The Importance of Precision and Specification in Torque Sensors

A torque sensor, also known as a torque meter or a torque transducer, is a device that measures the torque in a rotating system. Accurate torque sensors are critical to the development and testing of products such as motors, compressors, pumps or any power generating/consuming equipment. Inaccurate torque readings can lead to product inefficiencies and shortened overall product life. These in turn can lead to unexpected expenses and downtime. A quality torque sensor can help avoid the issues described above with accuracy and reliability.

Torque Sensor Operation

To determine the torque between a shaft input and its output, an in-line rotational torque sensor must be fitted into the shaft network. The exact measurement technology varies depending on sensor manufacturer, although the most accurate and common continues to be strain gage technology. Signal transfer from the rotating shaft had historically been through the use of slip rings. Slip ring technology features rotating, conductive rings that transmit data to stationary brushes. However, slip rings are wear components that must be replaced and are not optimal for high-speed and low-torque measurements.

These limitations are overcome by a rotary transformer, which delivers non-contact measurement signal transfer between rotating and stationary components. This is done via induction between conductors, with the strain gauge or sensor on the rotating side delivering the measurement signals to the stationary device. This enables high-speed torque readings and eliminates drag on the shaft, but adds emphasis on rotational precision. High-quality bearings carry some extraneous loads and ensure accurate readings, especially in tasks where shaft power transmission is not completely smooth, which is the majority of applications.

Accurate and Reliable
A quality torque sensor has a number of features that make it reliable and provide a long service life. Quality bearings are a large upfront cost for torque sensor manufacturers, but they result in long life and reduced measurement degradation over time. Quality signal conditioning using high-quality electronic components, and filtering provides exceptional accuracy in the sensor.

Inferior torque sensors may include contradictory features that are appealing as a one-size fits all solution. However, this creates a torque sensor with degraded accuracy and a shorter service life.
In general, the optimal torque sensor is likely to be cost-effective and conservatively designed for specific applications. Torque sensor technology is always innovating to provide more accurate measurements, longer service lives, and other features that make them easier to use in challenging applications. Prevailing torque sensor manufacturers are actively participating in their clients' applications and collecting feedback. They provide calibration services so equipment is maintained to OEM standards, while being able to troubleshoot issues and provide the measurement precision each application requires.

Torque sensor quality has a significant impact on financial resources. Inaccuracies during the R&D cycle can lead to delays and increased product development costs. Even miniscule inaccuracies can affect the longevity and reliability of a product over the long term. Accurate results lead to streamlined iterative prototyping and cost-effective resource management. Engineering teams are able to exactly dial in products, with no guesswork, providing a competitive advantage in time-to-market.

Accurate torque sensor readings are imperative to product quality assurance. Manufacturers are able to better, more quickly identify quality issues. This means that the manufacturing issue can be found and addressed sooner, limiting the expense of defective and reworked products. Critically, the potential for sending out defective equipment and components is minimized. Unsatisfactory parts in the hands of a loyal customer need to be replaced, and that customer relationship needs to be repaired as well.

Not only should a torque sensor have unmatched precision, but a variety of features and specifications help determine the correct torque sensor for a manufacturing application. The essential characteristics of a torque sensor are its torque range, accuracy, overrange, speed and overload. The torque range is the minimum to maximum torque the sensor is designed to measure, and it is expressed as N-m or lbf-in. Accuracy is generally expressed as a percentage of the torque range and is an important measure of the reliability uncertainty of the torque sensor measurement.

Torque sensor overrange is the highest torque at which measurement error is less than 0.1 percent of full scale. This is a critical consideration since rotary machinery usually exhibits pulsating as opposed to smooth torque. A torque sensor must have an acceptable overrange rating to be error-free over its entire operating range. Overrange is generally expressed as a percentage of the torque range. The speed of the torque sensor is the range of RPMs that the torque sensor can operate within and remain accuratereliable. Obviously, rotational speeds can vary greatly based on application and it is important that a torque sensor's speed range is well in excess of those speeds in order to maintain accuracya long operational lifetime.

Overload rating is the maximum torque that can be applied without yielding its element or otherwise producing permanent change in the torque sensor's performance. Like overrange, overload is measured as a percent of torque range. Since torque is rarely smooth, it is critical that users understand the nature of the torque they are measuring in order to select the appropriate overload rating. Regarding overload, it is helpful to consider the load service factor (LSF). LSF can fall into several groups, such as LSF=1 for smooth, constant load devices; LSF=2 for non-reversing, non-constant load; LSF=3 for high-variable shock or light reversing loads; and LSF=4-6 for heavy to full torque reversals (see image below).

Understanding the type of torque being measured is essential to accurately determine the overload needed. It is also important to understand the drive service factor (DSF), such as DSF=0 for smooth turbines and DC motors, all the way up to DSF=2-4 for diesel engines with less than six cylinders. LSF and DSF, along with an understanding of the running torque of your application, will help predict torque spikes and better understand the overload rating needed.

Torque sensor specifications may vary in name and definition among manufacturers and distributers. Often this "specmanship" is used to conflate dissimilar specifications and make a torque sensor appear of higher quality. Unnecessary or conflicting features are sometimes included to appeal to a specific market or end-use. It is imperative to try to understand how these specifications are defined and how they compare to other manufacturer's specifications. Typically, such specifications are designed to obfuscate the shortcomings of a torque sensor.

When specifying a torque sensor, the first and most important thing to consider is the manufacturer. The manufacturer should be focused on torque sensors, not selling them as one of many disparate product lines. Representatives and engineers should be willing to discuss particular use cases and help identify the correct torque range, accuracy, overload and other parameters for each application. They will be knowledgeable regarding the nuances of torque and will be able to suggest features that improve accuracy and reliability for particular applications - they will not attempt to deliver a one-size-fits-all solution. Finally, the manufacturer will undoubtedly provide details regarding calibration and maintenance. Once a quality manufacturer has been identified, carefully consider the use case and consult with them about purchase, installation and upkeep.

Torque is complicated and unforgiving, and inaccurate or unreliable torque measurement can have far-reaching consequences. A high-quality, appropriately specified torque sensor will provide reliable, accurate torque measurement over a long service lifetime. This will lead to innovative and efficient product development. A low-quality, generic torque solution will degrade quickly over time, producing unreliable torque measurements, which mislead R&D efforts and result in delays and expense. The end products produced with faulty torque sensors will be less efficient and have unpredictable lifetimes, which will hurt the manufacturer's reputation.

Putting in the time to establish a relationship with a quality torque sensor manufacturer now will pay dividends down the road. The quality of product and subsequent reputation of rotary manufacturers is built upon a foundation of accurate torque measurement. No manufacturer understands this better than S. Himmelstein Company, which has been doing this since 1960.

The Importance of Precision and Specifications in Torque Sensors
by Steve Tveter of S. Himmelstein and Company and Roger Pink of GlobalSpec

October 2018


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