3.

Hydraulic system
Chapter Overview
This covers system specifications of the hydraulic system used in the new electric truck. This section
helps understand more specified details of each function within this hydraulic system.

Learning Objectives
At the completion of this chapter, you will be to do the following:
 Component identification
 Demonstrating knowledge of system specifications
 Demonstrating knowledge of the mechanical hydraulic systems
 Demonstrating knowledge of the electro-hydraulic systems
 Demonstrate competency on finding the latest diagnostic and service information using Axcess
Online

Skills Objectives
At the completion of this chapter, you will be able to do the following:
 Demonstrating proper diagnosis and repair of the hydraulic system
 Demonstrating proper disassembly and reassembly procedures of hydraulic components.
1. Introduction of Hydraulic System
Basically the hydraulic system on BYD forklift is similar other forklifts. The hydraulic system controls
hydraulic functions: lift, lowering, tilt, and auxiliary functions. Difference in BYD forklift system is
that the steering system and hydraulic system are combined. The system used in this truck has one
hydraulic AC motor, controlled by a separated controller, powering a single hydraulic pump. The
control output is based on the input from the movement of steering wheel and manual levers or
mini levers.

Fig 2, Input of the hydraulics are manual levers or mini levers

The operators input will be converted into a controller oil flow by hydraulic system. The pump
motor and the valve are both commanded. The pump motor is controlled to deliver a specified
motor speed and the hydraulic control valve opens by the specified PWM signal.
BYD Forklift hydraulic systems includes: working system (hydraulic function system), steering system,
and braking system. Working system (hydraulic function system) allows forklift to complete mast
lifting/lowering function, tilting function, forks side-shifting function and other functions.

Fig3 Work system

Fig4 Steering system

 Fig5 Brake system

2. Hydraulic System Principle

Counterbalance forklift has two variants: basic model with manual hydraulic valve and comfort
model with e-hydraulic valve. Components in hydraulic system in all counterbalance forklifts are
basically the same, which are mainly: hydraulic oil tank, oil pump, steering control unit, multi-way
hydraulic valves, lifting cylinder, tilting cylinder, side-shifting cylinder, steering cylinder, hoses,
connectors and so on.

Description of Operation
When the key is turned on, and the operator protect system (OPS) detects the status of the (OPS)
switch, the hydraulic pump starts working at a low speed. The hydraulic pump supplies oil to the
steering control unit. Inside the steering control unit, the oil flow first passes a priority valve to
make sure that steering always has oil supply regardless of all the other functions. The oil flows
through the pump are arranged by the operator’s request during steering and when a hydraulic
controlled function is active. The priority flow goes directly to the steering control unit, as
mentioned in the chapter “Steering System”. The second output of the priority valve allows oil flows
into other hydraulic functions. Pump speed for oil flows into hydraulic functions is set in the
hydraulic controller. Other hydraulic functions settings can be set as well. The minimum pump
speed can be changed by the technician from 0 to maximum 100% using the computer.

Fig 6 Forklifts’ hydraulic diagram

3. Hydraulic components introduction
a) Pump
The hydraulic pump is inter-gear type and bolt on to the AC motor. The pump used on the truck is
designed to operate quietly. The drive axle of the pump is connected by spline. The spline type is
shown in fig 14.

Fig 13 Motor and pump

Fig 14 Spline’s size of pump

Fig 15 The internal structure of the pump

 Characteristics
 Internal gear pump with axial and radial gap compensation
 Radial compensation with segments

Fig 16 Connecting flanges

Spec. volume Vth [cm3/rev] 25

Continuous operating pressure [bar] 250

Max. speed [min-1] 3600

Nominal speed [min-1] 100-3000

Operating temperature [℃] -20 to +100

Efficiency η vol [%] 95

Efficiency η hm [%] 93

(measured in sound chamber) db[A] 64

Tab 1 Pump technical data

Hydraulic Pump Removal

Caution: Safe Parking before Working on Truck
1. Parking truck on a hard, level and solid surface, such as a concrete floor with no gaps or
breaks.
2. Put upright in vertical position and fully lower the forks or attachment.
3. Turn key switch OFF and remove key.
4. Apply the park brake and block the wheels.
The hydraulic pump is located on the side of the motor. The pump can be removed from the
truck without removing the pump and motor assembly.
Serviceable items are the pump, motor and the suction line filter. Other components such as
hose, fittings and clamps are non-serviceable and should be replaced if faulty.
1. Drain all oil out from the tank. Because the pump suction line is located under the oil tank,
if remove the pump, the oil will flow out from the oil tank. See “Changing the hydraulic oil”.
2. Place a drip pan under the hydraulic pump for oil that will drain from the pump and hose
when they are disconnected.
3. Disconnect suction line from pump and plug the line.
4. Disconnect outlet hose.
5. Remove the pump mounting fasteners. The ground strap will come off with the fasteners.
6. Remove the bolts of the pump inlet and outlet connection.

Hydraulic Pump Installation

IMPORTANT: Keep all components clean during installation.
1. Be sure new or serviced pump is well lubricated before installation.
2. Install pump onto motor. Install and tighten fasteners.
3. Connect pump outlet and inlet connection and tighten.
3. Slide suction hose and clamp into place and tighten.
4. Connect pump outlet hose and tighten. Use two wrenches to tighten hose fittings to prevent
twisting of lines.
5. Connect return oil hose and tighten.
5. Reinstall the drain plug into the oil tank and fill hydraulic oil to the level. See changing the
hydraulic oil for description of oil tank fill levels.
6. Check operation of hydraulic system.
7. Check oil level of oil tank again. Add oil as necessary.
Routine Maintenance
 No maintenance is necessary other than periodic checks for tightness of the mounting
bolts and port fitting plus visual examination for oil leaks. The unit should be kept external
clean.
 The unit must be operated on clean hydraulic oil. Directions for filter service in this group
must be adhered to. A oil cleanliness level of ISO4406 17/14 or better is recommended to
give maximum life.
b) Manual Control Valve
The manual control valve has 4 spools. Number 1 is the inlet section and is connected to the
pressure hose coming from the pump and is also connected to the return hose to the tank. Number
2, 3, 4 and 5 are the sections with all the hydraulic functions. The control valve P , H and R ports
type are G1/2’’, the control valve A/B ports are G3/8’’.

Fig 17 The manual control valve

1—Inlet section 2—Lift section 3—Tilt section 4—Auxiliary 1 section
5—Auxiliary 2 section 6—Solenoid valve 7—Relief valve

Fig 18 Manual control valve diagram

 Fig19 P/T/H Port size Fig20 A/B Port size

c) Hydraulic oil tank

When the hydraulic oil has to be drained out of the tank a drain plug is located under the tank.
When the tank must be refilled use only hydraulic oil which has a specification of ISO-VG46. This oil
is used in a temperature range of -39℃-100℃. The system has a capacity of 60L.

Fig21 The model of hydraulic oil tank（OLD）

 Fig 23 The module of tank（NEW）
d）Tilter
Advantage of tubular filter:
1. Improve the filtering quality of hydraulic oil from earlier 20μm to the current 10μm;
2. Tubular filter integrates with filter and breather, cost-saving and easy for maintenance;
3. Easy for checking if hydraulic is clean, thus better protection on hydraulic components;

Fig 24 The model of new tilter

 Fig 25 Return oil filter working schematics
Technical data:
Specifications Value
Rated pressure 10bar
Working temperature -30℃—+100℃
Pressure when warning is sent 2bar
By-pass valve opening presure 3bar
Filtering accuracy 10μm
Material of filter Fiberglass
Tab 2 Filter technical data
e) Lift cylinder
When the piston inside the cylinder moves faster than 0.2m/s, a buffering device must be
implemented. In the below figure, component pointed out as 1 is a buffering device. It could avoid
and reduce the impact caused by piston or piston rod to the bottom of cylinder. It allows them to
reduce speed gradually to 0 before reaching the bottom.

Component 2 is a shut-off valve, which prevents mast from falling when hoses blast. This shut-off is
installed in the bottom side of hydraulic ram. When the system is working normally, this shut-off
valve does not function; when there is a blasting hose, shut-off valve automatically cuts off oil flow
to avoid abrupt dropping of mast.

Fig 26 Lift cylinder

1—Buffer
2—Anti-burst Safety Valve
f) Tilt cylinder
In order to achieve the mast forward and backward movement, tilt cylinder is double-acting
cylinder.
Tilt of mast：ECB2.5: 5/8 deg ECB3.5: 5/8 ECB1.6: 5/7 deg
Attention: the port of cylinder is plane seal, and the size is G1/4.

The model of tilt cylinder

Tilt cylinder connection:
The tilt cylinder one side connects with body through a pin shaft, and another side connects with
mast through a pin shaft. The angle of mast and synchronization can be changed by the length of
the piston rod thread.
Note: the length of not mesh thread should not over 32mm.
g) Speed regular valve
The mast lowering speed is generated by the gravity of the mast. When the goods is heavy, the mast
will lower very fast without flow limited, so we should add speed regular valve in the hydraulic
system. The speed regular valve is composed of differential reducing valve and throttle valve. At
present, there are two kinds of speed regular valve, one is tube type one-way throttle and another
one is cartridge valve. Comprehensive safety and efficiency consideration, the lowering speed is
300mm/s≤V≤500mm/s.

a) Tube type one-way throttle valve（ECB25）

Tube type one-way throttle valve has advantage of simple structure and easy manufacture. The oil is
through the speed regular valve, 80% of the oil pass the valve through check valve.

Tube type one-way throttle valve

The diagram of speed regular valve

Parameter:
1. Rated pressure: 20MPa, rated flow:60L/Min
2. Forward pressure loss: 0.7MPa
3. Port type: metric female 24° connection O-ring
4. Port size: M36x1.5
 Pressure and flow diagram
b) Regular speed cartridge valve (ECB16/ECB35)

The model of the regular speed cartridge valve

kind of forklift ECB35 ECB16

rated pressure（bar） 240 240
rated flow（L/min） 70 40
A1，A2 G1/2 G3/8
port B G3/4 G1/2
M G1/4 G1/4
material of valve body Aluminum Aluminum
weight（Kg） 0.8 0.7
h) Hydraulic Hose
The hydraulic hoses are two wire braid hoses.

Fig 27, Hydraulic hose

1—Inner Tube: Oil Resistant Synthetic Rubber
2—Reinforcement: Two Braids of Wire Reinforcement
3—Rubber layer: Wear-resistant Synthetic Rubber
4—Cover: Oil and Weather Resistant Synthetic
i) Hose fitting
Hose Fitting Type Picture Size

METRIC FEMALE 24° M14X1.5

CONE O-RING L.T. M18X1.5
ISO 12151-2—DIN3865 M22X1.5

45°METRIC FEMALE 24° M14X1.5

CONE O-RING L.T. M18X1.5
ISO 12151-2—DIN3865 M22X1.5

M14X1.5
90°METRIC FEMALE 24° M18X1.5
CONE O-RING L.T M22X1.5
ISO 12151-2—DIN3865

JB METRIC BANJO

M22X1.5
M14X1.5

JB METRIC BOLT

Tab 3, Type of hose fitting

j) Fittings
Due to hydraulic components ports are different, so the types of the fittings are different. The
imperial fittings have two types O-ring seal and plane seal. If hydraulic element port is O-ring form,
we should choose the fittings with O-ring; if hydraulic element port is plane sealing form, we should
choose the fittings with combine sealing.
Note: two kinds of fittings can not interchangeable.
Both American fittings and metric fittings are O-ring fittings.

O-ring seal port

Plane seal port

The following hydraulic components ports are O-ring seal form:
1. Manual control valve (P/T/A/B/R)
2. Three-way valve block on the lift line
3. Lift cylinder port
4. Sider shifter port
5. Steering cylinder port
The following hydraulic components ports are plane seal form
1. Steering control unit (P/T/L/R/EF)
2. E-hydraulic valve (P/T/H/A/B)
3. Three-way valve block on the tilt way
4. Tilt cylinder port
5. ECB 35/ECB 16 Regulate speed valve port
Fitting type：
Adaptor Type Picture Size
1CG-M14-G3/8 1CG-M14-G1/4
DIN Adaptor 1CG-M18-G3/8 1CG-M22-G1/2
BSP THREAD STUD ENDS 1CG-M26-G1/2 1CG-M22-G3/4
WITH O-RING SEALING 1CG-M26-G3/4

1CG-M14-G3/8-Z
DIN Adaptor 1CG-M14-G1/4-Z
BSP THREAD STUD ENDS 1CG-M18-G3/8-Z
WITH COMBIND SEALING 1CG-M22-G1/2-Z
1CG-M22-G3/4-Z
1CG-M26-G3/4-Z
1CG-M26-G3/4

DIN Adaptor 1CG9-M14-G3/8 1CG9-M18-G3/8

90°ELBOW BSP THREAD 1CG9-M22-G1/2 1CG9-M26-G3/4
ADJUSTABLE STUD ENDS
WITH O-RING SEALING

DIN Adaptor 1CO-M14-9/16

UNF THREAD STUD ENDS 1CO-M22-3/4
WITH
0-RING SEALING

DIN Adaptor 1CH-M14

METRIC THREAD STUD ENDS 1CH-M14-M16
ISO-6149
 DIN Adaptor 1C-M14
STRAIGHT FITTINGS

PLUG 4B-G3/8
4B-G1/2

PLUG 4B-G3/8-Z

Flange Coupling 1CF9-55LK

1CF-35LK
Flange Coupling

Tab 4, Type of connection

Torque Requirements:
Hydraulic system requirements sealed type good. The deformation of O-ring or combine seal
between fittings and hydraulic components/Fittings and tubes will form a good seal. Fittings and
tubes connection torque has strict standard, and excessive torque will injury thread and seals. The
following table is a reference torque values Eaton standard.
 Suggested torque Suggested torque
Thread Size
values /N.M values /foot pounds
M14×1.5 35 26
M16×1.5 40 30
M18×1.5 45 33
M22×1.5 70 52
M26×1.5 100 74
9/16—18UNF 28 21
3/4—16UNF 53 40
G1/4 35 26
G3/8 45 33
G1/2 75 55
G3/4 125 92
4. Hydraulic models of the BYD hydraulic forklifts (basic)
a) Hydraulic model of the ECB3.5 basic

Fig 7 The model of manual hydraulic system (3.5)-OLD

1—Air filter 2—Filter 3—Tank 4—Pump 5—Tilt cylinder 6—Steering control unit
7—Valve

Fig8 The model of manual hydraulic system (3.5)-NEW

Design changes:
1. Tubular filter replaced old filter and breather;
2. Oil level gauge replaced old dipstick for oil level checking;
3. Changes made to 3-way block for tilting function and other oil line designs;
b) Hydraulic model of the ECB2.5 basic

Fig9 The model of manual hydraulic system (2.5T)-OLD

Fig10 The model of manual hydraulic system (2.5T)-NEW

Design changes:
1. Tubular filter replaced old filter and breather;
2. Oil level gauge replaced old dipstick for oil level checking;
3. Changes made to 3-way block for tilting function and other oil line designs;
4. Changes made to master brake cylinder and its tube designs;
c) Hydraulic model of the ECB1.6 basic

Fig 11 The model of hydraulic system (1.6T)

5. Hydraulic System Operating Principle (Basic)
Manual Hydraulic Function
When the operator uses one of the hydraulic functions, the hydraulic controller will receive a signal
from a position switch mounted in the manual valve. Depending on the lever/spool position, the
controller speeds up the AC motor and the pump. The spools are opened by the mechanical
movement of the levers. Depending on the function that the operator had requested, the cylinder
will be activated.

Fig12 Signal flow for manual hydraulic function

1—Lever moves the spool including a position sensor which gives a signal to Controller
2—Controller supplies AC voltage to the AC motor
3—AC motor and pump creates a oil flow
4—Hydraulic control valve opens spool and oil flows into cylinders
5—Cylinders are activated
Hydraulic Functions Working Principle
All the basic models share a similar hydraulic schematic, the difference is the relief valve pressure
setting. We use ECB35 to illustrate as to how the hydraulic oil flow in the forklift hydraulic system.
ECB35B (manual hydraulic valve) hydraulic system schematics are illustrated here below:

Fig 28, Manual Hydraulic Diagram

1—Pump 2—Motor 3—Steering Control Unit 4—Steering Wheel 5—Steering Cylinder
6—Lift Cylinder 7—Tilt Cylinder 8—Side Shift Cylinder 9—Valve 10—Filter 11—Tank
a) Open Center Line in the Manual Control Valve
When none of the function is used and the pump delivers an oil flow, the flow passes all spools
through an open center line to drain back to the tank. See figure 29.

Fig 29, Open center line in the manual hydraulic valve diagram
When forklift is switched on, the pump motor rotates at creep speed. As the result, hydraulic
oil flows through CF port on priority valve to steering control unit, through central orifice and
returns back to oil tank. Due to the drop of pressure in the central orifice, the pressure between the
two sides of orifice gets passed onto the priority valve and causes the spool to shift to its right side.
Oil flow through CF port is low at central position. Most of hydraulic oil flows through EF port and
central passage of hydraulic valves and returns back to the oil tank.
b) Lifting
The flow during lifting is visible in figure 30. When the lift function is activated the center line closes.
The oil flow will pass the priority valve, then the oil flows through the lift spool and the lift cylinder
will be activated.
When operator performs only the lifting function, the oil passage in the middle position of valves
gets closed, so oil flows through priority valve and hydraulic valve and then enters into lifting
cylinders. Other hydraulic functions will cause similar oil flow.

Fig 30, Oil flow during lifting in the manual hydraulic valve diagram
c) Lifting to Maximum
When the forks are totally lifted , the relief valve will open as shown in figure 31. The oil flow will
pass the priority valve and the load holding valve and load holding valve, the oil flows through the
lift spool and the lift cylinders. Pressure will increase in the whole system and also in the load
sensing line. The relief valve opens at 206 bars(2987PSI). The opening pressure is factory set can not
be changed by the technician.

Fig 31, Oil flow during lifting to maximum

d) Lowering
The flow during is visible in figure 32. When the lower function is activated , the center line stay
open. The weight of the forks and mast will force the oil out of the lift cylinders. The oil will pass the
lift / lowering spool and the filter. Finally the oil flows back to the tank. The fork will move
downwarde.

Fig 32, Oil flow during lowering in the manual hydraulic valve diagram
f) Emergency Lowering
In case of emergency the manual lower valve can be used in case the lift spool can not be operated
by the mini lever. The flowing during lowering is shown in figure 33. When the lower valve is open,
the weight of the forks and mast will force the oil out the lift cylinders. The oil will by pass the
lift/lowering spool and flows through the filter and finally back to the tank. The forks will move
downwards.

Fig 33, Oil flow during emergency lowering in the manual hydraulic valve diagram
6. Hydraulic System Instruction (Comfortable)
Description of E-Hydraulic System
The electrical hydraulic system (E-hydraulics) controls all hydraulic function of this truck. Similar to
the truck with manual levers is that in this system the steering system and hydraulic system are
combined. There is no separate pump for the steering system in this truck; the system used in this
truck has one hydraulic AC motor, powering a single hydraulic pump.
Description of Operation
The basic operation from a truck with E-hydraulic is similar to a truck with manual hydraulics. When
the key is turned on, and the OPS detects the operator with the seat sensor, the hydraulic pump
starts at a low speed. The hydraulic pump supplies the hydraulic control valve. Inside the steering
controller the oil flow first passes a priority valve to make sure that steering always has oil supply
the other functions. The oil flow through the pump is arranged by the operator’s request during
steering and when a hydraulic controlled function is activated. The priority flow goes directly to the
steering control unit, as mentioned in the chapter steering. The second output of the priority valve
is the connection that supplies the other hydraulic functions. For each hydraulic function , a value
for the pump speed is set in the controller; combinations of functions are set as well. The minimum
pump speed can be changed by the technician from 0 to maximum 100% using the display.
Operation of E-hydraulic System
When one of the E-hydraulic functions is used, the controller will receive a signal from the Mini
lever Module. The spool inside the hydraulic control valve opens electronically and the controller
speeds up the AC motor and the pump. This generates the flow in order to activate the function
which operator had requested.

Fig 34, Signal flow for E-hydraulic function

1. Lever at mini lever gives a signal to controller
2. Controller supply AC voltage to the AC motor
3. AC motor and pump creates a oil flow
4. Hydraulic control valve opens spool to cylinders
5. Cylinders are activated
a) Hydraulic model of the ECB3.5 comfortable

Fig36, The model of E-hydraulic (3.5T)

b) Hydraulic model of the ECB2.5 comfortable

Fig38, The model of E-hydraulic (2.5T)

Hydraulic Functions Working Principle
Fig 39 shows the complete overview of the hydraulic system when manual levers are mounted on
the truck.

Fig 39, E-hydraulic diagram

1—Pump
2—Motor
3—Pressure Filter
4—Steering Controller
5—Steering Wheel
6—Steering Cylinder
7—Lift Cylinder
8—Speed Regulator Valve
9—Tilt Cylinder
10—Side Shift Cylinder
11—E-hydraulic Control Valve
12—Filter
13—Tank
a) Compensator valve as Proportional Relief Valve
One function of the compensator valve is to connect the pressure line with the tank when a certain
pressure is reached. This is not a relief valve but it has some similarities to it. The main difference
with the relief valve is that the compensator valve opens proportionally. When none of the
functions is used and the pump delivers an oil flow, the flow stops at all spools. The E-hydraulic
control valve does not have an open center line, so the oil flow can only go to the compensator
valve. The load sensing line which is connected at the other end of the compensator valve is inside
the spools connected to the tank. The oil flow is stronger than the spring and the compensator
valve is pushed open to drain the oil back to the tank.

Fig 43, Oil flows in the E-hydraulic control valve diagram, when no function is used.
b) Lifting to Maximum
When the forks are totally lifted, the relief valve will open as shown in figure 44. The oil flows
through the lift spool and the lift cylinders. Pressure will increase in the whole system and also in
the load sensing line. The relief valve opens at 206 bars (2987 PSI). The opening pressure is factory
set and can not be changed by the technician.

Fig 44, Oil flows in the E-hydraulic control valve diagram, when maximum pressure is reached.
c) Emergency lowering
In case of emergency the manual lowering valve can be used in case the lift spool can not be
operated by the mini lever. The flow during lowering is shown in figure 45. When the flower valve is
open, the weight of the forks and mast will force the oil out of the lift cylinders. The oil will by pass
the lift/ lowering spool and flows through the filter and finally back to the tank. The forks will move
downwards.

Fig45, Oil flow in the E-hydraulic control diagram, during emergency lowering.
E-hydraulic system component description
a) E-hydraulic Control Valve
The electronic hydraulic valve has 3 or 4 spools. The picture gives an overview of the valve. Number
1 is the inlet section and is connected to the pressure hose coming from the pump and the return to
the bank. It has also the connections to the lift system. Number 2, 3, and 4 are for other hydraulic
functions. Number 5 are the lift solenoid and number 6 are the lowering solenoid. In case of
emergency the mast can be lowered manually by a small manual lowering valve(7) on front of the
hydraulic control valve. To avoid confusion, solenoids should be labeled to their function before
disassembly.

Fig 40, E-hydraulic control valve

1—inlet section 2—tilt section 3—Auxiliary 1 section 4—Auxiliary 2 section 5—lift solenoids
6—lowering solenoid 7—Manual lowering valve 8—counter balance valve
9—electric connection 10—Relief valve
E-hydraulics valve diagram
Figure 41 shows a complete overview of the hydraulics system when an E-hydraulics system when
an E-hydraulic control valve is mounted on the truck.
 Fig41, E-hydraulic control valve diagram
b) Pressure Filter
To better protect e-hydraulic valve, a high-accuracy filter has been installed.

Fig 42, Pressure filter

Parameters:
1—Connector type: G3/4’’
2—Rated Pressure: 420 bar
3—Temperature Range: -30℃—100℃
4—Filter material：Fiberglass
5—Filtering Accuracy：5μm