Cardan shaft critical speed problem

Cardan Shaft Critical Speed Problem

Resonance Institute technical team visited the factory with the following equipment to solve a vibration problem:
Cold rolling motor-reducer set
critical to production
Variable engine speed in the range of 900 – 1500 RPM
High amplitude 1X and 2X peaks were encountered at the gearbox output when the engine was running at 1500 RPM.

Initial estimates prove that the problem is due to propeller shaft misalignment. However, subsequent cardan shaft alignment control measurements showed that the misalignment level was within acceptable limits.
In order to make detailed analysis, an acceleration test was carried out on the gearbox. After this test, it was understood that the 1X and 2X drive frequencies of the cardan shaft entered the natural frequency region, and therefore, a resonance situation occurred as the motor approached 1500 RPM.

The existence of the resonance state has been proven, but it has been understood that the machine operational cycles cannot be changed to prevent the resonance state.
Generally, the resonance situation is caused by poor equipment bases. However, this equipment is located on a very rigid reinforced concrete base.
Since the cardan shaft length is ~5 meters, a question arose in the analyst's mind; “This observed resonance is caused by shaft critical velocities.

Based on the doubt in his mind, the analyst performed impact tests on the lower and upper cardan shafts. As a result of the tests, the existence of bending natural frequencies has been proven in the regions close to the frequencies observed in the acceleration tests.
Minor differences between the acceleration test and impact tests are thought to be due to the forces on the shaft in operational condition (Forces generated during the shaft acceleration phase may increase the shaft stiffness)

Change in Design: A change in shaft design can shift shaft critical speeds to a safer frequency range (away from 1X and harmonics). This can be achieved by varying the mass and stiffness ratios. This action will ensure that the root cause is resolved.
Cardan Shaft Carrier Structure Placement: Cardan shaft carrier structure can be applied to shift the resonance regions. This practice can also reduce the load on the gearbox and rolling bearings.
Elimination of Forced Vibrations: Small forces due to unbalance and/or misalignment can cause high amplitude vibrations when they act in the resonance region. In order to eliminate these forces, balancing and alignment studies can be performed on the shaft (correcting the shaft if there is a tilt in the shaft). These studies can help reduce vibration amplitudes.

Due to the high vibration levels, the rolling stand was not operating at full speed/load.
After the vibration analysis report was prepared, the company that designed and produced the cardan shaft sent a supervisor. With the recommendation of the supervisor, the cardan journal tolerances were changed, and this solved the vibration problem. As a result of this tolerance change, it is thought that the shaft critical speeds are shifted to a safer frequency region.
After this change, the customer stated that the vibration levels were also below the “Acceptable” limits. If the line is desired as it should be, it can go up to full cycle / load without any problems.