Bearings and bushings are necessary for optimal functioning of any wheel application. Without it, the bore of the wheel is subject to constant friction—wearing it out faster and shortening its life span. This is especially important in industrial wheels that carry heavy loads, such as metal wheels at lumber mills or manufacturing plants.
The word bearing is a mechanical term referring to the process of transferring the energy of a load between a wheel and axle. Bearing also refers to the specific device placed inside a wheel to minimize the friction between the wheel and axle. Bushings are shaped like a tube or sleeve, and help with motion by sliding, as opposed to the rolling motion of most bearings. Still, bushings are a type of bearing, as they aid in the efficiency of rotating.
From cart wheels to Ferris wheels—all wheels need bearings or bushings to operate smoothly in the long run. However, the type and size of the bearing or bushing required will be specific to each application and must be carefully selected.
How does a bearing work?
A bearing allows for easier motion by reducing unnecessary friction between two surfaces. Metal-on-metal contact is abrasive and causes material degradation. Bearings allow the two surfaces to roll over each other, instead of grinding against each other. Bearings use a smooth ball or roller that rolls against an inner and outer surface. The ball or roller takes on the weight of the load, allowing the device to spin.
Common load types
There are various bearing designs that control motion in different ways. Common load types include radial, axial, moment, and combined loads. Load types in the diagrams below show different loads acting upon the bearings—in these cases, ball bearings.
Types of bearings
Different bearings exist for a range of load types and speed capacities. The correct bearing design ensures maximum durability, efficiency, and performance.
Ball bearingsBall bearings use perfectly spherical balls as a rolling element to maintain separation between the bearing races. Races are rings with a groove, where the balls rest. The larger race that contacts the bore is the outer race. The smaller race that the shaft rides in is the inner race.
Ball bearings can spin in any direction due to their spherical shape. They can support combined loads—loads from above (radial loads), as well as side-to-side forces (axial loads). Ball bearings minimize rotational friction as the races that contain the balls work to transmit the loads. However, since perfectly round ball bearings have limited areas of contact, they are best for light to medium applications that are not subject to heavy loads or impact.
Straight roller bearingsStraight roller bearings, also known as cylindrical bearings, run in cylindrical races. They are low-friction and suitable for high-speed and high-radial use. Applications that hold heavier radial loads such as conveyor belts often use these bearings. In straight roller bearings, the contact point between the bearing and the race is a line instead of a point. Unlike ball bearings, the load is distributed over a larger surface area, allowing the bearing to handle a heavier load. To limit the chance of the bearing from skewing, the roller length is made to be longer than the diameter of the roller.
Tapered roller bearingsTapered roller bearings have rings and rollers that are tapered and shaped like truncated cones. Their unique shape allows them to simultaneously support axial and radial loads. The angle of the axes between the roller and bearing determines the load capacity. The greater the angle, the more axial loads that can be supported. The contact angle in tapered roller bearings are between 10–16 degrees—however, for higher thrust-load capacities, a 30-degree contact angle is necessary.
Bearing load and speed limitations
Bearings are generally more suitable for supporting wheels with higher velocities and lighter loads. There are three points to keep in mind when checking for bearing limitations:
- Consider the load condition and bearing type
As mentioned above, it is important to identify the type of load that will be applied onto the bearing. This also helps to determine which bearing type to use.
- Evaluate the allowable space
The allowable space for a bearing is limited and its constraints are outlined by the manufacturer. Ensure that the bearing will be able to fit into the intended space before progressing.
- Check the load capacity
Each bearing comes with a specific load capacity. The load that will be carried by the wheel and bushing cannot exceed this load capacity.
How does a bushing work?
Bushings, also called sleeve bearings, are just one specific type of bearings. They are made to slide over rods to provide an extremely low-friction motion. They are excellent for shock absorption and work to minimize energy usage, noise, and wear. Bushings are widely used for heavy-duty wheels that require high load-bearing tolerances.
Bushings are self-lubricating, and therefore ideal for low-maintenance or maintenance-free operations. Self-lubrication is achieved through a uniform pattern of indents on the surface of the bushing that act as grease reservoirs. Some nylon bushings can operate without the use of lubricants, and are especially useful in food and textile industries where dry applications are preferred.
Types of bushings
There are several bushing types on the market with different material compositions. Some can withstand corrosion, whereas others are suitable for controlling high pressure and impact. Select the best bushing type based on the application and operating environment.
Bronze bushings are typically manufactured from continuous cast bronze material to ensure a uniform bronze structure throughout the bushing. They are general-purpose bushings that offer excellent load carrying capabilities and protect against wear. They can endure high levels of heat and withstand temperatures of up to 450°F. They can be easily modified by adding oil and grease grooves, as well as self-lubricating graphite plugs. Bronze bushings can also be designed with flanges to handle combined loads.
Solid plastic bushings are increasingly popular due to their lightweight, corrosion-resistant properties. They require little to no maintenance, or additional lubrication. Modern plastics have been designed to overcome previous concerns, such as heat sensitivity, softening, wear, or reduced life at elevated temperatures. Nyloil bushings are one of the most common types of plastic bushings offering exceptional machinability and durability. They work well in wet or humid conditions, such as on dry kiln cart wheels at lumber drying facilities, where the water acts as a lubricant for the bushing.
Bushing load and speed limitations
Bushings are often used with heavier loads at slower speeds. Industrial metal wheels benefit from durable bushings that can withstand substantial load transfers and shock loading. When it comes to bushings, frictional build-up of heat must be considered. The two major factors that influence heat are unit pressure (P) and surface velocity (V). The product of unit pressure and surface velocity is pressure velocity (PV). To determine if a bushing is appropriate for any application, first find the limiting PV value from the manufacturer. For safe operation, the application’s calculated PV value must be lower than the manufacturer’s limiting PV value.
- To calculate the PV value of an application:
- To determine surface velocity (V):
Rpm = shaft revolutions per minute D = shaft diameter in inches
- To determine pressure (P):
P=Total Load (lbs)/Contact Area(in²)
Contact Area = D (shaft diameter in inches) x L (bushing length)
- To calculate the PV value of an application:
In an internal engineering study, Reliance Foundry’s R-3320 wheel was used with the Nycast Nyloil and C93200 bronze bushing to test the effects of the limiting PV value of the bushings. As mentioned earlier, bushings are typically used for heavier loads at slower speeds. This is evident in the graph as both bushings cannot handle substantial loads at high speeds. The faster the speed, the lower its load capacity. The P-max (psi) and V-max (fpm) values are provided by the manufacturer for each product. P-max is the maximum load at 0 rpm, and V-max is the maximum velocity (maximum shaft rpm) at light loads. For real-world applications, compare the calculated P, V, and PV values against the maximum allowances to determine if the bushing will work. Ensure that the bushing is working within the range below the limiting PV curve.
Bearing and bushing maintenance
Proper maintenance and inspection is critical. Periodic maintenance should include surveying operating conditions for any abnormalities. Any changes in noise, vibration, and temperature should be inspected. Bearing and bushing failures such as flaking, scoring, cracking, corrosion, and abnormal wear must be addressed immediately. The supply or replacement of lubricants should also be checked regularly for continued optimal performance.
Choosing between bearings and bushings
When selecting between bearings and bushings for wheel applications, the main consideration is speed and load capacity—will the wheels carry heavier loads at slower speeds, or lighter loads at higher speeds? Another area to keep in mind is lubrication. Some maintenance-free applications will require self-lubrication, especially in the food and textile industry where dry applications are sought after. A vast selection of bearings and bushings are available on the market with different sizes and material options—fit for a variety of wheel applications at varied price points.