Propeller shaft design

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kubark42

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I'm looking for information on propeller shaft design for a pusher prop. I have a belt-driven system where the pulleys mount directly to the propeller, so the shaft is more of a free-wheeling axle than it is a power-transmitting shaft. As such, I only have bending moments to worry about. Thus, calculating the loads and stresses is trivial.

No surprise that I want to make this as strong and light as possible. Design parameters AFAICT are two: 1) the distance between bearings and 2) the shaft's 2nd moment of area (i.e. inner vs outer diameters). (Cost isn't much of a factor, as a section of 4140 shaft material from McMaster is going to cost around $1/cm, so the final materials cost is well under $20.)

Where I need some input is calculating the design load limits as a function of desired TBO. On paper, the solid shaft is massively over-speced, and could almost be a thin-walled 4140 tube. Of course, that isn't accounting for 2nd order effects, such as resonance, fatigue, bearing point load deformation, etc... These are the things of which I have no knowledge.


Close up view:
Screen Shot 2021-01-16 at 2.08.12 AM.png

  • Gold: propeller
  • Red: bearings
  • Purple: pulleys
  • Blue: propeller shaft
Thoughts?

Side view:
Screen Shot 2021-01-16 at 2.08.31 AM.png
 

Dana

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The biggest (and hardest to calculate) loads are gyroscopic forces from pitch and yaw, and vibration from an out of balance prop.

A lot of small redrives use a non rotating shaft, with the bearings pressed into the pulleys. I had a paramotor with this arrangement... unfortunately the shaft was aluminum, machined in one piece with the eccentric that provided the tensioning. After a couple of years the shaft failed, cracked at the inside corner where it necked down to the bearing diameter.

The places you'll want to look at are any diameter changes where the shaft is machined down to retain the bearings, etc. Remember in this arrangement the berings must be a press fit onto the shaft and a slip fit into whatever housing you're using, unless you're using extended inner ring bearings with some kind of locking feature.
 

wsimpso1

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As designed, the shaft will see a fully reversed load cycle (from belt pull) on every revolution, so you will likely have to design for infinite life under that loading. If power pulses react into bending the shaft, you will have half as many power pulses as you have cylinders per turn of the crankshaft (four stroke) or as many per turn as cylinders (two stroke). Shrigley's chapter on fatigue is an excellent place to start. This might be why some would use a fixed instead of rotating shaft...

Then there is Dana's point on gyroscopics. You will need an upper bound on propeller inertia and a look at your entire operation envelope for prop speed combined with pitch-yaw rotation rate to calculate gyroscopic moment. Remember that speeds for gyroscopic moment are in radians per second. Spins definitely need to be covered, snap rolls need to be considered, and non-spin maneuver entries and exits can produce substantial pitch-yaw rates with the engine at power.

I would look over both solid and tubes for shafts. Your bearing at the prop will likely be much larger than the bearing at the other end of the shaft. Consider angular deflection of the shaft and in the bearings too - the bearing manufacturer will have advice on max angle that should be heeded...

Billski
 

Protech Racing

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I have an" Eagle UL" pusher redrive that has been cut off to about 1 ft. The prop shaft is 1in solid Aluminum.
I put a bout 150 hrs on it with bearing ,prop bolts etc preflighted every flight. It is still RTF . 54x27 wood prop .
 
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kubark42

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For background, the application is a self-launching glider, so TBO is around 25hrs. This lets me make certain choices which would otherwise seem underwhelming. In particular, no maneuvers aside from straightforward climb, slow turns, and recovery from the unforeseen (e.g. stall-spin) are expected.

The biggest (and hardest to calculate) loads are gyroscopic forces from pitch and yaw, and vibration from an out of balance prop.
I'm hoping there are some FAA guidelines for this, similar to the FAA's engine mount spec.

A lot of small redrives use a non rotating shaft, with the bearings pressed into the pulleys. I had a paramotor with this arrangement... unfortunately the shaft was aluminum, machined in one piece with the eccentric that provided the tensioning. After a couple of years the shaft failed, cracked at the inside corner where it necked down to the bearing diameter.
Lemme guess, you had an MZ34/MZ35? The aluminum shaft which, reportedly, single-handedly caused the replacement of every AC-5M powerplant, leading to the demise of the plane after Aviastroitel sold 3 dozen?

Not that I'm bitter.

If power pulses react into bending the shaft... This might be why some would use a fixed instead of rotating shaft...
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I would look over both solid and tubes for shafts.
I am using electric propulsion, so aside from minor torque ripple there's really no power pulse. I don't quite follow why using a fixed inner shaft solves the problem, since that moves the cyclic bending into the outer

The MZ35, despite its faults, has found a bearing system which works. It uses two identical 6206 bearings, and if that approach can survive the single-cylinder two-stroke engine, I figure it's definitely suitable for a motor application.

Regarding looking at both solid rods and tubes, is there any difference between the two aside from 2nd moment of area? And do you have a favorite recommendation for these other than McMaster?

How are thrust loads handled?
Probably the forward bearing will serve as the thrust bearing. In theory, it doesn't matter which bearing it is and mechanically it would be simpler to let the pulleys ride directly on the rear bearing. However, this means that the force application point is at the rear of the pylon, instead of the front. I fear this will increase the odds of oscillation.

I have an" Eagle UL" pusher redrive that has been cut off to about 1 ft. The prop shaft is 1in solid Aluminum.
I put a bout 150 hrs on it with bearing ,prop bolts etc preflighted every flight. It is still RTF . 54x27 wood prop .
The shaft issue @Dana and I are referring to is the teachable moment for why I'm focusing on 4140 instead of aluminum. 7075-T6 Aluminum vs. Normalized SAE-AISI 4140 :: MakeItFrom.com shows that the fatigue strength of the 4140 is almost 3x that of 7075-T6, so there's really no advantage even on paper for 3x lighter aluminum.

I could be convinced to use aluminum, since a solid shaft bought from McMaster is easier to manufacture than a bored out 4140 rod. What power engine are you using?
 

Dana

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Lemme guess, you had an MZ34/MZ35? The aluminum shaft which, reportedly, single-handedly caused the replacement of every AC-5M powerplant
No, it was a Solo 210 engine on a Walkerjet paramotor. When it broke (I never found the prop/pulley/bearings) I bored the remainder of the eccentric out, pressed a 4130 shaft into it, and made a new pulley.

Regarding thrust loads, they'll be significantly less than the radial loads from belt tension. I forget the specifics, but determined a radial bearing that could handle the radial loads would have plenty of thrust capacity, IIRC they can typically handle thrust loads of 10-20% of their rated radial loads.
 
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