Proper Use and Maintenance of Overhead Stirrers

By Aimee O'Driscoll, 20 August 2018

An overhead stirrer is one of simplest and arguably most useful pieces of equipment in the lab. However, incorrect use of these units could be a detriment to your application. What’s more, improper maintenance could lead to poor results and a shorter lifetime for your machine.

We’ll go into these in more detail below, but here are the steps you can take to use your overhead stirrer more efficiently and for longer.

  1. Use the right machine for your application.
  2. Ensure the mixer is properly mounted.
  3. Make sure the impeller is locked in place.
  4. Clean the unit regularly.

Let’s look at each of these in turn.

1. Use the Right Machine for Your Application

An overhead stirrer isn’t always the most suitable tool for mixing. Even when it is, it's important you choose a machine with enough power and select the right impeller, paddle, or blade for your application.

Of particular concern is high viscosity samples. Whether they are high viscosity to begin with or become that way during processing, mixing these materials could strain the drive motor of your mixer. This causes it to overheat and to wear more quickly, reducing the lifespan of your machine.

Some overhead stirrers have a feature that reduces the power if a certain torque is reached. For example, the MSA Analog Overhead Stirrer by Jeio Tech shown below uses a “best effort” function to maintain a particular stirring speed, even under a heavy workload. However, it can determine if the work becomes harmful, and in that case, will stop stirring.


An MSA Analog Stirrer.


Another feature in this machine is a small internal fan which can help to avoid overheating of the motor. Other stirrers have a red warning light that indicates when it is overloaded.

Regardless of these measures, it’s best to use the right tool for the job in the first place. For example, for certain applications, you might need to consider a more powerful mixer, a different impeller or paddle, or a different type of machine, such as a homogenizer. Factors to consider include the solubility of sample components, desired particle size, and the viscosity of the sample.

2. Ensure the Mixer Is Properly Mounted

Mixers are typically mounted on a post fixed to the lab bench or on their own separate stand. It’s important to ensure the mixer is properly mounted to avoid it slipping down and damaging the sample or causing injury.


The CS20 stirrer.

An OS20 Overhead Stirrer.

The vibrations caused by the mixer, especially when run at high speeds, can cause clamps to loosen slightly, so they should be tightened before each use. In some instances, the weight of the mixer resting against a clamp can make it feel as though it’s tightly in place even when it’s not. As such, it’s a good idea to try to move it at a few different angles to check that it is indeed tightly in place.

3. Make Sure the Impeller Is Locked in Place

Aside from ensuring the mixer clamps are tightened, you also need to check that your impeller is locked in place within the stirrer chuck. In most models, the jaws of the chuck close around the impeller shaft as you turn a key. Some models, such as the LH Overhead Stirrers by VELP Scientifica, don’t require the use of a key and can be tightened by hand.

While locking the impeller in place, it’s important to try to keep the shaft central so that the jaws close evenly. If not, it might feel like the impeller is locked in place even when it’s not. Then, once you start mixing your sample, the motion will cause the impeller to fall out, potentially breaking your glassware, ruining your sample, and creating a safety hazard. Certain stirrers, such as Heidolph’s Hei-Torque Value, have an audible signal to confirm that the impeller is tightly fitted.


Examples of overhead stirrers.

A Hei-Torque Value by Heidolph and an LH Overhead Stirrer by VELP Scientifica.

One way to avoid this is to close the gap between the jaws so that it’s just slightly larger than the size of the impeller shaft before inserting the impeller. You can also turn the machine so that you have full view of the closure.


Impeller examples.


One more thing to check is that the impeller blade is tightly attached to the end of the shaft. These can loosen over time and come off while mixing. This is especially common when mixing very viscous materials. A quick tighten using the appropriate tool before use should help you avoid this issue.

4. Clean the Unit Regularly

The main part of the stirrer that houses the motor should be cleaned regularly with a soft cloth and a solution of warm water with a standard lab detergent. Even if sample spatters don’t end up on the housing, it will tend to accumulate dust and grime from the lab environment. The dust in particular can be disturbed due to vibrations when the mixer is in use, and cause your sample to become contaminated.

The impeller should be cleaned between each use to ensure there's no cross-contamination between samples. If your mixer comes with a protective cover for the chuck, you should make use of it. This will prevent corrosive materials entering the machine, thus helping to prolong its lifespan.