Camshaft breakage, I may have found the root cause(long, boring,)
05-25-2005, 04:40 PM
#1
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Camshaft breakage, I may have found the root cause(long, boring,)
In doing some research on aircraft proplulsion systems, I think I may have stumbled across the reason for those cams that broke at the area of the hub. Many years ago I was in the vibraiton acoustics field, and we spent some time doing FFT (fast fourier transform) which is a way of determining the resonance of a rotating and pulsing system. Rather than bore everyone to death, to summarize, the pulses from the engine firing cause a torsion(twisting moment) of inertia. This twisting moment is translated via the belt to the various components of the cams, oil and water pumps, and tension dampener.
The dampener is located in the slack side of the belt, which make the drive a modified hard type system. Prior to the dampener in the circuit is the oil pump, cam, water pump, cam, and finally guide roller. Since the crank is the driven shaft, and has the vibraiton dampener mounted on it, the crankshaft assembly, including the dampener, and flywheel or TC has a pretty moderate modulus of elasticity. The oil pump circulates oil from a pair of gears, which although have a higher modulus, have a much lower mass, and also some elasticity in the entrained air in the oil. There may also be a cavitation action occuring on the low side of the pump(theoretical).
The cams. The cams have a decided disadvantage in terms of torsional vibration. First is the resonance problem. Typcally, a mech engineer would use a variation on the 'rule of sines' to determine the ratio of a drive system to select the gears and or pullies. This rule is similar to the chromatic scale that musicians may be familiar with. There are several reasons to use this, one of which is the known frequencies of resonance. In the cam system, it is impossible because the ratio must be two
ne, crank:cam(s). So, we are stuck with a fairly high probability that the resonant frequency of the belt circuit will fall in the range of the operating speed of the engine (600-6600RPM). Additionally, the cams are made of pretty hard stuff, and are also heat treated to further imrpove the Rockwell value. While the heat treating is probably not germaine to the overall brittleness, it may be that the location of the break is within the area of heat treating(nitriding?).
Now, torsional vibration in it's worst form would have the pulse of the next cylinder occuring just as the slack from the last cylinder firing is being unoladed from the camshaft. Add in the pulses from the openning and closing of the various valves, and things get pretty complex. In one mathematic progression, this torsional force would become thousands, if not tens of thousands of ft/lbs. Granted, it only presents for a very short time on the shaft, but it is multiplied by four pulses per rev by X RPM.
Eventually, the shaft fails. The failure mode would bear this out, in that it presents as a crystalization of the shaft at the break point, consistant with a shear, rather than a spal, or twist failure. The crystalization is a long term fatigue affect of molecular debonding at the point of weakness. The weakness is usually exacerbated by oxidation, or any minute artifact on the surface or even within the substrate of the shaft.
The two types of drive systems are hard and soft. The 928 belt drive is actually a soft system(tensioner) with elements of a hard system(no clutching to smooth pulses). Although failures are quite rare, they do occur, and without a major redesign of the whole system, they will be impossible to avoid entirely.
An intriguing question. Do chain drive systems on horizontally opposed engines experience this problem? The resonance would be important to know, and the softness of the 928 drive may acutally exacerbate the problem of torque reversal, bt allowing a bit of slack where no slack would be better.
So, If you've made it this far, you are a real masochist. Obviously it was a slow day at work, and I had another reason to start looking up elasticities and drive systems. If anyone sees another failure of the camshaft, please let me know the condition of the metal at the break.
Doc 90GT in resto.
The dampener is located in the slack side of the belt, which make the drive a modified hard type system. Prior to the dampener in the circuit is the oil pump, cam, water pump, cam, and finally guide roller. Since the crank is the driven shaft, and has the vibraiton dampener mounted on it, the crankshaft assembly, including the dampener, and flywheel or TC has a pretty moderate modulus of elasticity. The oil pump circulates oil from a pair of gears, which although have a higher modulus, have a much lower mass, and also some elasticity in the entrained air in the oil. There may also be a cavitation action occuring on the low side of the pump(theoretical).
The cams. The cams have a decided disadvantage in terms of torsional vibration. First is the resonance problem. Typcally, a mech engineer would use a variation on the 'rule of sines' to determine the ratio of a drive system to select the gears and or pullies. This rule is similar to the chromatic scale that musicians may be familiar with. There are several reasons to use this, one of which is the known frequencies of resonance. In the cam system, it is impossible because the ratio must be two
ne, crank:cam(s). So, we are stuck with a fairly high probability that the resonant frequency of the belt circuit will fall in the range of the operating speed of the engine (600-6600RPM). Additionally, the cams are made of pretty hard stuff, and are also heat treated to further imrpove the Rockwell value. While the heat treating is probably not germaine to the overall brittleness, it may be that the location of the break is within the area of heat treating(nitriding?). Now, torsional vibration in it's worst form would have the pulse of the next cylinder occuring just as the slack from the last cylinder firing is being unoladed from the camshaft. Add in the pulses from the openning and closing of the various valves, and things get pretty complex. In one mathematic progression, this torsional force would become thousands, if not tens of thousands of ft/lbs. Granted, it only presents for a very short time on the shaft, but it is multiplied by four pulses per rev by X RPM.
Eventually, the shaft fails. The failure mode would bear this out, in that it presents as a crystalization of the shaft at the break point, consistant with a shear, rather than a spal, or twist failure. The crystalization is a long term fatigue affect of molecular debonding at the point of weakness. The weakness is usually exacerbated by oxidation, or any minute artifact on the surface or even within the substrate of the shaft.
The two types of drive systems are hard and soft. The 928 belt drive is actually a soft system(tensioner) with elements of a hard system(no clutching to smooth pulses). Although failures are quite rare, they do occur, and without a major redesign of the whole system, they will be impossible to avoid entirely.
An intriguing question. Do chain drive systems on horizontally opposed engines experience this problem? The resonance would be important to know, and the softness of the 928 drive may acutally exacerbate the problem of torque reversal, bt allowing a bit of slack where no slack would be better.
So, If you've made it this far, you are a real masochist. Obviously it was a slow day at work, and I had another reason to start looking up elasticities and drive systems. If anyone sees another failure of the camshaft, please let me know the condition of the metal at the break.
Doc 90GT in resto.
05-25-2005, 04:53 PM
#3
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I think it makes sense ... I am not sure that is the cause though in and of itself. I have long suspected that slight mistuning of one cam in relation to the others could cause harmonic overstress on the cams and therefore failure at the weak point. So ... to fix, one would be best-off to have perfect cam timing IMHO.
05-25-2005, 05:17 PM
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Sounds to me like there are enough variables in the equation that it boils down to bad luck. How would you go about moving the resonance out of the rpm range or, how would you redistribute the forces? Would changing the length of the cam help?
