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Discussion in '2-Stroke Aircaft Engines' started by Jeff Higgins, Dec 9, 2019.
Slippage and breakage
Dude, I’m beyond qualified to machine and make my own redrive and prop hub. I don’t overlook steps and I know what’s needed for it to safely function and last. Prime example: people often try to use pillow block bearings and a collar with set screw but neglect to realize the amount of thrust force on the bearings, shaft and collar. I’m no dummy. I’ll be fine.
No offence, but you've not mentioned the issue that kills more beautifully and badly machined redrives than everything else put together...
Google 'torsional vibration'. It's not a simple task to design around. But it is possible. Machining a good design is the easy part.
I was an ASE certified master tech for 17 years so I am aware of it. It’s the reason that front crank pulleys have a large amount of rubber cast between the inner and outer pulley, to absorb “harmonics”.
Car engines, that is.
I measured 90 pounds static thrust with a 20hp 250cc Fuji Polaris engine. 30" diameter and wide chord prop, direct drive.
I think it weighs 30 or 40 pounds.
I guess I do not understand it.....all those PPG engines do they have more than just a couple of pulleys and a belt drive?
That’s typically all they have because they use such a short, robust stub of a shaft and everything is well balanced.
The Arctic Cat JAG 2000 275 Twin Free Air is 20hp@6000rpm with a Muffler, JAG 3000 340 is 30hp@6000rpm with a Muffler, and 440 was 40hp with a Muffler. A Twin is going to be Heavier than a Single. 20hp x 7cc = 140cc with a Tuned Pipe Designed for 6500rpm. Hirth makes a Direct Drive Hub for there Engines but I have never seen anyone using it, or a photo of it. A Kawasaki 292 Single was rated 20hp@5500rpm with a Muffler, and 22hp@6000rpm with a Muffler. Every Brand Name (25+) Snowmobile Engines made a 292 Single. Kawasaki, JLO, Sachs, others, had threaded holes on PTO Case to run a Belt Drive. Since your only using it for Takeoff of your Motor Glider. You could turn it 7000rpm for 2 Minutes for takeoff, then throttle back to gain your desired Altitude.
At 77F, using Calc: http://godolloairport.hu/calc/strc_eng/index.htm
A 32" x 8 (2) Blade can be turned 7000rpm and make 137.76 lbs Static Thrust! Needs 19.547 hp. 2 min Takeoff only!
A 32" x 8 (2) Blade can be turned 6000rpm and make 101.21 lbs Static Thrust! Needs 12.309 hp.
A 32" x 8 (2) Blade can be turned 5500rpm and make 85.04 lbs Static Thrust! Needs 9.481 hp.
A 32" x 13 (2) Blade can be turned 6000rpm and make 101.21 lbs Static Thrust! Needs 20.003 hp.
Do centrifugal clutches adsorb harmonics? (slippage?)
Where to mount it when there is no place to mount it. Anything can be done but doing anything while keeping weight down is the hard part.
Any specifications or information on the prop?
It was a Banks-Maxwell airboat prop, for low speed. About 1" thick of 1/8" laminations 4" constant chord. Tips about 1/2" thick. (very thick) Cost about $35 around 1975.
Clutches, deal with Slippage till they Lockup. Like the RK400 used on some Rotax engines with C Gear Drives only. Nothing really absorbs Harmonics. It usually happens in a very Narrow Band. Look up your Crank Bearing Spec's it usually gives the rpm range where the Harmonics Band happens.
The primary purpose of the RK400 clutch is to disengage the prop from the engine allowing it to start and operate at idle without vibration. When the engine is idling (below 2350 rpm) the prop is not turning. Installation of the clutch replaces the gearbox flywheel and the hard disk. The added weight is approx. 3 Lbs. At this time the clutch is only available in the "C" gearbox.
99.9% of the Belt Drives ever made for Airplane use, use a Small roughly 3" PTO Aluminum/Steel Pulley, with a Large Pulley. 1st Gen used V Belts 3-5 depending on hp. Then they went to Cog Belt usually Wider in 1-3 Belt configurations again depending on hp. The 3rd Gen newer Belt Drive's uses a Serpentine Micro V Belt Drive, again in 1-3 belt Configurations depending on hp used.
All of these Pulleys used on Belt Drives were Solid, no Rubber Dampner!
Torsional Vibration has nothing to do with Belt Drive Failures, it's Belt Tension and Time! The Theory of using multiple Belts is they won't all Fail at once, for redundancy. On a low hp, low 6500rpm engine like say a 277, one Belt may work, but they always used minimum (2). Cog Belts had better Grip than Old V Belts. The New Serpentine Micro V Belts have better Grip than the Cog Belts.
Most PPG Belt Drives are lightweight to push just a Person through the Air hanging onto a Parachute. As they get Bigger Rigs they get Heavier Drives.
A Rotax 277 (26hp) Back Plate starts at 3/4". A 582UL (65hp) Back Plate is usually 1.0"
Pulleys used today on Modern Belt Drives.
Like this New Drive Sold on eBay for a 503. He is using (4) Belts with 4PK means (4) Micro V grooves in each Belt. The 675 is the Lenght. 4 x 4 = 16 for 50hp. 50hp/4 = 12.5hp per belt. I just wish he had used a Smaller/Cheaper/Lighter, Jet Ski Starter.
A 4PK675 Belt is 14mm Wide, so 14mm x 4 = 56mm Wide PTO Pulley.
If you want to use a COG Belt their is some Raw Stock on eBay, enough to make 2-3 PTO Pulleys. A PTO Pulley 56mm Wide for just the Belt = 2.204718", but you probably need a longer 3.25" to 3.75"piece like this Challenger Belt Drive uses. Looks like around 2" Wide for the Belt Width. I can't read the number on the Belt. This one is for the 582UL 65hp.
I'm not an engineer; am a competent manual machinist, and work with spindles and precision spindles at times.
I find your spindle design more than a little scary.
With rounding error for convenience, there is 10" between the nut centers, with 2 unlikely bearings sandwiched loosely between that length, for preload.
Again in round numbers for quick mental calculation, steel shrinks/expands 6 millionths of an inch per °F. So with a 50F° change, the preload (or your design clearance) will change by .003 inches. That is quite large for the taper bearing.
Typically a long spindle is retained axially at one end only, and floats at the other end to accommodate the stress of constant length variation due to temperature. Facing taper rollers, if necessary for the load, at one end. lighter radial in which the shaft is a snug but not press fit a the other end, or a light press fit and the bearing outer race floats in the housing in a push fit; or in a roller bearing (not tapered). Alternately, an adequate deep row radial to take all the thrust at one end of the shaft, constrained in the housing and between collars on the shaft, and the other end floating in a plain radial.
I would guess for lightness if you want the long shaft that your design would function efficiently with a roller bearing (non-taper) at the prop end, and possibly a single deep row radial at the tail end. That design would take a shoulder on the shaft at the tail end to retain it in the bearing inner cone, and one on the prop end to seat the prop flange.
If your prop radial and thrust loads can be accommodated at the prop end with a single deep or conventional radial ball bearing, and a lighter radial with the shaft floating at the tail end, it could be made even lighter. That would require a shoulder at the prop end behind the bearing. But none at the tail end, and no nut at the tail end.
If you know the typical arrangements and believe your design is adequate anyway, at least put short keyways in the end threaded sections of the shaft that do not intersect the shoulder, use heavy tab washers, & use crennelated nuts with cotter pins.
Most Belt Drives have been Built the same way for 40+ years. Depending on the HP the Width of the Pulley they use 2-3 Bearings. Same Type Ball Bearings are used on the Cranks and in the Gear Drives. That Gold Challenger Belt Drive is used on 582UL's (65hp) at 6500rpm. I have never heard of a Bearing or Shaft or Pulley or Back Plate Failure on one myself.
Most Belt Drives have been Built the same way for 40+ Years. I agree I would use the Third Bearing vs a Spacer on a long Shaft. The 503UL uses a Gates PowerGrip HTD® (High Torque Drive) 960-8M-50 belt. I take it uses a 50mm Wide Belt. I don't find a different one for the 582UL.
Challenger Synchronous Belt Report
I have heard of a Hegar Belt Drive with a Cast backplate break. The main problem with most Belt Drives is the belt not being Tensioned right or replaced.
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