How to Maintain Laboratory Overhead Stirrers

Laboratory overhead stirrers are one of two widely used instruments to mix ingredients in product research labs. CAT Scientific lab overhead stirrers, also called overhead mixers, have motor-powered stirring tools immersed in the sample beaker*.  The configuration of the stirring tools – generally termed impellers, paddles and blades – is selected based on the task to be performed.  These units offer researchers the ability to control the stirring speed and time.

This post discusses key components of a lab overhead stirrer and provides suggestions on how to maintain them.

Key Components of an Overhead Lab Stirrer

Although offered in several models with differing features, CAT Scientific overhead lab mixers have several components in common. These are

  • A drive motor activated by an on-off switch and a speed control, a chuck and a chuck key to lock the stirring tool shaft to the motor housing.
  • A support rod and clamp to correctly position the stirring tools in the sample.
  • An instruction manual.

Optional components include

  • Stirring tools in 3 configurations:  impellers, paddles and blades
  • A drive shaft protector
  • A pedestal stand and crossover clamp to hold the unit’s support rod

How to Protect Your Lab’s Overhead Stirrer Investment

Protecting your investment in a laboratory overhead mixer starts with reading the instruction manual.  Research scientists and others who operate overhead stirrers should be thoroughly familiar with all aspects of how the instrument functions.

Individual company operations manuals should include use instructions and cleaning/maintenance procedures for all instruments.

Pay particular attention to intended uses to help avoid potential maintenance issues.

Match Overhead Mixer Selection to the Tasks

For example, the viscosity of the sample, either at the beginning or evolving during the stirring operation, is an important consideration. High viscosity samples can strain the drive motor, causing it to overheat.

CAT Scientific lab overhead stirrers are equipped to automatically reduce power should the allowable torque be exceeded.  Operators should not attempt to override this and instead concentrate on identifying the cause of the problem.

On some models a warning light illuminates.  If the overload is removed or overcome the unit will go back to full power; if not the motor switches off.

Similarly if the motor overheats for any reason power will be reduced.  If continuously overheated the motor will shut off then restart when it cools.

Be Certain the Lab Mixer is Firmly Mounted

Proper mounting provides the foundation for good maintenance by eliminating vibrations that could potentially cause the apparatus to become dislodged from its base.   If your lab does not have a pedestal stand and supporting system for the drive motor assembly CAT offers them as options.

It is also essential that the stirring shaft is firmly attached to the overhead mixer drive motor.  This helps to ensure that individual components such as the stirring tool does not inadvertently make contact with the bottom or the sides of the container resulting in breakage and loss of samples.

Select the Right Stirring Tool Configuration

This is particularly important, and depends largely on the nature of the material(s) being mixed and how their physical properties may evolve during the mixing process.   It can directly relate to overheating issues.

Most stirring tools ( impellers, paddles and blades) are permanently attached to a stainless steel rod that is fitted into and tightened to the stirrer chuck.  CAT stirrer rods should be selected based on the size of the mixing vessel.

For example, the impeller distance from the bottom of the vessel should be about 1/3 the diameter of the vessel. Similarly the diameter of the mixing blade should be approximately 1/3 the diameter of the vessel.

Rod diameters are selected based on sample viscosities and the diameter of the mixing blade.

Viscosity applies resistance to the impeller that is transmitted by the rod to the stirring motor and can be most pronounced when using a blade configuration due to its relatively large surface area.  This can cause the motor to overheat.

For more on this see our post on selecting CAT Scientific overhead stirrer impellers.

Dealing with Lab Overhead Stirrer Malfunctions

Onboard diagnostics in higher-end overhead stirrers alert users to possible failures and shut the system down as a precaution. Error messages are displayed on the LCD screen when the unit is turned back on. Common causes include overheating, a stuck rotor, expired time, and a malfunction in the temperature measurement component.

Do not attempt to fix these malfunctions.  Operators or lab managers should properly pack the unit and return it to CAT Scientific so that it may be professionally repaired.


The motor housing of laboratory overheard stirrers should never be immersed in water as part of the cleaning process.

Instead, warm water and soft cloths should be used with a laboratory-grade detergent to remove soil and debris from the casing. Rough surfaced materials such as steel wool should never be used because they may scratch essential parts such as membrane key pads or motor housings.

Stirring tool cleaning should be performed on a regular basis to prevent cross-contamination.

Please feel free to reach out to us at your convenience with concerns or questions proper maintenance of overhead stirring systems.


*The other option is a magnetic stirrer in which the mixing is done by a magnetic bar immersed in flasks holding samples and driven by a rotating magnet in the base of the instrument.


CAT Scientific

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