Caterpillar Global Mining

UNIT RIG MT3700 DC TRUCKS

STEERING SYSTEM

LEARNING OBJECTIVES:

Given the presentation of the Unit Rig MT3700 DC Off-Highway Truck

Steering System, the participant will take notes in the classroom on the
identification, location, and the basic functionality of the major
components, service points, and adjustment points of the truck’s
steering system.
SERV1997 - 12/13 -3- Module 4 - Steering System

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INTRODUCTION

̋""Kfgpvkh{"vjg"ockp"uvggtkpi"u{uvgo" This module explains the operation of the steering system. The steering system is a
eqorqpgpvu0 closed center, load sensing system, which controls two double-acting cylinders. Oil
is supplied to the cylinders by accumulators charged by a pressure compensated
piston pump.

The steering system uses hydraulic force to change the direction of the front wheels.
The system has no mechanical connection between the steering wheel and the
steering cylinders.

This image shows the location of the following main steering system components:

̋" Orbital valve (1)

̋" Steering column and wheel (2)
̋" Hnqy"cornkÝgt"*5+
̋" Steering cylinders (4)
̋" Steering accumulators (5)
̋" Hydraulic tank (6)
̋" Steering pump (7)
̋" Steering pressure gauge (8)
̋" Steering manifold (9)

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SERV1997 - 12/13 -4- Module 4 - Steering System

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4_1

Uvggtkpi"U{uvgo"J{ftcwnke"Icwigu

̋""Kfgpvkh{"vjg"uvggtkpi"u{uvgo" The steering valve gauge (1) and the steering accumulator gauge (2) are located in
j{ftcwnke"icwigu0 the hydraulics cabinet at the rear of the cab on the left side of the truck.

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SERV1997 - 12/13 -5- Module 4 - Steering System

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Uvggtkpi"Rwor

̋""Kfgpvkh{"vjg"uvggtkpi"rwor"cpf" The steering pump (1) is the smaller piston pump assembly mounted on the rear of
vcpfgo"ugevkqp"fwor"rwor0 the dump pump (2). The pumps are mounted on a special bracket assembly between
vjg"ockp"htcog"tcknu."lwuv"kp"htqpv"qh"cpf"cdqxg"vjg"pqug"eqpg"tgegkxgt0

The steering pump is a variable displacement, piston-type pump that supplies oil to
the steering and brake systems.

NOTE: A tandem section dump pump is shown. The MT3700 AC trucks are
equipped with a single section dump pump.

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SERV1997 - 12/13 -6- Module 4 - Steering System

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5 6_1

Uvggtkpi"Rwor"Kpvgtpcn"Eqorqpgpvu

̋""Kfgpvkh{"vjg"uvggtkpi"rwor" This image shows the internal components of the steering pump and the pump
kpvgtpcn"eqorqpgpvu0 regulator (1), located on the bottom of the pump housing.

The oil supply for the pump enters from the reservoir and is routed internally to the
pump piston assembly. As the pistons turn on the angled swashplate, they move in
cpf"qwv"qh"vjgkt"tgurgevkxg"dqtgu."rtqfwekpi"qkn"Þqy0"Vjg"qkn"gzkvu"vjg"rwor"vjtqwij"
outlet ports.

The steering pump is pressure compensated with self-contained, preset,

spring-loaded piston assemblies (2) that control the operation of the moveable
swashplate (3). The stroke of the pumping pistons is varied to increase or decrease
vjg"qwvrwv"Þqy"kp"vjg"u{uvgo0

Spring force on the counter piston (4) moves the swashplate to the left to upstroke
the pump. The control piston (5) is controlled by the pump regulator and extends
when system pressure reaches the relief valve setting. When the control piston
extends, the swashplate moves to the right and the pump destrokes.

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SERV1997 - 12/13 -7- Module 4 - Steering System

7_1

Uvggtkpi"Ceewowncvqtu

̋"Kfgpvkh{"vjg"uvggtkpi"ceewowncvqtu The steering accumulators (arrows) are mounted together on the outside of the
frame in front of the hydraulic tank.

The accumulators are piston-type accumulators charged with nitrogen and stored
oil under pressure. The accumulators are a ready source of pressurized oil for
the steering system and include an oil reserve in case of restricted or inadequate
Þqy"htqo"vjg"uvggtkpi"rworu0"Vjg"ceewowncvqtu"cnuq"cnnqy"vjg"uvggtkpi"u{uvgo"vq"
maintain a more constant pressure by absorbing high or low pressure surges.

A nitrogen charge valve is on the top of the accumulators. The accumulator should
be pre-charged to approximately 10340 kPa (1,500 psi), with no pressure in the
steering system.

NOTE: Steering accumulators remain charged when the cab master switch is in
the ON position. Discharge hydraulic pressure from the steering accumulators
before inspection or maintenance by switching off the master power switch on
the front of the cab before turning off the battery disconnect switch.

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SERV1997 - 12/13 -8- Module 4 - Steering System

8_1

Uvggtkpi"Ocpkhqnf"cpf"Uvggtkpi"Qkn"Hknvgt

̋""Kfgpvkh{"vjg"uvggtkpi"ocpkhqnf"cpf" Vjg"uvggtkpi"ocpkhqnf"*3+"cpf"vjg"uvggtkpi"qkn"Ýnvgt"*4+"ctg"oqwpvgf"kpukfg"vjg"nghv"
uvggtkpi"qkn"Þnvgt0 frame rail in front of the hydraulic tank near the drive system alternator.

Vjg"uvggtkpi"qkn"Ýnvgt"ku"gswkrrgf"ykvj"c"d{rcuu"xcnxg0"Vjg"d{rcuu"xcnxg"nkokvu"vjg"
oczkowo"fkhhgtgpvkcn"rtguuwtg"vjtqwij"vjg"Ýnvgt"gngogpv"yjgp"vjg"qkn"ku"eqnf"qt"vjg"
Ýnvgt"gngogpv"ku"tguvtkevgf0

Vjg" Ýnvgt" ku" gswkrrgf" ykvj" c" ogejcpkecn" kpfkecvqt" nqecvgf" qp" vjg" gzvgtkqt" qh" vjg"
Ýnvgt" cuugodn{" jgcf0" Vjg" vtwem" ecp" dg" gswkrrgf" ykvj" cp" qrvkqpcn" d{rcuu" uykvej"
that illuminates an indicator in the cab. The mechanical indicator and bypass switch
ctg"fgukipgf"vq"cevkxcvg"yjgp"vjg"fkhhgtgpvkcn"rtguuwtg"vjtqwij"vjg"Ýnvgt"gngogpv"ku"
restricted.

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SERV1997 - 12/13 -9- Module 4 - Steering System

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9 8 9_1

Uvggtkpi"Ocpkhqnf"Eqorqpgpvu

̋""Kfgpvkh{"vjg"uvggtkpi"ocpkhqnf" Qkn" htqo" vjg" uvggtkpi" rwor" Þqyu" vjtqwij" vjg" qkn" Ýnvgt" cpf" kpvq" vjg" uvggtkpi"
eqorqpgpvu0 manifold (1), which distributes the oil to the steering system, the lube manifold, and
the brake system.

The solenoid cartridge valve assembly (2) contains a solenoid and manual
pull-type valve. When the master switch is turned OFF, the solenoid is energized and
oil is directed from the steering accumulator to the tank. The manual pull-type valve,
which is spring loaded to the closed position, provides a method to manually release
pressure in the steering accumulators.

The relief cartridge valve assembly (3) limits pressure in the steering system and is
set at approximately 28270 kPa (4,100 psi).

Vjg"Þqy"eqpvtqn"ectvtkfig"cuugodn{"*kh"gswkrrgf+"*6+"eqpvtqnu"vjg"qkn"Þqy"vjtqwij"vjg"
brake return Venturi system to regulate the pressure reduction function of the brake
system.

The low steering pressure switch (5) monitors the steering system supply pressure
(accumulator pressure) and causes a cab dash low steering pressure indicator to
light.

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SERV1997 - 12/13 -10- Module 4 - Steering System

Additional components on the steering manifold are:

̋" Check valve cartridge assembly (6)

̋" Jqug"vq"Þqy"cornkÝgt"*9+

̋" Hoses from accumulators (8)

̋" Supply hose from steering pump (9)

The manual power supply pressure switch (not visible) monitors the system supply
pressure (accumulator pressure) and controls the automatic operation of the
cxckncdng"ocpwcn"rqygt"uwrrn{"u{uvgoÓu"46"XFE"rwor"cuugodn{0

NOTE: The steering manifold can be equipped with an optional output to the
hoist system, which assists the hoist system when the truck is parked and no
steering is required.

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SERV1997 - 12/13 -11- Module 4 - Steering System

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Uvggtkpi"Xcnxg

̋"Kfgpvkh{"vjg"uvggtkpi"xcnxg0 The steering wheel in the cab is connected to the steering valve (arrows). The
steering valve is also referred to as an orbital valve or a Hand Metering Unit (HMU).

Vjg"uvggtkpi"xcnxg"ogvgtu"vjg"coqwpv"qh"qkn"ugpv"vq"vjg"Þqy"cornkÝgt"d{"vjg"urggf"cv"
which the steering wheel is turned. The faster the steering wheel is turned, the higher
vjg"qkn"Þqy"vjcv"ku"ugpv"vq"vjg"uvggtkpi"e{nkpfgtu"htqo"vjg"Þqy"cornkÝgt."cpf"vjg"hcuvgt"
the wheels will change direction.

The following ports are located on the orbital valve:

̋" Vq"rqtv"N"qp"Þqy"cornkÝgt"*nghv+"*3+

̋" Vq"rqtv"T"qp"Þqy"cornkÝgt"*tkijv+"*4+

̋" Vq"rqtv"R"qp"Þqy"cornkÝgt"*uwrrn{+"*5+

̋" Vq"rqtv"V"qp"Þqy"cornkÝgt"*tgvwtp+"*6+

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SERV1997 - 12/13 -12- Module 4 - Steering System

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Uvggtkpi"Hnqy"CornkÞgt

̋"Kfgpvkh{"vjg"uvggtkpi"àqy"cornkÞgt Vjg"uvggtkpi"Þqy"cornkÝgt"*3+"ku"oqwpvgf"qp"vjg"kpukfg"qh"vjg"nghv"htcog"tckn"cv"vjg"
htqpv"qh"vjg"vtwem0"Vjg"uvggtkpi"tgnkgh"xcnxg"*pqv"xkukdng+"ku"nqecvgf"kp"vjg"Þqy"cornkÝgt"
and is set at approximately 18270 kPa (2,650 psi).

Vjg"Þqy"cornkÝgt"ku"rknqv"qrgtcvgf"htqo"vjg"uvggtkpi"xcnxg"nqecvgf"cv"vjg"dcug"qh"vjg"
steering column. Pilot hydraulic lines send pilot oil from the steering valve to shift the
urqqnu"kp"vjg"Þqy"cornkÝgt"0"Vjg"urqqnu"eqpvtqn"vjg"coqwpv"cpf"fktgevkqp"qh"rtguuwtg"
oil sent to the steering cylinders.

Eqppgevkqpu"vq"cpf"htqo"vjg"uvggtkpi"Þqy"cornkÝgt"ctg<

̋" To steering manifold STR port (2)

̋" To orbital supply port P (3)
̋" To orbital load sense port LS (4)
̋" To orbital right port R (5)
̋" To bottom bulkhead on the front axle (6)
̋" To top bulkhead on the front axle (7)
̋" To orbital left port L (8)
̋" To hydraulic manifold (9)
̋" To orbital return port T (10)
̋" To oil cooler (11)

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SERV1997 - 12/13 -13- Module 4 - Steering System

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Uvggtkpi"E{nkpfgtu

̋"Kfgpvkh{"vjg"uvggtkpi"e{nkpfgtu0 The steering cylinders are two-way hydraulic cylinders mounted under the frame
rails on the front beam axle. This image shows the right steering cylinder. All trucks
have a pin and clevis arrangement. A pin and clevis are used to connect the piston
rod to the steering arm.

Rtguuwtk|gf" qkn" ku" tqwvgf" vq" gcej" e{nkpfgt" htqo" vjg" Þqy" cornkÝgt" vjtqwij" c" jkij"
pressure hose entering through an inlet port at either the head or rod end. The oil
rtguuwtg"ecwugu"vjg"tqf"vq"gzvgpf"qt"tgvtcev0"Vjg"tgvwtp"qkn"ku"tqwvgf"dcem"vq"vjg"Þqy"
cornkÝgt0

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SERV1997 - 12/13 -14- Module 4 - Steering System

STEERING SYSTEM
HOLD
Dump Valve (25) Left Steering Right Steering
Cylinder (19) Cylinder (20)

Steering Accumulators (24)

Crossover Relief
Steering Manifold (23) Valves (13)

Main Relief
Brake Return Valve (12)
Venturi
(5)
T
Directional Spool (17)
S

Amplifier
Spool (16)
Low Steering
Steer Accum Valve (14)
Dump Dump
Press Flow Drain (10)
Pump System
Sw (6) Control
(2) (3)
(11)

Steering
Oil Filter (4) Steering Relief
Valve (18)
Priority
T
Spool (15)
S
Steering Manual
Pump Brake Power
(1) Manifold (7) Supply
Pressure
Switch Brake
Manifold Flow Amplifier (9)
(22)
(7)
LS Signal
Secondary Pressure (21)
Steering
M Pump (8)
(optional)

14_1

Uvggtkpi"U{uvgo"Qrgtcvkqp"/"JQNF

̋""Gzrnckp"vjg"qrgtcvkqp"qh"vjg" When the engine is started, oil for the steering system is drawn from the steering
uvggtkpi"u{uvgo"ykvj"vjg"u{uvgo" j{ftcwnke"vcpm"d{"vjg"uvggtkpi"rwor"*3+"cpf"ugpv"vjtqwij"vjg"uvggtkpi"qkn"Ýnvgt"*6+"vq"
kp"JQNF0 vjg"uvggtkpi"ocpkhqnf"*45+0"Kpukfg"vjg"uvggtkpi"ocpkhqnf."qkn"Þqyu"vq"vjg"ockp"tgnkgh"
valve (12) and through a check valve to the brake manifold (13) to supply the brake
u{uvgo0"Qkn"vjgp"Þqyu"vjtqwij"c"ugeqpf"ejgem"xcnxg"vq"vjg"ceewowncvqt"ftckp"*32+.""
vq"vjg"uvggtkpi"ceewowncvqtu"*46+."cpf"vq"vjg"Þqy"cornkÝgt"*;+0"

After the oil pressure increases to a predetermined pressure in the accumulators,

the steering pump will destroke. When a steering demand occurs, the accumulators
uwrrn{" vjg" pgeguuct{" qkn" Þqy" hqt" uvggtkpi." cpf" rtguuwtg" kp" vjg" ceewowncvqtu"
decreases. When the oil pressure in the accumulators decreases to a predetermined
level, the steering pump will automatically upstroke to maintain the oil pressure in the
accumulators that is required for steering.

Qkn"htqo"vjg"ceewowncvqtu"Þqyu"vjtqwij"vjg"rtkqtkv{"urqqn"xcnxg"*37+"vq"vjg"uvggtkpi"
xcnxg"*36+0"Kh"vjg"uvggtkpi"yjggn"ku"pqv"vwtpgf"*JQNF"rqukvkqp."cu"ujqyp+."vjg"qkn"Þqyu"
vjtqwij"vjg"Þqy"cornkÝgt"vq"vjg"vcpm0"Cnnqykpi"qkn"vq"ektewncvg"vjtqwij"vjg"JOW"yjkng"
the steering wheel is stationary provides a thermal bleed condition, which maintains
a temperature differential of less than 28°C (50°F) between the steering valve and
the tank. This thermal bleed prevents thermal seizure of the steering valve (sticking
steering wheel).

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SERV1997 - 12/13 -15- Module 4 - Steering System

STEERING SYSTEM
Dump Valve (25) TURN Left Steering
Cylinder (19)
Right Steering
Cylinder (20)

Steering Accumulators (24)

Crossover Relief
Steering Manifold (23) Valves (13)

Main Relief
Brake Return Valve (12)
Venturi
(5)
T
Directional Spool (17)
S

Amplifier
Spool (16)
Low Steering
Steer Accum Valve (14)
Dump Dump
Press Flow Drain (10)
Pump System
Sw (6) Control
(2) (3)
(11)

Steering
Oil Filter (4) Steering Relief
Valve (18)
Priority
T
Spool (15)
S
Steering Manual
Pump Brake Power
(1) Manifold (7) Supply
Pressure
Switch Brake
Manifold Flow Amplifier (9)
(22)
(7)
LS Signal
Secondary Pressure (21)
Steering
M Pump (8)
(optional)

15_1

Uvggtkpi"U{uvgo"Qrgtcvkqp"/"VWTP

̋""Gzrnckp"vjg"qrgtcvkqp"qh"vjg" When the steering wheel is turned, the steering valve (14) directs oil to one side of
uvggtkpi"u{uvgo"fwtkpi"c"VWTP0 the directional spool (17). The directional spool shifts and directs oil to the left
steering cylinder (19) and right steering cylinder (20). Depending on which direction
vjg"uvggtkpi"yjggn"ku"vwtpgf."qkn"yknn"Þqy"vq"vjg"jgcf"gpf"qh"qpg"uvggtkpi"e{nkpfgt"cpf"
to the rod end of the other cylinder. The action of the oil on the pistons and rods in
the steering cylinders causes the wheels to change direction. Displaced oil from the
uvggtkpi"e{nkpfgtu"Þqyu"vjtqwij"vjg"Þqy"cornkÝgt"vq"vjg"vcpm0

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