Torque Testing Information
Torque Testing
Torque Testing Overview
There are different torque testing systems and equipments in the market for the broad range of functional torque testing and measuring applications. Applications include automotive steering, drivetrain component testing and assembly, seat testing, bearing preload testing and torque-to-turn testing. The following is the typical torque testing for certain applications
Torque Testing: Bearing
The first test is to measure the breakaway torque as the outer race is held and the inner race is turned. When the bearing is rotating, the average torque is measured over several rotations.
Torque Testing: Seat Latch
The first task in the process is to perform a break-in function by exercising the part through its full motion a predetermined number of times. After the break-in the part is rotated through its full motion (one time) to measure the maximum and minimum torque values. The seat latch is then moved to the ship position.
Torque Testing: Gear Train
The resistance torque of the gear train is measured by driving and measuring the torque on the input shaft. Then the backlash throughout the gear travel is measured on the fly.
Torque Testing: Door hinge
The door hinge is first exercised in both directions and the peak torque is captured and stored. Then based on the peak torque measured, the spring tension is adjusted to increase or decrease the amount of torque needed to rotate the hinge over the detent.
Before torque testing: What is Torque?
A torque (τ) in physics, also called a moment (of force), is a pseudo-vector that measures the tendency of a force to rotate an object about some axis (center). The magnitude of a torque is defined as the product of a force and the length of the lever arm (radius). Just as a force is a push or a pull, a torque can be thought of as a twist.
The SI unit for torque is the newton meter (N m). In U.S. customary units, it is measured in foot pounds (ft•lbf) (also known as ‘pound feet’). The symbol for torque is τ, the Greek letter tau.
Before torque testing: Torque Explanation
The force applied to a lever multiplied by its distance from the lever’s fulcrum, the length of the lever arm, is its torque. A force of three newtons applied two meters from the fulcrum, for example, exerts the same torque as one newton applied six meters from the fulcrum. This assumes the force is in a direction at right angles to the straight lever. The direction of the torque can be determined by using the right hand grip rule: curl the fingers of your right hand the direction of rotation and stick your thumb out so it is aligned with the axis of rotation. Your thumb points in the direction of the torque vector.
Mathematically, the torque on a particle (which has the position r in some reference frame) can be defined as the cross product:
where
r is the particle’s position vector relative to the fulcrum
F is the force acting on the particle.
The torque on a body determines the rate of change of its angular momentum,
where
L is the angular momentum vector
t stands for time.
As can be seen from either of these relationships, torque is a vector, which points along the axis of the rotation it would tend to cause.
Before torque testing: Machine torque
Torque is part of the basic specification of an engine: the power output of an engine is expressed as its torque multiplied by its rotational speed of the axis. Internal-combustion engines produce useful torque only over a limited range of rotational speeds (typically from around 1,000–6,000 rpm for a small car). The varying torque output over that range can be measured with a dynamometer, and shown as a torque curve. The peak of that torque curve usually occurs somewhat below the overall power peak. The torque peak cannot, by definition, appear at higher rpm than the power peak.
Understanding the relationship between torque, power and engine speed is vital in automotive engineering, concerned as it is with transmitting power from the engine through the drive train to the wheels. Power is typically a function of torque and engine speed. The gearing of the drive train must be chosen appropriately to make the most of the motor’s torque characteristics.
Steam engines and electric motors tend to produce maximum torque close to zero rpm, with the torque diminishing as rotational speed rises (due to increasing friction and other constraints). Therefore, these types of engines usually have quite different types of drivetrains from internal combustion engines.
Torque is also the easiest way to explain mechanical advantage in just about every simple machine.
Relationship between torque, power and energy
If a force is allowed to act through a distance, it is doing mechanical work. Similarly, if torque is allowed to act through a rotational distance, it is doing work. Power is the work per unit time. However, time and rotational distance are related by the angular speed where each revolution results in the circumference of the circle being travelled by the force that is generating the torque. The power injected by the applied torque may be calculated as:
On the right hand side, this is a scalar product of two vectors, giving a scalar on the left hand side of the equation. Mathematically, the equation may be rearranged to compute torque for a given power output. Note that the power injected by the torque depends only on the instantaneous angular speed - not on whether the angular speed increases, decreases, or remains constant while the torque is being applied (this is equivalent to the linear case where the power injected by a force depends only on the instantaneous speed - not on the resulting acceleration, if any).
In practice, this relationship can be observed in power stations which are connected to a large electrical power grid. In such an arrangement, the generator’s angular speed is fixed by the grid’s frequency, and the power output of the plant is determined by the torque applied to the generator’s axis of rotation.
Consistent units must be used. For metric SI units power is watts, torque is newton meters and angular speed is radians per second (not rpm and not revolutions per second).
Also, the unit newton meter is dimensionally equivalent to the joule, which is the unit of energy. However, in the case of torque, the unit is assigned to a vector, whereas for energy, it is assigned to a scalar.
Conversion to other units
For different units of power, torque, or angular speed, a conversion factor must be inserted into the equation. Also, if rotational speed (revolutions per time) is used in place of angular speed (radians per time), a conversion factor of 2π must be added because there are 2π radians in a revolution:
where rotational speed is in revolutions per unit time.
Useful formula in SI units:
where 60,000 comes from 60 seconds per minute times 1000 watts per kilowatt.
Some people (e.g. American automotive engineers) use horsepower (imperial mechanical) for power, foot-pounds (lbf•ft) for torque and rpm (revolutions per minute) for angular speed. This results in the formula changing to:
The constant below in, ft•lbf./min, changes with the definition of the horsepower; for example, using metric horsepower, it becomes ~32,550.
Use of other units (e.g. BTU/h for power) would require a different custom conversion factor.
Information is provided by Xiang Yi Testing Instrument Ltd, a torque testing expert offers various torque measurement device.
