steering systems

  STEERING SYSTEMS

INTRODUCTION

          Steering is the collection of components, linkages, etc. which allows              any Vehicle (car, motorcycle, bicycle) to follow the desired course. An exception is the          case of rail transport by which rail tracks combined together with railroad switches (and        also known as 'points' in British English) provide the steering function. The primary              purpose of the steering system is to allow the driver to guide the vehicle.

                 

       

              Four wheel steering is a method developed in automobile industry for the effective turning of the vehicle and to increase the maneuverability. In a typical front wheel steering system the rear wheels do not turn in the direction of the curve and thus curb on the efficiency of the steering. In four wheel steering the rear wheels turn with the front wheels thus increasing the efficiency of the vehicle. The direction of steering the rear wheels relative to the front wheels depends on the operating conditions. At low speed wheel movement is pronounced, so that rear wheels are steered in the opposite direction to that of front wheels. At high speed, when steering adjustments are subtle, the front wheels and the rear wheels turn in the same direction. By changing the direction of the rear wheels there is reduction in turning radius of the vehicle which is efficient in parking, low speed cornering and high speed lane change.

         

          In city driving conditions the vehicle with higher wheelbase and track width face problems of turning as the space is confined, the same problem is faced in low speed cornering. Usually customers pick the vehicle with higher wheelbase and track width for their comfort and face these problems, so to overcome this problem a concept of four wheel steering can be adopted in the vehicle. Four wheel steering reduces the turning radius of the vehicle which is effective in confined space, in this project four wheel steering is adopted for the existing vehicle and turning radius is reduced without changing the dimension of the vehicle.

HOW THE STEERING SYSTEM WORKS

            The steering system converts the rotation of the steering wheel into a swivelling movement of the road wheels in such a way that the steering-wheel rim turns a long way to move the road wheels a short way.

            The system allows a driver to use only light forces to steer a heavy car. The rim of a 15 in. (380 mm) diameter steering wheel moving four turns from full left lock to full right lock travels nearly 16 ft (5 m), while the edge of a road wheel moves a distance of only slightly more than 12 in. (300 mm). If the driver swivelled the road wheel directly, he or she would have to push nearly 16 times as hard.

           The steering effort passes to the wheels through a system of pivoted joints. These are designed to allow the wheels to move up and down with the suspension without changing the steering angle.

          

         They also ensure that when cornering, the inner front wheel - which has to travel round a tighter curve than the outer one - becomes more sharply angled.

           The joints must be adjusted very precisely, and even a little looseness in them makes the steering dangerously sloppy and inaccurate.

           There are two steering systems in common use - the rack and pinion and the steering box. On large cars, either system may be power assisted to reduce further the effort needed to move it, especially when the car is moving slowly.

TYPES OF STEERING SYSTEMS

          Two main types of steering systems are used on modern cars and light trucks: the rack-and-pinion system and the conventional, or parallelogram linkage, steering system. On automobiles, the conventional system was the only type used until the 1970s. It has been almost completely replaced by rack-and-pinion steering. Many light trucks continue to use the conventional system. The two types of systems are discussed below.

              1. Rack and pinion
              2. Reciprocating ball and steering box

    

Rack-and-Pinion Steering
 

         Rack-and-pinion steering is the most common type of steering on cars, small trucks and SUVs.  A rack-and-pinion gear set is enclosed in a metal tube, with each end of the rack sticking out from the tube. A rod, called a tie rod or axial rod, connects to each end of the rack.
      
  
         
           The following diagram shows how the rack-and-pinion gear set interacts with the steering wheel. The pinion gear is attached to the steering shaft. When the steering wheel is turned, the gear spins, moving the rack. The tie rod at each end of the rack connects to the steering arm which is attached to the spindle.
       

The rack-and-pinion gear set has two purposes:
  - It converts the circular motion of the steering wheel into the linear motion needed to               turn the vehicle’s wheels.    
   - It creates a gear reduction, which makes it easier to turn the wheels.

  

Variable Ratio Steering

             Some conventional steering gears are equipped with variable ratio steering. Variable ratio steering gears use specially designed nut and sector gears. These gears taper as the sector gear moves away from the center position. The sector is moved a relatively small amount when it is in the center of its travel, which eliminates overreaction when the steering wheel is first turned,and reduces wander when traveling straight ahead. As the sector gear moves further, the increasing taper of the meshing gears increases the steering ratio. This makes hard turns easier, especially in low speed situations such asparking.

 There are two types of rack-and-pinion steering systems:

- End take off, where the tie rods exit from the end of the steering rack via inner axial joints

- Centre take off, where the tie rod ends attach to the centre of the rack with bolts

 

Recirculating-Ball Steering Gear

             The recirculating-ball steering gear is commonly used on modern vehicles. This steering gear is known by many names, depending on the manufacturer. Common names for this type of steering gear include worm and nut, worm and ball, and recirculating nut and worm. No matter what the name, the basic design is the same. The basic principle

of this type of steering gear is shown in Figure 9-27. The worm gear is the screw, and the ball nut rides up and down as the screw turns. Teeth on one side of the ball nut contact

matching teeth on the sector gear. When the steering shaft turns the worm gear, the ball nut moves on the worm gear shaft. Teeth on the ball nut cause the sector gear to turn. 

     

                                        RECIRCULATING BALL STEERING
            
        The ball bearings between the threads of the worm gear and ball nut reduce friction between the worm and the nut. The steering gear is called a recirculating-ball. Cutaway of a recirculating ball steering gear. Note as the sector gear moves, the nut travels with it, causing the worm gear to rotate. (Dodge)

            steering gear because the ball bearings move in a loop, or circuit, as the worm gear moves the ball nut. The ball bearings can recirculate because of ball guides installed on top of the ball nut assembly.                                      

     

            The worm and sector gears ride on ball or roller bearings to reduce friction. The steering gear may be lubricated by 90–140 weight gear oil, or it may require automatic transmission fluid. Seals at the input shaft and sector shaft keep lubricant from leaking out of the steering gear. The worm and sector gears contain thrust washers and spacers for proper clearance.

Worm-and-Roller Steering Gear

           The worm-and-roller steering gear is found on Asian vehicles, as well as some older Jeep vehicles and European cars. an illustration of a typical worm-and- roller steering gear. Unlike the recirculating-ball steering gear, the worm-and-roller steering gear does not use a ball nut. Instead, the worm gear turns against a roller installed in the sector gear. Turning the worm gear causes the sector gear to move. The rolling action between the worm gear and the sector roller reduces friction. Note that the worm gear is tapered, with the end sections being larger than the center. This worm gear design results in constant full

contact between the worm and roller, no matter what the sector position. The worm-and-roller steering gear may have a provision for adjustment.

                                 
                              

                              

                                    WORM AND ROLLER STEERING GEAR

 Worm-and-Follower Steering Gear

           The worm-and-follower steering gear is found on older European cars. Figure 9-29 shows a typical worm-and-follower steering gear. This design may be called the worm-and-peg, worm-and-cam, or worm-and- pin steering gear. Note that the worm gear works directly on the follower, causing the sector shaft to turn as the follower moves on the turning worm gear.

Power steering system

               Power steering mechanism is a driver assist feature equiped with all type of modern vehicles. Apparently it helps the driver to turn steering wheel generously at any load,speed,road conditions.

               Before we discuss how power steering is employed, we must know how do steering works.

               This is a typical steering system layout which shows almost all parts. Types of steering systems are,

       1. Rack and pinion

       2. Worm and sector

       3. Screw and nut

       4. Recirculating ball screw

       5. Cam and lever.

           are still in use for different vehicles.

              These types manual steering system enlighten the need of power system, by which power steering system draw power from the engine to assist the effort made by the driver.

Types of power systems are,

      1. Hydraulic

      2. Hydro static

      3. Electrohydraulic

      4. Electric

        All these types works in similarly the way they do it is different.

 Hydraulic steering system

          Hydraulic power steering systems work by using a hydraulic system to multiply force applied to the steering wheel inputs to the vehicle's steered (usually front) road wheels.The hydraulic pressure typically comes from a gerotor or rotary vane pump driven by the vehicle's engine. A double-acting hydraulic cylinder applies a force to the steering gear, which in turn steers the road wheels. The steering wheel operates valves to control flow to the cylinder. The more torque the driver applies to the steering wheel and column, the more fluid the valves allow through to the cylinder, and so the more force is applied to steer the wheels.

                 
                    

                                     HYDRAULIC POWER STEERING SYSTEM

         One design for measuring the torque applied to the steering wheel has a torque sensor – a torsion bar at the lower end of the steering column. As the steering wheel rotates, so does the steering column, as well as the upper end of the torsion bar. Since the torsion bar is relatively thin and flexible, and the bottom end usually resists being rotated, the bar will twist by an amount proportional to the applied torque. The difference in position between the opposite ends of the torsion bar controls a valve. The valve allows fluid to flow to the cylinder which provides steering assistance; the greater the "twist" of the torsion bar, the greater the force.

           Since the hydraulic pumps are positive-displacement type, the flow rate they deliver is directly proportional to the speed of the engine. This means that at high engine speeds the steering would naturally operate faster than at low engine speeds. Because this would be undesirable, a restricting orifice and flow-control valve direct some of the pump's output back to the hydraulic reservoir at high engine speeds. A pressure relief valve prevents a dangerous build-up of pressure when the hydraulic cylinder's piston reaches the end of its stroke.

        The strifffooster is arranged so that should the booster fail, the steering will continue to work (although the wheel will feel heavier). Loss of power steering can significantly affect the handling of a vehicle. Each vehicle owner's manual gives instructions for inspection of fluid levels and regular maintenance of the power steering system. 

The working liquid, also called "hydraulic fluid" or "oil", is the medium by which pressure is transmitted. Common working liquids are based on mineral oil. 

Some modern systems also include an electronic control valve to reduce the hydraulic supplypressure as the vehicle's speed increases; this is variable-assist power steering.

Electro-hydraulic steering system

In general elctro hydraulic power steering has rack and pinion but along with this there is one more arrangements for reducing drivers effort.

The arrangement includes a simple hydraulic arm which is connected with rack and in the same direction as that of rack

       
                       

                            ELECTRO-HYDRAULIC STEERING SYSTEM
                                  
And a torque sensor on steering column which sensor the steering torque provided by driver.

So as driver rotates the steering wheel torque sensor sense that and send signal to ECU which control the amount of fluid to be flow in hydraulic arm

If driver steer the steering wheel faster more current is send by torque sensor and more fluid is send to the hydraulic arm which move is same direction of rack.

As without power steering the rack was moving only by drivers effort but now rack is also moving by hydraulic arm thus it reduce the driving effort.

Electric steering system

Power steering systems supplement the torque that the driver applies to the steering wheel. Traditional power steering systems are hydraulic systems, but electric power steering (EPS) is becoming much more common. EPS eliminates many HPS components such as the pump, hoses, fluid, drive belt, and pulley. For this reason, electric steering systems tend to be smaller and lighter than hydraulic systems.

                      
                                   ELECTRIC  POWER STEERING SYSTEM

EPS systems have variable power assist, which provides more assistance at lower vehicle speeds and less assistance at higher speeds. They do not require any significant power to operate when no steering assistance is required. For this reason, they are more energy efficient than hydraulic systems.

• The EPS electronic control unit (ECU) calculates the assisting power needed based on the torque being applied to the steering wheel by the driver, the steering wheel position and the vehicle’s speed.
• The EPS motor rotates a steering gear with an applied force that reduces the torque required from the driver.

          There are four forms of EPS based on the position of the assist motor. They are the column assist type (C-EPS), the pinion assist type (P-EPS), the direct drive type (D-EPS) and the rack assist type (R-EPS). The C-EPS type has a power assist unit, torque sensor, and controller all connected to the steering column. In the P-EPS system, the power assist unit is connected to the steering gear's pinion shaft. This type of system works well in small cars. The D-EPS system has low inertia and friction because the steering gear and assist unit are a single unit. The R-EPS type has the assist unit connected to the steering gear. R-EPS systems can be used on mid- to full-sized vehicles due to their relatively low inertia from high reduction gear ratios.

         Unlike a hydraulic power steering system that continuously drives a hydraulic pump, the efficiency advantage of an EPS system is that it powers the EPS motor only when necessary. This results in reduced vehicle fuel consumption compared to the same vehicle with an HPS system. These systems can be tuned by simply modifying the software controlling the ECU. This provides a unique and cost effective opportunity to adjust the steering "feel" to suit the automotive model class. An additional advantage of EPS is its ability to compensate for one-sided forces such as a flat tire. It is also capable of steering in emergency maneuvers in conjunction with the electronic stability control.

        In current-day systems, there is always a mechanical connection between the steering wheel and the steering gear. For safety reasons, it is important that a failure in the electronics never result in a situation where the motor prevents the driver from steering the vehicle. EPS systems incorporate fail-safe mechanisms that disconnect power from the motor in the event that a problem with the ECU is detected.

      The next step in electronic steering is to remove the mechanical linkage to the steering wheel and convert to pure electronically controlled steering, which is referred to as steer-by-wire.