Viscosity matches torque
The selection of laboratory mixers should be based on the viscosity of the material and matched with the required torque. The higher the value, the greater the shear stress to be overcome during mixing, and the corresponding torque demand also increases. For example, low-viscosity liquids such as solvents or water are often configured with high speed and low torque; while high-viscosity systems such as coatings or colloids require low speed and high torque drive to prevent motor overload or stirring failure.
The relationship between torque (T) and viscosity (η) can be simplified to the formula:
T = k · η · n
Among them,T is the torque (in N·m),η is the dynamic viscosity (unit Pa·s),n is the stirring speed (in rpm),k is the shape coefficient of the agitator (dimensionless, determined by the blade structure). In practical application, this formula needs to be corrected in combination with specific mixing scenarios.
The viscosity grade corresponds to the torque
Referring to common standards at home and abroad, the selection of laboratory mixers can refer to the table below. The table lists the typical viscosity ranges and their recommended torque ranges for initial reference.
| Material viscosity range (mPa·s) | Recommended torque range (N·cm) |
| 1 – 100 | 2 – 10 |
| 100 – 10,000 | 10 – 50 |
| 10,000 – 100,000 | 50 – 200 |
| 100,000 or more | 200 or more |
The above table is a general guideline, and the non-Newtonian fluid characteristics of the material should be considered in the actual selection, such as shear thinning or shear thickening behavior, at which time the constant speed torque output needs to be dynamically adjusted.
Operating temperature impact
Temperature changes can significantly alter viscosity values. Most liquids lose viscosity with temperature, reducing torque requirements. It is recommended to determine viscosity at the target operating temperature or to estimate it from the temperature-viscosity curve. For example, the viscosity of a resin system is 5,000 mPa·s at 25°C, and if it is heated to 60°C, the viscosity may drop to 800 mPa·s, and the corresponding torque demand can be reduced from about 35 N·cm to less than 10 N·cm.
Agitator geometry
The type of paddle affects the torque transmission efficiency. Propeller or turbine type is often used for low viscosity, and anchor or frame type is preferred for high viscosity. Shape factorkThe value varies with the diameter of the blade and the angle of inclination. For example, at the same speed, a large diameter straight blade requires 2 to 3 times the torque of a small diameter inclined blade. When selecting, make sure that the torque range that the instrument can provide covers the actual load.
Safety margin setting
To account for batch variations or instantaneous thickening, it is recommended to set a torque margin. The usual practice is to add a margin of 20 to 30 percent to the calculation of the demand value. For example, if the material needs 50 N·cm under the most demanding conditions, it is advisable to choose a mixer with a maximum torque output of at least 65 N·cm. Also confirm the motor overload protection function to avoid damage.
Testing and validation steps
After completing the selection, it is recommended to validate:
1. Use the target material to conduct a torque test in a small-scale mixing device and record the actual torque value.
2. Gradually increase the speed to a predetermined value and observe whether the torque change is stable.
3. Compare the specifications of the instrument and confirm that it does not exceed the rated range during the continuous operation period.
Common misunderstandings and clarifications
Myth 1: High speed equals high stirring effect. In high-viscosity materials, too high a rotational speed can lead to idling or local shear, which can reduce efficiency. The correct approach is to match the viscosity and torque and set the appropriate rotational speed.
Myth 2: Select only according to the power of the motor. Power is the product of torque and speed, and high power may come from high speed rather than high torque. Laboratory mixing should prioritize torque capability over power values.
Cited source
1. ASTM E2503 – Standard Test Method: Performance Evaluation of Laboratory Agitators
2. ISO 3219 – Plastics Viscosity Determination in Liquid or Emulsion State
3. Overview of Mechanical Mixing Technology (2021)
4. Non-Newtonian Fluid Stirring Dynamics Study (2022 Technical Report)