Technology

Torque Applied InManufacturing

扭力應用於製造業

The function of any threaded fastener is to provide sufficient clamping
force onto a joint that when properly tightened will achieve andmaintain the desired clamp load for its intended application. Any type of
joint movement must be overcome to prevent joint failure. This clamp load isachieved by means of torquing a fastener.

R e c e n t l y R iv i a n A u t o m o t ive h a s r e c a l l e dapproximately 13,000 of its vehicles for a loose steering
assembly. It was reported that the fastener connecting thefront upper control arm and steering knuckle “may nothave been sufficiently torqued.”

In a bolted joint, torque is measured in two different planes thatare perpendicular to one another. Torque is a combined relationshipbetween force, times distance and tension. Torque measures thereaction of rotational movement of the fastener, while tension
measures the longitudinal movement of the same fastener.

The question to be asked is, why wasn’t there
suff icient torque applied? How was the assembly
tightened? Is there a calibration issue? Was the output
data not interpreted correctly?

China Fastener World no.68/2023

The longitudinal movement (tension) is constrained by the joint members
between the first engaged threads and the bearing surfaces of the bolt and nut.
A portion of the rotational energy (torsion) is used to stretch (tension) the bolt
while, at the same time, causes the joint members to compress.
The amount of energy that is applied to the bolt from rotation to create
tension is determined by the energy losses due to the friction between thesurfaces of the rotating parts and the joint. The relationship between theapplied torque and the tension generated in the joint is the torquetension relationship.
While torque may seem universal in use, the torque-tension relationshipis applied differently to three specific industries; mass assembly, structuralassembly and in maintenance and repair.
The mass assembly of products include the automotive, truck and bus
industries to name a few, excluding the electronics industries because vehicles
experience heavy loading, vibration and shock. Also, because vehicles canbecome very dangerous to life and property if something breaks or loosens.

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惠達雜誌

Automotive assembly lines (Fig. 1) use a varietyof programmable, digital power tools for assemblythat many will transmit data, such as torque and angle,
to the data processor for error-proofing. The torqueangle measurement is a most accurate tool used forsolid and stiff joints. In most cases the turn will negate
friction variables, which is why a pre-torque is first
applied to establish a hard base without any further joint
compression.
However, in some cases it will not always mean thatthe preload has been achieved. This depends upon how
the data is monitored. If the torque and angle have beenachieved within a certain per cent window, then the joint
is good.
However, if the angle goes significantly beyond
the expected tolerance window to achieve the proper
torque, this means the joint was not solid and there wasinterference between the joint, such as a burr, unexpectedTechnology

Fig. 1. Automotive assembly line
lubrication or even a non-conforming part that produced compression or resistance.
If the torque becomes high and the angle was not reached, then this could
mean debris in the threads, cross threading or something else that needs to
be investigated. If none of these anomalies are noticed and the operator just inputs
on torque alone, then there are problems later on, which is why there are recalls.
Automotive manufacturers are basically just assemblers of pre-made andassembled parts from other manufacturers. They also use a variety of fasteners
and special fasteners besides the common bolt and nut. Many stampings will haveextrusions where threads are tapped which will accommodate a threaded fastener
and there are times where several stampings will be stacked together, some of which
are designed to move about the fastener.
When several manufacturers are producing parts that are to be assembled
together, tolerances and torque accuracy become paramount. Because movementof the assembled parts is involved, a thread locking chemical is used to help withvibrational loosening.
Loosening occurs in the presence of transverse and cyclic loads. It is subject tothe relative slip between the threads and the joint surfaces. Since threads are a helix
angle, which is nothing more than an inclined plane, there is a natural tendency forthreads to relax and rotate in the downward or counter clockwise “off” position,
which will cause joint loosening under cyclic, vibration or impacts to the joint.

China Fastener World no.68/2023

To overcome any external forces of transverse loads, bending, vibration andtensile loads, the joint must develop and maintain a clamp load in excess of any ofthese external loads. Any significant amount of loss of clamp load will cause thejoint to become loose.
For example, let’s assume one of the extrusions has a burr that is just slightly
over tolerance. In this case, a burr that is only 0.43 mm (0.0169”) over tolerance canbe most significant. The most important reason for loss of clamp load is not being
able to maintain the required clamp load due to embedment during tightening andfurther embedment under service loads. These losses cause joint relaxation andlooseness.
During assembly, torque has been applied by use of a positioning electronic
torque wrench. It has a preset torque value which the results are stored for reference
and error-proofing. There is no fault with the assembly method. However, once thetorque is applied, the joint may still relax and lose clamp load.
The electronic torque wrench also measures rotation angle. Parameters were set
for the amount of torque required to achieve a certain preload with respect to theviscosity of the thread locking material on the threads of the screw and with theamount of torque angle that should be achieved.

110惠達雜誌Fig. 2. Thread locking chemical
The assembly torque data is monitored tothe extent that if the required assembly torqueis not reached upon the second time, that bolt
is discarded. However, the mistake here is notrelating to the torque angle for potential problems.
For example, if the required torque was 25 lbft and took only 35 degrees of angular rotation toachieve clamp load, acceptable tolerances would
be ± 5 lb-ft and ± 10 degrees rotation. However,suppose the torque was achieved at 24 lb-ft but
at 90 degrees rotation or more. This should be animmediate red flag to the operator.
Unfortunately, in this case, the operators
failed to observe the torque angle as a significant
component of the entire assembly. The excessive
rotation resulted in a loss of clamp load fromembedment and weakened threads which permitted
transverse loading. Adhesives are good at resistingrotational movement and cyclic tensile impacts, notlateral impacts. Therefore, the fastener becomesloose and joint failure occurs.
Error-proofing does not mean fool-proof.
There are still variables that can cause errors.
Article by Guy Avellon
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