Back to basics: the ball bearing
2017 August 14, 09:00 GMT
First, a bit of history. The earliest recovered example of a rolling element bearing is a wooden ball bearing that supported a rotating table from the remains of a Roman ship in Lake Nemi, Italy. The wrecks were dated to 40BC.
Over a millennium and half later, Leonardo da Vinci incorporated drawings of ball bearings for cart axles around the year 1500. The first modern recorded patent on ball bearings was awarded to Philip Vaughan, a British inventor and ironmaster who created the first design for a ball bearing in Carmarthen in 1794. His was the first modern ball-bearing design, with the ball running along a groove in an axle assembly.
Fast forward a century or more, and the first major ball bearing development of the modern industrial era was patented on 6 June 1907 by inventor, Sven Wingquist, the first managing director of Aktiebolaget Svenska Kullagerfabriken (a company we now know as SKF). This was the double-row self-aligning ball bearing, the basic design of which SKF has continued to develop to the present day.
In the following years, from its base in Gothenburg, SKF promoted this innovative ball bearing design internationally, opening branch offices in Germany and France, and appointing agents initially in Finland, Switzerland, Belgium, Denmark, Austria and Australia. Agents in many more countries were to follow.
A subsidiary, SKF Ball Bearing Co, was established in New York in 1909. A year later the Skefko Ball Bearing Co. Ltd was registered in the UK and work started on a plant in Luton. Here some 180 bearing types were eventually being produced daily. By 1913, the company had 3,200 employees around the world, producing 1.3 million bearings a year.
What is a ball bearing?
Since then ball bearing technology has advanced apace, thanks to continuing research and development by companies such as SKF. But before we examine the many different modern variations and combinations of bearing geometries and materials of construction, let’s take a moment to define exactly what a ball bearing is, its basic mechanical construction and the forces that act upon it when in service.
In their basic form, ball bearings are an assembly of four parts: a large outer ring, a smaller inner ring, the balls, sized so that they are free to rotate between and make contact with the raceways of the rings, and a cage, the purpose of which is to retain the balls, preventing them from touching one another. Variations of this basic design have given rise to a range of special types of ball bearings (of which more later) designed to meet specific application needs.
The main purpose of a ball bearing is to reduce friction in a rotating axis – for example, between a fixed shaft and a component (such as a wheel) rotating about that shaft. The more precise the internal geometry of the bearing is, the lower the friction will be. Friction will also be affected by speed of rotation, lubrication, load and other factors. Lubrication is a very important topic and a subject in its own right, and will be covered in a future article in this series.
Basic ball bearings will handle both radial and axial loads, but as the point of contact with the balls and the raceways is very small there is considerable pressure created, so loads must be limited in order to avoid damage to the balls and raceways. Ball bearings are thus better suited to lower load applications. Roller type bearing designs overcome these limitations, as we shall be discussing at a later date.
Calculation of the loads for a particular application and operating conditions is an important step, as a miscalculation is likely to result in premature bearing failure. Choosing the correct type and size of ball bearing can be a complex task. In addition to the loads they are subject to, there may be other parameters to consider such as high or low extremes of operating temperature, lubricant type, presence of corrosive agents, and even stray electrical currents which can cause damage to the raceways.
Modern types of ball bearing
Let’s now take a look at modern ball bearing configurations and how their particular geometries and materials meet the demands of different applications and working environments. For the purposes of this article, we’ll be considering 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
Single row deep groove ball bearings are the simplest and most versatile of all ball bearing designs, and so they tend to be the most widely used type. They are suitable for high and very high speed applications, and are robust in operation, requiring little maintenance. This bearing type has deep, uninterrupted raceway grooves that have a close osculation with the balls (maximum contact between the curved surfaces), enabling them to accommodate radial loads, axial loads and combinations of both. Variations include hybrid types comprising ceramic balls and steel raceways (offering insulation against stray electrical currents) and polymer bearings for applications subject to contact with corrosive agents.
Double row deep groove ball bearings
With very similar properties to single row versions, these 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 considerably higher load capacity.
Self-aligning ball bearings
The bearing type that placed SKF at the forefront of modern bearing design and development over one hundred years ago, the modern self-aligning bearing has two rows of balls running in two inner raceway grooves and a common single spherically contoured raceway in the outer ring. This novel design allows the bearings to accommodate angular misalignment (shaft relative to the housing) of 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
Angular contact ball bearings have raceways in the inner and outer rings that are displaced relative to each other in the direction of the bearing axis. This means that they are designed to accommodate combined loads (simultaneously acting radial and axial loads). The axial load carrying capacity of angular contact ball bearings increases with increasing contact angle, the latter being 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.
Standard configurations provided by SKF include single row angular contact ball bearings (accommodating radial loads as well as axial loads in one direction only), double row angular contact ball bearings (accommodating radial loads as well as axial loads in both directions) and four-point contact ball bearings supporting only axial loads in both directions).
Thrust ball bearings
Thrust ball bearings are manufactured as single direction or double direction units that are designed to accommodate axial loads only and 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(s), 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 are able to accommodate axial loads in one direction. Double direction thrust ball bearings consist of one shaft washer, two housing washers and two ball and cage assemblies, the housing washers and ball and cage assemblies being identical to those used in single direction bearings. Double direction thrust ball bearings can accommodate axial loads in both directions.
All of the ball bearing types mentioned in this article are available from SKF, together with expert application advice concerning special configurations, materials, sizing, and load and life expectancy calculations. In the next issue, we will be going back to basics with roller bearing configurations, which offer useful alternatives to their ball bearing counterparts when higher load bearing capabilities are needed.