The torque sensor can function like a mechanical fuse and is a key component to get accurate measurements. However, improper installation of the torque sensor can damage the device permanently, costing money and time. Hence, the torque sensor has to be properly installed to ensure better performance and longevity. This article provides the tips to be followed in the following areas:
- Arrival - handling, moving, shipping
- Driveline design - critical speed, parasitic forces and design
- Mechanical installation - couplings, mounting and alignment
- Cabling - construction and noise immunity
- Electrical setup - shunt value, calibration data and onsite calibration
Handle with Care
Rotating torque sensors and reaction torque sensors have to be handled with care, as they are very sensitive measuring devices. Low-capacity torque sensors below 100Nm need to be handled gently as they can be overloaded or wrecked easily.
Tip 1 – Threaded holes might be available for some of the heavier, larger-capacity torque sensors and can be used to attach eyebolts and straps for smooth handling of the sensor.
Tip 2 – Packing the torque sensor in protective packing material is recommended while shipping the device.
Ensure Peak Performance with a Thoughtful Driveline Design
The performance and longevity of the torque sensor and its reading accuracy will be affected by the design of the driveline. The shaft becomes unstable at the critical speed of the driveline and causes torsional vibration, which can damage the torque sensor. It is necessary to direct the strain to an exact point for accurate torque measurement. This point is typically the weakest point of the sensor structure. Hence, the torque sensor is purposely designed to be one of the weaker components of the driveline.
Four driveline factors that contribute to critical speed are length, stiffness, weight and RPM.
Tip 3 – Several mechanical components might be present in a long, complicated driveline, causing errors and problems. Hence, keeping the driveline as short, light and stiff as possible is crucial to achieve improved performance and protect the sensor from damage by avoiding any critical speeds (Figure 1).
Tip 4 – Placing the torque sensor as close as possible to a bearing rather than to the center of the driveline is a good practice (Figure 1). This helps to shift the critical speed away from the measuring range of the test.
Figure 1. A short, simple driveline
Test results of a long drive shaft from a diesel engine on an engine dynamometer are presented in Figure 2, showing the importance of rapidly passing through a critical speed or preventing one altogether in order to achieve testing accuracy and protection for the torque sensor.
Figure 2. The test output of a long drive shaft from a diesel engine on an engine dynamometer
Balancing the Torque Sensor
Tip 5 – Balancing the torque sensor and driveline is another method of avoiding the torsional vibration. Hence, users must ensure with the manufacturer whether the torque sensor was balanced at the plant.
The presence of keyways on both ends of the shaft of a circular shaft-style torque sensor creates an imbalance and backlash when there is change in the torque during the application. Hence, it is necessary to ensure the proper fitting of the keys into the keyways.
The entire driveline needs to be balanced after installation due to the possibility of creating an imbalance by the keys. Balancing the entire driveline subsequent to the installation of rotors is crucial for an advanced flange-mount telemetry torque sensor application.
Minimizing Parasitic Extraneous Loading
Torsional vibrations can also be caused by extraneous forces or off-axis loads such as:
Lateral (shear) limit force, which which can be X or Y - is the maximum permissible radial force
Longitudinal (thrust or tensile) limit force, which is the maximum permissible axial force
Bending moment, which is the maximum permissible bending force and is the weight times the distance away from the end of the torque sensor.
Most torque sensors have data sheets providing a parasitic load chart with the longitudinal, lateral and bending moment limits for the sensor. Torque reading with minimal error can be achieved if the parasitic load limits are maintained.
Tip 6 – Approaching 100% of the cumulative parasitic load limit causes roughly a 0.3% full-scale error on a torque sensor reading.
Understand Proper Mechanical Installation
Torque information can be effectively transmitted if the driveline is more rigid. Hence, having a torsionally rigid coupling is beneficial in some applications, whereas a less rigid coupling is recommended in applications that do not require peak torque measurements. This will cause dampening of the torque spikes and consequently the sizing of the torque sensor becomes closer to the average torque.
Mounting the Torque Sensor
Foot-mounted and floating are the two ways of mounting a torque sensor.
Foot-Mounted Torque Sensors
A foot-mounted torque sensor features a square “foot” for mounting to a pedestal, thus avoiding the floating of the sensor in the driveline (Figure 3). Parallel and angular misalignments need to be taken into account due to the presence of bearings in foot- mounted torque sensors. As a result, a full coupling or dual-flex coupling is required on both sides of the sensor.
Figure 3. A foot-mounted slip-ring (rotary transformer) torque sensor
Tip 7 – The unsupported shaft length can be reduced and the critical speed can be changed using a foot mount. However, proper alignment is imperative while employing a foot mount.
Figure 4 shows a mounting application that requires only one dual-flex coupling.
Figure 4. A foot-mounted sensor with two dual-flex couplings and a foot-mounted pulley sensor with one dual-flex coupling
Floating Torque Sensors
A floating torque sensor floats in the driveline and does not require bearings (Figure 5). It can be placed between two half couplings rather than a dual-flex coupling. Since the floating sensor handles angular misalignment, only parallel misalignment has to be taken into account.
Figure 5. A floating telemetry-style torque sensor
Nevertheless, it is not recommended to use two half couplings due to the possibility of sagging of the weight of the torque sensor when it is in the middle of the driveline and in between two flexible couplings. As a result, the sensor begins to “jump rope” while rotating, causing severe damage under the wrong conditions (Figure 6).
Figure 6. A torque flange floating between two half couplings
Tip 8 – The torque sensor needs to be mounted close to a bearing block and the two dual-flex couplings have to be placed on one side of the torque sensor, typically on the load side of the sensor.
Installing Integral Coupling Assemblies
Integral coupling assemblies are supplied by some torque sensor manufacturers.
Tip 9 – The two flex packs are recommended to be placed on the load side of the torque sensor with the drive side rigidly mounted for configuration of an integral coupling assembly. Though acceptable, it is not recommended to use one flex pack on either side of the torque sensor due to the possibility of sagging of the sensor between the two flex members. This leads to a critical speed problem.
Mounting and Aligning Stator and Rotor
Tip 10 – Proper bolts (grade-eight bolts or better) need to be used to mount the rotors. Tightening of the bolts is based on the torque sensor capacity (Figure 7).
Figure 7. Rotor mounting bolts torque chart
Adaptor flanges have to be made by hardening them to a Rockwell (RW) hardness close to the hardness of the torque sensor. It is necessary to maintain proper clearances while mounting the rotor and the stator. The air gap between the rotor and the stator is generally 4mm in total or about 2mm on either side from the center.
Tip 11 – The chart shown in Figure 8 can be used to align the male and female pilots in the case of hard mounting the rotor into the driveline without using couplings. However, it is recommended to use couplings to get very precise alignment.
Figure 8. Rotor pilots chart
Users can refer their torque sensor manual for aligning the stator to the rotor. A light indicating proper alignment is available for most stators, showing green light for proper alignment and communication between the stator and rotor and red light for improper alignment and no communication.
Tip 12 – It is possible to mount or orient the stator in any fashion around the rotor of the torque sensor, including upside down. This is useful in space constraint applications. The orientation of the stator to the rotor is insignificant in terms of the operation for most of the telemetry torque sensor systems.
Allowing Proper Metal Clearance Around the Sensor
Nearby metal objects may affect some older telemetry systems as they draw the induced power away or affecting the data communication between the rotor and stator by acting as secondary antennas. Hence, user manual must contain a chart on the proper clearances around the system antennas for proper operation of the torque sensor.
This problem may not be available in advanced telemetry systems like digital telemetry systems. However, users must check with manufacturer whether the nearby metal object will cause any problem to the torque sensor.
Make Cabling in the Right Way
The following tips are useful for making cabling in the proper manner.
Tip 13 – The cable jacket must be ensured for its resistance to all corrosive fluids in the test cell.
Tip 14 – For improved data accuracy, it is necessary to have conductors of the cables with low- resistance and low-capacitance levels.
Tip 15 – Color codes mentioned in the sensor manuals must be strictly adhered to avoid guesswork in making cables.
Tip 16 – users must ensure that soldering or crimping wires into connectors is carried out without any broken connections.
Tip 17 – Conductors can be broken by a vibrating or bending cable due to fatigue. Hence, it is necessary to properly attach the strain relief when completing the cable.
Tip 18 – Ground loops need to be avoided by properly shielding the cabling in terms of electrical-mechanical interference. Moreover, data lines must be kept away from high voltage lines to alleviate electrical noise or interference.
Shunt Calibration, Calibration Data, and Spanning Electrical Set-up
The strain gauge output of older torque sensors is typically mV or V. A strain gauge amplifier is required for this output to power and condition the signal. This is then converted into a usable high-level output. Conversely, telemetry torque sensors are generally self-amplified and therefore strain gauge conditioning is not required. They are also available with a range of high-level output types (typically a +/-10vdc or a frequency output).
Tip 19 – The functionality of the amplifier can be checked with the shunt cal value in the case of a self-amplified torque sensor. Output changes can also be checked with the shunt value, which is obtained during the calibration process from the full-scale output of the torque sensor. Hence, the change in the shunt cal value is an indication to the change in the full-scale output of the torque sensor.
Accurate spanning of instrumentation to the output of the torque sensor can be achieved with a shunt cal value depending on the accuracy of the shunt network. A “Test Protocol” must accompany with each torque sensor, including shunt value, linearity and hysteresis, and the output at specific torque levels.
Tip 20 – The torque sensor’s full-scale output can be used in place of the shunt value to span instrumentation. This can be found in the calibration data sheet.
A dead-weight calibration can be performed with a lever arm and calibrated weights to span instrumentation in case of not using either the full-scale output or the shunt value. However, this is difficult to perform as it requires breaking of the driveline, locking down of the shaft, and addition of a calibration arm. In addition, it is required to have the correct number of calibrated weights and a person to hang the weights, which has the chance to cause injury. The torque sensor may be subjected to a parasitic load due to bending of the arm while hanging the weights, resulting in erroneous reading. Furthermore, calibration of the arm, instrumentation , and weights is typically required once in a year.
Tip 21 – A reaction torque sensor and hydraulics are recommended to be used instead of the lever arm and the calibrated weights to perform a calibration. This is a simple and convenient approach when compared to using a lever arm and weights.
This information has been sourced, reviewed and adapted from materials provided by HBM, Inc.
For more information on this source, please visit HBM, Inc.