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1National Fluid Power CentreSteering SystemsCarlton RoadWorksopNotts S81 7HPTel (01909) 504709Fax (01909) 484571www.nfpc.co.ukHYDROSTATICSTEERING SYSTEMSPresented by John R Savage Director NFPCIssue 04/05EDUCATION AND TRAINING BY PROFESSIONALS 2National Fluid Power CentreSteering SystemsThe purpose of these notes is to introduce the subject of HYDROSTATIC STEERINGThese notes are not intended to give a total explanation but merely provide an overview of a verycomplex subject To study this subject in greater detail you are recommended to visit the Sauer Danfosswebsite on www.sauer-danfoss.com and the Eaton Hydraulic website on www.hydraulics-eaton.comWe wish to thank our Advisory and Strategic Planning Group, representing:Bosch Rexroth, Eaton Corporation, Denison Hydraulics, Hydac Technology, BFPA, Moog Controls, Linde Hydraulics, Rotary Power and Witham Oils, for their kind support in giving their permission for us to use some of their illustrations in the formulation of notes presented by the National Fluid Power Centre.AcknowledgementContents Page • Introduction 3 • General Machine Layout 4• Hydrostatic Steering and Associated Parts 5• Basic Steering Operation 8 • Why Use This Type of Steering System? 11• Types of Systems Available 11• Introduction to Load Sensing 12• Priority Flow Control Valves 15• Priority Valve Operating Principles 16 • Full Load Sensing System 21• Dynamic Steering System 22 • Dual Displacement System 25 • Load Reaction System 27 3National Fluid Power CentreSteering SystemsIf we look at a typical modern wheeled excavator we see that the hydraulic system is sub-divided into a number of individual circuits: (1) the closed hydrostatic circuit for providing machine travel (in some cases machine movement is accomplished by an open circuit transmission.(2) The auxiliary circuit ( usually supplied by an axial piston swash plate pump) incorporating load sensing and used in conjunction with a multi-function direction control valve block for operating the various services such as boom lift and lower, arm and bucket operations and machine swing. This pump may be driven independently by the engine or as a “piggy-backed” unit driven from a through shaft on the main hydrostaticpump.(3) The steering and braking circuit. It is also normal practice to incorporate an additional fixed displacement pump ( usual gear type) to provide a supply to the steering system. In some cases this pump also provides the supply of oil to the service brake system.A GENERAL MACHINE LAYOUT OF SUCH A SYSTEM IS SHOWNIntroduction 4National Fluid Power CentreSteering Systems(General Machine Layout SIMPLIFIED)Closed Hydrostatic System (Machine Travel Circuit)Parking Park BrakeGearbox AxleService BrakePriority ValveMain Hydrostatic PumpAuxiliary PumpDisplacementControlSteering / BrakesPumpSteeringControlUnitService ValvesProportionalBang - BangPilot Pressure CircuitCFEFMotors CylindersLoad Holding and Motion Control ValvesPressure Relief andUnloading ValvesDirectionFlowPressureSteering CylindersService Brake ControlAuxiliary Services 5National Fluid Power CentreSteering SystemsLet us now bring all the parts together to form an open centre systemLets now look at the individual parts needed to create the steering function.First of all we need a steering wheel and steering column (although some machines use a joystick arrangement) and instead of a mechanical connection to a steering rack, in a hydrostatic steering system the steering column is connected to the steering control unit (SCU).The service ports of the SCU are then piped to the steering cylinders and these cylinders provide the mechanical connection to the steering linkage on the axleWhen the steering wheel is turned the SCU provides a metered volume of pressurised oil to the steering cylinders.Let us now focus on the major component parts of a HYDROSTATIC STEERING SYSTEM.The system is fully fluid linked, this means that there is no mechanical connection between thesteering control unit, the steering pump and the steering cylinders.A suitable axle arrangementincorporating one or two hydrauliccylinders provides the steering action.Some machines have both front and rear wheelsteering with the steering actions workingtogether, opposed or individually depending onthe machine requirements and terrain Hydrostatic Steering and Associated PartsSCU 6National Fluid Power CentreSteering SystemsHydrostatic Steering and Associated PartsSplined Coupling tofit the STEERING COLUMN Steering Cylinder PortsPressure / Tank PortsCardan ShaftNeutral Position SpringsRotary Servo Valve(spool and sleeve)Rotary Metering Unit (gerotor set often termedorbit set )STEERINGCONTROLUNITInternally, the SCU comprises a manually operated rotary servo valve and manually operatedrotary metering unit both housed in a common body. The initial deflection of the steering wheelcauses the rotary valve to open up a supply of pressurised oil to the rotary metering unit (alsoconnected to the steering wheel). Rotation of the steering wheel rotates the gerotor set via thecardan shaft connection and thus allows a precise volume of oil ( based upon its displacement)to be metered to the steering cylinders. Oil will be directed to the left or right cylinder portsdepending on which direction the steering wheel is turned.The rotary servo valve comprises a spool and sleeve incorporating a complex series ofinterconnecting galleries linked to the main ports. The rotary spool and sleeve are held in a neutral position by a group of leaf springs. When the steering wheel is turned the viscous dragand gerotor forces within the housing creates angular deflection between the spool and sleeve.This relative movement represents the opening of the valve to allow oil to flow to the gerotorset below.A maximum deflection of up to 15 degrees relative movement between the spool and sleevecan occur. The degree of opening is based upon the rate of steering which in turn sets thedemand for pressurised oil to meet the steering operation. If power fail should occur or there is insufficient flow of oil from the steering pump then manual operation can be achieved but with a greateroperator effort. 7National Fluid Power CentreSteering Systems1. Check valve2. Shock valve3. Relief valve4. Housing withanti-cavitationvalves5. Spool6. Neutral positionsprings7. Sleeve8. Cross pin9. Cardan shaft10. Distributor plate11and 12. Gerotor Set13. End coverOVPRLTPExploded view of the SCUHere we see the rotary valve and gerotor set in more detail. The amount of oil displaced by the gerotor set per revolution depends upon its displacement.Typical sizes range from 32 to 1000 cm3 per revolution.Rotary Servo Valve in Detail Rotary Servo Valve SimplifiedHydrostatic Steering and Associated Parts4 8National Fluid Power CentreSteering SystemsHYDROSTATIC STEERINGSTEERING UNITopen centre non reactionfixed displacementpumpmain relief valveshock valvesand anti - cavitation check valvesback pressure check valveto prevent cavitation incylinders(OSPC ON)The diagram above shows the typical arrangement of an open centre non-reactive system.The flow from the fixed displacement pump returns to the reservoir when the steering wheel is in the neutral position. Rotating the steering wheel opens a rotary servo spool valve inside the steering unit, thereby connecting the relevant service ports as shown below. HYDROSTATIC STEERINGfixed displacementpump RUNNINGsteering to left Basic Steering OperationNEUTRAL POSTION 9National Fluid Power CentreSteering SystemsWhen the steering wheel of a vehicle is turned the unit provides a metered flow of oil to the steering cylinders based upon the degree of opening of the rotary servo valve and the displacement of the rotary metering unit. At this stage it may be better to think of this as an adjustable throttle valve (rotary in design) that will open between zero flow and maximum flow based upon a deflection ofup to15 degrees between the inner and outer spool and sleeve .The degree of deflection being a function of the related to the physical steering rate.The diagram shows a simple open center-non reaction steering unit.With this type of system the flow rate though the unit is LOAD DEPENDENT.HYDROSTATIC STEERINGfixed displacementpump RUNNINGsteering wheel in neutralposition and vehiclemovingExternal force acting on wheelsFclosedportsshock valvelifts as pressure risediverting fluid toopposite side of cylindersOverload / Shock Protection. If the wheels are subjected to an external force withthe steering valve in the neutral position the shock valves will react as cross line relief valves.Where differential area steering cylinders are employed the re-distribution of fluid isachieved via the inline check valves. Any shortfall in fluid necessary to prevent cavitation isobtained from the return line.Basic Steering Operation 10National Fluid Power CentreSteering SystemsSo far we have introduced the basicopen centre steering system, but thereare alternative systems-Tractors-Loaders- Combine Harvesters- Forklift TucksTypical Applications for the Open Centre SystemSteering Control UnitPriority valveSteering cylindersShock valve blockMany machines employ load sensinghydraulic systems and include the operation of the steering system withinthis arrangement. Priority flow is given to the steering system based upon its importance and at this stage a valve termed a PRIORITY VALVE is incorporated with the design of the circuitry.When the steering system demand for fluid by the steering system is zero the priority valve directs the pump flow along its excess flow line. This oil may be used to charge the accumulators for the service brake system or be used to supply a auxiliary valve bank 123doc.vn