While visiting a pump rebuild shop at a Pulp and Paper mill the other day, I was asked about the difference between the styles of Double Row Angular Contact Ball Bearings that are available from bearing manufacturers.
First, what are the designs?
- Full complement or max type
- Conrad type
- Two-piece inner ring
Full complement or max type
The full complement type has a filling slot machined on each ring on one side. This allows the maximum number of balls to be loaded in-between the rings. This gives a high load-carrying capacity. But the downside is that this bearing can only take an axial load in one direction. If the bearing is installed the wrong way (in reference to the axial load direction), it fails quickly as the balls run over this slot.
The Conrad type refers to the Conrad method of ball bearing assembly in which the inner and outer ring are placed with outer and inner eccentrically placed together and the space is filled with balls to the point that the rings can be re-centred with the balls evenly distributed around the circumference. This assembly can take a thrust load in either direction but it has an overall lower rated capacity.
Two-piece inner ring
The third type is the two-piece inner ring type denoted by the suffix “D.” This type has a single double-row outer ring with two inner ring races. This design allows more balls to be used than the Conrad style, but typically not as many as a max type. Due to the advances in material and internal design, a “D” type has a similar capacity to the maximum of 40 years ago, often when the pump using a max-type bearing was designed.
So the question was raised: Has the user found using the “D” type less reliable than the max type in a certain make of pump? I looked at the pump assembly and discovered that in the original design the bearing abutted against a shaft shoulder on one side but there was no axial retention (lock nut) against the inner ring on the other side of the bearing.
For a max type bearing this is fine since there is only one (double) inner ring. For a bearing with one double outer ring and one double inner ring, the clearance (axial and radial) is set at the factory. So this bearing does not need axial retention in the opposite direction to set the clearance. However, in the case of the “D” style, the inner rings need to be clamped together to achieve the proper axial play set at the factory.
Without the clamping pressure against the inner ring of the unloaded set of balls, they likely suffered from a lack of loading and skidded, which caused the reported poorer reliability. The issue, therefore, is not that the “D” style bearing is less reliable, but rather that the pump design is such that a “D” style should not be used without there being a means to clamp the two-piece inner ring together.
Douglas Martin is a heavy industry engineer based in Vancouver. He specializes in the design of rotating equipment, failure analysis and lubrication. Reach him by email at email@example.com.
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