Bearings Research
A bearing is usually found between two adjacent surfaces and allows for rotation and transmission of load between those two surfaces. As well as this, bearings typically feature the ability to attach to structures quickly and simply.
A typical Bearing 1
Bearings can be found in numerous amounts of applications, for example, sliding mechanisms in a set of drawers (linear), roller beds in a factory (linear), bearings in a turbocharger (rotary), wheel bearings on a car, motorbike, bicycle (rotary).
For the crane the group is designing, we are utilising a bearing on which the crane will pivot. This bearing will have to support horizontal movement, and must allow the crane to pivot through a full 360 degree movement with ease. The forces involved with this will be a compressive axial load, torque, and some minimal friction. The compressive axial load will be from the main beam of the crane and the load at the end of the crane. This will vary depending on the load at the end of the crane. The torque force will be from the movement around the cranes axis. The friction is found between the moving parts but generally has a very small rolling coefficient of friction. The speed at which the crane will be pivoted will be relatively slow, therefore bearings with high speed capabilities will not be necessary.
There are several types of bearing available, the two that would be applicable for the crane would be a Thrust Roller Bearing or a Four-Point Contact Bearing.
Thrust Roller Bearing
Thrust Roller Bearing Cutaway 2
A thrust roller bearing may be appropriate for the crane because they can withstand high thrust forces (i.e. a downward force from the weight of the crane and load). The bearing consists of cylindrical bearings which are lubricated with oil, and sandwiched between two circular (upper and lower) races.
The advantages of using a Thrust Roller Bearing are that there is a rolling coefficient of friction of around 0.005. This is very low and allows for easy pivoting of the crane. The bearing can operate at very high speeds, although the crane will not be used at high speeds, rendering the bearing over engineered for the application. The lifespan of the bearing is high as long as it is well maintained, this would involve making sure the bearings are well lubricated and each individual bearing functioning correctly. There are no significant disadvantages of using a bearing of this kind, making it even more applicable to the crane.
Four-Point Contact Bearing
Four point contact ball bearings are capable of resisting radial, thrust, and moment loads, it is usually subjected to a combination of two or more of these loads wherever it is applied. In our case it will resist all three loads. A Four-Point Contact Bearing is a type of roller bearing which uses spherical bearings. This allows for the device attached to the bearing to spin very freely due to low resistive qualities.
Below is an exploded view of a typical Four-Point Contact Bearing.
A: Inner Race
B: Outer Race
C: Cage for Bearings
D: Bearing (Spherical, conical, cylindrical)
The advantages of using a Four-Point Contact Bearing are that they can take high moment, thrust and radial forces. They can be commonly found in cranes of varying sizes. The bearing also has a very low rolling coefficient of friction. The Bearing can be supplied with teeth on the outer or holes in the outer ring for mounting purposes. Both would be suitable ways of attaching the bearing to the crane.
1 http://www.engineeringsurplus.co.uk/images/uploads/bearing.jpg
http://science.howstuffworks.com/bearing2.htm
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Magnetic bearings are ideal for high speed applications as they support the load using magnetic levitation which has zero friction and requires no maintenance.
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