Sorry you are confused
DanH said:
<<Screws in TENSION is not good.... SHEAR IS GOOD, Tension is bad from fatigue and pull-up stand point.>>
Complete nonsense.
With UNF thread fasteners, the tension rating is almost always higher than the shear rating. Only small UNC (coarse) thread fasteners have a lower tension rating, a function of their smaller thread root diameter.
Moot anyway, because properly designed true shear joints do not apply any shear load to the threaded portion of the fastener. This is not possible with a standard short-length machine screw; there is no unthreaded shank.
Even if this application was a true shear joint (screw with a shank, no shear loads on the threaded portion of the fastener) the bearing strength of a hole in fiberglass is way, way lower than the fastener shear strength. The hole will elongate.
All ordinary screw-fastened joints are intended to place the fastener in tension. The load vector of the joint may indeed be shear, but the load transfer between the parts is via friction, a function of the clamping force provided by the tension fastener and the coefficient of friction of the clamped surfaces.
As for fastener fatigue in tension, go back and re-read FrankH's post regarding pre-tension of fasteners. He is completely correct.
Pre-tension aside, let's assume you can't get much clamp force in the joint with a Phillips drive screw. In this spinner application the crush strength of the fiberglass under the head of the screw may be too low anyway. No problem; increase the coefficient of friction between the clamped parts, and thus eliminate relative motion (and fastener fatigue) at the lower fastener tension. As Kahuna suggested, put some proseal in the joint...and clean out any residual grease or mold release agent before you re-assemble.
First Dan it is apparent that you have no clue what the difference is between ultimate load and fatigue load is.
You are also confused about the word pre-load and pre-tension. I agree 100% with FrankH's post. I never said different. I said a screw in tension is not good. I should have said
cyclic tension. YOU are confused with bolt PRE-TENSION and pre-loaded structure. Frank correctly describes pre-loaded structure not bolt pre-tension. Bolt pre-tension is used in joints like rod cap bolts. A screw is not a designed for pre-tension or fatigue.
I also never said Kahuna's idea would not work or did not have merit.
PULL-UP IS A KILLER AND IS WAY DIFFERENT THAN PRE-TENSION OF FASTENERS. Trust me there are very tight controls regarding pull-up and pre-loaded structure (ie bent into shape to assemble) in aerospace structure.
Pre-tension is for fatigue improvement and done extensively (with fatigue quality fasteners not screws). You are badly confused about the difference.
Pre-tension is done to keep the fastener in tension always, by "stretching" the bolt and compressing the surrounding structure by torquing the bolt. You improve fatigue life of bolts because the bolt stress is always positive or in tension. Dan... in the spinner we are talking SS screw in sheet metal/fiberglass not a rod caps or jet turbine case.
PULL-UP or
pre-load stress (bending sheet metal and fiberglass) is a problem because the pre-loaded structure vaires the tension in the fastener greatly under load (as the structure deflects per FrankH's comments). This allows the bolt or screw to totally relax. THIS IS BAD and causes high fatigue cycles.
The fastener loaded by the pre-loaded structure being twisted and sprung is bad. As the spinner rotates the flange flexes and the pull-up relaxes and then stretches the screw again. This is exaggerated by the prying of the heal toe action of the flange increasing the load. Trust me. It is a poor LOAD PATH.
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Hardware store quality (SS) stainless steel fasteners builders use in "appearance" locations are NOT structural but decorative boat quality. The spinner is a bad place for them. Even Mil-Spec or AN spec stainless steel screws are not structural or intended for high loads or severe vibration. Yes they are strong but they are brittle. In general the stronger a steel is the lower it's fatigue life, durability and crack resistance is. A low carbon steel fastener is a better choice than SS.
MS (mil-spec) or AN (army navy, old designation) hardware come in different tensile strength like 60,000 psi (same as 60 ksi), 80 ksi and 125 ksi typically. Lower tensile strength screws are fine. We don't need strength as much as fatigue durability. You also can get countersunk -8 or -10 screws or bolts with a shoulder so threads are NOT is shear (
NAS1202/NAS1203). Needless to say threads in shear is BAD.
Dan I have a Masters in Mechanical Engineering and was a senior lead structural engineer at Boeing before flying Boeing jets. My job was calculating stress, strain, fatigue, and fatigue life of mechanically fastened joints. I know what I am talking about.
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Your statements about
load transfer and bolted joint design are mostly incorrect. I may be wrong about many things Dan, but this is not one of them. You know a little, enough to be dangerous with your limited understanding load transfer and bolted joint mechanics.
You are right, a GOOD shear joint does not put the shear thru the threaded portion. That is not the case here with fully threaded screws. So its not moot. It is bad to use threaded fasteners in structural applications. A SS screw on the spinner can cause damage as we see.
Shear is better than tension any day for fatigue. This is engineering 101. A shear joint is 1000 times more efficient than tension joints. Fatigue is predominately a tension or tension/compression reversal mechanism, NOT shear and never compression. This is basic aircraft structures knowlege. All aerospace structural engineers strive for shear load paths into structure.
As far as friction taking the shear, that is NEVER used or assumed in design, EVER NEVER EVER. I know I did it for a living. It's not that it's not there, just that it can't be counted on.
Friction is involved in ALL joints, but ALL airframe engineers ignore friction in the design and analysis 99.9% of the time. So what is the shear capability of the friction in your JOINT? You don't know. That is why engineers typically do not design aircraft structure with FRICTION! Geeee
Pro seal is wonderful stuff (we called it Boeing Material Spec, BMS- 5-95). You could do the same with fiberglass and microballoons. Grease the bulkhead and fill the fiberglass "land" for better fit. However the pro-seal trick is good and it will in fact increase friction and help unload the screws. Along with that use structural screws. Regardless good bulkhead/spinner fit (no pull-up) and min spinner run-out (wobble) is more important.
You say fiberglass has less bearing strength than the screws shear strength. It all depends on the fiberglass thickness. Fiberglass can shear a screw, trust me. As one fastener yields it loads the next more and pop pop pop. It is called the zipper effect.
You can't pre-tension screws in fiberglass. This is a big issue in aerospace and composite structure, so inserts are often used to get proper clamp-up.
As far as tension / shear rating of fasteners, it varies and depends on fastener diameter and head design. Not all fasteners have higher tension rating than shear. There are spacific tension and shear head fasteners made by name and purpose.
I am sorry you took the "complete nonsense" attitude, but I'll take the high road. The only nonsense is your lack of civility.
Merry Christmas
