If you’re building a rotating display platform, designing a lazy Susan, or tackling a demanding industrial application, the bearings you choose for the rotation are more important than you may expect. That’s because the wrong choice can lead to inconsistent, premature failure, or a complete failure while in use. However, the correct choice will make your design work reliably and quietly for years.
What are they and Why Does the Choice Matter?
Turntable bearings are called slewing rings or lazy susan bearings, and they are circular bearing rollers which can carry axial, radial, and tilting load all at the same time. These differ from a typical shaft bearing because they sit flat and can allow two surfaces to rotate freely against each other.
The types, sizes, and variations of turntable bearings available are vast. It’s easy to mistakenly choose the wrong one for your application if you don’t understand how they’re constructed and what they’re capable of.
Loading Comes First
The most important step to correctly choose a bearing for your application is understanding your load. This includes understanding not just the total load on the bearing, but how that load is distributed, and what forces and moments are applied to the load while it’s rotating.
Axial load occurs when forces are applied vertically to a component, parallel to its primary longitudinal axis, and, most simply, means the load that is bearing downward due to weight. Radial load occurs when forces are applied laterally, perpendicular to the primary longitudinal axis. Moment load occurs when leverage is applied to a component. An example of a moment load is when a crane places a load toward the end of its arm causing a load to be applied to the crane’s rotational bearing.
The first resource to consult when evaluating the load-bearing capacity of a joint should be the load rating table of the bearing in question. If the load in your application is a shift or an eccentric load — a rotating arm with a heavy end load, for instance — a moment load should be of primary concern when applying a bearing. A moment load is almost always the unseen factor in an application.
Size, Mounting, and Clearances
Once the loads have been determined, the next consideration is physical sizing. A standard bearing may have a short, medium, and long option when it comes to the overall height of the bearing. The standard bearing may not have some of the other necessary parameters (outer or inner bore diameters). Custom bearing orders take a long time (usually greater than two weeks) and may also be costly.
Mounting holes are also an adaptation in design. Misalignment of bearings is a common and early failure of a bearing. Bearings are pre-drilled with bolt circle patterns, but the patterns may not be what a given application requires.
Clearances must also be taken into consideration when a bearing is mounted within a housing. The bearing must not contact adjacent components and also must not bind in an obstructed position.
Material and Environment
Standard turntable bearings are often produced using medium carbon steel and suffice in common settings. Where corrosion is an issue – in outdoor and marine environments, or in food manufacturing – the poly or stainless steel options justify the expense.
Lubrication matters too. Severely used bearings typically come ready to go, sealed and pre-greased. For ongoing use, bearings with grease nipples are necessary in order to replenish lubrication.
Speed and Precision
Most turntable bearing applications involve minimal rotation – typically a few RPM. High rotation requirements, or even systems that require high precision and repeatability (like camera turntable rigs or automated equipment) involve consideration of the accuracy class and running torque. Accuracy is the key aspect, and a higher precision is indicated by a higher price.
Getting the Specification Right
When first defining the requirements for turntable bearings, it helps to draw up your complete list before dealing with suppliers. Requirements for loads, environments, and speeds (among others) work together. A bearing that appears to meet most of your list (like five of six) is likely still the wrong choice.
Takings to suppliers for specifications to meet requirements (like loads or speeds) will help to ensure that you will reduce the risk of a high-cost mistake purchase. Most suppliers will be happy to provide you with the free guidance you require.