It is not reasonable to expect an assembly the size of a screw compressor package…
Vibration vs Crankangle
The purpose of vibration analysis is to display the vibration amplitude vs crankangle and to analyze the pattern. The vibration is composed of many different components as they vibrate at various frequencies, amplitudes, and degrees of crankshaft rotation.
If you know the exact degree at which each event occurs, its amplitude of vibration, and its shape, you can determine if this event is normal for this type cylinder, the condition of the various components, and even predict failures or schedule the exact repairs as needed before the unit fails.
Refer to Figure 800-8 for typical vibration patterns and problems associated with power cylinders. Although not discussed, similar patterns are evident on compression cylinders. Figure 800-9 shows typical compression-analysis signals.
Peak Pressure Vibration. The flame front is moving at maximum velocity at this point and usually causes this normal vibration. If detonation occurs, this vibration becomes a high-amplitude sharp spike. Piston slap also occurs at or near this point, since the piston will rock in a worn cylinder under the maximum pressure.
Excessive wear in the wristpin or bushing is often seen at this point.
A badly worn rod bearing will knock at this point or slightly later, normally detected at BDC long before it appears at this point. The same is true of a worn wristpin. We normally look for the rod bearing, wristpin, or a piston loose on the hanger at 10 degrees before and after BDC when it is in the early stages of wear.
Top Ring Enters the Exhaust Port. If the top ring is doing its job, the pressure will be released when this event occurs. If this ring becomes worn, stuck, broken, or the piston or cylinder port area wear, this vibration becomes a high-amplitude, sharp spike. If something happens to the top ring, the second ring will hold most of the pressure, resulting in a vibration spike when it enters the port. (This spike will occur earlier, since the second ring enters the port at an earlier degree of crank rotation.)
Evaluate the ring condition by watching this area. Also watch for carbon buildup in the ports, which will cause the rings to clip. This can occur in the intake port as well as the exhaust. Normally, a ring is not picked up as it enters the intake port unless there is a problem. The same is true of the rings going back up through the ports on the compression stroke. Since there is no pressure to hold the rings against the cylinder walls, they do not clip in the ports on the compression stroke unless there is a problem with the rings or the cylinder port.
If one of the lower rings is broken, it will cause a sharp spike in the exhaust port on the power stroke, indicated by the degree at which it occurs.
Watch these ring vibrations, and they will increase in amplitude as the condition gets worse. A new set of rings will clip in the ports until they wear in. Then the vibration will drop down to a normal level until they begin to deteriorate.
Exhaust Blowdown—This is vibration of the gas as it leaves the cylinder. This vibration will be present when the cylinder fires on a normal cycle and will disappear when the cylinder has no combustion within the cycle. Use the exhaust blowdown as a standard to compare all other vibrations within the pattern. If the mounting or transducer changes, it will affect the whole pattern amplitude, which the operator will note if he compares to a standard within the pattern.
The exhaust will elongate if the ports are restricted by carbon.
The operator may also note that on V-type units with a common exhaust manifold between the V, the exhaust blowdown from the cylinder on the other bank may carry over in the vibration trace. This blowdown (or ghost vibration) will always be there and in the same place or crankangle. The operator can disregard it once he identifies it.
Injection Valve Opens—This vibration is caused by the slack taken out when the valve train activates the injection valve. The operator can get a fix on the CAM timing and lobe condition from this vibration and the closure vibration. Some units with hydraulic lifters will not have this vibration unless there is a problem with the CAM or lifter. To pick up this vibration on such a unit, put the pick-up directly on the rocker arm pin.
This vibration will be excessive if there is any wear in the valve actuation assembly. Pressure applied to the rocker arm sometimes will eliminate much of the vibration and allow the operator to make a true analysis of the rest to the pattern, possibly distorted by this vibration.
Injection Valve Closes—The front (flat) side of this vibration is the degree at which the valve hits the seat. The higher the amplitude, the harder the valve hits the seat. The wider the vibration spike, the wider the mating surfaces. This is a good indication of valve lash, CAM timing, injection valve, and seat condition.
If the vibration fades into the baseline, it is a good indication that the valve has sealed. If it balloons out or continues to vibrate for too long, the valve is leaking. Note the pattern in Figure 800-10.:
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