The difference between bushings and bearings
There isn’t really a difference: a bushing is a type of bearing. In general, a “bearing” facilitates movement between two parts while reducing friction. Simple in design, the typical bearing has two surfaces that roll over each other, enabling two mating components to move friction-lessly.
They are broadly divided into radial bearings, thrust bearings and linear bearings, depending on whether they’re working with rotational or linear movements.
A “bushing” is a specific design of bearing used for certain applications. To summarise: a bushing is a bearing, but a bearing is not always a bushing.
Now that is cleared up, we will take a closer look at the different types of bearing, and how a bushing differs from its rolling counterparts.
The main types of bearing
Different types of bearing have the same basic structure and purpose: they have two surfaces that move over each other, enabling mating parts to move easily and without friction. The main difference is whether they are working with rotational or linear motion. Here’s our at-a-glance guide to the main bearing types:
Ball Bearings
Ceramic or steel balls are fitted between inner and outer rings to create the classic ball bearing. These are designed to support rotating shafts, while reducing friction between the static and moving parts.
Ball bearings support rotating shafts while cutting down on the fraction between these and static components. Spheres (often steel balls but sometimes ceramics, which perform at high temperatures) are fitted between an inner ring and an outer ring. The bearings assembly can have a single row or multiple rows, depending on the application.
There are broadly two ball bearing types: deep groove ball bearings which have a high radial load capacity, and angular contact ball bearings which can manage both high radial and axial loads. Ball bearings are used in a great range of applications, from complex aerospace engineering to the everyday skateboard.
Roller Bearings
Roller bearings can take heavier loads than ball bearings. They are cylindrical or tapered in shape, and are fitted between inner and outer races. Their role is to minimize friction between moving shafts and static components.
Cylindrical roller bearings are generally made from stainless steel, and are intended for heavy radial loads. They are ideal for high speed applications, as their low-friction design doesn’t generate heat or noise. You’ll find cylindrical roller bearings in gearboxes, electric motors and pumps, in a wide range of industries, including automotive, mining and construction.
For lower speed applications, use spherical roller bearings. These self-aligning bearings can cope with serious misalignment, vibrations and sudden shocks, and when installed on a double row, can manage extremely heavy radial and axial loads. Roller thrust bearings have similar misalignment managing properties.
Needle Bearings
Needle roller bearings may be small, but they’re tough. They get their name from their thin, needle-like appearance, and are used for reducing friction within smaller applications. The rigid bearings are perfect for applications with oscillating movement and are handy if you need to keep down weight or have a limited clearance area. They are found in all manner of applications, including domestic appliances.
Tolerance Rings can be used to secure and mount ball or roller bearings, using radial forces. The corrugated “waves” in the ring’s circumference hold the ball bearings in place within the housing, acting as springs. The more you compress the ring, the greater the force it creates, which also determines load-bearing capacity.
Rod End Bearings
The rod end bearing is compact, light and easy to fit, and they’re a reliable choice for heavy alternating loads. The basic design is a round head with an integral shank, which houses spherical plain bearings. Rod ends don’t have the misalignment properties of some other bearings as they’re fixed.
Rod ends are a type of mounted bearing, a group of bearings that are housed inside bolted-on components, including pillow bocks. They are typically used in machines with exposed rotating shafts. Cam followers are another variation. These have thick outer rings, softer inner rings to accept shock, and work with the cam to transfer rotary into linear motion.
The spherical bearing housed in the rod end needs a constant area of contact to minimize stick-slip and keep the friction stable. Lubrication is essential, which can cause issues in applications where impurities simply can’t be allowed.
Our NORGLIDE® MP is a maintenance-free material that combines the strength of its flexible metal mesh with the self-lubricating properties of high-quality plastic. Used as a liner for spherical bearings, it reduces the risk of impurities while dampening noise and vibration, providing stable friction and torque, as well as removing the need for lubrication.
Sleeve Bearings
And now we come to the missing link between the bearing and the bushing: the sleeve bearing.
Sleeve bearings are also called bushings, journal bearings or plain bearings, and are the simplest design of bearing. Sleeve-shaped, they are usually found in applications that use rotating or sliding parts. They are made from a range of materials, with plastic being the lowest-cost option.
All-metal bearings have a low coefficient of friction but can cause rattling at high speeds. Steel sleeve bearings can be lubricated; however, a quieter, lower-maintenance and reliably low-friction alternative is to use a self-lubricating material such as PTFE.
A simple cylindrical bearing is ideal for a radial load, while a flanged sleeve bearing takes axial loads. For a high load, the combination of thrust washers and a cylindrical bearing works well.
What are the best bushing materials?
The design may be simple; however, the performance and function of a bushing can vary greatly depending on its material.
Bronze bushings
Bronze bushings have exceptional durability and a long service life. They don’t corrode and are not easily deformed. However, bronze bushings do need plenty of lubrication during operation, making them higher maintenance than plastic.
Plastic Bushings
Plastic bushings are popular, being a low-cost, low friction and low maintenance option. They are self-lubricating so are good for hard-to-reach parts, and as there’s no risk of corrosion, they can be used in wet environments.
We take a look at the pros and cons to plastic as a bearing material.
Advantages of plastic bushings
Plastic bushings have a lot of operational advantages. They are self-lubricating, thus they are maintenance free and have a low coefficient of friction. Plastic lends itself best to simple plain bearings, because it reduces both weight and maintenance, and you won’t see any brinelling. Although, we’re seeing an increase in plastic ball bearings as well as bushings.
In general, plastic has many advantages over metal. It can be used in wet environments, and is easier to keep hygienic in clean room situations. FDA approved food grade plastics may be specified for certain industrial applications, while polyethylene has been traditionally used for orthopaedic bearings.
When specifying plastic bearings, it’s easier to find off-the-shelf options that can go straight into the shopping cart, and bespoke options can also be relatively inexpensive to produce.
Limitations of plastic bearings
Plastic is not the recommended bushing material for heavy loads. Plastic bushings can crack under a high load, which as well as damaging the bearing itself can harm the mating parts and increase the friction. Another issue can be a tendency to misalign if the manufacture or assembly is not absolutely precise – a real issue with off-the-shelf bushings.
Plastic may be quieter than metal, but it’s not inaudible, and hard plastic sleeve bearings can rattle as much as metal if they’re not a precise fit or if they don’t have the right clearance.
An unexpected issue can be damage caused by high-temperature e-painting, which can cause the plastic material to soften, resulting in permanent thinning. A solution to this is to use a stainless-steel backing, which undoes the benefits of a purely plastic bush but does improve wear resistance and enables e-painting. Find out more disadvantages of plastic bushing in our blog “5 reasons to replace plastic bushings”.
However, there’s plastic and there’s plastic. If plastic is specified, the engineering plastic material of choice is usually PTFE.
PTFE Bearings
PTFE (polytetrafluoroethylene) is a low friction, self-lubricating plastic, found in many everyday items under its better-known brand name, Teflon™. PTFE is a popular engineering plastic as it has a low coefficient of friction, maintains constant torque through a range of tolerances, and (a great plus point for the automotive industry where rattles are deathly) has noise-dampening properties.
With its resistance to high temperatures, corrosive chemicals and water, PTFE can be used in a huge range of applications. It’s an excellent bushing material for high-performance bearings – but, unlike other plastic bearings, it’s not always a low-cost option. It can also be difficult to join, and isn’t suitable for environments with high radiation levels.
NORGLIDE®: reinforced PTFE
NORGLIDE® Bearings are made from PTFE, which is reinforced with metal to manage heavy duty applications and to ensure a long and reliable service life. We recommend specifying NORGLIDE® for sleeve bearings for its superior load bearing properties. If you like the noise-dampening and self-lubricating properties of plastic, but require a heavy-duty component, this is the perfect solution.
Why is tribology important when designing bearings?
Tribology is the study of friction, and it plays a huge part in Saint-Gobain’s research and development. Through tribology, we can better understand the wear processes of materials and apply this to the design of our bearings. For example, we collaborate with automotive engineers to create consistent controlled friction that reduces noise, vibration, corrosion and wear.
What options are there for lubrication?
Lubrication is a whole sub-branch of tribology. Chemical tribology looks at the viscosity of the lubricants to determine the optimum formulation for sliding components.
Getting the lubrication correct is key to the efficacy of mating components. Yes, you can use manually lubricating steel for example, but this isn’t an appropriate option in applications that can’t afford contaminants.
Choosing a lubricated material like PTFE removes any need for additional greasing. Our NORGLIDE® range is made from a self-lubricating PTFE compound combined with a strengthening metal.
Talk to the experts in bearings
Not sure what material to specify for your bearings? From a simple flange to heavy duty roller bearings, the material you use for your application can make all the difference to its operational performance and service life.
Talk to the experts at Saint-Gobain, engineer to engineer. We’ll discuss options with you, and together, we’ll make sure your custom bearings are robust, reliable, and just right for your application.
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