All about oil viscosity

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What is oil viscosity?

In order to minimize increased wear and maximize the performance of systems, hydrodynamic lubrication is generally used for the continuous operation of bearings. This means that the lubricant is conveyed into the lubrication gap by the relative movement of the contact surfaces to each other. As a result, the rolling element and the track are separated from each other. The lubricant must have a certain oil viscosity so that a stable lubricating film can form in the contact surfaces between the rolling elements and tracks.

Oil viscosity can be described as a measure of the resistance of a fluid to deformation or flow. This means that the higher the viscosity, the thicker and more viscous the oil. Furthermore, oil viscosity is temperature-dependent. This is because higher temperatures cause the oil to become thinner and flow more easily. The temperature behavior of an oil is described by the viscosity index. Accordingly, a high viscosity index means that the oil maintains its viscosity better over a wider temperature range.

In cold environments, a higher oil viscosity can have a negative impact on lubrication efficiency, as the oil becomes more viscous and flows less smoothly. It is therefore important to select a lubricant with an oil viscosity that meets the temperature requirements of the specific application. Furthermore, under high mechanical loads, increased oil viscosity may be necessary to maintain a constant lubricating film and prevent damage to the machines.

The ageing of the oil also influences the oil viscosity. Contamination, water absorption and high temperatures can reduce the oil viscosity, which reduces the protective effect of the lubricant. Regular checking or constant monitoring of the oil viscosity is therefore essential to ensure optimum lubrication and smooth operation of the machines.

In summary, oil viscosity is a critical parameter for the effective lubrication and protection of machinery. Careful monitoring and design of the parameter will help to increase efficiency, extend equipment life and avoid costly downtime.

In summary, oil viscosity is a critical parameter for the effective lubrication and protection of machinery. Careful monitoring and design of the parameter will help to

Increase efficiency
Extend equipment life
And avoid costly downtime.

The viscosity ratio κ

While the oil viscosity of lubricants is specified by manufacturers, the viscosity ratio κ is not so easy to determine. In general, the viscosity ratio is described as a measure of the quality of lubricant film formation in rolling bearings. It is derived from the ratio of the actual kinematic viscosity of the lubricant and the reference viscosity of the lubricant.

In general, the viscosity ratio depends primarily on the following parameters:

Bearing size/bearing type
Lubricant and its additives
Rotation speed
Temperature
Bearing load
Degree of contamination of the bearing

Although there are calculation methods for determining the viscosity ratio, these often only provide rough estimations that can deviate considerably from the actual conditions. The main reasons for this are the additives in the lubricant, the variable bearing load and the degree of contamination of the bearing, which are difficult to determine precisely.
The viscosity ratio is an important indicator for determining the lubrication condition in the bearing. This is defined so that a minimum separating lubricating film occurs between the contact surfaces from a viscosity ratio of κ = 1. Accordingly, assuming a high level of cleanliness, the nominal lifespan of the rolling bearing can probably be achieved from a value of κ ≥ 1. However, by using lubricants with special additives, this can be achieved even at lower values. Full lubrication can be assumed from a value of κ ≥ 4, whereby excessive viscosity also leads to increased flow resistance and energy losses. Figure 1 visualizes the significance of the viscosity ratio on the lubrication condition and the associated load carried by the lubricant.

Figure 1: Segmentation of the viscosity ratio κ into relevant sub-ranges

Our solution for you

By using machine learning in combination with countless series of tests, HCP Sense has managed to determine the viscosity ratio of a wide range of lubricants in real time using its unique measurement process.

Figure 2: A series of tests to determine the viscosity ratio

Figure 2 shows a series of tests to determine the viscosity ratio, in which different rotation speeds and temperatures were used. The transitions from boundary friction to mixed friction and from mixed friction to hydrodynamic lubrication are demonstrated. It should be particularly emphasized that these measurements are not only possible on special test benches, but can also be successfully carried out in series applications thanks to the implementation of our measurement technology.