How to fix excessive servo motor noise & vibration
18 December 2020
It’s easy to dismiss unusual noise and vibration in a servo motor as a small nuisance — after all, a certain level of both in rotating equipment is to be expected. However, when these exceed a critical threshold, they become a clear signal that something is not working properly and that action should be taken before the motor is irreversibly damaged.
First of all, it’s useful to distinguish vibration from noise. Although strictly related, they are the symptoms of different malfunctions. It is the general rule that, in normal operating conditions, one kHz is what separates vibration from noise — if the measured frequency is one kHz or less, the problem is vibration, while anything above that should be treated as noise.
A chattering or rattling noise can indicate a positioning error, especially if the model you’re working with has an auto-tune feature. It might be that the drive has been accidentally reset and if this is the case, following the set-up procedure again should fix the problem. If this doesn’t stop the noise, there’s probably a failure in the feedback unit and you will need the assistance of an authorised repair centre.
On the other hand, a high-pitched noise usually indicates a bearing problem, such as a lack of lubrication or a worn bearing. Usually, servo bearings are protected in an enclosure, but dust can still creep in and dry out the lubricant. If the level and quality of lubricant are correct, then the servo probably needs a bearing change.
Measuring the vibration levels and frequencies of your servo motor is an effective and affordable way of detecting potential issues. Depending on the problem, vibration data can be taken from different parts of the servo with different tools — for example, for vibrations that originate from the motor frame, you should gather housing vibration data with an accelerometer. On the other hand, for vibration coming from the rotors, you should collect shaft vibration data with a shaft stick or proximity probe.
Vibration can be caused by several mechanical problems, such as loose or unlubricated bearings. if action is not taken, the vibrations can accentuate the original problem and cause indentations on the bearing raceways. Vibration can also be caused by misaligned gears, which can happen when a gear’s teeth are worn down and lose contact.
Another common reason of excessive vibration is a weight imbalance in one of the rotating components of the servo motor. This can be caused by a manufacturing fault such as a casting flaw, or by maintenance issues such as dirt build-up on the fan blades.
Finally, excessive vibration can happen when a servo motor is not securely attached to its foundation. This can lead to motor wear and fatigue and ultimately to flaking or cracking of the external material. Ensuring that the servo is aligned and securely fastened to its foundation will minimise not only vibration but also the resulting extra mechanical loading that can result from it.
Stradivari violins have stood the test of time — after several centuries, the quality of their sound is still undisputed. Your servo motor won’t last that long, but with proper maintenance and prompt repairing it might serve you for decades.