"The Teeter Bolt and NDT

The recent posts regarding fractured teeter bolts brings up an issue that I have had to address. During a recent annual inspection I removed and inspected my favorite teeter bolt, after more than 500 faithful hours. It looked a bit worn, at least the coating, but otherwise looked okay. It was a Grade 8 bolt and I thought that it was time to replace it but when I tried to buy a similar bolt I found that the bolt had been cut down to 5 inches from a larger size, so the grip (the nonthreaded portion) was sized for the rotorhead and bearings. The option was to replace it with a 3/8" AN bolt. The expert at the aircraft hardware store assured me that AN bolts and Grade 8 bolts were identical. This assurance began a chain of events which led to this post. The two bolt types are very different. AN bolts are MIL spec bolts ("Army-Navy&quot that have specific material, construction and inspection criteria. The minimum tensile strength is 125,000 psi. Grade 8 bolts are commercial bolts, also designed as high strength bolts, but are not held to the same construction and inspection standards. The ASTM standard is for a minimum of 150,000 psi tensile strength. There is a higher commercial bolt, L9, which is rated to 180,000 psi. So, the question arose, do I use the higher rated Grade 8 bolt or the higher spec'ed AN bolt?

The answer requires a bit of math. The psi tensile strength must be recalculated for the size bolt used. In this case, a 3/8" bolt has a cross-sectional area of 0.1104 sq in. So a 3/8" AN bolt has a rated tensile strength of 13,800 lb (125,000 x 0.1104). And a 3/8" Grade 8 bolt has a rated tensile strength of 16,560 lb. Plenty strong, right? Well, not so fast. Tensile strength is the force needed to induce failure along the long axis of the bolt, along the length of the bolt. Most forces in aircraft are applied across bolts, referred to as a shear force. And the shear strength of a bolt is usually about 60% of the tensile strength. So a 3/8" AN bolt has a shear strength of about 8280 lb and a Grade 8 bolt about 9936 lb.

How much force can a gyro develop? Chuck Beaty and I once had a discussion and decided that in almost all cases the maximum G force a gyro can generate is about 2 G's. My gyro, with me and 48 gallons of fuel would weigh a little less than 1000 lb. So I need to figure about 2000 lb.

In many things I do, I like a 5:1 safety factor. And the above calculations do not figure in wear and tear. So the numbers weren't looking too good for the AN bolt. But wait!!! The 60% shear force calculation is based on a SINGLE POINT shear. And the rotor head generates a double point shear, the force is applied simultaneously to two places on the bolt. In short the G forces are split between two points. So the safety factor for the AN bolt jumps from 4:1 to 8:1. Much better.

But how do I know that the bolt is safe? NDT stands for Non Destructive Testing. It can be quite complicated and can involve many different techniques including Xray, Ultrasound, Dye Penetrant and Magnetism. But for most purposes Dye Penetrant and Magnetic testing are used. Dye testing involves cleaning the bolt, for this discussion, and soaking the bolt in a penetrant which will be absorbed into any small cracks by capillary action. The residual penetrant is removed and a developer is placed on the bolt which draws out the penetrant from the crack, creating a pattern that is wider than the crack so it can be seen by the human eye. Visual dye systems such as Magnaflux's Spotcheck are cheap (about $28 for the Spotcheck Jr.) and can be used easily. Fluorescent UV dye systems such as Magnaflux's Zyglo system use a black light and a fluorescent dye which is more easily seen and therefore is more sensitive to small cracks. Both systems only work on surface cracks. There is a more complex method involving magnetism if the material tested is ferrous, ie. made of iron, like our bolt. The part is magnetized and sprayed or soaked in small particles. If there is a crack, either at the surface or even just below, then the magnetic field in the bolt "jumps" over the defect causing a clumping of particles at the surface, which can be seen. The main advantage to this system is that subsurface flaws can be seen.

So, what am I using? Since the magnetic system requires a huge investment in the yokes or coils used to create a magnetic field, I have opted for the UV system. I only have to pay for the chemicals and the black light. It will find very small cracks and allow me to sleep better when I'm flying.

I am using an AN bolt at this time. I figure that the trade of 16% shear strength is made up by better documentation and testing of AN bolts. And well within the safety margins that I've set. One other factor plays into the equation. AN bolts tend to bend and elongate prior to fracture. Grade 8 bolts tend to fracture quickly. I am much more likely to pick up deformation on inspection.

What about that 500 hour teeter bolt? Stay tuned." ~ Bill Clem; Rotary Wing Forum.

".... in the nearly 35 years I've been hanging around gyros, I have never heard of a 3/8" teeter bolt failing." ~ Doug Riley; Rotary Wing Forum

"Scott, NAS bolts are fine. They are stronger than AN bolts. They're also a pain to work with, however, because they have extremely short threads. Properly-made Grade 8 bolts have rolled threads, too. A rolled thread is easy to spot: at the point just above the beginning of the thread, the shank of the bolt tapers down below the nominal diameter, and then below that point the material has obviously been "squished" back up to the nominal diameter. Cut threads look just like the threads you make with a hand die... because that's what they are." ~ Doug Riley; Rotary Wing Forum

"My interpretation was that AN bolts were the equivilent of grade 5 but with closer tolerances, inspection processes, rolled threads and cad plated. I thought that grade 8 are more designed for tension aplications and grade 5 and AN more for shear applications. Do I have the wrong understanding? " ~ Aussie Paul; Rotary Wing Forum

"An AN6 bolt is actually rated for only 10100 lb. in tension. The bolt will fail in the threads and not the cross-section. Tension loadings should be avoided where ever possible. An ideal joint is in double shear with a minimum of 4 fasteners with the load passing directly through the fastener pattern.... " ~ Rwilco; Rotary Wing Forum
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This is the US Patent and Trademark Office, where bolt manufacturers have their insignia recorded.