Dictionary A B C D E F G H I J K L M N O P Q R S T U V W X Y Z B Bainite hardening Through hardening heat treatment method of bearing rings which gives extreme toughness.   Ball bearings Bearings that have balls as rolling elements. May contain one or more rows of balls. They have very low friction and can operate at high speed. Used at moderate loads. The picture shows a deep groove ball bearing. Ball bearings versus roller bearings The main difference in the performance of these two bearing types is that ball bearings have lower friction than roller bearings, while roller bearings have higher load carrying capacity. Ball screws Ball screws have one or more circuits of balls interspaced between a screw shaft and nut. As the screw or nut is rotated, the balls roll in helical grooves formed by the threads.Each circuit has a return device that recirculates the balls to form a closed path. The high efficiency screws offer an opportunity to displace loads by transforming rotary motion into linear motion. The balls transfer the load under very low friction, whereas the Acme sliding screw typically consists of a steel shaft with trapezoidal thread and a bronze nut.Replacing sliding friction with rolling friction increases the efficiency of the screw from ~30% to ~90%. See also http://linearmotion.skf.com Balls Of all components mass-produced within industry, the precision demands placed on balls for ball bearings are probably the most rigorous. For example the surface roughness of the ball has a tolerance of 0.01 micron (= 0.00001mm). Basic dynamic load rating When calculating bearing life, the basic dynamic load rating C is used. It expresses the bearing load, which will give a life according to ISO (L10) of one million revolutions. See also Basic rating life.   Basic rating life The bearing life calculated with the most simple of ISO equations. Basic static load rating When the bearing is stationary or makes slow oscillating movements or is subjected to heavy shock loads under rotation, the bearing size should be selected on the basis of the basic static load rating C0. This is the load that corresponds to a certain stress level, according to ISO, at the centre of the most heavily loaded rolling element. Bearing Wherever there is rotation, there is a need for some form of bearing. The function of a bearing is to minimise the friction between moving machine parts and to carry a load. The majority of bearings nowadays are rolling bearings consisting of an inner ring, an outer ring, a number of rolling elements (balls or rollers) and a cage. Most bearings are made of steel, but other materials are also used, such as ceramics. In addition to rolling bearings, SKF produces spherical plain bearings, with different sliding contact surface combinations e.g. steel-on-steel or maintenance-free with special sliding layers and also magnetic bearings where friction is eliminated because the moving parts are separated by a magnetic field. The SKF standard product range comprises more than 5,000 variants covering all the principal bearing types. Bearing accessories Sleeves, nuts, spanners and washers for locking, sealing and spacing are called bearing accessories. Bearing components The bearing components are the inner and outer ring, the rolling elements and the cage. Bearing failure analysis SKF has the means for calculating the life of a bearing with considerable accuracy. If for various reasons however, a bearing does not attain its calculated life (see Bearing-failure), the failed bearing should be examined to find the cause of the damage. Corrective action can then be taken to prevent any recurrence.   Bearing failure due to electric current Passage of electric current across the rings and rolling elements damages the contact surfaces and the lubricant. A process similar to electric arc welding, with very high localised temperatures occurring, causes this damage. In the picture, the ball on the left has a dull surface due to multiple micro craters from passage of current. For comparison, an undamaged ball is shown on the right. Bearing failure Only a fraction of all bearings in use fail, and the main reasons are: - Poor lubrication - Contamination - Faulty mounting - Careless handling - Fatigue The picture shows flaking due to faulty mounting. The mounting force has passed through the balls and made indentations in the raceway, causing flaking. Bearing fatigue Fatigue cracks in the raceways can originate from sub-surface stress raisers like slag inclusions or from an indentation in the surface. As the rolling elements pass over the cracks, fragments of material break away and this is known as flaking. The flaking increases over time and eventually leads to bearing failure. The pictures show the gradual development of flaking. Bearing housing The structure in which a rolling bearing is mounted for protection and support. Often made of cast iron or plastic and used in many different applications, such as fans, papermaking machines etc. A wide range of standard bearing housings simplifies bearing arrangements. Bearing internal clearance The total distance that one bearing ring can be moved relative to the other in either the axial or radial direction. In the figure, the left shows radial clearance and the right, axial clearance. Bearing life Bearing basic rating life is defined as the number of revolutions a bearing is capable of enduring before the first signs of flaking occur on one of the rings or rolling elements.But there are also other ways to define bearing life: - Service life = Real life in operation - Specification life = Specified e.g. by the electric motor manufacturer Bearing basic rating life L10 = Millions of revolutions or operating hours at a given speed Bearing steel The steel from which rolling bearings are produced is vital to bearing quality, and SKF's Ovako Steel produces the cleanest steel in the world. The steel must be suitable for hardening, and have good fatigue strength and wear resistance. Precise analysis and freedom from inclusions are factors critical to the operational life of the bearing, and the structural and dimensional stability of the bearing components must be satisfactory at the operating temperatures which can be expected. See also www.ovako.com Bearing units Products in which the bearing has been integrated with other components in a single unit. The bearing is often greased for its entire lifetime. The picture shows a hub bearing unit for cars. Bearing precision classes Rolling bearings are precision products, but sometimes even higher than normal precision is required e.g. in machine tool spindles. For such applications, several higher precision tolerance classes are available according to SK17, ISO or ABEC specifications. (ABEC = Annular Bearing Engineers Committee). Examples of higher precision tolerance classes of equal value: SKF ISO ABEC PA9A ISO 2 ABEC 9 BEAST BEAring Simulation Tool, developed by SKF, is a very advanced computer-based 3D tool used to simulate different types of bearings. It replaces expensive and time consuming laboratory tests and is regarded as a virtual test rig. Bogie-monitoring system (BoMo) SKF and Sécheron have jointly developed a highly sophisticated bogie-monitoring system, BoMo. The axleboxes are equipped with compact and integrated multifunction sensors. The sensors for instance, monitor wheel condition and risk of derailment, and generate warning signals. BoMo See Bogie-monitoring system.   Brake-by-wire SKF and Brembo have jointly developed an electromechanical brake calliper which incorporates SKF actuators.The braking-system control is mounted on the Guida (see By-Wire Technology) and activated by squeezing handgrips. The mechanical design of the driver's braking controls incorporates progressive resistance and a small but clearly discernible free-play at the beginning of the movement. This provides the driver with a tactile indication that the brakes are starting to operate. Each brake is controlled as an individual sub-system under an overall control arrangement for the complete vehicle braking system. Prototype SKF and Brembo Brake-by wire calliper Brembo S.p.A Italian brake manufacturer collaborating with SKF in brake-by-wire development.   Bushings SKF now has an extensive range of bushings comprising many different types - from traditional solid bronze to maintenance-free SKF Filament Wound bushings. A wide stock assortment ensures prompt delivery. By-wire technology In by-wire systems, direct mechanical control of a machine is replaced by electronic control. The drive-by-wire system follows the fly-by-wire concepts used successfully by the aerospace industry. In conventional control, the movements the driver makes with the steering wheel are transmitted mechanically to the front wheels. In a by-wire system, the driver's physical movement on the steering wheel is sensed and converted into a digital electronic signal that is transmitted to a smart electro-mechanical actuation unit that controls the wheels. The same principle can be applied to the braking and gearbox systems. This is aircraft technology applied on automotive applications. See also Drive-by-wire. Guida - drivers control - with the brake-by-wire calliper, steering actuator and clutch actuator. Top of page