3Inspecting theTightening
Torque403-1. Retightening Torque Methods
Retightening torque methods ── 42

3-2. Proposal of New Retightening Method (T-point Method)
Proposal of new retightening method(T-point method) ──────── 43

TOHNICHI TORQUE HANDBOOK Vol.8

TECHNICAL DATA

Chapter

Inspecting theTightening Torque

3Methods of Inspecting the Tightening Torque■ Methods of inspecting the tightening torque
Estimate how much torque was applied in the screw tightening, and carryout an inspection of the tightening operation with the following methods.
■ Loosening Torque■ Retightening Torque■ Marking■ T-point

Table 3-1. Methods of inspecting the tightening torque
MethodMeasuringmethodMeasured torque /Tightening torque = αLoosening Torque Retightening Torque
Loosen the bolt using atorque wrench and readthe torque when the bolt
starts moving.

Tighten the bolt further
to determine the applied
torque. Read the torquewhen the bolt starts
moving again.

0.6 〜 0.9 ※（0.8）

0.9 〜 1.2 ※（1.05）

MarkingT-Point

Mark the position of the When continuing to tighten a bolt thattightened bolt, loosen it
has already been tightened with a torqueand read the torque when wrench until the bolt begins to turn again,
retightening it up to thethis torque value can be calculated bymarked position.
using the θ - t wave formula.
0.9 〜 1.1 ※（1.0）

Relatively easy tomeasure. It is necessary

0.9 〜 1.1 ※（1.0）

Accuracy is obtained if the Takes time and labor.
point at which rotation
After the inspection, thebegins is distinct. No
bolt is maintained at its
Often used for sizes of M4 further work is necessary original torque.
or less.
after the inspection.

Advantages/
to retighten the bolt.
Disadvantages

α : Ratio between measuring torque and tightening torque※ : Approximate value obtained by experiment

Figure 3-1. Variation of tightening torque and loosening torque

Tightening torque
Loosening torqueTm : Measuring torqueTt : Estimated tightening torqueα : CoeﬃcientTechnical Data

Measurement will be most accurate
when the test piece is secured. Thebolt can be left "as it is" after theinspection is complete and no individual
variations will result.

Tm = α・Tt41

Chapter

3-1

Inspecting the Tightening Torque

Retightening Torque Methods■ Retightening torque methods
The retightening method requires that additional torque is applied to a bolt or screw which has already beentightened, and then measures the torque value when the bolt just begins to move again. For standard screws:● Torque to overcome the static friction of the screw (A point)

Figure 3-2. Typical torque change
on retightening● Torque when the screw starts to turn continuously (B point)● Maximum torque for the inspection (C point)

TC point

A point

These measuring methods are used according to the accuracy required.
TtB point(1) A-point method
The torque value when the bolt just begins to move again measurement
is relatively easy and is not greatly aﬀected by individual interpretationor performance. But because this method measures the torque value
by static friction, the value will be higher than the tightening torque(Tt) and the relationship is not clear. Sometimes the maximum value by
static friction (A point) may not exist for some screws (Figure 3-3).

(2) B-point method

Figure 3-3.
T

T: ｔｏｒｑｕｅθ：ａｎｇｌｅTtθTechnical experience is necessary to measure this point. The retightening torque will reduce temporarily andthe reading accuracy is not very high, but this value is the closest to the tightening torque (Tt). Sometimes aclear minimum torque may not exist for some screws.

(3) C-point method
Measurement is easy by checking the maximum torque using a memory pointer. But the measurement value
may change considerably depending on the operator s sensation and where they stop when the screw
starts turning, so individual interpretation and performance can be a large factor. Generally, tightening torquemeans the C point method. The C point may sometimes show the A point.

42TOHNICHI TORQUE HANDBOOK Vol.8

TECHNICAL DATA

3-2

Inspecting the Tightening Torque

Proposal of New Retightening Method (T-point Method)
Chapter■ Proposal of new retightening method (T-point method)
The retightening torque ﬁrst starts with the rotation of the head only, and then causes the rotation of the screw,
shifting from static friction to dynamic friction, before friction whip subsides to become a stable straight line. (Figure3-4) Basically this straight line is an extension of the torque angle line ﬁgure obtained during the tightening.
Figure 3-4. New retightening method

Tightening process

Retightening inspection process

Unstable

StableCompared with the conventional A, B, and C point methods, the new retightening method (T-point method)
will have less dispersion in the measurements, and in addition its central value will almost match thetightening torque. Unlike the A, B, and C point methods, compensation using oﬀsets will not be required.
The general oﬀsets and dispersions of the tightening torque for each method in situations where there is noloosening or galling are shown in Figure 3-5.
Figure 3-5. Distribution of retightening torque
Tightening torque

Retightening torque inspected by CTB model
Retightening torque by bottom
method (B point)
Retightening by peakmethod (A, C point)

Advantages of the New Retightening Method
Anyone can measure the tightening torque easily.
No individual interpretation or performance variable is involved inmeasuring the torque.
● Requires less time to perform the measurement.
● Dispersion of data is small.
●●Technical Data3
Inspecting theTightening Torque

Chapter43