STEERING SYSTEM Chapter 5: STEERING SYSTEM I PRE-READING QUESTION How to turn direction of a moving car? What component(s) helps the car change its direction? How about the speed of the front wheels in the car when they turn? II READING TURNING THE CAR You might be surprised to learn that when you turn your car, your front wheels are not pointing in the same direction 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 you 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 Fig 5.1: Turning car with their steering system STEERING TYPES 2.1 Rack-and-pinion Steering a System introduction Rack-and-pinion steering is quickly becoming the most common type of steering on cars, small trucks and SUVs It is actually a pretty simple mechanism A rack-and-pinion gearset is enclosed in a metal tube, with each end of the rack protruding from the tube A rod, called a tie rod, connects to each end of the rack Fig 5.2: Rack-and-pinion steering Technical English for AUTOMOTIVE ENGINEERING The pinion gear is attached to the steering shaft When you turn the steering wheel, the gear spins, moving the rack The tie rod at each 28 STEERING SYSTEM end of the rack connects to the steering arm on the spindle b Advantages and disadvantages: + The advantages: Fig 5.3: Top view of the rack and pinion steering of the front-wheel drive Opel (Vauxhall) Astra (up to 1997) and Vectra (up to 1996) • simple construction • economical and uncomplicated to manufacture; • easy to operate due to good degree of efficiency; • contact between steering rack and pinion is free of play and even internal damping is maintained • tie rods can be joined directly to the steering rack; • minimal steering elasticity compliance • compact (the reason why this type of steering is fitted in all European and Japanese front-wheel drive vehicles); • the idler arm (including bearing) and the intermediate rod are no longer needed; • easy to limit steering rack travel and therefore the steering angle + The main disadvantages: • greater sensitivity to impacts; • greater stress in the case of tie rod angular forces; • disturbance of the steering wheel is easier to feel (particularly in front-wheel drivers); • tie rod length sometimes too short where it is connected at the ends of the rack • size of the steering angle dependent on steering rack travel; • decrease in steering ratio over the steer angle associated with heavy steering during parking if the vehicle does not have power-assisted steering; • cannot be used on rigid axles 2.2 Power Rack-and-pinion steering When the rack-and-pinion is in a 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 higherpressure fluid to one side of the piston forces the piston to move, which in turn moves the rack, providing the power assist Fig 5.4: Power Rack-and-pinion Technical English for AUTOMOTIVE ENGINEERING 29 STEERING SYSTEM 5.3 Recirculating-ball Steering Recirculating-ball steering is used on many trucks and SUVs today The linkage that turns the wheels is slightly different than on a rack-and-pinion system 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 (Fig 5.5) The steering wheel connects to a threaded rod, similar to a bolt, that sticks into the hole in the block Fig 5.5: Recirculating-ball steering 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 Instead of the bolt directly engaging the threads in the block, all of the threads are filled with ball bearings that recirculate through the gear as it turns The balls actually serve two purposes: • reduce friction and wear in the gear • reduce slop in the gear Slop would be felt when you change the direction of the steering wheel without the balls in the steering gear, the teeth would come out of contact with each other for a moment, making the steering wheel feel loose Fig 5.6: Recirculating ball gearbox Power steering in a recirculating-ball system works similarly to a rack-and-pinion system Assist is provided by supplying higher-pressure fluid to one side of the block Fig 5.7: The steering damper is supported on the one side at the intermediate rod and on the other side on the suspension subframe Technical English for AUTOMOTIVE ENGINEERING 30 STEERING SYSTEM 5.4 Power Steering There are a couple of key components in power steering (Fig 5.8) in addition to the rack-andpinion or recirculating-ball mechanism Fig 5.8: Power Steering a Pump The hydraulic power for the steering is provided by a rotaryvane pump (Fig 5.9) 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 must be designed to provide adequate flow when the Fig 5.9: Rotary-vane pump engine is idling As a result, the pump moves much more fluid than necessary when the engine is running at faster speeds The pump contains a pressure-relief valve to make sure that the pressure does not get too high, especially at high engine speeds when so much fluid is being pumped b 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 (Fig 5.10) Technical English for AUTOMOTIVE ENGINEERING 31 STEERING SYSTEM 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 The input from the steering shaft forms the inner part of a spool-valve assembly It also connects to the top end of the torsion bar The bottom of the torsion bar connects to the outer part of the spool valve The torsion bar also turns the output of the steering gear, connecting to either the pinion gear or the worm gear depending on which type of steering the car has Fig 5.10: The rotary valve 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 (and therefore to the steering wheel), 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 III NEW WORDS direction smoothly wheel intersect geometry linkage tie rod pinion gear steering shaft spindle Recirculating-ball steering spin rotary-vane pump steering rack threaded hole Power Steering steering arm Power Rack-andpinion pitman arm hydraulic pulley Retractable vane adequate Rotary Valve torsion bar spool-valve assembly assist twist exerting force torque slop belt pressure-relief valve sense pinion inner part connect rotate rack-and-pinion gearset steering wheel angular forces perpendicular Rack-and-pinion steering worm gear IV COMPREHENSION QUESTION When you turn your car, are your front wheels pointing in the same direction? How to turn your car smoothly? Technical English for AUTOMOTIVE ENGINEERING 32 STEERING SYSTEM Why does Rack-and-pinion steering become the most common type of steering on cars? In Rack-and-pinion steering, what is attached to the steering shaft? What is/are the main advantages of Rack-and-pinion steering? What is/are the main disadvantages of Rack-and-pinion steering? What is/are the difference(s) between Power Rack-and-pinion steering and Rack-andpinion Steering? Where are the Recirculating-ball steerings applied? What is/are the difference(s) between Recirculating-ball steering and Rack-and-pinion Steering? 10 What is/are components in power steering? V TRUE/FALSE The front wheels in a car are pointing in the same direction ( True False) For turning smoothly, each wheel must follow a same circle ( True False) Rack-and-pinion steering is a sophisticated system ( True False) Rack-and-pinion steering is always applied on trucks ( True False) Power Rack-and-pinion steering is a power assisted steering system ( True False) Recirculating-ball steering is used on many trucks and SUVs today ( True False) The recirculating-ball steering gear contains a worm gear ( True False) Power steering in a recirculating-ball system works similarly to a rack-and-pinion system ( True False) Pump and Rotary Valve are key components in power steering ( True False) 10 The hydraulic power for the steering is provided by a rotary-vane pump ( True False) Technical English for AUTOMOTIVE ENGINEERING 33 STEERING SYSTEM VI WORD(S) SELECTIONS Select ONE word(s) in the below box and fill in the gap in column B with its meaning word in column A a A type of steering system uses Pump and b To cut into Rotary Valve to turn the front wheels f A type of steering system uses rack & g A device to pinion to turn turn the car the front wheels d A rack-andpinion steering system assisted by hydraulic cylinder with a piston e A type of steering system is used on many trucks and SUVs h A pump uses vane & i A type of oil rotor to using in steering control oil system flow j The device that senses the force on the steering wheel c Right angle No Column A perpendicular Column B No Steering wheel Power Rack-andpinion steering Recirculating-ball steering rotary-vane pump 10 Technical English for AUTOMOTIVE ENGINEERING Column A intersect Rack-and-pinion steering Column B Power Steering Rotary Valve slop 34