PresentationOn

Power Steering

By Sahaj Harnal07AU626

STEERING SYSTEM

The steering system allows the operator to guide the vehicle along the road and turn left or right as desired. The system includes the steering wheel, which the operator controls, the steering mechanism, which changes the rotary motion of the steering wheel into straight-line motion, and the steering linkage. Most systems were manual until a few years ago. Then power steering became popular. It is now installed in most vehicles manufactured today.

The steering system must perform some important functions like –

•Provide precise control of front-wheel direction. •Maintain the correct amount of effort needed to turn the front wheels.

STEERING LINKAGESteering linkage is a series or arms, rods, and ball sockets that connect the steering mechanism to the steering knuckles. The steering linkage used with most manual and power steering mechanisms typically includes a pitman arm, center link, idler arm, and two tie-rod assemblies.

How Car Steering Works?

Turning the Car

For a car to turn smoothly, each wheel must follow a different circle. Since the inside wheel is following a circle with a smaller radius, it is actually making a tighter turn than the outside wheel. If we draw a line perpendicular to each wheel, the lines will intersect at the center point of the turn. The geometry of the steering linkage makes the inside wheel turn more than the outside wheel.

While turning, the inside wheel turn more than the outside wheel

Turning the Car

STEERING RATIO One purpose of the steering mechanism is to provide mechanical advantage. In a machine or mechanical device, it is the ratio of the output force to the input force applied to it. This means that a relatively small applied force can produce a much greater force at the other end of the device.

The steering ratio is a number of degrees that the steering wheel must be turned to pivot the front wheels 1 degree. The higher the steering ratio, the easier it is to steer the vehicle, all other things being equal. However, higher the steering ratio, the more the steering wheel has to be turned to achieve steering. With a 30: 1 steering ratio, the steering wheel must turn 30 degrees to pivot the front wheels 1 degree.

"Variable steering ratio" means that the ratio is larger at one position than another. Therefore the wheels are turned faster at certain positions than at others. At the center or straight-ahead position, the steering gear ratio is high, giving more steering control. However, as the wheels are turned, the ratio decreases so that the steering action is much more rapid. This design is very helpful for parking and maneuvering the vehicle.

Variable steering ratio

MANUAL STEERING SYSTEMS

Manual steering is considered to be entirely adequate for smatter vehicles. It is tight, fast, and accurate in maintaining steering control. However, larger and heavier engines, greater front overhang on larger vehicles and a trend toward wide tread tires have increased the steering effort required. Steering mechanisms with higher gear ratios were tried, but dependable power steering systems were found to be the answer. There are several different types of manual steering systems, which are as

follows:

1. Worm and sector 2. Worm and rotter 3. Cam and lever 4. Worm and nut 5. Rack and pinion

Worm and sector type steering system

Worm and roller type steering system

Cam and lever type steering systemWorm and nut type steering system

Manual Steering System

Manual Steering System

Rack and Pinion The rack-and-pinion steering gear has become increasingly popular on smaller passenger vehicles. It is simpler, more direct acting, and may be straight mechanical or power-assisted.

In the rack-and-pinion steering system the end of the steering gear shaft contains a pinion gear, which meshes with a long rack.

As the steering wheel is rotated, the pinion gear on the end of the steering shaft rotates. The pinion gear moves the rack from one side to the other. This action pushes or pulls on the tie rods, forcing the steering knuckles or wheel spindles to pivot on their ball joints. This turns the wheels to one side or the other so the vehicle can be steered.

The rack-and-pinion gearset does two things:•It converts the rotational motion of the steering wheel into the linear motion needed to turn the wheels.•It provides a gear reduction, making it easier to turn the wheels.

Rack-and-pinion steering gear

Rack-and-pinion steering gear

POWER STEERING SYSTEMS 

Power Steering

Power steering systems normally use an engine-driven pump and hydraulic system to assist steering action. Pressure from the oil pump is used to operate a piston and cylinder assembly. When the control valve routes oil pressure into one end of the piston, the piston slides in its cylinders. Piston movement can then be used to help move the steering system components and front wheels of the vehicles.

Pump The hydraulic power for the steering is provided by a rotary-vane pump.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. As the vanes spin, they pull hydraulic fluid from the return line at low pressure and force it into the outlet at high pressure. The amount of flow provided by the pump depends on the car's engine speed. The pump contains a pressure-relief valve to make sure that the pressure does not get too high. The pump must be designed to provide adequate flow when the engine is idling. So, the pump moves much more fluid than necessary when the engine is running at faster speeds.

Rotary ValveThe device that senses the force on the steering wheel is called the rotary valve.

The key to the rotary valve is a torsion bar. 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 bottom of the torsion bar connects to the outer part of the spool-valve assembly. As the bar twists, it rotates the inside of the spool valve relative to the outside. Since the inner part of the spool valve is also connected to the steering shaft, the amount of rotation between the inner and outer parts of the spool valve depends on how much torque the driver applies to the steering wheel.

When the steering wheel is not being turned, both hydraulic lines provide the same amount of pressure to the steering gear. But if the spool valve is turned one way or the other, ports open up to provide high-pressure fluid to the appropriate line.

Working of the rotary Valve

Power Rack-and-pinion

In the rack-and-pinion power-steering system, the rack has a slightly different design. Part of the rack contains a cylinder with a piston in the middle. The piston is connected to the rack. There are two fluid ports, one on either side of the piston. Supplying higher-pressure fluid to one side of the piston forces the piston to move, which in turn moves the rack, providing the power assist.

Power Steering Leakage

A common problem with power steering systems is fluid leakage. With pressure over 1,000 psi, leaks can develop easily around fittings, in hoses, at the gearbox seals, or at the rack-and-pinion assembly.

In Steering gear box, leakage occurs from:-

1.Upper Pinion seal.2.Rack bush side seal.3.Pinion side seal.

Power Steering Assembly line

The main processes on the Power Steering assembly line are:-

Parts washing (cylinder tube, pinion housing). Rack housing and bearing Pressing. Rack bar washing, grease application and insertion. Rack bush assembly. Pinion assembly. Assembly of feed tubes. Tests (Air leak test, Rack twist adjustment, internal oil leak test). Oil purge. Assembly of IBJ. Assembly of bellows and OBJ. Final inspection.

Electronic Power Steering

(EPS)

Electronic Power Steering Electrically powered steering uses an electric motor to drive either the power steering hydraulic pump or the steering linkage directly. The power steering function is therefore independent of engine speed, resulting in significant energy savings.

In electro-hydraulic steering, one electrically powered steering concept uses a high efficiency pump driven by an electric motor. Pump speed is regulated by an electric controller to vary pump pressure and flow, providing steering efforts tailored for different driving situations. The pump can be run at low speed or shut off to provide energy savings during straight ahead driving

Electronic Power Steering (EPS) is an advanced power steering system. It eliminates the need for a power Steering pump, hoses, hydraulic fluids, and a drive belt and pulley on the engine. As a result, electric power steering is more energy efficient.

Electronic Power Steering (EPS)

Control block diagram for EPS system

Working A "steering sensor" is located on the input shaft.The steering sensor is actually two sensors: a "torque sensor" that converts steering torque input and its direction into voltage signals, and a "rotation sensor" that converts the rotation speed and direction into voltage signals. There is an "interface" circuit that converts the signals from the torque sensor and rotation sensor into signals that are sent to a microprocessor. Inputs from the steering sensor are digested by a microprocessor control unit that also

monitors input from the vehicle's speed sensor. The sensor inputs are then compared to determine how much power assist is required according to a preprogrammed "force map" in the control unit's memory. The control unit then sends out the appropriate command to the "power unit" which then supplies the electric motor with current. The motor pushes the rack to the right or left depending on which way the voltage flows (reversing the current reverses the direction the motor spins). Increasing the current to the motor increases the amount of power assist.

Classifications of EPS - EPS can mainly classified in to 3 kinds depending up on the position of the motor: 1. Column assist type.

2. Pinion assist type.

3. Rack assist type.

Column-Assist Type:

•The power assist unit, controller and the torque sensor are attached to the steering column.•This system is compact and easy to mount on vehicle.•An integrated pivot/mount and integrated controller option increase mounting flexibility.•Shafts offer long-term durability performance at much higher torsion loading.

Pinion-assist type:

•The power assist unit is attached to steering gears pinion shaft.•The power assist unit is outside the vehicles passenger compartment, allowing assist torque to be increased greatly without raising interior noise.•Combined with a variable ratio steering gear, this system can suffice with a compact motor and offer superior handling characteristics.

Rack-assist type:

•The power assist unit is attached to the steering gear rack.•The power assist unit can be located freely on the rack, allowing great flexibility in layout design.•The power assist units’ high reduction gear ratio enables very low inertia and superior driving feel.•Electric power steering rack assist is a scalable system, suitable for midsize cars to full-size trucks.

Advantages of EPS over Hydraulic Power Steering:

Reduced driver fatigue.

Enhances dependability and safety.

Compact, modular design and flexible tuning capability.

Helps improve fuel economy by reducing the pressure the pump has to work against during straight-ahead highway speed driving.

Accommodates most vehicle platforms.

Reduced steering system operating temperature.

Lower noise under all driving conditions.

EPS has got upper hand compared to Hydraulic power steering because: