The unit of torque is usually the SI units which are kilogram-per-meter or kg/m in short.
What torque measures are the amount of force per unit length of an object about its rotating axis.
It is of great importance in mechanical designs, and mechanics in general. And, below is a list of methods under different categories where you might need to measure torque.
How to Measure the Torque of a Rotating Shaft?
In case of a rotating shaft, look for the bearings or the ‘Cradling’ and start with mounting the load on it. When you measure the reaction force in ‘F’ and arm length in ‘L’ you can calculate the torque. Or, you can also directly calculate the torque in the rotating shaft itself using the angular twist or strain on the shaft.
- You can use a torque sensor from the market or local hardware store.
- You must follow the instructions on the manual and attach it in the right direction, clockwise or anticlockwise.
- When coupled to the shaft, the sensor will give the actual reading which is a combination of the load and the shaft itself.
- To understand the theory behind measuring torque on a shaft, have a look at this video from a research group.
- When you use strain gauges on a shaft, the shaft itself becomes the transducer.
1. Using Contact-Type Torque Sensors
- Slip rings are a common trait of such sensors and they apply power to the strain gauges and also retrieve power from the same. However, slip rings are susceptible to wear and tear, so they don’t last long.
- To keep them durable enough, you have to maintain an oil-free slip ring, but many industrial applications inhibit these uses.
- Both slip-ring types and brushes support internal sensors, which consist of moment arm, rotary transformer, and slip-range methods for calculating torque. They often find use in constant movement shafts.
2. Non-Contact Radio Telemetry Method
In this case, radio signals are induced from strain gauges and this excites the stationary-rotating gap, which induces power in a loop antenna on the rotating shaft/axis. Next, a shaft radio transmitter sends the signals back to the stationary antenna.
- The radio antenna gap is normally about 3/4 inch, which makes them reliable and easy.
- They cost more and need no maintenance.
How to Measure Torque of a Motor?
To measure the starting torque on a motor, you have to know the rotor speed. Because it is zero when in neutral, N, the equation of torque begins with putting the value of time as one. In the case of a three-phase induction motor, the starting torque is also known as standstill torque and follows the same principle.
- It is usually measured in N/m (Newton per meter) and often applies to power screwdrivers and stuff.
- You can simply note the rating by the manufacturer such as volts, amperes, rpm, and these are readily available on the nameplate on the appliance.
- To derive the torque, you have to multiply the volt with the amperes and calculate the watt at which the appliance is running.
- Now, you have to divide the wattage by 746, to convert it into horsepower, to get the horsepower rating of the motor, and then divide by the rpm.
1. In-Line Wireless Torque Cells Method
There are tons of ways to measure torque, and with the digital age blooming ahead, you can use the wireless medium as well. These wireless cells consist of a rotary transformer and it is connected to in-line standard flanges. This is in turn connected to the stationary portion of the assembly using wireless transmission.
- Now, a looped antennae design helps to induce power from the stationary part to the embedded antennae. This antenna on the rotating torque cell measures the voltage.
- The generated voltage puts a strain on the gauges and transmits power to the radio transmitter mounted on the torque sensor. Because it is a digital sensor, it can measure minute changes.
- Now, the radio transmitter modulates this signal and transmits it via the antennae to the receptor. In the case of an in-line torque cell, high torsion means a high torque.
2. Manual Calibration Method
These digital instruments are pre-calibrated and supply the same through internal calibration.
- You can adjust it if you wish, using the same telemetry support.
- The wireless telemetry feature is useful because it eliminates the need to support bearings and the maintenance of the same from time to time.
- Further, in-line torque sensors require cutting the shaft or lengthening the machine train to accommodate the sensor, so that may be an add-on burden if you are ready to take.
How to Measure Torque in a Car?
Because a lot of people confuse between torque and horsepower when we talk about new cars and their specs, here is a video that debunks the major myths and doubts. Next, let’s see how to calculate the torque given other specifications and ratings.
- In the case of a car, or an automobile, the torque is a twisting force that is proportional to the engine’s rotational force and measures how much of a twisting force it can apply.
- While horsepower is a measure of the power of the engine, the torque is a measure of pounds per feet (in the US markets) towing capacity.
- Horsepower measures how rapidly the car’s engine can speed up and pickup while the torque says how much the car can pull or tow.
- For example, high-end sports cars have very high horsepower, but big tractors and agri-machines have high torque.
Torque Calculation Method for a Car
- First, you need to find an appropriate sensor; digital is the norm of these days, so go with a good one.
- The mathematical formula for calculating torque is the horsepower of an engine multiplying with 5252. The number 5252 is a constant, so don’t bother about it.
- The problem with measuring torque in a car is that you have to do it twice and that too at two different places first, directly from the engine, and second, from the drive wheels.
- Because these two are different, you need to know their values for specific requirements, for example finding if it can fit a certain load.
How to Measure Torque on a Bolt?
When using a bolt, you can measure the torque very easily. All you need is a wrench and a classic adjustable torque wrench, if possible. A torque wrench is a device or tool which allows you to adjust the torque on the bolt directly by reading the values on the same.
- In theory, you have to loosen the bolt using a torque wrench and read the torque reading on the tool as the bolt starts to move. This is the base reading.
- Now, you have to tighten the bolt but rotate it again and check the value on the torque wrench again.
- Now, you must rotate it a little further till the bolt moves and check the reading on the bolt. Do not move a lot. This is the final reading of the bolt.
- Just watch this short video to understand how the torque wrench works and how to do a test on a bolt.
Clamp-On Torque Cells Method
It works wirelessly too but it is immune to oil and dirt, and also does not need cutting the shaft. Comes pre-calibrated from the factory and has a bending beam between the two collars. It clamps on the rotating shaft and handles diameters from 3 to 32 inches, easily.
- The installation takes less than 30 minutes.
- It can handle horsepower as much as 100,000 HP.
- Ideal for cases when torque monitoring systems are an integral part of the system.
What is Used to Measure Torque?
There are several small and big tools you can employ to measure the torque in a system, whether it is a motor, car’s engine or your bolt. The best means available these days are as follows.
- An LTT torque tester is a useful kit; it helps perform the three model tests on a bolt.
- Then, you have the PTT torque meter, which is a digital meter and gives an accurate real-time reading.
- Then, you can use your torque screwdriver which is a handy tool but a little less accurate, but still quite useful and fast enough.
- Finally, there are a set of rotary torque and angle sensors that most people use even in-line wireless sensors. They are not easy to repair or reinstall but are standalone and hassle-free once you fix them in place.
How does a Torque Meter Work?
When a rotating body moves about an axis, it offers resistance, and measuring this value can provide the torque applied on the shaft. Usually, it is a transducer that converts this mechanical resistance to electric signals, and the strain gauge bonded to the shaft or the axis delivers this torque. You can measure it in ‘Pounds-Feet’ in the FPS system, classically using a Wheatstone Bridge method.
- In daily use, mechanics and electricians might need to measure the torque of their electric motor inside their power drill or screwdriver.
- There are digital as well as analog meters that can help measure torque.
- In many cases, you can calculate the torque from other ratings and specifications on a nameplate of the appliance.
- Use the correct unit system such as SI, Imperial, or FPS before reporting the same.