Special Feature

Strategies toSuppress theVariations in
Axial Force
Fig 1Q1.

Is there an easy
way to improve
the accuracy of TorqueControl Method?

The biggest cause of the variation in axial bolt force
is the scatter in the coefficients of friction on the threadsurface and the nut bearing surface. The mechanism
was explained in the article "Reviewing Torque Control
Method” before the previous one. As a measure to improve
the accuracy of Torque Control Method, the author has
proposed "the use of flat washers with the same
surface treatment that is applied to bolts and nuts”
in the relevant academic society. Since bolts and nuts
have usually undergone the same surface treatment, thevariations in the coefficient of friction on the thread surface
are considered relatively small. In contrast, the coefficient offriction on the nut bearing surface varies greatly dependingon the material and surface treatment of the fastened object
in contact. Then, the use of a flat washer describedabove can reduce the variation because "the surfaces
with the same surface treatment are in contact.”
Experiments conducted by use of a large number of bolts
and nuts have shown that the variation in axial force can be
reduced to about 10%, as shown in Figure 1. Finite element
analysis, shown in Figure 2, confirms that there are no
particular problems with the loosening performance
when flat washers are used, unless the fasteners are
subjected to severe vibrations or impact loads.

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Fastener World no.202/2023 惠達雜誌

Fig 2

Special Feature

Q2:

Is the friction ofcoefficient theonly factor that affects
the accuracy of TorqueControl Method?

Basically, the nut bearing surface and the bolt headbearing surface are "perfectly flat," so that they are closelyin contact with the fastened plate surface. However, thenut bearing surface is sometimes slightly tapered inward
and the bolt head bearing surface is tapered outward
within the tolerance of the JIS standard. When the taper
angle is large, the axial force may be reduced up to10% in case of the torque being applied to the nutside, even if the friction of coefficient is the same. Incontrast, the axial force may be increased up to 10%
when tightening from the bolt head side, as shownin Figure 3. The reason for this is that the equivalentfriction diameter changes due to the inclination of thebearing surface, and then the frictional work being
consumed at the bearing surface changes. In addition,
when a powered torque wrench is used and the tightening
operation is performed from the bolt head side, the axial
force may be low. This is because part of the torque isconsumed by frictional work when the bolt body and bolt
hole come into contact due to the effect of counter-torque.

Fig 3

Q3:

Please explain whyhigh tightening
accuracy cannot be attainedeven with a hydraulictensioner.

Including thin plates, when applying a hydraulic tensioner tobolted joints with short grip lengths, the tightening accuracy reduces.
In the tightening operation with a hydraulic tensioner, the axial
bolt force to be generated is lower than the tension initiallyapplied, because the nut bearing surface deforms so thatit sinks into the fastened part when the work is completed.
If the grip length is short, the ratio of the deformation of the nut
bearing surface to the bolt's elongation increases, resulting in a large
reduction in axial force. At the same time, the variation of axial
force also increases. Although it is difficult to give an exact figure,hydraulic tensioners should be used for bolted joints whose grip
length is several times or more larger than the nominal diameter ofthe bolt.

Q4:

What precautions
should be taken
when tightening by use of bolt
heaters?

In order to ensure that the bolt, which has been elongated byheating, generates the target axial force by contraction duringcooling, the snug torque of the nut bearing surface should belarge enough so that the average surface contact pressure
on the bearing surface is 10 MPa or higher. Another important
point is that the bolt heater should be kept inserted in the holeuntil the snug operation is completed. If it is removed before that,the temperature of the bolt drops rapidly and the bolt will not haveenough elongation, resulting in a decrease in the value of the final
axial force.

Q5:

What are theimportant points
when tightening is performed
using he nut rotation angle as
an indicator?

Reference
1. Toshimichi Fukuoka, “The Mechanics of Threaded
Fasteners and Bolted Joints for Engineering and Design”,
pp.81-86, pp.93-96, pp.256-261, ELSEVIER. (2022)

Since Elastic Range Rotation Angle Method is used together with
Torque Control Method, it should be applied to the bolted joints
whose target axial bolt stress is above a certain level (e.g. 300MPa). The amount of axial stress generated by the torque should be
at least 50 MPa to 100 MPa. In addition, as with Tension Method
and Thermal Expansion Method, it is not desirable to apply thismethod to bolted joints with short grip lengths.

Article by Toshimichi Fukuoka
Copyright owned by Fastener World惠達雜誌 Fastener World no.202/2023

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