Most wrong hydraulic parts are ordered because somebody looked at a machine instead of a diagram. A valve body can carry three ports that look identical, two part numbers on one casting, and a plug where another market's machine has a fitting. The diagram resolves all three, and reading it costs nothing once you know the conventions. Guangzhou Shenyue Machinery Technology Co., Ltd. treats the diagram as step one of every enquiry.
Five line types, five meanings
A solid heavy line is main pressure or main return, carrying full flow. A solid thin line is usually a drain or case drain, low pressure, often to tank through a cooler. A dashed line is pilot pressure, the joystick signal that shifts a spool. A dash-dot line is a mechanical connection such as a linkage or cable. A boxed line marks a housing boundary, telling you which components share one body.
The distinction with commercial consequences is pressure versus drain. A case drain mistakenly connected to a return line carrying backpressure pushes oil past a motor shaft seal, and the seal is then blamed for a plumbing error. When a customer reports seals that keep failing, the first question is what the drain is teed into.
The symbols you will actually use
A circle with a filled triangle is a pump or motor, the triangle giving flow direction; two triangles in one circle means reversible, which is how a bidirectional swing or travel motor is drawn. A rectangle with an arrow is a cylinder. A box with an arrow and a spring is a relief valve, the spring side being adjustable. A ball on a seat is a check valve, a diamond with a cross is a filter, a square with a diagonal arrow is a variable orifice.
Solenoids appear as a rectangle on the end of a spool with a diagonal line through it, and the count tells you whether a replacement must be single or double coil. The spring on the other end gives the valve's normal position, which is what the machine does when the coil is dead, the single most useful fact when diagnosing an interlock fault.
Port letters and what they mean
P is pressure in, T is tank or return, A and B are the work ports, and Dr or D is the case drain on a pump or motor, the one people skip. Pi is pilot inlet, and on Kobelco machines pilot pressure is usually derived from the main pump through a reducing valve, so low pilot pressure can originate in the main circuit. Ps is pilot signal, used with load sensing.
Port position on the drawing is not port position on the casting. Use the diagram to identify which port does what, then locate it from the parts page or a photograph of the casting. Castings are shared between models and only the plugs differ, which is why one housing can appear under two numbers.
Tracing a circuit from pump to actuator
Start at the pump and follow the heavy line to the main control valve, drawn as a stack of sections, each a spool with its own checks, reliefs and make-up valves. The section feeding your function is the one whose A and B ports continue to the actuator. Then follow the dashed pilot lines back to the joystick: that line usually passes a solenoid valve for lockout, a common hiding place for an intermittent fault.
On the return side, follow the oil out of the actuator to the valve, then to the return filter and cooler. If the diagram shows a negative control orifice on that path, you have found what sets standby pressure, and a partly blocked orifice explains a machine slow to build pressure yet healthy at full relief.
From diagram to part number
The diagram gives a function, not a number. Numbers live in the parts book, where the same valve appears at three levels: complete assembly, sectional body, individual cartridge. Ordering at the wrong level is the costliest mistake in hydraulic buying. Our catalogue reading guide explains how index and figure numbers connect, and the prefix guide separates a pump assembly number from a regulator number.
Two warnings. Diagrams are revised by serial range, so a diagram matching the model name may not match the machine, and the serial should always be quoted. Where our records list three numbers for one valve body, a photograph of the casting settles it.
Guangzhou Shenyue Machinery Technology Co., Ltd. will read the diagram with you before quoting, and every line of the answer is marked genuine, OEM or aftermarket. Start from the hydraulic components page or send the diagram and serial through the contact page. Kobelco publishes service literature per model at kobelco.co.jp, and the flow and pressure figures there are the ones to write on your diagram before going to the machine.
Frequently Asked Questions
What do the different line styles mean on a Kobelco hydraulic diagram?
Solid heavy lines are main pressure and return, thin solid lines are drains, dashed lines are pilot pressure, dash-dot lines are mechanical linkages, and boxed lines mark a housing boundary. Confusing a drain with a return is the expensive misreading, because a drain on a pressurised return will blow shaft seals repeatedly.
What do the letters P, T, A, B and Dr mean on a hydraulic valve?
P is pressure in, T is tank or return, A and B are the work ports, Dr is the case drain, and Pi and Ps are pilot inlet and pilot signal. The case drain is the port most often overlooked, and it must return at low pressure.
How do I find the part number for a valve I have identified on the diagram?
The diagram gives the function and the parts book gives the number at three levels: complete assembly, sectional body and individual cartridge. Quote at the level you actually need, and include the machine serial, because diagrams are revised by serial range.
Can Guangzhou Shenyue Machinery Technology Co., Ltd. identify a part from a diagram?
Usually yes. Send the diagram page, machine model and serial, and a photograph of the casting with its markings. Where our records show more than one number for the same body, the casting marking decides it, and the quotation states whether the part is genuine, OEM or aftermarket.