Site work & quality
The machine will not reach pressure: troubleshooting hydraulics, WeldControl and CNC
Pressure stops below the calculated value: how to distinguish between setting errors, pressure relief valve issues and true hydraulic faults - and what the difference between WeldControl and CNC means for the fault.
Pressure stops: what the number on the display means
When a butt welding machine will not increase in pressure any further, it is almost always one of three things: the pressure is set lower than calculated, a valve is open, or the hydraulics cannot deliver. These three faults feel identical on site, but they require very different solutions - and the expensive one is rarely the correct one.
First, locate the figure. The display shows gauge pressure, i.e., the pressure present in the hydraulic system, not the welding force calculated in newtons. On a WeldControl machine, you set the maximum pressure yourself, and the machine may therefore stop precisely at that point because you instructed it to. On a CNC machine, the control unit calculates the pressure based on the entered pipe data and stops if there is a deviation.
Therefore, look up in the welding tables what pressure your dimension and SDR require, and compare it with the machine's current setting. If the set maximum pressure is below the table value, there is no fault in the hydraulics - only a setting that needs correction.
WeldControl and CNC are not the same
The terms are often used interchangeably, but the two levels operate differently, which changes where you look for the fault. Data logging is largely identical for both: pressure, temperature, times and operator are saved per joint. The control of the welding cycle is what differs.
On a WeldControl machine, you set the maximum pressure yourself and physically operate the lever for the hydraulic motor when pressure needs to be applied or released. The machine assists in monitoring, logging and reducing pressure in auto mode, but it does not move the carriage for you. On a CNC machine, the control unit sets the pressure and moves the carriage automatically - and with an automatic heating plate, you can initiate the weld and attend to other tasks while the cycle runs autonomously.
This means that a CNC machine refusing to build pressure most often has a fault in its programmed parameters or in the hydraulics - it has no levers that you could have forgotten. A WeldControl machine is just as likely to have a fault caused by operator handling of the lever and valves.
| WeldControl (assisted control) | CNC | |
|---|---|---|
| Max. pressure | Set manually on the machine | Calculated and set automatically by the control unit based on pipe data |
| Carriage | You operate the lever for the hydraulic motor manually | The carriage moves automatically through the cycle |
| Heating plate and phases | You change the heating plate and follow the phases on the display | Automatic heating plate can run the cycle while you work on other tasks |
| Logging | Same type of logging: pressure, times, operator, PDF | Same type of logging, frequently with export to documentation software |
| Typical cause of missing pressure | Max. pressure set too low, or pressure relief valve not tightened | Pipe data or SDR entered incorrectly, or hydraulics reaching its limit |
The levels describe functionality, not a specific manufacturer. The workflow is identical across all manufacturers.
The pressure relief valve: the overlooked cause on a WeldControl
On a WeldControl machine, the pressure relief valve is used when welding manually: it is used to relieve pressure from the system when you are operating the lever yourself. If the valve is open or not fully tightened, the hydraulics cannot build up the set pressure - and the machine stops somewhat below the calculated value without anything being wrong with either the pump or the oil.
This is therefore the first thing you should check when pressure will not build up: is the pressure relief valve tightened (closed) before the cycle starts? It takes a single second and is by far the most frequent explanation for pressure failing to reach the required level.
Do not confuse the pressure relief valve with the hydraulic system's pressure limiting valve, which is the mechanical safety valve on the manifold. This is factory-set and must not be adjusted on site - it is a safety feature, not a setting.
Manual mode: the best troubleshooting tool on a WeldControl
A major advantage of WeldControl machines is that pressing the right and left arrow keys switches the machine to manual mode. In manual mode, the display shows only gauge pressure and heating plate temperature, and the machine behaves precisely like a manual hydraulic machine where you control the pressure yourself using the lever.
This makes manual mode the fastest test on site: if you can build pressure using the lever alone with the automated cycle deactivated, the hydraulics are in proper working order, and the fault lies in the setting, the valve or the cycle. If you cannot build pressure in manual mode either, there is a physical fault - oil, air or a leak - and the machine must not be used for a joint.
Troubleshooting sequence
Perform the checks in this order. The first three take no time and catch the vast majority of cases before you start dismantling anything.
1. Read the display: is the displayed pressure the set maximum pressure, or does the machine stop below it? Compare with the table for your dimension. 2. Tighten the pressure relief valve and try again. 3. Switch to manual mode and check whether pressure can be built up using the lever. 4. Check the oil level and air in the system, and inspect hoses and couplings for weeping. 5. If there is still no pressure build-up, the hydraulics must be assessed at a workshop.
On CNC machines, the sequence starts elsewhere: check the entered pipe data - diameter, SDR and material - and read the error message before inspecting the hydraulics. The control unit often stops because it has received incorrect data, not because it is failing.
| Symptom | Typical cause | Initial action |
|---|---|---|
| Pressure stops somewhat below the calculated value | Max. pressure set too low, or pressure relief valve not tightened | Compare with table, tighten valve, retry the cycle |
| Pressure drops again when the lever is released | Pressure relief valve open or internal leakage in valve/cylinder | Close the valve completely; if this does not help, send the hydraulics to a workshop |
| No pressure build-up at all, pump sounds abnormal | Incorrect phase rotation on 400 V supply or air in the system | Check phase rotation and bleeding according to the manual, see power on the job site |
| Pressure is generated, but the carriage does not move | Clamps binding or drag pressure too high | Clean and lubricate the clamps, check the drag pressure |
| CNC reports deviation or stops before reaching pressure | Pipe data, SDR or material entered incorrectly | Verify data against the pipe and read the error message |
| Pressure fluctuates from joint to joint | Low oil level, air in the system or leaking hose | Inspect oil level and hoses, bleed the system |
| Display shows only pressure and temperature | Machine is in manual mode following right/left arrow key press | Switch back to the controlled cycle if the joint is to be logged |
For guidance only. Adjustment of pressure limiting valves, hydraulic repairs and interventions in the control system must be performed at an authorised workshop.
When the machine must go to the workshop
If pressure cannot be built up in manual mode, or if it drops again without the valve being open, there is a physical fault: worn seals, a valve failing to seat properly, air in the system or insufficient oil. This must not be resolved in a trench with a commissioning deadline pending.
The same applies if the machine has suffered an impact, has been idle for a long time or has been opened by unauthorized personnel. Following a repair, the machine must be calibrated before being used for documented work, see calibration of welding machines. If the machine is already inbound, you can use the service form to describe the fault so the workshop knows what to look for.
Many of these faults can be prevented through routine maintenance as outlined in service and maintenance of welding machines - oil levels, hoses and clamps are rarely inspected until they cause issues.