The accuracy of the valve depends on the size of the two orifices, the spring force and the leakage across the spools. If the tolerances on these items are kept to a minimum then accuracies of +/- 3% can be achieved.
Most production valves state an accuracy of +/- 10% on inlet flow. In some applications this can cause a problem when the cylinders are not flexible enough to accommodate this inaccuracy.
With this kind of design there is also a minimum flow at which the valve will operate. The relationship between the orifice diameter and the spring force opposing the movement of the spool means that there is a minimum flow before the spool will move and start to compensate.
If for some reason the flow from either leg is restricted then the spools will react to the offset pressure drops causing the spools to move to one end of the cartridge blocking off both outlets. This can be overcome by placing relief valves down stream of the flow divider to allow the flow to continue through a blocked or restricted outlet.
In cylinder applications the cylinders may not reach the end of stroke together. There will be a small make up flow but if relief valves are used the slower leg will catch up at 50% of the inlet flow. There are versions of flow divider that have extra holes to increase the make up flow. These however, are less accurate as the pressure difference between the two legs increases.
It is not necessary to fit flow dividers of this type on the inlet and outlet lines. The flow divider combiner will maintain equal division in both directions but care must be taken to size the flow divider to suit the outlet flow if it is in the full bore side of the cylinder as the flow will be increased by the rod/bore ratio of the cylinder.
It is not practical to cascade these valves to control more than two cylinders because the inaccuracy of the valves will be additive so you could end up with 20 to 30% difference in the flow.
It is also important that the valves are not over flowed. When in the dividing mode the pressure drop through the spools acts directly on the ‘lugs’, to rip the two spools apart. The normal factor of safety is 4:1 on ultimate tensile strength and as pressure drop through an orifice raise as the square of the increase in flow, so putting twice the rated flow through the valve will produce 4 times the pressure drop and probably break the ‘lugs’ The normal and most common division ratio is 50/50 but it is possible, by having different diameter orifices in the opposing spools, to produce offset ratios. The ratio of the orifice area in each spool will determine the offset flow ratio.
In spite of these draw backs there are many applications where the performance is good enough and therefore provide a cost effective solution to the problem of providing effective division of flow despite varying pressures in each actuator.
The accuracy of the valve depends on the size of the two orifices, the spring force and the leakage across the spools.
A typical example is on the arms of a tarpaulin cover for tipping trucks. The arms, either side of the lorry, have to extend together first and then rotate together to unroll the tarpaulin and stretch it over the insecure load in the skip.
One of the most common applications for flow dividers is for wheel motors in transmission circuits to give an element of ‘Diff-lock’. The flow divider will ensure that there is always traction to both wheels even when one of them is over soft or slippery ground.
Figure 3 shows a typical circuit where the flow divider is switched in when needed. The flow divider works on pressure drop so is intrinsically inefficient, even though the pressure drop is low. In a transmission circuit this pressure drop would create excessive heat so it is necessary to have a system to select the diff-lock when required.