Dynamic Versus Static Torque 101
In mechanical testing and control, the torque transducer is a critical component for ensuring performance, efficiency, and safety in a range of industries. Torque transducers measure the twisting force, or moment, applied to an object.
While often referred to as torque sensors, the measurement devices fall into two fundamental categories based on the measurement type: Dynamic (Rotary) and Static (Reaction).
For any engineer designing a test bench or choosing a sensor, the first question must always be functional. Are you measuring the torque transmitted through a rotating element or the torque reacting against a stationary mount?
Recap of Torque Transducer Fundamentals
Before examining the categories, it is helpful to establish core terminology for torque. Torque is the rotational equivalent of linear force. It is defined as the product of a force and the perpendicular distance from the point of application to the axis of rotation.
A transducer transforms the mechanical strain caused by this torque into an electrical signal that can be measured. In modern sensors, this conversion process relies on strain gages, which are resistive elements attached to the sensor’s housing or shaft, whose electrical resistance increases or decreases in proportion to the applied mechanical strain.
Shaft-style sensors have an integral shaft that connects directly to the drive train, whereas flange-style sensors are more compact and include mounting flanges for direct attachment between rotating parts.
REFERENCE: Interface provides a comprehensive torque review in Interface’s Technical Library. The Torque Measurement Primer is detailed here or available for download.
Dynamic Torque Transducers
Dynamic (rotary) torque measurement is required when the sensor must rotate with the shaft, typically when the component operates at high speeds and/or undergoes significant angular acceleration. The sensor measures the applied or transmitted torque through the rotating shaft.
Measurement Principle and Signal Transmission for Dynamic Torque
The dynamic transducer is designed as a rotating coupling or shaft that is mounted in-line with the rotating drive train. Dynamic sensors provide two primary outputs: torque and rotational speed.
Couplings connect the torque sensor to the drive and load shafts. These components transmit torque while isolating the sensor from extraneous loads, such as bending and vibration.
Shaft connections can be keyed or smooth. Keyed shafts offer positive engagement but may cause backlash, clearance, or lost motion in a mechanical linkage, making disassembly more difficult. Smooth shafts with clamping couplings provide consistent torque transfer and avoid keyway-related issues. Mounting configurations include floating and fixed mounts.
In dynamic transducers, the main technical challenge is transferring the electrical signal from the rotating shaft to the stationary data acquisition system. Slip rings provide physical electrical contact via rings and brushes and are best suited for lower-speed, lower-cost applications. Wireless telemetry offers a rotating antenna and transmitter that sends the signal wirelessly to a fixed stator and receiver. This method is preferred for high-speed, high-accuracy, and maintenance-free operation, as used in our AxialTQ.
Engineers must also consider the sensor’s mass and inertia, as these factors affect the rotating system and can influence its natural frequency and dynamic response.
Dynamic Applications
Dynamic sensors are vital for measuring continuous power transmission in operational systems. They are commonly used in drivetrain testing to measure the torque transmitted through a vehicle’s drive shaft between the engine and gearbox. They are essential in turbomachinery for monitoring the torque and power output of high-speed gas or steam turbines. They are the preferred sensors in high-speed testing to evaluate the durability of engine components and pumps. In process control, they monitor the mixer or extruder torque to ensure material consistency.
Static Torque Transducers
Static (reaction) torque measurement occurs when there is no or negligible angular acceleration of the rotating element, or more commonly, when the measurement is taken on the non-rotating reaction element of a system.
Measurement Principle and Form Factor for Static Torque
The core principle of a reaction torque transducer is to measure the static (reaction) torque created by a rotating component, such as an electric motor or pump, against its stationary mounting point. The primary output of this type of sensor is torque only.
The sensor itself is a fixed block or housing mounted along the force path and does not rotate. This fixed configuration significantly simplifies the entire test setup.
The most notable technical advantage is signal transmission: power and data are sent via a simple hard-wired cable connection. Since the sensor is fixed, there is no technical challenge in bridging a rotating element to a data acquisition system. Consequently, the sensor’s mass and inertia are largely irrelevant to the measurement, as they do not affect the dynamic behavior of the rotating drivetrain.
Static Applications
Static sensors are ideal for tests where the rotational component is fixed or moves slowly. Typical applications include motor and actuator testing on a dynamometer, where the transducer is bolted to the reaction arm or frame. They are helpful for brake testing, measuring the holding torque against a fixed anchor. For fastener analysis, these static sensors measure the torque applied to a stationary joint. They are also used in quality checks, such as measuring the torque required to turn a fixed steering column.
Watch this Engine Dynamometer Application Using AxialTQ
Technical Synthesis of Dynamic vs. Static
The decision between static (reaction) and dynamic (rotary) torque sensors hinges on two primary factors: the component’s state and the method of signal transfer.
From a mechanics and installation perspective, the static sensor is simpler. It is fixed and easy to install, and its inertia is negligible. The dynamic sensor, however, rotates with the shaft, requiring precision alignment and forcing engineers to account for the added inertia in their system modeling.
Regarding signal handling, the static sensor uses a simple, robust cable, which contributes to its generally lower cost and complexity. The dynamic sensor requires sophisticated, and therefore more expensive, signal-transfer hardware, which increases both the complexity and the system’s maintenance demands.
A static sensor is the logical choice for measuring holding forces, motor efficiency at low speeds, or any situation where a fixed structure can adequately absorb torque. Conversely, a dynamic sensor is essential when high-speed performance, angular acceleration analysis, or continuous monitoring of power transmission is needed.
Understanding the distinction is essential for selecting the correct sensor for any given application. Use our Torque Transducer Selection Guide to review all options. Use Interface’s ForcEDU Torque Transducer Cheat Sheet for terms and references.
InterfaceIQ Podcast on Torque Transducer Basics
InterfaceIQ Podcast 🔴 Everything About Torque Transducers
ADDITIONAL RESOURCES
3AFM110 3-Axis Force/Torque Load Cells
3AFM70 3-Axis Force/Torque Load Cells
Accelerating Advanced Automotive Testing Using Torque Transducers
AxialTQ™ Wireless Rotary Torque Transducer System
Torque Measurement in Ship Lifts
The Future of Automotive Performance Requires Precise Torque Measurement
