Advanced Vehicle Technology 2 Episode 2 ppsx

the tractor unit, just sufficiently back to clear the rear tractor road wheels when the trailer is coupled and the combination is being manoeuvred (Fig. 1.28(a)). To provide additional support for the legs, bracing stays are attached between the legs and from the legs diagonally to the chassis cross- member (Fig. 1.28(b)). The legs consist of inner and outer high tensile steel tubes of square section. A jackscrew with a bevel wheel attached at its top end supported by the outer leg horizontal plate in a bronze bush bearing. The jawscrew fits into a nut which is mounted at the top of the inner leg and a taper roller bearing race is placed underneath the outer leg horizontal support plate and the upper part of the jackscrew to minimize friction when the screw is rotated (Fig. 1.28(b)). The bottom ends of the inner legs may support either twin wheels, which enable the trailer to be manoeuvred, or simply flat feet. The latter are able to spread the load and so permit greater load capacity. To extend or retract the inner legs, a winding handle is attached to either the low or high speed shaft protruding from the side of the gearbox. The upper high speed shaft supports a bevel pinion which meshes with a vertically mounted bevel wheel forming part of the jackscrew. Rotating the upper shaft imparts motion directly to the jackscrew through the bevel gears. If greater leverage is required to raise or lower the front of the trailer, the lower shaft is engaged and rotated. This provides a gear reduction through a com- pound gear train to the upper shaft which then drives the bevel pinion and wheel and hence the jackscrew. 1.6 Automatic chassis lubrication system 1.6.1 The need for automatic lubrication system (Fig. 1.29) Owing to the heavy loads they carry commercial vehicles still prefer to use metal to metal joints which are externally lubricated. Such joints are kingpins and bushes, shackle pins and bushes, steering ball joints, fifth wheel coupling, parking brake linkage etc. (Fig. 1.29). These joints require lubricating in proportion to the amount of relative movement and the loads exerted. If lubrication is to be effective in reducing wear between the moving parts, fresh oil must be pumped between the joints frequently. This can best be achieved by incorporating an automatic lubrication system which pumps oil to the bearing's surfaces in accordance to the distance travelled by the vehicle. 1.6.2 Description of airdromic automatic chassis lubrication system (Fig. 1.30) This lubrication system comprises four major com- ponents; a combined pump assembly, a power unit, an oil unloader valve and an air control unit. Pump assembly (Fig. 1.30) The pump assembly consists of a circular housing containing a ratchet operated drive (cam) shaft upon which are mounted one, two or three single lobe cams (only one cam shown). Each cam operates a row of 20 pumping units disposed radially around the pump casing, the units being connected to the chassis bearings by nylon tubing. Power unit (Fig. 1.30) This unit comprises a cylinder and spring-loaded air operated piston which is mounted on the front face of the pump assembly housing, the piston rod being connected indirectly to the drive shaft ratchet wheel by way of a ratchet housing and pawl. Oil unloader valve (Fig. 1.30) This consists of a shuttle valve mounted on the front of the pump assembly housing. The oil unloader valve allows air pressure to flow to the power unit for the power stroke. During the exhaust stroke, however, when air flow is reversed and the shuttle valve is lifted from its seat, any oil in the line between the power unit and the oil unloader valve is then discharged to atmosphere. Fig. 1.27 Ball and socket caravan/trailer towing attachment 32 Fig. 1.28 (a and b) Semi-trailer landing gear 33 Air control unit (Fig. 1.30) This unit is mounted on the gearbox and is driven via the speedometer take-off point. It consists of a worm and wheel drive which operates an air proportioning control unit. This air proportioning unit is operated by a single lift face cam which actuates two poppet valves, one controlling air supply to the power unit, the other controlling the exhaust air from the power unit. 1.6.3 Operation of airdromic automatic chassis lubrication system (Fig. 1.30) Air from the air brake auxiliary reservoir passes by way of the safety valve to the air control (propor- tioning) unit inlet valve. Whilst the inlet valve is held open by the continuously rotating face cam lobe, air pressure is supplied via the oil unloader valve to the power unit attached to the multipump assembly housing. The power unit cylinder is sup- ported by a pivot to the pump assembly casing, whilst the piston is linked to the ratchet and pawl housing. Because the pawl meshes with one of the ratchet teeth and the ratchet wheel forms part of the camshaft, air pressure in the power cylinder will partially rotate both the ratchet and pawl housing and the camshaft clockwise. The cam (or cams) are in contact with one or more pump unit, and so each partial rotation contributes to a proportion of the jerk plunger and barrel pumping cycle of each unit (Fig. 1.30). As the control unit face cam continues to rotate, the inlet poppet inlet valve is closed and the exhaust poppet valve opens. Compressed air in the air con- trol unit and above the oil control shuttle valve will now escape through the air control unit exhaust port to the atmosphere. Consequently the com- pressed air underneath the oil unloader shuttle valve will be able to lift it and any trapped air and oil in the power cylinder will now be released via the hole under the exhaust port. The power unit piston will be returned to its innermost position by the spring and in doing so will rotate the ratchet and pawl housing anti-clockwise. The pawl is thus Fig. 1.29 Tractor unit automatic lubrication system 34 Fig. 1.30 Airdromic automatic chassis lubrication system 35 able to slip over one or more of the ratchet teeth to take up a new position. The net result of the power cylinder being charged and discharged with com- pressed air is a slow but progressive rotation of the camshaft (Fig. 1.30). A typical worm drive shaft to distance travelled relationship is 500 revolutions per 1 km. For 900 worm drive shaft revolutions the pumping cam revolves once. Therefore, every chassis lubrication point will receive one shot of lubricant in this distance. When the individual lubrication pump unit's primary plunger is in its outermost position, oil surrounding the barrel will enter the inlet port, filling the space between the two plungers. As the cam rotates and the lobe lifts the primary plunger, it cuts off the inlet port. Further plunger rise will partially push out the secondary plunger and so open the check valve. Pressurised oil will then pass between the loose fitting secondary plunger and barrel to lubricate the chassis moving part it services (Fig. 1.30). 36 2 Friction clutch 2.1 Clutch fundamentals Clutches are designed to engage and disengage the transmission system from the engine when a vehicle is being driven away from a standstill and when the gearbox gear changes are necessary. The gradual increase in the transfer of engine torque to the transmission must be smooth. Once the vehicle is in motion, separation and take-up of the drive for gear selection must be carried out rapidly without any fierceness, snatch or shock. 2.1.1 Driven plate inertia To enable the clutch to be operated effectively, the driven plate must be as light as possible so that when the clutch is disengaged, it will have the mini- mum of spin, i.e. very little flywheel effect. Spin prevention is of the utmost importance if the vari- ous pairs of dog teeth of the gearbox gears, be they constant mesh or synchromesh, are to align in the shortest time without causing excessive pressure, wear and noise between the initial chamfer of the dog teeth during the engagement phase. Smoothness of clutch engagement may be achieved by building into the driven plate some sort of cushioning device, which will be discussed later in the chapter, whilst rapid slowing down of the driven plate is obtained by keeping the diameter, centre of gravity and weight of the driven plate to the minimum for a given torque carrying capacity. 2.1.2 Driven plate transmitted torque capacity The torque capacity of a friction clutch can be raised by increasing the coefficient of friction of the rubbing materials, the diameter and/or the spring thrust sandwiching the driven plate. The friction lining materials now available limit the coefficient of friction to something of the order of 0.35. There are materials which have higher coeffi- cient of friction values, but these tend to be unstable and to snatch during take-up. Increasing the diameter of the driven plate unfortunately raises its inertia, its tendency to continue spinning when the driven plate is freed while the clutch is in the disengaged position, and there is also a limit to the clamping pressure to which the friction lining material may be subjected if it is to maintain its friction properties over a long period of time. 2.1.3 Multi-pairs of rubbing surfaces (Fig. 2.1) An alternative approach to raising the transmitted torque capacity of the clutch is to increase the number of pairs of rubbing surfaces. Theoretically the torque capacity of a clutch is directly propor- tional to the number of pairs of surfaces for a given clamping load. Thus the conventional single driven plate has two pairs of friction faces so that a twin or triple driven plate clutch for the same spring thrust would ideally have twice or three times the torque transmitting capacity respectively of that of the single driven plate unit (Fig. 2.1). However, because it is very difficult to dissipate the extra heat generated in a clutch unit, a larger safety factor is necessary per driven plate so that the torque capacity is generally only of the order 80% per pair of surfaces relative to the single driven plate clutch. 2.1.4 Driven plate wear (Fig. 2.1) Lining life is also improved by increasing the number of pairs of rubbing surfaces because wear is directly related to the energy dissipation per unit area of contact surface. Ideally, by doubling the surface area as in a twin plate clutch, the energy input per unit lining area will be halved for a given slip time which would result in a 50% decrease in facing wear. In practice, however, this rarely occurs (Fig. 2.1) as the wear rate is also greatly influenced by the peak surface rubbing temperature and the intermediate plate of a twin plate clutch operates at a higher working temperature than either the fly- wheel or pressure plate which can be more effect- ively cooled. Thus in a twin plate clutch, half the energy generated whilst slipping must be absorbed by the intermediate plate and only a quarter each by the flywheel and pressure plate. This is usually borne out by the appearance of the intermediate plate and its corresponding lining faces showing evidence of high temperatures and increased wear compared to the linings facing the flywheel and pressure plate. Nevertheless, multiplate clutches do have a life expectancy which is more or less related to the number of pairs of friction faces for a given diameter of clutch. For heavy duty applications such as those required for large trucks, twin driven plates are used, while for high performance cars where very 37 rapid gear changes are necessary and large amounts of power are to be developed, small diameter multiplate clutches are preferred. 2.2 Angular driven plate cushioning and torsional damping (Figs 2.2±2.8) 2.2.1 Axial driven plate friction lining cushioning (Figs 2.2, 2.3 and 2.4) In its simplest form the driven plate consists of a central splined hub. Mounted on this hub is a thin steel disc which in turn supports, by means of a ring of rivets, both halves of the annular friction linings (Figs 2.2 and 2.3). Axial cushioning between the friction lining faces may be achieved by forming a series of evenly spaced `T' slots around the outer rim of the disc. This then divides the rim into a number of seg- ments (Arcuate) (Fig. 2.4(a)). A horseshoe shape is further punched out of each segment. The central portion or blade of each horseshoe is given a per- manent set to one side and consecutive segments have opposite sets so that every second segment is riveted to the same friction lining. The alternative set of these central blades formed by the horseshoe punch-out spreads the two half friction linings apart. An improved version uses separately attached, very thin spring steel segments (borglite) (Fig. 2.4(b)), pos- itioned end-on around a slightly thicker disc plate. These segments are provided with a wavy `set' so as to distance the two half annular friction linings. Both forms of crimped spring steel segments situated between the friction linings provide Fig. 2.1 Relationship of torque capacity wear rate and pairs of rubbing faces for multiplate clutch Fig. 2.2 Clutch driven centre plate (pictorial view) 38 progressive take-up over a greater pedal travel and prevent snatch. The separately attached spring segments are thinner than the segments formed out of the single piece driven plate, so that the squeeze take-up is generally softer and the spin inertia of the thinner segments is noticeably reduced. A further benefit created by the spring segments ensures satisfactory bedding of the facing material and a more even distribution of the work load. In addition, cooling between the friction linings occurs when the clutch is disengaged which helps to sta- bilise the frictional properties of the face material. The advantages of axial cushioning of the face linings provide the following: a) Better clutch engagement control, allowing lower engine speeds to be used at take-up thus prolonging the life of the friction faces. b) Improved distribution of the friction work over the lining faces reduces peak operating tempera- tures and prevents lining fade, with the resulting reduction in coefficient of friction and subse- quent clutch slip. The spring take-up characteristics of the driven plate are such that when the clutch is initially engaged, the segments are progressively flattened so that the rate of increase in clamping load is provided by the rate of reaction offered by the spring segments (Fig. 2.5). This first low rate take-up period is followed by a second high rate engage- ment, caused by the effects of the pressure plate springs exerting their clamping thrust as they are allowed to expand against the pressure plate and so sandwich the friction lining between the flywheel and pressure plate faces. 2.2.2 Torsional damping of driven plate Crankshaft torsional vibration (Fig. 2.6) Engine crankshafts are subjected to torsional wind-up and vibration at certain speeds due to the power impulses. Superimposed onto some steady mean rotational speed of the crankshaft will be additional fluctuating torques which will accelerate and decel- erate the crankshaft, particularly at the front pulley Fig. 2.3 Clutch driven centre plate (sectional view) Fig. 2.4 (a and b) Driven plate cushion take-up 39 end and to a lesser extent the rear flywheel end (Fig. 2.6). If the flywheel end of the crankshaft were allowed to twist in one direction and then the other while rotating at certain critical speeds, the oscillating angular movements would take up the backlash between meshing gear teeth in the transmis- sion system. Consequently, the teeth of the driving gears would be moving between the drive (pressure side) and non-drive tooth profiles of the driven gears. This would result in repeated shockloads imposed on the gear teeth, wear, and noise in the form of gear clatter. To overcome the effects of crankshaft torsional vibrations a torsion damping device is normally incorporated within the driven plate hub assembly which will now be described and explained. Construction and operation of torsional damper springs (Figs 2.2, 2.3 and 2.7) To transmit torque more smoothly and progressively during take-up of normal driving and to reduce torsional oscillations being transmitted from the crankshaft to the trans- mission, compressed springs are generally arranged circumferentially around the hub of the driven plate (Figs 2.2 and 2.3). These springs are inserted in elongated slots formed in both the flange of the splined hub and the side plates which enclose the hub's flange (Fig. 2.3). These side plates are riveted together by either three or six rivet posts which pass through the flanged hub limit slots. This thus provides a degree of relative angular movement between hub and side plates. The ends of the helical coil springs bear against both central hub flange and the side plates. Engine torque is therefore transmitted from the friction face linings and side plates through the springs to the hub flange, so that any fluctuation of torque will cause the springs to compress and rebound accordingly. Multistage driven plate torsional spring dampers may be incorporated by using a range of different springs having various stiffnesses and spring loca- tion slots of different lengths to produce a variety of parabolic torsional load±deflection characteris- tics (Fig. 2.7) to suit specific vehicle applications. The amount of torsional deflection necessary varies for each particular application. For example, with a front mounted engine and rear wheel drive vehicle, a moderate driven plate angular movement is necessary, say six degrees, since the normal trans- mission elastic wind-up is almost adequate, but with an integral engine, gearbox and final drive arrange- ment, the short transmission drive length necessit- ates considerably more relative angular deflection, say twelve degrees, within the driven plate hub assembly to produce the same quality of take-up. Construction and operation of torsional damper washers (Figs 2.2, 2.3 and 2.8) The torsional energy created by the oscillating crankshaft is partially absorbed and damped by the friction washer clutch situated on either side of the hub flange (Figs 2.2 and 2.3). Axial damping load is achieved by a Belleville dished washer spring mounted between one of the side plates and a four lug thrust washer. Fig. 2.5 Characteristics of driven plate axial clamping load to deflection take-up Fig. 2.6 Characteristics of crankshaft torsional vibrations undamped and damped 40 The outer diameter of this dished spring presses against the side plate and the inner diameter pushes onto the lugged thrust washer. In its free state the Belleville spring is conical in shape but when assembled it is compressed almost flat. As the fric- tion washers wear, the dished spring cone angle increases. This exerts a greater axial thrust, but since the distance between the side plate and lugged thrust washer has increased, the resultant clamping thrust remains almost constant (Fig. 2.8). 2.3 Clutch friction materials Clutch friction linings or buttons are subjected to severe rubbing and generation of heat for relatively short periods. Therefore it is desirable that they have a combination of these properties: a) Relatively high coefficient of friction under operating conditions, b) capability of maintaining friction properties over its working life, c) relatively high energy absorption capacity for short periods, d) capability of withstanding high pressure plate compressive loads, e) capability of withstanding bursts of centrifugal force when gear changing, f) adequate shear strength to transmit engine torque, g) high level of cyclic working endurance without the deterioration in friction properties, h) good compatibility with cast iron facings over the normal operating temperature range, i) a high degree of interface contamination toler- ance without affecting its friction take-up and grip characteristics. 2.3.1 Asbestos-based linings (Figs 2.2 and 2.3) Generally, clutch driven plate asbestos-based lin- ings are of the woven variety. These woven linings are made from asbestos fibre spun around lengths of brass or zinc wire to make lengths of threads which are both heat resistant and strong. The woven cloth can be processed in one of two ways: a) The fibre wire thread is woven into a cloth and pressed out into discs of the required diameter, followed by stitching several of these discs together to obtain the desired thickness. The resultant disc is then dipped into resin to bond the woven asbestos threads together. b) The asbestos fibre wire is woven in three dimen- sions in the form of a disc to obtain in a single stage the desired thickness. It is then pressed into shape and bonded together by again dip- ping it into a resin solution. Finally, the rigid lining is machined and drilled ready for riveting to the driven plate. Development in weaving techniques has, in certain cases, eliminated the use of wire coring so that asbestos woven lining may be offered as either non- or semi-metallic to match a variety of working conditions. Asbestos is a condensate produced by the solidi- fication of rock masses which cool at differential Fig. 2.7 Characteristics of driven plate torsional spring torques to deflection take-up Fig. 2.8 Characteristics of driven plate torsional damper thrust spring 41 [...]... diaphragm spring by the action of the pedal selfadjustment mechanism In consequence, there is no pedal free movement adjustment required (Fig 2. 22) 54 Fig 2. 19 Fully automatic centrifugal clutch 55 Fig 2. 20 Fully automatic centrifugal clutch characteristics Fig 2. 21 Hydraulic clutch operating system with over-centre spring When the pedal is released, the adjustment pawl is no longer engaged with the teeth... engagement is provided To facilitate gear changes whilst the vehicle is in motion, a conventional clutch release 2. 12 Semicentrifugal clutch (Figs 2. 17 and 2. 18) With this design of clutch lighter pressure plate springs are used for a given torque carrying capacity, making it easier to engage the clutch in the Fig 2. 17 Semicentrifugal clutch 53 Fig 2. 18 Semicentrifugal clutch characteristics lever arrangement... tonne and the maximum power produced by the engine amounts to 120 kW b) A fully laden articulated truck weighs 38 tonne and a 29 0 kW engine is used to propel this load 120 ˆ 100 kW/tonne a) Power to weight ratio ˆ 1 :2 290 b) Power to weight ratio ˆ ˆ 7:6 kW/tonne 38 3.1.4 Engine torque rise and speed operating range (Fig 3 .2) Commercial vehicle engines used to pull large loads are normally designed... pedal has been depressed approximately 25 mm of its travel, the over-centre spring change over point has been passed The over-centre spring tension is then applied as an assistance to the foot pressure Adjustment of the clutch is carried out by adjusting the pedal position on the master cylinder push rod 2. 14 .2 Clutch cable linkage with automatic adjuster (Fig 2. 22) The release bearing is of the ball... obtained between the bearing housing cover face and clutch brake i.e 9.5 mm for 355 mm 13 mm for 355 mm 13 mm for 29 4 mm Ð Ð Ð New engaged position (Fig 2. 14(a)) The spring thrust horizontal component of 2. 2 kN, multiplied by the lever ratio, provides a pressure plate clamping load of 13 .2 kN (Fig 2. 14(a)) The axial thrust horizontal component pushing on the pressure plate does not vary in direct proportion... pedal is pushed down, the fluid from the master cylinder is displaced into 2. 14.3 Clutch air/hydraulic servo (Fig 2. 23) In certain applications, to reduce the driver's foot effort in operating the clutch pedal, a clutch air/ hydraulic servo may be incorporated into the actuating linkage This unit provides power assistance 56 Fig 2. 22 Clutch cable linkage with automatic adjuster the servo hydraulic cylinder... the weight of the vehicle and any load it is expected to propel Consequently, the power developed per unit weight of laden vehicle has to be known This is usually expressed as the power to weight ratio i.e Ratio span ˆ Calculate the ratio span for both a car and heavy commercial vehicle from the data provided Type of vehicle km/h/1000 rev/min Top First 0.7 2. 9 39 9.75 Commercial vehicle (CV) There... plate clamping load 2. 14 Clutch pedal actuating mechanisms Some unusual ways of operating a clutch unit will now be described and explained 2. 14.1 Clutch pedal with over-centre spring (Fig 2. 21) With this clutch pedal arrangement, the overcentre spring supplements the foot pressure applied when disengaging the clutch, right up until the diaphragm spring clutch is fully disengaged (Fig 2. 21) It also holds... speed to be reduced whilst in top gear cruising, fuel 62 Lubrication to the mainshaft gears is obtained by radial branch holes which feed the rubbing surfaces of both mainshaft and gears Table 3.1 Typical four and five speed gearbox gear ratios Five speed box Four speed box Gear Ratio Gear top 4 3 2 1 R 0.8 1.0 1.4 2. 0 3.5 3.5 top 3 2 1 R 3 .2. 2 Five speed and reverse single stage synchromesh gearbox... shaft to the secondary sun gear 2. 13 Fully automatic centrifugal clutch (Figs 2. 19 and 2. 20) Fully automatic centrifugal clutches separate the engine from the transmission system when the engine is stopped or idling and smoothly take up the drive with a progressive reduction in slip within a narrow rising speed range until sufficient engine power is developed to propel the vehicle directly Above this speed . clutches are preferred. 2. 2 Angular driven plate cushioning and torsional damping (Figs 2. 2 2. 8) 2. 2.1 Axial driven plate friction lining cushioning (Figs 2. 2, 2. 3 and 2. 4) In its simplest form. 2. 2, 2. 3 and 2. 8) The torsional energy created by the oscillating crankshaft is partially absorbed and damped by the friction washer clutch situated on either side of the hub flange (Figs 2. 2. its friction charac- teristics and deterioration in wear resistance. 2. 3 .2 Asbestos substitute friction material (Figs 2. 2 and 2. 3) The DuPont Company has developed a friction material derived from