By: Stan Riddle, VibrAlign
In my job, I teach shaft alignment training to over 500 people per year, in dozens of facilities-everything from manufacturing, to mining, to energy, to municipalities. All of these facilities have one thing in common-they use the same types of couplings. And many of them fail before their time. This article is not to focus on a particular flexible coupling manufacturer or design, but on the most common errors in coupling installation and maintenance that I have observed (or been a willing participant in).
A shaft coupling is an extremely efficient method of power transmission. They are typically simple to install and maintain. Many need little maintenance at all beyond installation, other than an occasional inspection. Most are also tolerant of slight misalignment, temperature extremes, humidity, loads, and so on. Most coupling manufacturers will even assist engineers with specifying the proper coupling type for the application.
So, why do couplings keep failing?
Misalignment
I am amazed that shaft misalignment is still such as issue. When I started in maintenance, there were no laser alignment tools. Mechanics used straightedges, key stock, or calipers. The real craftsmen used dial indicators. Then laser alignment tools came along. Today, laser tools can align shafts to extreme levels of accuracy, well beyond the limits of dials. Yet, misalignment happened then, and continues to happen today. Why?
Some mechanics don’t align at all. They assume that if the coupling will assemble, it must be good enough. Or they don’t even know it needs to be aligned. I believe the number of coupling failures would decrease if mechanics would just use a straightedge. It would decrease more if they would learn and use dial indicators properly. And even more if they would use modern laser alignment tools-properly.
Couplings for the most part are relatively inexpensive. The problem is that the forces generated by misalignment impact not only the coupling, but the bearings, seals, gears, and other machinery components.
Improper Assembly
I often ask mechanics if they read the little piece of paper folded and rolled up into the coupling insert. Maybe one out of fifty says yes.
Why? Maybe they’re too busy. Or maybe they are in a hurry, because they need to get the machine back on line. Or maybe they think they already know what it says. Or they just don’t think it’s that important. But I would say it’s a fair assumption that if every mechanic would read the installation instructions, and do what it says, they might not be changing so many couplings in the first place.
The coupling manufacturer’s engineering department knows more than the maintenance mechanic about the couplings performance characteristics-so the installation directions should be followed.
Lack of lubrication
Couplings that require lubrication – require lubrication. It’s that simple. Few people would run their automobile engine until the oil runs out, but they will run a lubricated coupling until it is bone dry. There should be sufficient down time on machinery to perform the necessary preventive maintenance procedures, and this includes coupling lubrication. If it can’t be shut down for lubrication, then a different type of non-lubricated coupling should be used.
Improper Coupling Specification
There are many different types of flexible couplings. And almost all coupling manufacturers will assist in design specifications. It is the design specifications that allow the coupling to do its job, namely, transferring power from the driver to the driven, and serving as a “weak link” in cases of overload.
However, couplings are sometimes used because:
- We needed something quick, and it would “fit”.
- The company wanted to minimize spares, so they just buy one or two types of couplings
- The maintenance mechanic or department was not aware of the design specs.
If couplings are failing more often than expected, and alignment and maintenance have been performed, it is good to review the design of the machine, and make sure the coupling is correct for horsepower, start-up torque, load, temperature, etc.
Poor Maintenance Practices
A coupling should be treated like any other precision machinery component, even if it looks like it isn’t. The hubs should not be beaten on and off with a hammer. Old gaskets and sealing rings should be replaced. They should be cleaned and re-lubricated. Alignment should be checked as part of the PM. The shaft onto which the coupling mounts should be of the proper size (and not sanded down 1-2 mils every time the coupling is changed). When the elastomer is deformed, twisted, or worn, it should be replaced. Set screws and bolting in the coupling should be checked.
The same level of precision and good maintenance practice used on the pump, gearbox, or bearing assembly should be used on the coupling.
Education
If you have read down to this part, I must assume you are interested in knowing more about why couplings fail. My job is, in essence, a shaft alignment educator. Almost every maintenance mechanic I have met wants to do a good job, and they take great pride in their work, skills, and experience.
By knowing more about the reasons couplings fail, and making changes to maintenance procedures and practices, we all can improve the reliability of couplings. In doing so, we improve the reliability of the machines, and the plants, products, and services we maintain.
The latest in Couplings
Couplings are simple devices designed to connect two shafts and transmit torque between them, but they provide a critical function to a variety of machines. Machines like pumps, for example, rely on couplings to ensure that they have the power needed to move liquids and gasses wherever they need to go. As industries continue to try and increase their output, pumps are being pushed to handle ever-increasing flow rates. As a result, couplings are being driven to increase their performance.
R+W’s Survivor series of couplings is one such example of this performance-driven change. The Survivor series disc pack couplings are a little different from most traditional disc pack couplings. They transmit torque across the disc pack assemblies purely by friction, thus helping to avoid problems associated with stress concentration, backlash, and chatter, all of which can result from transmitting torque across the shanks of shoulder bolts, and can lead to premature failure.
Another way to increase process flow rates is to take the same size pump or compressor and increase its operating speed. To accommodate the higher equipment speed, the coupling speed rating must also increase. The easiest way to accomplish this is to make the coupling smaller in diameter to try to avoid expensive balancing costs. Rexnord’s Thomas XTSR series couplings fit this idea. The 3-piece design allows for the smallest possible package for an application. The hubs are pilot fitted to the factory assembled center member. The design allows for repeatable installations without special tooling. Additional modifications may be made to reduce coupling weight, or special mountings to make it an economical option on various critical and high-speed applications.
The coupling also features an anti-flail ring to confine the center member in the event of a failure, overload bushings to limit stress during start up to extend disc pack life, and an optional torque overload protection system.
Couplings do more than just help move liquids and gasses however; they also play important roles in generating electrical power. Because of this, bearings must face the added challenge of dealing with stray electrical charges that could damage mechanical components. CD Couplings from Zero-Max are designed specifically to address this problem. The flex element designed into Zero-Max CD couplings provides electrical insulation while protecting mechanical components from system overloads. In addition to protecting from stray electrical current, CD couplings protect generators in a system from transferring lower reaction loads to the generator bearings. The coupling’s composite discs withstand all types of environmental elements, including temperature extremes from -40°C to +70°C, and also moisture and chemicals.
There is no point in generating power if you can’t get that power where you want it to go. For this task, consider simple rigid couplings like those produced by Oren Elliott Products. OEP Couplings manufactures the rigid coupling in steel, aluminum, stainless steel, and Ultem, which offers electrical insulation. Shaft attachment options for the rigid coupling include setscrew with or without keyway and clamping with or without keyway. These couplings come in three variations: The Type RS, a set screw variation; The Type RC, a one-piece clamping variation and the Type RT, a three-piece clamping variation.
Rexnord
www.rexnord.com
Zero-Max
www.zero-max.com
VibrAlign
www.vibralign.com
For information on the GERWAH product line, click here.