Strain gauge sensors all work the same way, whether the input is a push or a twist. Here the input is twist: applied torque winds an internal flexure by a barely perceptible amount, bonded foil gauges wired into a bridge register the strain, and a calibration turns the resulting millivolts into inch-ounces.
Down at the small end of the torque range, that description hides the actual difficulty. Torque is unusually easy to produce by accident. A force sensor mostly has to worry about force arriving in the wrong direction; a torque sensor has to contend with the fact that half the things in a test fixture generate a twisting moment of their own, and the sensor cannot tell them apart from the thing you are trying to measure.
Anything that resists rotation adds to your reading. A bearing supporting the rotating part. An O-ring or lip seal dragging on a shaft. Grease that is stiffer this morning than it was yesterday afternoon. The sensor's own cable, if it is routed so that it has to twist. Each of these produces a genuine torque, so it does not appear as noise or as an offset that can be engineered away — it is arithmetically added to the number you read. At five inch-ounces of full scale, a single seal can be most of your range.
An unbalanced fixture produces torque that changes with angle. Mount something whose centre of mass is not on the axis and gravity applies a torque that varies as it turns — largest at the horizontal positions, zero at top and bottom. On a static measurement that is a fixed offset you can zero out at one position. On anything that moves through an angle it is a sinusoid riding on your data, and it is frequently mistaken for a real characteristic of the part being tested.
And weight hanging off the sensor is a separate problem entirely. Torque is not the only thing a fixture applies. Its mass presses down, pulls sideways and bends the sensor about axes it was never meant to measure. Those extraneous loads show up two ways: as cross-talk, where a fraction of a load the sensor cannot measure appears in the reading anyway, and as a structural limit, because a sensor built to resolve fractions of an inch-pound is not built to carry very much.
Which is why this series publishes what it will tolerate. Maximum overhung moment, maximum shear and maximum thrust are given for every model, alongside the torque rating. Those figures deserve as much attention as the capacity, and on the higher-capacity models rather more — a point the specification table makes quietly and this page makes explicitly further down.
Sensors of this kind are used in automotive engine assembly and component testing, manufacturing and assembly of torque-sensitive products, aerospace fastener work, research and material characterisation, quality verification, medical device assembly, robotics and automation feedback, product development and calibration reference work.
Nothing unusual happens electrically. Three sets of numbers on this page repay proper attention, though, and one of them contains a genuine surprise.
Signal. A 350 ohm bridge excited at 10 VDC returns 1.5 mV/V at rated torque — about fifteen millivolts at full scale, expressed per volt because the output is a ratio of the supply rather than a fixed voltage. Behind the four conductors sits a temperature compensation and balance network, and it is that rather than the flexure which keeps the drift figures where they are. A ten foot mating cable is supplied with the sensor rather than being a separate item to remember.
The extraneous load limits do not scale with the torque rating, and that matters enormously. Read down the table and the capacity climbs two hundredfold across the series while the maximum overhung moment grows only eightfold. The consequence is worth stating plainly: on the smallest models the moment the sensor will tolerate is many times its torque rating, while on the largest two the maximum overhung moment is roughly half the rated torque. A fixture that applies a bending moment equal to the torque being measured is comfortably inside the limits on a five inch-ounce sensor and well outside them on a thousand inch-ounce one. The structure is much the same size throughout; only the sensing range changes.
So weigh your fixture, and measure how far out it sits. The published figures are a weight limit and a weight-times-distance limit, and guidance for this class of sensor is explicit that you must not exceed either — whichever is reached first — or you risk both measurement error and structural damage. On the smallest model in the range the shear limit corresponds to about two and a half pounds, so anything heavier bolted to the sensor is outside specification before a single inch-ounce of torque is applied. Work it through both ways: a fixture at the weight limit can sit only an inch and a half from the mounting face, while one a quarter of that weight can reach six inches.
The limits assume one load at a time. Specifications of this kind are quoted for loads applied singly. Where a fixture produces a moment and a shear and a thrust together — which is the normal case — the allowable value of each is lower than the published figure. Manufacturers generally do not compensate for cross-talk from extraneous loads because doing so is expensive, so what leaks into your reading stays there. If you know you have significant side loading and need it quantified rather than merely minimised, that is a conversation worth having with us.
Torsional stiffness is published, and the sensor is a spring. The figures run from 725 in-oz per radian at the bottom of the range to 72,350 at the top. Divide capacity by stiffness and you find full-scale twist is well under a degree on every model — small, but real angular compliance added to whatever drivetrain the sensor sits in. That matters in two places: it sets how much lost motion the measurement introduces into a positioning or servo arrangement, and together with the inertia bolted to it, it determines the torsional natural frequency of your fixture and therefore how quickly a reading settles.
Choosing a low capacity reaction torque sensor is mostly a matter of being realistic about the fixture, and it is worth doing with our application engineers rather than from the capacity column alone.
Estimate the parasitic torque before the working torque. Ask what else in your arrangement resists rotation — bearings, seals, brushes, a gearbox, a wiring loom — and get a feel for how large that is next to the torque you actually want. Where the parasitic share is significant, the sensor still works but your measurement becomes a difference between two numbers, and the difference deserves more thought than either number alone.
Then check the mechanical limits, not just the torque range. Weigh the fixture, measure the distance from the mounting face to its centre of mass, and compare the result against both the shear and the overhung moment figures for the model you are considering. This is the check most often skipped and it is the one that damages sensors. If the numbers are marginal, the fix is usually mechanical — support the fixture separately, shorten the overhang, lighten it — rather than a larger sensor, because the mechanical limits barely improve as capacity rises.
Size torque with the same discipline as any sensor. Accuracy figures are proportions of rated output, so a sensor chosen well above your working torque spends its resolution on range you never use, and the fine steps across this series exist so you can land close. Leave sensible margin above your credible peak against the safe overload figure, remembering that a hand-applied torque can overshoot considerably.
Decide how the sensor will be driven and reacted. The four-bolt pattern with optional pilots takes care of the mounting side; what attaches on the measuring side depends on the job. Where the work is fastener or driver related, socket adapters in male and female configurations are available and are usually the tidiest route to a defined, repeatable interface. Where it is a motor or a mechanism, tell us how it couples and we will talk through the alignment.
Think about how the cable will run. A ten foot mating cable is supplied. On a small torque measurement, route and secure it so it never has to twist as the fixture moves and never hangs its weight on the sensor — guidance for this class recommends strain relieving the connection and clipping the cable to the structure so it cannot whip. On the largest sensors in a catalogue this is housekeeping; at five inch-ounces it is a measurement decision.
Then choose the instrumentation. Fifteen millivolts at full scale needs conditioning: an amplifier signal conditioner module for a voltage or current output into a controller or logger, or a digital display where an operator needs the reading, a peak hold, or a setpoint to act on. Specify Cal-Teds plug and play if the sensor will be shared across benches — its calibration data then rides along on the sensor instead of living in a folder.
Give us the torque you are after, the weight and overhang of your fixture, and an idea of what else in the rig drags — that is enough for us to point you at the right model. Capacities are stocked, and educational and research purchases are discounted.
RTS Series Torque Sensor Applications.
The Transducer Techniques RTS Series torque sensors are versatile instruments designed to accurately measure torque in a variety of applications.
- Automotive Testing: In the automotive industry, RTS Series torque sensors can be used to measure torque during engine assembly, component testing, and quality control processes. They ensure that fasteners are tightened to the correct specifications, preventing over-tightening or under-tightening.
- Manufacturing and Assembly: These torque sensors are valuable in manufacturing and assembly lines, where torque-sensitive components are assembled, such as electronic devices, medical equipment, and consumer products. They help maintain product quality and consistency.
- Aerospace and Aviation: In the aerospace sector, RTS Series torque sensors play a critical role in the assembly and maintenance of aircraft components and systems. They ensure that fasteners on aircraft engines, wings, and other critical parts are tightened accurately.
- Research and Development: Researchers and engineers can use these torque sensors in various R&D applications, such as testing the torque characteristics of materials, prototypes, and mechanical components. They are essential for designing and improving products.
- Quality Control: The RTS Series sensors are ideal for quality control processes across industries, including electronics, manufacturing, and aerospace. They help verify that products meet torque-related specifications and standards.
- Material Testing: In material testing applications, torque sensors are used to measure the mechanical properties of materials, including their torsional strength and behavior under various torque loads.
- Medical Devices: In the medical device industry, torque sensors are utilized for the assembly and testing of medical instruments, ensuring that components are securely fastened and meet regulatory requirements.
- Automation and Robotics: Torque sensors can be integrated into automated systems and robotic arms to provide feedback on the torque applied during tasks such as screwing, gripping, and manipulating objects.
- Calibration and Instrumentation: The RTS Series torque sensors can serve as reference standards for calibrating other torque measurement devices and instrumentation, ensuring their accuracy and reliability.
- Product Development: Engineers and product designers use torque sensors during the development and testing of new products to understand how components interact under different torque loads. This information is crucial for optimizing designs.
- Educational and Training: Educational institutions and training facilities use torque sensors to teach students about torque principles, measurement techniques, and their real-world applications.
The RTS Series torque sensors are known for their stiffness, sensitivity, and durability, making them suitable for precise torque measurements in a wide range of industries and research fields.
Frequently Asked Questions
What torque range does the RTS Series cover?
Eight capacities in inch-ounces: 5, 10, 25, 50, 100, 200, 500 and 1,000 in-oz — which is 0.312 to 62.5 inch-pounds. That places the whole series below the range most torque products start at, and it is the point of it. Small torque is a genuinely different measurement problem from large torque, because the friction and drag already present in a fixture stop being negligible and start being a significant share of what you are trying to read.
What do the maximum overhung moment, shear and thrust figures mean?
They are the limits on everything the sensor is not measuring. Shear is the weight acting across the sensor's axis. Overhung moment is that weight multiplied by how far it sits from the mounting face, so it is quoted as weight times distance. Thrust is the load acting along the axis. Guidance for this class of sensor is explicit that you must not exceed either the moment or the shear, whichever is reached first, and that going past them risks measurement error and structural damage — not just a bad reading. They deserve to be checked with the same care as the torque rating.
Why is the moment limit lower than the torque rating on the larger models?
Because those limits are set by the structure, and the structure does not grow at the rate the sensing range does. Across the series the torque capacity climbs two hundredfold while the maximum overhung moment grows only eightfold, so the relationship inverts: on the smallest sensors the tolerable moment is many times the torque rating, and on the largest two it is roughly half of it. In practice that means the higher the capacity you choose, the more carefully you have to design the fixture relative to the torque you are measuring — which is the opposite of most people's instinct.
How do I check my fixture against those limits?
Weigh it, then measure from the sensor's mounting face to its centre of mass. The weight goes against the shear figure and the weight multiplied by the distance goes against the overhung moment figure, and both have to pass. On the smallest model the shear limit works out at about two and a half pounds, so a heavier fixture is outside specification before any torque is applied. Working the moment limit both ways on that model: a fixture at the full weight limit may sit only about an inch and a half from the face, while one a quarter of that weight can reach six inches out.
What is torsional stiffness and why is it published?
It describes how much the sensor twists for a given torque, quoted in inch-ounces per radian, and it runs from 725 at the bottom of the range to 72,350 at the top. Divide the capacity by the stiffness and full-scale twist comes out well under a degree on every model. That figure matters in two places: it is angular compliance added to whatever mechanism the sensor sits in, which affects positioning and servo behaviour; and together with the inertia you bolt to it, it sets the torsional natural frequency of your fixture, which is what determines how quickly a reading settles after a change.
How accurate is the RTS Series?
Nonlinearity and hysteresis are each 0.1% of rated output, nonrepeatability 0.05%, and zero balance 1.0%, with safe overload at 150%. Those are solid figures for the capacity range. The realistic caveat on small torque work is that the sensor is often not what limits you — friction in the fixture, an unbalanced mass, and cross-talk from side loading are all capable of contributing more than a tenth of a percent, and none of them is the sensor's doing.
What is the RTS made from and how does it mount?
Sensor-grade aluminium with an anodized finish, carrying bonded foil strain gauges. Aluminium is the right choice at these capacities: it produces usable strain from very small torques, which a stiffer material would not. Mounting is a four-bolt pattern with optional pilots, so the sensor locates positively rather than relying on the bolts to hold position. Bolt it to something flat and rigid — a mounting face that flexes puts strain into the sensor before any torque arrives.
Does the RTS come with a cable?
Yes — a ten foot AMX-4 mating cable is supplied with the sensor rather than being a separate item to order. That is worth knowing if you are comparing against products where the mating assembly is a mandatory extra. On small torque work, how that cable is routed matters as much as the fact that it exists: keep it from twisting as the fixture moves and do not let its weight or stiffness reach the measuring side.
What are the socket adapters for?
They give the sensor a standard drive interface in male and female configurations, which is the tidiest way to connect it to fastener and driver work — screwdrivers, small drivers, torque tools and the fixtures built around them. The value is repeatability as much as convenience: a defined drive interface introduces torque the same way every time, where an improvised adapter is one more thing that can shift between tests. Tell us the drive size and the direction of the work and we will confirm which suits.
How does temperature affect an RTS reading?
The compensated range is 60° to 160°F with safe limits from −65° to 200°F, and drift is 0.005% per °F on both output and zero. Those figures are modest in absolute terms, but on small torque work there is a second temperature effect that usually dominates them: lubricant in your own fixture gets thinner as it warms, so bearing and seal drag falls through a test session. If a reading changes over a morning, look at the grease before you look at the sensor.
Questions From The Field
My reading includes torque I am not applying. Where is it coming from?
Almost certainly from something in the fixture that resists rotation, and the sensor is reporting it accurately because it is genuinely torque. Work through the candidates: bearings supporting the rotating part, any O-ring or lip seal on a shaft, grease and its viscosity, brush or contact drag, a wiring loom that has to flex, and the sensor's own cable if it is routed so that it twists. None of these can be filtered out electrically, because they are the same physical quantity you are measuring. The answer is either to reduce them mechanically or to characterise them and subtract, which is covered separately below.
How do I subtract my fixture's own drag from the measurement?
Run the fixture without the part under test and record what it takes to turn it — that is your baseline, and it should be measured across the same speed and angle range the real test uses rather than at a single point, because friction is rarely constant. Then decide honestly whether subtraction is good enough for your purpose. It works well where the drag is stable and repeatable; it works poorly where it changes with temperature, wear or run-in, which is common. Where the parasitic torque is a large share of the reading, reducing it mechanically buys more than any amount of arithmetic.
My fixture is heavier than the shear rating. What are my options?
Do not simply move up a capacity in the hope of more mechanical margin — the moment and shear limits improve far more slowly than the torque rating does, so a larger sensor is usually the wrong fix. Better answers are mechanical. Support the fixture on its own bearing so its weight goes to the structure and only torque reaches the sensor. Shorten the overhang so the centre of mass sits closer to the mounting face. Or lighten the fixture, which is often easier than it sounds once it is being designed against a number. If none of those is possible, send us the arrangement and we will look at it with you.
Readings change when I reposition the fixture, even at the same torque.
Two effects, and it is worth separating them. If the fixture has mass off the axis, gravity applies a torque that varies with angle — largest when the offset mass is horizontal, zero at top and bottom — which is real torque and appears in the reading legitimately. If the change follows how the fixture is bolted or supported rather than its angle, you are seeing cross-talk from side load or bending, where a fraction of a load the sensor cannot measure shows up in the output anyway. Manufacturers generally do not compensate for cross-talk, so the remedy is to remove the extraneous load rather than to correct for it.
Can I mount the RTS with its axis horizontal?
You can, but it changes which limit you are up against. With the axis vertical, the fixture's weight acts along the sensor's axis and counts against the thrust rating, which is the most generous of the three figures. Turn the axis horizontal and that same weight becomes shear and overhung moment, which are much tighter — on the smallest model the thrust allowance is several times the shear allowance. So an arrangement that is comfortably within specification vertically can be outside it horizontally without anything else changing. Check against the right column for the orientation you are actually building.
Can I tighten a fastener through the sensor?
Only with care, and the risk is not the torque. Tightening applies thrust along the axis and often a side load from however the tool is held, and both of those are separate limits that can be reached long before the torque rating is. The practical rules are to apply torque smoothly rather than with a sudden impact, never to use the sensor as a mechanical stop or a lever, and to keep the tool aligned so it is not levering on the drive. If a fastener application is what you are building, describe it to us — the fixture design usually matters more than the sensor selection.
Does the sensor cable really affect a small torque reading?
It can, at the bottom of this range. A cable is a spring in torsion, and if it is routed so that it must twist or bend as the fixture rotates, it resists that motion and the resistance is torque. Guidance for this class of sensor recommends strain relieving the connection at the sensor and clipping the cable to the surrounding structure so it cannot move or whip. Route it so that the run near the sensor is straight and unstressed, secure it to something stationary, and leave the slack somewhere it cannot be dragged. It costs nothing and removes a variable that is very hard to spot afterwards.
How do I tell whether extraneous loading has damaged the sensor?
Start with the unloaded output and compare it against the zero balance specification — a zero that has moved well outside it and stayed there is the usual first sign of a sensor that has been overstressed, whether by torque or by a moment. Then check bridge resistance across the excitation pair against the nominal 350 ohms, since a large discrepancy or an open circuit indicates a damaged gauge or lead. Finally verify against a known torque, because a sensor can settle to an acceptable zero and still have lost span. If any of those is off, recalibration is the honest next step rather than continuing and hoping.