Clever. Hmm, taking them in sequence:There are numerous and sensible reasons that many people opt to buy a redrive from someone who has already done the engineering...
Now, while I do now have a day job hitting million dollar machines with a hammer, I do not have Billski's luxurious experience with million dollar test gear. So my approach to problems can sometimes be a little different...
If you can estimate or measure the stiffness of the crankshaft and the MOI of parts, you can calculate resonant frequencies.
Somewhere, I saw that a very rough guide for the stiffness of a crankshaft is to assume that it is a straight shaft with diameters of the journals. Yeah, rough, but it puts you in the ballpark and lets you know if you need to work a bit harder, or can risk the jump to testing.
For masochists, like myself:
Buy a good book. I recently acquired Den Hartog's 'Mechnaical Vibrations', but haven't read it yet. I expect it to be very useful. Maybe Billski could suggest one or two from his library, but not a $500 one
Some ideas I've had that I may use when testing my redrive.
If it's a ribbed belt, it will slip if there is appreciable TV. That will heat it up. That can be measured with a laser thermometer. Run the engine with a club at all RPMs from idle to full power. Preferably a little higher, and see what the laser thermometer says. Do this with a range of tensions. Toothed belts may warm up too, but they have an annoying habit of breaking things, too.
It's also occured to me that TV harmonics are going to be audio frequency. There are a lot of handy little sensors and transmitters that could be used to build a battery power transmitter to be mounted to spinning parts to sense and transmit torsional vibrations to a radio then your PC for analysis.
Close. You can as an EAA'er get SolidWorks for free and model individual parts, get mass, inertia, and even resonant modes and frequencies for the individual parts, but you can not just assemble the parts and run them on the free version. There are ways to mimic the assembly in a single part and run the Eigen solver, but you have to be clever...TV: (belt, crankshaft, prop): Is the problem even practical to address for a homebuilder? It would seem that the vibrational attributes of the most important moving parts are not easily known by a homebuilder, and the results of interactions between them would require tools and techniques that are not available, either.
1) Have I got that about right?
I call this monkey-see monkey-do engineering and it can work. You have to closely imitate the big inertias, the primary spring rates, and the architecture as these set the Eigen modes and the frequencies. Architecture includes support of shafts - length between supports, diameters, if cantilevered or not, etc. You also have to avoid adding any new big inertia or new soft springs as these will change things. This is why I find it so important that each drive maker track the engine side and prop side inertia of their successful applications and their unsuccessful ones, then plot the inertia against each other noting which are which, and showing good and bad regimes.2) Is there a "prudent approach" that can be taken, absent a complete TV analysis, that can get us into a likely safe window? For example:
a) If successful PSRUs are known (based on field experience), can we determine the applicability of that history to another (new) PSRU/engine/prop combination? What attributes would need to the same to make that previous history a good guide?
BIG TOPIC. I will get into this in a separate replyb) Avoiding first-order prop/crank/PSRU resonance: as this often occurs below "usable for flight" engine RPMs, can we achieve usable improvements through use of belt slip/idler pulleys, etc, or are sprag clutches, Guibo "donut" dampers, or other measures required?
I covered this in other posts on this thread. Possibilities exist for the clever. The only ones that I KNOW will be successful likely cost more than the engine and drive will (Rotec used by a torsional vibe guy for a couple days). Not reasonable for a one-off, but definitely in range for somebody producing and selling the things...c) Assuming that even a thorough analysis will require testing, what would a thorough test program for a one-off PSRU look like? Tricks (e.g. strobe photography useful for belt/torsional dampers? Can commercial "knock sensors" be used to tell us something about gear lash issues?)
I know this goes outside the treatment billski probably intended with his (very handy) tutorial. Maybe it can be discussed in another thread, if useful.
I wasn't suggesting using a microphone, but a microphone interface (preamp + analog to digital converter + pc interface like USB). Check out those links I posted if you haven't already, and you will see that many of the sensing techniques are pretty simple and can potentially be done on a small budget. Though I don't know how well audio equipment would work for something that outputs a square wave, at least not without some additional signal processing before going through an [F]FT.My audio idea is not to use a microphone, it's to use either a pair of accelerometers (to cancel linear vibrations), or a rotating weight on a strain gauged spring, then use cheap and available audio electronics to get the signal to a PC. Small FM transmitters can be had in small sizes for peanuts and seem ideal for this. A modern version of torsiograph used to diagnose Wrights woes.
Rotec is out of my budget. But, if I get far enough to consider selling stuff, I would definitely consider a professional test to check for things that my improvised methods may have missed.
FWIW, as someone who is responsible for taking this discussion off the original "sticky-worth" path, I have >no< objection to moving my posts elsewhere, or to deleting them entirely if that serves the greater interest.I'm wondering if all of this recent posting should be moved to a separate thread rather than loading up a sticky?
Since this is wsimpso1's domain I'd suggest that he and the moderators separate the basics and the discussion - then leave the basics here and reference the new discussion thread with a link?
Might also be a good idea for the other stickies as well?
I wasn't able to find that video. I understand in concept how that might work (since the idler is on the "slack" side of the belt, it would allow the prop to smoothly "overrun" the engine's drive pulley in between torque pulses of the engine). But (and this is just "gut engineering", take it for what it is worth) that these pulses would be of such high frequency that it would seem improbable that the idler actually displaces appreciably in the time available. And the idler would not serve to disconnect the prop hub/pulley from excitation by the engine during the periods of increasing torque.There is a YouTube video where the late Gene Smith explains the one way clutch effect of his belt idler tightener.
I watched him fly year after year at Oshkosh, which was impressive to me.
The redrive is no longer available, as far as I know.
Just from what I've read about the rigor needed to assure a geared redrive stays in the "stiff" mode (strict attention to gear lash reduction, etc), is it conceivable that a poly-v belt system could be stiff for our present purposes? I would guess (and that's what it would be), that any poly-v belt system would be "soft" forour purposes.This is the whole thing of figuring out if the first torsional mode frequency is significantly above max firing frequency, significantly below idle firing frequency, or within the range of firing frequency. Here's why you might need to know this:
If you have a "stiff" system, where first torsional mode is above the operating range, the entire system accelerates and decelerates with the crank, in phase and somewhat amplified. The amount of prop shaft acceleration is decreased relative to the engine side shaft, but it still cycles with the engine and is somewhat amplified, so you have to design everything to stand the firing order accels and thus torques, including the bearings. Firing torques are much larger than the mean torque of the engine. Props and flywheels bolted directly to the crankshaft flange are really beefy for exactly this reason - there has to be enough torque capacity in the joint to keep the parts from slipping;
If you have a "soft" system, where first torsional mode is below the operating range, everything from the crank to the springy element is accelerating and decelerating with the crank, while everything downstream is seeing a small fraction of the firing oscillation, and can thus be designed to a level modestly above the mean torque in the system. This is why transmission components and driveshafts with big gear ratios can get by with bolted joints that are lighter than the crank to flywheel or prop bolts;
This seems to be the very toughest nut for a homebuilder thinking of designing his/her own "soft" system. And when we throw in the uneven firing pattern of a V-twin (boom- 270 deg - boom - 450 deg - boom . . .), the potential system resonances get yet more complex.And if your firing frequency coincides with any of your resonant mode frequencies, well, it is likely to tear itself apart;
That's very generous--thanks.If someone has the spring rates and inertia of the parts and sends them to me, I will find a way to run it and get the modes and frequencies. I might even be able to calculate spring rates and inertia with enough detail on the parts...
You may have already been down this road, but this spring rate info may be of interest, even if it is not tech info for the belt you are looking for:Optibelt got back to me. They do not disclose the spring constant of their belts to customers :/
I did find it for gates polychain gt 8M toothed belts, but not from Gates. I was looking at largish pulleys and a 63mm belt for a stiff 627 redrive. I would be able to dispense with the flywheel entirely, saving lots of lbs, but thats a big and expensive set of pulleys and belts.
Now, that info sheet is almost 30years old. It is likely that newer belts (esp those with Aramid fibers) have tighter tolerances.Stiffness. The dynamic elongation of a belt, as well as the stiffness, can be best quantified by the use of EA or spring rate charts. Simply stated, the EA product is Young’s Modulus (E) times the Cross Sectional Area (A). A 1-inch-wide belt is placed on a test apparatus and its elongation is measured under various tension loads. Several belts of the same size and construction are tested and the average elongation is plotted on the EA chart, Figure 9.[in the PDF] The deviation from this curve by any single belt can be as great as ±30% due to normal material variations