Interface offers an extensive range of torque transducers. Keith Skidmore provided an essential guide to torque transducers in our popular and timeless Torque 101 recorded webinar. Here is a quick review, plus a handy glossary of essential torque terms.

Fundamentals of Torque Measurement

Torque is a rotational force applied at a distance from a center of rotation, causing a twisting tendency (Torque = Force x Distance). Unlike linear force, which is measured by load cells, torque is measured using torque sensors or transducers.

The fundamental difference between torque and force lies in their nature: force is a linear push or pull, while torque is a rotational twisting force. Torque transducers are designed to measure this twisting force or moment applied to an object.

Types of Torque Transducers

Torque transducers can be broadly categorized into reaction and rotary torque sensors. Reaction torque sensors are typically stationary, measuring the reaction torque produced by a rotating element. These sensors utilize a cable for power and signal transmission and find applications in tests like steering column analysis, where the sensor is fixed and the column rotates. Rotary torque sensors, on the other hand, are designed to rotate with the shaft they are measuring. They require methods such as wireless telemetry or slip rings to transmit signals.

Tip: Review our 101 articles, Reaction Torque 101 and Rotary Torque 101, for further details and model examples. Use our Torque Transducer Selection Guide to review all options.

The measurement principle of a rotary torque sensor relies on strain gages on a flexure, which measures the strain induced by the applied torque, not the rotation itself. The “rotary” aspect is the signal transmission method, effectively eliminating cable wrap-up issues.

Furthermore, torque sensors can be classified as shaft-style or flange-style. Shaft-style sensors have an integral shaft connected to the drive train, while flange-style sensors are more compact and feature mounting flanges for direct integration between rotating components.

Installation and Mounting Considerations

Proper installation and mounting are crucial for accurate torque measurements. 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. Different types of couplings, including flexible and rigid, are selected based on specific application requirements, preventing damage and minimizing measurement errors from misalignment.

Shaft connections can be keyed or smooth. Keyed shafts provide positive engagement but may introduce backlash, clearance, or lost motion in a mechanical linkage and complicate disassembly. Smooth shafts with clamping couplings offer uniform torque application and avoid keyway-related issues. Mounting configurations include floating and fixed mounts.

Floating mounts support the sensor only by shaft connections, providing tolerance for misalignment. Fixed mounts rigidly attach the sensor housing, requiring double flex couplings and offering better support and stability, particularly in high-speed applications.

Torque Measurement Considerations

Accurate torque measurements depend on several factors. Accuracy refers to the closeness of the measured torque to the true value, expressed as a percentage of the sensor’s capacity. It encompasses factors like linearity, the deviation from a straight-line relationship, and hysteresis, the difference in readings between increasing and decreasing loads.

Resolution is the smallest discernible change the sensor can detect. Noise, random signal fluctuations from sources like electrical interference and mechanical vibration, can obscure measurements. Filtering techniques remove unwanted frequencies from the signal, improving accuracy. Bandwidth, the range of frequencies a sensor can accurately measure, is another critical consideration.

Overload, the maximum torque a sensor can withstand without damage, must be carefully considered to prevent sensor failure. Understanding the difference between average running torque, the steady-state torque during normal operation, and peak torque, the maximum transient torque from startup or impact loads, is essential for proper sensor selection. Undersizing based only on average torque can lead to overloading.

Dual range sensors offer two different measurement ranges, useful for applications requiring both high peak load capacity and precise measurement of smaller torques.

Finally, scalability, the ability to adjust the output signal range or sensor units to match different instrumentation or testing requirements, adds flexibility. For example, changing the output voltage per unit of torque.

Glossary of Essential Torque Terms

• Torque Transducer: A sensor that measures the twisting force or moment (torque) applied to an object.
• Reaction Torque Sensor: A torque sensor that is typically stationary and measures the reaction torque produced by a rotating element.
• Rotary Torque Sensor: This type of torque sensor is designed to rotate with the shaft it measures torque on, requiring methods for wireless or slip-ring signal transmission.
• Accuracy: The degree of closeness of a measurement to the actual value of the measured quantity, often expressed as a percentage of the sensor’s full-scale output.
• Average Running Torque: The calculated steady-state torque required to operate a system at its normal speed and load.
• Backlash: The clearance or lost motion in a mechanical linkage, such as a coupling or keyed shaft connection, can lead to torque measurement errors.
• Bandwidth: The range of frequencies a sensor or measurement system can accurately measure or respond to.
• Bearingless Torque Sensor: A rotary torque sensor that lacks internal bearings and relies on external alignment from connected machinery. See: AxialTQ
• Coupling: A mechanical component that connects the torque sensor to the driving and driven shafts, transmitting torque while often providing flexibility and compensating for misalignment.
• Dual Range Sensor: A torque sensor that provides two different measurement ranges, allowing for high peak loads and precise measurement of smaller torques.
• Filtering (of a signal): Removing unwanted frequency components (noise) from a measurement signal.
• Fixed Mount: A mounting configuration in which the housing of the rotary torque sensor is rigidly attached to a stationary base or structure.
• Flange-style Torque Sensor: A more compact torque sensor with mounting flanges that can be bolted directly between two rotating components.
• Floating Mount: A mounting configuration allowing the rotary torque sensor to be supported solely by its connections to the input and output shafts, enabling slight movement.
• Hollow Torque Sensor: A torque sensor featuring a central bore that permits the passage of shafts, cables, or fluids.
• Keyed Shaft: A shaft with a keyway (slot) that fits with a key in a hub or coupling to transmit torque.
• Noise: Random or unwanted fluctuations in the sensor’s output signal that can obscure accurate measurements.
• Overload Range: The maximum torque a sensor can endure without sustaining permanent damage, typically specified as a percentage above the sensor’s capacity.
• Peak Torque: The maximum transient torque experienced in a system, often significantly higher than the average running torque due to startup, impact loads, or dynamic effects.
• Pedestal Mount (Foot Mount): A type of mounting where the sensor housing has feet or a base that can be bolted to a stationary surface; primarily used for positioning rather than bearing support.
• Resolution: The smallest change in the sensor’s measured quantity can be detected and indicated.
• RPM (Rotations Per Minute): A unit of rotational speed indicating the number of complete rotations an object makes in one minute.
• Scalability (of a sensor): The capability to modify the output signal range or sensor units to meet different instrumentation or testing requirements.
• Shaft Style Torque Sensor: A torque sensor featuring an integral shaft connected to the drive train.
• Slip Rings: Electromechanical devices that facilitate the transfer of electrical power or signals between rotating and stationary parts.
• Smooth Shaft: A cylindrical shaft without a keyway, typically used with clamping-style couplings.