DESIGN: Electrotor-MicroLite ~ Power Train - Miscellaneous
Bolt Load Calculations:
The following must be checked.
Bolts Holding Crown Gear, Spacer w/ Bearing and Inner Hub-bar.
Assumption: total of 4 blades and a GW of 300 lbs.
Hovering load is 75 lbs per blade.
Moment arm is 0.75R where R is 3'-6" [42"] = 31'5"
For pitch and roll using Limit load factor for flapping moment is 3.0
"Assume the craft is flying at maximum speed with full torque applied to the rotor, and a maneuver made with full control displacement. Limit load factor for rotor thrust loads = 4.0" ~ [Source ~ MDD p.11]
Safety factor is 1.5
Using 4 and 1.5; Ultimate moment is 75 x 31.5 x 4 x 1.5 = 14,175 lb-in.
Distance from bolt radius across to Hub's OD = 4.625"
The moment is shared by 2 bolts therefor the tensile loading per bolt is 14,175 / (4.625 X 2) = 1,532 lbs.
An AN4 steel bolt has an Ultimate tensile strength at root diameter of 4,080 lbs. and a Yield tensile at root diameter of 3,130 lbs.
An AN5 steel bolt has an Ultimate tensile strength at root diameter of 6,500 lbs. and a Yield tensile at root diameter of 4,980 lbs.
OTHER: Mechanics - General - Hardware
An AN5 bolt; Area of minor diameter = 0.0580 sq in. Minimum yield strength = 109,000 PSI. 0.0524 x 109,000 = 5,712 lbs ~ OTHER: Mechanics - General - Hardware
The weight of a 5/16" dia. x 4" bolt = 0.087 lbs. The weight of a 1/4" dia. x 4" bolt = 0.056 lbs. 12 bolt means a difference of 0.372 lbs.
The blades on the coaxial configuration will be subjected to the additional loading of rotor to rotor at various RRPM due to the n/rev (NP) vibration, which will likely induce self-exciting vibration.
If only one bolt take the load there is no additional safety factor.
If 3-blade rotors are used the moment should be less but not 2/3s less.
This probably does not consider the tension placed on the nut and bolt from the installation torque. Perhaps AN5 would be better. Or, alternatively, would increasing the number of AN4 bolts help.
The smaller bolts help with the design and there is no difficulty in adding more bolts at the start or later on.
What if bolt count was doubled to 12. Then if the bolts that center on the hub bar azimuths had slightly less torque the the bolts at 30º on each side would this be more inclined to share the tension load on the bolts???
- Relevant or Irrelevant Musings???
- An AN5 bolt; Tensile strength area = 0.0580 sq in. AN minimal tensile strength = 125,000 PSI. 0.0580 x 125,000 = 7,450 lbs ~
Aluminum Hub-bar Calculations re Bolt head pulling through aluminum:
From above Bolt Load Calculations; The moment is shared by 2 bolts therefor the tensile loading per bolt is 14,175 / (4.625 X 2) = 1,532 lbs. For one bolt it will be 2 X 1,532 = 3,064 lbs.
The bearing yield for 6061-T6 is 65k psi and the ultimate is 80k psi. The outside diameter of an AN960-416 washer is 0.50". This area is 0.1964 sq-in. giving a bearing yield of 65k x 0.1964 = 12.8 K. This probably assumes that the bolt is not near the edge of the aluminum, which in this case it is.
Universal Joint: [Sources ~ MDD p.72- 74]
- "For general use a factor of from 4 to 4.5 times the maximum operating load will be used to determine the ultimate strength of the universal joint."
- "The maximum operating angle for a Cardan type of universal joint under full load is about 7º." This will not be a problem since the Universal joint is only being considered for the Intermeshing configuration.
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Last Revised: November 23, 2009