Lift Capacity
The maximum load a three-point linkage lifts at a given measuring point; pressure, cylinder area, arm geometry and relief valve setting decide it together.
Lift capacity is the maximum load that a three-point linkage can lift at a particular measuring point. It is not the property of a single part; it is a result determined together by the hydraulic working pressure, the cylinder bore, the geometry of the lift arm and the setting of the relief valve. That is why two tractors at the same pressure give a different capacity if their arm geometry differs.
Where the force comes from
Pressure on its own does not determine the capacity. The force the cylinder produces is the pressure multiplied by the piston area; that force is carried to the linkage by the lever ratio of the lift arm and the lift rod. The relief valve setting draws the upper limit as well: the moment the valve opens, the oil returns to the tank and the lifting stops. What order of pressure and flow a tractor's hydraulics work at is dealt with under a separate heading.
Every link in this chain can be the limiting one on its own, and their symptoms differ. If the pump cannot deliver enough flow the linkage rises slowly, but it still rises. If the pressure is low it does not rise at all beyond a certain load. The lever ratio, on the other hand, is fixed; it does not change with maintenance, an oil change or an adjustment, it comes with the design of the tractor. That is why the request to "make the hydraulics stronger" is most of the time unanswerable: the geometry that determines the capacity stays where it is.
Why the catalogue value and the field value diverge
| Factor | Effect on the capacity |
|---|---|
| Measuring point | The value measured at the pin ends is higher than the value taken 610 mm back from the pins |
| Arm geometry | A different lever ratio at the same pressure gives a different capacity |
| Relief valve setting | If the valve opens early, the lifting stops before the capacity is reached |
| Condition of the hydraulics | If the oil level is low or the pump tired, you cannot reach the catalogue value |
| Centre of gravity of the implement | The further the centre goes from the pins, the lower the weight that can be lifted |
The first row is the place most often confused; how the difference between the two measuring points arises is explained in the definition of lift at the pin ends.
One more distinction: that the linkage can lift a load off the ground does not mean it can carry that load across the field. Lifting is a static operation; in carrying, the shocks from the ground come into play and the load on the linkage grows momentarily. The difference between them is explained under the difference between lifting and carrying capacity.
If the capacity has dropped, where to look first
In the case of "it used to lift, now it does not", the hydraulic oil level, the filter, the leaks and the behaviour of the relief valve are looked at in that order. What kind of symptom an oil leak produces on the kit is under the heading oil leakage. Loss of capacity most often comes on gradually; that is why the difference is only felt with a heavy implement.
There is a simple check that tells them apart: fit an implement of known weight, raise the linkage from fully down to fully up and watch two things — the speed of the lift and the point where it stops. A slow but complete lift generally points to the flow side, while stopping and straining at a certain height points to the pressure or the relief valve side. Try it separately with the oil cold and after it has warmed up; straining that is seen only when cold has to do with the viscosity of the oil, not with the capacity.
Retrofit kits do not change the capacity
The LFT20, the LFT54 and the ELF-812 make the control side electronic; the lifting work is still done by the tractor's own hydraulics. The kit neither increases nor decreases the capacity and does not touch the relief valve setting. If the linkage cannot lift a heavy implement, the answer is not in the kit but in hydraulic maintenance and in comparing the weight of the implement with the capacity of the linkage.
A practical rule: know the weight of the implement and how far its centre of gravity is from the pins, read the value in the catalogue that corresponds to that distance, and leave a margin in between. Working at the limit of the capacity tires both the hydraulics and the linkage mechanism.
The context in which this term is used: Converting a tractor hydraulic lever to electronic control