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Engine mount longeron bolts

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Gareth

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Oct 14, 2018
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Brisbane
Hi all
please see hand drawn sketch of proposed bolts to longeron for engine mount support, what is the best material to machine these 4 bolts from
Cheers Gareth from Aus
 

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TiPi

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Aug 25, 2019
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Peeramon (AUS)
Hi all
please see hand drawn sketch of proposed bolts to longeron for engine mount support, what is the best material to machine these 4 bolts from
Cheers Gareth from Aus
Step 1: work out the load on the bolts (tensile, shear)
Step 2: calculate the stress (combined tensile & shear incl pre-load)
Step 3: add the safety factor
Step 4: check out material data to find a material that has the strength for meeting/exceeding the stress levels calculated and has suitable properties for the application
 

wsimpso1

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I do not particularly like the scheme on several levels.

The conventional hex bolt going through a tubular end on both the mount and the fuselage is torqued, putting the bolt in tension that is above the largest load that would open the joint. This preload prevents the threaded regions from seeing large load variations, which largely precludes fatigue, and is all standard fastener design stuff. LOTS of history...

This design puts two1/4" bolts or pins in double shear on the fuselage end. That may appear strong enough, but they will see all of the cyclic load applied to the mount, so in addition to analyzing for stress, mean load, cyclic load, stress concentrations, and fatigue, you get to analyze the pins or bolts for all that too. Bolts should not see cyclic loads - they work far far better under lots of preload and then friction and joint stiffness preventing fatigue.

At the other end, you appear to be applying a nut to clamp the mount to the flange, and that is good practice, but I still have a great big concern. Usually this is done with a fastener that has its threads rolled in, then heat treated and Cd plated. This produces a bolt of consistent and fairly high strength, excellent fatigue resistance (for a bolt) , and excellent corrosion resistance. None of this can be said for your part. I suspect it will be turned and the thread cut. Maybe it will be heat treated, maybe sprayed with corrosion preventative.

As to materials, few on here might recommend a material, but you are designing this piece, you are responsible for the design, and you will have your neck on the line. Specifying material and heat treat to ensure long term survival of several of these per airplane really requires detailed review of the design space for all fixed and variable loads, cyclic loading, etc, then analysis of the design under this spectrum of loads to select suitable material, heat treat, and preservative in use.

Billski
 

wsimpso1

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Step 1: work out the load on the bolts (tensile, shear)
Step 2: calculate the stress (combined tensile & shear incl pre-load)
Step 3: add the safety factor
Step 4: check out material data to find a material that has the strength for meeting/exceeding the stress levels calculated and has suitable properties for the application
Step 2.5: Calculate min and max values on both ends of the system given preload or lack thereof, then perform fatigue study
 

Gareth

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Oct 14, 2018
Messages
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Brisbane
The solid steel(mild steel) round bar with a 6mm flat bar welded to it is used on a australian kit Morgan sierra over 50 flying with many different engines installed, uses 2 3/16 bolted through longerons as described in my sketch and 1 1/4 bolted through flat bar to engine mount and even when a Morgan crashed into a ferris wheel in Aus the bolts did not let go, i would not use mild steel for my build but may use the same system using 4140, from what i can deduct from tables found ,thanks for your imput
 

wsimpso1

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The solid steel(mild steel) round bar with a 6mm flat bar welded to it is used on a australian kit Morgan sierra over 50 flying with many different engines installed, uses 2 3/16 bolted through longerons as described in my sketch and 1 1/4 bolted through flat bar to engine mount and even when a Morgan crashed into a ferris wheel in Aus the bolts did not let go, i would not use mild steel for my build but may use the same system using 4140, from what i can deduct from tables found ,thanks for your imput
That's nice. Is the Morgan same or smaller engine weight, same or smaller distance from longerons, same or larger spread between longerons in both vertical and horizontal directions? If it is the same, monkey-see monkee-do might be OK, depending upon how good the copied design is.

Either go conventional or engineer it. Conventional works. Engineering works. But eyeballing frequently ends up either way heavy or weak, can easily be both heavy and weak. The thing drawn in the OP is a brick for weight, and WEIGHT IS THE ENEMY.

Billski
 
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