Mbeya Institute of Science and Technology

Department of Mechanical Engineering

Automotive Technology

STEERING SYSTEM

STEERING SYSTEM

• A vehicle is not much use if it cannot be steered or guided. The act of guiding the vehicle 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.

STEERING SYSTEM

•The function of a steering system is to convert the rotary movement of the steering wheel in driver's hand into the angular turn of the front wheels on road. •Additionally, the steering system should provide mechanical advantage over front wheel steering knuckles, offering driver an easy turning of front wheels with minimum effort in any desired direction.

• 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.

Steering layout

• The following are commonly used layouts• Swinging beam steering• Fixed beam steering• Ackermann steering principle

Swinging beam steering

• In this layout the front axle is mounted onto the vehicle by means of a central turntable which allows the whole axle to pivot

• As each of the steered wheels is at right angles to the centre of the turn, the vehicle can follow a curved pathway. This gives true rolling motion in which the wheels has minimal resistance.

Swinging beam steering

Swinging beam steering

Fixed beam steering

• In fixed beam steering the front axle is unable to turn. Instead the wheels are mounted on pivoting sub axles, mounted on each end of the axle beam.

• This method is the basis of all steering systems. Vehicles with independent suspension have their stub axles mounted in the same relative positions.

Fixed beam steering

Ackerman steering principle

• To ensure that the front steered wheels rotate around a common centre the inner and outer roadwheels must be moved by differnt amounts. This achieved by setting the steering arms at an angle so that their projected centrelines meet on or near the centre of the rear axle.

Ackerman steering principle

Ackerman steering principle

Ackerman steering principle

• The intention of Ackermann geometry is to avoid the need for tyres to slip sideways when following the path around a curve.[2] The geometrical solution to this is for all wheels to have their axles arranged as radii of a circle with a common centre point. As the rear wheels are fixed, this centre point must be on a line extended from the rear axle. Intersecting the axes of the front wheels on this line as well requires that the inside front wheel is turned, when steering, through a greater angle than the outside wheel

Ackerman steering principle

Wheel alignment

• Wheel alignment is part of standard automobile maintenance that consists of adjusting the angles of the wheels so that they are set to the car maker's specification

• The purpose of these adjustments is to reduce tire wear, and to ensure that vehicle travel is straight and true (without "pulling" to one side).

Wheel alignment

• Alignment angles can also be altered beyond the maker's specifications to obtain a specific handling characteristic. Motorsport and off-road applications may call for angles to be adjusted well beyond "normal" for a variety of reasons

Wheel alignment

Wheel alignment

Wheel alignment

Steering angles

• The steering geometry of a vehicle is purposely designed with certain steering angles these have an important effect on how the vehicle steers and handles

1.Caster angle2.Camber angle3.Kingpin inclination(KPI)

Caster angle

• Caster angle or castor angleis the angular displacement from the vertical axis of the suspension of a steered wheel in a car, bicycle or other vehicle, measured in the longitudinal direction. It is the angle between the pivot line (in a car - an imaginary line that runs through the center of the upper ball joint to the center of the lower ball joint) and vertical. Car racers sometimes adjust caster angle to optimize their car's handling characteristics in particular driving situations

Caster angle

Caster angle

• When an automotive vehicle's front suspension is aligned, caster is adjusted to achieve the self-centering action of steering, which affects the vehicle's straight-line stability. Improper caster settings will cause the driver to move the steering wheel both into and out of each turn, making it difficult to maintain a straight line.

Caster angle

Camber angle

• Camber angle is the angle made by the wheels of a vehicle; specifically, it is the angle between the vertical axis of the wheels used for steering and the vertical axis of the vehicle when viewed from the front or rear.

• It is used in the design of steering and suspension. If the top of the wheel is farther out than the bottom (that is, away from the axle), it is called positive camber; if the bottom of the wheel is farther out than the top, it is called negative camber

Camber angle

Camber angle

• Camber angle alters the handling qualities of a particular suspension design; in particular, negative camber improves grip when cornering. This is because it places the tire at a more optimal angle to the road, transmitting the forces through the vertical plane of the tire, rather than through a shear force across it. Another reason for negative camber is that a rubber tire tends to roll on itself while cornering. If the tire had zero camber, the inside edge of the contact patch would begin to lift off of the ground, thereby reducing the area of the contact patch. By applying negative camber, this effect is reduced, thereby maximizing the contact patch area. Note that this is only true for the outside tire during the turn; the inside tire would benefit most from positive camber.

Camber angle

Camber angle

Camber angle

Kingpin inclination

• The kingpin, also king-pin and king pin,[ is the main pivot in the steering mechanism of a car or other vehicle. Originally this was literally a steel pin on which the moveable, steerable wheel was mounted to the suspension.

• On most modern designs, the kingpin is set at an angle relative to the true vertical line, as viewed from the front or back of the vehicle. This is the kingpin inclination or KPI (also called steering axis inclination, or SAI).

Kingpin inclination

• KPI is the angle at which the centrline of the kingpin or the axis of the steering swivels leans inwards from the vertical.

Kingpin inclination

Kingpin inclination

Kingpin inclination

Steering gear

• The purpose of steering gear is to enable the driver to alter the vehicles steered direction with a minimum of effort. Three of the most common types in use are

1.Worm and peg2.Recirculating ball3.Rack and pinion

Steering gear

• Worm and peg steering• A worm drive is a gear arrangement in which

a worm (which is a gear in the form of a screw) meshes with a worm gear (which is similar in appearance to a spur gear, and is also called a worm wheel). The terminology is often confused by imprecise use of the term worm gear to refer to the worm, the worm gear, or the worm drive as a unit.

Worm and peg steering

Worm and peg steering

• With with this type of steering mechanism the lower part of the steering column shaft is machined with a worm-type screw thread, which meshes with a peg protuding from the arm of the cross-shaft

• When the steering wheel is rotated, worm moves the peg along , and the arm transfered to a steering drop arm and linkage to move the roadwheels.

Steering gear

Recirculating ball steering

• The recirculating ball steering mechanism contains a worm gear inside a block with a threaded hole in it; this block has gear teeth cut into the outside to engage the sector shaft (also called a sector gear) which moves the Pitman arm. The steering wheel connects to a shaft, which rotates the worm gear inside of the block. Instead of twisting further into the block, the worm gear is fixed so that when it spins, it moves the block, which transmits the motion through the gear to the pitman arm, causing the roadwheels to turn.

Recirculating ball steering

Recirculating ball steering

Recirculating ball steering

• The recirculating-ball steering gear contains a worm gear. You can image the gear in two parts. The first part is a block of metal with a threaded hole in it. This block has gear teeth cut into the outside of it, which engage a gear that moves the pitman arm (see diagram above). The steering wheel connects to a threaded rod, similar to a bolt, that sticks into the hole in the block. When the steering wheel turns, it turns the bolt. Instead of twisting further into the block the way a regular bolt would, this bolt is held fixed so that when it spins, it moves the block, which moves the gear that turns the wheels.

Recirculating ball steering

Steering gear

Power steering

• Power steering, assists the driver of an automobile in steering by directing a portion of the vehicle's power to traverse the axis of one or more of the roadwheels. As vehicles have become heavier and switched to front wheel drive, particularly using negative offset geometry, along with increases in tire width and diameter, the effort needed to turn the steering wheel manually has increased — often to the point where major physical exertion is required.

Power steering

• To alleviate this, auto makers have developed power steering systems: or more correctly power-assisted steering — on road going vehicles there has to be a mechanical linkage as a fail safe. There are two types of power steering systems—hydraulic and electric/electronic. A hydraulic-electric hybrid system is also possible.

Power steering

• A hydraulic power steering (HPS) uses hydraulic pressure supplied by an engine-driven pump to assist the motion of turning the steering wheel. Electric power steering (EPS) is more efficient than the hydraulic power steering, since the electric power steering motor only needs to provide assistance when the steering wheel is turned, whereas the hydraulic pump must run constantly. In EPS, the assist level is easily tunable to the vehicle type, road speed, and even driver preference.

Power steering

• There are a couple of key components in power steering in addition to the rack-and-pinion or recirculating-ball mechanism

• Pump The hydraulic power for the steering is provided by a rotary-vane pump (see diagram below). This pump is driven by the car's engine via a belt and pulley. It contains a set of retractable vanes that spin inside an oval chamber.

Power steering

Power steering

• Rotary Valve A power-steering system should assist the driver only when he is exerting force on the steering wheel (such as when starting a turn). When the driver is not exerting force (such as when driving in a straight line), the system shouldn't provide any assist. The device that senses the force on the steering wheel is called the rotary valve.

Power steering

• The key to the rotary valve is a torsion bar. The torsion bar is a thin rod of metal that twists when torque is applied to it. The top of the bar is connected to the steering wheel, and the bottom of the bar is connected to the pinion or worm gear (which turns the wheels), so the amount of torque in the torsion bar is equal to the amount of torque the driver is using to turn the wheels. The more torque the driver uses to turn the wheels, the more the bar twists.

Power steering

Power steering

Steering linkage

• The steering linkage is a combination of rods, and arms, that transmit the movement of the steering gear to the front wheels.

• It must transmit this movement to the front wheels, while still allowing for any up-and down movement they may make, while the vehicle is in motion.

• The type of steering mechanism, and the number of linkages, depends on the type of steering box, its location, and the type of suspension on the vehicle.

Steering linkage

Steering linkage

Ball joint

• In an automobile, ball joints are spherical bearings that connect the control arms to the steering knuckles. More specifically, a ball joint is a steel bearing stud and socket enclosed in a steel casing. The bearing stud is tapered and threaded. It fits into a tapered hole in the steering knuckle. A protective encasing prevents dirt from getting into the joint assembly. Motion control ball joints tend to be retained with an internal spring, which helps to prevent vibration problems in the linkage. Commonly found in automotive throttle linkages, throttle body set ups, these are also widely used on construction equipment, the end of gas springs and in children's toys.

Ball joint

Ball joint

Purpose of ball joint

• Ball joints are the pivot between the wheels and the suspension of an automobile. Ball joints play a critical role in the safe operation of an automobile's steering and suspension. Ball joints can also be found in most linkage systems for motion control applications, and should not be confused with spherical rod end bearings, which are a different design.

Ball joint

END OF PRESENTATION

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