Viscosity – the great unknown of hydraulic oil
March 25, 2024
Technical articles
We regularly receive enquiries about hydraulic fluids and take orders for this essential system component, which is primarily responsible for power transmission in mobile and stationary hydraulic systems.
When asked which hydraulic oil is the right one, we usually ask in return what viscosity is required or specified. Time and again, we receive the reply: “Yes, just the standard one.” But is there such a thing as “the standard hydraulic oil” or “the most commonly used hydraulic fluid” at all? Is the viscosity of hydraulic oil really so important that we always ask about it? How do you determine the correct viscosity, and who decides what is correct? As is so often the case with lubrication issues, questions cannot always be answered clearly with a simple ‘yes’ or ‘no’, but require a bit more explanation.
The viscosity of a hydraulic oil is not really a great unknown, given that ISO 3448 clearly stipulates how lubricants are to be classified into ISO viscosity grades.
The machine or component manufacturer usually specifies which ISO viscosity grade is suitable for a machine or system. The lubricant manufacturer is subsequently responsible for developing and producing the appropriate hydraulic oil with the specified ISO viscosity and for making it available to the machine or plant operator. Ultimately, the correct viscosity of a hydraulic fluid is crucial to whether a hydraulic system delivers the required performance efficiently and over the long term. Our blood group serves as a useful analogy: we only function properly with the right blood group. An incorrect blood group – or, in this case, the wrong viscosity of the hydraulic fluid – can lead to malfunctions in the hydraulic system.
Below is a simple, illustrative explanation of the potential consequences of using the wrong viscosity grade.
Hydraulic oils that are too thin can result in a loss of performance in the hydraulic system and lead to wear on hydraulic components, as, on the one hand, sufficient pressure cannot be built up in the system and, on the other hand, a potential breakdown of the lubricating film can lead to increased wear and, in extreme cases, even pump failure.
Hydraulic oils that are too viscous primarily result in poor cold-start performance of the hydraulic system and prevent the system from operating efficiently. In some cases, the hydraulic oil may be so viscous that the hydraulic pump cannot draw it in or deliver it, causing the pump to ‘starve’. Permanently excessive viscosity during operation can lead to sluggish or delayed responses from the hydraulic system and is fundamentally inefficient, as more energy is required to pump the more viscous hydraulic oil.
Our enquiry regarding the required viscosity class of the hydraulic oil is therefore well-founded and serves to ensure the operational safety of your plant and machinery. To check the condition of the hydraulic fluid, we recommend regular oil analyses at the LTC LAEMMLE Tec Centre laboratory. Alongside many other parameters, the measurement of viscosity at 40 °C and 100 °C forms part of the standard scope of testing and is one of the most important assessment criteria. So, should you in future receive an LTC test report containing the comment: “The oil does not meet any ISO viscosity grade or does not meet the specified one. If the viscosity grade does not comply with the manufacturer’s specifications, the oil must be changed,” you will know what to do.
Check the correct blood group – and, of course, the ISO viscosity class – using the plant/machine documentation; contact our LTC Service + Support (techsupport@laemmle-ag.ch/044 956 65 65) or consult the ROXOR Oil Finder.
Author: Martin Ruch, Head of Technical Support
Figure 1: Close-up of the laemmle-chemicals ROXOR drum