Mike Everman wrote:If there were any delay, you could easily find out by re-running the test in the opposite direction. The bi-sector of the CW and CCW result would be the actual heavy spot. In my mind, this is an essential test. Were there any phase lag or delay, this would be the way to deal with it operationally.
I'm curious if you have done this yet, Ash?
Already been there and done that.
The Lm339 has a response time of 0.5us. At the rotational speeds I've used to balance the parts, the time delay is magnitudes smaller than the rotational diatance traveled by the piece. The P-trim turbine has an exducer diamter of 2.544", which is a circumference of 7.992".
At the ~6000RPM I've spun the parts up to, that's 799.2" linear distance per second at the circumference. The LM339 has a response time of 0.5 microseconds, which is 0.5X10^-6s. One microsecond of rotational distance on this turbine wheel amounts to 0.0007992 inches, or basically, 8-ten thousandths of an inch.. Half of that is 4-ten thousandths of an inch, which is utterly undetectable by the human eye.
The one thing I do not know is the response of the strobe circuit. However, I can tell you that it must also be within the realm of the microseconds as well. When I swapped the drive motor leads with the test shaft with known off-balance, I could not visually detect a difference in location. The strobe always fired off when the off-weight "pin" in the shaft end was pointing directly downward regardless of rotational direction.
I recently spoke with my turbo builder about this rig and he pointed out a few things I'd like to mention here. The concept of "dynamic balancing" takes from the fact that a spinning piece has essentially an infinite number of "cross sections" of which act upon "balance" as the piece rotates. Example: The very back end of the turbine wheel, where the hexagonal "nut" shape is located is on a balance plane different than that of the other side of the turbine wheel where the shaft is connected. In "static" balancing, you are taking the sum of all imbalances across the length of the entire part and effectively averaging them out, and then removing weight in "some place" along the length of the part in order to achieve balance.
The problem with static balancing is that if you have a part with sufficient length to it, such as our beloved turbine wheels, you cannot account for imbalances across every cross-sectional plane with static balancing. You might get the part to a point in static balancing where you cannot find a heavy side but when you actually spin the piece up to sufficient speed, you will start to see an oscillating vibration of opposing phase on each end of the part. This is why dynamic balancing rigs use TWO sensors rather than just one - a sensor being located out at each end of the part in order to detect this imbalance.
I I had the good fortune about 5 years ago to be "schooled" in the art of dynamic balancing by a high-performance engine builder. The guy was completely wacko, but he knew how to build engines.
I was in the process of building my current 300ZX-TwinTurbo's engine and wanted to shave some mass from the engine's rotating assembly, namely the crankshaft. I built a fixture to mount the 40 lb crankshaft into a Bridgeport mill, used a "Cushman" 3-wheeled golf-cart steering gearbox as a mechanical drive to rotate the crank, and fabricated a degree wheel to attach to the crankshaft for milling purposes. The fixture:
This fixture allowed me to accurately machine the counterweights and rod bolsters of the crankshaft. The crank you see is the first "test run" of the equipment - it was an unusable, damaged crankshaft. In the process of milling this crank, the piece went back and forth from the mill to the balancer probably 30 times. This effort helped me get a feel for the process, figure out what amounts of material I could remove and still achieve balance, and to familiarize myself with the balancing equipment.
At that point I loaded up the actual crankshaft to be milled. The final part also incorporated knife-edging of the counterweights to reduce aerodynamic and fluid-dynamic losses of the crank spinning within the gases and oil of its environment. About two weeks of "after-work" time was spent on this part and it transferred from the mill to the balancer probably 50 times before I had it nailed down.
Here is the crankshaft on the balancing rig (bob-weights attached to mimic connecting rod and piston assembly mass):
The balancing machine has two transducers at each end of the crankshaft to detect vibration from imbalance. At the left end of the crankshaft is a position sensor. When everything was said and done, here were the final results:
Kindof hard to see, but it shows "LEFT" and "RIGHT" values, just below the notations is the actual crankshaft position in degrees, and below that is the actual mass imbalance. The LEFT end is 0.6 grams light and the RIGHT end is 0.5 grams light. Typical industry standard is within 1.0 grams of balance. I spent the time on balancing to the point where each time I spun the part, the machine would register +/- 0.5 grams - I was at the limit of the balancing rig. At this point I was some 60 hours in on this crankshaft, not to mention the 40 hours or so spent building the fixture and playing with the bad crankshaft to get it down pat.
Perhaps I've gone a really long way around to my point here, so please excuse me for doing so. But I have only done so in order to fully illustrate what the difference is between dynamic and static balancing. An engine crankshaft is quite long and it is easy to see that achieving balance is far more than just finding the heavy side of a part - you literally have to find the heavy side of a part across its entire length.
Interestingly enough, I've been working on a hovercraft project with my 7-year old son. I got ahold of a 34cc 2-stroke engine from a leafblower that my neighbor gave to me which didn't work. I took it apart to find that the only thing wrong with it was that the fuel line had age hardened and broke, preventing the engine from getting fuel. I dismantled the engine and machined several of the parts to simplify its design for use as a drive motor for the main ducted fan for the hovercraft. One of these pieces I chose to modify was the flywheel/magneto. I machined a good bit of "junk" material from the flywheel, namely the fins of the centrifugal air pump used to cool the engine. In the hovercraft this will be unnecessary as the engien will be placed behind the main fan and receive more than ample airflow for cooling. I fired the engine up with the modified (unbalanced) flywheel and it just about "jumped" out of my bench vise I had it locked down into - and this was in an idle condition, LOL.. The vibration was horrendous!
So I machined a shaft to mount the flywheel into the balancing rig and went to town. I reached a balance "resolution" with the flywheel spinning about 3KRPM where the sensitivity of the comparator was at its maximum. At this point the strobe pulses actually appear to be totally random.
So I reinstalled the flywheel and fired the 2-banger back up in the vice. I will pre-emptively state that there are some inherent "imbalance" factors involved with reciprocating engines since the impulse energy from combustion is inconsistent, however, when spinning the engine up to full bore, probably some 9KRPM, those effects minimize. With the engine turning out every bit of RPM it was capable of, she spun with no obvious imbalance issues.... at least, none of my tools were dancing around on the workbench and falling on the floor anymore.
Unfortunately a gas turbine rotates about 10X faster than this and while my balancer appears to work just fine for 2-stroke engines turning ~9KRPM, I cannot yet say that my balancer will be suitable for pieces rotating more than ten times this rate.
I'm trying to make time to get the T04GT engine back together but there are two major parts needing rebuilding; the bearing tube and the NGV. Both of these items are rather complex and require the utmost attention to detail - so I'm just waiting for a clearing in the "weather" to be able to fabricate these pieces and get her back up and running. And at that point, I will be able to come back here and say whether or not my balancing rig is suitable for the application.
FWIW: I shaved 8lbs out of a 40lb crank in my current 300ZX engine. It produced 693RWHP and 604RWTQ at the wheels, which is about 820 horsepower at the crankshaft. 28,000 miles so far and no problems.