Aug, 17 2009

Hydraulic

It is quite challenging to become an expert troubleshooter on a hydraulic system without first mastering some of its most basic design parameters. So this month let's start to think about how fluid can be made to flow through more than one parallel path at a time.

Most every industrial hydraulic system has more then one flow path. That is to say that once the pump has drawn in fluid from the reservoir, there is more than one potential path through valves and cylinders/motors for fluid flow back to the reservoir. The flow paths or branches are said to be in parallel with each other. In the following example, spring loaded check valves are used as generic loads on a hydraulic circuit branch. The check valves can easily be replaced by cylinders or motors that have heavy loads to move. The spring loaded check valves are only used as simple examples for teaching the concepts.

The Path of Least Resistance

Consider a system with three parallel flow paths. Each path has a simple spring loaded check valve. Each check valve’s spring has a different strength expressed in equivalent PSI. Also assume that the hoses/tubes on each path are sized large enough to take the entire flow of the pump without adding restriction.

Fluid in this hydraulic system will follow the path of least resistance. With more than one potential flow path (parallel paths), the pressure will only rise to the level needed to take the easiest path.

Most hydraulic systems have more than one parallel flow path. If the system needs to build pressure to a certain level to move a loaded actuator, but due to a malfunction an easier flow path develops, flow will now be redirected and consequently motion will cease at the actuator.

Flow Through Multiple Paths is Possible

If you work with a system that has more than one parallel branch with different load values, and yet you are sure that they both operate simultaneously, there are several possible explanations.

Swamping the Circuit

If the hoses and directional valves are undersized so that no single branch can handle the pump’s full displacement without restriction, then two paths with different springs or load values can be passing fluid simultaneously. However, the flow rates in these two paths will not be the same. If this design seems inefficient, you are correct. It is, however, an approach to design that has been used for a long time, even with its excess heat and energy consumption. These systems, when used with over sized variable displacement pumps and with accumulators, are very responsive to parallel branches with valves that open up and require immediate flow.

Two more design possibilities for allowing simultaneous flow include:

The Flow Divider and Load Sensing Hydraulic Circuits

We'll cover those devices and designs in the next issue of Newsletters that Teach. Until then feel free to login and work with the basic parallel circuit teaching animation. You'll be able to change the size of the pump, and you can click on the ball valves to close off parallel branches.