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November 2007

Caring for Aging Parts

Corrosion and wear can cause serious problems in your key machinery.

Written by Neville W. Sachs, P.E. | 0 comment

In an ideal world, if ball and roller bearings could be kept clean, dry, and regularly lubricated, they would last forever. But in the real world that rarely happens. Given the complexity of modern equipment, most resort managers know that corrosion and wear can cause serious machinery problems, and that it’s important to respect the possible dangers.

All materials, including metals, are elastic. When we put a load on them, they stretch or elongate to some degree. If we put too much load on them, they either bend or break. When an engineer designs a piece of machinery, he calculates the expected loads, then—using either the material’s yield strength, the point at which it bends, or the tensile strength, the point at which it breaks—applies a safety factor to make sure the part doesn’t fail during operation.

This works well when the load on the part is relatively constant, because metals don’t “age.” That is, as long as they don’t corrode (rust) or wear and are not subjected to greatly elevated temperatures, the basic strength and physical properties never change.

However, if a component is subjected to a lot of cyclical stresses, like a grip spring on a lift or a bearing roller in a bull wheel, it has to be designed based on its fatigue strength. Most data show that a metal’s fatigue strength is about half of the tensile strength and, as long as the stress is below that, the part will never fail—unless corrosion or wear act to slowly and continuously reduce that fatigue strength.

Corrosion affects equipment in three ways:

• It can thin the material, slowly reducing the structural strength until failure occurs. Fortunately, this process is something that can usually be monitored and predicted.

• It will reduce the fatigue strength of metals.

• With hardened materials, it can cause hydrogen induced cracking (HIC).

Corrosion and Fatigue Strength
Fatigue strength varies depending on:

• the number of stress cycles. If the corroded part has only seen a few hundred severe stress cycles, like a lift tower in a sheltered area, the decrease is minimal and probably not worth worrying about. But if it has seen millions of stress cycles, like the rusty input drive shaft on the bull wheel, that reduction can be more than 50 percent.

• the severity of the corrosion. A mildly corrosive atmosphere will have an effect on metal. Severe corrosion would decrease the fatigue strength even more.

• the strength of the part. If the metal is relatively ductile, like the mild steel of a chair bale, the reduction is much less than would be seen on a high-strength part like a grip or a Grade 8 bolt.

For corrosion to occur, there has to be moisture present, and the rate can vary considerably. For example, an unprotected piece in the northeast U.S. or eastern Canada would be attacked far more than an identical part in Utah or the Canadian Rockies, because:

• The eastern pieces see more rain and are continuously exposed to higher humidity.

• Eastern precipitation is more acidic and contains more sulfides and chlorides, known corrosion accelerants.

A piece in a valve house, or a similar poorly ventilated, partially underground enclosure with high humidity, would be much more corroded than an identical part out in the air.

The more heavily corroded the piece is, the more the fatigue strength is reduced, and as long as the corrosion continues to occur, the fatigue strength continuously decreases.

Corrosion and Hardened Parts
Corrosion’s effects on hardened parts are diabolical. For example, on a high strength stainless grip or a high strength bolt, the most serious corrosion damage is hydrogen induced cracking (HIC), which is frequently impossible to see with the naked eye.

Hydrogen damage? The science is a bit complicated, but the key fact is: corrosion generates hydrogen atoms. Most of these hydrogen atoms immediately combine with other atoms to form hydrogen gas. But some of them remain single atoms and wander off through the metal. If it is ductile, like the mild steel we use to make buildings, towers, and motor shafts, those atoms do little or no damage. However, on hardened pieces like bearings and grips, the hydrogen atoms can cause HIC.

Even corrosion scientists don’t completely understand all the details of how this degradation occurs, but we know that:

• The atomic hydrogen generated by the corrosion process is the culprit.

• Some of the hydrogen atoms stop along the grain boundaries.

• At these grain boundaries the stray atoms create high stresses that then combine with the normal operating stresses to cause cracking.

• The probability of damage appears to depend on the hardness of the material, the corroding chemicals, and the age of the parts.

So, with a hardened part in a corrosive atmosphere, a small amount of corrosion can cause tiny cracks that slowly grow larger.

Wear and Bearing Problems
The other mechanism that causes serious aging issues is wear. Sometimes it is easy to see the damage, such as when bull wheel liners slowly get thinner. Other times it is insidious, as when the shaft fit of a bearing slowly gets looser.

Sometimes the cure is simple, but a worn bore and a bearing loose in the housing presents challenging problems:

• The movement from the loose bearing fit will cause additional wear and continuously get looser.

• Permanent repair of a loose fit is difficult and expensive. The parts have to be removed from position and either welded or sleeved, and this requires precision machining. In the case of a worn bull wheel bearing housing, it entails removing the wheel from the site and moving it to a machine shop.

• The loose bearing fit is potentially damaging because of the failure mechanisms it accelerates. With low-speed bearings, such as found on bull wheels, the poor fit leads to outer race cracking and the relatively rapid catastrophic failure of the bearings. In high-speed applications, such as on lift drive motors, pumps, compressors, and snowmaking fans, loose fits lead to elevated internal bearing temperatures and greatly shortened bearing lives.

How to Prevent or Reduce Aging Effects
Be sure your maintenance people are “doing the job right” and:

• Don’t let the machinery get rusty. Keep it well oiled and greased. Even cables on counterweights should be periodically lubricated!

• For those parts that can’t be oiled and greased, clean them down to bare metal and cover them with high-quality coatings.

• Conduct periodic vibration analysis on critical bearings, using permanently installed transducers for better accuracy, and trend the results.

• Check the lubricants in reducers and large bearings regularly to be certain they aren’t contaminated and that the additives are still doing their jobs. (Even tiny amounts of water in the oil will damage bearings and will also cause corrosion when the machine is shut down. Also, some lubricant additives break down, forming acids that accelerate the corrosion.)

• Monitor wear items carefully and be very careful about changing materials.

A Summary Box
1. Most nonmetallic materials degrade either by slow destruction of their chemical bonds (plastics) or by oxidation.

2. Metals don’t “age.” The tensile, yield, and impact properties don’t change with time, but corrosion will reduce the fatigue strength.

3. Corrosion’s effects are difficult predict because the variables include:
• the metal composition, i.e., the alloying elements, and the hardness
• the corrosivity of the atmosphere
• the frequency and severity of the stress cycling
• the component temperature

4. The trade-off for using high strength materials: they are more rapidly affected by corrosion fatigue and HIC.

5. Ball and roller bearings are extremely complex devices and even 0.001” shaft or housing wear can be very important.

6. Degradation of lubricants can cause bearing and gear damage from accelerated wear and corrosion due to:
• water in the lubricant
• chemical degradation of additives
• water combining with additives or degradation products to form acids

7. Chemicals that cause corrosion of metals frequently have little or no effect on polymers.

8. Conditions that cause polymer degradation usually have little effect on metals.