hydraulic principle

  • work is force acting through a distance
  • power is the amount of work done in a specific time
  • potential : pressure
  • kinetic : flow rate (velocity)
  • in pipe : potential + kinetic = constant
  • cylinder: flow rate = cyl vol / time
  • 1 gal = 231 cubic inches
  • power loss
    • pressure loss: potential energy is change to heat energy. small conductor, abrupt change cross session, direction
    • flow loss : slower hyd func, heat, too much clearance, wear
    • back pressure is the resistance to flow on the return side of comp
  • series: same flow rate, split pressure: limit torque
  • para: split flow rate, same pressure : more torque
  • viscosity : resistance to flow
    • low: oil film thinner, improve flow characteristics
    • high: oil film thicker
    • need: adequate film strength for lub and able to flow to small clearance
  • vi : ability resist changes in viscosity as temp changes (at least 90)
  • ester base fluid
    • produce acid on exposing to water
    • react with zinc, brass, bronze
    • swell nitrile-based seal (Buna N)
    • compatible with Viton (EPR) seals
  • replace filter after 100 hours to remove built-in contamination
  • faulty air breathers draw dirt to reservoirs
  • leaked pump inlet cause air and contamination entering system
  • rod seal and wipers are wear ingress more contamination
  • uncleaned maintenance schedule
  • strainer in res at least 200 mesh
  • catastrophic, intermittent, degradation failures

pumps

  • gear : 6 * W * (2D - L) * (L - D) / 2
  • vane : 12 * W * {(L + D) / 4} * {(L - D) / 2}
    • inlet ports are paced 180 apart, same as outlet ports
    • vane pockets increase in vol in inlet, decrease vol outlet ports
    • the high point of bevelled tip is the leading edge in the rotation
    • oil passage is drilled from behind the vane (on bevelled face)
    • can have arrow on housing or on rotor
    • require to flip the rotor and vane if its is bevelled tip
  • piston pump
    • pressure compensated pump varies its displacement in response to system pressure
      • bias spring want full flow
      • stroking piston sense system pressure want zero displacement
  • pump flow rate: pump displacement (cubic meter) * rpm
  • pump efficiency:
    • mechanical : friction
    • volumetric: inter nal leakage
    • overall = mech + vol
  • cavitation
    • restricted inlet

actuators

cyl

  • piston seal leaks
    • reduce cyl speed
    • cyl drift in the extension direction
    • pressure intensification when the load is applied on piston rod in the retraction direction
  • piston relief allows oil bypassing the piston when the piston reaches the end of its stroke
    • used in mobile equipment that has limited res vol to prevent heat up the oil
  • single acting cyl
    • ram type: guide ring support the inner end of the piston tube, but does not seal the piston in the barrel
  • cyl cushion
    • slow down cyl rod when it reaches the end of its stroke
    • prevent shock load when piston come to its end
    • restrict oil when the piston reach its end stroke
  • cyl return drill
    • allow oil is returning though small drilling in the cyl barrel when the piston end its stroke.
    • control deceleration of cyl movement
  • quick drop valves
    • quick drop or extend the piston by pumping oil from rod end to base end
    • power down: when the blade hits the ground, stop pumping oil from rod end to base end, thus, providing max down force
    • normal drop by feather dcv, (not regenerate)
  • apply light coating of hydraulic oil to the seals and surfaces they must slide over to make installation easy, and prevent seal stretch or damage
  • after installing cyl, check and adjust system relief (especially if the old cyl was failed because of excessive pressure)
  • if cavitation occurs on rod side, (pitch hole on rod), air and contamination could be pulled into the cyl past the rod seal and wiper

motor

  • speed depends on flow rate and motor displacement
  • torque depends on inlet pressure and motor displacement which influence the surface area
  • gear motor: poor torque
  • gerotor or orbit motor : high torque & low rpm (1 revolution of shaft, all lobes must be exposed to pressurized oil)
    • the spool valve directs the flow of oil to and from the valleys in the rotor element
  • case drain allows some internal leakage (lub and cool parts) returning to res
  • piston pump:
    • larger swash plate angle, motor displacement increases => torque increase, speed decrease
    • smaller swash plate angle, motor displacement decreases => torque decrease, speed increase
    • pressure compensated motor varies its displacement in response to changes in the load
    • bias spring try to keep minimum swash plate angle => max speed, min torque
    • comp spool sense inlet pressure want maximum swash plate => max torque, min speed
  • motor will hydraulically lock and stall in the zero displacement position
  • disassemble
    • housing should be marked
    • external gear pump is non-hunting, should be marked
    • pistons should be kept in in their original bores (cylinder barrel)
  • overloading
    • excessive pressure
    • crack housing
  • contamination
    • sandblasted
    • heavy core
    • inspect filter, strainers
  • cavitation
    • pitted erosion
    • the load overrun the motors, motor rotates faster than the pump supplies; inlet pressure falls too low
  • lack of lub
    • heavy wear
    • blue color
    • improper oil

hydraulic valves

pressure control valves

  • direct acting : ball, poppet, guide poppet
    • high amount of override pressure, causing waste power, generates unwanted heat
    • used low flow rate applications or infrequent duty cycles
  • double acting
    • less override pressure required
    • pilot valve has big spring deal with high pressure, low flow rate
    • other valve has ligh spring deal with low pressure, high flow rate
    • less prone to chatter
    • operate through a wider pressure range
  • applications
    • pressure relief
      • system relief: limit system pressure
        • double acting
        • pass full flow rate as little pressure override
        • excessive override pressure cause relief valve to crack well within operating pressure range
        • adjust @ high idle
        • hold maximum pressure; max load on the pump, max energy converted to heat
      • circuit relief : limit pressure in one branch or circuit, protect comp from pressure pike (shock load)
        • does not require to handle full flow
        • adjust @ low idle to avoid valve override cause inaccurate reading
        • can use hand pump to adjust
  • unloader valves
    • allow full flow return to res and maintain minimum pressure at pump outlet
    • work with a check valve
    • pilot sense from downstream of check valve (high pressure side)
    • pilot operated to handle full flow
    • closed center hyd sys with fix dis pump unloader
    • demand type hyd sys with tandem fix dis pump (hi-low configuration)
      • normally closed, provide more flow with light load
      • when the load is high, the valve opens first pump’s oil to tank, as we don’t need too much flow tandem demand type
  • pressure reducing valves
    • allow a circuit to have a max operating pressure that is less than the max system pressure
    • normally open
    • sense downstream of the valve
    • pilot operated to handle large flow rates
    • externally drain
    • drain line allows internal leakage return to the res preventing an internal hyd lock that would hold the valve in the open position prv
  • sequence valves
    • cause action occur in order
    • normally closed
    • sense upstream of the valve
    • externally drain
    • either direct acting or pilot operated sv
  • backpressure valves: counterbalanced, brake valves, and lock valves
    • control actuator movement by restricting return oil from the actuator
    • counterbalance valves
      • control safely lowering the load
      • normally closed
      • sense upstream of the valve
      • internally drain
      • set maximum pressure on the vale match intended load to prevent cavitation rod end.
      • work well with constant load
      • pressure intensification can happen if a restriction is put to return line of the rod end cbv
    • brake valves
      • prevent the load from overspeeding the motor and modulate deceleration no matter what the lad
      • 2 sense lines counteract spring force
        • first senses upstream of the actuator, acting on large piston surface
        • second senses upstream of the valve or downstream of the actuator, acting on small piston surface
        • support cushion the load when suddenly changing to neutral, prevent shock load by using second pressure
      • internally drain bkv
    • lock valves or pilot operated check valves (basically, a safety valve)
      • hold load when hyd line rupture, prevent overspeeding, and drifting due to leakage
      • used in blade lift circuit of graders to avoid cyl drift during operation
      • used in outriggers to avoid crane roll over if there’s leakage lckv

accumulator

  • weight (constant regardless of vol oil), spring, gas (piston, bladder)
  • purpose:
    • store engergy
    • shock absorption
    • gradual pressure build up (applications like handling fragile load, e.i. eggs)
    • constant pressure maintenance: absorb oil when pressure rise and deliver oil when it falls

flow control valves

  • actuator speeds depends on its capacity and flow rates
  • non-pressure compensated flow control use a variable orifice to feather the oil flow
    • adv : work best with constant pressure
    • dis : speed of actuators varies as the load changes
  • pressure compensated flow control maintain constant oil flow by maintaining constant pressure drop across orifice pcfcv
  • bypass type flow control
    • used with fixed displacement pump
    • excess flow can be wasted btfc
  • restrict type flow control
    • used with pressure comp pump
    • normally open valves
    • closed when experience high pressure, send signal to destroke pump. rtfca rtfc
    • meter in
      • load pulling the actuators may cause cavitation
      • control the speed of cyl extension
      • work best if the load oppose the cyl movement mi
    • meter out
      • work well despite the load changes and directions
      • overruning load can cause pressure intensification mo
  • flow dividers
    • priority flow dividers
      • divert all flow to a primary circuit (steering) before flow to implement circuit
      • normally closed implement circuit pfd pfda
    • proportional flow dividers
      • ensure the same amount of oil flow passes through each of outlet ports regardless of pressure variations
      • load on outlet 1, restrict on outlet 2; thus reducing flow on outlet 2 pfda

directional control valves

  • poppet valve: check valve
  • rotary spool valves
    • higher internal leakage
    • used in low pressures and flow rate application, like controlling pilot oil flow
  • slide spool valves
    • handle high pressures and flow rate with minimum leakage
    • the goose trap oil for better lub and sealing, allowing pressurized oil to act around the spool, centering, preventing binding, and excessive wear
    • notches help metering oil flow
    • diverter valves : 2 positions dcv
    • float mode : work ports connect to tank (loader boom, grade blade lift) usually use detent to hold in float position
  • center types
    • open center dcv
      • work with system relief
      • fixed displacement pump
      • open center, closed port
      • open center, open port (motor or float center)
        • use with 2 brakes valve on working lines to prevent shock load and drift
    • closed center dcv
      • fixed displacement with unloader valves
      • pressure comp var dis pump
  • dcv actuators
    • mech actuation (lever, foot pedal, push button)
    • pilot operated (hyd or air)
      • reduce effort to move dcv
      • remote actuation
      • sliding pool type pilot dcv: does not regulate pressure applied on main dcv
      • pressure regulating controller: provide accurate dcv movement by regulating pilot pressure to the dcv
    • electrical (selenoid)
      • electric-over-pilot : easy & economical remote control with greater pilot force
  • dcv detenting
    • mech
    • electromagnetic can work with proximity switch on loader to stop the boom movement
  • dcv accessories
    • system and circuit relief
    • load check valves: prevent a suspended load from momentarily dropping when dcv is moved from hold to raise
    • anti cavitation valves:
      • prevent a void forming in the actuator when the load overrun the pump
      • located between work port and return passage in the dcv atcv
    • flow compensator
      • provide accurate flow (speed) to the actuators regardless of the load
      • ensure constant pressure drop across the dcv spool by sensing pressure before and after dcv spool
      • use double check valves on working lines to assist compensator spring regardless of the dcv position
        • ensure that the highest load pressure is sent to the compensator compv
  • multi dcvs
    • unibody: compact, less point for external leakage, hard to maintenance
    • valve stacks
  • multi dcv conns
    • open center dcv conns
      • series passageway
        • have a passage through the valves to the res
        • priority given to the first pump spss
      • tandem center conn
        • return oil back to passageway
        • actuators move together as both dcvs are actuated tdcn
      • series-parallel conn
        • oil flow to series passageway in neutral
        • oil flow to operators drawn from the para passageway
        • actuators move together, the lightest load actuator will have priority spcnn
    • closed center dcv conns
      • don’t have a series passageway
      • valves connected para, including aux ccdccn
  • methods to connect an auxiliary dcv to open center hyd sys
    • diverter valve
    • power beyond sleeve
    • flow divider

hydraulic system types

open center (constant flow sys)

fix dis pump and load check valves

  • cheap (repair and purchase)
  • used of load check valves
  • used for low percentage of machine’s duty cycle, such as haul truck hoist circuit, wheel loader
  • always have flow in the series passageway allowing oil return to tank in neutral

constant horsepower open center (used pressure comp pump)

  • regulate pump flow based on pump output pressure, excavator
  • low pressure -> upstroke to get high displacement
  • high pressure -> destroke to get low displacement (to limit max power drawn from engine)
  • designed to operate engine at a constant speed
  • used side-by-side pressure comp pump
  • upstroke and destroke based on
    • power shift pressure generated by PRV (power shift proportional reducing valve)
      • based on selected speed
      • PRV pressure decrease, pump output increases
    • negative flow control (NFC) from main control valve GP (group)
      • NFC pressure decrease, pump output increases
    • cross sensing signal pressure from out put of the two pumps
      • cross sensing pressure decrease, pump output increases

close center

  • fix dis pump with accumulator and unloader valve
    • similar to “open center with fix dis pump & load check valves”
    • there’s constant oil flow in neutral
  • demand flow system or pressure comp hyd sys
    • used pressure comp pump
    • flow is provided when it’s required for work
  • load sensing hyd sys
    • provide precise amount of flow and pressure to perform work demanded by actuators
    • minimum waste energy
    • actuator speed will not change as the load changes
    • keep the pump outlet pressure slightly higher than the load pressure, margin pressure (150 - 450psi)
    • load sense spool or flow comp or margin spool
      • control flow (affect actuator speed)
      • margin pressure: 350 psi
    • pressure limiting spool or pressure comp or cutoff spool
      • control pressure (2900 psi)
    • dozer
    • stages
      • engine off
        • flow comp dump oil in the control piston
        • max wash plate angle
      • stand by
        • pump outlet pressure force flow comp destroke control piston reduce wash plate angle
        • minimum wash plate angle
        • produce just enough oil to make up for leakage
        • stand by pressure is slight higher than margin pressure to overcome flow comp’s spring to activate the control piston
      • upstroke
        • load pressure signal stops “flow comp” keeping the control piston pressure
        • oil in the control piston is dumped to tank
        • bias spring wins
        • max wash plate or pump upstroke
      • constant flow
        • pressure drop across dcv is stable (dcv is fixed at a position)
        • flow comp’s spring pressure + load pressure = pump output pressure
        • the contained oil in the control piston will be pump at fixed rate
      • destroke
        • load pressure drops
        • pump output pressure wins
        • flow comp open passage for oil going to the control piston
        • pump is destroked
      • high pressure stall
        • stall actuators
        • pump is upstroking to handle the load
        • flow comp’s spring pressure + load pressure is less than pump output pressure
        • flow comp blocked oil going to the control piston => max wash plate angle
        • pressure comp notice that max pressure is reached 2900psi
        • pressure comp allows oil going to the control piston => dewash the pump
        • pressure comp overrides flow comp to destroke the pump
    • adjust flow comp and pressure comp springs
      • fcs weaken : result low flow rate.at any load and dcv position
      • pcs weaken: result machine weaker, function normally at light load, but can not create higher work pressure
      • fcs tension increase
        • at higher load, pump overdraw engine power, thus stall it
      • pcs tension increase
        • machine can be stronger than before because it is gonna create more pressure
        • but power demand could exceed the supply

hydrostatic transmission

hst

  • the most common setup: closed circuit with var dis, rev pump
  • open loop circuit: returning oil from actuator go to tank (loader)
    • res interrupt path of oil
    • good for dissipate heat
  • closed loop circuit/HST: oil return to pump inlet
  • can have var dis pump or motor
  • infinite rpm
  • load can not overrun the motor
  • charge pump
    • deliver oil to either side of closed loop circuit
    • deliver oil to dcv control the pump direction and displacement
    • provide cool oil to closed loop circuit
    • make up internal leakage
    • prevent cavitation for main pump
    • supply oil for aux attachment like hoist or lift circuit
  • flushing valve
    • draw cooler oil from output side supplying to motor
    • orifice on the valve will regulates the amount of oil used to flush the motor
    • the flushing relief valve ensures that minimum pressure is maintained on the low side of closed circuit, this protect HST pump from cavitation

hyd systems service

  • gather information
    • service manuals, bulletins, operator manuals
  • talk to operators
    • what job were you doing with the machine when the problem occurred?
    • when was the machine last serviced?
    • have any repairs or modifications been done to the machine recently?
    • have the problem been evident for a long time or did it just appear suddenly?
    • did you notice anything unusual, noises or erratic movements before the machine failed?
  • visual inspection
    • check oil level, correct oil?
    • check oil filter, check quantity and type of contamination trapped
    • oil condition (foamy, milky, dirty)
    • oil leaks?
    • overheating (burnt paint, blue color)
    • check damage (dent cyl barrel, bent, scored cyl rods, collapse hose, crushed tubing)
  • operational test
    • check fluid level first!
    • should be safe to operate
    • steps
      • warm up hyd oil by stalling actuator @ wide open throttle (150-180F)
      • operate
        • response slow, jerky?
        • unusual smell, noises?
        • smoke?
        • @ what point, problem occurs
      • measure actuator speed (flow issue)
        • to determine if there are excessive flow losses (too much leakage)
        • check cycle times
        • take 3 or 4 measurement and get avg
        • check motor rpm (both direction)
      • perform stall test (pressure issue)
        • result depends on what type of hyd sys used
        • to check engine rpm when hyd sys is @ max load
        • used tachometer when a actuator is stalled @ W.O.T
        • engine rpm drop excessively
          • engine lacks of power
          • relief valve is set to high
          • misadjust unloader valve or pump comp
        • engine rpm drop slightly
          • engine power has increased
          • relief valve is set to low
  • pressure testing
    • pressure gauge
      • snubbers are orifices placed in the gauge inlet to dampen pressure pikes, protecting the gauge components
      • range should be twice the expected pressure
    • tee test
      • gauge installed in para with the tested area
      • shut-off valve is protect the gauge, open when measuring pressure
      • need to open to put the tee; thus, contamination can enter
      • builtin pressure taps need to be cleaned before installing gauge
    • common location for installing pressure gauge
      • close center
        • before the pump
          • low pressure (cavitation on outlet)
            • inlet restricted
            • driveshaft speed too high
            • wrong oil used
          • high pressure (air in system, cause jerky operation)
            • inlet leaking, air drawn to system
        • after the pump
          • neutral pressure @ W.O.T
            • filter, fitting,return line
            • localize hotpots
          • working pressure (NO load)
            • resistance dcv, fitting, actuators, linkages, etc.
          • working pressure (max load or pressure)
            • stalling actuators @ W.O.T
            • pressure too high => adjusting system relief
      • closed center load sensing
        • check for pilot pressure on dcv or bw double check valves
      • HST
        • closed circuit pressure:
          • one read charge pressure
          • one read working pressure
          • check max closed circuit pressure by stalling motors @ W.O.T
        • charge pressure
          • insufficient pressure cause cavitation
        • case drain pressure
          • pressure’s too high causing pump or motor damage
  • flow testing
    • slow cycle time
    • check of hotpots, e.i. cylinder barrel
    • small leakage: check cyl drift by using a tape to make the rod
    • check cyl seal leaks
      • fully extend cyl
      • remove the rod end hose
      • raise (extend) cyl @ W.O.T
      • measure oil from removed return line in “10 seconds”
    • check motor internal leakage
      • drain line secure to test container
      • stall motor @ W.O.T
      • measure oil from drain line in “10 seconds”
    • hydraulic test unit
      • including flow meter, pressure, temp, and load control
      • pressure gauge should be installed upstream of load control
      • always backoff load control before test
      • series conn: test pump efficiency
        • 2 types of connections: bypass & inline conn
        • flow rate @ no load is slower than specs
          • restricted inlet
          • aeration
        • flow rate @ full load (increase load control to the pressure specified by manufacture)
        • efficiency less than 80%, pump should be serviced
      • tee test: test circuit leakage
        • perform this test after confirm pump efficiency
        • can used to check system relief
          • there is wide pressure difference bw cracking and full flow pressure, the relief valve is broken
        • adjust load control slightly below cracking pressure of system relief valve
        • compare this flow rate with flow rate @ full load during pump testing
        • circuit leakage = pump full load flow - circuit flow

          electrohydraulics

  • clamping diode
    • dissipate current flow created by induce voltage upon energize, protect relay contact
  • proportional solenoid
    • varying current causes varying valve spool movement
    • use PWM to reduce the heat generated by large voltage drop
  • electronic control system types
    • open loop
      • there’s no feedback signal to the ECU indicating the extent of output action
    • closed loop
      • include sensors that send feedback signal to ECU
      • provide more accurate control of its position, velocity, or acceleration
      • servo valves can be used (wheeled vehicle steering system)
  • electronic control module (ECM) or programmable logic controller (PCL)
  • duty cycle is a percentage of on time compared to off time
  • proximity switches are commonly used in loader bucket and lift height control systems.
  • when deflector plate is in close proximity to the switch, the switch is closed
  • the loader bucket kickout position and lift height can be adjusted by adjusting the position of the proximity switch in relation to the deflector plate
  • switch sends a discreet voltage signal
  • the force created by a proportional solenoid is proportional to output voltage signal from ECU
  • using dither to keep the spool moving or vibrating sightly, stiction (or hysteresis) is prevented

misc

  • close center hyd system often called demand flow
  • axial displacement pump
    • the center line is para with the pump drive shaft
  • water glycol fluid contain 40% of water and 60% of glycol (more glycol)
  • water/oil emulsion contains 60% of water and 40% oil (more water)
  • invert emulsion contains 40% of water and 60% of oil (more oil)
  • most multiple pump conf use a common inlet
  • with control valves connections are often referred as ways
  • cam radial piston motors, the cam surface is ground with ramps space 90 degree apart
  • flood lamps on a machine is controlled by the key switch control circuit
  • thrust pin on differential carrier helps prevent movement of differential carrier during different loads
  • thrust crew prevents the bevel gear deflecting during high load applications
  • metallic components can use steam cleaning or Varsol
  • bearing/friction disc clean with Varsol (mineral spirit)
  • carrier internal components clean with petroleum products
  • accumulator suspension is set too high cause driveline problems
  • bronze coloured particles in filters means clutch failure
  • powershift trans : cycling applications e.i dozer, loader(fixed stator)
  • automatic trans : hauling applications e.i scrapers (freewheeling stator)
  • circuit breakers
    • type 1:
      • law for headlights
      • auto reset to keep light on
      • typically gold
    • type 2:
      • has heater (resistor)
      • once tripped will not reset until load removed or power discontinued
      • used for all circuits other than headlamps
      • typically silver in color
      • there’s wire connect 2 terminal, current run though this wire to keep bimetall warm, not to auto reset
      • if we have short-to-ground, it draw more current. as long as fix the short, current drawn drops
    • type III
      • manual reset
      • look exactly like type II with a button on top to reset the breaker
  • battery disconnect switch prevent the discharge of the battery group when equipment is parked
  • flywheel checks
    • crankshaft end play
    • flywheel housing bore eccentricity (base on fw)
    • flywheel housing face squareness (base on fw)
    • crankshaft hub eccentricity (base on housing)
    • crankshaft hub face squareness (base on housing)
  • torque divider
    • ring - turbine
    • input sun + impeller
    • out pc.
  • as track wears, the track pitch increases, sprocket wear causes the disctance bw theeth to decrease
  • drive axle
    • drive on convex side (), coast on concave side )(
    • moving ring gear in or out affects backlash
      • moving ring gear towards pinion gear will lower the pattern (move from heel (outside) to toe)
        • used when it has excessive heel
    • moving pinion in or out affects face and flank; also affect backlash
      • moving pinion towards ring gear will low the pattern (move from face(top) to flank(bottom))
        • will decrease backlash
        • used when it has excessive face

          1 cm = 0.394 inches 1 m = 39.37 inches 1 micron = 1 / 1 million meter = 1 psi = 6.89kPa 1 bar = 100kPa 1 Mpa = 1000kPa 1 atm = 14.69psi