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1. AN INTRODUCTION:-STEERING SYSTEMS IN AUTOMOBILES

A vehicle is not much use if it cannot be steered or guided. The act of guiding the vehicle in the intended direction is called steering. Wheeled vehicles are steered by aiming or pointing the wheels in the direction we want the vehicle to go. The driver of a car or truck guides it by turning the steering wheel. The steering system of cars and trucks consists of levers, links, rods, and a gearbox and sometimes a hydraulic system that assists the driver's steering effort.The steering system is of critical importance in the safe operation of the vehicle. There must be no looseness between the steering wheel and the front wheels if the driver is to keep control over the direction the wheels point. The tires must meet the road at the correct angle to get good traction and to prevent unnecessary tire wear. Also, the driver should be able to hold the wheels in the straight-ahead position and change them to the right or left with very little effort.The steering wheel of a car is one of those instruments which help us achieve the required operation.

1.1. BASIC STEERING COMPONENTS Figure 1.199% of the world's car steering systems are made up of the same three or four components. The steering wheel is connected to the steering column which is connected to the track rod, which connects to the tie rods, which connect to the steering arms. The steering system can be one of several designs, but all the designs essentially move the track rod left-to-right across the car. The tie rods connect to the ends of the track rod with ball and socket joints, and then to the ends of the steering arms, also with ball and socket joints. The purpose of the tie rods is to allow suspension movement as well as an element of adjustability in the steering geometry. The tie rod lengths can normally be changed to achieve these different geometries.

1.2. STEERING SYSTEM CONDITION

Figure 1.2In the simplest form of steering, both the front wheels always point in the same direction When a car goes around a corner, the outside wheels travel further than the inside wheels. In the case of a transmission, it's why you need a differential, but in the case of steering, it's why you need the front wheels to actually point in different directions.In order for that to happen without causing undue stress to the front wheels and tires, they must point at slightly different angles to the Centre line of the car. The diagram to the left shows the same thing only zoomed in to show the relative angles of the tires to the car. It's all to do with the geometry of circles. This difference of angle is achieved with a relatively simple arrangement of steering components to create a trapezoid geometry (a parallelogram with one of the parallel sides shorter than the other). Once this is achieved, the wheels point at different angles as the steering geometry is moved. Most vehicles now don't use 'pure' Ackermann steering geometry because it doesn't take some of the dynamic and compliant effects of steering and suspension into account, but some derivative of this is used in almost all steering systems.Another steering mechanism is Davis steering mechanism. This is not to be discussed in our scope.

1.3. STEERING RATIOSEvery vehicle has a steering ratio inherent in the design. If it didn't you'd never be able to turn the wheels. Steering ratio gives mechanical advantage to the driver, allowing to turn the tyres with the weight of the whole car sitting on them, but more importantly, it means you don't have to turn the steering wheel a ridiculous number of times to get the wheels to move. Steering ratio is the ratio of the number of degrees turned at the steering wheel vs. the number of degrees the front wheels are deflected. So for example, if you turn the steering wheel 20 and the front wheels only turn 1 that gives a steering ratio of 20:1. For most modern cars, the steering ratio is between 12:1 and 20:1. This, coupled with the maximum angle of deflection of the wheels gives the lock-to-lock turns for the steering wheel. For example, if a car has a steering ratio of 18:1 and the front wheels have a maximum deflection of 25, then at 25, the steering wheel has turned 25x18, which is 450. That's only to one side, so the entire steering goes from -25 to plus 25 giving a lock-to-lock angle at the steering wheel of 900, or 2.5 turns (900 / 360).This works the other way around too of course. If you know the lock-to-lock turns and the steering ratio, you can figure out the wheel deflection. For example if a car is advertised as having a 16:1 steering ratio and 3 turns lock-to-lock, then the steering wheel can turn 1.5x360 (540) each way. At a ratio of 16:1 that means the front wheel deflect by 33.75 each way.For racing cars, the steering ratio is normally much smaller than for passenger cars that is closer to 1:1 - as the racing drivers need to get fuller deflection into the steering as quickly as possible.

1.4. STEERING SYSTEM DESIGNS: RACK AND PINION

Figure 1.3This is by far the most common type of steering you'll find in any car today due to it's relative simplicity and low cost.In a rack and pinion system, the track rod is replaced with the steering rack which is a long, toothed bar with the tie rods attached to each end. On the end of the steering shaft there is a simple pinion gear that meshes with the rack. When you turn the steering shaft using the steering wheel, the pinion gear turns, and move the rack from left to right. Changing the size of the pinion gear alters the steering ratio. Thus we are implementing the variable steering ratio mechanism with the rack and pinion.

1.5. VARIABLE STEERING RATIO MECHANISMA variable-ratio steering, is a system that uses different ratios on the rack, in arack and pinionsteering system. At the center of the rack, the spaces between the teeth are smaller and the space becomes larger as the pinion moves down the rack. In the middle of the rack you'll have a higher ratio and the ratio becomes lower as you turn the steering wheel towards lock. This makes the steering less sensitive, when the steering wheel is close to its center position and makes it harder for the driver tooversteerat high speeds. As you turn the steering wheel towards lock, the wheels begins to react more to your steering input.Some variable steering mechanisms say a disadvantage of a fixed-ratio system is that towards the lock positions, more effort is needed by the driver. This is because the angle of the steering arms reduces their effective length, and that reduces the leverage on the wheels. To overcome this, many rack-and-pinion systems use variable ratio steering. The ratio is made variable by changing the shape of the teeth on the rack, between the centre and the outer edges of the rack. Then, as the steering moves away from the straight-ahead position, the ratio, and therefore, the mechanical advantage, increases progressively. As the pinion turns, and moves on the rack, the gear contact point between the pinion, and the teeth on the rack, changes. This change in tooth contact changes the effective diameter of the pinion. Then, for the same amount of steering wheel rotation, the rack move s a shorter distance near the ends of the rack than near the centre. Effort needed to turn the wheels stays approximately the same through the whole range of movement.The variable steering ratio we are dealing with doesnt deals with changing the profile of the rack gears at the rack ends to make it more sensitive. We will use a altogether a different concept i.e. an introduction of steering gear box in between to alter the steering ratios according to the drivers needs and condition requirement.

2.LITERATURE REVIEW

A lot of work has been done till now in this field, obviously to attain variable steering ratio depending upon various conditions. Some have used just different teeth ratio while others using certain other mechanisms. Some of these attempts have been described below

2.1. RACK AND PINION VARIABLE RATIO GEAR

INVENTOR Arthur E. Bishop (1971) ABSTRACTA variable ratio steering mechanism for a vehicle having an axially movable rack meshing with a helical pinion the axis thereof making an angle with the axis of the said rack, the rack having a group of teeth at its center of varying form and varying inclinations with respect to the axis of the rack, the inclination of the teeth of said group becoming less closely aligned with the pinion axis as the teeth are more remote from the center of said rack, said teeth thereby meshing with the teeth of the pinion at varying effective pitch radii in a predetermined manner.

BACKGROUND OF THE INNOVATIONThe present invention relates to a variable ratio steering mechanism of the rack and pinion type. The characteristics and advantages of rack and pinion steering are well known, as are the advantages of providing a variable steering ratio and it is therefore unnecessary to explain these in the present specification. Considerable problems have, however arisen in devising a practical construction which incorporates variable ratio steering in a mechanism of the rack and pinion type.

SUMMARY

Figure 2.1It is proposed in the present invention to use a substantially concentrically mounted helical pinion the teeth of which are substantially identical in shape meshing with a rack having teeth some at least of which are such as to provide a variable steering ratio, it being preferred that the majority of the teeth of the rack provide a low steering ratio and be of constant section across their width, a small group only of teeth at the center of the rack being of warped configuration to provide, in cooperation with the helical pinion, a change of steering ratio from a high ratio to the low ratio.The use of a helical pinion permits the variation of the steering ratio by as much as two to one and whereas a non-helical pinion under these circumstances would give discontinuous tooth action in the low ratio region at either end of the rack and would result in poor strength of the teeth in the high ratio region, by the use of a helical pinion it is possible to design the teeth at the center of the rack so as to have adequate strength. Although this results in the use of a pressure angle in the teeth towards the ends of the rack of about 60, an angle which would be entirely unacceptable for a straight out pinion, this is acceptable here, due to the fact that the resulting discontinuous tooth action can be accommodated by a helical pinion, since tooth engagement is staggered across the width of the rack.If the helical pinion fails to span the coarse low ratio teeth of the rack for some parts of its rotation this will occur only in one part of the width of the rack and full engagement will occur across the remainder of the width. Furthermore, tooth engagement being staggered across the width of the rack compensates for the variations of efficiency in rack tooth action.While the use of a helical pinion has the advantages referred to above, its use does present some problems in manufacture of the rack.An examination of the problems has, however, shown that teeth of suitable form can be generated, for example, by the use of a cutter which is an identical facsimile of the pinion, a technique well known in the manufacture of geared pairs. The difficulties and expense of the method can be considerably reduced by designing the rack so that the majority of its teeth can be formed by a straightforward broaching process, only a small number of teeth near the middle of the rack having to be generated. The design may be such that it is possible to broach teeth of the form used at the ends of the rack over the whole length of the rack and thereafter modify the teeth at the center of the rack to the required form, broaching of the central teeth would, of course, be to only a relatively shallow depth. This reduces the use of the more elaborate generating technique required for these teeth to a minimum. So this invention claims a variable ratio rack and pinion steering mechanism having an axially movable rack meshing with a helical pinion the axis of which makes an angle with the axis of the rack, the rack being characterized by having a group of teeth at its center said teeth being of varying forms and varying inclinations with respect to the axis of the rack, the inclination of the teeth of said group nearest the center being the most closely aligned with the axis of the pinion, the inclination of teeth of said group becoming less closely aligned with the pinion axis and more closely perpendicular to the axis of the rack as they move away from the center of the rack, the inclinations and forms of said group of teeth being associated with engagement of the said teeth with the pinion at varying effective pitch radii, the effective pitch radius being least at the center of the rack and increasing on either side thereof.

2.2. VARIABLE GEAR RATIO STEERING DEVICE

INVENTOR - Yasuo Shimizu. Saitama-ken. Japan (1995)

ABSTRACT

In a variable gear ratio steering device comprising a rack and pinion gear mechanism. the rack of the rack and pinion gear mechanism is provided with tooth pitch distribution for producing a variable rack gain property in relation to a steering input which includes, in each rotational sense. a low fixed rack gain region near a neutral position. a high fixed gain region in a large steering input region, and a progressively increasing rack gain region connected to the low fixed rack gain region via a first point of inflection and to the high fixed rack gain region via a Second Point of transition from a fixed low rack gain region to a region of a the play of the steering mechanism. the vehicle operator would not be affected by the presence of the point of inflection because as soon as the steering wheel is turned beyond the play or the dead zone near the neutral position of the steering wheel the point of inflection has already been passed. Another point of inflection or the point of transition between the region of the progressively increasing rack gain and the fixed high rack gain region also would not affect the vehicle operator because it is l bocated well outside the range of normal operation in a medium to high speed range. And it will not be noticeable in a low speed range even When the steering angle passes this point

BACKGROUND OF THE INVENTION

Figure 2.2Figur shows a conventional rack and pinion steering mechanism for motor vehicles which is provided with an electric motor for reducing the effort required for turning the steering wheel. In this electric power steering device. The steering wheel I is integrally attached to a steering shaft 2, and a lower end of the steering shaft 2 is connected to a pinion 4 via a connecting shaft 3 including a pair of universal joints 3a and 3b. The pinion 4 meshes with a rack 5 extending laterally of the vehicle body and the rack 5 is connected at its two ends to a pair of front wheels 7 via tie rods 6 and knuckle arms, respectively. Thus according to this steering device, the rotational movement of the steering wheel 1 is converted into the lateral linear movement of the rack 5 which is in turn converted into the steering movement of the front wheels 7. The electric motor 8 for providing an assisting torque is provided with a hollow rotor through which the rack 5 is passed. and actuates a ball screw mechanism 9 extending along the rack 5 which converts the rotational movement of the output shaft of the electric motor 8 into a linear movement of the rack5. More specifically the rotor of the electric motor 8 is integrally provided with a helical drive gear 80 which meshes with a helical driven gear 9b integrally attached to an axial end of a screw shaft 9a of the ball screw mechanism 9. The nut of the ball screw mechanism is connected to the rack 5. Inside the steering gear box is provided a steering torque detector 10 including a torsion bar for detecting the manual steering torque applied to the pinion 4. and the output signal from the steering torque detector 10 is supplied to a control unit 11 which controls the electric motor 8 according to this output signal. This conventional steering device consists of a variable gear ratio steering device whose effective gear ratio changes according to the magnitude of the steering input. More specifically in a normal range of operation corresponding to a medium to high vehicle speed range (involving. For instance no more than 150 degrees of rotation of the steering shaft 2 on either side of a neutral position). the displacement of the rack 5 is relatively small for a given steering input. Or the rack gain is relatively low. 0n the other hand, in a low vehicle speed range where a large steering input is often necessary, the displacement of the rack for a given steering input is increased and the rack gain is increased. Such a conventional variable gear ratio steering device can be achieved, for instance, by using a rack and pinion mechanism in which the rack teeth are formed as given in

Figure 2.3Figure In this rack the pitch of the rack teeth is progressively increased as the distance from the neutral position is increased.

Figure 2.4In FIG the pinion 4 in the neutral position is indicated by imaginary lines while the pinion 4 placed away from the neutral position is indicated by solid lines. The rack gain of a rack and pinion mechanism is defined as a displacement of the rack for each turn of the pinion. Thus according to this conventional variable gear ratio steering device the point of inflection at which the rack gain moves on from a region of a fixed level R1 to a region of a progressive increase occurs at the steering angle of 61 which is typically :40 degrees away from the neutral position which falls into the range of normal operation (i150 degrees from the neutral position). This scheme is based on the consideration to achieve a uniform handling of the steering device in the region near the neutral position, and smoothly connect this region of a small rack gain to the region of a large rack gain. Thus, according to this conventional device. it is possible to achieve a low rack gain in the region of small steering input and a high gain in the region of large steering input or, in other words. a stable handling in a medium to high speed range and a favorable maneuverability in a low speed range can be achieved at the same time. Therefore. when a relative large steering maneuver is executed at a medium to high speed range the steering device may be steered beyond the point of inflection. In this case as the steering angle passes the point of inflection. the vehicle operator inevitably experiences a sudden change in the steering reaction of the steering device, and the yaw movement of the vehicle body, and may not feel very comfortable. To eliminate such a problem. it may be conceivable to abolish the region of a fixed rack gain near the neutral position and place the point of inflection right on the neutral position. However in reality achieving such an arrangement in a rack and pinion steering device has been considered, at least, impractical because of various restrictions imposed on the fabrication of the gear mechanism. And the need for some amount of tolerance in the dimensional precision of the gear members. In particula an offset in the neutral position of the steering device is highly perceptible to the vehicle operator. and a region of a fixed rack gain of a certain width has been therefore considered necessary to make any such offset acceptable to the vehicle operator. Furthermore, when such a conventional variable gear ratio steering device is used on a FF (front engine front drive. i.e. a front wheel drive,) vehicle which tends to have an under steer tendency. the vehicle operator needs to turn the steering wheel by a larger angle than anticipated and may experience a discontinuous impression so that the vehicle operator may feel somewhat uncomfortable. By taking an example of a FF vehicle the steering properties of the conventional variable gear ratio steering device are compared with those of the conventional fixed gear ratio steeringDevice. In other words the conventional steering device produces an excessive under steer tendency which is not desirable for the comfort of the vehicle operator.

SUMMARY

In view of such problems of the prior art. a primary object of the present invention is to provide a variable gear ratio steering device which can provide a uniform steering property regardless of the vehicle speed while maintaining the advantages of a variable gear ratio steering device. A second object of the present invention is to provide a variable gear ratio steering device which can provide uniform steering property regardless of the vehicle speed without complicating the fabrication process. A third object of the present invention is to provide a variable gear ratio steering device which can control the under steer tendency of the vehicle. According to the present invention, such objects can be accomplished by providing a variable gear ratio steering device comprising: a pinion functionally coupled to a steering wheel; and a rack meshing with the pinion; the rack having a tooth pitch distribution for producing a variable rack gain property in relation to a steering input which includes in each rotational sense. a low fixed rack gain region near a neutral position a high fixed rack gain region in a large steering input region. and a progressively increasing rack gain region connected to the low fixed rack gain region via a first point of inflection and to the high fixed rack gain region via a second point of inflection; the first point of inflection being located within a first angle at the neutral position of an amount within a play in an overall steering system of a vehicle on which the steering device is installed For instance the first point of inflection may be located within ten degrees of the rotational angle of the steering wheel from the neutral position. For the convenience of fabrication. the first point of inflection may be located five degrees away from the neutral position in terms of the steering input in view of the difficulty involved in fabricating the variable gear ratio rack and pinion gear mechanism. Thus, the inconvenience of passing the first point of inflection dining a sensitive maneuver in a medium to high speed range can be avoided. Typically the present invention is applied to a power steering device which includes torsion bar means for detecting a steering torque applied to the steering wheel and actuating an actuator for the power steering device according to a magnitude of a twisting angle of the torsion bar means, and the first point of inflection is located within a certain angular displacement of the steering wheel away from the neutral position which is smaller than a maximum twisting angle of the torsion bar means. The second point of inflection is located outside a range of normal operation in a medium to high speed range, and would not interfere with the operation of the vehicle in a medium to high speed range. The second point of inflection will be encountered in a low is so stable and insensitive that the vehicle operator would not experience any inconvenience. The play mentioned above may include any combination of a play in a gear mechanism of the steering device. A play in a mechanical linkage in a torque transmitting path of the overall steering system an elastic deformation of parts involved in a torque transmitting path of the overall steering system and a twisting deformation of vehicle tires. In a front engine front drive vehicle which often has an under steer tendency. The present invention is particularly useful because the virtual absence of any fixed low rack gain region near the neutral position prevents an insufficiency in the steering angle of the tires when the vehicle makes a turn in a medium to high speed range.

2.3. VARIABLE GEAR RATIO TYPE RACK BAR AND STEERING APPARATUS FOR VEHICLESINVENTORS Kwang Ho YANG, Ki Che Park (2011)ABSTRACTDisclosed are a variable gear ratio type rack bar, and a steering apparatus having the same. According to the present invention, it is not required to fabricate a separate mold for processing rack-formed parts with different inter-tooth spaces, the rack-formed parts can be formed by using various processing methods, and it is possible to obtain precise tooth shapes through one step processing, Figure 2.5

BACKGROUND OF THE INVENTION

Field of the InventionThe present invention relates to a variable gear ratio type rack bar and a steering apparatus having the same, and more particularly to a variable gear ratio type rack bar, and a steering apparatus for a vehicle including such a variable gear ratio type rack bar, the rack bar having two or more rack-formed parts with different inter-tooth spaces, wherein the rack-formed parts make it needless to fabricate a separate mold for processing them, the rack-formed parts can be formed by using various processing methods, and the rack-formed parts make it possible to obtain precise tooth shapes through one step processing. Figure 2.6Description of the Prior ArtFigure. 1 schematically shows the construction of a conventional rack-and-pinion type steering apparatus for a vehicle. As shown in the drawing, the conventional rack-and-pinion type steering apparatus includes: a steering wheel 100 positioned at a driver seat side; a steering shaft 106 connected to the steering wheel; a steering column 103 adapted to allow the steering shaft to be fastened to a vehicle body; a gear box 130 including a rack 110 and a pinion 120 for converting torsional force input from the steering shaft 105 to rectilinear movement; a rack bar 140, on which the rack 110 is formed; and a pair of tie rods 150 integrally formed with balls of inner ball joints 135, respectively, the inner ball joints being provided at the opposite ends of the rack bar 140, respectively, and the tie rods 150 being connected with outer ball joints 155, respectively, so that power is transmitted to knuckles 159 so as to steer tires 158.In general, a steering apparatus for a vehicle is an apparatus for allowing a driver to change the running direction of a vehicle in accordance with the driver's intention while the vehicle is being run, wherein as shown in FIG. 1, the steering apparatus includes: a steering wheel capable of being rotated by the driver; a steering shaft having an end coupled to the steering wheel to be rotated together with the steering wheel; a pinion provided at the lower end of the steering shaft and positioned within a gear box, the pinion being rotated together with the steering shaft; and a rack bar, on which a rack is formed to be engaged with the pinion, the rack bar conducting rectilinear movement in accordance with the rotation of the pinion. With the steering apparatus configured as described above, as the steering angle of the steering wheel is increased, torque applied to the pinion is increased due to the tires' friction and the geometry of the vehicle, wherein in order to reduce the torque applied to the pinion, a hydraulic power steering system increases hydraulic pressure, and an electric power steering system increases the power of a motor.In general, the angle for manipulating the steering wheel for a vehicle is typically limited within 90 while the vehicle is running on a road, and the steering wheel may be manipulated over 90 only when parking or U-turning the vehicle. However, since the gear ratio of the rack and pinion of such a steering apparatus is determined to be constant, inconvenience to necessarily manipulate the steering wheel to a large extent is accompanied at the time of parking or U-turning. In addition, since in increasing hydraulic pressure or motor power for reducing torque applied to a pinion, there was a limitation as well as a problem of inefficiency, variable gear ratio (VGR) systems have been developed in order to solve these problems. Here, a VGR system means a system in which the gear ratio of a pinion, to which steering wheel manipulating force is input, and a rack cooperating with the pinion is variable in such a manner that the input angle at the opposite side parts of the rack differs from that at the central part of the rack, so that when the manipulating extent of the steering wheel is large, the moving distance of the rack is increased, thereby reducing the required manipulating extent of the steering wheel at the time of parking or U-turning, which helps a driver to conveniently drive the vehicle.The above-mentioned VGR system is a system having a rack, the shape of which is modified in the following manner: inter-tooth spaces of the teeth of the rack are increased as approaching to the opposite ends from the central part of the rack, so that as the steering angle of the steering wheel is increased, the moving distance of the rack is also increased, thereby reducing the torque applied to the pinion. Such a conventional VGR steering system has problems in that when forming rack-formed parts on a rack bar body using a bobbing machine or a broaching machine, it is impossible to process the rack-formed parts with different inter-teeth spaces at once, and even if the rack-formed parts are processed through plural separated steps, the processing error of the rack-formed parts become severe, which in turn causes the precision of the rack to be deteriorated. In addition, if a forging process is employed so as to process such rack-formed parts at once, there are problems in that the precision is deteriorated in terms of the shape and size of the rack, which in turn causes a driver's steering feeling to be deteriorated when the driver manipulates the steering wheel. Furthermore, if a precise forging process is employed for securing the precision in terms of the shape and size of the rack, there are problems in that the length of production time is substantially increased, and the manufacturing cost is very high.

SUMMARYAccordingly, the present invention has been made to solve the above-mentioned problems occurring in the prior art, and an object of the present invention is to provide a variable gear ratio type rack bar having two or more rack-formed parts with different inter-tooth spaces, wherein the rack-formed parts make it needless to fabricate a separate mold for processing forming-parts having inter-tooth spaces, the rack-formed parts can be formed by using various processing methods, and the rack-formed parts make it possible to obtain precise tooth shapes through one step processing, whereby it is possible to reduce the number of steps for manufacturing a various gear ratio type rack bar and the manufacturing cost of such a various gear ratio type rack bar.In order to accomplish this object, there is provided a variable gear ratio type rack bar including: two or more rack-formed parts on a side of a rack bar body, the rack-formed parts having different inter-tooth spaces, wherein the rack-formed parts are continuously arranged, and at least one of the rack-formed parts is formed separately from the rack bar body to have a predetermined thickness from a rack-formed side's end, and then coupled to the rack bar body.In accordance with another aspect of the present invention, there is provided a variable gear ratio type rack bar including: three rack-formed parts on a side of a rack bar body, the rack-formed parts having different inter-tooth spaces, wherein the rack-formed parts are continuously arranged, the rack-formed parts consist of a variable rack-formed part arranged at the central area, and basic rack-formed parts arranged at the opposite sides of the variable-rack-formed part, and the variable-rack-formed part is formed separately from the rack bar body to have a predetermined thickness from a rack-formed side's end, and then coupled to the rack bar body.In accordance with another aspect of the present invention, there is provided a steering apparatus for a vehicle including a steering wheel, a steering shaft connected to the steering wheel, and a variable gear ratio type rack bar for converting rotational force into rectilinear movement, the rotational force being input to the rack bar from the steering shaft while changing the extent of manipulating the steering wheel, wherein the variable gear ratio type rack bar includes: two or more rack-formed parts on a side of a rack bar body, the rack-formed parts having different inter-tooth spaces, wherein the rack-formed parts are continuously arranged, and at least one of the rack-formed parts is formed separately from the rack bar body to have a predetermined thickness from a rack-formed side's end, and then coupled to the rack bar body.According to the present invention as described above, it is not required to fabricate a separate mold for processing rack-formed parts with different inter-tooth spaces, the rack-formed parts can be formed by using various processing methods, and it is possible to obtain precise tooth shapes through one step processing, whereby the number of steps for manufacturing a variable gear ratio type rack bar and its manufacturing cost can be reduced.

3. Our Aim and Project3.1. What we are going to do and how we are going to do? Single rack-pinion assembly. Design a unique gearbox mechanism with 3 or more gear ratios.

3.2.Basic design problems and their solutions Design rack and pinion gear using involute profile helical gears. Design gear sets of gear box which must be spur gears. Design a mechanism to move the lay shaft in the gearbox and also a locking mechanism for it. Design a gearbox case to accommodate the shafts, gears and locking mechanism. Design the steering wheel and mesh it with the lay shaft using spur gear which can be slided. Design and install universal joints between steering wheel and pinion gear to provide flexibility. Design and install ball joints at the connecting rods at both the ends of the rack to provide for movement of suspension. Design the complete mechanism in solid works and finalize material for different parts of the mechanism. Do analysis and simulation of the mechanism in solid works.

3.3. Basic fabrication problems Gear cutting for spur gear on milling machine. Gear cutting for helical gears on hobbing machine. Fabrication of rack and connecting rods. Fabrication of the gear box case. Assembly of all the components.

Figure 3.1 -Basic design of differential ratio steering

3.4. How the mechanism is going to work?As shown in figure there is a gearbox between the steering wheel and the rack pinion steering mechanism. Earlier the degree of rotation of pinion gear was the same as the degree of rotation of the steering wheel. Now this can be changed by putting a gearbox between them.The steering wheel is attached to a long spur gear with length L3 equal to 4X as shown in the figure. Now there are two shafts as shown in the design. The shaft to the right is known as lay shaft and is having two degrees of freedom. One is it can slide in the vertical direction and the second is it can rotate about the axis of the lay shaft.The shaft to the left is known as drive shaft and it is having only one degree of freedom. It can only rotate about the axis of the drive shaft. The steering wheel and the shaft attached to it also have only one degree of freedom to rotate about the axis passing through the steering wheel.Now initially the first gear of the lay shaft is meshed with the first gear of the drive shaft. This is known as first steering ratio. The lay shaft is locked in this position by an appropriate locking mechanism. Now to move to the second steering ratio, the lay shaft is unlocked and is slided downwards by a distance X such second gear of lay shaft meshes completely with the second gear of drive shaft. Now again the lay shaft is locked in this position and we get the second steering ratio.Similarly to obtain the third steering ratio, the lay shaft is unlocked again and is slided by a distance 2X such that the third gear of lay shaft completely meshes with the third gear of drive shaft . Now the lay shaft is locked in this position and we get third steering ratio.As shown in the figure the distance between second and third gears on drive shaft is 2X and is denoted by L1. The distance between the first and second gears on drive shaft is denoted by L2 and is 3X. These distances are such that at a time one and only one gear of lay shaft meshes with its pair on drive shaft.The length L3 that is the length of the gear attached to steering wheel is such that it is always meshed with the lay shaft irrespective of its movement.All these gears are spur gears so that they can slide with respect to each other in the axial direction.The drive shaft is finally attached to a simple rack and pinion arrangement.One important point to be noted is steering ratio depends on the gear ratio of the gear box as well as the mounting points of the connecting rods on the wheel hub.

3.5. Calculations To Manufacture Rack and Pinion Assembly

Wheel base1650

Track1150

Distance of CG from rear axle660

Radius of turn to be executed5000

Length of tie-rod400

Tyre angle from vertical(radians)0.360177026

Steering arm length70

Final rack travel24.6945457

Steering ratio4.3

Rotation till stop1.548761212

Pinion PCR15.94470826

Pinion PCD31.88941653

Rack diameter20

Helix angle15

Pressure angle20

Min pinion teeth17.11390231

Teeth taken18

Module1.771634252

Pinion addendum diameter35.43268503

Pinion deddendum diameter27.81465775

Teeth height3.809013641

Pinion face width23.88799598

Here, the values in green are required or assumed values and the value in red are the calculated values.

3.6. Formulae used Tyre angle from vertical: = tan-1 [ wheelbase / {(Radius of turn2 -Center of gravity from rear axle2 )} -(track/2) ] Final Rack Travel = {(steering arm length2)(1-cos)2+(tie rod length + steering arm lengthsin)2} tie rod length

Rotation Till Stop = Steering Ratio*

Pinion Pitch Circle Radius = Rack travel / Rotation Till Stop

Maximum Pinion Teeth = 2 / ( Sin ( Pressure angle / 180 )2 )

Module = Pinion pitch circle diameter / Teeth Taken

Standard Module taken = 1.75

Pinion Addendum= 2.Module + Pinion Pitch Diameter

Pinion deddendum = Pinion pitch circle diameter- (Module 2 1.15)

Tooth height = 2.15 module

3.7. Work done till now

Rack and pinion fabricated successfully according to the given calculations.

3.8. Our Project-Practical Implementation and its Future scope Our project steering system can be implemented in daily production cars and luxury cars. Different steering ratios can be adjusted for i.e. off-roading (high ratio) and racing (low ratio) according to the driving conditions. Similar system can be used to adapt drivers ability and needs, he can adjust the ratio according to required steering effort, steering sensitivity, steer time etc. Luxury cars or cars running at high speeds on highways can use the low steering ratio mode and SUVs or Gypsys can use the high ratio mode for off-roading to lower bump steer. An efficient driver could use low steering to increase sensitivity and thus decreasing drivers fatigue as well which is least at low ratios. Learners could use the high steering ratio mode/low sensitivity thus decreasing the risk of over steering which could be combined with Variable rack body to avoid under steer at ends. Thus improving the concepts and designs of previous attempters. Even more development can be made in this project in order to relate it with electronic circuits in order to make the system automatic in place of manual operation. Speed sensitive electronic system can be used with it combined with the road/condition type sensor as well in order to shift from one gear to another automatically.

REFERENCE

Figure 1.1 www.carbibles .com Figure 1.2 - www.carbibles .com Figure 1.3 - www.carbibles .com Figure 2.1 - US Patent number 3753378 Figure 2.2 - US Patent number 5687811 Figure 2.3 - US Patent number 5687811 Figure 2.4 - US Patent number 5687811 Figure 2.5 - US Patent Number 0204588 Figure 2.6 - US Patent Number 0204588 Figure 3.1 - Self made Figure 3.2 - Self captured