Parts of Electric Power Steering System

Figure 2: Electric Power Steering.

The main components of the electric power steering system (see Figure 2) are as follows,

1. Motor
2. Electronic Control Unit (ECU)
3. Torque sensor
4. Rotating (or) Steering angle sensor
5. Reduction gear box.

Motor

The motor employed for Electric Power Steering (EPS) system gear assembly is a permanent magnetic field DC motor. This motor generates steering assisting force required to turn the wheels. The motor should be able to produce torque without turning and also, to reverse the rotation rapidly.

Electronic Control Unit (ECU)

ECU is an important device of EPS system. The ECU controls the input and output signals of steering sensor and vehicle speed sensor. In EPS system, ECU has three primary roles or functions. They are,

1. To control the motor current and steering functions.
2. For self-diagnosis and fail-safe functions i.e., for monitoring the controls of EPS system and its components.
3. To communicate between the ECU and EPS system i.e., the data stored in ECU can be read by EPS.

Torque Sensor

A torque sensor in EPS system is used to convert the input torque of steering and its direction into signals of voltage. Torque sensor consists of input and output shaft, which are connected by a torsion bar. The input shaft has splines and output shaft has slots. The displacement of input and output shaft produces the torque in the torsion bar, which is magnetized and changes into the signals of voltage.

Rotating or Steering Angle Sensor

It is located in the steering box and is employed to convert the rotational speed and its direction into signals of voltage.

Working of Electric Power Steering System

During steering operation, the inputs from the vehicle speed sensor and steering sensor are sent to ECU. The ECU will compare the input signals with the assisting force of steering, which is pre-programmed and sends the appropriate signals to the current controller. The controller supplies a sufficient amount of current to the electric motor. Therefore, the supply of current causes the motion of rack towards the left or right, depending upon the direction of the current. The motor can be protected from being overloaded and surges of voltage, due to faulty alternator or charging problem by an Electronic Steering Control unit (ESC). The ESC unit is a self-diagnosing device, which is capable to monitor the input and output of the system and supply of driving current to the driving motor. If a problem exists in the system, then by actuating a fail-safe relay in power unit, the entire system is turned off through the ESC unit. Thus, the steering system reverts back to the manual steering system and a red light is indicated on dash board as an indication to the driver

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Figure 1: Sliding Mesh Gearbox

Construction & Working of Sliding Mesh Gearbox

The sliding mesh gearbox consists of three shafts namely input shaft, output shaft and lay shaft. The input and output shafts are in line, whereas lay shaft is parallel to them and lies below them. The function of input shaft is to take power from engine and transmit to output shaft through a series of gears mounted on layshaft. The function of output shaft is to transmit power obtained from input shaft to the propeller shaft for onward transmission to road wheels. Schematic layout of sliding mesh gear box is shown in Figure 1.

Input shaft has a clutch gear mounted on it. Clutch gear is in constant mesh with gear A on the layshaft. Gear A is larger than clutch gear. Layshaft has second gear, first gear and reverse gear pinions mounted on it. Pinions and gear A are integrally constructed with layshaft so they cannot be slided on the shaft. Output shaft has two gears mounted on it and has splinted on it. These two gears can slide on output shaft. These gears have forks attached to them on the output shaft and can be moved by a desired selector. Each fork has one selector. Selectors are moved by a control lever at the desire of driver. The control lever motion gives direction as 1, 2, 3 or R depending upon the gear to be engaged like first, second, third or reverse. Its position in neutral gear is marked as N. The direct drive dogs are on the input and the output shafts. These are coupled to each other for engaging top gear In this sliding mesh gear box third gear is the top gear Idler (a pinion) is always in mesh with pinion on a layshaft and is used for engaging the reverse gear. It causes the reversal of rotation.

For changing the gear, clutch should be depressed and lever should be moved till the selector pinion on the main shaft (output shaft) engages with its mating gear on layshaft. When clutch is released, the drive from the engine will be transmitted through the gear box.

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Fig. 1: Wilson-Hartnell Governor

Working of Wilson Hartnell Governor

When the load on the engine decreases, then the radius of rotation of flywheel increases due to increase in speed, the balls try to fly out (move away from the governor axis). The main springs exert an inward pull force on the balls and the sleeve moves upwards closing the throttle valve to decrease the speed. Exactly opposite phenomenon is observed, if load on the engine increases.

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(a)

(b)

Fig. 1: Pickering Governor

Working of Pickering Governor

Fig. 1 (a) shows the Pickering governor, when it is at rest. When load on engine decreases, then the speed of governor and hence speed of spindle increases, the weights move outwards due to the action of centrifugal force. It causes the leaf springs to bend outwards, since they rotate about the spindle axis with increasing speed. Refer Fig.1 (b). The sleeve can rise until; it reaches a stopper, whose position can be adjusted. Due to raising sleeve, supply of fuel decreases and speed of governor and hence, engine speed can be reduced till it reaches the desired speed.

Application of Pickering Governor

Pickering Governor is mostly used for driving gramophone to adjust speed of turn-table.

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Construction of Hartnell Governor

Hartnell governor consists of a casing, in which, a pre-compressed spring of helical compression type is fitted so as to apply force on the sleeve. The sleeve can slide – up and down on the vertical spindle. The casing is keyed to the spindle. Therefore, when the spindle rotates, the casing along with the spring also rotates about the axis of governor. Two bell crank levers are fitted on the frame of casing. Each bell crank lever carries a fly-ball at one end [vertical arm OB] and a roller at the other end [horizontal arm OR]. The rollers fit into a groove in the sleeve. Bell crank levers are pivoted at point ‘O’ to the frame. Therefore point ‘O’ acts as a fulcrum. A helical compression spring exerts equal downward forces on the two rollers through a collar provided on the sleeve. Refer Fig. 1.

Fig. 1: Hartnell Governor

Working of Hartnell Governor

When the load on engine increases

The speed of governor decreases and the fly-balls move inwards, i.e. towards the governor axis. Due to this, the bell crank lever moves about the fulcrum and its roller end lowers the sleeve. This downward movement of sleeve is transferred to throttle valve of an engine through suitable links (not shown in figure), so that, the supply of working fluid is increased in order to increase the speed of governor up to original value.

When the load on engine decreases

The speed of governor increases and the fly-balls move outwards, i.e. away from the governor axis. Due to this, the bell crank lever moves about the fulcrum and its roller end lifts the sleeve against the controlling force. This upward or lifted movement of sleeve is transferred to throttle valve of an engine, so that, the supply of working fluid is decreased in order to decrease the speed of governor up to original value.

Note: Here, the controlling force is spring force along with the weight of sleeve. This spring force can be adjusted by tightening or loosing the nut.

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Construction of Proell Governor

In Proell governor, comparatively small diameter fly-balls can be used for same height. In Proell governor, the fly-balls are mounted on the extension links of lower arms, whereas in case of Porter governor, the fly-balls are attached at the junctions of upper and lower arms. The lower arms with extension links in Proell Motion of sleeve governor act as ‘Bell crank lever’.

Fig. 1: Proell Governor

Under normal conditions, the extension links remain vertical; however, they change their positions with the variations in speed caused due to variations in load on the engine.

Working of Proell Governor

Proell governor works in the same manner as that of porter governor. An increase in speed of rotation (due to decrease in load) increases the radius of rotation of fly-balls and raises the sleeve, thus reducing the amount of working fluid (fuel) supplied to the engine. Conversely, decrease in speed of rotation (due to increase in load) decreases the radius of rotation of fly-balls and lowers the sleeve, thus increasing the amount of working fluid (fuel) supplied to the engine.

Application of Proell Governor

Proell governors are suitable for ‘medium speed range’ applications.

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Fig. 1: Double Block (Shoe) Brake.

Construction of Double Block (Shoe) Brake

Double block or shoe brake consists of two blocks or shoes situated at opposite ends of circumference of brake drum (wheel). This arrangement reduces the unbalanced force acting on the shaft, when brake is applied. The upper ends of brake arms are set together by using a spring.

Working of Double Block (Shoe) Brake

When brake is in released position (i.e. brake is disengaged)

Using an electromagnet or solenoid, electric current is supplied to apply an actuating force ‘P’ to the lever. Due to this actuating force ‘P’ applied to the lever, the spring gets compressed and brake gets released. At this condition, brake drum (wheel) will rotate continuously, since brake is disengaged due to current being supplied.

When brake is to be applied (i.e. brake is to be engaged)

For braking action, current supply is stopped. Due to this, actuating force ‘P’ acting on the lever suddenly vanishes and becomes zero. This makes the spring to elongate freely. Therefore, brake gets automatically engaged causing both blocks or shoes to move inward, i.e. towards the brake drum. Contacting or frictional surfaces of blocks start retarding the motion of brake drum and finally, it is stopped. Unless and until, supply of electric current is not restarted, brake will not be released or disengaged.

Application of Double Block (Shoe) Brake

Electric cranes.

In these brakes, the braking torque is given by,

T_B = (F_t1 + F_t2) × r = μ (R_N1 + R_N2) × r

where, F_t1 and F_t2 = Braking forces on the two blocks in ‘N’, and

R_N1 and R_N2 = Normal reactions on the two blocks in ‘N’.

Limitations of Double Block (Shoe) Brake

1. As brake drum assembly is enclosed, it does not allow the cooling air to enter the assembly. Due to this, heat generated during action of braking is not dissipated to atmosphere with enough rate of heat transfer required. This heat generated remains inside the brake assembly for longer period. This causes the radius of the drum to increase more than radius of brake shoe. This reduces braking ability of a vehicle by 15 – 20
2. Enclosed assembly of brake drum does not allow to use water as cooling medium, because water carrying the heat generated in drum assembly finds difficulty in expelling out rapidly. Also, water use makes the brake cavity wet, which reduces frictional properties of brake system and hence, braking ability of vehicle.
3. Use of many clips and springs make the overhauling or maintenance activity much time consuming in addition to fatigue.
4. Dust of asbestos gets collected in the brake cavity due to enclosed assembly

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