Selecting and Sizing Geared Motors

This post shows how to size C-Face gear reducers and integral geared motors.

To make the selection, I will use KEB’s free software sizing program called KEB-DRIVE. Click the link to download a copy of the software to follow along.

1. Consider the Application Requirements

Before selecting a geared motor, know the application requirements. Identify the minimum and maximum amount of torque and speed, and what duty, control, and mounting are required.

Torque & Speed

• What torque and speed are required at the output of the gearing?
• What does the typical torque profile look like?
• Is the loading more or less steady – or will the gearing experience shock loading?
• What electrical power do I have (three-phase, 50 or 60Hz, Voltage)?

Duty

• What is the duty of the application?
• If I need more motor overload – can I rate for a reduced duty like S2 or S3?

Control

• Does the application require position, torque, or speed control?
• Will the motor be run across the line or with a VFD?
• Will the motor hold the load at 0 speed indefinitely (e.g. hoisting application)?
• Does the motor need a spring-set brake on it?
• Does it need a feedback device like an encoder?

Mounting

• • How will the gearmotor be mounted to the machine – foot, shaft, or flange mounted?
  • Are there space constraints?
  • Does the output axis need to be inline or at a right angle?

Environment

• What environment is the gearmotor entering (e.g., caustic washdown, saltwater, sensitivity to noise, etc.)?
• What are the ambient temperature ranges during operation?
• Does it require special ingress protection (washdown, outdoors, etc.)?
• Is this a food processing application that will require food-grade lubricants and grease?

2. Select the Correct Gear Technology for the Application

The video above gives you a visual overview of 5 typical types of gearing.

Configurations in the KEB-DRIVE selection software start at the top left. On the left, there is a drop-down to select the gear type and size. KEB offers 4 main types of gearing:

Table 1
4 Types of KEB Gearing

Helical-Worm gearmotors and Helical-Bevel gearmotors will provide right-angle outputs. The Helical-worm can be cost-advantageous for larger reduction ratios. Helical bevel has the advantage of better efficiency, which can equate to a smaller motor.

There is an option in KEB-DRIVE to leave off the gearing. This is common when people are interested in selecting a servo or induction motor.

3. Motor Selection (Size, voltage, frequency)

Working to the right, select the size of the motor. Options for both Induction motors and AC Servo motors are listed. Here is a comparison of the advantages between servo and induction motors.

A tab allows you to select the stator winding information – specifically, the rated voltage and frequency. KEB can offer special windings outside of what is listed. Last year, I worked on a 380V/60Hz installation in South Korea – who knew?!?

Larger motors will provide more torque. As you increase the motor size, the torque information is updated in the torque/speed drop-down.

4. Adjust the Torque/Speed selection

Is it a speed reducer? Or a torque Increaser? It’s both – higher gear ratios will provide lower output speeds and higher torques. Use the drop-down to see all possible configurations with the selected gearbox/motor combo.

There is a limit to the options that KEB-Drive will show. Only options with a service factor of 1.0 or greater are displayed.

Note: If you have selected a motor winding rated for both 50 and 60Hz, you will see two values listed for the speed and torque. The 50Hz rating will be reflected with the slower rpm output.

5. What is the gearing Service Factor, and why is it important?

The gearing service factor (SF) is the ratio between the:

In other words, the Service Factor provides a relative comparison to how much capacity the gears have in the current configuration.  An SF of 1.0 means the gears will have a nominal output torque equal to that of their rating. Selecting a motor/gear configuration with an SF of less than 1.0 is not advised. This means the gears will be undersized when operated at the nominal point. This could also indicate that the motor selected is too large.

Sometimes, the application is very difficult with regards to Duty, Shock loading, Temperature, etc. (look at step 1 – application requirements). In this case, it is advisable to select a very high SF that compensates for the factors that will stress, wear, and possibly damage the gearing.  Gear motor manufacturers typically provide a table of typical applications. Many also list a multiplication factor based on duty. It is advised that difficult applications like a Rock Crusher have an SF in the 3.0 range or higher. Relatively easy applications like fans with light duty might have an SF closer to 1.0. This often becomes a trade-off between safety factor and cost.

At this point, if the selection cannot meet the required torque, speed, and SF of the application, you’ll need to return to step 2 and select a different gear and motor combination. Or conversely, if the motor or gear appear to be oversized, then you can return to step 2 in order to optimize.

6. Select Gearmotor Options (Mounting Style)

This section allows users to select how the geared motor will be mounted. The flexibility of mounting is one reason the KEB integral gearmotor solution has been so popular. Users can select a unit with an output shaft. Or a shaft-mounted unit with a hollow bore. Mounting feet and mounting flanges can also be selected.

For shaft mount applications, a good option is shrink disk mounting. KEB’s shrink disc mounting provides a zero-backlash connection between the gearbox and the machine shaft. Assembly and disassembly are straightforward.

To go along with the flexibility-theme, users can select either English or Metric units on the shaft/bores. If you don’t see the exact option you want in the configurator, then I suggest contacting a KEB engineer to explore what is possible. We are able to offer a lot of customer flange and bore designs if the customer wants.

This section also includes the lubrication used in the gearbox. A number of different lubricants are possible depending on the application requirements. Consult the KEB gearmotor manual but the temperature ranges for the lubricants is listed. For food and packaging applications, a USDA food-grade lubricant is available.

Below that is a list of checkboxes that can be selected. Gearing options like low backlash and protection covers are possible.

7. Choose the Motor Options

This is where a user selects the motor options. Options like a motor on the brake. Or, the type of encoder for closed loop applications. Even the type of motor fan can be selected here.

8. General Options

Next, the general options can be defined. These are special requirements that can be easily selected in the configurator. This includes a second nameplate, condensation drain hole, etc, The conduit box orientation and the fitting location are also defined here.

Finally, the paint treatment can be selected here. The number of applications and thickness of paint is defined. Consult the KEB gearmotor catalog for the specifications. Added protection is possible with the P1, P2, or P3 options. These would be good options for washdown, outdoor, and maritime gearmotor applications.

The paint color is also defined here. Gray and black are most commonly requested so they are standard options. But we can paint to whatever color the customer specifies. In the case of a special, contact KEB.

9. Gather 3D models and dimensional drawings

The last option is the mounting position of the gearmotor. This is important as it determines the oil fill level of the gearmotor and venting.

At this point, the user can explore additional tabs in KEB-DRIVE in order to get more information. Of note, the customer can get extra motor information including the efficiency values, nominal torque, and inertia values.

The Dimensions tab includes a drawing with critical dimensions and 3D .step models.

KEB – High Performance Gearmotor Solutions

KEB-Drive is a useful tool because it is easy to use and selections can be made very quickly. However, please note that these are only the commonly requested features. There are many more possibilities KEB can offer like absolute or safety encoders, quiet brakes for theatre or elevator applications, custom flanges, etc.

If you have specific needs feel free to consult with a KEB gearmotor engineer and we’d be happy to help with your application.