FOUR-WHEEL STEERING SYSTEM
ABSTRACT
THIS PAPER
DEALS WITH THE DETAILS OF FOUR WHEEL STEERING(4WS)SYSTEM.WITH THE HELP OF THIS
SYSTEM ALL THE FOUR WHEELS CAN BE TURNED TO ANY DIRECTION USING THE
STEERING.THUS THE VEHICLE CAN BE CONTROLLED MORE EFFECTIVELY ESPECIALLY DURING
CORNERING AND PARKING.ALSO THE SPEED OF THE VEHICLE CAN BE INCREASED OR
DECREASED.
THERE ARE
THREE TYPES OF PRODUCTION OF FOUR-WHEEL STEERING SYSTEMS:
1 .MECHANICAL
4WS
2 .HYDRAULIC
4WS
3. ELECTRO /HYDRAULIC
4WS
THE
MECHANICAL 4WS USES TWO SEPARATE STEERING GEARS TO CONTROL THE FRONT AND REAR
WHEELS. THE HYDRAULIC 4WS USES A TWO-WAY HYDRULIC CYLINDER TO TURN BOTH THE
WHEELS IN THE SAME DIRECTION.IT IS NOT POSSIBLE TO TURN IN THE OPPOSITE
DIRECTION.THE ELECTRO/HYDRAULIC 4WS COMBINE COMPUTER ELECTRONIC CONTROLS WITH
HYDRAULIC TO MAKE THE SYSTEM SENSITIVE TO BOTH STEERING ANGLE AND ROAD SPEEDS.
THIS SYSTEM
INDS APPLICATION IN OFF – HIGHWAY VEHICLES SUCH AS FORK LIFTS,AGRICULTURAL AND
CONSTRUCTION EQUIPMENT AND MINING MACHINERY.IT IS ALSO USEFUL IN PASSENGER
CARS,MAINLY SUVS.FOUR-WHEEL STEERING,4WS,ALSO CALLED REAR-WHEEL STEERING OR ALL
– WHEEL STEERING, PROVIDES A MEANS TO ACTIVETY STEER THE REAR WHEELS DURING
TURNING MANEUVERS.IT SHOULD NOT BE CONFUSED WITH FOUR-WHEEL DRIVE IN WHICH ALL
FOUR WHEELS OF A VEHICLE ARE POWSERED.IT IMPROVES HANDLING AND HELP THE VEHICLE
MAKE TIGHTER TURNS.
PRODUCTION-BUILT
CARS TEND TO INDERSTEER OR, IN FEW INSTANCES, OVERSTEER.IFF A CAR COULD
AUTOMATICALLY COMPENSATE FOR AN UNDERSTEER/OVERSTEER PROBLEM, THE DRIVER WOULD
ENJOY NEARLY NEUTRAL STEERING UNDER VARYING CONDITIONS.4WS IS A SERIOUS EFFORT
ON THE PART OF AUTOMOTIVE DESIGN ENGINEERS TO PROVIDE NEAR – NEUTRAL STEERING.THE
FRONT WHEELS DO MOST OF THE STEERING. REAR WHEEL TURNING IS GENERALLY LIMITED
TO 50-60 DURING AN OPPOSITE DIRECTION TURN.DURING A SAME DIRECTION TURN, REAR
WHEEL STEERING IS LIMITED TO ABOUT 10-1.50.
WHEN BOTH
THE FRONT AND REAR WHEELS STEER TOWAARD THE SAME DIRECTION, THEY ARE SAID TO BE
IN-PHASE AND THIS PRODUCTS A KIND OF SIDEWAYS MOVEMENT OF THE CAR AT LOW SPEEDS.WHEN THE FRONT AND REAR
WHEELS ARE STEERED IN OPPOSITE DIRECTION,THIS IS CALLED ANTI-PHASE,COUNTER –PHASE
OR OPPOSITE –PHASE AND IT PRODUCES A SHARPER,TIGHTER TURN.
WHY FOUR –
WHEEL STEERING SYSTEM?:
TO
UNDERSTAND THE ADVANTAGES OF FOUR – WHEEL STEERING, IT IS WISE TO REVIEW THE
DYNAMICS OF TYPICAL STEERING MANEUVERS WITH A CONVENTIONAL FRONT – STEERED
VEHICLE.THE TORES ARE SUBJECT TO THE FORCES OF GRIP,MOMENTUM,AND STEERING INPUT
WHEN MAKING A MOVEMENT OTHER THAN STRAIGHT-AHEAD DRIVING.THESE FORCES COMPETE
WITH EACH OTHER DURING STEERING MANEUVERS.WITH A FRONT – STEERING VEHICLE,THE
REAR END IS ALWAYS TRYING TO CATCH UP TO THE DIRECTIONAL CHANGES OF THE FRONT
WHEELS. THIS CAUSES THE VEHICLE TO SWAY.AS A NORMAL PART OF OPERATING A
VEHICLE, THE DRIVER LEARNS TO ADJUST TO THESE FORCES WITHOUT THINKING ABOUT
THEM.
WHEN TURNING,THE
DRIVER IS PUTTING INTO MOTION A COMPLEX SERIES OF FORCES.EACH OF THESE
MUST BE BALANCED AGAINST THE OTHERS.THE
TIRES ARE SUBJECTED TO ROAD GRIP AND SLIP ANGLE.GRIP HOLDS THE CAR’S WHEELS TO
THE ROAD,AND MOMENTUM MOVES THE CAR STRAIGHT AHEAD. STEERING INPUT CAUSES THE
FRONT WHEELS TO TURN.THE CAR MOMENTARILY RESISTS THE TURNING MOTION,CAUSING A
TIRE SLIP ANGLE TO FORM.
ONCE THE
VEHICLE BEGIINS TO RESPOND TO THE STEERING INPUT,CORNERING FORCES ARE
GENERATED. THE VEHICLE SWAYS AS THE REAR WHEELS ATTEMPT TO KEEP UP WITH THE
CORNING FORCES ALREADY GENERATED BY THR FORNT TIRES.THIS IS REFERRED TO AS REAR
– END LOAG,
BECAUSE
THERE IS A TIME DELAY BETWEEN STEERING INPUT AND VEHICLE REACTION.WHEN THE
FRONT WHEELS ARE TURNED BACK TO A STRAIGHT-AHEAD POSITION,THE VEHICLE MUST
AGAIN TRY TO ADJUST BY REVERSING THE SAME FORCES DEVELOPED BY THE TURN.AS THE
STEERING IS TURNED,THE VEHICLE BODY SWAYS AS THE REAR WHEELS AGAIN TRY TO KEEP
UP WITH THE CORNERING FORCES GENERATED BY THE FRONT WHEELS.
THE IDEA
BEHIND FOUR – WHEEL STEERING IS THAT A VEHICLE REQUIRES LES DRIVER INPUT FOR
ANY STEERING MANEUVER IF ALL FOUR WHEELS ARE STEERING THE VEHICLE.AS WITH
TWO-WHEEL STEER VEHICLES, TIRE HOLDS THE FOUR WHEELS ON THE ROAD.HOWEVER WHEN
THE DRIVER TURNS THE WHEEL SLIGHTLY,ALL FOUR WHEELS REACT TO THE STEERING
INPUT,CAUSING SLIP ANGLES TO FORM AT ALL FOUR WHEELS.THE ENTIRE VEHICLE MOVES
ON ONE DIRECTION RATHER THAN THE REAR HALF ATTEMPTING TO CATCH UP TO THE
FRONT.THERE IS ALSO LESS SWAY WHEN THE WHEELS ARE TURNED BACK TO A
STRAIGHT-AHEAD POSITION.THE VEHICLE RESPONDS MORE QUICKLY TO STEERING INPUT
BECAUSE REAR WHEEL LAG IS ELIMINATED.
MECHANICAL
4WS
IN A
STRAIGHT-MECHANICAL TYPE OF 4WS,TWO STEERING GEARS ARE USED –ONE FOR THE REAR WHEELS.A STEEL SHAFT CONNECTS THE
TWO STEERING GEARBOXES AND TERMINATED AT AN ECCENTRIC SHAFT THAT IS FITTED WITH
AN OFFSET PIN.THIS PIN ENGAGES A SECOND OFFSET PIN THAT FITS INTO A PLANETARY
GEAR.
THE
PLANETRY GEAR MESHES WITH THE MATCHING TEETH OF AN INTERNAL GEAR THAT IS
SECURED IN A FIXED POSITION TO THE GEARBOX HOUSING.THIS MEANS THATTHE PLANETRY
GEAR CAN ROTATE BUT THE INTERNAL GEARR CANNOT.THE ECCENTRIC PIN OF THE PLANETRY
GEAR FITS INTO A HOLE IN A SLIDER FOR THE STEERING GEAR.
A
120-DEGREE TURN OF THE STEERING WHEEL ROTATES THE PLANETRY GEAR TO MOVE THE
SLIDER IN THE SAME DIRECION THAT THE FRONT WHEELS ARE
HEADED.PROPORTIONATELY,THE REAR WHEELS TURN THE STEERING WHEEL ABOUT 1.5 TO 10
DEGREES.FURTHER ROTATION OF THE STEERING WHEEL,PAST THE 120-DEGREE POINT,
CAUSES THE REAR WHEELS TO START STRAIGHTENING OUT DUE TO THE DOUBLE-CRANK
ACTION(TWO ECCENTRIC PINS)AND ROTATION OF THE PLANETRY GEAR.TURNING THE
STEERING WHEEL TO A GREATER ANGLE,ABOUT 230 DEGREES,FINDS THE REAR WHEELS IN A
NEUTRAL POSITION REGARDING THE FRONT WHEELS.FURTHER ROTATION OF THE STEERING
WHEEL RESULTS INTHE REAR WHEELS GOING COUNTER PHGASE WITH REGARD TO THE FRONT
WHEELS.ABOUT 5.3 DEGREES MAXIMUM COUNTER PHASE REAR STEERING IS POSSIBLE.
MECHANICAL
4WS IS STEERING ANGLE SENSITIVE.IT IS NOT SENSITIVE TO VEHICLE ROAD SPEED.
HYDRAULIC
4WS
THE
HYDRAULIC OPERATED FOUR-WHEEL-STEERING SYSTEM IS A SIMPLE DESIGN,BOTH IN
COMPONENTS AND OPERATION.THE REAR WHEELS TURN ONLY IN THE SAME DIRECTION AS THE
FRONT WHEELS.THEY ALSO TURN NO MORE THAN 11/2 DEGREES.THE SYSTEM ONLY ACTIVATES
AT SPEEDS ABOVE 30 MPH (50 KM/H) AND DOES NOT OPERATE WHEN THE VEHICLE MOVES IN
REVERSE.
A TWO-WAY
HYDRAULIC CYLINDER MOUNTED ON THE REAR STUB FRAME TURN THE WHEELS.FLUID FOR
THIS CYLINDER IS SUPPLIED BY A REAR STEERING PUMP THAT IS DRIVEN BY THE
DIFFERENTIAL.THE PUMP ONLY OPERATES WHEN THE FRONT WHEELS ARE TURNING. A TANK
IN THE ENGINE COMPARTMENT SUPPLIES THE REAR STEERING PUMP WITH FLUID.
WHEN THE
STEERING WHEEL IS TURNED,THE FRONT STEERING PUMP SENDS FLUID UNDER PRESSURE TO
THE ROTARY VALVE IN THE FRONT RACK AND PINION UNIT.THIS FORCES FLUID INTO THE
FRONT POWER CYLINDER,AND THE FRONT WHEELS TURN IN THE DIRECTION STEERED.THE
FLUID PRESSURE VARIES WITH THE TURNING OF THE STEERING WHEEL.THE FASTER AND
FARTHER THE STEERING WHEEL IS TURNED,THE GREATER THE FLUID PRESSURE.
THE FLUID
IS ALSO FED UNDER THE SAME PRESSURE TO THE CONTROL VALVE WHERRE IT OPENS A
SPOOL VALVE IN THE CONTROL VALVE HOUSEING.AS THE SPOOL VALVE MOVES,IT ALLOWS
FLUID FROM THE REAR STEERING PUMP TO MOVE THROUGH AND OPERATE THE REAR POWER
CYLINDER.THE HIGHER THE PRESSURE ON THE SPOOL, THE FARTHER IT MOVES.THE FARTHER
IT MOVES,THE MORE FLUID IT ALLOWS THROUGH TO MOVE THE REAR WHEELS.AS MENTIONED
EARLIER,THIS SYSTEM LIMITS REAR WHEEL MOVEMENT TO 11/2 DEGREES IN EITHER THE
LEFT OR RIGHT DIRECTION.
REFERENCE
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