Inertia Considerations

When Sizing Bell-Everman Rotary Stages

Continuous Torque Examples  Inertia Mismatch Capabilities 

When specifying rotary stages, engineers understandably tend to focus on allowable payload. However, the actual mass of the payload on a ServoBelt Rotary (SBR) stage doesn’t come into play because we use extremely robust duplex pair bearings that handle hundreds of pounds both statically and dynamically in the axial and radial directions without reducing service life. When sizing SBR stages, it’s more important to pay attention to the payload’s rotational inertia—also known as the polar or mass moment of inertia. The rotational inertia of the payload affects stage sizing because it determines torque requirements and affects servo authority.

Determines Torque Requirements

Continuous Torque Examples

The rotational inertia of the payload determines the amount of torque the stage will need to provide to accelerate and decelerate the payload according to the desired motion profile and duty cycle. The following charts illustrate SBR’s continuous torque capabilities. Horizontal dashed lines represent payloads that are one-inch thick circular aluminum plates of varying diameters. The motor designations indicate the NEMA size in the first two digits and the stack length in the last digit. Performance values are for Bell-Everman house motors.

Affects Servo Authority

Inertia Mismatch Capabilities

The rotational inertia influences the inertia mismatch between the payload and motor, which in turn affects the servo system’s ability to control motion. While keeping the inertia mismatch below a 10:1 ratio is a good rule of thumb for applications using plug-and-play motors and drives, modern control systems can remain stable at far higher mismatches as long as you have no disturbances and are able to adjust the PID tuning. An SBR can easily be tuned for stable indexing at up to 200:1 and beyond.

Sample payloads are shown below with dotted lines representing one-inch thick aluminum plates of various outer diameters. This information can serve as a quick guide to controllable inertia ratios for different SBR and motor combinations. Other cases can be quickly assessed by mentally moving a dashed line downwards in proportion to your payload thickness. For example, if your table has a 12-inch diameter and 0.5-inch thickness, shift the 12-inch dashed line down by half to end the mismatch for a given house motor. Or, consider your actual payload inertia if known.

Configure A Stage

Off-The-Shelf and Customized Stages For Any Application

While off-the-shelf rotary stages will do the job in most cases, sometimes your application requirements call for a customized stage. It may be something as simple as a special bolt hole pattern on the top of the stage. Or you may need a way to secure a workpiece in the stage center opening. Or you may need to squeeze a rotary positioning device into a tight space.

Whatever your requirements, we can help. We’ve introduced accessories and modular design features that make it easy to customize our standard Servo-Belt Rotary (SBR) and Direct Drive Theta (DDT) rotary stages. And in cases where our standard rotary platforms can’t be modified to fit your requirements, we can often design and build one-of-a-kind rotary motion systems to your specifications.

ServoBelt Rotary

ServoBelt Rotary (SBR) is a series of compact, high-torque rotary stages that offer through hole size and speed comparable to direct-drive rotary tables costing thousands more. Standard SBR models are available in five different through hole sizes.


Direct Drive Theta

Direct Drive Theta (DDT) stages feature a compact mechanical design that makes them easy to integrate into metrology systems and other machines that need precision positioning. Standard DDT units are available in two sizes, both with ample space in the middle to bring power and utilities to the top of the stage.


Go Custom

Download our guide to ServoBelt Rotary and Direct Drive Theta modification or visit us online to learn more about customization options and how to build a one-of-a-kind system that meets your exact specifications.

Download The Guide