What is Torque (& Tension)?

Torque = T

Force = F

Torque = Force by Turning or Twisting.

Tension = Power by Creating a Straight Pull.

 (but, differs from Torque)

 so that: Torque is used for creating Tension.

How to Make a Torque Evaluation.

Torque = Force x Distance

T = F x L so that:

 T1 = F1 x L1 = 10 lbs x 2 ft = 20 ftlbs

 T1 = T2

 T2 = F2 x L2 = 20 lbs x 1 ft = 20 ftlbs

Torque Units
 International System = S.I.  Metric System
 N·m (Newton Meter)  kgf·m (Kilogram Force Meter)
 N·cm (Newton Centimeter)  kgf·cm (Kilogram Force Centimeter)
 N·mm (Newton Millimeter)  gf·cm (Gram Force Centimeter)

Imperial System (American Size)
in.oz (Inch Ounce)
in.lbs (Inch Pound)
ft.lbs (Foot Pound)

Why is Torque Necessary?

(A) In case of threaded objects: Example: car wheel bolts or nuts
Lightly tightened (lack of torque)
Bolts will be loosened gradually and car accident might occur.
Strongly tightened (over torque)
Bolts are tightened just before breaking point, so bolts might be suddenly broken, due to highter tension caused by temperature or shocks.
Properly tightened (suitable torque)

(B) In case of unthreaded objects: Example: A Channel Selector for TV Sets
Lightly tightened (lack of torque)
Easy to turn/ bad contact / unstable picture.
Strongly tightened (over torque)
Difficult to turn / large friction / short life, difficult to use.
Properly tightened (suitable torque)

What is Suitable Torque?

(A) When the joint does not receive external force or vibration
 Q = (0.5 to 0.7) P
(B) When the joint receives external force or vibration
 Q = (0.5 to 0.6) P
 Note: Q = Suitable Torque     P = Breaking Torque

Bolt Tightening Method:

There are three ways to make sure that tightening is correct.
(A) Torque control method
Using hand torque tool (wrench or driver), or power operated torque tool (screwdriver or nutrunner) to tighten screws, nuts and bolts.
(B) Part-turn method
Bolts are tightened until joint surfaces are in close contact. At that point, just before solid tightening starts, nut and protruding bolt threads are marked to show their relative positions. Then, correct tightening is completed by turning the nut a half or 3/4 turn.
(C) Bolt elongation measuring method
When bolts are tightened at the same time, the elongation of the bolts are measured.

Units of Torque and Conversion Values

To measure the tightening torque for screw, kgf·cm has often been used in the past.
This now becomes N·m in terms of SI units. Therefore, their relationship is given as:

1 kgf·cm = 0.098066N·m=0.098N·m

1N·m = 10.1972kgf·cm=10.2kgf·cm

See:  Torque Conversion Factors  for torque conversion charts.

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