Bearings for Passenger Vehicles
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BALL and roller bearings are of interest to operators cf road transport, because the fundamental principle of their operation is identical with the basic feature on which road vehicles are designed, i.e., rolling motion. As the starting and tractive resistances of vehieles are reduced by road wheels, instead of the sleds of primitive times, so are the starting effort and running friction of a bearing reduced by the provision of rolling elements in place of metallic rubbing contact, or a film of lubricant which is often uncertain and rarely becomes effective until after a certain period of running.
As regards design, there is point contact in ball bearings and line contact in roller bearings. Under load, deformation of the surfaces occurs, the points becoming spots and the lines areas. The magnitude of the area of contact governs load capacity, so that a roller bearing can support ,heavier loads than a ball bearing of the same dimensions. The contact is small and the stresses high, frequently approaching the elastic limit. The metal must, therefore, be of intense hardness and of sufficient toughness to withstand the incalculable momentary overloads caused by shocks.
The rolling elements are usually caged, which is essential where silence is important. A cage is also necessary with hall bearings where the load is appreciable rI the speed high, to prevent grooves being worn in the balls by the rubbing of surfaces moving in opposite directions. Cylindrical roller bearings, on the other hand, can run satisfactorily without cages.
The most widely used type is the single-row ball journal with tracks conforming closely to the ball contour, so that the ball cannot accommodate appreciable errors of alignment, but can take some end thrust.
Self-aligning properties can he obtained in a double-row hearing by making the outer track spherical, whilst another type can have a spherical outside race and a correspondingly shaped seating in a separate ring.
Sometimes a roller bearing is selected for ease in mounting, but chiefly to obtain greater load-carrying capacity without increasing external dimensions. It is important, B30 for efficiency and life, that the rollers should be free to rotate on their axes without any tendency to skew Or creep endwise. When made without lips on one track, roller bearings cannot take end thrust, but they can with one or two lips on both races; as, however, the movement is sliding, a separate ball bearing should be provided if the speed be high or the end thrust heavy, but this type serves satisfactorily for the axles and wheels of buses and trolley buses.
A bearing with tapered tracks and rollers will support end thrust and journal load, whilst meeting the condition of maximum load capacity, but even a perpendicular load 'will cause thrust and press the roller large ends against the retaining lip on the inner race.
Double-row self-aligning roller bearings in which the crest of each barrel-shaped roller has a smaller radius than the spherical outer track, are also made. In Germany this
type is made with symmetrical rollers, but, as generally used here, they are .clesaxee and larger at one end than the
other. One type of bearing has rollers formed as hollow helices. Thus small errors of alignment can be accommodated by roller deflection, but load capacity is reduced. The needle bearing is the latest. It has no cage and its tracks are nearly full of long rollers, usually 2-3 ram. diameter.
Needle bearings do not function well unless there is appreciable slackness between tracks and rollers after
mounting. Hardened-steel end. plates keep the needles in
position. They may be loose plates or lips integral with the inner or outer races. Copious lubrication is required
—soft grease for low speed, but oil for high. The type has advantages over the older pattern for oscillating movements, or where the fluctuation of speed or load is continuous.
One type of ball bearing can deal with journal awl thrust loads in one direction, separately, or in combina tion. It resembles the ordinary ball bearing, but has greater diametric slackness, allowing the balls to make contact with the sides of the tracks, thus reducing the wedging effect of thrust load. One lip of the outer race is cut away to enable more balls to be inserted. This type must be used in pairs, the bearings being adjusted endwise against one another, unless there is continuous thrust in cite direction.
Other types similar in principle, but able to deal with thrust in either direction, are known as duplex, and can support journal load in combination with, thrust, provided that the former is less than the latter, but they cannot take journal loads alone, as the balls would slide down to tracks until they made contact at two points on both inner and outer races, when grinding would occur. This type is made with either the outer or inner races in two parts, and was developed by the Hoffmann Manufacturing Company about 1909, when it was known as the four-point bearing. It has become a useful type for rear axles.
Correct mounting of a bearing is as important as its quality or the selection of the suitable type for the conditions. The main principles are as follow : cleanliness and freedom from moisture ; freedom from direct hammer blows ; round seatings to avoid distortion; races square with axis of rotation (unless provision be made for self-alignment) ; rotating races for journal loads should be an interference fit on their seatings, stationary races can be a push fit ; races of bearings only for thrust can be a push fit; location in one direction to be effected by only one bearing.
Both grease and oil form satisfactory lubricants, but, usually, grease is preferable. It should have a mineral base with no free mineral acid, alkali or solid matter such as graphite, talcum or resin. The free fatty acid, moisture and ash contents must be low.
In gearboxes or axles, a thin disc is frequently fitted close to the revolving race, usually the inner, which acts
as a baffle and filter, preventing flooding with oil and the discharge of metallic particles or grit.
Roller bearings owe to pneumatic tyres their general adoption in the hubs of large commercial vehicles, and in the rear wheels, particularly to the use of twin tyres. Plain bearings were difficult to lubricate and in the event of punctures in the inner tyres the loads became overhung and seizures were of frequent occurrence.
Hub bearings usually contain cylindrical or tapered rollers. With the latter, endwise adjustment is permissible, but it carries the disadvantage that over-adjustment may be effected by untrained operators.
Wear due to dirt is recognized by the greyish matt lapped appearance of the tracks, and by indentations if the trouble be due to hard metallic particles, as from worn gear teeth. Corrosion may appear as rust stains or pitting. Creep may be recognized by the seating showing highly burnished marks, with corresponding markings on the shaft or housing. Lack of squareness in mounting results in a wobbly track, wear on the ends of rollers, and diagonally opposite markings on the lips of the stationary race.