You’re designing an application and trying to find the best bearing to support its operational requirements. Does this sound like you? If so, you’re probably doing your research and have run into some information about radial ball bearings, a.k.a. deep groove ball bearings, and are wondering if this is the right bearing to use for your application.
This radial ball bearing guide will help you know if you need a radial bearing for your application.
Radial Ball Bearings Are The Most Common Type of Bearing
The bearing world considers radial ball bearings to be the “duct tape” of bearings. Good for many different applications and easily found. Applications from skateboards to dental drills (more on that later) use radial bearings.
The reason they’re so popular is because they are widely available and support a broad range of:
- and more
What Are Radial Ball Bearings?
Radial ball bearings carry radial loads (forces perpendicular to the shaft) in both directions, back-and-forth or side-to-side.
Radial bearings don’t need to be purchased in pairs like angular contact bearings. Radial bearings are a good choice if space is limited in your application.
There are three key pieces of information you should know before deciding if a radial ball bearing is best for your application.
- Internal Clearance
- Bearing Fits on Shaft and Housing Units
- Radial Bearing Material
1. Internal Clearance
Internal clearance is the amount of distance the inner ring moves relative to the outer ring from the geometric center of the bearing. Axial clearance is the allowable movement in the direction of the shaft and radial clearance is the allowable movement perpendicular to the shaft.
Knowing the internal clearance of your bearing is important because it affects the load distribution, vibration produced, and the rolling elements of the bearing.
These clearances have been standardized in the bearing industry and are represented as “C” numbers. The higher the number the more relative movement allowed between the inner and outer ring. Here are the standardized clearance ratings from tightest to loosest:
The clearance of a bearing before it’s fitted on a shaft is called the initial clearance (or initial radial play).
The clearance of a bearing after it’s fitted on a shaft is called the operational clearance (or operational radial play).
2. Bearing Fits on Shaft and Housing Units
Internal clearance can be affected by the machined shaft and housing tolerances, not just after the application’s thermal effects, but before.
A tight press fit on a machined shaft that is slightly bigger than the inner ring of the bearing will reduce the bearing clearance dramatically.
A reasonable rule of thumb is for every 3 microns of ID (inner diameter) that is press fit on the shaft, 1 micron of internal clearance is lost. Same rule of thumb applies to the housing tolerance too.
This may seem like a small number, but if a bearing only has 5 microns of internal clearance and a 15-micron press fit, the bearing could lock up at a low RPM.
If either the shaft or housing is pressing on both the inner and outer ring of the bearing, then be sure there’s enough internal clearance left for a successful operation, making sure to take the application loads and RPMs into account also.
Internal Clearance and Max RPM
The other thing to think about is the percentage of time you’ll be running the bearing at its maximum RPM limit.
For example, a common 608 radial ball bearing has a max speed rating of 40,000 RPM. If your application is running at 35,000 RPM, then a tolerance analysis may tell you to leave ~5 microns available for some ball movement at this high of an RPM utilization.
If your application allows some shaft movement and its precision requirements are low, then a more liberal tolerance strategy can be used. If the application is going to be giving root canals, then the bearings need to be locked down to a couple of microns!
To conclude about internal clearance, choosing the right internal clearance for your application can be tricky. You’ll need to look at multiple metrics and calculations like:
- Application temperatures
- Mating part coefficient of thermal expansion
- Rigidity requirements
- Duty cycles of application forces
- A few other metrics may also be required
Not sure where or how to start? We’ve developed numerous engineering tools and automatic calculators that can help you find some of these answers.
If doing manual calculations doesn’t interest you, our bearing engineers would
geek out love to help you with your analysis.
3. Radial Bearing Material
Bearing Cage Material
Radial ball bearing cages are commonly made from either:
- Phenolic (TA cage) is a good choice for most applications since it supports max operating temperatures up to 248°F (120°C)
- Strip Steel (J cage or “ribbon cage”) is a good choice for an application that may have increased operating temperatures. A J cage made of strip steel can support max temperatures of around 428°F (220°C)
Phenolic material is made of layered fibers, like fiberglass on a corvette or a boat, but the phenolic fibers made for a bearing are tightly controlled without voids, inclusions, or porosity.
There are other less common cage material options, like injection molded plastics or other machined cages made with various metals, plastics, or even glass. We recommend, that if your application is operating in a room temperature environment, then a J or TA cage will be best. Plus, these cages will save you from having supply chain issues and cost headaches.
Sealing a Radial Bearing
The best ‘plug and play’ option is to buy your radial bearing sealed and grease lubricated for life.
There are two different types of sealing closures for radial bearings:
- Shields: A metal cover either stamped or fastened with a c-clip to the outer ring of a bearing
- Seals: A rubber cover that contacts both the inner and outer ring of a bearing
The most common type of shield is the Z shield made of a low-cost metal that’s stamped or c-clipped to the outer ring of the radial bearing. If stamped, then the bearing can’t be regreased in the field. If the shield is fastened with a c-clip then you can regrease and re-shield the bearing as needed.
Shields (including Z-shields) do make contact to the outer ring of the bearing but not the inner ring, like a seal. Shields usually cause less friction and drag when compared to a seal.
The most common type of radial bearing seal is the RS seal made of a low-cost metal that’s coated with rubber. This seal will contact the inner and outer race of the bearing which could slow down the speed of rotation and produce extra heat during operation.
Both the RS seals and Z shields are low-cost options that work well with an application that has operating temps < 212°F (100°C). There are many different types of materials available for the sealed radial ball bearing and most can support high or low operating temperatures, chemical exposures, and dirty environments.
Best Applications for a Radial Bearing
After taking all this information into consideration, now we will look at the applications that generally work best using a radial ball bearing.
The most common industrial application that uses radial ball bearings is an electric motor. This is because radial bearings carry loads and movement between the shaft and housing in both axial directions. An axial direction is defined as rotating, or moving, in the direction of the axis. Radial bearings can accept load in both directions along the axis (back and forth), whereas an angular contact bearing can’t.
A lower cost electric motor may have a bit of electrical flux that’s offset from the rotor to the stator. This could cause some axial force to get produced within the motor to the shaft. A radial bearing can handle this kind of sporadic force with no issues. Even if that flux happens to change directions, a radial ball bearing can handle that because it supports axial loads in both directions.
This comes in handy for many pumps where liquids surge or cavitate and cause the axial load to switch directions for just a second before going back to its original direction.
Other kinds of simple applications that rely on radial ball bearings include:
- Conveyor systems
- Bicycles and skateboards
Not all radial bearing applications are slow. In fact, most handheld rotating dental tools are made with radial bearings and can spin well over 100,000 RPM, some even close to 200,000 RPM. The next time you hear the buzz of the dental drill, remember radial ball bearings are present, keeping that tool steady.
This article has covered a lot and hopefully shed some light on radial ball bearings. If you need help understanding these concepts or picking out the right ball bearing for your application, reach out to our onsite engineers either with our contact us form or by giving us a call at 800.323.5725.