B297

OTHER: Mechanics - Safety Factor

Overview:

An attempt to get a clearer understanding of the safety factors that are used on the mechanical and structural components of helicopters.

I am coming to the impression that certified rotorcraft are looking at a 2000 hour life-limit whereas homebuilt rotorcraft are looking at a 250 - 750 hour life-limit.

This page needs to be expanded, but the information is hard to get.

Perhaps for gears the factors should be Durability S.F. = 0.9 and Strength S.F. = 1.0. This should result in faster wear but decreased chance of out-right breakage. These factors could be increased of decreased depending on the criticalness of the gears.

Drive for Intermeshing Rotors: [Source ~ MDD p.77]

The drive between the engine and. the rotors for a twin-rotor craft having intermeshing rotors should be given special consideration, particularly the portion of the mechanical system which interconnects the rotor hubs. The transmission between the engine and the rotor interconnecting portion may be designed to the same factors used in connection with standard transmission design, namely, 4 to 4.5 on torsional operating loads. The interconnecting portion should, however, have an additional safety factor inasmuch as a failure in this portion of the mechanism would result In the rotors moving from their proper relationship, thus permitting interference between the blades. An additional factor of 1.5 should be used In the design of the shafting and similar parts. The gearing and mounting should also be designed to lower operating stresses to guard against the possibility of failure in this part of the system.

Statement in Journal of the AHS, April 2003, page 74:

During maneuvers, the value of the torque can be 2 to 4 times the value in hover for single rotor helicopters.

Posting on Eng-Tips:

Intermesher (Aeronautics)

Can anyone advise on how gear sizes are determined for aerospace application?

When I reverse engineer a gear used in a helicopter and then use catalog calculations, the safety factor comes in a 1 or less. The safety factor must obviously be greater than 1, therefore I assume that there is an adjustment made, which is related to the life of the gear.

My questions are;
~ How is the life factor entered in to the calculations?
~ Is the life factor based on Durability or Strength?

israelkk (Aerospace)

Look for AGMA standards. http://www.agma.org/index.asp?content_id=31

plasgears (Mechanical)

The PowerGear program for gear design handles these questions with authority. Gear teeth fatigue of course is the limiting element.

What you may be missing is the principles of modified gears, like extended addendum on pinions, full fillet radius, thicker pinion tooth/thinner gear tooth, etc. When you iterate your design based on various mods, you will be surprised at the reductions in stress. I have found 1.2 extended addendum and 54% pinion tooth thickness to be very beneficial.

From Kaman Paper: [Source ~ HTD p.7]

Bearings - AFBMA System

Deflection Criteria - Roller bearing misalignment is limited to 0.0005 inches/inch max.

Life Criteria - Bearings are sized for 4000 hours L10 using a life enhancement factor of 3 for CEVM M50

Notes:

Where feasible, it is desirable to provide a minimum life of 1000 hours for the bearings of a transmission. This will give upwards of 2500 hours average life. When designed to this life the bearings need be carefully inspected only at about 500 hour intervals. [Source - MDD p.68]. Duan Klettke mentioned that bearings are rated for 10,000 hours.

Re Planetary Gearing:

Often the gear itself may be manufactured with a hardened and ground bore which will serve as the outer race of the bearing. The inner race may be a hardened pin. or if space permits, a hardened and ground sleeve may be used. Rockwell C-60, or greater. It is recommended that rollers be used in this design.[Source - MDD p.68]

Tentative design criteria:

 Critical location Reliability: 99% Fatigue life: 1000 hours Non critical location Reliability: 98% Fatigue life: 1000 hours

Calculations of fatigue life of bearing below are done on [FORM: PT - Bearing Calculations]

Outside Helicopter

Safari (formally Baby Belle):

"The main gears are 1/2" wider for stronger gear teeth and increased gear life (guaranteed 1000 hours at certified aircraft standards)".

Notes:

The V belts drives in the Robinson R22 and the Schweizer 300 appear to be handling far more loading then the V-belt manufactures recommendations. Obviously, the life factor is playing a large role. Complete information and calculations on the Helicycle's primary drive are located on page ????

Local Page:

 0881 DESIGN: UniCopter ~ Rotor - Hub - Design Considerations B011 DESIGN: SynchroLite ~ Power Train - Secondary Reduction (planetary drive)

Web Pages:

SynchroLite

Consider having the Safety Factor at 1.0 (at least for the rotor) for the SynchroLite; so that it is able to be <= 254 lbs. It will obviously have a reduced life-limit.

UniCopter

Consider having the Safety Factor at 1.5 (at least for the rotor) for the UniCopter; so that it complies with [Source ~ MDD] life-limit.

Dragonfly

Consider having the Safety Factor at 1.0 (at least for the rotor) for the Dragonfly; so that it is able to be <= 254 lbs. It will obviously have a reduced life-limit.

Last Revised: May 24, 2008