Shaft Strength Reduction Due to Fretting Corrosion
Why Do The Secondary Shafts Fail?
The design flaw in the secondary shaft is fretting corrosion around the bearing and under the sleeve used to keep the shaft from moving. This fretting corrosion causes a reduction in shaft strength (endurance limit) to the point that the endurance limit is less then the rotational bending stress. When this occurs the shaft will ultimately begin to crack and fail in rotational bending fatigue. Once the shaft cracks, the crack will grow across the shaft until failure, resulting in a loss of power to the main rotor and tail rotor.
Figure 1 shows a failed secondary shaft. The most significant point is the rusty colored line around the shaft. All of the secondary shafts that have been evaluated by Pro-Drive have shown a red coloration at the point of failure. This red color is called "bleeding" and is an indicator of severe fretting. All of the failed shafts evaluated by Pro-Drive have shown fretting induced rotational bending fatigue failure.
Figure 1. Fretting corrosion on a secondary shaft.
Figure 2 shows the face of a failed part. Please notice two distinct types of surface. On the right side of the part is a large textured area, the fatigue zone. The fatigue zone is a crack that starts at the fretted edge and grows across the face of the part. With each rotation of the shaft the crack gets slightly deeper in the part. At some point the stress in the remaining portion of the shaft is so high that the shaft snaps, causing a rapid failure of the shaft in the plastic zone. The plastic zone is on the left and is characterized by the appearance of tearing, as the part twists/bends off. An important aspect is the ratio between the plastic zone and the fatigue zone. Notice that the plastic zone is only about 25% of the total area. Also, this part is the worst one we could find, most of the failed shafts show less then 10% of the total area failing in the plastic zone. What this means, is that the part has a safety factor of at least 4. The shaft is four times stronger than it needs to be, but it still fails. The thing to understand is that the design of this shaft should not be about strength, it should be about eliminating the fretting that causes the crack to start.
Figure 2. Face of a failed shaft.
St - tensile strength - Typically the ultimate strength of a material
Se - endurance limit - Also known as fatigue strength. This is the highest stress allowable for an infinite life part (allowable stress may be lower due to high mean stress)
Figure 3 shows five failed shafts with fretting evident at each crack initiation. One of the shafts was so damaged by fretting that it actually had two cracks growing at the same time.
Figure 3. Failed shafts showing fretting corrosion.
Fretting corrosion causes a significant reduction in the fatigue strength of part. According to Figure 4, a shaft with 130,000 psi tensile strength (St) will have an endurance limit of 65,000 psi (Se). According to reference 1 this 65,000 psi endurance limit is then modified according to the following equation:
Se = Ka * Kb * Kc * Kd * Ke * Se'
Ka, Surface factor = .875 (modifies the endurance limit due to the quality of the part's surface)
Kb, Size factor = .841 (modifies for geometry issues with changes in diameter, relative to the tested sample)
Kc, Load factor = 1 (modifies for cyclic torsion)
Kd, Temp factor = 1 (modifies for high temperature operation)
Ke, Miscellaneous effects factor
Therefore, the 65,000 PSI Se' becomes Se = (0.875)(0.841)(1)(1)(65000)Ke
Which is 47,800 psi, not including the Ke.
Reference 1 then shows that Ke can range from 0.24 to 0.90 due to fretting.
This is what causes the part to fail: with the stress operating near 35,000 psi and the endurance limit at 47,800 psi, any Ke value less then 0.73 will cause the part to fail. RotorWay's engineers, who will not allow themselves to be identified, suggests that Ke for a fretted part should be 0.80. A convenient number that allows the part to appear safe.
Another way to look at the problem of endurance strength (Se) is to look at an ASME handbook, reference 2. In that book, they show the effects of notching and corrosion on the endurance strength of a part. For our steel, 9310, the tensile strength (in the area near the bearings) will be approximately 130,000 psi, therefore, according to Figure 4, the shaft should have an endurance limit of 65,000 psi, which is then modified as previously described. However, with a notched or corroded part, the endurance limit drops significantly. The surface damage caused by corrosion, fretting or atmospheric, will cause the endurance limit to drop to less then 20,000 psi. Well below the stress in the shaft.
Figure 4. Fatigue Behavior of Steel due to Notching or Corrosion.
So the reduction in endurance limit (Se) is what allows a crack to start in the secondary shaft, once the crack starts there is approximately 15 minutes before the shaft fails. (By the way, Pro-Drive spent a significant amount of money to determine how long after the crack started, would the shaft fail. It was thought that a sensor could be made to allow sufficient time to land the aircraft, however, the sensor would only provide about 3 minutes before shaft failure, therefore the value would be too small).
Why Does Fretting Occur?
If we can prevent fretting corrosion, we can then maintain the high endurance limit, providing an infinite life part. Reference 1 reports that fretting corrosion "is the result of microscopic motions of tightly fitting parts or structures." As the two parts move back and forth, under pressure, the small surface irregularities between the two parts will weld together due to heat and pressure. Then as the motion continues, the small micro-welded points will rip apart. These very hard particles (welded) will oxidize and become a red powder or discoloration at the interface between the two parts. As the particles damage the surface the shaft material will become rough, causing more surface irregularities, causing more welding, oxidation, and damage. The process continues until the surface is so rough, a small crack appears at the valley where a piece of material has been removed and then a fatigue failure begins.
The concern is the rate of fretting corrosion development. The rate of development is related to a number of factors and is very difficult to predict. However, what is known is that fretting corrosion increases with the amount of relative motion; the greater the motion, the greater the damage. Oxygen is also known to accelerate fretting corrosion, but reference 3 explains that fretting corrosion will continue in a vacuum or an inert atmosphere, possibly at a slower rate.
The worst case would be high horsepower causing high shaft bending (thus relative motion between the shaft and the bearing) and a loose bearing fit where the bearing and shaft easily move relative to one another. It disassembly of RotorWay secondary assemblies, it is evident that some bearings have an extremely low fit between the bearing and shaft. This variation in bearing-shaft fits is what causes the confusion as to why one system fails and another lasts for long period of time. It is the bearing fit that variations that make the failures look random.
Examples of Fretting Corrosion on Unbroken Shafts!
Check out these photos of unbroken, disassembled secondary shafts.
Click on picture to see a larger view.
33 Hours, 35 mm 280 Hours, 30 mm 25 Hours, 35 mm 100 Hours, 35 mm
How Can We Prevent Fretting Corrosion?
While there are a number of techniques to reduce fretting corrosion, the best choice is to reduce the relative motion between the two parts by moving the bending stress of the part into a different portion of the shaft. With approximately 2000 pounds of tension pulling on the shaft (regardless of chain or belt) the shaft will flex. That flex is best put somewhere other then the area under the bearing. Therefore, by stress relieving the bearing-shaft interface and moving the bending away from the bearing, fretting can be eliminated. The concept is shown clearly in reference 3 and is included in Figure 5.
Figure 5. Standard design practice for reducing fretting and improving fatigue strength.
1. "Mechanical Engineering Design, edition 5" by J.E. Shigley and C.R. Mishke, published by McGraw-Hill 1989.
2. ASME Handbook, Metals Engineering-Design, McGraw-Hill Book Company, 1953
3. "Metal Fatigue in Engineering" by H.O. Fuchs and R.I. Stephens, John Wiley & Sons, 1980