The Engineering of Angular Contact Ball Bearings: What the Number of Balls Reveals about the Bearing

Customers may have asked you, “How many balls are in the bearing?” While this question may initially be irritating, since roller bearings are standardised, the number of balls can actually vary significantly, influencing the bearing’s maximum load capacity. The first thing to look at is the item designation, but this does not directly indicate the number of balls. It does contain other useful information instead, however. Let’s take a closer look:

First of all, it’s important to know that the presence of a filling slot may already indicate the number of balls. The problem is that the designation doesn’t indicate the existence of a filling slot for all bearing types. In double row angular contact ball bearings, such as item 3307, the main designation doesn’t specify the design method used.

The clue is actually found in the suffix.

The material used for the cage reveals much more with regard to the properties of a double-row angular contact ball bearing. With a sheet steel cage, a filling slot can enable the bearing to accept the maximum number of balls. For example, the NSK type 3307 JC3 has 12 balls per row. The disadvantage to this design, however, is that the notches in the filling slot on the inner and outer rings severely restrict the axial load capacity in one direction. Under high axial loads, the indentations have an effect similar to that of “potholes”, impairing the concentricity of the bearings and significantly reducing their function and longevity. These bearings are therefore best suited to radial loads.

There are also double-row angular contact ball bearings with sheet steel cages that use split inner rings instead of a filling slot to accommodate more small balls, such as FAG 3307 DA, with 12 balls per row. Alternatively, designs like the SKF 3307 A use fewer balls (8 per row) but with a larger diameter. A major advantage of the sheet steel cage is its heat resistance, remaining stable even when regularly exposed to temperatures above 120 degrees Celsius.

Plastic cages are often used in double-row angular contact ball bearings as well. One advantage of this material is its elastic deformability, allowing it to do without a filling slot. During assembly, the rolling elements are pressed into the bearing through “snap cages” that are open on one side. However, this method accommodates fewer rolling elements compared to sheet steel cages. For example, the FAG 3307-BD-XL-TVH contains 7 balls per row but uses larger diameter rolling elements to ensure maximum load capacity despite the smaller number of balls. In addition, this bearing is not subject to the same restrictions in terms of axial load capacity. Bearings with a plastic cage are not as well suited to high-temperature applications, however.

So your customer’s query as to the number of balls in the rolling bearing is quite justified, although it could be phrased more generally, as “Are there few large or many small rolling elements?” The suffix in the item designation indicates whether the design includes a filling slot or not.

We have listed the cage materials by suffix in the table below to help identify them. You can also find extensive technical information available for download in our online shop.

• = not explicitly specified

This will help you to work with your customer to find the most suitable design for the bearing’s intended point of use. It is essential to select the right cage material for the application.

Are you familiar with the various deep groove ball bearing design types? The filling slot plays an important role in these as well.

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