STEERING HYDRAULICS 210171Fig. 1

STEERING SHAFT

CHECK VALVE

STEERING UNIT(HAND METERING UNIT)

FLOW AMPLIFIER

C/LOF

MACHINE

210171CH_C

OUTLINE OF CAB

HYDRAULIC MANIFOLDUNDER CAB

FLOWAMPLIFIER

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STEERING HYDRAULICS

The steering system is a dual, flow amplified, load sensing, split hydraulics arrangement. Thereason the system is split is for safety. If something happens to one of the amplifiers, then theother one will still allow the vehicle to be steered left or right so it can safely return for repairs.

When the steering wheel is turned, the Steering Metering Pump meters an oil volume proportionalto the amount of turn.

The oil then passes through the dual flow amplifiers where the oil flow is amplified (8 to 1 ratio)and is then led to the appropriate side of the pistons in the steering cylinders. When one cylinderis pushing, the other is pulling the whiffletree.

The amplifiers are equipped with a built-in priority valve. The priority valve ensures the neces-sary oil flow is led to the Steering Metering Pump and to the flow amplifier spool in all steeringsituations.

There is also a built in back pressure valve which ensures a return pressure of 73 PSI (5 bar) toimprove make-up oil to the low pressure side of the steering cylinder pistons during steering. Thesole purpose of the pilot relief valve is to protect the steering system.

The Steering Metering Pump automatically returns to its neutral position when steering wheelrotation stops.

If the oil flow from the accumulator system fails, the Steering Metering Pump automatically func-tions as a hand pump, i.e. manual steering (emergency steering) of the vehicle is maintained.

Max. pressure in steering system 2900 PSI (200 bar)Control spring pressure in priority valve 100 PSI (7 bar)Pressure relief valve setting 2045 PSI (210bar)Shock valve setting 3916 PSI (270 bar)Counterpressure (back pressure) valve setting 73 PSI (5 bar)

STEERING METERING PUMPThe steering metering pump (also called the steering unit) is located at the base of the steeringcolumn. It is made up of two sections: 1) Control section 2) Metering section . The two sec-tions are connected inside the unit both hydraulically and mechanically.

Accumulator oil from the amplifier valve enters through inlet “P” into control section. When thesteering wheel is turned, the control section sends oil to metering section. Metered oil is thendirected by control section to either left turn port “L” or right turn port “R.” This oil thenbecomes pilot oil for the flow amplifier valve. The metering section is a small hydraulic pump. Itproduces a specific amount (metered) of oil flow.

OIL FLOWThe control section of the steering metering pump is a closed center type. When the steeringwheel is in the NEUTRAL (non-steering) position, there is no alignment between the holes insleeve and the passages in spool. However, a small amount of pump oil from inlet “P” is allowedto flow through the center position of the steering metering pump. This small amount of oil flow

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(thermal bleed) keeps the steering metering pump full and ready for a quick response to steeringdemands. The thermal bleed also helps keep the pilot oil lines to the steering control valve andthe steering metering pump warmed during cold weather operations.

If the steering metering pump is taken apart for any reason, it must be putback together with the relationship between rotor and pin as shown in the illustration to the right,otherwise erratic steering will result.

LOAD SENSINGThe steering metering pump has a load sensing port in addition to the ports already described.The load sensing port is connected internally to inlet port “P” through an orifice. Oil pressure ininlet port “P” is felt in the load sensing line. This signal pressure is then communicated to the pri-ority valve (2) in the amplifier valve.

AMPLIFIER VALVESThe amplifier valve is pilot operated through the five hydraulic lines connecting it to the steeringmetering pump. These lines direct the pilot oil from the steering metering pump to shift thespools in the amplifier valves. The spools control the amount and direction of pressure oil sent tothe steering cylinders. The main components of amplifier valve (1) are:

1. Priority spool (Item #2)2. Amplifier spool3. Combiner/check spool (inside amplifier spool)4. Directional spool (Item #6)5. Relief/makeup valves (Item #5)6. Back pressure valve (Item #4)

The following is a list of abbreviations and their meanings for the illustrations in this section.

HP High pressure T Tank R RightP Pressure HT Tank L LeftLS Load Sensing PP Pilot PressureCL Cylinder Left steer CR Cylinder Right steer

OIL FLOWThe split system allows one accumulator system to be connected to one amplifier valve. If oneaccumulator system fails, the other system is still supplying pressure oil to one amplifier to steerthe vehicle to a safe stop.

Pressure oil from the large and small accumulator systems flows through port “HP” of the individ-ual amplifier valves. This oil then flows by the priority spool (2), and is blocked by the amplifierspool. Pressure oil is communicated to the steering metering pump through port “P.” Once allpassages fill with pressure oil, the priority spool (2) shifts left, but remains slightly open to allowa small amount of oil flow (thermal bleed) to the steering metering pump through port “PP.”

With the vehicle in the neutral (non-steering) position, the left and right pilot oil ports are ventedto return to tank oil port (T) in the steering metering pump. With no pilot oil acting on it, the

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directional spool (6) is maintained in the center position by springs. Any rolling resistance (oppo-sition), acting on the steering cylinders, creates a pressure increase felt in port “CL” or “CR.” thispressure increase acts upon the relief/makeup valve (5) in that port.

If the pressure increase exceeds 27000 kPa (3916 PSI), then the relief/makeup valve (5) allowsthe excess pressure to bleed into the tank passage to the back pressure valve (4). Pressure greaterthan 500 kPa (72 PSI) will open the back pressure valve (4) and the oil be allowed to return totank through port “HT.”

STEERING LINES 210171Fig. 2

AMPLIFIERVALVE

AMPLIFIERSPOOL

4STEERING ACCUMULATOR OIL

2900 PSI

STEERING ACCUMULATOR OIL

2900 PSI

73PSI

73PSI

3045PSI

3045PSI

3916 PSIMAX

5

5

STEERING

CYL. #2

STEE

RIN

G C

YL. #

1

WHIFFLETREE

3916 PSIMAX

5

AMPLIFIERVALVE

210171CHa_C

6

TO TANK

2

SYSTEMRELIEF

4

AMPLIFIERSPOOL

7STEERINGMETERINGPUMP

6

SYSTEMRELIEF

2

1

5

1

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STEERING LINES 210171Fig. 3

AMPLIFIERVALVE

3916 PSIMAX

RIGHT STEERSTEERING CYL. #2

WH

IFFL

ETRE

E

5

5

6

AMPLIFIERSPOOL

2

4

SYSTEMRELIEF

STEERINGMETERINGPUMP

STEERINGACCUMULATOR

OIL 2900 PSI210171CHc_C

73PSI

3045PSI

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RIGHT STEERWhen the steering wheel is turned to the right, steering metering pump sends right turn pilot oilthrough port “R.” Pilot oil moves the directional spool (6) to the right. When the directionalspool has moved far enough, pilot oil flows to amplifier spool.

At the same instant, pressure in pump port “P” at the steering metering pump is sensed (felt) andcommunicated to load sensing port “LS.” The increased pressure in the load sensing line causesthe priority spool (2) to move to the right. This allows more oil to flow to the steering meteringpump through passage “P.” Load sensing pressure will vary with steering load (resistance). Thepriority spool (2) will move accordingly, allowing sufficient oil flow to meet steering require-ments.

Pilot oil from the directional spool (6) flows to the left end of the amplifier spool through a con-necting pin hole and a stabilizing orifice. This causes the amplifier spool to move to the right.Accumulator oil at the spring end (right end) of the amplifier spool flows through a mid-connect-ing pin to the left side of the amplifier spool and also causes the amplifier spool to move to theright.

Pilot oil from the directional spool (6) also flows through orifices into passage of combiner/checkspool (inside amplifier spool). Pilot oil is momentarily blocked until the amplifier spool movesfar enough right to allow partial oil flow through an orifice (one of eight orifices) to the direc-tional spool (6).

When the amplifier spool moves to the right, pressure oil flows through orifices forcing the com-biner/check spool left. Pressure oil flows through the balance of orifices (seven of eight orifices)into passage where pilot and pressure oil merge. Oil flows across directional spool (6) (which hasalready shifted) into port “CR” for a right turn. This is where the 8 to 1 flow ratio is achieved bythe 1 portion coming in as pilot oil having 7 portions added to it to go the cylinders.

The faster the steering wheel is turned, the farther the directional spool (6) and amplifier spool areshifted (moved). A higher flow rate is available to cause the vehicle to turn faster. The ratio ofpilot and pressure oil that merge is always the same because one orifice is dedicated to pilot flowand seven orifices are dedicated to pressure flow.

Return oil from the cylinders enters port “CL,” flows across directional spool (6) into the tankpassage, around relief/makeup valve (5), forces the back pressure valve (4) open, and returns totank through port “HT.”

If while turning the vehicle, a front wheel would strike a large obstruction that cannot move, oilpressure in that steering cylinder and it's oil line increase. Oil flow to the cylinder is reversed.This pressure spike (sharp increase) is felt in the amplifier spool. The combiner/check spoolmoves to the right and closes off (blocks) the seven pressure oil orifices. The amplifier spoolmoves to the left and blocks the pilot oil orifice. This stops pressure oil flow to the steering cylin-ders. The pressure spike is not felt at the steering metering pump. If the pressure spike is largeenough, the relief/makeup valve (5) will dump the pressure oil to tank as described before.

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LEFT STEERThe operation of the amplifier valve for a left hand turn is basically the same as a right hand turn.The only difference is pilot oil from the steering metering pump now is felt in port “L.” Thisshifts directional spool left. The machine will now turn left. The rest of the valve operates thesame.

RETURN TO NEUTRAL POSITIONWhen steering effort stops, the steering metering pump returns to the neutral position. Ports “L,”“P,” “LS,” and “R” are vented (connected) to tank port. Pilot oil flow to the directional spoolstops. Oil flow to and from cylinder ports is closed off as springs re-center the directional spool.Loss of pilot oil to the end of amplifier spool allows the spring to shift it to the left.

OPERATIONAL CHECKSThe front wheels must be on a dry, smooth hard surface, and the hydraulic oil in the steering sys-tem must be warm.

Test the steering system with the engine at low idle, or “off,” as directed in the procedures. Forthis test the engine is running at low idle with both accumulator systems at 2900 PSI. Use a stopwatch or timer to measure the time it takes for the front wheels to be turned from stop to stopwhen the steering wheel is turned at a speed of approximately one revolution per second. If thetime is more than approximately 10 seconds to turn the wheels from full right to full left or fullleft to full right, there could be a problem in the hydraulic steering system.

Be sure the accumulators are at 2900 PSI before each right or left steer test. Thetest times are to be equal.

Position steering linkage against the right hand stop. Build accumulator pressure at 2900 PSI.Turn the engine “off.” Turn key back to “On” position (Do Not Start Engine). Steer to the full leftturn stops, back to right hand stops, back to left hand stops, etc. until the accumulator system hasno oil pressure remaining. The steering system should be capable of at least 2 complete right turnto left turn and back to right turn cycles for this test.

The failures in the steering system can be one or more of the items that follow:

• Broken oil line or a leak in an oil line connection• Hydraulic pump that is worn• Steering metering pump that is worn• Relief/makeup valves have wrong pressure setting or leak• Spools in the steering control valve do not shift properly• Steering cylinders that are worn• Accumulators with low Nitrogen precharge

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TROUBLE SHOOTINGTrouble shooting processes usually need to follow a logical and systematic sequence. With thatthought in mind, try to complete the steps in number order (or printed order if not numbered) soback-tracking will not be needed. The items immediately following are general maintenanceitems sometimes easily overlooked. More specific items are listed later in the trouble shootingchart.

Have a diagram (electrical/hydraulic) of the system you are diagnosing with you when you start.

1. Is the operating oil level correct?2. Is the state of the oil and the filters acceptable?3. Are pressures (hydraulic & nitrogen), amounts of flow, and flow directions in accordance with

specifications?4. Is the oil temperature too high or too low (oil viscosity)?5. Are there undesirable-vibrations or noise (cavitation)?

If the operator of the vehicle is present, he should be thoroughly questioned about:

1. The nature of the fault and its effect on the system.2. How long have forewarnings been noticed?3. Has someone been “tampering with/adjusting” the system components?

The following quick test method of steering systems is recommendable:

1. Start the motor (pump) and let it operate for a couple of minutes.2. Drive slowly in a figure eight and notice any percussions or vibrations in steering wheel or

steered wheels. Notice whether the steering wheel movements are immediately followed bycorresponding corrections of the wheel movements.

3. Stop the vehicle and influence the steering wheel with small, quick movements in both direc-tions. Let go of the steering wheel after each movement. The steering wheel must immedi-ately go to neutral, i.e. no “motoring” tendencies. The vehicle still being at standstill, turn thesteering wheel from stop to stop and count the number of steering wheel turns in both direc-tions:

4. Stop the motor (pump) and turn the steering wheel again from stop to stop. Count the numberof steering wheel turns and compare the figures with the previous ones.

5. An important difference (1 turn or more) indicates too much leakage in cylinder, gear-wheelset, shock valves, or suction valves.

In large vehicles which can not be emergency steered, the steering wheel move-ment must be slow with the motor running at low idle speed.

6. If there is leakage, remove a hose from one of the cylinder ends, plug this end and the hose,and influence the steering wheel again. If the steering wheel cannot be turned, the cylinder isdefective; in the opposite case, the Steering Metering Pump or perhaps the valve block isdefective.

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STEERING LINES 210171Fig. 4

AMPLIFIERVALVE

3916 PSIMAX

LEFT STEERSTEERING CYL. #2

WH

IFFLETREE

5 5

6

AMPLIFIERSPOOL

2 4

SYSTEMRELIEF

STEERINGMETERINGPUMP

STEERINGACCUMULATOR

OIL

210171CHc_C

73PSI

3045PSI

When you have identified a defective component for sure, it must be cleaned, together with itsclosest surroundings, before dismantling. Remove loose paint from lines and fittings in order toprevent dirt and dust from entering the system. All holes, hoses, and pipes must be plugged orcovered immediately after dismantling. Never dismantle a hydraulic component in the open air.Dismantling should always take place in a work shop, at a clean work table. Cover the table topwith newspapers, if necessary.

Always make sure to be in possession of a DANFOSS SERVICE MANUAL for the productinvolved and strictly follow the instructions both for dismantling and reassembly in order to avoidserious errors.

For a few assemblies, special tools are a must!

TROUBLESHOOTING: STEERING SYSTEMS WITH OSQA/B AND OSPBX-LS

FAULT POSSIBLE REASON CORRECTION

Amplification too high 1) Dirty, leaky or missing system relief valve (1)2) Priority valve (2) piston sticking

1) Clean or replace check valve2) Clean and check that piston

moves easily

Amplification too small 1) Priority valve (2) piston sticking2) Priority valve (2) piston fitted wrongly (only

OSQA/B-5)

1) Clean and check that piston moves easily

2) Rotate the piston 180 degrees around its axis

No end stop in one or both directions

1) One or both shock valves (5) set too low2) One or both suction valves (5) leaky or

sticking3) Missing end stop plate(s) for directional valve

(6)

1) Setting without special equipment is very time consuming. Please contact theclosest service shop

2) Clean or replace complete shock/suction valve(s)

3) Fit end stop plates

No/low oil pressure build-up 1) LS pressure relief valve adjusted too low2) Spool and sleeve in OSPBX steering units

wrongly composed3) Emergency steering ball in steering metering

pump missing4) Compensator valve (in accumulator system)

pressure setting is too low5) The accumulator system pump has too much

wear6) Blowdown valve for accumulators is bad

1) Remove plug and adjust to specified pressure

2) Remove the spool/sleeve set and turn the spool 180 degreesin sleeve. (See Service Manual)

3) Fit a new ball4) Readjust (see Sect 3)5) Check flow, repair/replace6) Repair/replace as necessary

“Hard” point when begin-ning to turn the steering wheel

1) Air in LS and/or PP lines2) Spring force in the priority valve (2) too weak3) Orifices in LS or PP ports clogged

1) Bleed the lines2) Replace spring by a stronger

one. There are 3 sizes: 4-7 and10 bar

3) Remove and clean orifices

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Too much force needed to turn steering wheel

1) Low oil level in hydraulic oil tank2) Accumulator system oil pressure is low (worn

pump/compensator valve?)3) Steering metering pump end cover is too tight4) Steering column components interfere with

Steering metering pump (7) shaft5) Steering metering pump (7) is not working

correctly6) Operation of relief/makeup valves (5) are not

correct7) Oil is cold8) Load sensing line (LS) is kinked or leaks9) Dirty orifice in LS-port10) Dirty orifices in directional valve (6)11) Dirty orifice in priority valve (2) in PP-port

1) Add as required.2) Repair as necessary3) Adjust per Danfoss service

manual4) Check/adjust for proper

alignment5) Check for contamination6) Clean, repair and/or replace7) Operate until at operating

temperature, then retest8) Repair/adjust as necessary9) Clean or replace orifices10) Clean or replace orifice11) Clean or replace priority

valve

Truck does not turn when steering wheel is turned

1) Air in the system2) Cylinder relief valves (5) malfunction3) Steering metering pump (7) has a lack of oil or

is worn4) Steering column shaft is disengaged from the

steering metering pump (7)

1) Bleed lines and valves2) Repair/replace as necessary3) Check accumulator psi and

pressure to amplifier valve4) Repair/replace as necessary

Truck wanders or steers erratically

1) Air in the system2) Steering linkage is worn3) Worn or loose cylinder piston4) Cylinder relief valves (5) malfunction5) Timing of steering metering pump (7) geroter

and pin not correct6) Contamination in steering metering pump (7)7) Column components interfere with steering

metering pump (7)

1) Bleed lines and valves2) Repair/replace as necessary3) Repair/replace as necessary4) Replace valves5) Adjust per instructions6) Disassemble, clean and

assemble per Danfoss service manual

7) Check/adjust for proper alignment

Truck turns too slow in both directions

1) Not enough oil flow from accumulators2) Cylinder relief valves (5) malfunction3) Failed accumulator4) Priority (2), amplifier, or directional (6) spool

shift only enough to allow partial flow

1) Check and repair as req'd (see Sect 10)

2) Check and repair as req'd3) Repair (see Sect 10)4) Check, clean, replace as

necessary

Truck turns too slow in one direction

1) Directional spool (6) does not shift fully due to restriction at one end

2) Restriction in steering metering pump (7)3) Cylinder relief valves (5) malfunction

1) Check, clean, replace as necessary

2) Clean, reassemble, test3) Check, clean, replace as

necessary

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Steering wheel does not return to center position cor-rectly

1) Steering metering pump (7) cover is too tight2) Steering column components interfere with

steering metering pump (7) shaft3) Restriction between spool and sleeve in

steering metering pump (7)4) Steering metering pump (7) centering springs

are damaged or broken

1) Adjust per Danfoss service manual

2) Adjust as necessary3) Check, clean, replace as

necessary4) Repair/replace as necessary

Steering wheel turns without operator turning it

1) Foreign material causes the steering metering pump (7) sleeve/spool to stick

2) Steering metering pump (7) centering (leaf) springs are broken or damaged

1) Clean, repair/replace as necessary

2) Repair/replace as necessary

Steering is soft or spongy 1) Air in the steering hydraulic circuit2) The oil level in the tank is low3) Accumulator has failed and released nitrogen

into the hydraulic system

1) Bleed lines & valves2) Add as necessary3) Repair/recharge (see Sect 10)

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