1. Description

The tilt steering mechanism allows selection of the steering wheel position (in the vertical direction) to match the driver’s driving posture. The tilt steering mechanisms are classified into the upper fulcrum type and the lower fulcrum type. Here, the lower fulcrum type is explained.

2. Construction

The tilt steering mechanism consists of a pair of tilt steering stoppers, tilt lock bolt, breakaway bracket, tilt lever, etc.

3. Operation

The tilt steering stoppers turn together with the operation of the tilt lever. When the tilt lever is in the lock position, the peaks of the tilt steering stoppers are lifted up and the stoppers push against the breakaway bracket and tilt attachment, locking the break away bracket and tilt attachment. On the other hand, when the tilt lever is moved to the free position, the height difference on the tilt steering stoppers is eliminated, and the steering column can be adjusted in the vertical direction.

1. Description

The telescopic steering mechanism allows forward or backward adjustment of the steering wheel position to suit the driver’s posture.

2. Construction

The telescopic mechanism consists of the sliding shaft tube, two wedge locks, stopper bolt, telescopic lever, etc.

3. Operation

The wedge locks move together with operation of the telescopic lever. When the telescopic lever is in the lock position, the telescopic lever presses the wedge locks against the sliding shaft tube, locking the sliding shaft tube. On the other hand, when the telescopic lever is moved to the free position, a gap is created between the wedge locks and the sliding shaft tube and the steering column can be adjusted in the forward or backward direction.

When a vehicle involved in a collision, this mechanism helps prevent the steering main shaft from injuring the driver in two ways: by the breaking at the time of the collision (primary shock); and by reducing the secondary shock imposed upon the driver’s body when it hits the steering wheel due to inertia. Impact-absorbing steering columns are classified into the following types.

• Bending bracket type

• Ball type

• Sealed-in pulverized silicon-rubber type

• Mesh type

• Bellows type

1. Description

The piston in the power cylinder is positioned on the rack, and the rack moves due to fluid pressurized by the vane pump acting on the piston in either direction. Fluid pressure leakage is prevented by a seal ring on the piston. Also, there is an oil seal on both sides of the cylinder to prevent external leakage of the fluid. The control valve shaft is connected to the steering wheel. When the steering wheel is in the neutral (straight-ahead) position, the control valve is also in the neutral position, so the fluid from the vane pump does not act on either chamber but flows back to the reservoir tank. However, when the steering wheel is turned in either direction, the control valve changes the passage so the fluid flows into one of the chambers. The fluid in the opposite chamber is forced out and flows back to the reservoir tank by way of the control valve. Currently, there are three different types of control valves which perform this changeover action of the passage; spool valves, rotary valves and flapper valves. All types have a torsion bar between the control valve shaft and pinion, and the control valve functions in accordance with the amount of twist applied to the torsion bar.

2. Type of control valve

A control valve is located in the gear housing. The gear housing houses a rackand- pinion type power steering mechanism or a recirculating- bail type power steering mechanism. The control valve is one of three types: a rotary valve type, a spool valve type, or a flapper valve type. Currently, rotary valve types are used in many models.

3. Construction

Here, the rotary valve type is explained. The control valve in the gear housing determines to which chamber the fluid from the vane pump goes. The control valve shaft (to which steering wheel torque is applied) and the pinion gear are connected by means of a torsion bar. The rotary valve and pinion gear are secured by a pin and rotate integrally. If no vane pump pressure is applied, the torsion bar is fully twisted and the control valve shaft and pinion gear make contact at the stopper so the control valve shaft torque is applied directly to the pinion gear.

4. Operation

A restriction in the hydraulic circuit is formed by rotary movement of the control valve shaft in relation to the rotary valve. When the steering wheel is turned to the right, pressure is restricted at orifices X and Y. When it is turned to the left, a restriction is formed at X’ and Y’. When the steering wheel is turned, the control valve shaft rotates, turning the pinion gear via the torsion bar. In contrast to the pinion gear, as the torsion bar twists in proportion to road surface force at this time, the control valve shaft rotates only to the extent of the amount of twist, and moves to the right or left in relation to the rotary valve. Thus orifices X and Y (or X’ and Y’) are formed and a difference in hydraulic pressure between the right and left cylinder chambers is created. In this manner, rotation of the control valve shaft directly performs changeover of the passages and regulates the fluid pressure. The fluid from the vane pump enters from the outer circumference of the rotary valve, and the fluid returning to the reservoir tank passes between the torsion bar and the control valve shaft.

(1) Neutral position

As the control valve shaft does not revolve, it is in a neutral position in relation to the rotary valve. Fluid supplied by the pump returns to the reservoir tank through port “D” and chamber “D”. The right and left chambers of the cylinder are slightly pressurized but as there is no pressure difference between the two, no power steering assist occurs.

(2) Turning right

When the vehicle makes a right turn, the torsion bar is twisted and the control valve shaft revolves to the right accordingly. Fluid from the pump is constricted by orifices X and Y of the control edge in order to stop flow to ports “C” and “D”. As a result, fluid flows from port “B” to sleeve “B” and then to the right cylinder chamber, causing the rack to move to the left and resulting in power steering assist. At the same time, the fluid in the left cylinder chamber flows back to the reservoir tank via sleeve “C” -> port “C” -> port “D” -> chamber “D”.

(3) Turning left

In the same manner as for a right turn, when the vehicle makes a left turn, the torsion bar is twisted and the control shaft rotates to the left accordingly. The fluid sent from the pump is constricted by orifices X’ and Y’ of the control edge in order to stop flow to ports “B” and “D”. As a result, fluid flows from port “C” to sleeve “C” and then to the left cylinder chamber, causing the rack to move to the right and resulting in power steering assist. At the same time, the fluid in the right cylinder chamber flows back to the reservoir tank via sleeve “B” -> port “B” -> port “D” -> chamber “D”.

1. Description

The new PPS(Progressive Power Steering) varies, the steering operation effort in accordance with the vehicle speed. The steering effort is lighter at low speed and heavier at high speed.

2. Operation

The hydraulic reaction type progressive power steering uses a thinner torsion bar than in ordinary power steering to reduce the steering effort required during stationary steering or when traveling at a low speed. However, this causes the steering required effort to become too small (that is, the steering wheel feels too “light”) when the vehicle speed increases. To prevent this, the required steering effort is increased as when there is a thicker torsion bar, by the provision of a hydraulic reaction chamber for suppressing the rotation of the control valve shaft (in the control valve housing) by means of four hydraulically operated plungers. The hydraulic pressure applied to the hydraulic reaction chamber is low at low vehicle speeds and high at high vehicle speeds. It also increases as the hydraulic pressure in the power cylinder rises as a result of steering operation. In the hydraulic reaction type progressive power steering, the required steering effort varies according to the vehicle speed and steering operation.

1. Description

EPS (Electric Power Steering) generates the assist torque by the motor for steering operation and reduces the steering effort. Hydraulic power steering uses the power of the engine to generate the hydraulic pressure and obtain the assist torque. Since EPS uses a motor, it does not require the power of the engine and improves fuel economy.

2. Construction & Operation

(1) EPS ECU

The EPS ECU receives signals from various sensors, judges the current vehicle condition, and determines the assist current to be applied to the DC motor accordingly.

(2) Torque sensor

When the driver operates the steering wheel, the steering torque is applied to the torque sensor input shaft via the steering main shaft. Detection rings 1 and 2 are positioned on the input shaft (steering wheel side) and detection ring 3 is positioned on the output shaft (steering gear side). The input shaft and output shaft are connected via a torsion bar. Also, the detection rings have non-contact detection coils on their outer circumferences in order to form an excitation circuit. When the steering torque is generated, the torsion bar is twisted, generating a phase difference between detection rings 2 and 3. Based on this phase difference, a signal proportional to the input torque is output to the ECU. Based on this signal, the ECU calculates the motor assist torque for the vehicle speed and drives the motor.

(3) DC motor & Reduction mechanism

The DC motor consists of the rotor, stator, and motor shaft. The reduction mechanism consists of a worm gear and a wheel gear. The torque that is generated by the rotor is transmitted to the reduction mechanism. Then, this torque is transmitted to the steering shaft. The worm gear is supported by the bearings in order to reduce noise. Even if the DC motor breaks down, the rotation of the steering main shaft and the reduction mechanism is not fixed therefore the steering wheel can be steered. (4) ABS ECU Vehicle speed signal is outputted to EPS ECU. (5) Engine ECU Engine speed signal is outputted to EPS ECU. (6) Combination meter In case of a malfunction in the system, turns on the warning light. (7) Relay Supplies power to DC motor and EPS ECU.

1. Description

To improve driving comfort, most modern automobiles have wide, lowpressure tires which increase the tire-to-road surface contact area. As a result of this, more steering effort is required. Steering effort can be decreased by increasing the gear ratio of the steering gear. However, this will cause a larger rotary motion of the steering wheel when the vehicle is turning, making sharp turns impossible. Therefore, to keep the steering agile and, at the same time the steering effort small, some sort of a steering assist device became necessary. In other words, power steering, which had been chiefly used on larger vehicles, is now also used on compact passenger cars.

2. Type of power steering

There are hydraulic type and electric type power steering. Currently, hydraulic power steering is used on almost all models. The three main components of hydraulic power steering are the vane pump, control valve, and power cylinder.

3. Operation of hydraulic power steering

The power steering system uses the power of the engine to drive the vane pump that generates the hydraulic pressure. When the steering wheel is turned, an oil circuit is switched at the control valve. As oil pressure is applied to the power piston in the power cylinder, the power needed to operate the steering wheel is reduced. It is necessary to inspect for leakage of power steering fluid periodically.

Vane Pump

Power steering is a type of hydraulic device requiring a very high pressure. It uses the power of the engine to drive the vane pump uses that generates this hydraulic pressure. Vanes are used in this pump, so this name is used for this type of power steering.

The discharge volume of the vane pump increases proportionately as engine speed rises. The amount of steering assist provided by the power piston of the power cylinder is determined by the volume of fluid from the pump. As the pump speed increases, the flow volume becomes greater, providing more steering assist and, consequently, less steering effort is required. In other words, the steering effort varies in accordance with the change in speed, which is a disadvantage from the standpoint of steering stability. Therefore, it is necessary to maintain a constant fluid flow volume from the pump regardless of the speed, and this is the function of the flow control valve. Normally, When the vehicle is running at high speed, there is less tire resistance and, consequently, less steering effort required. Therefore, with some power steering systems, there is less assist provided during high speeds so that an appropriate steering effort can be obtained. In short, the flow volume from the pump to the gear housing is reduced during high-speed driving and there is less power steering assist. Discharge volume of the pump increases with a rise in pump speed, but the flow volume of the fluid to the gear housing is reduced. This is referred to as the speed-sensing power steering and consists of a flow control valve with a built-in control spool. ‘

<1> At low speeds (Pump speed: 650-1,250rpm)

Pump discharge pressure P1, is applied to the right side of the flow control valve and P2 is applied to the left side after passing through the orifices. The pressure difference between P1, and P2 becomes larger as the engine speed increases. When the pressure difference between P1, and P2 overcomes the flow control valve spring tension (A), the flow control valve moves to the left. This opens the passage to the pump suction side so the fluid returns to the pump suction side. Fluid volume to the housing is kept at constant in this manner.

<2> At medium speeds (Pump speed: 1,250-2,500rpm)

Pump discharge pressure P1 is applied to the left side of the control spool. When the pump speed is above 1,250 rpm, pressure P1 overcomes the spring tension (B) and forces the control spool to the right, so the fluid volume passing through the orifices is decreased, causing a lowering of pressure P2. Consequently, the pressure difference between P1 and P2 increases. In this manner, the flow control valve moves toward the left so that the fluid returns to the pump suction side, reducing the fluid volume to the gear housing. In other words, when the control spool moves to the right, the fluid volume passing through the orifices decreases.

<3> At high speeds (Pump speed: over 2,500rpm)

When pump speed exceeds 2,500 rpm, the control spool is forced all the way to the right, half closing the orifices. At this time, pressure P2 is determined by only the amount of fluid passing through the orifices. Fluid volume to the gear housing is kept at constant (small value) in this manner.

<4> Relief valve

The relief valve is located in the flow control valve. When pressure P2 exceeds specified level (when turning the steering wheel fully), the relief valve opens to lower pressure. When pressure P2 drops, the flow control valve is forced to the left and controls the maximum pressure.

1. Description

A steering linkage is a combination of the rods and arms that transmit the movement of the steering gear to the left and right front wheels. The steering linkage must accurately transmit the movement of the steering wheel to the front wheels as they move up and down while the vehicle is running. There are various linkage arrangements and joint constructions designed to do this.

2. Construction

The steering linkage consists of the following parts.

(1) Tie rod

(2) Tie rod end

(3) Knuckle arm

(4) Pitman arm (R/B type)

(5) Relay rod (R/B type)

(6) Steering knuckle (R/B type)

(7) Idler arm (R/B type)

(8) Drag link (R/B type)

1. Description

This type of steering column adjusts the tilt mechanism and telescopic mechanism electrically. A motor is used for each mechanism, and operations with a switch.

2. Construction

The power tilt mechanism section consists of the tilt motor, tilt worm shaft, tilt worm gear, and slider. The power telescopic mechanism section consists of the telescopic motor, sliding tube, and telescopic screw.

The switches for operating these motors are on the steering column cover.

HINT: If the tilt away auto switch is ON, when the ignition key is removed, the tilt position of the steering column automatically moves to the highest position and the telescopic position moves to the shortest position for the driver to get on and off comfortably. Also, since the ECU stores the column position read by sensor into memory, when the ignition key is inserted again, the steering column returns to its original position.

3. Operation

(1) Power tilt operation

Operating the switch up or down operates the tilt motor. The tilt worm gear and tilt worm shaft start to rotate and the slider slides. The steering column linked with the linkage tilts up or down.

(2) Power telescopic operation

Operating the switch left or right operates the telescopic motor. The telescopic worm gear starts to rotate and the sliding tube slides forward or backward.