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I was visiting a ski area and planning on combining a little business with pleasure. I had about two hours of predictive maintenance (PdM) work to do, and then I could spend the rest of a gorgeous day skiing. We climbed into the tower above the loading station so we could look at the lift drive. Using a predictive maintenance tool called a “shock pulse meter,” I took a series on readings on the motor, the jackshaft, and the bullwheel bearings. I didn’t like what I saw. I turned to the hill maintenance supervisor and told him that the jackshaft bearings were in the early stages of failure caused by lubrication problems.

He said, “That can’t be true! Those are new bearings and we just greased both of them.” We talked a little more about what was happening and eventually realized that the grease was not designed for use with ball and roller bearings. Five minutes of analysis averted a disaster and more than paid for my day. Made me feel like a hero, too.

We all use predictive maintenance every day. In our cars, we look at the gas gauge and when it gets low, we go to the filling station. Some of us look at the tires occasionally and, if one looks soft, we check the pressure in it. We all know that the gas tank doesn’t instantaneously go from full to empty, and we know that tires very rarely fail without some warning. We look at these indicators so our car is more reliable and we can live more comfortably.

Similarly, there are predictive maintenance tools that can make our ski areas more reliable and all but eliminate unanticipated mechanical problems. The motors, gear reducers, pumps, wire, and bearings we use don’t fail rapidly; weeks or even months in advance, predictive maintenance tools can detect impending failures and the need for corrective action. They let us both understand the condition of the machines and look into the future, so we can anticipate problems and correct them on our schedule, instead of the machine’s schedule.


WHY IT WORKS
The graph below, left, shows the probability of failure for a typical piece of complex mechanical machinery, like a motor or a snowmaking pump.

There are three distinct time periods in the life of a machine:

• an infant mortality period, where problems typically result from errors in assembly

• a long normal run period when there are relatively few failures

• a slowly increasing failure rate during the wear out period. (This curve applies to relatively complex mechanical machinery; for example, solid-state electronic equipment shows essentially no purely time-based failure rates.)

The beauty of PdM is that it lets you understand the condition of the machinery, without any human intervention. It allows you to run a machine until the symptoms indicate that action is needed. This is far preferable to simply following a set maintenance interval.

For example, I once worked in a plant where preventive maintenance was the way of life. Every three or four years we took our large (over 40 hp) motors out of service, cleaned the windings and replaced the bearings. Today we know that those motors will run at least 125,000 hours (15+ years) between repairs, but we didn’t understand that. As shown in the chart (facing page, below right), by doing unneeded work, we were paying to increase the chance of failure!

Several studies have found that typically, 14 percent of all machinery is significantly defective as installed. (The range we’ve seen across North America has been from 5 percent to 22 percent.)


BENEFITS OF PdM
The primary goal of most PdM programs is to increase profitability, and they have generally been shown to cut machinery maintenance costs about in half. But a significant side benefit is that they also result in a safer workplace, because PdM eliminates emergency jobs and crisis-oriented reactions to major breakdowns.

The first large-scale industrial applications of PdM occurred in the mid-1970s. The common early techniques involved vibration analysis, shock pulse monitoring, and oil analysis. Over the years the sophistication and application of those technologies has expanded, and other techniques, such as infrared scanning, motor current analysis, and other ultrasonic monitoring tools have been developed. Let’s take a look at some of these.

Vibration analysis. This is by far the most common diagnostic and PdM tool. All operating mechanical equipment vibrates to some degree or another. With rotating components, some of the vibration happens at 1x the rotating speed, some at 2x, some at 3x, etc. A vibration analysis measures this vibration and compares those readings both with what has been found to be generally acceptable and with past readings. It is used to analyze the condition of almost any rotating mechanical equipment—motors, reducers, bearings, gears, shafts, and the like.

Acoustic Emission Monitoring. This includes several similar technologies, some called shock pulse monitoring and other called ultrasonic monitoring. AEM is extremely effective in finding early-stage bearing degradation and in measuring lubricant film thicknesses in bearings. Also, for many low-speed bearings, AEM is more effective than vibration analysis in detecting problems.

Oil Analysis. This technique began over 60 years ago and has become invaluable in monitoring complex machines. The basic analysis is used to understand the condition of the base oil and the additives, and to evaluate the contamination. A more sophisticated wear particle analysis looks at both the condition of the oil and the wear particles in the oil. Using a microscope, the analyst can tell how and when the wear particles were generated, and whether abnormal conditions exist. For example, if there are metallic particles in a reducer oil, the analyst could tell the difference between normal break-in wear or fatigue damage to the gears or bearings.

With engine-driven equipment, analysis of the crankcase oil can easily diagnose a multitude of problems, ranging from defective air filter systems to excessive blowby and coolant leakage.

With the gear reducers used in outdoor applications like ski lifts, we usually recommend the use of synthetic lubricants, dessicant breathers to prevent moisture from contaminating the oil, and oil analyses twice per year. We know of one area that is going on 10 years between reducer oil changes, and the oil analyses and vibration analyses are ensuring that the reducers are in good condition.

Infrared scanning. This is an extremely effective tool for finding poor electrical connections, overloaded circuits, and other problems that result in temperature differentials. IR scanning is a versatile tool: in one instance, when the level monitoring system on a critical process water tank failed, we used an IR scanner to monitor the tank levels until the sensor could be repaired.

Motor Current Analysis. These devices connect to the motor’s power leads at the switchgear. Early versions basically just analyzed the condition of the motor insulation, while later ones can be synchronized with an operating motor and analyze potential electrical and mechanical problems.


DEVELOPING A PROGRAM
In every area of PdM there are both basic tools and much more sophisticated ones. Some can be used “out of the box” to get an approximate idea of what is happening, while others, in the hands of an expert, can tell what is happening to amazing detail. (One of our people could consistently tell the size of a flaw in a 15-inch diameter bearing running at 500 rpm.) In the last 30 or so years, they all have become much easier to use and much more accurate in their application.

The frequency of the analysis really depends on the speed and criticality of the application. For most ski area equipment, a typical program might involve:

• Vibration analysis at the end of the season, to determine what should be worked on over the summer, and a second analysis a few weeks before start-up as a quality control check on the repaired machinery.

• Acoustic emission analysis on the critical motor and pump bearings every two months.

• Annual infrared analysis and motor current analysis on the critical electrical motors and switchgear.

How should you get started? Hire a pro on PdM and ask him to visit your site, explain the options, and set up a comprehensive program. There are some basic things that your people should do, such as taking routine readings on motors using a shock pulse meter. But some analysis is best performed by an experienced professional—for example, vibration readings on the lift drive reducers.

Over the last few years we’ve all come to rely on noninvasive (read PdM) medical tests like MRIs and EKGs. We use them because they are less expensive, offer less chance of damage, and are more informative than invasive “exploratory surgery.” It’s time to use the same approach on your machinery.


Neville Sachs (sachscracks@att.net) is a founder and principal in Sachs, Salvaterra & Associates, Inc., which provides specialty technical and inspection services.

VIBRATION ANALYSIS
By Steve Rogan, Senior Service Engineer, Artec Machine Systems

Few predictive methods have improved as much over the years as vibration analysis. Early on, technicians placed screwdrivers in their ear and held the blade firmly to the bearing housing, or removed their shoes and stood upon the equipment, to detect an abnormal shake, rattle or roll. Those remain valid ways to sense changes in rotating equipment, but state of the art vibration analysis today is a bit more refined.

While vibration monitoring has been prohibitively expensive for all but the biggest resorts and their most critical equipment, that is changing. Newly installed lifts now usually include monitoring and trending collection instrumentation that involves vibration collection capabilities. Existing lifts can be inspected using portable equipment employed at regular intervals to give a trending over time.

We have found that the most common use for vibration analysis has not actually been for the “last minute save before all operation ceases with a catastrophic failure,” but mainly for peace of mind for the administration and manufacturers that the equipment is running properly. The types of problems with ropeway equipment that cause noticeable vibration are usually discovered by routine maintenance, such as the everyday operational procedures that bring personnel into close proximity with the rotating equipment.

When we do employ PdM, we try to combine an historical background check with a complete visual inspection and, if possible, collect vibration data for frequency and phase analysis. This combination of preventive and predictive maintenance practices reinforces the conclusions of the visual inspection and allows us to predict condition, remaining life and what parts need attention upon the next maintenance action.

And there are instances in which vibration monitoring can provide unique insights. On one occasion, we performed vibration monitoring to analyze a used lift that was relocated from one mountain to another. This verified the good working order of the unit, checked to detect problems in the infant mortality phase of the installation, and measured any differences between the old and new installations. There can be many influences that project themselves through the vibration check, including such obvious things as bearings that are wearing or a misalignment in the train, but also a weak structural foundation that, left unattended, can shorten the life of most components.

Vibration analysis has historically been complex and expensive, but modern advances in communication and computation have made it feasible for equipment that traditionally never had it before. We make it a normal task in most of our inspections, as it supplies more engineering depth to truly understand the condition and to predict the equipment’s remaining life or to identify additional maintenance tasks.