hydraulic principle

• work is force acting through a distance
• power is the amount of work done in a specific time \(power={flow rate (l/min) * pressure (Mpa) * 0.000583 \over efficiency}\) \(power={flow rate (gal/min) * pressure (psi) * 0.0167 \over efficiency}\)
• 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
  • 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
• 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
• sequence valves
  • cause action occur in order
  • normally closed
  • sense upstream of the valve
  • externally drain
  • either direct acting or pilot operated
• 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
  • 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
  • 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

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
• bypass type flow control
  • used with fixed displacement pump
  • excess flow can be wasted
• restrict type flow control
  • used with pressure comp pump
  • normally open valves
  • closed when experience high pressure, send signal to destroke pump.
  • meter in
    • load pulling the actuators may cause cavitation
    • control the speed of cyl extension
    • work best if the load oppose the cyl movement
  • meter out
    • work well despite the load changes and directions
    • overruning load can cause pressure intensification
• flow dividers
  • priority flow dividers
    • divert all flow to a primary circuit (steering) before flow to implement circuit
    • normally closed implement circuit
  • 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

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
  • 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
• 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
    • tandem center conn
      • return oil back to passageway
      • actuators move together as both dcvs are actuated
    • 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
  • closed center dcv conns
    • don’t have a series passageway
    • valves connected para, including aux
• 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

• 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