First off, you need to know that most bearings with a rolling element fall into two broad groups:


  1. Ball bearings

  2. Roller bearings


There are sub-categories of bearings within these groups that have unique features or optimized designs to enhance performance.


Find the Bearing Load & Load Capacity


Bearing loads are generally defined as the reaction force exerted by a component on a bearing when it is in use.

When selecting the right bearing for your application, you should first determine the bearing's load capacity. The load capacity is the amount of load it can withstand and is one of the most important factors to consider when selecting a bearing.

Bearing loads can be axial (thrust), radial, or a combination of the two. When force is parallel to the shaft's axis, it is an axial (or thrust) bearing load. When force is perpendicular to the shaft, this is referred to as a radial bearing load. When parallel and perpendicular forces produce an angular force relative to the shaft, this is a combination bearing load.


How Ball Bearings Distribute Loads?


Ball bearings are spherical balls and have a medium-sized surface area for distributing loads. They are more effective for small-to-medium-sized loads because they distribute loads through a single point of contact.


The type of bearing load and the best ball bearing for the job are listed below as a quick reference:

  • Light and radial loads: Radial ball bearings are the best option (deep groove ball bearings). Radial bearings are among the most common bearing types on the market.

  • Axial (thrust) loads: Selecting thrust ball bearings

  • Loads in radial and axial directions: Angular contact bearings are the best choice. The balls make oblique contact with the raceway, which improves the support for combination loads.


Roller Bearings & Bearing Load


As opposed to ball bearings, roller bearings are made with cylindrical rollers that can distribute loads over a larger surface area. They are more effective in heavy-duty applications.

The type of bearing load and the best roller bearing for the job are listed below as a quick reference:

  • Loads that are perpendicular to the shaft are referred to as radial loads. Specify standard cylindrical roller bearings.

  • Axial (thrust) loads: Select a cylindrical thrust bearing.

  • Loads in radial and axial directions: Select a tapered roller bearing.

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Rotational Speeds


When selecting a bearing, the next consideration is the rotational speed of your application. Ball bearings are usually the preferred choice for high rotational speeds applications. They outperform roller bearings at higher speeds and have a greater speed range.

One reason for this is that the contact between the rolling element and the raceways is a point rather than a line of contact in a ball bearing, as in roller bearings. Because rolling elements press into the raceway as they roll across the surface, point loads from ball bearings cause much less surface deformation.


Centrifugal Force and Bearings


The presence of centrifugal forces is another reason why a ball bearing is preferable for high-speed applications. The definition of centrifugal force is a force that pushes outward on a body moving around a center and is caused by the body's inertia.

Because it causes radial and axial loads on a bearing, centrifugal force is the primary limiting factor to bearing speed. Roller bearings produce more centrifugal force than ball bearings of the same size because they have more mass.


Reduce Centrifugal Force with Ceramic Balls Material


Sometimes the speed of an application exceeds the speed rating of a ball bearing. If this occurs, a simple and common solution is to change the material of the ball bearing from steel to ceramic, which maintains the same bearing size while providing a 25% increase in speed. Ceramic balls produce less centrifugal force for any given speed because they are lighter than steel.


High-Speed Applications Work Best with Angular Contact Bearings


For high-speed applications, angular contact bearings are the best choice. One reason is that the balls are smaller, and smaller balls weigh less and produce less centrifugal force when rotated. Angular contact bearings also have a built-in preload that works with centrifugal forces to properly roll the balls in the bearing.

If you're creating a high-speed application, you'll need a high-precision bearing, typically in the ABEC 7 precision class.

Compared to a high precision bearing, a lower precision bearing has more dimensional "wiggle room" during manufacturing. As a result, when the bearing is used at high speeds, the balls rapidly roll over the bearing raceway with less reliability, potentially resulting in a bearing failure.

High precision bearings are manufactured to strict standards and have a minimal deviation from the specifications when produced. Because they ensure good ball and raceway interaction, high precision bearings are reliable for fast-moving applications.


Bearing Runout & Rigidity


Bearing runout is the distance a shaft orbits from its geometric center while rotating. Some applications, such as cutting tool spindles, will tolerate only minor deviations in their rotating components.

If you are designing an application like this, use a high precision bearing because the tight tolerances the bearing was manufactured will result in smaller system runouts.

Bearing rigidity is the resistance to the force that causes the shaft to deviate from its axis, and it is important in minimizing shaft runout. The interaction of the rolling element and the raceway causes bearing rigidity. The higher the rigidity, the more the rolling element is pressed into the raceway, resulting in inelastic deformation.

Bearing rigidity is usually categorized by:


  • Axial rigidity

  • Radial rigidity


When in use, the higher the bearing rigidity, the more force is required to move the shaft. Consider how this works with precision angular contact bearings. These bearings usually have a manufactured offset between the inner and outer raceways. The offset is removed when the angular contact bearings are installed, causing the balls to press into the raceway without any outside application force. This is known as preloading, and it increases bearing rigidity even before any application forces are applied to the bearing.


Bearing Lubrication


Knowing your bearing lubrication requirements is critical for selecting the right bearings and should be considered early in the application design process. One of the most common causes of bearing failure is improper lubrication.

Lubrication forms an oil film between the rolling element and the bearing raceway, which reduces friction and overheating.

Grease, an oil with a thickening agent, is the most commonly used lubricant. The thickening agent holds the oil in place, preventing it from escaping from the bearing. The thickening agent separates from the grease as the ball (ball bearing), or roller (roller bearing) rolls over it, leaving only the oil film between the rolling element and the bearing raceway. The oil and thickening agent recombine after the rolling element passes through.

Oil mist systems, which mix oil with compressed air and inject it into the bearing raceway at metered intervals, are another lubrication option for high-speed applications. This method is more expensive than grease lubrication because it necessitates an external mixing and metering system and filtered compressed air. On the other hand, oil mist systems allow bearings to operate at higher speeds while producing less heat than greased bearings.

Oil baths are commonly used in low-speed applications. When a portion of the bearing is submerged in oil, this is an oil bath. A dry lubricant can be used instead of a petroleum-based lubricant for bearings that will operate in extreme environments. Still, the bearing's lifespan is typically shortened due to the lubricant's film breaking down over time.


Conclusion


When selecting the bearing that is right for your machine, it is important that the bearing be appropriate for the requirements of the usage environment and easily acquired for replacement.

In summary:

  1. Select the right bearing type based on the magnitude and direction of the load.

  2. Select a bearing that matches the dimensions of the shaft or housing from the bearing boundary dimensions table.

  3. Check that the type of bearing you have selected is appropriate by using the “Performance comparison of bearing type” with the bearing usage criteria.

Other factors that must be considered in the selection are operating speed and temperature, as they dramatically impact bearing selection.


To view the full selection of oilfield Bearings products, visit their page on the DrillingParts website here.