Reduction drives

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Neil Unger

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Oct 27, 2019
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I see that I have been misquoted re my redrives, possibly not intentionally. My redrives are already on the market and I admit to me at a price far too dear. $5500 USD. I spent many months arguing with machine shops -- to no avail. The redrive is a copy of tracy's with some improvements I believe. For CNC numbers are critical in cost. For 10 or less the cost is usually dearer than manual machining. There can be 3 hours set up for each operation, all to be recouped in the end article. Hence the price.
To suggest that I only had a complete engine for sale (with a RWS redrive) is false. yes I do have that unit still for sale but also another 4 redrives. The USA distributor is Randy Kempf of Rptorpower. I also have a brand new Tracy redrive for sale as well. Garry I would prefer you just say my redrives are too dear for you. Honesty hurts no one.
 

Vigilant1

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Prop flange horsepower times prop efficiency sets thrust times airspeed possible . . .
Yes, and (just to expand a little) this is why, even though propeller efficiencies tend to increase with increasing airspeed (to a point), the thrust we get decreases with increasing airspeed.

HP x prop efficiency x 375 (a constant, if using MPH and lbs) / airspeed (MPH) = thrust (lbs)
 

GESchwarz

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Considering that the reliability of these PSRU's are associated with torque spikes overstressing the gear teeth, has anyone considered adding a shock absorbing device between the engine and the PSRU?

The idea I have is a simple spring and dashpot assembly. It could be made from the common automotive clutch plate which already has the springs in place. All that would be needed is a similar arrangement of dashpots or a single rotary dashpot to dampen the relative rotational motion of the input and output.
 

cheapracer

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has anyone considered adding a shock absorbing device between the engine and the PSRU?
I like the cam - shafts (not camshafts) you find in the drive of some older shaft drive motorcycles.

I forgot the name of them, torsional shock damper(?), the connecting shafts have dual lobes each, and a spring that allows limited independent rotation spreading the shafts apart against the spring.

R80 shaft.jpg


Some bikes use this blade style now

R100GS-Driveshaft.jpg
 

Erik Snyman

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Hello Neil
What would your price be for the Tracy Crook drive you have for sale?
Erik in Parkes, NSW.
 

AdrianS

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I like the cam - shafts (not camshafts) you find in the drive of some older shaft drive motorcycles.

I forgot the name of them, torsional shock damper(?), the connecting shafts have dual lobes each, and a spring that allows limited independent rotation spreading the shafts apart against the spring.

View attachment 89816


Some bikes use this blade style now

View attachment 89817
That lower one looks similar to those lightweight rubber-sprung trailer trailing arms.
Easy to tune the stiffness by varying the length of the elastomer, whatever it is.
 

cheapracer

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That lower one looks similar to those lightweight rubber-sprung trailer trailing arms.
Easy to tune the stiffness by varying the length of the elastomer, whatever it is.
Yeah, same principle, any system that squashes rubber*!

* rubber is rarely used of course, it's all plastic alloys now, neoprenes, teflons, etc, we just still call it "rubber".
 

pictsidhe

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Yeah, same principle, any system that squashes rubber*!

* rubber is rarely used of course, it's all plastic alloys now, neoprenes, teflons, etc, we just still call it "rubber".
For a drivetrain, vulcanised rubber is still the best option. It can have high resilience. Synthetic options cannot match it. A drivetrain needs a low loss spring, anything that absorbs much energy will rapidly overheat.

I was looking at making a rubber torsional absorber for my redrive, but it would need real rubber, which would be a PITA for me to prototype. Now, engine mounts, I can cast those from urethane kits.
 

harrisonaero

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Very good responses! Thank you very much.

So I have been building my scratch design airplane for 10.5 years based on the Mazda rotary engine which I purchased 10.5 years ago. So now it's time to start working firewall fwd and I can't seem to get my hands on a PSRU.

Where can I find a Tracy Crook PSRU?

I believe Neil Unger took over that product line, but when I asked him for one, all he seemed to have is a completed system, engine and all, and it's sitting on a pallet somewhere on the other side of the Pacific Ocean.
Get on Paul Lamar's email list and ask him. rotaryen@earthlink.net

Tracy's PSRUs come up for sale sometimes.

There's also plans for building one yourself out of a planetary (C6 or similar). A bit lighter than Tracey's. I built one and didn't use it so sold it a couple decades ago. Don't remember the name of the plans author but he was in Canada so you might search old websites with Wayback Machine.
 

GESchwarz

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Get on Paul Lamar's email list and ask him. rotaryen@earthlink.net

Tracy's PSRUs come up for sale sometimes.

There's also plans for building one yourself out of a planetary (C6 or similar). A bit lighter than Tracey's. I built one and didn't use it so sold it a couple decades ago. Don't remember the name of the plans author but he was in Canada so you might search old websites with Wayback Machine.
Where do they come up for sale?
 

Neil Unger

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Garry, re your inquiry re the RWS reduction drive. Price is $2750 USD Currently parts have been loaned out, so the complete unit will not be available for a month or so.
 

Lendo

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GES,
Tracy Crook used C6 Ford gears sets, he used 2 different ratios, the 6 stands for 6 planetary gears that spread the load better than the previous 4 planets. They are all Helical cut gears, I believe designed for quietness - however they impart a large Thrust Load that must be managed effectively. Some have used a number of flat needle bearings - I hear not terribly successfully.

The nature of the Planetary Gear set allows for more Lash than a stiffer straight cut Spur Gear system, like the Powersport PSRU, which has an internal Spur Gear- an ingenious arrangement as gears must be held firmly at both sides, difficult to do with an internal Spur Gear, running inside a Ring Gear.

I also believe the Lash exacerbates the torque loads, Tracy used large diameter soft Rubber Donuts to absorb those torques i.e. (Torsional Isolator or Damper system).

However if you prefer a Planetary gear (as Misted used), there is a larger stronger Ford gear set, but I can't remember the details at the moment - in my notes somewhere. Some research might find it.
George
 

wsimpso1

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These are ramp type spring dampers, very similar in design to what is in the Rotax 912 family of engines. Mostly, it is a spring, and the shape of the ramps can give either a single spring rate or a rising rate. There is some friction on the ramps, but not much, leading to little damping.

These are elastomer torsional springs that give rising spring rate with rising torque and with significant energy loss when the elastomer springs are cycled.

Both design types also appear to have significant length of a shaft with a reduced diameter for some length. These are quill shafts and have been used to put a soft spring in the system for vibration isolation in many of the older piston engines. Combining a quill shaft with another spring device may provide low enough resonant frequencies to where only minor damping is also needed.

Billski
 

pictsidhe

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I pulled a dual mass flywheel off a Mini Cooper S today. It has a surprisingly soft spring. DMFs may be worth investigating as a soft redrive sprung element.
 

wsimpso1

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I pulled a dual mass flywheel off a Mini Cooper S today. It has a surprisingly soft spring. DMFs may be worth investigating as a soft redrive sprung element.
The DMF is interesting. It is generally applied in lieu of the spring center clutch disc, but for the same reasons - to isolate the downstream components (transmission internals, final drive, shafts, etc) from the engine vibrations and from clutch engagement impulses as well. Because it is a manual trans setup, you can go down to and below idle, so really low spring rates are needed to get resonance frequency really low. To get the low spring rates, they use arc springs that curve around and rub in the housing, and as they rub, they raise the effective spring rate, requiring even lower spring rates designed in. The friction goes up with torque and with square of engine speed, so the effective spring rate goes way up as we get into the flight speeds we run. So you have to make sure that you not only put resonance safely below 2x firing at idle, but you have to model the springs (some way, not necessarily FEA) under torque and centrifugal loading to get spring rates as the engine runs at flight rpm range and make sure that resonance is sufficiently removed from forcing functions everywhere you will operate.

In a manual tranny car, the big inertia (as reflected to the engine)in the system is the crankshaft and flywheel, with the tranny, and driveline being much lower. Putting some more inertia on the downstream side of the springs can really help with isolation. Automatic tranny cars and trucks have more inertia on both the engine side and the downstream side - some of the torque converter and clutch turn with the engine, some is downstream with the tranny, and then the tranny inertia is bigger too. DMF's are not applied to hydraulic automatic trannies.

When we go to an airplane, the prop inertia is much bigger than anything else. Yeah, it does not weigh more than a flywheel, but props have a lot of radius and radius is squared in the inerta. So airplanes already have plenty of inertia downstream of the springs. We end up needing more inertia on the engine side in many systems. Stick a DMF in as your soft spring element, and you have moved inertia from the engine side - where you need more inertia - to the prop side - where you usually do not need more. It is usually directionally wrong for driving a prop.

Now MAYBE your airplane can stand the added mass and maybe your airplane has enough engine side inertia, but that will take calculating the values and doing the analyses or risking it on a build (that this powertain vibration guy knows will go in the garbage). If you are getting educated and doing all the math, sure, put in some DMF's and see how they do. I would bet that you will get way better results with engine side flywheels and springs downstream, like the other successful units are doing.

If anyone wants to math model these systems, we can talk about that too, including arc springs and all of the effects on spring rates.

Billski
 

Vigilant1

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After reading about PSRU math and analysis, I offer this:. Pictsidhe, maybe take the honorable, cowardly route to get those large, long prop blades on your scale ultralight Hurricane:
1) Direct-drive 4 blade 46" diameter prop.
2) Four very realistic XPS-and-fiberglass slip-on Rotol blades that store right behind the seat. Maybe 3 lbs total.
You could have those things in place 30 seconds after engine shutdown.:)
 

Neil Unger

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Oct 27, 2019
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Erik, The tracy PSRU is in the US, so freight becomes a huge issue for price. I have already mortgaged a kidney to get it there. Still I have a few options re redrives. I have a prototype here that eventually I will get together. It is one of the originals where I used bronze bushes for the prop shaft bearings before I went to timken tapers. Then I have a tracy PSRU on my engine coming home now near Christmas. Could change that one for the prototype. All takes time which at the moment I just do not have as still putting a turbo on one engine. THat requires ditching the water pump in favour of an electric to allow room. Again time as the renesis water pump cannot be simply replaced with a plate like a 13B. Tracy reduction is 2,85 and mine is 3,17. In hindsight what difference that makes I do not know, but at the time it seemed significant. I do know that the extra reduction was very expensive. in fact everything I did cost far too much. I could have "tooled" up for production myself, but the volumes are just not there. Good job for you Erik!!
 

Erik Snyman

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Erik, The tracy PSRU is in the US, so freight becomes a huge issue for price. I have already mortgaged a kidney to get it there. Still I have a few options re redrives. I have a prototype here that eventually I will get together. It is one of the originals where I used bronze bushes for the prop shaft bearings before I went to timken tapers. Then I have a tracy PSRU on my engine coming home now near Christmas. Could change that one for the prototype. All takes time which at the moment I just do not have as still putting a turbo on one engine. THat requires ditching the water pump in favour of an electric to allow room. Again time as the renesis water pump cannot be simply replaced with a plate like a 13B. Tracy reduction is 2,85 and mine is 3,17. In hindsight what difference that makes I do not know, but at the time it seemed significant. I do know that the extra reduction was very expensive. in fact everything I did cost far too much. I could have "tooled" up for production myself, but the volumes are just not there. Good job for you Erik!!
No problems, my friend. It is not very serious, as I will not need an engine for a while, and my main wish is for a GTD-350 turbine, anyway.
Stay safe,
Erik.
 
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