The Motor Vehicle 2010 Part 13 pptx

865Universal joints and driving steered wheels members A are secured to the shafts that are connected by the joint and carry bushes B. These are positioned by the projecting lips C of the yokes, which fit machined portions of the bushes and by the keys D integral with the bushes and which fit in keyways formed in the yokes. The keys D transmit the drive and relieve the set screws E of all shearing stresses. The two yokes are coupled by the cross member F which consists of a central ring portion and four integral pins G. The ends of the latter bear on the bottoms of the bushes B thus centring the joint. Between the pins G and the bushes B are needle bearings H. The hole in the centre of the cross member F is closed by two pressings J and forms a reservoir for lubricant which reaches the bearings through holes drilled in the pins G. The cork washers K form a seal at the inner ends of the pins and also serve to retain the needle rollers when the joint is taken apart. A single filling of oil suffices for practically the whole life of the joint. Any excessive pressure in the reservoir which might lead to oil being forced out past the seals K is prevented by the relief valve L. Fig. 30.3 shows a ring-type joint. The member A is bolted to one shaft by its flange and the fork B is secured to the other shaft by splines. The two members are coupled by the ring C. This ring is made of two steel pressings each forming half the ring and being bolted together by the nuts on the trunnion portions of the four bushes D whose shape is clearly seen in the separate plan view. The pins of the fork member B fit in two of the bushes and the ends of the pin E, which is fixed in member A, fit in the other two bushes. The space inside the ring forms a reservoir for oil which may be introduced through a nipple not shown. The joint between the two halves of the ring is ground to form an oil-tight joint and escape of oil at the points of entry of the pins is prevented by the compressed cork washers F. The shafts D G H B K A D J F L A C B C A E D X X Y Y A C B Fig. 30.1 Hooke’s universal Fig. 30.2 Cross-type universal joint joint 866 The Motor Vehicle A G D C B F H C F D E G Fig. 30.3 Ring-type universal joint are centred relatively to the ring by reason of the fitting of the pins on faces G accurately machined inside the ring, and not by the cork washers. The nuts securing the ring are locked by a pair of tab washers H. Another universal joint construction is shown in Fig. 30.4. It consists of a ball A having two grooves formed round it at right angles. In these grooves the forked ends of the shafts E and F fit. Obviously when the joint is put together the shaft E can slide round in its groove, thus turning about the axis XX. Similarly the shaft F can slide round in its groove, thus turning about the axis YY. This type of joint was used at one time in front wheel brake linkages. The arrrangement of the shaft bearings has to be such that the shafts cannot move away from the centre of the ball, otherwise the joint would come apart. It is not suitable for use in the transmission. 30.2 Flexible-ring joints A joint that acts by the flexure of a flexible ring is much used to connect shafts between which the angular displacement will not be very great. Such a joint is shown in Fig. 30.5. The shafts are provided with three-armed X X Y Y F E A Fig. 30.4 Fig. 30.5 867Universal joints and driving steered wheels C D E A B D A spiders, the arms of which are bolted to the opposite faces of a flexible ring, the arms of one spider being arranged mid-way between the arms of the other. The flexible ring is usually made of one or more rings of rubberised fabric made in a special way so as to provide the necessary strength. A number of thin steel discs are sometimes used instead of the fabric rings. When the shafts are revolving about axes which are not coincident there is a continuous flexing of the ring. This type of joint has several advantages over the universal joints described above, the principal of which are the elimination of the need for lubrication and cheapness of manufacture. The joint connot cope with such large angular displacements as the universal joints and when the torque to be transmitted is large it becomes very bulky. 30.3 Rubber-bushed flexible joints Joints of this kind are now widely used and there are several forms of them, three being shown in Figs 30.6 to 30.8. The first of these is the original Layrub; it is basically a ring type of joint. The shafts that are connected by the joint carry the two-armed spiders A and B projecting from which are bolts carrying special rubber bushes C. These bushes are housed inside the coupling ring D which is made of two exactly similar steel pressings bolted together. The rubber bushes by distorting slightly enable any misalignment of the shafts to be accommodated. Angular misalignments up to 15° can be allowed for but generally the misalignment is limited to about half that amount. The joint can also accommodate a considerable amount (up to 12.5 mm) of axial movement of the one shaft relative to the other and when two of them are used, one at each end of a propeller shaft, it is usually possible to dispense with the sliding joint that is essential when all-metal joints are used. Fig. 30.7 Fig. 30.6 868 The Motor Vehicle Since the only connection between the shafts is through the rubber bushes, the joints also assist in smoothing out vibrations; this property has been used to give a flexible clutch plate in a single-plate clutch the driven plate being connected to the clutch centre by four Layrub bushes. The bushes are made with concave ends as shown in order to keep the internal stresses in them approximately uniform and to increase their flexibility. They are made with a metallic gauze insert on their insides and are forced on to the sleeves E which are made somewhat larger than the holes in the bushes. The outside diameters of the bushes are also greater than the diameters of the pockets in the ring D in which the bushes are housed and so when the coupling is assembled the bushes are compressed to such an extent that although when the joint flexes the distance between the sleeve E and the ring D may increase on one side the rubber remains in compression and is never in tension. The sleeves E have spigots which fit into holes in the spiders so that the bolts are not called upon to transmit the torque and are not subjected to any shearing. Figure 30.7 shows the very effective Metalastik unit in which the rubber bushes are bonded to the spherical pins and are compressed when the two metal pressings which form the ring of the joint are assembled together. These pressings are held together by spinning the lips of one of them over those of the other. The design in Fig. 30.8 is basically a cross-type joint and is made by the Moulton company. The rubber bushes are bonded on the inside of the tapered portions of the arms of the cross and on the outside to steel shells. The latter fit into depressions formed in the flanges of the joint and are held in place by stirrups which are bolted up to the flanges. 30.4 Constant-velocity joints The Hooke’s type of universal joint suffers from a disadvantage which is obviated in some other types of joint. It is that supposing one of the shafts AA α α α α 2 α (a) (b) Fig. 30.8 Fig. 30.9 S S 869Universal joints and driving steered wheels connected by a Hooke’s joint is revolving at an absolutely constant speed then the other shaft will not revolve at a constant speed but with a speed that is, during two parts of each revolution, slightly greater and, during the other two parts of the revolution, slightly less than the constant speed of the first shaft. The magnitude of this fluctuation in speed depends on the angle between the axes of the two shafts, being zero when that angle is zero but becoming considerable when the angle is large. This disadvantage becomes of practical importance in front wheel driven vehicles and in the drives to independently sprung wheels where the angle between the shafts may be as large as 40°. It can be obviated by using two Hooke’s joints arranged as shown in Fig. 30.9(a) and (b), the intermediate shaft being arranged so that it makes equal angles with the first and third shafts and the fork pin axes of the intermediate shaft being placed parallel to each other. The irregularity introduced by one joint is then cancelled out by the equal and opposite irregularity introduced by the second joint. Examples of front wheel drives using this arrangement are shown in Figs 30.16 and 30.17. A slightly different arrangement using the same priniciple is given in Fig. 37.9. Constant-velocity joints are joints which do not suffer from the above disadvantage but in which the speeds of the shafts connected by the joint are absolutely equal at every instant throughout each revolution. Although such joints have been known for very many years they have not been used to any extent until relatively recently. The Tracta joint, manufactured in England by Bendix Ltd, is shown in Fig. 30.10, from which the construction will be clear. The joint is a true constant-velocity joint but the theory of it is beyond the scope of this book and those who are interested in this theory and in those of the joints described below, are referred to an article by one of the authors in Automobile Engineer, Vol. 37, No. 1 Another true constant-velocity joint, the Weiss, which is used to a considerable extent in America, where it is manufactured by the Bendix Products Corporation, is shown in Fig. 30.11. It consists of two members each with two fingers or arms in the sides of which are formed semi-circular grooves. When the two members are assembled the fingers of the one fit in between the fingers of the other and balls are inserted in the grooves of the fingers and form the driving connection between them. The formation of the Fig. 30.10 Bendix Tracta universal joint 870 The Motor Vehicle grooves is such that the balls lie always in a plane making equal angles with the axes of the shafts connected by the joint, this being a fundamental condition that must be satisfied if the drive is to be a constant-velocity drive. This joint has the property that the shafts connected by it may be moved apart axially slightly without affecting the action of the joint and this axial motion is accommodated by a rolling of the balls along the grooves in the fingers of the joint members and so takes place with the minimum of friction. A third example is shown in Fig. 30.12. It is the Rzeppa (pronounced Sheppa) and it consists of a cup member A with a number of semi-circular grooves formed inside it and a ball member B with similar grooves formed on the outside. Balls C fit half in the grooves A and half in B and provide the driving connection. For true constant-velocity operation the balls must be arranged to lie always in a plane making equal angles with the axes of rotation of the members A and B. This is ensured by the control link D and the cage E. The former has spherical ends one of which engages a recess in Fig. 30.11 Bendix Weiss universal joint D G F E B C X X X X A B θ 2 θ Fig. 30.12 Rzeppa universal joint Fig. 30.13 Birfield constant-velocity universal joint A 871Universal joints and driving steered wheels the end of the member B while the other is free to slide along a hole formed inside A; the link is kept in place by the spring F. The spherical enlargement G of the link engages a hole formed in the cage E which has other holes in which the balls C fit. When the shaft B swings through an angle relatively to A the link D causes the cage E and the plane XX of the balls C to swing through half that angle and thus the balls are caused to occupy the required positions for the correct functioning of the joint. In some designs of this joint, intended for use where the angular deviation of the shafts is small, the control link D is omitted. A joint developed by Birfield Transmissions Ltd which gives constant- velocity ratio transmission and allows for a plunging motion of one of the shafts relative to the other is shown in Fig. 30.13. The inner member is grooved to carry the balls that transmit the motion and its outer surface is ground to a sphere whose centre is at the point A. The balls are housed in recesses in the cage and this is ground on its inside to fit the outer surface of the inner member while its outer surface is ground to a sphere whose centre is at the point B. The outer member has a cylindrical bore with grooves formed in it to take the balls and the outer spherical surface of the cage fits the cylindrical surface of the outer member. The inner member can therefore move bodily along the bore of the outer member thus giving the plunging motion required in the drives to most independently sprung wheels and which usually has to be provided by sliding splines. The off-setting of the centres of the spherical surfaces of the cage keeps the plane of the balls at all times in the plane bisecting the angle between the shaft axes as is necessary for the maintenance of a constant-velocity ratio. 30.5 Driving and braking of steered wheels Various methods of driving a steered wheel are shown in Fig. 30.14. In the examples (a) and (b) a rigid driven axle is assumed, but in the others independent suspensions are shown. The arrangement at (a) is the simplest, a single universal joint U being provided to accommodate the steering motion of the stub axle S. Unless this joint is of the constant-velocity type, there will be an S H U A U 1 C U 2 A B U 1 U 1 R R (a) (b) (c) (d) (e) S U 2 U 2 Fig. 30.14 I A 872 The Motor Vehicle B C D L X E A F M Y E F irregularity in the drive to the wheel whenever the stub axle is turned for steering purposes while, if the wheel is given any camber and the wheel shaft A is inclined to the half-shaft H, the irregularity will always be present. This constructional arrangement was adopted by the Four Wheel Drive Company in their lorries, which were among the earliest four-wheel-driven vehicles, and although the details of this arrangement are obsolete the general arrangement still represents current practice. An ordinary live axle is used so far as the final drive, differential and axle casing surrounding those components are concerned, but at their outer ends A, Fig. 30.15, the drive shafts are carried in bushes and are forked to form one member of a Hooke’s type universal joint. The other shaft of this joint is seen at B and conveys the drive to the hub cap of the road wheel. The hub is carried on bearings on the stub- axle member, which is made in three pieces D, E and F bolted together as shown in the right-hand view. The inner spherical surfaces of the portions E and F touch the corresponding surfaces of the end of the axle casing in order to make the housing oil tight and to exclude mud and dust, but those surfaces do not carry any loads. E and F are carried on the projecting swivel pins of the axle casing. In the arrangement shown in Fig. 30.14(b), two universal joints are used and are symmetrically disposed relative to the king-pin axis OO. When the stub axle is turned about that axis for steering purposes, the angles between the intermediate shaft I and the steel shaft A and half-shaft H respectively will be maintained equal as in Fig. 30.9, and so a constant-velocity drive will be obtained. An example of this construction is shown in detail in Fig. 30.16. It is the design of the Kirkstall Forge Engineering Company and incorporates a second reduction gear which is housed in the wheel hub. This second reduction is between the pinion which is splined to the end of the shaft A and the annulus C which forms the hub cap of the road wheel and is bolted to the hub of the latter. The intermediate pinions B are carried on pins D, which are supported in the member E. The latter fits the cylindrical extension of the stub axle and a key prevents rotation. Because the intermediate member coupling the two universal joints is rigid, and the forks of the joints are rigidly attached to the half-shaft H and wheel shaft A respectively, one of Fig. 30.15 Details of the FWD stub axle 873Universal joints and driving steered wheels these shafts must be left free to float axially. This will be seen from Fig. 30.19, in which the full lines show the position when the wheels are in the straight-ahead position, and the dotted lines the position when the stub axle is turned for steering. It is clear that the distance X 1 Y 1 is less than the distance XY. This variation is accommodated by leaving the shaft A, Fig. 30.16, free to float. It is therefore carried in a parallel roller bearing at the right end and is supported by the contacts with the three pinions B at the left end. The omission of a bearing at the left end ensures equal division of the driving torque between the three pinions. The example shown in Fig. 30.14(c) is a conventional double-arm type of suspension, in which a stub axle carrier C connects the two arms. The drive shaft S is provided with universal joints U 1 and U 2 . The first of these accommodates the steering motion of the stub axle and, in conjunction with the second, allows for the vertical motion of the wheel assembly. Because the distance between the centres of the universal joints cannot be kept constant, the shaft S must be provided with some axial freedom. This is usually done by leaving one of the universal joint forks free to slide on the splines of its shaft. Obviously, U must be a constant-velocity joint. In the example shown in Fig. 30.14(d), the stub-axial carrier is omitted and the stub axle is carried directly by the arms RR, to which it is connected by ball and socket joints which accommodate the steering motion as well as the vertical motion of the road wheel. The joint U 2 now has to be supported from the stub axle through the joint U 1 , and the construction of a joint which provides this support is shown in Fig. 30.17. The joint is made by the Glaenzer Spicer Company, of Poissy, France. The forks A and B, integral with their Fig. 30.16 Kirkstall steered axle B D E C A E H 874 The Motor Vehicle Fig. 30.17 Glaenzer Spicer axle shafts, are coupled by four-armed spiders and an intermediate member C. The shaft B is supported relative to A by the ball and socket DE. The ball D is free to slide along the spigot shaft F, and the socket E is integral with the spigot G. The connection keeps the two universal joints and the intermediate member in the correct relationships to provide a constant-velocity drive, as described above. Figure 30.14(e) shows a swinging-arm type of independent suspension, in which the arm A which carries the stub axle is pivoted to the final drive casing B on the axis O. Two universal joints are necessary; one (U 1 ) to accommodate the steering motion and the other to allow for the swinging of the arm. The arrangement does not provide a constant-velocity drive unless both the joints are of the constant-velocity type. The casing B is carried by the frame of the vehicle. In the arrangement shown in Fig. 30.18 there is a gear reduction between the driveshaft and the road wheel. This makes the speed of rotation of the driveshaft higher than that of the wheel and reduces the torque the universal joint has to transmit. The driveshafts are more exposed and difficult to protect from mud and dust but, being higher than the axle, are more out of the way of damage from the striking of obstacles. The arrangement is only very occasionally used on special types of vehicle. B C A B C X X 1 Y Fig. 30.18 Fig. 30.19 Fig. 30.20 A G A C E D B F [...]... rotate the spider relative to the differential cage it causes the ends of the pins to ride up the flanks of the V-shaped slots in which they seat This forces outwards the two parts of the cage and thus applies pressure axially to the clutch plates, which are splined alternately to the differential cage and the bosses of the differential gears Consequently, the greater the applied torque, the tighter is the. .. wheels on the other On the other hand, if rear wheel spin occurs where both inter-axle and rear traction control differentials are fitted, the lost traction is transferred to the other wheel on the same axle, the front wheels continuing to transmit their 882 The Motor Vehicle share of the drive However, if either front wheel spins, all the traction is transferred to the rear axle This layout is particularly... tangential to the discs as shown, then if the beam does not turn about the vertical axis, or if it turns about that axis with uniform velocity, the forces at the ends of the beam must be equal and each will be equal to P/2 The reactions of the forces acting on the ends of the beam act on the discs, hence equal forces are applied to the discs at equal distances from their axes, and therefore the twisting... are splined on to the shafts which drive the road wheels Of course, the gear ratio between D1 and E is the same as the ratio between D2 and F The action of the differential is just the same as in the conventional axle, but the reduction of speed now occurs between the differential and the road wheels instead of between the propeller shaft and the differential cage The differential therefore runs at... simply by the suspension springs 32.5 Driving thrust Again, according to Newton’s third law of motion, the driving thrust, or tractive effort, of the road wheels is reacted by the vehicle structure, the reaction being the inertia of the mass of the vehicle if it is accelerating, or rolling resistance of the other axle plus the wind resistance if it is not – the rolling resistance of the tyres of the driving... between the teeth of the bevel pinion and the bevel wheels 878 The Motor Vehicle 31.1 Another arrangement of the bevel final drive The bevel final drive is sometimes arranged in a different manner from that described previously, generally because some other difference in axle construction necessitates the change The principle of this other method is shown in Fig 31.3 The propeller shaft is coupled to the. .. rear-wheel-drive Scorpio, on the other hand, the gearing is such that 66% of the drive goes to the rear axle and only 34% to the front, so the rear wheels are the more likely to spin If this happens all the torque is transmitted to the front wheels On this model the viscous coupling is on the rear end of the gearbox, and the torque is transmitted through a multi-strand toothed chain drive to a shaft,... tangential to the two differential gears, and if the beam either does not swing, or swings at a uniform velocity, it follows that the forces at the ends of the beam will be equal to P/2 Hence, equal forces are applied at equal distances from the centres of the differential gears, and therefore the torques they transmit to the halfshafts are equal Clearly, the force P represents the pressure between the differential... the other wheel is spinning Alternatively, if the traction potential of the previously effective wheel is exceeded, the load on the coupling is again relieved because it, too, will spin If, on the other hand, neither of these reliefs comes into operation, the engine stalls and the load on the coupling is once more effectively shed 890 The Motor Vehicle There are two ways of installing this device in... regards the equality of the torques, the torque on the wheel A is due to the pressure of the teeth of the pinion E1 This pressure tends to make the pinion E1 revolve on its pin, and this tendency is opposed by a pressure between the teeth of the two pinions at the centre This last pressure tends to make the pinion F1 revolve on its pin, and this tendency is opposed by the pressure between the teeth of the . integral pins G. The ends of the latter bear on the bottoms of the bushes B thus centring the joint. Between the pins G and the bushes B are needle bearings H. The hole in the centre of the cross member. splines. The off-setting of the centres of the spherical surfaces of the cage keeps the plane of the balls at all times in the plane bisecting the angle between the shaft axes as is necessary for the. pressure between the bevel pinion and its pin while the forces P/2 appear as pressures between the teeth of the bevel pinion and the bevel wheels. 878 The Motor Vehicle 31.1 Another arrangement of the bevel