The Standard by Which Torque is Measured - Torque Wrench Calibration Systems

     

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WHAT IS TORQUE?

The term "torque" as applied to hand tools that measure a turning force is actually a misnomer. "Torque measuring" or "torque indicating" would be more descriptive since all tools used to turn or twist are involved with torque. A simple screwdriver or wrench could be called a torque tool.

"Torque" is a turning force or twisting force distinctly different from "tension" which is a straight pull.

Simple leverage is the same as torque and both are measured in terms of force and distance (distance is the length of the lever, force is the amount of pulling or pushing applied at the end of the lever). Example: length of the lever is 10 inches, pulling pressure applied is 6 pounds.

LET: T =Torque (inch-pounds, inch-ounces, foot-pounds, etc.)
F = Force (pounds or ounces)
L = Length of lever arm (inches, feet)
L x F = T
10 inches x 6 pounds = 60 inch-pounds

APPLICATION

Since it is known that distance times force will give us the torque of a given application it is a simple matter to turn the formula around and build a tool for a known torque requirement. Using the same example and knowing only the torque requirements of a tool: 60 inch-pounds = L x D

We know that the tool can have a leverage of:
  • 10 inches with a 6-pound pull
  • 12 inches with a 5-pound pull
  • 6 inches with a 10-pound pull
The tool can be built to fit the application whether a long or short lever is required, a curved lever to fit a hard-to-reach fastener, a self-contained tool or one that will take many adapters or accessories.
TORQUE FORMULAE

When adapters are used that rotate about a center not located at the center of the tang of the torque wrench, it is necessary to compensate for the additional length. An extension will always increase the capacity of the wrench.

LET: F = Force (pounds)
IT = Wrench length (inches) measured from the middle of the handle to the center of the tang.
IA = Extension length (inches) measured from center of socket to center of drive square.
L = IT +IA (if extension is mounted as in figure [B] measure L as shown or calculate using L = IT      IA (cos a).
TA = Torque exerted at extension.
TT = Torque setting of wrench.

A.

To find torque setting (TT) when desired amount of torque exerted at extension (TA) is known.

B.

To find torque exerted at extension for a particular torquo setting

EQUATION: TT =

TA x IT

L

EQUATION: TA =

TT x L
IT

AMOUNT OF TORQUE REQUIRED

A torque wrench is used in conjunction with a threaded fastener for the single purpose of controlling the clamping ability of the fastener. The stresses induced in the body of the bolt or screw by tightening provide the force which does the clamping.

Since the torque applied to the head of the fastener is directly proportional, or nearly so, to the load applied, it is possible to measure bolt stress by means of a torque wrench.

The first requirement in determining the amount of torque to apply is a knowledge of the desired bolt stress. It this stress is not dictated by the function of the assembly it is common practice to base the limit on the yield strength of the bolt material.

Theoretically, screws and bolts may be tightened to the yield point. However, in order to avoid fastener failure during the assembly process it is recommended that the induced stress not be allowed to exceed 80% of the yield strength.

In the original design of a fastener which will be subject to external loading, whether static or dynamic, it will be necessary to establish bolt size and allowable stress in accordance with current engineering practice.

The mathematical relationship between torque applied and the resulting tension force in the bolt has been determined to be as follows:

LET: T = Torque required (inch pounds)
F = Bolt tension desired (pounds).
D = Nominal bolt diameter.
EQUATION: T = .2 D F

This relationship is based on the assumption that commercial, steel, semi-finished, regular series nuts with rolled threads, semi-finished steel bolts are used, acting on steel surfaces without lubrication.

Tests have shown these conditions will result in a coefficient of friction of .16 at both the thread and head bearing surfaces.

For the usual application this equation can be used to approximate the required torque. Tests can then be conducted to determine a more exact value, if required. In any case, it is important to maintain conditions on the job as consistent as possible to those used at the time of determination. 

     
     

AKO Torque Specialties - 50 Baker Hollow Road, Windsor CT 06095 - Tel. 860-298-9765 or 860-749-7441 Toll Free: 800-462-1906