Back to basics with ball bearings
07 March 2017
The ball bearing is the best-known – and most widely used – bearing type, and its many variants can be applied to a range of specific application. Phil Burge, Country Communication Manager at SKF explains the fundamentals.
For people who have no involvement in engineering, there is only one type of bearing – the ball bearing – which is found everywhere from skateboard wheels to toy cars.
Of course, there are many other types of bearing – and many variants of ball bearing. However, it’s worth going back to basics by defining what a ball bearing is – its basic mechanical construction, and the forces that act on it – backed by some modern (and ancient) history.
According to the American Bearing Manufacturers Association, the earliest known example of a ball bearing dates from around 40BC: the remains of a Roman ship, found in Italy, included a rotating table that used a wooden ball bearing. In the 16th and 17th centuries, Leonardo da Vinci and Galileo both described ball bearings. The first ball bearing patent was filed in the late 18th century, by British inventor Philip Vaughan. These were the seeds from which modern bearing designs grew.
In their most basic form, ball bearings are an assembly of four parts: a large outer ring; a smaller diameter inner ring; the balls, which are sized so that they are free to rotate between – and make contact with – the raceways; and a cage, which retains the balls and prevents them from touching one another. Variations of this basic design have given rise to special types of ball bearings that meet specific application needs.
The main purpose of a ball bearing is to reduce friction in a rotating axis – such as between a fixed shaft and a component like a wheel that is rotating about that shaft. Generally, the more precise the internal bearing geometry, the lower the friction. Friction is also affected by factors such as rotation speed, lubrication and load.
Basic ball bearings can handle both radial and axial loads. However, the point of contact with the balls and the raceways is very small – which creates considerable pressure – so loads must be limited in order to avoid damage to the balls and raceways. This makes ball bearings more suitable for lower load applications.
Calculating the loads for a particular application and operating conditions is an important step. Any miscalculation can lead to premature bearing failure. Choosing the correct type and size of ball bearing can be a complex task. Other parameters must also be considered, including high (or low) operating temperatures, lubricant type, the presence of corrosive agents, and even stray electrical currents – which can cause damage to the raceways.
The design and construction of modern ball bearings allows them to meet the demands of a variety of applications and working environments. This article will consider five key types of modern ball bearing: single row deep groove; double row deep groove; self-aligning; angular contact; and thrust.
Single row deep groove ball bearings are the simplest, most versatile of all designs – so tend to be the most widely used. They can be used in high-speed (and very high speed) applications and are robust in operation, requiring little maintenance. They have deep, uninterrupted raceway grooves that have a close contact with the balls – ensuring maximum contact between the curved surfaces – which enables them to accommodate radial loads, axial loads and combinations of the two.
Variations include hybrid types comprising ceramic balls and steel raceways (which offer insulation against stray electrical currents) and polymer bearings – which can withstand corrosive agents.
Double row deep groove ball bearings are essentially a tandem arrangement of the single row deep groove type, where a single ring combines two raceways. They are suitable for bearing applications where the load-carrying capacity of a single row bearing may not be sufficient. For the same bore and outside diameter, double row bearings are, by design, slightly wider than single row bearings, but have a much higher load capacity.
Self-aligning ball bearings usually have two rows of balls running in two inner raceway grooves and a common single spherically contoured raceway in the outer ring. Their novel design renders bearings insensitive to angular misalignment up to a maximum of three degrees.
Typically, self-aligning ball bearings are an ideal choice for light- to medium-load conveyor applications as they manage varying loads well with minimal friction, enabling higher conveyor speeds and longer bearing service life.
Angular contact ball bearings have raceways in the inner and outer rings that are displaced relative to one another in the direction of the bearing axis. In this way, they can accommodate simultaneous radial and axial loads. The axial load-carrying capacity of angular contact ball bearings increases with the increasing contact angle. This is defined as the angle between the line joining the points of contact of the ball and the raceways in the radial plane – along which the load is transmitted from one raceway to another – and a line perpendicular to the bearing axis.
Finally, thrust ball bearings are manufactured as single direction or double direction units that can accommodate only axial loads – so must not be subjected to any radial load. Unlike other ball bearing types, thrust ball bearings are separable – that is, the shaft washer, housing washer (or washers), ball and cage assemblies can be mounted separately. Shaft washers have a ground bore to enable an interference fit, while the bore of the housing washer is turned and always larger than the shaft washer bore.
Single direction thrust ball bearings consist of a shaft washer, a housing washer and a ball and cage assembly, and can accommodate axial loads. Double direction thrust ball bearings have one shaft washer, two housing washers and two ball and cage assemblies, and can accommodate axial loads in both directions.
Laypeople will continue to use the term ‘bearing’ and ‘ball bearing’ interchangeably – and be blissfully unaware that many other variants exist. However, the simplicity – and versatility – of ball bearings means that this is likely to continue for a long time.
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