Sep, 22 2009

Hydraulic

Last time we started to look at how hydraulic flow can occur in more than one flow path at once. With the understanding that moving fluid always follows the path of least resistance, we found that a simple way to have two paths with simultaneous flow is to force a disproportionately high flow volume (oversized pump) into the system where the hoses and tubes, and/or perhaps the valve openings themselves are now undersized. The undersized hoses and valves become points of restriction and resistance. They literally become loads that increase the total system pressure. This is not the most efficient design, even with a pressure compensated pump. But it is popular and it does allow for simultaneous parallel flow paths.

In this edition of Newsletters that Teach let's continue on with parallel flows. Understanding these concepts helps a lot when you're troubleshooting any hydraulic system.

Fluid Power Principle:

First let's make sure we've reviewed the source of pressure in a hydraulic system. To be very clear, the pump does not determine the system pressure value. However, the pump may have a mechanism attached to it that limits the system pressure (pressure compensated and load sensing pump designs). While it is largely true that without the pump forcing a volume of oil into a system there would be no pressure, the level of that system pressure is determined by loading (work at the cylinders or motors) and by restrictions (hose sizes, valve openings etc.). We are assuming that there is an open flow path to a cylinder or motor and that we are not sending fluid over a relief valve.

In this image we see that the pressure at the end of the hose is more or less zero. Back near the pump's outlet the pressure is close to 250 PSI. This pressure is the effect of forcing one gallon per minute into 100 feet of 1/4" hose. If we put the gauge at the middle of the 100 foot hose, we'd have a 125 PSI reading.

The Flow Divider

The example in Newsletter 16 of how to create more than one simultaneous parallel path was inefficient. (Parallel Flow Paths Part I.) It may also be important to note that the flow rates through the different paths would not be the same.

Another possibility for feeding more than one parallel path simultaneously is the use of a flow divider. This device comes in a few different design styles. We'll look at the pressure compensated flow divider. This device is designed to split the pump's flow into two flow paths with the same flow rate in each (some models will produce other ratios of flow).

The fact that each flow path may have a different amount of resistance (different loads at the cylinder) is not an issue for a pressure compensated flow divider. A compensating spool inside the flow divider will constantly shift back and forth as the two outlet load values (pressures) change. The compensating spool creates a variable "pinch point" or load that is added in series to one or the other outlet path.

On the outlet where the load pressure becomes higher (harder work at the cylinder), that higher pressure is used to shift the compensator spool and add restriction to the path that had a lower load pressure. The net effect is two flow paths that have exactly the same total resistance and therefore the same flow rates.

The flow divider is more efficient than the swamping method (as covered in the previous edition).

Load Sensing Hydraulic Circuits

Load sense hydraulic circuits that are common in many mobile machines have a much more energy efficient method of handling simultaneous parallel flow paths. The basic assumption of a load sense design is that the system pressure should never be more than a few hundred PSI higher than the heaviest parallel load. This saves input energy.

The operator of an excavator may wish to move both the boom and the stick cylinders simultaneously. If they represent different load pressures, the maximum system pressure will be set slightly higher than the greater of the two loads. This adjustment is done automatically with a sensing line from the valve bank that adjusts the pump.

With this adjustment made we would still expect the easier path to get all of the flow. But that is not the case. First of all the directional valves will be of the proportional type where the flow opening can varied. This means that a directional valve that is just barely open (for slow cylinder speed) will be a load on that branch circuit in addition to the cylinder load.

Secondly, as the system pressure is increased to support the heavier cylinder, a pressure compensator inside the valve sections with the lighter loads will pinch down a little.

This is a very dynamic system design. We'll get into it in more detail in the next edition of Newsletters that Teach .

Until then.....