the two shafts were rigidly coupled, all of the vibration and axial forces would be transmitted from one rotor to the other which, in most cases, is an undesirable situation.
Over the years, various designs of “flexible” couplings have been marketed that claim to have extraordinary benefits for the user. Alignment is no longer critical, and as long as you can bolt the coupling up, everything is okay.
One thing must be clearly understood: “flexible” couplings should not be used to compensate for any significant misalignment between the driver and driven shafts during normal operation. Even though the coupling may not be adversely affected, the effect on the other drive train components, such as bearings and mechanical seals, can be catastrophic.
A flexible coupling should only be considered as a device to permit adjustment between the driver and driven shafts during the warm-up period when machines are started up and coming to operating temperatures. Often times there is an initial shaft offset between the driver and driven units to allow for thermal growth. Sometimes, such as on refrigeration compressors, this initial offset is significant.
A flexible coupling will also minimize thrust loads on one unit or the other, especially where sleeve bearing units are concerned. It will be forgiving if one shaft has a bearing failure and may prevent damage to the other unit.
The important point to remember is that a coupling is not an alignment tool and should not be treated as such. Properly sized and applied, and with good alignment, a coupling should last as long as the machine it drives.
WHAT IS ALIGNMENT?
The common terminology used in industry is “Coupling Alignment”. You do not align couplings. We must remember the definition of a coupling – the element that “transmits torque from the driver unit to the driven unit.” When analyzing what the coupling component actually is, you will find that what is normally referred to as the coupling is actually the coupling hub. The coupling hub is affixed to the shaft, and therefore becomes an integral part of the unit. The coupling is actually the spacer piece, or the shroud bolts that bolt the two hubs together. In a grid coupling, it is the grid. In a shim pack coupling, it is the shim pack. Without the attaching piece, no torque could be transmitted.
So, what do you align when the boss says, “Go align the coupling on number 2 compressor”? Do you align the coupling, or the compressor? No, you align the shaft centerline of the driver unit to the shaft centerline of the driven unit. More fundamental than that, you align the centerline of the bearing housing fits of the driver unit to the centerline of the bearing housing fits of the driven unit, using the shaft centerline as a tool to do so.
The closer to each other the shaft centerlines are lessens the bearing preload on both units due to binding of the shaft. Excessive preload is one of the primary causes of premature hydrodynamic bearing failure.
In rotating element bearings, excessive preloading normally results in the bearings running at higher than normal temperatures. This excessive temperature leads to lubrication failure, which in turn leads to early bearing failure.
Statistics gathered by a number of companies in recent years indicate that alignment-related problems are the number one cause of failure in small horsepower equipment. The higher the speed, the more rapid the failure.
These statistics show that approximately $80 per horsepower per year is spent on lower horsepower equipment as compared to $20 per horsepower year on larger, un-spared equipment. If alignment is the number one cause of these failures, then correcting alignment problems should be an easy way of controlling maintenance costs in this class of equipment.
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