Introduction

In this article we continue our exploration into the world of powder metallurgy by focusing on Sintered Oil-Impregnated Bearings.  Read the two previous installments, Introduction to Powder Metallurgy and Introduction to Metal Injection Molding, to get an idea of the general principles of powder metallurgy.

Sintered Oil-Impregnated Bearings are a specific type of powder metallurgy product that takes advantage of the porous nature of powder metallurgy by filling those voids with lubricant.  The result is a self-lubricating bearing that emits oil/lubricant when rotating but reabsorbs the lubricant when it ceases to rotate.

Let’s dive in!

Types of Bearings

For our purposes, we will break down the world of bearings into two distinct categories – Roller Bearings and Plain Bearings.  Roller Bearings are generally ball bearings, but sometimes the rolling element is something other than balls.  Plain bearings encompass plastic bearings, fluid bearings and more, including, our focus, Sintered Oil-Impregnated Bearings.

The above image details the differences between ball bearings and oil-impregnated bearings.  Ball bearings are made up of an outer and inner ring separated by the balls.  They are commonly used in home appliances, automobiles and industrial machinery.  Oil-impregnated bearings, on the other hand, are usually just one piece and can be found in electronics, HVAC systems and automobiles.  The other major difference you will notice is the contact that the bearing makes with the shaft.  A ball bearing makes full contact with the shaft.  An oil-impregnated bearing will leave a very small gap between the bearing and the shaft.  This gap will be filled with lubricant when the bearing rotates.

Sintered Oil-Impregnated Bearings made via powder metallurgy have some distinct advantages over their main competitor, ball bearings.  The self-lubricating nature of oil-impregnated bearings allows them to decrease noise and vibration levels during operation and protects them from electric corrosion.  Additionally, by its very nature, the powder metallurgy process allows oil-impregnated bearings to be more cost-effective and have greater freedom of shape than ball bearings.  Beyond a variety of shapes, it is also possible to achieve miniaturization in the form of micro bearings with powder metallurgy.  Something that is generally not possible with roller bearings.  Sintered Oil-Impregnated Bearings are not able to match the strength properties of ball bearings.  This being the main advantage that ball bearings have over oil-impregnated bearings.

The Science Behind Sintered Oil-Impregnated Bearings

Many of you are probably asking “How is self-lubrication possible?”  It is a good question, and one that comes up quite a bit when discussing Sintered Oil-Impregnated Bearings.  The ability to impregnate the bearing with oil is a by-product of the powder metallurgy manufacturing process.  Because powder metallurgy utilizes tiny metal particles that are then pressed together, there are naturally occurring microscopic voids in the finished part.  These voids are then filled with oil.  We will discuss the manufacturing process in more detail below.

Now that we know a little about how the bearing holds the oil, let’s discuss how that oil goes to work when the bearing is in use.  As the bearing begins to rotate against the shaft, frictional heat causes the oil volume to expand.  At this point, oil begins to seep out of the inner diameter of the bearing forming a thin oil film around the shaft, allowing the bearing to perform optimally.  When the bearing stops rotating, the oil cools and the oil volume shrinks.  At this point, the oil returns to the bearing’s pores through a process known as Capillary Force.  You can envision Capillary Force as like a sponge near water in that it just naturally soaks up the nearby water into its available pores.

Sintered Oil-Impregnated Bearings Manufacturing Process

The manufacturing process for Sintered Oil-Impregnated Bearings is very similar to the normal powder metallurgy manufacturing process with a few slight variances.  In this article, we will mostly focus on what makes the process unique for Oil-Impregnated Bearings.  To review the basic process for powder metallurgy, check out our Introduction to Powder Metallurgy article.

In general, powder metallurgy follows the basic process of (1) powder mixing, (2) compacting and (3) sintering.  After these three basic steps, a variety of secondary operations can be completed depending on the requirements for a given component.  In the case of Sintered Oil-Impregnated Bearings, the most important secondary operation is vacuum oil immersion.

To achieve oil impregnation, the sintered bearings are placed in a vacuum immersion tank with the desired lubricant.  First, the pressure is reduced in the tank.  After decompression, the pressure in the vacuum tank is returned to normal.  As the pressure returns to normal, the oil is allowed to enter the vacant space (pores) in the bearings.  When the bearings are removed from the tank, the oil remains in the pores of the bearings, creating a Sintered Oil-Impregnated Bearing.

Product Examples

Sintered Oil-Impregnated Bearings are used in a variety of industries including home appliances, home electronics, medical devices, HVAC systems and more.  They are ideal for applications where periodic lubrication of a bearing is not practical.  This is why it is so common for these bearings to be used in household products.  The average person is not going to oil the bearings inside their vacuum cleaner.  Assuming someone did open their vacuum and oil the motor bearings, imagine the possibility that this opens for dirt and other debris to enter the motor.  By building the vacuum with self-lubricating bearings, the manufacturer ensures that the consumer does not need to perform this maintenance and reduces the possibility of damage caused by dirt getting into sensitive parts of the motor.

Below you will find examples of many products that contain Sintered Oil-Impregnated Bearings.

Conclusion

The unsung heroes of the bearing world, Sintered Oil-Impregnated Bearings are found in many products that we all use daily.  But most never notice their importance.  Being both self-lubricating and having freedom of shape in the design stage makes these bearings adaptable to a variety of circumstances.

Talk with our experts and see how you can make Sintered Oil-Impregnated Bearings work for you.  Get started here.