A short while ago, my wife went to the local health club for her normal workout. On the way out with some friends, she stopped at the front desk and noticed that the club was offering “metabolic testing” at no charge. She was absolutely delighted to find that her “metabolic age” was the youngest of the group, close to half of her chronological age, and substantially younger than some of her friends.

We all know 60-year-olds who ski and bike like they were 30, and we know some 30-year-olds that act and think like they were 90. We all know people who treat their cars poorly and wonder why they have engine problems at 100,000 miles, while the person down the street is still smiling while driving a Honda that has 300,000 miles on it.

The same sort of aging occurs with ski lifts. There are a tremendous number of 50- or even 60-year-old lifts that run day in and day out without skipping a beat. Similarly, one of the areas we ski at runs a groomer that’s more than 30 years old. Part of that is the design—there are a lot more 50-year-old Mercedes than there are Fiats—but a very big part is the maintenance.


How a Machine Is Designed
Whether it is a snowmaking pump, a car, or a ski lift, most mechanical designs start with the designer calculating the expected loads. These are then multiplied by a safety factor, and the machine is built to that spec.

With large, heavy assemblies such as lifts and towers, the basic structure usually sees some small variations in loading, with the variations not substantial when compared with the design load.

In contrast, rotating components, such as shafts and bearings, and light frame components, such as that of a snowmobile, see tremendous changes in their loads. Figure 1 compares the load variation a tower structure might see to the load variation a shaft inside a gearbox typically experiences.

Components with continuous large load variations are designed with consideration for the fatigue strength of the material in these conditions. Figure 2 shows how the fatigue strength of a typical alloy varies with the number of stress cycles.

Many of our machines use ball and roller bearings. Figure 3 shows a curve used to describe their life in a given application. The common design point is the L10, the point where a huge number of statistical analyses say that, with clean, dry, and well lubricated operating conditions, 10 percent of the bearings can be expected to fail. Under those same operating conditions, we see that the L50 point, the average bearing life, is about 5 times the L10.

It is interesting to see that, regardless of the design, an occasional bearing will fail shortly after installation, while others will last almost forever. For greater reliability, the designer could specify an L5, an L1 or even an L0.5. (Predictive maintenance can easily identify failing bearings, which can then be replaced. Changing bearings prematurely would be a waste of money, and could introduce human error.)

An important point to be considered is that ball and roller bearings are greatly affected by the load. Doubling the load on a ball or roller bearing will cut the life by a factor of eight or 10. So it’s critical that the designer has a very good understanding of the actual loads the machine will see, and that the operator maintains those loads in operation.

For example, the bearings in a lift drive gear reducer are designed for specific loads. But when the input shaft is misaligned because of careless assembly, or the reducer base is distorted because it wasn’t properly shimmed, the bearing loads go up, and the life expectancy drops rapidly.


Simplicity Enhances Reliability
One of the good things about older lifts is their lack of complexity. Modern lifts, with much more sophisticated designs and many more components, are more likely to experience a failure. The more components a machine has, the greater the chance one of them will fail.

If we look back to Figure 3 and apply the statistical analysis used for determining bearing life, we can understand why that happens. For example, look at the four wheel bearing sets on a truck, where each of the bearings was designed with an L10 life of 225,000 miles. The formula for determining the average life of those four wheel bearings is:

1
Average = _________________________________________________________________
Bearing 1 Life + Bearing 2 Life + Bearing 3 Life + Bearing 4 Life

Using this approach, if we look at the combined life of the two bearing sets on one axle, it would be 130,000 mi., and the average life between failures of the four wheel bearing sets would be about 62,000 miles. As you can imagine, as the number of bearings gets larger, the average time between failures drops. If we had a truck with eight sets of wheel bearings, each with a design life of 225,000 miles, the average life between failures would be about 37,000 miles. So the number of bearings in a machine can greatly affect the reliability and maintenance requirements.


Maintenance and Operating Philosophy
Maintenance is critical on ski area machinery. Aside from normal wear, two areas that contribute tremendously to reduced life are corrosion and a lack of precision in maintenance work. Both of these, corrosion and maintenance precision, seem as though they should be easy to address, but all too often a lack of awareness and training combine to cause problems.

Metal components don’t age. Except for a few uncommon alloys that we seldom get involved with, as time goes on the strength, ductility, and most other properties don’t change. A cast iron structure built 200 years ago (or a steel lift tower built 50 years ago) is as strong and sturdy as the day it was made.

But for those components subjected to fatigue loading, life is very different. If there is any corrosion, even light surface rust, the fatigue strength drops, roughly in proportion to the severity of the corrosion. If such a component is rusty and subjected to fatigue stresses, it will eventually fail, regardless of the safety factor in the original design. It’s not that the metal “ages,” but that corrosion weakens it.

Some corrosion is easy to see and to prevent. For example, parts such as shafts and support pieces that are out in the open can be painted or coated to prevent rusting. But in hidden areas inside components—such as reducers, bearings, and wire ropes—that corrosion is much more difficult to stop. Using high-quality seals on bearings and desiccant breathers (or bladders) on reducers will reduce the chance of moisture ingress and materially lengthen their lives.

Fretting is a particularly insidious form of corrosion. It is caused by microscopic back-and-forth movement on parts such as bearings, gears, and wire ropes. The basic mechanism of fretting occurs when two similar smooth metal pieces move back and forth by a few microns and eventually squeeze out the lubricant between the pieces. Then, solid phase welding takes place, and the continued movement causes tiny particles to be pulled out of the mating pieces, greatly reducing the fatigue strength. Fretting is a common failure cause with both bearings and wire ropes that are allowed to sit unused for months at a time. It can easily be eliminated by operating the machine once every six weeks or so.

The benefit from precise maintenance adjustments is that they reduce the loads on the machinery and increase its life. Every mechanic knows that the wheels of his truck or car have to be well aligned for good tire life, and that impact loads from serious potholes damage that alignment. The same principles apply to lifts. Above, we mentioned that doubling the load on a ball bearing cuts the life by a factor of eight. Reducing the load has a similar effect in the other direction: Dropping the load by just 10 percent will increase average bearing life by a third.

Some examples where precision adjustments will result in substantial life increases include:

• Lift tower and bullwheel alignments. Ensure that the rope path is truly perpendicular to the axle, the rope is well centered, and there are no parasitic side loads.

• Moving assemblies. Make adjustments so that the incoming and outgoing engagements are smooth and impact-free. Impacts can be injurious (see box).

• Drive coupling and belt alignments. Misalignment of drive couplings creates forces that are absorbed by the supporting bearings, and drive belt misalignment creates stresses within the belt. In both cases the mating parts’ lives are shortened.

• Bolting procedures. Good bolting procedures are critical. (How many bolts are put on a Nascar vehicle without a torque wrench? None. And the guns they use during the race are kept in a temperature-controlled compartment to ensure accuracy.)

• Replacement bearings. Pay close attention and make sure the fit is accurate for the application.

Several years ago, my wife and I visited a small ski area. The entire place was neat and clean and looked great: The chairs were nicely painted, the drive hummed quietly, and the chairs ran smoothly through the loading and unloading stations. My wife asked if the lift was new. I had to tell her that it was close to 50 years old. It was running like a new lift, because the owners understood the value of precision maintenance and controlling corrosion.