As presented in Article 1: Introduction to the amazing world of Fluid Power – Part 1; we discussed the concept of using fluid to transfer mechanical energy into fluid power to perform a “Work” function.  Touching on  the four key fluid power elements  Energy,  Flow, Pressure and Fluid.

In this article we are going to expand further on this substance called; “Fluid”

The principal of using fluid to drive an actuator to create a work function to lift or turn an object has been a around for hundreds of years. The topic of Fluid in a fluid power setting can springboard into two key different discussion points:

  • understanding the associated impact of selecting a fluid type
  • maintaining the fluid quality

Before we explore these two key points, let’s ask a question; Fluid what is it really about?  The core purpose is to use this fluid to transfer power from one location to another. As we look at this principal deeper, the technicalities of this substance can be seen in 4 key levels:

1. Power; Transfer and Control – (a hydraulic pump will generate a flow of fluid through the circuit to transfer the power requirement)
2. Lubricate the system – (this same fluid will lubricate the components in the system)
3. Heat dissipation – (heat will be dissipated by the fluid as it travels through the circuit)
4. System sealing – (this fluid will also seal the internal components of the circuit)Ahydraulic drive System

In an application like a piece of earthmoving equipment that incorporates a hydraulic drive system, it will have a dedicated hydraulic pump to supply flow to the drive motors. The type of hydraulic pump and motors used in this system will come with a clear recommendation of the fluid Viscosity level required for this application.
Fluid Viscosity is a very important term to understand in hydraulic and lubrication fluids.

Viscosity is a quantity expressing the magnitude of internal friction in a fluid, expressed as a measurement in a centistoke unit. This is  measured by the force per unit area resisting uniform flow. To help explain this, a reference is captured from the Wikipedia site, which articulates: “Viscosity as a physical property of fluids. It shows resistance to flow. In a simple example, water has a low viscosity, as it is ‘thin’. Syrup or tar, on the other hand has a high viscosity, as it is ‘thick”
Why is this critical to a hydraulic system?  Viscosity of hydraulic fluid will change with temperature; for example,  in an environment where the ambient temperature is low this has a direct impact on the fluid viscosity states.  The following chart illustrates this point clearly.

Viscosity Chart













Why is this important to understand? Simply put, if the wrong Viscosity of fluid type is used, the system could be subject to unnecessary stress while trying to pump a heavy oil through it’s components, causing premature wear of its core components and certainly see the operation of the machine performance slow/sluggish.

An example of this situation; let’s say a machine was design to operate in the heart of the Australia’s hottest regions, where ambient temperatures are in the mid 40 degree Celsius range. A typical hydraulic fluid viscosity chosen for this type of application is 68cst. Now, if that machine was sold and moved down to the coast where the typical ambient temperature was in the low 20 degree c range, this same oil will now have viscosity rating of approx 200cst, which would labour the poor the hydraulic system and create a range of unnecessary issues. We will explore these issues in a separate discussion article.

For majority of the population this simple fluid function is just a given fact that machines work, brakes in cars just work so why should we care about the detail? It’s a fair question; why should I care about fluid? The great answer to the general public is that they don’t. It’s the engineers and designers of the equipment who need to take into consideration the many facets of what is the correct fluid for an application.

From a circuit design point of view, incorrectly chosen components like; pipe size or valve selection can also contribute to unnecessary heat being generated in the operating temperature of the fluid. This will also consequently cause the design Fluid type operating in a temperature range that will cause massive fluid degradation issues.

For the maintenance teams, responsible for keeping the equipment operating, fluid knowledge is so important.  Key terms to know:

1. Fluid Viscosity, as we have been discussing.

2. Lubricity is the measure of the reduction in friction and or wear by a lubricant, is there the right lubricity in the fluid for the application?

3. Oil degradation is a general term used to describe the destructive mechanisms that cause physical and chemical changes to fluids while in service. Cleanliness Levels, again separate discussions will focus on each of these points.
This article, just touches the surface of the key Fluid aspects of hydraulic or lubrication system. Fluid is the life blood of the hydraulic system and over the coming articles we will explore fluids in more detail. If the hydraulic system is designed well and the fluid is maintained correctly a hydraulic system will last a very long time.