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Industrial lubricants work under extreme operating conditions and stress that can severely affect their structure, behavior, and performance—anticipating this is critical for appropriate lubricant selection. In this article, Interlub provides a brief introduction to the field of rheology, the science that studies how fluids move under different circumstances.
Rheology is a specialty area within Physics that analyses the stress-deformation ratios of fluent materials. These fluidity and deformation behaviors depend, mainly, on three factors:
1. A material’s internal structure
2. The external stress that acts on materials
In rheology, there are materials that aren’t liquids nor solids, but viscoelastic. When a viscoelastic material has a greater liquid proportion, it’s considered a viscoelastic fluid (it has a viscous behavior). On the other hand, when a viscoelastic material has a greater solid proportion, it’s considered a viscoelastic solid (it has an elastic behavior).
Image 1: Viscoelastic materials have a behavior that is between liquid and solid.
When materials with viscous behavior become deformed, they lose energy and don’t come back to their original form (see yellow shape below).
When materials with elastic behavior become deformed, they store energy and use it to return to their original form (see red shape below).
Image 2: Energy experiment on materials with viscous and elastic behavior.
Rheology helps predict the behavior of industrial lubricants that submit to external stress. This helps in dealing with the fact that greases usually have the properties of viscoelastic solids with an elastic behavior, while industrial oils can behave like Newtonian fluids or like viscoelastic fluids, depending on their formulation.
Rheometry is the science that takes rheologic measurements. That is, the science that measures the fluidity and deformation of all materials. A rheometer is an instrument that measures both a material’s stress and its deformation history. A rheometer is useful to determine a material’s function.
Image 3: Rheometer
A rheometer helps to evaluate the viscosity and elasticity of materials through the measurement of stress and deformation.
A rheometer takes its measurements using the Parallel Plates model. With it, the instrument can relate the variables of Shear Stress and Shear Rate to take the necessary measurements.
Video 1: Rheometer test setup
This equipment has two parallel plates working together, separated by a set height. The upper plate has an oscillating or rotating movement, while the lower plate is static. In between these two plates is the lubricant to analyze.
The viscosity of a fluid is the measure of its resistance to deformation. When a liquid deforms, the material’s molecules crash against themselves, generating internal friction that causes resistance to flux. The more resistance a fluid offers, the more viscous it is.
A rheometer helps to measure the viscosity of a fluid, where Viscosity (η) is a relation between Shear Stress (τ) and Shear Rate (γ).
When analyzing viscosity measurements against rotation speed, a viscosity curve emerges, identifying three types of viscoelastic behavior that fluids can present: Newtonian, shear-thinning (pseudoplastic) or shear-thickening.
Image 4: Viscoelastic behavior of fluid
The Newtonian behavior of a fluid is when its viscosity is independent from Shear Rate, thus remaining constant.
The shear-thickening behavior of a fluid is when it offers more resistance to flux as speed increases. Viscosity increases as speed increases.
The shear-thinning (pseudoplastic) behavior of a fluid is when it loses viscosity as speed increases.
This is relevant to industrial oils, as the most important parameter to select an oil for a specific application is its operational viscosity. When an oil loses its viscosity through its use, it can cause problems in the equipment that uses it. The same can happen when not considering an oil’s increased viscosity as part of its selection.
A rheometer can also test a viscosity curve against temperature, which allows observation of materials’ viscosity behavior when they heat up or cool down. To create a viscosity curve, speed (shear rate) stays constant and temperature is variable. Materials’ viscosity changes as temperature changes.
The deformation of an elastic material is directly proportional to the stress it undergoes. When stress deforms an ideal elastic material, it goes back to its original form when the stress stops. Solid viscoelastic materials, such as greases, don’t return to their original form completely.
A rheometer can measure the deformation that a viscoelastic solid suffers under shear stress. Deformation being the relation between the material’s displacement distance (s) under stress, and the height (h) of separation between the rheometer’s plates.
Once a deformation percentage is available, the applied shear stress relates to the resulting deformation, which delivers a Complex Modulus (G*). This Complex Modulus consists of two elements: An Elastic Modulus (G’) (the proportion of the material that behaves like a solid) and a Viscous Modulus (G’’) (the proportion of the material that behaves like a liquid).
The relation between the two moduli defines where in the solid-liquid spectrum a material is.
Greases have a greater elastic proportion, which explains why they don’t flow freely but stay in place. This, however, depends on the grease’s consistency: the more consistent it is, the higher its elastic proportion.
An amplitude sweep test describes how a material deforms and what happens when its structure breaks. A material’s structure breaks when it goes from having a greater elastic modulus to a greater viscous modulus.
During this test, a material’s deformation amplitude varies at a constant frequency and temperature. A resulting graph puts the elastic and viscous moduli against the deformation percentage or applied stress. Three things come up as a result:
1.The lineal viscoelastic range. This range is present when the application of shear stress begins. Within this range, the proportion of the elastic and viscous moduli doesn’t change, no matter how much the material deforms or how much applied stress there is.
2.Yield point. After crossing this point, the proportion of the elastic modulus starts to fall. The greater the elastic modulus, the more rigid the material is.
3.Flow point. As a material deforms, it reaches its flow point, where its elastic proportion is less than its viscous proportion. At this point, the material has lost its structure and isn’t storing energy. In the case of greases, this is where they lose their form and start to flow.
Image 5: Deformation and shear stress impact on grease structure
This is an example of the rheological behavior of different greases as the result of an amplitude sweep test, where each grease has a different consistency:
Evaluating material structure deformation under different temperatures
A rheometer can also make a temperature sweep test, which helps observe the behavior of a material’s structure at different temperatures.
For this test, a material’s temperature varies at a constant frequency and amplitude.
In this test, it’s important to identify any crossings between the elastic and viscous moduli. Crossings mean that temperature makes the grease lose its structure. Also, the more lineal the behavior of the grease, the less temperature affects its structure and consistency.
The use of rheometers provides Interlub’s consultants a sharper understanding of their products’ performance under different circumstances. This, in turn, has helped them make more effective product recommendations that have led to significant reductions of total maintenance costs.
If you’d like to explore opportunities for improvement, Interlub’s experts can run a 360° diagnosis of your operating conditions to recommend the ideal lubricant.