Motor Speeds Explained: When to Use a Gearbox

Thursday - 01/11/2018 14:28
If you missed part one of this series, you might want to take a quick look at it before going forward. We discussed how motor speeds differ between AC and DC motors and the ways that speeds can be changed.
Motor Speeds Explained: When to Use a Gearbox
Motor Speeds Explained: When to Use a Gearbox
This post will discuss gearboxes and how they can help to dramatically change speeds when the previous options aren’t enough.
 
What is a gearbox?
 
A gearbox otherwise known as a gear reducer or speed reducer is a set of gears that can be added to a motor to drastically decrease speed and/or increase torque. Groschopp offers four different types of gear reducers: Planetary, Parallel Shaft, Right Angle Worm and Right Angle Planetary (Bevel). Each gearbox type works in unison with a motor to achieve the desired speed-torque output.
 
Gearboxes & Speed
 
For motor-only applications, the slower the motor speed, the larger the motor needs to be. Large motors can be expensive and may not fit within the allowable envelope space of an application. Using a gearbox with a smaller motor enables the unit to operate at lower speeds.
 
In order to understand the importance of using a gearbox to decrease motor speed, consider this scenario. You have a pastry conveyor that adds cherry filling as the pastries go by. Because this is a fairly light load, you would not want the conveyor running at 5000 rpm. If it was, there would be pastries and cherry filling everywhere!
 
Gearboxes & Torque
 
Torque is directly proportional to the volume of a motor (diameter squared times length). This means that a very large motor and a relatively small gearbox could provide the same amount of output torque.
 
Let’s look at our example conveyor application again. Say your customer overloads torque on the conveyor motor. If you have a stand-alone motor, it may not have enough torque to move 50 heavy bottles. You will need to have the additional torque provided by a gearbox.
 
Torque Output of Motors vs. Gearmotors
 
Let’s look at these concepts another way. The chart below shows that you typically gain less than 5 in-lbs of torque for each size increase of the motor. In order to greatly increase torque, a gearbox would be the smaller and likely more cost-effective solution.
 
Motor Voltage Speed Torque
PM6013 12v 915 rpm 1.54 in-lbs
PM8014 12v 934 rpm 4.75 in-lbs
PM8018 12v 982 rpm 6.8 in-lbs

 

Let’s dive deeper and isolate the middle size, a PM8014 12v DC motor running at 934 rpm and 4.75 in-lbs of torque.

Motor Voltage Speed Torque
PM6013 12v 915 rpm 1.54 in-lbs
PM8014 12v 934 rpm 4.75 in-lbs
PM8018 12v 982 rpm 6.8 in-lbs
 
Under a light load, this motor will operate effectively and at a fast speed; if it is overloaded, the motor will struggle to have enough torque to move the load. This could damage the motor. If a parallel shaft gearbox with a 10:1 ratio is added to the motor, the speed will decrease from 934 rpm to 93 rpm, and the torque will increase to 36 in-lbs. This is a much larger jump in torque without having to significantly increase the motor’s size.
 

This chart shows how adding a gearbox can affect speed and torque.

Motor Voltage Speed Torque
12v PM8014 No gearbox 934 rpm 4.75 in-lbs
12v PM8014-PS 10:1 93.4 rpm 40.45 in-lbs
12v PM8014-PS 15:1 62.3 rpm 59.81 in-lbs
 
As you begin searching for which motor or gear motor is best for your application, it’s important to consider which factor is more important: speed or torque. If high speed and low torque (or vice versa) are needed, a fractional horsepower gear motor may be a good solution for you. But, if the application needs to move quickly and carry a heavy load, a much larger motor with higher torque outputs may be the route to go. Make sure to consider physical space and cost when comparing your motor options.

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