Introduction to bearings


Let’s take a quick look at bearings and check out how they work, their basic components, the different types of bearings, and how to find replacements for worn out bearings using part numbers.

As you probably already know, bearings reduce the friction between different parts so that they can move freely at different speeds and loads.

Motorcycles use different sizes and types of bearings to keep their engines, transmissions, wheels and other rotating assemblies running smoothly and efficiently.

TL;DR – watch the video instead: 

Parts of a Bearing

Most modern bearings have the same basic design and components. Even if you’re working on vintage motorcycles, you’ll find that their bearings will look very similar to those used in newer bikes.

Here’s a quick rundown of the basic components that you’ll find in pretty much every bearing:

  • The Outer Race or outer ring determines the outside diameter and width of the bearing.
  • An Inner Race or inner ring that fits inside the outer ring and is much smaller in size. This gives us the bore size of the bearing.
  • Rolling Elements like Balls or Rollers in between the two rings are the key components that allow the bearing to spin freely.
  • A Cage in between the two rings keeps the balls and rollers in place and prevents them from falling out.
  • A Dust shield or Seal keeps dust and other contaminants out, which can grind down and wear out the interior components of the bearing much faster.
  • Lubricants such as grease and oil minimize the friction between the bearing’s internal components. Lubricants create a thin film around the surface of the bearing internals that prevents, or at least reduces, metal to metal contact thus reducing friction, heat and wear. Lubricants also help keep contaminants and moisture out, and prevent corrosion of the bearing components.

Identifying Bearing Part Numbers

Bearings from different manufacturers use a common part numbering scheme so you can easily find the correct replacement parts using just the part numbers. Bearing part numbers also tell you exactly what type of bearing you have, its size and many other specifications.

Let’s take a look at this bearing part number as an example. You can find these numbers stamped on the bearing itself or on the box it comes in.

The combinations of numbers and letters give us basic information about the bearing such as its type, series and bore size.

Bearing sizes are “standardised”. The “L446343” shown in the photo above will be the same dimensions and construction no matter which manufacturer you purchase from.

You can find lots of resources online that go into a lot more detail as to what every digit or character means. Simply search for ‘bearing numbers’ or ‘bearing designation system’ if you want to learn how to decode the part numbers on your own.

You can also search online for a bearing’s part number to find its exact specifications using the manufacturer’s published specifications and spec sheets. This should give you more detailed information about the bearing such as its outside diameter, width, basic load ratings, limiting speeds for grease and oil, the size of the balls or rollers, and its weight.

If we use the photo above as an example, and type “Timken L44643” into our favorite search engine, we’ll find a page such as this:

If you can’t find the bearing specifications or have doubts about their accuracy, you can usually find more complete and accurate spec sheets from the websites of well-known bearing manufacturers like Timken, NSK or SKF.

In most cases, the replacement bearings from unknown or generic brands will have the same dimensions as bearings from more reputable brands. However their quality will not be as consistent as a well-known brand. You get what you pay for!

When you buy more expensive bearings from the top brands and manufacturers, you have assurance that you’re buying a quality product -using high quality steel, that’s been precision ground and heat treated making it more reliable and durable.

I don’t skimp on bearings – I go to my local industrial bearing shop – not cheap online sources!

Whether you choose a brand-name bearing or a cheaper version, it’s still vital you look after them by keeping them clean and properly lubricated.

Comparing Ball Bearings and Roller Bearings

The most common types of bearings you’ll find in a motorcycle are ball bearings and roller bearings.

Ball bearings use spherical balls in between the inner and outer rings to reduce friction. The contact points between the balls and the rings are very small which allows the bearing to spin freely at high RPMs.

Roller bearings use small cylindrical rollers instead of spherical balls to allow the inner and outer rings to spin freely. Using rollers helps spread the weight of heavier loads across a larger surface.

Axial vs. Radial Loads

It’s helpful to understand the two types of loads a bearing will be subjected to during its operation, namely axial loads and radial loads.

Axial loads or thrust loads occur along the horizontal axis or parallel to the rotating shaft or assembly.

Radial loads are applied on a vertical axis along the external surface of the outer ring, which also means that it is perpendicular to the axis of rotation.

Bearings and Loads

When used for lighter loads, ball bearings can easily handle both axial and radial loads. However, the smaller contact points of the spherical balls don’t provide much support against heavier radial loads, which can deform the shape of the different bearing components.

Roller bearings are more suitable for heavy radial loads, but do not spin as freely along its rotating axis – reducing its capacity for axial loads.

Tapered roller bearings are typically used for applications where bearings are subjected to heavy axial and radial loads at the same time.

These share the same basic components as straight roller bearings, but have a conical or tapered shape that allow it to support both heavy radial loads and moderately high RPMs. Due to its design, tapered roller bearings are designed to be very durable and long lasting.

In the next article, we’ll talk more about bearing maintenance, as well as how to clean and pack bearings.

About the author 

Matt McLeod

I teach people how to build custom motorcycles by helping them build skills and confidence with my coaching, articles and training videos.
I provide better technical information for custom motorcycle builders. And I shorten the learning curve getting you there.

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