An operator reports that the arm is slow. A fitter tests the arm alone and finds it fast. Both are right, and neither is describing a fault yet. On a Kobelco, single-function speed and combined-operation speed are different numbers by design, because two pumps and one valve block have to divide flow between whatever the operator asks for. Guangzhou Shenyue Machinery Technology Co., Ltd. supplies the valve, pump and regulator parts involved, and the first job is deciding whether the machine is behaving normally.
How two pumps share one job
The 20-tonne class runs a tandem pump: two pump sections on one housing, each feeding groups of functions in the control valve. Work one function alone and the valve can merge both sections into that circuit, which is why a single function feels strong. Work two functions and the flow divides, so each one slows. That behaviour is designed in, and it is the reason a combined-operation complaint needs a measurement before a part.
The straight travel valve
Travel is the exception the designers had to solve. Without correction, using the boom or arm while tracking would starve one track and the machine would veer. A straight travel spool inside the control valve holds flow to both tracks when an attachment function is used, so the machine keeps tracking straight. When that spool sticks, the symptom is a machine that pulls to one side only while another function is working.
Symptoms and what they point to
| Symptom | Most likely area | First test |
|---|---|---|
| Arm slow only while swinging | Normal flow split, or swing priority spool | Compare with a sister machine before buying parts |
| Both tracks slow when boom is raised | Straight travel spool or its pilot line | Travel alone, then travel with boom raise |
| Machine veers only under load | One pump section weak, or a relief leaking | Main pressure at both pump sections |
| All functions slow when a third is added | Pump delivery or regulator not destroking | Negative control pressure at neutral |
The test order that saves a week
Start with both pump main pressures at relief, taken at the test ports and written down, not remembered. Then negative control pressure at neutral, which should rise when the levers return to centre; a pump that never destrokes runs hot and steals flow. Then pilot pressure at the slow function, then regulator spool freedom, then the relief valves. Only after those five readings agree should the control valve come apart.
Two readings taken on the same machine, hot, tell you more than a week of swapping parts between machines.
What is normal, and what is not
Some slowdown is designed: a 20-tonne machine asked to swing and raise the arm at once will divide flow, and the arm will be slower than it is alone. What is not normal is a function that is slow in combination and fast alone when a third function is also used, or a machine that has changed behaviour since a repair. Compare against the same model, not against a larger one.
The parts involved
Flow sharing lives in the control valve, the two pump regulators and the relief valves that cap each circuit. We stock control valve assemblies such as YY30V00058F2, regulators 29070821-4927 / YN10V01006F2 and YN10V01009F4 / GYT6K, main relief valves in the V9510318049-T and YN22V00001F4 families, travel pilot valves YN30V00080F1 / YY30V00061F2 / 10181-0000A and check valve YN21V00025P1, all quoted against the serial.
What to send us
Send the model, serial, the two functions that fight each other, the four pressures, and whether the behaviour changed after a repair or arrived with the machine. We hold three thousand part types on the shelf and over one hundred thousand part numbers available to order through original supply channels, each marked genuine, OEM or aftermarket.
Useful background sits in the SK200 model page, the hydraulic components range and the work mode and pump flow guide; send pressures through the contact page. Circuit diagrams and system descriptions for each model are published by the manufacturer at kobelco.co.jp.
Guangzhou Shenyue Machinery Technology Co., Ltd. is an independent supplier of Kobelco parts, not affiliated with Kobelco Construction Machinery Co., Ltd. Genuine parts are supplied through original channels alongside quality OEM and aftermarket alternatives at distributor-level prices.
Frequently Asked Questions
Is it normal for the arm to slow down when I swing?
Yes. Both pump sections feed the arm when it works alone, and flow divides when swing is added, so the arm slows. It is a fault only when the same function is fast alone and abnormally slow in combination, or when behaviour changed after a repair.
What does the straight travel valve do?
It keeps flow even to both tracks while an attachment function is operated, so the machine tracks straight instead of veering. A sticking spool shows up as a machine that pulls to one side only when the boom or arm is working.
Which pressures should I measure first?
Both pump main pressures at relief, then negative control pressure at neutral, then pilot pressure at the slow function. Guangzhou Shenyue Machinery Technology Co., Ltd. quotes valve and regulator parts only after those readings, because guessing at a control valve is expensive.
Can a weak pump cause a combined-operation complaint?
Yes, and it is common. One tired pump section loses delivery, so functions that share it slow down while functions on the other section stay normal. Regulators 29070821-4927 / YN10V01006F2 and 29070821-4927 / YN10V01009F4 are the usual replacements.