SANY Drilling Rig Mast Cylinder Sequence Valve Diagnosis

A mast function that refuses to move under pressure can easily lead to the wrong component being replaced. In this field case, the left mast cylinder of a SANY rotary drilling rig would not retract during mast lowering, although system pressure could reach 30 MPa. The rig was level, but recent rain had left the ground badly settled and made moving or testing the machine more difficult.

The symptom could have come from the electronic control, a limit signal, the proportional valve, the auxiliary valve, a restricted hose, the counterbalance valve, the sequence valve, or the cylinder itself. The useful part of this case is not simply the final replacement. It is the sequence of comparison tests that removed one possible cause at a time before the failed valve was identified.

The following account is adapted from the supplied field report. It preserves the measured pressure, diagnostic order, component swaps, and confirmed repair result while omitting private service access information and personal identifiers.

The Symptom and the Initial Risk

During the mast-lowering operation, the left mast cylinder did not retract. The right cylinder could still move, and the circuit pressure reached 30 MPa. High pressure without the expected movement indicated that the fault could be downstream of the pump, but pressure alone did not prove that the cylinder had failed.

Because the machine was standing on soft ground after several days of rain, unnecessary movement and repeated uncontrolled testing would have added risk. The inspection therefore began with control information and accessible electrical checks before progressing into the hydraulic circuit.

Observed function: left mast cylinder failed to retract during mast lowering.
Comparison available: the right mast cylinder remained able to move.
Measured condition: hydraulic pressure reached 30 MPa.
Site constraint: wet, settled ground limited movement and made disciplined testing important.

A Diagnostic Path Built Around Elimination

The technicians listed the plausible causes before opening the circuit. These included incorrect joystick parameters, a mast limit condition, no electrical output to the proportional valve, a sticking or relieving auxiliary-valve section, a failed sequence valve, an internally blocked hose or steel line, a sticking counterbalance valve, and a jammed hydraulic cylinder.

Display parameters and alarm information were checked first and appeared normal. Input and output signals changed when the mast-control joystick was operated. Voltage measured at the relevant auxiliary-valve connectors was approximately 10 to 12 V. These checks supported the conclusion that the controller and command circuit were responding.

Check controls, parameters, limits, and alarms before disturbing hydraulic components.
Verify that the joystick command produces a corresponding electrical input and output.
Measure the relevant solenoid-valve supply instead of judging the circuit by indicator status alone.
Use a working function on the same machine as a comparison wherever the procedure can be performed safely.

Connector and Hose Swaps Narrowed the Fault

The left and right mast-cylinder control connectors were exchanged temporarily. With the connectors swapped, the right cylinder could retract. The connectors were then returned to their original positions. This comparison showed that the electrical control path could command movement and made an electronic fault less likely.

Next, the hydraulic hoses for the left and right mast cylinders were exchanged at the auxiliary valve. The left cylinder still failed to retract while the right cylinder operated normally. This result made the auxiliary-valve section an unlikely cause. Operating only the right cylinder still allowed pressure to reach 30 MPa, which added another useful comparison point.

These swaps were diagnostic tests, not permanent modifications. Their value came from changing one relationship at a time and observing whether the fault followed the electrical command, the hydraulic supply, or the physical cylinder.

The fault did not follow the exchanged electrical connectors.
The fault did not move to the right cylinder after the auxiliary-valve hoses were exchanged.
The working side provided evidence that the command and pressure supply remained available.
All temporary connections were restored after each comparison.

Mast-cylinder hydraulic hose connections used during isolation testing

Counterbalance Valve, Lines, and Cylinder Were Ruled Out

The counterbalance valve was inspected next. After pressure was released, the left mast-cylinder counterbalance valve was removed, cleaned, and exchanged with the counterbalance valve from another cylinder circuit. The left mast cylinder still did not retract, so the symptom did not follow the counterbalance valve.

The hose and steel-line path to the left cylinder was then checked for an internal obstruction. No blockage was found. At that stage, the cylinder remained a concern, but the remaining sequence valves also needed to be tested before the cylinder could be condemned.

The four sequence valves on the boom were temporarily removed from the circuit and the lines were connected for a controlled test. Mast movement returned to normal. This was the decisive result: the symptom changed only when the sequence-valve group was bypassed. The faulty sequence valve was replaced, and the machine's mast movement returned to normal.

Cleaning and exchanging the counterbalance valve did not change the fault.
The hose and steel line showed no internal blockage.
Controlled removal of the sequence-valve group restored movement.
Replacing the faulty sequence valve resolved the problem.

Sequence valves inspected during the mast-cylinder fault diagnosis

What This Case Means for a Used-Rig Buyer

A non-moving mast cylinder is not enough evidence to price a cylinder replacement. The same symptom can come from the command circuit, valve control, hose routing, load-holding components, or the cylinder. A buyer should ask what was measured, what was exchanged for comparison, whether the symptom followed the exchanged part, and how the final repair was verified.

For remote inspection, request one continuous video showing both mast-cylinder directions, pressure response, left-to-right symmetry, and operation after the repair. Photographs should identify the inspected valve group and hose connections without hiding temporary repairs. If any part of the mast circuit cannot be tested safely, that limitation should be recorded rather than described as passed.

Do not treat high pressure as proof that the cylinder itself has failed.
Ask for comparative test evidence from the working and non-working sides.
Confirm that temporary bypasses have been removed and the specified circuit restored.
Verify the completed repair through repeated, controlled mast movement.

Evidence Before Replacement

This case was resolved because the diagnosis continued after several likely components had been cleared. The electrical command, auxiliary valve, counterbalance valve, hoses, steel lines, and cylinder path were evaluated before the sequence valve was confirmed as the fault.

Kepler Machinery uses this type of evidence-led inspection to help overseas buyers understand what a used rotary drilling rig can do, what has been repaired, and which findings still require attention before shipment.

Safety Note

Mast systems can move suddenly and contain high-pressure oil. Testing, hose exchange, pressure release, valve removal, and repair must be performed by qualified technicians using the correct hydraulic diagram, approved supports, clean procedures, and a controlled work area.