Tractor Hydraulic Working Pressure
The maximum pressure the relief valve limits in the linkage circuit; typically 180–220 bar. If it will not lift look at pressure, if it is slow at flow.
The tractor hydraulic working pressure is the maximum pressure that the relief valve limits in the circuit feeding the linkage and the auxiliary outlets. On tractors it is typically in the 180–220 bar range, and this value determines the lifting force that can be produced. The speed of the movement, on the other hand, is determined not by the pressure but by the pump flow; in the 60–100 HP class typically 60–90 L/min is seen.
Pressure or flow? From symptom to diagnosis
Separating these two values is very useful in the field, because the same complaint can point to different places.
| Symptom | Probable side | Where to look |
|---|---|---|
| A heavy implement does not rise at all | Pressure | Relief valve setting, tired pump, internal leakage |
| It rises but very slowly | Flow | Engine speed, blocked filter, shared functions |
| It rises, then comes down on its own | Internal leakage | Control valve and cylinder sealing |
| Heavy when cold, normal once warm | Viscosity | Oil type and oil temperature |
The last row is often skipped: oil temperature and viscosity change the response of the lever directly. A measurement taken in the first hour of the morning and one taken in the afternoon can come out differently on the same tractor.
Rules for measuring
- Wait for the oil to come up to working temperature; on cold oil the values mislead.
- The measurement is made with a gauge of a suitable range and at the right port; it is not decided by guesswork.
- Taking the relief valve setting above the manufacturer's value puts the cylinder and the hoses at risk. Raising the pressure gains no capacity, it takes away life.
- Take the measurement both off load and under a real load; internal leakage most often shows itself only under load.
- Watch out for confusion over units; the catalogue may have used psi or MPa instead of bar.
Do not skip one further point: the pressure limit of the auxiliary hydraulic outlets does not have to be the same as that of the linkage circuit. On some tractors there is a separate relief valve or a flow restrictor on the auxiliary line. Do not carry the value you read at the linkage over to it; once you have decided which outlet the implement will be fed from, measure that outlet separately.
Why the pressure falls over time
There is usually no single part behind a fall in the working pressure. As the pump wears the internal leakage increases and the efficiency at high pressure falls; this produces a system that looks normal off load and comes up short under load. A tired relief valve spring, or a valve not seating fully in its housing, also limits the pressure early. The third possibility is the sealing of the control valve and the cylinder: the load rises but cannot be held. To separate these three possibilities the pressure has to be read at different points of the system; a value taken at a single point does not tell you where the loss is.
How it relates to the electronic retrofit kits
The kits do not touch this circuit at all. The LFT20, the LFT54 and the ELF-812 work on the control side; the pressure and the flow come from the tractor's own hydraulics. That has a side benefit which is useful in fault finding: take the kit out of service and work the lever by hand. If the hydraulic behaviour stays the same, the problem is not in the kit but in the hydraulics. This single step cuts the search in two.
The choice of kit is independent of these values as well. The quantities to look at are the force needed to move the control lever and the distance the lever travels; both are in the lever mechanism, not in the hydraulics.
What to do in practice
With a lifting performance problem, measure the hydraulics first, not the kit. Use the typical ranges here only as a starting reference; what is binding is the value in the manual of your own tractor. How the numbers are to be interpreted and how they relate to lift capacity is explained in detail on the pressure and flow page.
The context in which this term is used: Converting a tractor hydraulic lever to electronic control