Written by Mike Santora for Design World – Bearing Tips / Question and answer with Richard Picard, NMB Application Engineer
Unchecked vibration and noise can have real consequences in a motion system. And it’s not just the obvious reduction in bearing life that first comes to mind. Bearing noise in medical applications can create distractions for patients. It also affects the fans in air-conditioned seats in the automotive industry. In short, there are many reasons noise is a problem for engineers. Let’s start with the basic relationship between vibration and noise.
Q – What are some concerns that we might want to think about when designing a motion system?
A – Yes, so I’ll start with just a bit of an abstract on the relationship between noise and vibration as it relates to ball bearings. When an application is noise or vibration sensitive it’s important to take bearing noise characteristics into account. It’s also necessary to understand the unique needs specific to that application. Whether it be vibration, noise, a load induced application, contamination sensitivity or other application criteria. Once the proper bearing chassis, and the specification for the bearing is selected for a noise, or vibration sensitive application, it’s equally important to ensure that proper installation, and handling procedures are followed. These precautionary measures help to circumvent brinelling in the bearing raceways, and or contamination ingress that may lead to other defects causing vibration and noise.
Q – Can you expand on that and tell us what you consider the most important for an engineer to understand when you considering vibration noise for an application?
A – In general engineering terms noise is a function of vibration. In many applications very low noise levels are critical to customer satisfaction. One example would be oxygen concentrators; the concentrator is typically in very close proximity to the patient day and night. In this case, bearing noise that exceeds other noise factors inherent in the application can lead to customer dissatisfaction.
Vibration is also potentially damaging to a bearing, and to the application as well. Vibration can lead to increase rate of fatigue, and shortening bearing life. Vibration in a bearing, or a machine, can occur in many different frequencies, and they can all contribute to wear, and defects. We need to take all these vibration frequencies into account in our measurements of vibration.
As application engineers, it’s important to understand the noise level requirements for the application at the beginning of life, as well as factors that may cause noise and vibration throughout the useful life of the application. Again, loading conditions that cause brinelling of the bearing raceways, or contamination ingress that can lead to debris denting or issues such as lubrication deterioration over time.
Q – In relation to noise and vibration, do you have engineers come to you with any recurring questions or challenges?
A – Some of the recurring challenges are not necessarily based on the selection of the bearing and specification because much of our focus as bearing Applications Engineers is to understanding all the application analysis criteria, various calculations for fit, things of that nature. However, one of the things that customers don’t understand, or at least don’t put enough emphasis on, is the care, and handling of the bearing as well as the installation. We can spec a bearing very specific to the application needs however if the bearing is not assembled properly using proper methods, damage to the bearing can easily occur.
The damages that I’m talking about are defects such as axial brinelling. These types of defects can occur when the bearing is not supported properly. If it’s an interference fit by design to the shaft, when you are applying the force to the bearing onto the shaft, the load has to take place on that inner ring itself only, and not the outer ring. If it’s done the opposite way, what we run into is issues called axial brinelling, which are indentations in the raceway. Which then translate to noise, and vibration in the dynamic conditions.
Q Is axial brinelling, a common problem?
A – Axial brinelling is very common. Axial brinelling is, again, a load that is in excess of the capability of the bearing material. It’s a load in the axial direction. When that bearing is loaded to such a degree that it exceeds the contact stress levels of that bearing, the bearing then exhibits what’s called plastic deformation, and/or in this case, axial brinelling. These are indentations of the ball, into the inner, and/or outer raceway. These indentations then are what lead to noise and vibration when the bearing is in the dynamic condition.
Q – Once your bearing has undergone a certain amount of axial brinelling, is there any way to remedy that problem outside of simply replacing the bearing?
A – In all instances, once the bearing is brinelled, the bearing should be discarded. It’s damaged beyond repair, and it immediately needs to be replaced. Same thing applies to radial brinelling. That’s a load in the radial direction. That could occur, for instance, if bearings are dropped. If they’re dropped onto the ground, or there’s a force impact load on the bearing, that can create, a radial brinell. Radial brinelling also contributes to noise and vibration of the raceway.
Q – Any other recurring questions on vibration and noise. We sidetracked you there for a minute.
A – Of course, yes. I receive many questions about the need for bearing shields, or seals depending on contamination, and the types of bearing lubrication.
The need for specific enclosures such as contact seals, is a critical factor in ensuring contamination ingress does not create bearing defects leading to noise and vibration down the road. It’s also important to understand the loading condition, versus the noise requirement for the application to ensure the optimal lubrication is selected. Some types of lubrication are inherently less noisy. If we can choose a lubrication that meets the load criteria, but is an inherently less noisy grease, that would be optimal for the customer, and for the application.
Another factor consider is raceway finish. For extremely noise sensitive applications we offer what is called and MT specification. This specification is an electric motor quality noise specification. This spec involves additional steps in the manufacturing process to achieve a high finish and polish level, on the inner and out raceways to further minimize noise.
Q – And what kinds of applications would require a bearing with this particular specification?
A – In some types of units, for instance, the seat cooling fan in a vehicle. Many vehicles these days have heated and cooled seats. In the cooled seat there are 2 fan assemblies. One in the seat back, and one in the seat cushion. If the bearing noise were to exceed fan noise, which is quite low, then the customer could be dissatisfied hearing the noise from the bearing as it’s in its free rotational movement. That’s one example. Another example that I mentioned earlier would be in an oxygen concentrator. You would want that extra step in the manufacturing process to create that highest finish or polish level of the inner and outer raceways. The patient using the oxygen concentrator has the device next to them all day, all night, including while they’re sleeping. In a case like this we would advise our customer to go with what we call an electric motor quality noise specification.
Q – Lastly, is there anything in relation to vibration and noise that I have not asked you about, that we have not covered, that you think is important?
A – Well, there are several, but one priority that we would want to consider is seals versus shields on the bearing. That’s really a whole other topic, however. Seals come in contact and non-contact designs. Seals are made of rubber versus shields which are made of metal. There is no contact between the shields, and the inner ring, which means that there can be contamination ingress into the bearing. When you have contamination ingress into the bearing, that contamination then gets in to the grease, and the rolling elements in the dynamic state are then rolling over that contamination. This causes potential debris denting in the raceway. That debris denting in the raceway then leads to excessive noise, and vibrations. In a case like that, instead of going with a metal shield we’ll often times suggest a rubber seal where you have a seal contact lip to the inner ring preventing contamination ingress, into the bearing. Again, that is a large topic to cover in and of itself.