Technology >>Structures do not have to be only buildings
and bridges; they are anything larger thana PC board that is assembled outside of a massassembly line. This would include cranes, lift
trucks, wind turbines, storage tanks, amusement
rides, etc. Each structure has special assembly
needs to assure safety and performance of thefinished product.
The only accurate method of producing
a predictable amount of clamp load is bymeasuring the actual stretch of the fastener.
This may be achieved by several different
means and products that produce a low
amount of error. Those methods include
load cells, load washers, load indicating cap
screws, ultrasonic transducers and even theturn-of-the-nut method.
Load cells are used on site to ‘qualify’ theprojected performance of job-site bolts and nuts.
This, of course, is used in conjunction with adigital or click-type torque wrench. This method,
like others, does not guarantee installation
performance due to some external variables.
However, the per-cent of errors are low.

Hardware & Fastener Components - no.60/2023

Load indicating washers, or Direct TensionIndicators (DTI) are quite simple to use, are a bit
expensive but very useful on critical assemblies.
Their accuracy is only as consistent as theoperator.

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Ultrasonic transducers are also an extremely
accurate means of measuring the stretch of thefastener. This method is very expensive and is
not a permanent fixture as is a load washer. The
transducer head can be moved from one fastenerto the next, but the only drawback is that the headand point end of the cap screw must be smoothand f lat to accept the transducer head. This
would mean grinding off the grade markings andmanufacturer’s identification marks: removing all
traceability of the fastener.
Torque multipliers a re ver y usef ul a nd
accurate for large diameter bolts requiring hightorque values. As torque values increase, so does
the lever requirement of the wrench. Torque ismeasured in the amount of force (pounds forexample) times the distance of the lever (footfor example). Therefore, we have a pound-foot.
Some torque wrenches are up to three feet long toprovide the leverage needed to tighten the nut orbolt. Beyond this, a torque multiplier is used.
Torque multipliers are extremely useful when
mounting wind generator platforms or rotating
cra ne str uctu res. Some units a re ma nuallyoperated while others are either electric or
pneumatically actuated.

惠達雜誌

Tightening
Strategies for
Structural Bolting
結構物專用螺栓的緊固策略
Another great tool used in structures is by not using torque at all,but by turning the nut a certain degree of rotation. This method works
with coarse threaded fasteners only and on solid metal-to-metal
joints whose joint compression has been removed by pre-tightening
with a short handled wrench. This will not directly translate to metricbolts, as the same thread pitch is used on other metric bolt diameters.
Actual experimentation is needed for a particular size, thread pitch and joint
thickness.
How this works so well is because the threads of any standard bolt is aperfect helix. Therefore, when the mating helix threads of the nut are rotatedalong the threads of the bolt, the nut travels in a consistent linear direction.
As the nut rotates against a solid joint surface, the bolt is being tensioned
proportionately with respect to the rotation of the nut.
This reasoning is derived from Hooke’s Law. Work is now being done ona ‘linear’ scale: if the nut is moved the same distance each time, or all of thenuts in the connection move the same distance, the stretch of the fastener isreproduced the same amount on each fastener. This gauged amount of stretch
will produce a clamping force that is greater, more consistent and is morepredictable with any fastener grade, surface condition or length of fastenerusing common assembly methods.
So, what happens to torque? Torque is still present; we just do notmeasure it as such. Torque is always present in the form of friction as it is theenergy we must overcome in order to move the nut a certain distance. Torque
will not be measured here because we are more concerned with the distancethe nut moves, not how we get there.
For example: consider using four 1/2-13 Grade 8 fasteners; three areplated, the fourth is not. Of the plated bolts, one is dry to the touch, one islubricated and the last one has a thread nick. If all four bolts are installedtogether in a connection and torqued to the same 105 lb-ft, none of the clamp
loads will be remotely close to each other due to all of the friction variables
present.

Technology >>Fig 1

The lubricated bolt will be the tightest because there was a tremendous
reduction in friction. This bolt will probably produce over 18,000 pounds of clamp
load and is just into yield. The dry plated bolt will be the closest to producing
12,800 pounds of clamp load, while the one with the thread nick will be around
10,000 lbs. The non-plated bolt will only produce approximately 8,000 pounds
due to its rough surface finish. This is conservatively a 10,000 pound scatter.
If after making sure the joint was soundly compressed and the bolt head wasrestrained from turning and all four UNC bolts were tightened by rotating thenut using the same angular degrees of rotation, then all of the fasteners would bestretched an equivalent amount. This would produce clamping forces that would
only be a few hundred pounds apart, not thousands.

Fig 2

The difference to the operator is how much energy he must expend to cause thenut to rotate the prescribed amount. The non-plated fastener and the one with thenicked thread will require more ‘torque’ energy to turn the nut the same distance
X as the others. Of course, the lubricated fastener will require the least amount ofeffort.
The key element here is that all of the nuts have been rotated to X, not X-1 orX+2. This is the principle of the Turn-Of-the-Nut (TON) method.
Structural joints use the ASTM A325 and A490 fasteners. Since Hooke’sLaw states that stress is directly proportional to strain, and using the modulus ofelasticity for steel, we find that if a bolt is stretched 0.001”, per loaded inch, it willproduce a load of approximately 30,000 psi.

Fig 3

Therefore, according to the AISC Manual, the TON method uses theamount of rotation applied to the nut as a function of the diameter of the bolt
based on the overall bolt length. This method, as shown in Fig. 1, provides
the following three general rotations with respect to the diameter of thefastener and the joint thickness.
First, if the fastener length is no larger than four times the diameter of thefastener, the nut is turned 1/3 of a turn or 120˚ after the nut has been snug
tightened. Longer fastener lengths and their rotations are noted in Fig 1.

Tightening the bolt to the TON method of 120º, as in Fig. 1 for example, will
stretch the bolt into yield as shown in Fig. 3. By doing so, the bolts will produce
a maximum amount of clamp load with a minimal amount of clamp load scatter
while input error does not matter.

Fig 4

Since the fasteners are into yield, they can never be reused again. This isalso the principle of Torque-Turn-to-Yield (TTY) used by engine builders andothers to maximize the load of the joint and minimize load scatter.
First, the joint must be set rigid so all of the initial compression is removed.
After this, the fastener is assured to be placed into immediate stretch whileturning the nut. The TTY method uses a pre-torque to set the joint, the TON
method uses a short handled hand wrench to ‘snug’ the nut until it is felt the joint
has come together and any joint compression is removed.
Again, these assembly methods place all the fasteners into yield. This same
method used for ASTM structural fastener grades (ASTM A325 and A490) can beused for their strength equivalents of the SAE J429 Grades 5 and 8. The A325 orGrade 5 has a longer elastic curve than the 150 ksi A490, so both grades can use
the same turn.

Hardware & Fastener Components - no.60/2023

By looking at Figure 2, we can see the relationship of the helix thread to thestretch of a bolt using standard torque methods and the turn method. A torquevalue may stretch the bolt to less than 90º whereas using the turn method to rotate
the nut 90º will have produced a much higher clamp load.

The only caveat I have when using any of these methods in a multiple bolt
pattern is to tighten the bolts incrementally and in a cross pattern. (Fig 4)
Copyright owned by Fastener World / Article by Guy Avellon

惠達雜誌105