|The MGA With An Attitude
HARMONIC DAMPER Crankshaft Pulley -- CS-208
(Often called Harmonic Balancer)
At 12:08 PM 9/26/2010 -0400, J. D. Sutcliffe wrote:
"About the pulley; it isnít an O ring, it is a vibration reduction device, right"?
Refer here: www.mossmotors.com/graphics/products/PDF/433-696.pdf
Yes, the Crank Pulley with Damper is often called "Harmonic Balancer", but it doesn't really do any balancing (like adjusting wheel weights). It does damp out some harmonic vibration in the crankshaft, but not the kind of vibration you could ever feel. With each power stroke the crankshaft speeds up slightly. With each compression stroke the crankshaft slows down slightly. The flywheel with high mass tends to suppress this change of speed. On the power stroke the downward force from the connecting rod will twist the crankshaft very slightly clockwise (working against the mass of the flywheel). This torsional twist will spring back when the power stroke is finishing. On the compression stroke the connecting rod applies a lesser torque in the opposite direction twisting the crankshaft slightly anti-clockwise. Approaching end of compression stroke the twist will spring back.
Due to length of the crankshaft, con-rods near the front of the engine will impart more twist to the crankshaft than con-rods farther back (working against mass of the flywheel). Final result is the front end of the crankshaft rotating very slightly fore and aft relative to the flywheel end. Due to the multi-cylinder arrangement, this torsional vibration varies in timing and amplitude at various points along the crankshaft, but the greatest torsional vibration occurs at the front end. With a 4-cylinder 4-stroke engine you have 4 power strokes in 2 revolutions. Each of the 4 power pulses will impart a slightly different amplitude to the vibration, but the 4 pulse cycle will repeat with each 2 revolutions of the crankshaft. Running 4 cylinders at 3000 rpm with produce 6000 vibration cycles per minute, or 100 cycles per second (100 Hz). This small amplitude 100 Hz vibration is mostly torsional, not radial, so the "noise" is not transmitted to the engine block, which is why you don't hear it. In essence the twisting of the crankshaft rings like a bell at 100 Hz at 3000 rpm, and the ringing frequency varies directly with engine speed.
The crankshaft, like any other mechanical part, will have a certain natural resonance frequency. If you strike it in a radial direction with a small hammer it will vibrate and ring like bell at a fixed frequency (or several frequencies at the same time). If you can strike it in a torsional direction it will ring torsionally (rotational twisting) at a different frequency. It is this torsional ringing that is addressed by the Harmonic Damper.
When the frequency of the input excitation energy pulses come very close to the natural resonance frequency of the crankshaft, or some integer multiple or division thereof, the vibration will be enhanced and amplitude will increase. This is the same effect as giving a kid's swing a small push at just the right time in the swing cycle to make it swing a bit higher. Enough successive small pushes and the swing can go very high as it gains energy. The same thing happens to the crankshaft. At these certain selective resonance frequencies the input energy can increase the amplitude of the torsional vibration considerably.
This enhanced vibration of the crankshaft can eventually lead to fatigue failure of the crankshaft. An engine that is driven a lot to high accumulated mileage is more likely to incur a broken crankshaft. I had one fail like that in 1989, see photo here: http://mgaguru.com/abused/ab1.htm.
This part had accrued about 165,000 miles at time of failure. History of the car for the first 150,000 miles is a mystery, but I believe it was a "normal" life for a sports car, probably never used for regular competition (before I got my hands on it). This failure occurred about 15,000 miles after complete restoration of the vehicle, including regrinding of the crankshaft. The crank journals were standard size (although somewhat worn) when I got the car, so this was the first regrind. This fracture was NOT due to a sharp corner from the grinding operation, as the journal grinding did leave the proper small radius in the corners.
Such crankshaft fractures are not terribly common, but do happen often enough to be well known. The fracture is most common near the front end of the crank as shown in my picture. It can sometimes occur farther back, but statistically less likely. In most cases like mine the engine continues to run with a loud knocking noise as the crankshaft flexes slightly. The noise gets louder with more throttle and higher engine speed. If this occurs at high speed with high power, like full power racing, then the crankshaft might flex enough for the piston nearest the break point to hit the cylinder head. That could result in instant destruction of the piston and perhaps more catastrophic failure as the connecting rod may bash up the cylinder wall, or other internal engine parts.
The Harmonic damper has a drive hub and an outer ring. The outer ring is similar to a flywheel in that it can absorb and release energy, or it can simply resist a change of motion (in this case a change of rotational speed). As the crankshaft speeds up and slows down (at high frequency) the outer ring tends to run at a constant speed. The small relative change of rotational position between the inner hub and outer ring makes the intermediate vulcanized rubber part flex like a spring. This constant flexing of the rubber dissipates energy in the form or heat. It is this process of dissipating energy that dampens vibration of the crankshaft (similar to a shock absorber dissipating energy of motion in the suspension).
As the rubber part warms up it can transfer heat into the mating iron parts which in turn serve as heat sink and thermal radiator to keep the rubber relatively cool. After many years of vibration and heat cycling the vulcanized rubber may break loose from one of the metal parts. When this happens the outer ring may turn in relation to the inner hub, and the timing mark on the pulley can be disoriented. Then it is time to replace the part with a new one.
You may notice in the Moss Motors information sheet there is what appears to be a shallow key slot across the face of the steel parts (rear face I believe). This is used in a fixture to align the parts properly during the rubber molding process. It also serves as a visual check for proper alignment (at times when you can see the rear side of the part).
Buying a used harmonic damper is a little risky. It's like a used light bulb where some unknown amount of its life has been used up, and you don't know how much longer it may last before failure. Like any other used rubber part, it is a piece you might consider replacing as part of a general engine overhaul, if you want the engine to be entirely like new with expected long life.
The MGA, and very early production MGB, in stock form do not have this problem, because they do not have a harmonic damper. They are however more likely to incur a crankshaft fatigue crack failure due to constant vibration. The harmonic damper weighs more than the original sheet metal crank pulley, so prepared race cars may use the earlier part (accepting the small increased risk of crankshaft failure). This damper pulley can be retrofit to earlier engines like the MGA. Aside from the weight and risk of rubber failure, the damper pulley is also thicker that the sheet metal pulley (same diameter). This may interfere a bit with the steering rack during installation of the engine in the car. It is more likely to scrape paint off the rack housing, less likely to incur damage to the pulley flange. In some cases people may prefer to R&R the damper pulley before R&R of the engine to avoid the interference issue. I'd rather use a bit of touch up paint if needed.
In fact I have never bothered to install a harmonic damper on any MGA, but I must admit I did have the one crankshaft failure due to fatigue crack. So I leave this up to anyone as a personal choice. For a high mileage car it may be a good idea (ignoring the expense to purchase the part and violating concours standards).