16 17 i 18 Lekcija Otkucana
Post on 12-Apr-2016
BRAKES AND SPRINGSBRAKES. Brakes, like clutches, depend upon the friction between two surfaces for their action, the difference being that clutches are used to keep the driving and driven members moving together, whereas brakes are used to stop a moving member or to control its motion by absorbing kinetic energy, i. e. by absorbing in friction the energy possessed by the moving car. In so doing they convert the energy into heat.There are two types of brakes, the drum brake and the disc brake. Either or both types may be fitted, but where both types are used it is usual for the disc brakes to be fitted to the front wheels.DRUM BRAKES. The drum brake consists of a pair of semicircular brake shoes mounted on a fixed back plate and situated inside a drum. This drum is fixed to the road wheel and rotates with it. One end of each shoe is on a pivot and a spring holds the other end in contact with the piston of a hydraulic cylinder. Each shoe is faced with material, known as brake lining, which produces high frictional resistance.The hydraulic system comprises a master cylinder and the slave cylinders, which are the cylinders on the road wheels. The slave cylinders are connected to the master cylinder by tubing and the whole sytem is filled with hydraulic fluid. A piston in the master cylinder is connected to the brake pedal, so that, when the driver depresses the pedal, the fluid is forced out to each slave cylinder and operates their pistons. The fluid pushes the pistons out of their cylinders. They, in turn, push against the inner ends of the brake shoes and force them against the brake drums in each wheel. We say that the brakes are on. This friction of the shoes against the drums, which are fixed to the road wheels, slows down or stops the car. As the brake pedal is allowed to come up, the hydraulic fluid returns to its original position, the pistons retract, and a spring attached to each brake shoe returns it also to its original position, free of the brake drum. Now we say that the brakes are off.Drum brakes are prone to a reduction in the braking effort, known as fade, caused by the overheating of the linings and the drum. However, fading is unlikely to occur except after the brakes have been used repeatedly in slowing the car from a high speed or after braking continuously down a steep hill. Descending such a hill, it would have been preferable to use engine braking by changing down into a lower gear. Drum brakes can be made less prone to fade by improving the cooling arrangements, by arranging for more air to be deflected over them, for example.DISC BRAKES. The disc brake consists of a steel disc with friction pads operated by slave hydarulic cylinders. The steel disc is attached to the road wheel and rotates with it. Part of this steel disc is enclosed in a caliper. This caliper contains two friction pads, one on each side of the disc, and two hydraulic cylinders, one outside each pad. The pads are normally held apart by a spring, but when driver depresses the brake pedal, pistons from the hydraulic cylinders force the pads against the sides of the disc. Because the disc is not snclosed all the way round, the heat generated when the brakes are applied is dissipated very much more quickly than it is from brake shoes which are entirely enclosed inside a drum. This means that disc brakes are less prone to fade than drum brakes. SPRINGS. Springs are devices used a) as cushions to absorb shock, as in automobile springs, machine supports or airplane landing gear; b) as a source of power, as in clocks or watches, where energy is stored up in them and later delivered as driving power; or c) to provide a force to maintain pressure between contacting surfaces, as in friction clutches and brakes.There are various types of springs for various applications. Compression springs are more desirable fos heavy loads than extension springs, because of the possibility of stress concentration in the loop of the extension spring. Compression springs can, however, be employed for tensile loading. Conical coil springes, if properly designed, may be compressed flat under load. Disc springs represent a recent development that is being extensively employed for heavy loads. Laminated or leaf springs are used in vehichels of varyous types, although coil springs are now being used in automotive applications. Coil springs may be made of square, rectangular, or round wire.
BEARINGSA BEARING is a mechanical term used to denote that part of a machine which bears the friction occasioned when parts are in contact and have relative motion. Bearings are employed to support, guide, and restrain moving elements. They may be classified as bearings for rotating and oscillating elemnts, and as bearings for reciprocating elemnts. Bearings for rotating or oscillating elemnts may be further classified as PLAIN BEARINGS ( also called sliding bearings ) and ANTIFRICTION BEARINGS ( also called rolling bearings ).PLAIN BEARINGS, as distinguished from the rolling bearings to be discussed later, are usually classified as plain journal ( of sleeve ) bearings and plain thrust bearings. Plain journal bearings ( henceforth journal bearings ) support a load in the radial direction. Plain thrust bearings ( henceforth thrust bearings ) take a thrust load in the direction of the axis of the shaft.A journal bearing is composed of two essential parts, the journal, which is the inner cylindrical or conical partand which usually rotates, and bearing or surrounding shell, which may be stationary, as in the case of lineshaft bearings, or moving, as in a connecting rod bearing. The simplest form of journal bearing embodies a shaft rotating in a hole in a frame or bracket. For this reason, bearing holes are generally supplied with sleeves or bushings so that a comparatively inexpensive replacement is possible. The contac surfaces are lubricated through oil holes in the bushing.Thrust bearings are used for axial restraint, and are very important where heavy axial loads may prevail, as in vertical hydraulic turbogenerator stes and worm gearing. In such instances, a collar on the rotating shaft rests on a series of freely pivoting segments attached to the stationary bearing. The rotation of the collar induces a wedge-shaped oil film between its lower surface and the topof the segments to provide thick-film lubrication. For light thrust loads, one or more thrust washers, restarined by separate collars or integral shoulders on the shaft, are statisfactory.ANTI-FRICTION BEARINGS. Ball and roller bearings are known as anti-friction bearings, and have certain adventages over journal bearings.The basic ball bearing consists of four parts: 1. the outer ring, 2. the inner ring, 3. the balls, and 4. the separator ( that is, ball retainer ). The separator serves the purpose of always keeping the balls separated and thereby preventing them from rubbing against each other. Ball bearings are categorized into three areas, namely, 1. radial ball bearings, which support a load at right angles to the shaft axis, 2. thrust ball ebarings, which take a trust load in the direction of the axis of the shaft, and 3. angular-contact ball bearings, which are designed to take a combination of radial and thrust loads, and should be used in pairs unless the load is pure thrust.Roller bearings serve the same purpose as ball bearings, but they can support much higher loads than comaparably sized ball bearings because they have line contact instead of point contact. Most types of radial roller bearings cannot resist thrust loads of any significant magnitude and, with the exception of the cylindiral type, opetare at speeds lower than those for ball bearings. Careful consideration must be given to the lubrication of roller bearings, although many can function adequately under limited speed and load combinations with only a periodic greasing.Roller bearings can be classified into four basic types: 1. cylindrical roller bearings, with rollers whose diameters are approximately aqual to their lengths. 2. needle roller bearings, having cylindrical rollers of small diameter and considerable length, and operating without a cage or retainer ( They occupy very little diametral space in relation to their load-carrying capacitiy, and are therefore coming into extensive use in gear mountings, and as piston pin bearings in large internal combustion engines, 3. tapered roller bearings, which are extensively used for machine tool and automotive applications, and are capable of taking heavy unidirectional thrust loads in addition to large radial loads, and 4. spherical roller bearings.
VISCOSITY, LUBRICATION AND METHODS OF LUBRICATION( Bearing Maintenance)A major problem faced by engineers through the years has been how to prevent the loss of useful energy due to friction. It has been estimated that of all the energy produced throughout the world as much as one third to one half is wasted because of friction. FRICTION may be defined as the opposing force that is developed when two surfaces move relative to each other. LUBRICATION has then been ddefined as the art of reducing the friction developed between two surfaces moving relative to each other. This is usually done by placing a substance between the moving surfaces. Although the material that is called a lubricant is usually in the liquid state, solids and gases are also used as lubricants.One of the fundamental problems involved in lubrication theory is the effect of the internal resistance of the fluid being used as a lubricant. The expression for this internal resistance, called VISCOSITY, can be obtained by applying Newton's law of viscous flow to the system shown in Figure 18-1. In the figure, the lower plate is stationary, while the upper plate is moving to the right with a velocity U. The two parallel plates are separated by a lubricant film of thickness h.When two plates, having relative motion, are separated by a lubricant (in this case an oil film), a flow of oil takes place. In most lubrication problems, conditions are such that the flow that occurs is laminar. By LAMINAR FLOW we mean that the fluid is in layers or laminae, which are maintained as the flow progresses. When this condition is not met, we speak of the flow as being TURBULENT.Under conditions pf laminar flow, we may assume that perfect adhesion is achievable. In other words, the lamina immediately adjecent to the moving plate moves with the same velocity as the plate, whereas that immediately adjecent to the stationary plate has zero velocity. The intermediate laminae move with velocities that vary linearly from 0 to U. Because the laminae have different velocities, each layer must slide upon the adjecent layer. A force, F, is required to produce this sliding. The resistance the fluid offers to this force, is known as the shear stress, which in turn causes fluid friction.There are a variety of mathods used to lubricate bearings. The method chosen for a specific problem depends to a large extent upon the type of service the bearing is to perform and also how important the bearing is to the system of which it is a part.Bearings that are to be used in low-speed light-load appliciations can be lubricated by the HAND OILING METHOD. A good general rule to follow is that this method should be used only if no other alterantive is available, and then only if the bearing plays a relatively minor rule in the operation of the system it is a part of.The WICK-FEED OIL METHOD is better than the hand oiling method because of he more uniform supply of oil.The DROP-FEED OIL METHOD, in which oil from a reservoir flows through a needle valve to the bearing, is better than the hand oiling method because of a more uniform rate of flow. However, it does have the danger of the valve clogging, if any impurities are present in the oil.