Directly measure the torque, rotational speed, and power transmitted by a tractor's power take-off (PTO or TDF) It allows you to know precisely how much mechanical power an agricultural implement actually receives. This information is especially useful in machinery testing, implement development, energy optimization, precision agriculture, and test benches.
A system based on a rotary torque sensor installed directly between the tractor's power take-off and the implement shaft This allows obtaining this information without the need to internally instrument the transmission. A study published in Applied Engineering in Agriculture He validated this method by comparing it with a reference dynamometer and obtained differences of only 3 to 23 N·m During the verification tests, there were no statistically significant differences between the two measurement systems.
Key result: Direct measurement of torque at the PTO shaft can provide a reliable estimate of the power transmitted to the implement. In the system studied, the calibration showed a virtually perfect linear relationship, with R² = 0.9999.
What is measured in a tractor's power take-off (PTO)?
The power take-off, known internationally as PTO (Power Take-Off) and also as a PTO, it transmits mechanical power from the tractor to machines and implements such as mowers, shredders, balers, fertilizer spreaders, mixers, pumps, tillage equipment or tillage equipment.
To determine the transmitted mechanical power, it is not enough to simply measure the shaft's revolutions. It is necessary to know two quantities simultaneously:
- Torque (T), usually expressed in N·m.
- Rotation speed (n), expressed in revolutions per minute, rpm.
From both quantities, the instantaneous mechanical power transmitted by the shaft can be calculated:
P = T × ω
P [kW] = T [N m] × n [rpm] / 9550
For example, a couple of 1,000 N·ma 540 rpm corresponds approximately to 56.5 kW. The same pair 1,000 rpm represents approximately 104.7 kW. For this reason, torque and rpm must be recorded simultaneously when analyzing the actual power demand of an implement.
How to measure torque directly on the PTO shaft
One of the most direct solutions is to install a rotary torque transducer in line with the power take-off. The sensor is located between the tractor's PTO output and the implement's or dynamometer's drive shaft.
As the shaft rotates, the transducer measures the torque produced by the transmitted force. If the system also records the rotational speed, it is possible to determine simultaneously. torque, rpm and mechanical power.
Scientific validation of a torque sensor in the PTO
Roeber, Pitla, Hoy, Luck, and Kocher developed and evaluated an acquisition system designed to directly measure the torque and speed of a tractor's power take-off (PTO). The work, titled Tractor Power Take-Off Torque Measurement and Data Acquisition System, was published in 2017 in Applied Engineering in Agriculture.
The objective was to verify whether a torque sensor installed directly on the PTO shaft could provide results sufficiently close to those obtained using the reference measurement system of the Nebraska Tractor Test Laboratory.
Configuration used in the test
| Torque sensor | NCTE 7000 Series used in the studio setup |
| Torque range | Up to 5,000 N·m |
| Maximum sensor speed | Up to 3,600 rpm |
| Stop signal | 0–10 V |
| Power take-off used | 1 3/4″ PTO configuration, 20 splines, 1,000 rpm |
| Tractor | AGCO Allis 9695 |
| Reference system | Nebraska Tractor Test Laboratory's 522 kW eddy current dynamometer |
| Acquisition | National Instruments CompactDAQ and LabVIEW |
| Recorded variables | Torque, PTO speed, and data needed for power analysis |
This configuration allowed simultaneous comparison of the signal provided by the rotary transducer with the torque determined by the laboratory dynamometer.
Simultaneous acquisition of torque and speed
The acquisition system used in the study was developed around a platform National Instruments CompactDAQ. The analog signal corresponding to the torque and the rotational speed information were recorded synchronously using an application developed in LabVIEW.
Synchronization is especially important in dynamic applications. In an agricultural implement, the load is rarely constant: it changes with crop density, working depth, forward speed, terrain irregularities, and the machine's own mechanical cycles. Recording only average values can mask relevant torque peaks and transient phenomena.
Sensor calibration against the dynamometer
Before using this type of measurement system in field tests, it is essential to check the relationship between the electrical signal of the transducer and the actual mechanical torque.
During calibration, different load conditions were applied, and the sensor output was compared with the value obtained using the dynamometer's reference system. The linear fit obtained in the study showed a coefficient of determination R² of 0.9999, which indicates a practically linear relationship between both signals within the tested conditions.
The regression obtained by the researchers for the specific configuration of their system was:
Par = −1240.9 × V + 6412.5
This equation applies exclusively to the configuration and calibration described in the study. It should not be used as a universal equation for other sensors.
Each sensor must use its own sensitivity, calibration curve, or parameters supplied by the manufacturer.
Results: What accuracy was obtained by measuring the PTO pair?
In the verification tests at full load and different speeds, the difference between the torque obtained by the PTO sensor and that determined by the dynamometer was approximately 3 to 23 N·m in the first series of trials.
In relative terms, the observed errors remained small compared to the transmitted pair. The researchers also applied statistical tests to the differences obtained and determined that There were no statistically significant differences with respect to zero in the samples analyzed.
The result is especially relevant because it demonstrates that a rotary torque sensor mounted directly on the PTO line It can be used to perform field measurements and machinery tests without relying exclusively on a fixed laboratory dynamometer.
From the tested NCTE 7000 Series to current PTO sensors
It is important to distinguish the specific device used in the 2017 study from the versions currently on the market. The technology, electronics, interfaces, and available configurations have evolved since that trial was conducted.
NCTE currently markets transducers based on its magnetostrictive non-contact measurement technology, designed to reduce wear and maintenance in rotating applications. The range includes high-torque solutions such as the 7000 Series and models specifically designed for agricultural PTO applications such as the NCTE Series 7300 / PTO 7300.
| Feature | NCTE 7000 Series current | NCTE PTO 7300 |
|---|---|---|
| Nominal torque | 3,000 to 5,000 N·m depending on the version | Up to 3,000 N·m |
| Speed | Up to 3,600 rpm | Up to 1,100 rpm |
| Published accuracy | ≤ ±0.5 % according to configuration | ≤ ±0.5 % |
| Protection | IP50 / IP65 options depending on configuration | IP65 |
| Available departures | 0–10 V, 4–20 mA, CAN and other options depending on version | 0–10 V, 4–20 mA, CAN or USB depending on version |
| Main application | Large pairs, test benches, vehicles and machinery | Power take-off of agricultural machinery |
| PTO Mounting | Configurations and accessories according to application | Direct connection 1 3/8″, 6 splines |
The choice between the two families depends primarily on the Expected maximum torque, shaft speed, PTO geometry, available space, environmental protection, and data acquisition type.
Alternative technologies for measuring torque in a PTO
Torque measurement in a power take-off (PTO) can be achieved using various architectures. The market offers rotary transducers with integrated electronics, wireless telemetry systems, instrumented strain gauges on the shaft, and sensors based on non-contact measurement technologies.
Specialized manufacturers such as Datum Electronics and Michigan Scientific They also offer specific solutions for PTOs. Published characteristics should not be compared solely on the basis of a "precision" figure, as each manufacturer may specify different parameters—system accuracy, nonlinearity, hysteresis, full scale, or uncertainty—and test conditions are not always equivalent.
| Solution | Approach | Key aspects |
|---|---|---|
| NCTE 7000 Series | Magnetostrictive rotary transducer | Up to 5,000 N·m and 3,600 rpm; industrial outputs; suitable for test benches and high torque machinery. |
| NCTE PTO 7300 | Compact PTO-specific sensor | Up to 3,000 N·m, 1,100 rpm, IP65 and direct mounting on 1 3/8″ agricultural PTO. |
| Datum PTO Torque Sensor | Torque measurement with integrated telemetry | PTO solutions with wireless data transmission and power calculation. |
| Michigan Scientific PTO | Instrumented transducer with telemetry / encoder | Solutions of 2,500 and 5,000 N·m specially oriented to dynamic testing and acquisition. |
To select the right system, you shouldn't rely solely on the sensor's nominal range. Other important factors include dynamic response, acquisition frequency, permissible overload, mechanical compatibility, temperature, dust and water protection, and electrical integration with the acquisition system.
What information should be known before selecting a PTO torque sensor?
- Nominal torque and expected maximum torque, including potential spikes and blockages.
- Operating speedFor example, a PTO of 540 or 1,000 rpm.
- Type and dimensions of the grooves entry and exit.
- Direction of rotation and the need for bidirectional measurement.
- Available space between tractor and implement.
- Environmental protection required for dust, mud, moisture or washing.
- Output type: voltage, current, CAN, USB or other interface.
- Bandwidth and acquisition speed when transients or impacts are to be studied.
- Need to also measure the rpm to calculate power.
- Calibration and traceability requirements of the essay.
Applications of PTO torque and power measurement
Directly instrumenting the power take-off (PTO) allows for the study of the actual energy behavior of virtually any machine mechanically driven by the tractor. Common applications include:
- testing and homologation of agricultural machinery;
- development and validation of new implements;
- measuring the power demand of crushers, mowers, balers and tillage equipment;
- comparison between different machine configurations;
- optimization of the tractor-implement relationship;
- energy efficiency analysis;
- identification of load peaks and mechanical overloads;
- University research and technology centers;
- development of control strategies and precision agriculture;
- durability tests and transmission characterization.
Dynamic measurement: more information than an average power value
One of the main benefits of placing the sensor directly on the transmission is the ability to observe how torque changes during operation. Two implements can have similar average power output and yet exhibit completely different dynamic behaviors.
A sufficiently fast acquisition system can detect torque peaks, periodic oscillations, impacts, momentary blockages, or variations associated with the agricultural process. This information is important for sizing transmissions, clutches, shafts, and protective elements, and can help identify working conditions that would not be visible through an average measurement.
Frequently asked questions about torque measurement in a PTO
How do you calculate the power of the power take-off from the torque?
If the torque in N·m and the speed in rpm are known, the power can be calculated by P [kW] = T [N m] × n [rpm] / 9550. Therefore, a PTO power measurement system must simultaneously record torque and speed.
Is measuring torque the same as measuring PTO power?
No. Torque represents the torsional force applied to the shaft. Power also incorporates the rotational speed. The same torque transmits different power at 540 rpm than at 1,000 rpm.
Is it possible to measure torque without internally modifying the tractor?
Yes. There are transducers designed for external installation between the PTO output and the driveshaft or test equipment. Compatibility depends on the spline pattern, available space, and the specific geometry of the installation.
What torque range should the sensor have?
The sensor should be selected considering the expected operating torque and, especially, the load peaks. Using an excessively large range can reduce the useful measurement resolution, while a sensor that is too small can be subject to overload.
Can the sensor be used during actual field work?
Yes, provided the transducer, couplings, cabling, and acquisition system are specified for the environmental and mechanical conditions of the application. For agricultural work, protection against moisture and dirt, vibration, temperature, and transient loads is particularly important.
What is the difference between a 540 rpm PTO and a 1,000 rpm PTO?
These are different nominal operating speeds. For the same torque value, a 1,000 rpm PTO transmits more power than a 540 rpm PTO. The selected sensor must be compatible both mechanically and with the maximum test speed.
Conclusion: Measuring the PTO torque allows us to know the actual power transmitted.
Direct measurement of torque and speed at the power take-off (PTO) provides a practical way to determine the actual mechanical power the tractor delivers to the implement. Validation published by Roeber et al. demonstrated that a properly installed and calibrated rotary transducer can provide results closely correlated with a laboratory dynamometer.
The current availability of compact sensors, integrated electronics, and various industrial interfaces further expands the possibilities of this technique for R&D, machinery testing, precision agriculture, energy optimization, and transmission monitoring.
GUEMISA supplies NCTE torque sensors and can help size the transducer and acquisition electronics depending on the torque, rpm, PTO type, required signal and working conditions of each application.
Technical reference
Roeber, J.B.W. Pitla, SK; Today, RM; Luck, J.D.; Kocher, M. F. (2017). Tractor Power Take-Off Torque Measurement and Data Acquisition System. Applied Engineering in Agriculture, 33(5), 679–686. DOI: 10.13031/aea.11994.
The experimental procedure was developed at the Nebraska Tractor Test Laboratory and was based on procedures applicable to tractor performance testing. OECD tractor test codes continue to include power take-off (PTO) performance measurement as a fundamental characterization element.