CH01 Bearings

  • View
    222

  • Download
    0

Embed Size (px)

Text of CH01 Bearings

  • 8/6/2019 CH01 Bearings

    1/27

    Ch.#01

    B E A R I N G S I N T R O D U C T I O N

    BEARINGS1. INTRODUCTION1.1 What is a Bearing?A bearing is a component which supports and locates a rotatingshaft, or which guides one component as it slides over another.

    Furthermore, bearings are generally designed to permit relativemotion with minimal friction.

    1.2 Types of BearingBearings can easily be classified into either rolling element orplain types. A rolling element bearing contains a number of rollingelements (either balls or rollers) and these form the mechanicallink between the relative moving surfaces. A plain bearing has noadditional moving parts and relies upon large area surface

    contact to permit sliding of one surface over another. Specialmaterials are used for the bearing surfaces to reduce friction andwear. Within these two categories exist many further sub-categories based upon construction, load direction and operatingconditions.

    Bearings can also be classified according to the direction of theload that they carry. A journal bearing supports a rotating shaftand constrains radial motion; primarily it carries radial load. Athrust bearing constrains axial shaft motion, and carries primarily

    axial load. A linear bearing permits relative motion in a straightline, and carries primarily perpendicular load.

    2 THE BASICS OF ROLLING ELEMENT BEARINGS2.1 The Operation of Rolling Element BearingsA rolling element bearing is comprised of two races, betweenwhich a number of rolling elements (balls or rollers) is held. Therolling elements are themselves separated by a soft metal or

    polymer cage, which ensures that they are evenly spaced aroundthe races. The area of contact between the rolling elements andthe races is very small, and hence the frictional resistance to

    1

    Chapter

    1

  • 8/6/2019 CH01 Bearings

    2/27

    motion of this type of bearing is also small, which is why they aresometimes called anti-friction bearings.

    In contrast to a plain bearing, where the relative motion at thebearing faces is sliding, the relative motion in the case of a rolling

    element bearing is rolling. This underlies the need for verydifferent construction materials. Rolling element bearings aremade of exceptionally hard bearing steels. This is because theharder the material, the less the deformation caused by thecontact forces, hence the less the area of contact, and hence theless the friction. Even though the principle of operation is one ofrolling, some sliding does still occur, and for this reasonlubrication of rolling element bearings is an important as it is forplain bearings.

    2.2 Types of Rolling Element BearingA fundamental distinction in types of rolling element bearing iswhether or not the bearing uses balls or rollers. Ball bearings tendto be used for lower load, higher speed, unidirectional loadapplications, whereas roller bearings tend to be used for higherload, lower speed, multi-directional load applications. In general,ball bearings are cheaper than rolling element bearings, theyoffer lower frictional resistance, and they are more widelyavailable.

    The operational and performance distinctions between ball androller bearings all stem from the differences in contact area thatthey have between rolling elements and races. In theory, aspherical ball on a circular raceway has a single point of contact,whereas a circular section roller on a circular raceway has a lineof contact. This means that the load carried by a roller bearing isspread over a greater area than that of a ball bearing, giving it agreater load carrying and impact resistant capability, but also agreater frictional resistance than an equivalent ball bearing.Many type of ball and roller bearings exist. Simplest distinctioncomes from whether they are intended for axial, radial or multi-directional load transmission. Bearings intended for transmittingprimarily radial load are referred to as journal bearings, whereasthose for primarily axial load are termed thrust bearings.Over a dozen different types of journal bearings exist, rangingfrom the general purpose single row deep groove ball bearing, tothe specialist self-aligning spherical roller bearing. Similarly,thrust bearings range from single row ball bearings to spherical

    roller bearings.The selection of an appropriate bearing is itself awhole field of study, but primary factors in the decision-making

  • 8/6/2019 CH01 Bearings

    3/27

    Ch.#01

    B E A R I N G S I N T R O D U C T I O N

    process include the relative ability to support radial and/or axialload, the tolerance to misalignment, the speed range ofoperation, and the available methods of location. There exists afurther specialist category of rolling element bearing called the

    linear bearing, and again this represents a whole field in itself.2.3 Constraint of Rolling Element BearingsIt is normal practice that a rolling element bearing is mounted sothat its rotating race is a press fit into housing or onto a shaft,and its stationary race is a sliding fit. The specification of the fitsto use can be found in bearing handbooks and internationalstandards.

    To permit thermal expansion during operation, without imposingundesirable axial load on a bearing, it is normal practice on a two-

    bearing shaft to have one bearing fixed axially and the other freeto move.

    Bearings are normally fixed to a shaft with shaft nuts and tabwashers or with slotted nuts and split pins. Larger bearings maybe fixed to a shaft with an end plate. Other possibilities includethe use of an end cap, a circlip or a tapered adaptor / withdrawalsleeve.

    3 ROUTINE MAINTENANCE OF ROLLING ELEMENTBEARINGS3.1 Best Practice

    The majority of best practice measures amount to little more thangood common engineering sense. They include appropriatestorage and handling, maintenance of cleanliness, and pre-installation lubrication.

    It is important when mounting and dismounting bearings that thecorrect tools are used and that they are in good condition.Makeshift arrangements or badly worn tools are likely to lead todamage to the bearing or bearing assembly. Bearings should bewrapped in oil-proof paper when not in use. Only clean solventsand flushing oils should be used for bearing cleaning. Beforeinstalling a rolling element bearing both the housing and shaftshould be carefully inspected for burrs, nicks and scratches thatmay interfere with the fitting of the bearing. Bearings that are dryand unlubricated or have not been cleaned should not be spun. Ifcompressed air is used to clean a bearing care should be taken to

    avoid spinning the bearing. Cotton waste or dirty rags should notbe used to clean bearings, only clean, lint free rags.

    3

  • 8/6/2019 CH01 Bearings

    4/27

    The slushing compound used to protect a bearing in storage neednot be removed if it is petroleum based unless it has gone hard orbecome contaminated. The force applied when mounting ordismounting a bearing should always be applied to the ring with

    the interference fit - it should never be applied in such a way thatthe force is transmitted through the rolling elements! Never strikea bearing directly with a hammer or mallet or with a soft metaldrift. Remember that all sealed and shielded bearings must bemounted cold. Bearings should never be heated with a nakedflame.

    3.2 Tools and equipmentThe maintenance of rolling element bearings requires specialisedtools and equipment without these many tasks are extremelydifficult. Mounting dollies and sleeves - When mounting bearingswhich are a press fit it is important that pressure is appliedevenly. If the bearing is cocked due to uneven force then surfacescan be damaged and the bearing ring distorted. A set of mountingdollies and sleeves will ensure that bearings are driven homeevenly. Dollies and sleeves can be purchased directly from thebearing manufacturers, or can be made up to suit a particularapplication.

    Hammers should be made from steel or soft material and shouldbe free from burrs. Copper or synthetic resin is suitable materialsfor soft hammers but lead and tin should not be used. Woodenmallets should not be used because of splinters. Only steel driftsshould be used and they should only be used for bending orstraightening tabs on locking washers or for driving shaft nuts. Anarbor press ensures that bearings are driven evenly especiallywhen the fit is tight. It should be used in preference to othermethods where practicable. Bearing pullers are essential for

    dismounting. There are several types available. Pullers should bemaintained in good condition, so that they operate freely and theclaws are free of burrs. When handling large, heavy bearings it isimportant to have suitable lifting gear available such as tongs orslings.

    Hook and impact spanners are needed for use with adaptersleeve nuts, withdrawal nuts and shaft nuts. Modern inductionheaters are much cleaner and easier to use than an oil bath whenbearings have to be heated for a shrink fit. When induction

    heaters are not available, the traditional oil bath should be used.Various types of gauges are needed for measuring diametricalclearances, housing bores and shaft diameters. Ordinary feeler

  • 8/6/2019 CH01 Bearings

    5/27

    Ch.#01

    B E A R I N G S I N T R O D U C T I O N

    gauges are sufficient for clearances but a special set of boregauges may be required for measuring the bore of housings.Alternatively, standard inside and outside micrometers can beused for measuring diameters. The listed tools are the minimum

    required for bearing maintenance. Where bearings are mountedon tapered shafts or on sleeves, hydraulic equipment may also berequired.

    Where shafts have been provided with oil ducts, hydraulicpressure can be used for mounting an