The Pros and Cons of Gear Reducers for Positive Displacement Pumps

High viscosity liquids, shear sensitive liquids, abrasive liquids, or any combination of these often require the pump speed to be reduced from synchronous motor speeds. Thus, positive displacement pumps often require reduced speed operations depending on the application. 

One of the simplest solutions to reducing speed in positive displacement pumps is via a gear reducer.  These simple machines consist of one or more pairs of gears.  The driver (typically an electric motor) turns the smaller gear at high speed.  The smaller gear (drive gear) rotates a larger gear (driven gear) with a greater number of teeth.  The ratio of driven gear teeth to drive gear teeth determines the ratio of the gear reducer.  As an example, a reducer with a 10-tooth drive gear and 30-tooth driven gear has a ratio of 3:1.  The speed of the output shaft (and connected pump) would be 1/3 that of the driver shaft (and connected motor).

Gear reducers are simple, reliable, and highly efficient.  But, there are downsides.

Cons of Using Gear Reducers for Positive Displacement Pumps

• Increased costs for purchasing the unit because it's an additional piece of equipment
• Added length and weight of the pump unit because it's an additional piece of equipment
• More frequent oil changes and oil monitoring are required because gear reducers are typically oil-lubricated

These counterpoints can lead some to look for other options to reduce the speed of their pump.  One way to address all three is to reduce the speed of the motor and one of the most common ways to do that is to use a VFD (variable frequency drive). It is possible to remove the gear reducer and use an AC motor with a VFD drive, but to do so requires oversizing of the motor to the application. And depending on the speed reduction required, it may be a more expensive and larger overall size solution.

Let’s think of the gear reducer in another way.  Think of the gear reducer as a torque converter.  Power in = Power out.

On the output side, the speed is low / the torque high.

On the input side, the speed is high / the torque is low.

As a result, the coupling on that low speed / high torque side between the reducer and the pump is larger and capable of transferring higher torque. The reducer allows the user to run a low speed / high torque load (the pump) with a low torque / high speed motor.

So what happens if the “torque converter” is removed the end-user runs the pump directly from the motor via a VFD?

As you’re likely aware, motors run with VFD’s provide constant torque.  This can lead customers to an incorrect conclusion that they provide constant HP.  Let’s look at the basic torque equation:

Torque = [Power x constant] / Speed

If speed is decreased, but torque is constant, then available power decreases.  In other words, if a motor is running at half speed (via the VFD) it is capable of only half its rated power.  Below is a chart to help explain this.