GX200 External Thrust Bearing

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Ontarioflyer

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I wanted to post this idea I had and get some feedback before I start testing. My 185's are not worth rebuilding (probably for the best) and I have decided to go with two GX200's on my Lazair. I have read many posts and saw the You-Tube videos with and with-out the re-drives. I know Ace-Aviation is the turn-key solution, but you know, price/shipping, etc. I have done the math with Torontogear for "timing belt pulleys and it is very do-able. Which brings me to my "plan-B" direct drive design. I don't have any cad-like software to draw something out, so I will do my best to explain the idea.
Depending on available crank shaft length, starting with one or two 3/4" split ring shaft collars directly next to the crank seal leaving a small space. Next is a 3/4" thrust bearing with the polished washer on each side. Four threaded rods (1.5 - 2" long) attached to the block around the crank-shaft that will hold a 3" x 3" x 3/8" aluminum plate with a 7/8" centered hole that will fit over the crank and fit up against the thrust washer, of course in the vertical position. So the split ring collar(s) are friction fit, the thrust washer will press against the aluminum plate as axial load increases. Then of course mount the prop on the crank in a direct drive application.
Honda will not give me the crank axial load specs (of course) and I am not a fan of the crank taking this load. I know the one re-engine Honda clone guy is having luck, but I suspect that it is just a "matter of time" before crank bearing failure will happen.
Thoughts, supporting math loads, nay-sayers welcome.
 

Vigilant1

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If all of this is on the PTO end of the crankshaft, what mass is on the other end? Will you be keeping the stock flywheel with alternator and ignition magnets, lightening it up, etc? It seems to me that crank failure due to torsional issues/stresses if there is considerable mass on both ends of the crank will be a bigger concern than stresses due to the rather moderate axial thrust loads. This affects your plan because all the extra washers, bearings, split-ring collars and plates added to the PTO end reduces available shaft length to mount a lightened magnet wheel for ignition on the same end as the prop.
These small engines break cranks if there is appreciable moment of inertia on opposite ends of the crankshaft.
 
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wsimpso1

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I give my standard commentary on hanging a prop off a crank not designed for it. The prop will put several loads on what it is attached to:
  • Thrust - this axial force on the prop flange can be estimated by using a prop calculator;
  • P-Factor - Look up ways to calculate this bending moment on the flange due to the prop axis not being aligned with the direction of airflow. This one occurs at prop rotation rate, and so fatigue for this accumulates pretty rapidly;
  • Mean Torque - get out the engine curves and look this pure torsion load on the shaft;
  • Firing Torque - this one is a little harder to estimate, but a normally aspirated gasoline fueled piston engine have peak firing accels of about 2500 rad/s/s. Multiply that by prop mass moment of inertia, and you should have a reasonable estimate. This one happens at firing cycle rate, so its contribution to fatigue accumulates rapidly;
  • Gyroscopic Moment - this is the product of Mass Moment of Inertia * max combined pitch and yaw rate * prop rotation rate. Rotation rates are in rad/s, so MMOI needs to be in consistent units. This one also occurs at rotation rate and so can accumulate rapidly, but we tend to spend pretty small amounts of time in snap rolls and power-on spin entries...
You can get all of this simultaneously, but you need to start with reasonable estimates for your bird, engine, prop, etc, then make sure your system will carry all of that.

As to whether your system can carry all of that for long or not, I have no idea. I do know that a couple of the drive makers have calculated for it and one has published on hba.com about their capabilities under such loads. No should iterate your design as you feel is needed. SolidWorks is your friend. on all of this. So is monkey-see monkey-do engineering, as long as you EXACTLY COPY the schemes you KNOW to work with your engine. Start deviating, and who knows where you are actually stepping off the cliff...

Billski
 
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rv7charlie

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I bow to Bilski's expertise & experience, but I'd bet that thrust loads are the least of worries; they tend to be the smallest of all the forces on the crank.

As to loads opposite the flywheel, every small 'industrial'engine I'm aware of, from lawn mowers to generators to pumps to etc etc, has the PTO opposite the flywheel end of the engine. They also have extended length bearings on the PTO end (just like an a/c engine) to keep pulley bending loads (effectively, gyro loads from the prop in an a/c) away from the crank throws.

Charlie
 

wsimpso1

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The PTO end of many industrial engines is beefy for taking belt loads, hanging a mower blade that looks a lot like a prop, etc. While the industrial engine guys plan for an offset load from the belt and firing oscillation on the mower blade, they do not plan for as much inertia as a prop might have, nor do they expect any significant gyroscopic effects. It MIGHT be beefy enough to take a real prop, but you better make sure unless you like the idea of having the prop depart the airplane in flight at inconvenient times.
 

proppastie

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Weights vs. 185?......and if you can do what you are looking to do I would think that you can rebuild the 185s
 

TiPi

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This is from the Honda GX240-GX390 technical manual. As they are splash-lubricated, many engines have a deep-groove ball bearing in the engine cover and block (6206 for the GX240).
Bearing data: SKF1618704480216.png
 

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REVAN

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Honda will not give me the crank axial load specs (of course) and I am not a fan of the crank taking this load. I know the one re-engine Honda clone guy is having luck, but I suspect that it is just a "matter of time" before crank bearing failure will happen.
Thoughts, supporting math loads, nay-sayers welcome.
I've been experimenting with a Tillotson 212cc engine. I am directly mounting the propeller to the crankshaft. I've only got about 40 hours on the test stand so far, so nothing is conclusive at this point. However, I decided to forgo the development of a thrust bearing when I investigated similar bearings to what is installed in the engine that did list specs for axial load. Everything I could find had load specs upwards of 200 pounds for a single bearing. Most were in the 250 to 400 pound range, with some deep-race bearings getting up into the 600 pound range (though I don't know that the ones used in the engine qualify as "deep-race"). Since I'm not expecting more than about 60 pounds thrust from the propeller, I figured there is a good chance that a separate thrust bearing isn't needed for direct driving the propeller with this engine. As far as when the bearing will fail, you are right that is is only a "matter of time". But if the time is far enough out, then that is okay. The task now is to find out how long these engines will last before overhaul or replacement. At $200 for a stock 9.3 HP engine, these engines may be quite attractive for a TBR plan.
 

Vigilant1

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I've been experimenting with a Tillotson 212cc engine. I am directly mounting the propeller to the crankshaft. I've only got about 40 hours on the test stand so far, so nothing is conclusive at this point. ... At $200 for a stock 9.3 HP engine, these engines may be quite attractive for a TBR plan.
Thanks for the info, and looking forward to more reports as you test more. Have you had an opportunity to mount a test club to the prop hub to get a direct HP measurement?
 

proppastie

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Since I'm not expecting more than about 60 pounds thrust from the propeller,
one issue with propellers on aircraft is the Gyroscopic forces when there is a change of attitude, (pitch, roll) .......these forces subject the crank to bending loads.....examination of successful re-drives as regard prop mounting shaft size, bearings, hp, prop size etc. might change your mind as regards the need for strengthening. ....See post 5 above.....Good Luck.
 

REVAN

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one issue with propellers on aircraft is the Gyroscopic forces when there is a change of attitude, (pitch, roll) .......these forces subject the crank to bending loads.....examination of successful re-drives as regard prop mounting shaft size, bearings, hp, prop size etc. might change your mind as regards the need for strengthening. ....See post 5 above.....Good Luck.
The propeller is very lightweight compared to the flywheel, so it's contribution to gyroscopic forces on the engine's shaft will be limited. If I replace the iron flywheel with an aluminum wheel of half the weight (something I suspect is likely before flying it), these gyro loads on the shaft will be reduced further. Also, the gyro forces will contribute radial loads to the bearings which are the type of loads these bearings are built to take. These things combined lead me to believe that running into issues from gyroscopic forces is unlikely.

I'm more concerned with the axial loads becoming an issue. However, since I was unable to find a bearing in the style and size of what is used in this engine that was rated for less than 200 pounds in axial load, I'm encouraged that even this may turn out to be a non issue. Given this is a bearing for an engine's crankshaft, I doubt they went for the weakest bearing they could find. My guess is that it is actually a fairly good bearing, rating in the upper half of the spectrum for bearings of this size. The engine may very well wear out in other places before the bearings reach the end of their life. That's my hope, but only time and testing will tell.

My testing has been a mixture of static testing with a tractor propeller and dynamic testing with a pusher propeller. The dynamic testing is done on a bike with the motor mounted in a pusher configuration. So, the engine is being subject to a variety of loads from thrust, gyroscopic, and inertial forces. The inertial loads can be big as the bike has very little suspension and the roads are not very smooth. I've already had to replace a couple broken wheel spokes from the added weight of the engine and lack of suspension. Presently I'm back to static testing, as I took some FOD through the prop and damaged my pusher propeller.
 
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REVAN

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Thanks for the info, and looking forward to more reports as you test more. Have you had an opportunity to mount a test club to the prop hub to get a direct HP measurement?
I got my engine data from the folks at GoPower Sports, where I purchased the engine. The motor is marketed as a 10 HP engine. However, when they had it on the dyno, GoPower recorded outputs in the range of 9.3 to 9.5 HP for the stock engine. With an improved carb, intake and exhaust, the engine can be made to output 11+ HP without much cost or effort. Presently, I am only testing the stock engine configuration.
 
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Vigilant1

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Based on what I've read, with the direct drive industrial engines the crank itself, exterior to the bearing, is a more likely failure location than the bearing or the interior portion of the crank. This is particularly of concern if the crank there is of relatively small diameter, if the shaft is slotted and/or deeply tapped, and especially if there is a significant inertial load on the other end of the crank. A tapered shaft without a slot appears to be preferable. On the twins, many successful direct drive installations put the prop on the flywheel end-- the bearing isn't as stout, but the crankshaft is of larger diameter with fewer stress risers and the (lightened) flywheel can remain where it is. Still, I'm sure putting the prop on the PTO end can also work, and does have advantages (that's what Tipi is doing).
 

TiPi

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The propeller is very lightweight compared to the flywheel, so it's contribution to gyroscopic forces on the engine's shaft will be limited. If I replace the iron flywheel with an aluminum wheel of half the weight (something I suspect is likely before flying it), these gyro loads on the shaft will be reduced further. Also, the gyro forces will contribute radial loads to the bearings which are the type of loads these bearings are built to take. These things combined lead me to believe that running into issues from gyroscopic forces is unlikely.

I'm more concerned with the axial loads becoming an issue. However, since I was unable to find a bearing in the style and size of what is used in this engine that was rated for less than 200 pounds in axial load, I'm encouraged that even this may turn out to be a non issue. Given this is a bearing for an engine's crankshaft, I doubt they went for the weakest bearing they could find. My guess is that it is actually a fairly good bearing, rating in the upper half of the spectrum for bearings of this size. The engine may very well wear out in other places before the bearings reach the end of their life. That's my hope, but only time and testing will tell.

My testing has been a mixture of static testing with a tractor propeller and dynamic testing with a pusher propeller. The dynamic testing is done on a bike with the motor mounted in a pusher configuration. So, the engine is being subject to a variety of loads from thrust, gyroscopic, and inertial forces. The inertial loads can be big as the bike has very little suspension and the roads are not very smooth. I've already had to replace a couple broken wheel spokes from the added weight of the engine and lack of suspension. Presently I'm back to static testing, as I took some FOD through the prop and damaged my pusher propeller.
Sorry, you are wrong about the propeller being only a minor contributor to gyroscopic forces. It might be much ligther than the flywheel but the larger diameter more than offsets that. I measured the rotational inertia of a flywheel and a 2-blade wooden prop (2.7kg and 1.25m dia) and the prop had about 2.5 times the rotational inertia of the 7kg flywheel.
As V1 said, gyroscopic forces break the crankshaft. Add large rotational inertia components on opposite ends of the crankshaft makes the forces much worse (different resonant frequencies coupled by the crank).
 

REVAN

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Sorry, you are wrong about the propeller being only a minor contributor to gyroscopic forces. It might be much ligther than the flywheel but the larger diameter more than offsets that. I measured the rotational inertia of a flywheel and a 2-blade wooden prop (2.7kg and 1.25m dia) and the prop had about 2.5 times the rotational inertia of the 7kg flywheel.
As V1 said, gyroscopic forces break the crankshaft. Add large rotational inertia components on opposite ends of the crankshaft makes the forces much worse (different resonant frequencies coupled by the crank).
My heavy prop (14 pitch) only weighs 0.54 Kg with a diameter of 0.91m (36 inches). My other prop is lighter (12 pitch), but it's mounted to the engine so I can't weigh it right now. That, and the propeller has a lot of taper with comparatively little mass at the tips relative to other propeller designs I've looked at. So, my heavy prop is 1/5th the weight and 73% the diameter of your example. What do you think?

PS - I just noticed you referenced a 7 Kg flywheel. What engine are you using? My engine has a 3 Kg flywheel that I expect to be replacing with an aluminum flywheel of about half that weight.
 
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proppastie

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I've only got about 40 hours on the test stand
were you turning a prop?....was it a model prop and what rpm?....you might be able to test the gyroscopic forces by putting the stand on a pivot and rotating it 30 degree while running to simulate a turn or dive.

certainly the 27mm dia. crank is better than my 12mm crank on my 3w-200
 
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REVAN

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were you turning a prop?....was it a model prop and what rpm?....you might be able to test the gyroscopic forces by putting the stand on a pivot and rotating it 30 degree while running to simulate a turn or dive.

certainly the 27mm dia. crank is better than my 12mm crank on my 3w-200
Yes. It is a Xoar 36x14 pusher propeller. Static it gets up to 4050 RPM. At speed on the bike testbed, I think it can get up to about 4200 RPM.

I run a 36x12 tractor propeller on the static test stand. It gets up to about 4250 RPM.

FYI: I'm at about 3000 feet altitude here.
 

TiPi

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My heavy prop (14 pitch) only weighs 0.54 Kg with a diameter of 0.91m (36 inches). My other prop is lighter (12 pitch), but it's mounted to the engine so I can't weigh it right now. That, and the propeller has a lot of taper with comparatively little mass at the tips relative to other propeller designs I've looked at. So, my heavy prop is 1/5th the weight and 73% the diameter of your example. What do you think?

PS - I just noticed you referenced a 7 Kg flywheel. What engine are you using? My engine has a 3 Kg flywheel that I expect to be replacing with an aluminum flywheel of about half that weight.
I'm working on a B&S V-twin: B&S 49-series (810cm3/49ci) - TiPi's conversion for aircraft use
Rotax has published a simple procedure for measuring propeller inertia. Your prop is off the chart but the spreadsheet can be used to calculate it for any hanging configuration.
 

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Vigilant1

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Your prop is off the chart but the spreadsheet can be used to calculate it for any hanging configuration.
And about that spreadsheet...Are you responsible for the units cited in sheet 1, cell A15 ( an ostensible conversion of kgm^2 to 'slug.acres")? Someone is having a bit of fun at the expense of the noble and fine elvish units! :)
 
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TiPi

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And about that spreadsheet...Are you responsible for the units cited in sheet 1, cell A15 ('slug/acres")? Someone is having a bit of fun at the expense of the noble and fine elvish units! :)
I usually totally and completely ignore the non-metric stuff. Slugs arer snails as far as I'm concerned :)
I'm sure this spreadsheet was from a HBA member.
 
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