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Zen and the Art of Gear Drive Design : Part 2.
This is a continuation of last issues’ discussion of gear reduction design choices and the difference between my drive and the Ross Aero drive.
We’ll start with the choice of the coupling system between engine and gear drive input shaft. The Ross drive uses what is widely referred to as a clutch damper plate. It uses pre-loaded springs to absorb shock loads between the engine and output spline. The actual part used is a marine engine damper but it works the same as an automotive one.
Damper used in the Ross re-drive. The springs are pre-loaded so that they are not active during normal engine operation.
The term ‘damper’ is not really accurate here. Springs do not actually absorb or dissipate energy, they just store it. This would be fine if clutch dampers actually worked the way most people assume. It is widely believed that these parts work by compressing the springs during the engine’s torque peak and then expand when the torque declines after the power stroke. This would presumably level the torque output and provide a smooth power delivery to the gear drive. There is not room here for a thorough engineering analysis, but a look at the basic design of the damper is enough to reveal that this view is erroneous.
In order for the spring damper to work in the manner described, the spring rate and travel would have to be carefully selected to work with the other components. The weight of the crankshaft, flywheel and propeller would drastically affect the design. The engine used would also be a critical factor. A six cylinder engine would require a much different “damper” than a 4 cylinder. Even after all this careful design, you would be very disappointed to find that it works only at one particular rpm and power setting. Clearly, this is not an acceptable design for an automotive application where the engine speed varies by a factor of 10 and power by an even larger amount. Aircraft applications require a smaller range of operating conditions but still much too wide for this approach to be workable.
At this point it should be obvious that our original assumption about the intended purpose of the damper is wrong. The engineers in Detroit are not stupid and they don’t put in relatively expensive parts for no reason, so why do manual transmission equipped cars have a damper? The main reason is to absorb unexpected torque overloads. This happens only on rare occasions like when someone gets overly aggressive with the throttle and suddenly releases the clutch. The springs store the energy of the shock load and release it in a more controlled fashion in order to avoid breaking drive-train parts.
I’ve heard some airplane builders with a bit of engineering background comment that something is wrong with the design of some redrives because the torque required to cause any compression of the clutch damper is far more than the engine is capable of putting out. Their observation is correct, but their conclusion that a lower spring rate is needed is all wrong, due to the factors already mentioned. So the next question is, why put a clutch damper in an aircraft gear reduction drive? You may have noticed that there is no clutch pedal in an airplane, so we need to look a bit deeper.
The other redrive makers I’ve talked to were all relatively knowledgeable people, but most of them still thought the damper’s function was to absorb engine torque peaks. It did not seem to me that they had thought this through very thoroughly, because they had no answer when confronted with the evidence about the miss-match between engine torque and damper rating. I suspect the real reason they used it was because it was a readily available part that could be used to couple the engine to a splined shaft. Still, there is a pretty good record of success while using the clutch damper in redrives. For what its worth, here is my explanation of how and why it works.
As I have explained in the past, it is not torsional vibration per se that causes props, crankshafts and other driveline components to break. It is torsional resonance which can cause the problem. The resonant frequency of a driveline is determined by the spring rate and mass of the entire system, along with any play such as spline or gear lash. It may seem odd to think of something like a crankshaft as a spring, but it actually is. Keep in mind that the spring rate of the damper is NOT a factor under normal circumstances because the springs are pre-loaded and the input and output plates are against their stops. It acts like a solid disk until the damper is loaded enough to further compress the springs.
At some rpm, engine torsional vibration (or some harmonic of it) is going to coincide with the system’s resonant frequency. If the amplitude of the vibration is higher than the losses in the system, the system starts to “ring” at higher and higher amplitude until something breaks. It is during this condition that the damper has a chance to intervene and stop this destructive sequence. When the amplitude of the ringing reaches the point where the springs start to compress, conditions change radically. When the damper plates are no longer against the stops it no longer looks like a solid disk. The springs are now part of the driveline and affect the system resonant frequency. The additional spring rate lowers the resonant frequency and halts the build-up of vibration.
This method of resonance control prevents catastrophic damage to the drive-train but if the system were to be operated at this point for any significant period of time, another factor would rear it’s ugly head to ruin our day. The clutch damper was not designed to operate continuously with its springs being compressed and released. There is considerable sliding contact between metal parts when the springs are active, which would quickly wear them out. It is imperative that resonance not occur anywhere in the normal operating range of the engine if the spring damper is to have a reasonable life.
I lack the expensive equipment needed to accurately measure the resonant point of the drive-train but through careful inspection and audible clues I have concluded that the resonant frequency of a typical 13B installation using the Ross drive occurs at an engine speed of around 1300 – 1500 RPM. At this speed, the signs of resonance can be heard as a rattling noise as the gears and input shaft are slammed back and forth. (Note - resonance at higher engine speeds would not necessarily be audible.) To avoid resonance at these low speeds, it is a simple matter of idling the engine above this point. The other conditions that cause resonance to occur are rough running during engine start and any condition causing the engine to run on just one rotor. Running on one rotor causes an alarming amount of shaking and vibration and once you have experienced this first hand, it will eliminate any fantasy you may have of using “one rotor operation” as a backup mode.
Inspection of the Ross drive damper shows that it very rarely compresses the springs. The tell-tale signs of sliding metal to metal contact can be seen but are very minor. I accepted this as proof (after 856 hours of flight testing) that resonance is not occurring at normal operating speeds.
The BMW Seduction
OK, we have now pretty well analyzed the operation of the clutch damper and concluded that it is a workable solution, so why go looking for a different way? Part of the answer has to do with my own evolving tastes and preferences when flying. When I first “tasted flight” almost 30 years ago, it was in a Benson Gyrocopter. That instant when the wheels first lifted off the ground is still clear in my mind and forever hooked me on flying. Just the mere fact that I was controlling a machine capable of traveling through the air made me feel like every wish I’d ever made had been granted. I wanted nothing more. At least I wanted nothing more until six hours later when the engine seized a piston. Thus started a never ending quest for more reliable engines and other refinements to my flying machines.
Cars have never been a high priority for me, but it was a short drive in one that got me to thinking about human perception of “quality”. At the time, my personal car was an aging Ford Escort that cost me $600.00. Its paint was faded and the hood was severely wrinkled where I had inadvertently felled a tree on it while clearing the land for my house at Shady Bend. In spite of its 130,000+ miles, it was reliable as an anvil and I could not understand why anyone would pay 50 times more for something that did exactly the same job. The new cars that I sometimes rented on business trips only reinforced my opinion.
A friend and I were working on a project one day when I needed some supplies from the local hardware store. My car was blocked in the driveway by his and rather than move it, he threw me the keys and said to take his. I’d never driven a BMW before but I was immediately impressed with the “feel” of it. I use the word feel because I didn’t know what else to call it. It wasn’t particularly fast or quiet or superior to my Escort in any definable way (well OK, the paint was nicer), but there was no denying that the driving impressions were worlds apart. BMW was probably counting on that feeling to motivate drivers to go out and buy one for themselves, but I just wanted to know how they did it. I don’t know everything they did to create the “BMW driving experience”, but one important factor was the use of a “gebo”. This is a rubber isolator mounted between the transmission and drive shaft. Its sole function is to modify the noise and vibration signature of the drive-train and even though the change is fairly subtle, it has a major effect on the human perception of smoothness and quality of the car.
Lately, the carmakers in Detroit have gotten the message as well. When the decision is made to design a new engine or transmission, the most important factor has nothing to do with horsepower, torque, reliability or any of the other factors that we gearheads care most about. The first topics to be discussed at a Detroit engineering meeting are the NVH (Noise, Vibration & Harshness) factors. Power to weight ratio and reliability are still at the top of my list but I do have a better appreciation of these other factors after my short drive in the Beemer. So, there you have the long winded version of why I chose to use an elastomeric (fancy word for rubber) damper in my redrive. To borrow an old Mazda advertising slogan, “It just feels right”.
This damper has worked well, but I did have to make one change from the original design. The rubber isolators used in the first production unit were cut from shock absorber bushings. This was acceptable, since I assumed the drive would be built in relatively small numbers. The number of drives ordered has far exceeded my expectations so the isolators were among innumerable details which had to be re-thought in order to make the drive easier to produce. I was forced to have a custom molded rubber part produced in order to eliminate the labor intensive job of accurately cutting and trimming the donuts from shock bushings.
RD-1 Damper assembly installed on flywheel. The capturing hardware from the top-right damper has been removed to show the rubber isolator.
Input Shaft Thrust Bearing
The decision to incorporate a thrust bearing was already discussed in the last issue and was totally vindicated by the results of my engine teardown. I have done two internal inspections during the 175 hours of flight testing on the new drive and everything, including this thrust bearing, looked perfect.
One thing still bothered me however. The bearing I used was of the same type and similar in size to the thrust bearing in the engine (which wore out while using the Ross drive). Since the maximum thrust rating on the bearing was well above what it sees in this application, I took a closer look at what was going on. The problem turned out to be RPM, not the thrust. The rating of this bearing (2900 RPM) was only half of what it was being asked to endure. This is not a problem in the automotive application, where the bearing has no thrust load at all except when the clutch is disengaged. Using the bearing life formulas from SKF Bearing’s engineering guide, it looked like the life expectancy would only be about 300 hours under the conditions expected in our application. Since I had already bought parts for the redrives, I was reluctant to change my design. I reasoned (rationalized ?) that changing the bearing is a relatively easy job requiring only about 2 hours. (Changing the bearing in the engine is a different story). After waking up several times in the middle of the night agonizing over this (Note from Laura: I can vouch for that! It is not always easy living with an engineer..), I finally got up, turned on the computer, and designed in a different bearing. I had to eat the cost of the unused bearings and my production schedule was wrecked again, but I (we) slept soundly afterwards.
This is the original thrust bearing used in the first production RD-1 re-drive. It is similar in size and type to the one in the Mazda 13B engine. The surface velocity is significantly different at the inside and outside ends of the needles. This causes a sliding contact and limits the RPM at which this bearing type can be used.
This ball bearing replaced the needle bearing in the RD-1. It does not have the RPM limitation of the needle bearing but it’s greater thickness required a number of design changes. Life expectancy is over 4000 hours.
The adapter between engine and the gear case is traditionally a cast bell housing and this is what is used on the Ross drive. As in most cases, the design of any given part is affected by all the other parts. I had decided early on that I wanted more distance between the bearings of the prop shaft in order to reduce the stress on them and to hold prop shaft wobble to an absolute minimum. This required a longer gear housing than on the Ross. If the overall length of the drive was to remain the same, this meant that the adapter had to be shorter.
Another decision was to use the automatic transmission flywheel instead of the custom aluminum part used on the Ross. It was an attractive choice because it’s available, the price is reasonable and some builders already have them. (The RD-1 price is lower if you supply your own.) Many builders were concerned about this choice because of the many reported cracking failures on Blanton’s Ford V6 re-drive, which also used an automatic transmission flex plate. On the Blanton drive the flex plate is used only for the starter. In the car, it is bolted to the torque converter which damps any ringing so cracking is not a problem. On the RD-1, the flex plate is bolted to the damper which will also eliminate any ringing and subsequent cracking (another example of resonance induced failure). In any case, the flex plate is larger in diameter than the manual transmission flywheel (and 24 pounds lighter) which made the design of a bell housing adapter more difficult. The section thickness near the mounting holes came out a little too thin for a sand casting. Mazda uses a die casting for their bell housing which gives higher strength and better tolerances but is far too expensive for low volume production.
In addition, the shallow depth of the bell housing required for the RD-1 meant that it would resemble a large frying pan and have a flat face rather than the conical form of most bell housings. To achieve sufficient strength with a sand casting, the flat face would have to be either very thick or have a lot of reinforcing ribs. Either way it would add excessive weight.
Since the design called for a short, flat adapter, the obvious choice at this point was to make it from structural plate. This material is significantly stronger than cast aluminum and there is no question about possible casting flaws. Half inch 6061 T651 was chosen for the plate. Spacers made from ľ” plate (same alloy) are used to mount the plate at the correct distance from the engine.
Torsional rigidity is provided by two wide spacers which capture two bolts each. These spacers also have counter-bores that mate with locating dowels on the engine to precisely align the adapter. Two additional spacers capture single bolts.
The resulting structure does not totally enclose the flywheel and damper assembly. Some objected to this arrangement because they felt that loose parts could be launched out of the adapter at very high speed and do considerable damage. My position is that if there are loose parts in the adapter, the damage has already been done. Having the ability to visually inspect and re-torque the fasteners in this area is the best way to insure that parts don’t get loose to begin with. During my testing it was also very nice to be able to inspect the rubber damper parts without removing the drive from the engine. I now think of the lack of total enclosure as an advantage rather than a draw-back. If the idea of an exposed flywheel is unacceptable to you, it would be easy to fabricate sheet metal panels to close the openings between spacers. There is plenty of material in the spacers to allow drilling and taping screw holes to attach the panels.
RD-1 adapter plate & spacers. The two longer spacers provide plenty of torsional rigidity. The large hole on the right is for mounting the starter. (A starter for 1986 – 1988 RX7equipped with manual transmission is required).
Adapter plate, Main Gear & planet carrier housing for the RD-1. The threaded holes in the gear housing are for oil return lines.
The decision to use a casting for the main gear housing rather than machining it from solid billet was based on weight and man-hours of machining. This housing is secured to the adapter plate by 12 long bolts which go all the way through the outer wall of the big end. I didn’t feel comfortable with a relatively thin flange to do the job. This probably cost a couple of extra pounds in weight but it is still lighter than a solid billet housing would have been.
These two articles were intended give you an overview of the major design considerations but there are innumerable design details left out due to space considerations.
I also want to make it clear that this was not a “cost is no object” design. The best design in the world is worthless if you can’t afford it. The goal for the RD-1 was always to design the most reliable drive possible for what I perceived as the price range that most builders using the 13B engine were willing to spend. Of course it also had to meet one other important criteria. It had to be something I was willing to fly behind while carrying my own wife and kids. I know this does not sound specific enough for many builders. Some have called and asked if the drive would meet the requirements of Part 23 of FAA regulations regarding certification of aircraft. They emphasized that this was important to them if they were to be comfortable flying their own families. I must make it clear that I have not studied these requirements and have no intention of certifying this or any other product that I make. The only guarantee I offer is the one printed on our sales brochure - “We Fly What we Build.”
Operational LimitationsThe results of this effort was an unqualified success. The parts are beautiful and even the best machinist cannot achieve the accuracy and consistency that computer controlled machine tools can deliver. I must admit that I was very nervous about committing thousands of dollars to parts that would be useless scrap if my drawings were wrong. I just got finished inspecting the parts and still haven’t gotten over how incredible it seems to send off a bunch of data and get back parts that fit. My sincerest thanks to Gaylen Lerohl for his invaluable help with this.
PREVIEW of the RD-1(A)
This is the new propeller shaft for the RD-1(A). The shaft diameter is 45mm as opposed to the 35mm shaft on the original design. It is a single piece rather than the separate hub and shaft used previously. Material is 4340 which is thru heat treated. It is machined from a forging which is stronger than if machined from solid billet due to the better grain flow in the forging. It is an expensive part but it gives the capability of using metal props and competition level aerobatics. The shaft is hollow to save weight and will make a constant speed version possible in the future. All drives ordered after 4-1-00 will use this shaft. This was the reason for the price increase on the RD-1.
The prototype RD-1(A) was installed on our RV-4 test mule (the RVotter) in early July, 1999 and flown to Oshkosh and back (about 2200 miles round trip). The drive performed as expected and production has begun.
For more details on the RD-1(A) and other redrive development news, see Zen and the Art of Gear Drive Design part III.
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