Rotary viscometer / Rotary rheometer

Flow properties determine themselves or have an essential influence on two the most important functions of lubricating materials:

First of all, quality control serves for acquisition of reproducible and methrologically correct data about flow properties. Rheological parameters measured should have good comparability in production range of a producer, as well as be methrologically significant for consumers, suppliers and quality control authorities.
Reproducible and really comparable results of measurement require use of standard measurement methods which foresee preliminary thermal and mechanical treatment of the investigated material. Acknowledged algorithms of results processing must be used for assessment.

Figure 1: Relationship of viscosity against temperature for motor oils

Figure 2: Dynamic viscosity of greases against shear rate at different temperatures

Figure 3: Viscosity depending on load (number of cycles)

In practice lubricating oils are exposed to very high shear forces. They result in irreversible viscosity loss in case of lubricating materials with additives. Loss of viscosity is determined by means of comparison of viscosity value of unloaded lubricating material with the viscosity values after different loads.

Figure 4: Flow curve: shear stress as function of shear rate

Greases are a disperse system of lubricating materials. Dispersed components could form interrelated framework. If this framework will be elastic deformed during small shear stress, it has the yield point. It is necessary to measure the yield point in relationship against temperature and previous mechanical load. The yield point measurements could be done only with the help of rheometer that permits to carry out measurements with controlled stress (CS-tests).

Notes:

Yield point and high apparent viscosity at small shear stress are characteristic property of flow of greases. Owing to that, grease run-out from lubricating point is prevented.

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Determination of structure destruction or formation and study of shear rate-dependent and time-dependent flow properties (thixotropy)
Determination of influence of grease disperse phase on relationship of viscosity reduction and viscosity increase against time, as well as on equilibrium viscosity depending on shear rate, temperature and previous mechanical load

Figure 5: Relationship of viscosity against time at different shear rates

Figure 6: CR-ramps in forward and backward direction (hysteresis curve)

Notes:

Thixotropical properties of greases are decisive criterion of their implementation and quality. The end value of viscosity is achieved very rapidly in lubricating clearance of a bearing with grease. This is residual viscosity that depends upon shear rate and is a parameter of hydrodynamic conditions of the grease and gives possibility to determine how long it could operate normally.

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Back to Rheometer RHEOTEST^® RN

	sealing of spaces (e.g., combustion chambers of internal-combustion engines)
	hydrodynamic force transmission
	flowing to lubrication point and backwards
	cooling of lubrication point
	bearings protection from dirt and humidity

	viscosity as a function of temperature, shear rate and duration of shear
	yield point as a function of temperature and previous load

	the flow limit depends on previous load and temperature
	viscosity depends on shear gradient, shear duration and temperature

•	Measurement method to determine viscosity dependence against temperature and shear rate
	Controlled Rate Tests - (CR-tests)

	Setting of constant shear rate and change of temperature in order to measure viscosity against temperature
	Example: see Figure 1

	Equilibrium flow curves at different temperatures in order to determine dependence of viscosity against shear rate and temperature
	Example:see Figure 2

•	Measurement method to determine stability to shear
	Controlled Rate Tests - (CR-tests)

	Setting of constant shear rate and change of preliminary load to measure relative, irreversible viscosity losses
	Example: see Figure 3

•	Measurement method to determine flow limit
	Controlled Stress Tests - (CS-tests)

	Linear ramps with controlled stress to measure the yield point against temperature and previous load
	Example: see Figure 4

•	Measurement method to determine shear rate-dependent and time-dependent flow
	Controlled Rate Tests - (CR-tests)

	friction decrease
	wear minimization

	Step-change tests with controlled shear rate to determine structure destruction or formation
	Example: see Figure 5

	Linear ramps with controlled shear rate in forward and backward direction to study shear rate-dependent and time-dependent flow properties (thixotropy)
	Example: see Figure 6

	Maximum viscosity at the lowest temperature
	Minimal viscosity at the highest temperature