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What is Torque?

According to Webster:

  • A twisting or wrenching effect, or moment, exerted by a force acting at a distance on a body, equal to the force multiplied by the perpendicular distance between the line of action of the force, and the center of rotation at which it is exerted.

  • A force, which tends to produce rotation. The measurement of torque is based on the fundamental law of the lever.


Basic Torque Formula
L (length) x F (force) = T (torque)




Example: A two foot lever at a right angle to the fastener with 200 pounds at the end will produce
400 foot/pounds of torque.

Torque Formula: L x F = T

What are we trying to achieve with a torque wrench?

Answer: Proper Clamping Force

  • Torque is expressed in commonly used units of measurement such as:

  • in. lbs. = inch pounds
  • in. ozs. = inch ounces
  • ft. lbs. = foot pounds
  • Nm = Newton meter
  • cNm = Centi Newton meter

Torque and Clamping Force
Controlling the torque applied in tightening threaded fasteners is the most commonly used method for the application of clamping force. There are many factors which may affect the relationship between torque and clamping force of threaded fasteners. Some of these are: the type of lubricant used on the threads, the material from which the bolt and nut are made, the type of washers used, the class and finish of threads and various other factors. It is not possible to establish a definite relationship between torque and clamping force which will be applicable for all conditions.

Torque Versus Clamping Force

Only a small part of the torque applied to a fastener contributes to clamping force. The remaining, as much as 90% of the total applied torque, is used to overcome friction under the fastener head (or between nut and washer) and friction in thread engagement.

TORQUE

Head Friction:
45% - 55%

Thread Friction:
35% - 45%

Clamping Force:
10%

TORQUE
1. Head Friction
2. Thread Friction
3. Clamping Force


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Torque Wrench Safety

These precautions should always be taken when using any torque wrench to avoid possible injury:

  • Read instruction manual completely before using torque wrench.

  • Safety glasses or goggles should be worn at all times when using any hand tool.

  • Always pull, DO NOT PUSH, to apply torque and adjust your stance to prevent a fall.

  • A "cheater bar" should NEVER be used on a torque wrench to apply excess leverage.

  • Do not use with sockets or fasteners showing wear or cracks.

  • Ratchet mechanism may slip or break if dirty, mismatched or worn parts are used.

  • Make sure direction lever is fully engaged.

  • All mechanical torque wrenches are calibrated from 20% to 100% of full scale, therefore, they should never be used below or above those limits

  • To determine which torque wrench capacity is best suited for an application, many factors must be considered. However, as a recommendation, use a torque wrench in the middle 50% of the overall capacity of the tool. This will result in longer tool life, ease of use for the operator and increased accuracy from "clicker" type torque wrenches

  • Always grasp handle firmly in the center of the grip

  • Approach final torque slowly and evenly

  • Stop pulling wrench immediately when target torque is reached

  • Never use a torque wrench to break fasteners loose

  • Should be cleaned and stored properly

  • Should always be stored at it’s lowest torque setting

  • Wrenches should be re-calibrated if dropped. Should never be used in excess of it’s capacity

  • Torque wrenches should be "exercised" a minimum of three times at 100% of full scale before use

  • The wrench selected should be calibrated in the same torque units that are specified

  • Use of a "cheater bar" will result in an inaccurate reading and can possibly damage the wrench

  • Torque wrenches will last longer if reasonable care is taken. Always unwind handle to the lowest setting after each use. Do not attempt to lubricate the internal torque mechanism. Clean torque wrench by wiping, do not immerse. The wrench should be sent to a qualified calibration lab once every year or every 5000 cycles for re-calibration

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Torque Conversion

Easy-to-use Torque
Conversion Table

To
Convert
From
To Multiply
by
in. oz. in. lb. 0.0625
in. lb. in.oz. 16
in. lb. ft.lb. 0.08333
in. lb. cmkg 1.1519
in. lb. mkg 0.011519
in. lb. Nm 0.113
in. lb. dNm 1.13
ft. lb. in. lb. 12
ft. lb. mkg 0.1382
ft. lb. Nm 1.356
dNm in. lb. 0.885
dNm Nm 0.10
Nm dNm 10
Nm cmkg 10.2
Nm mkg 0.102
Nm in. lb. 8.85
Nm ft. lb. 0.7376
cmkg in. lb. 0.8681
cmkg Nm 0.09807
mkg in. lb. 86.81
mkg ft. lb. 7.236
mkg Nm 9.807

Common Torque Abbreviations

Foot Pounds – ft. lbs.
Inch Pounds – in. lbs.
Inch Ounces – in. ozs.
Newton Meter – Nm
Centi-Newton Meter – cNm
Meter Kilogram – Mkg

Torque Conversion Calculators

To convert torque values for other units, click on the unit you are starting with below and a popup window calculator will provide you a quick way to make the conversions:

Use of Adapter

Formula:
TA x L
L + A
= TW
Length (L) =
Effective length of the wrench as described below.
Dial Wrenches =
The measured distance from the center of the square drive to the center ring or notch on the handle.
Micrometer Wrenches =
The measured length from the center of the square drive to the center of the handle, with the wrench set at the desired torque reading.
Desired Torque (TA) =
The torque value designated for the fastener with or without an adapter.
Added Length of Adapter (A) =
The measured length from the center of the adapter drive to the center of the wrench square drive.
New Setting (TW) =
The torque setting on the wrench allowing for the added length of the adapter. This reading will be lower than the desired torque.
Example:
250 ft. lb. Dial Wrench using a 2” long crowfoot adapter

L = Effective Length: 18.75”
Desired Torque = 250 ft. lb.
Length of Adapter = 2”
Result:
18.75" x 250 ft. lb.
18.75" + 2"
= Pull Wrench to 226 ft. lb.

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Indicated Value vs. Full Value

Issues to consider when selecting an electronic torque tester:
1 Accuracy: Generally there are two ways of stating accuracy:

A. % of full-scale deflection or FSD
B. % of indicated value or reading

The following example will show the difference between the two methods:

Case 1 - Assume you have a 100 ft. lb. tester (maximum), and that the stated accuracy is +/- 0.5% of full scale.

At 100 ft. lb. +/- 0.5% full scale error = .5 ft, lb. This represents the “best case” error of the system. However, when a lower range is utilized, this .5 ft-lb becomes more significant. That is, on the same 100 ft. lb. tester;

      at 50 ft. lb. - .5 ft. lb. error = 1% accuracy
      at 10 ft, lb. - .5 ft. lb. error = 5% accuracy
      at 1 ft. lb. - .5 ft. lb. error = 50% accuracy

Therefore, what looks to be a good accuracy reading at full-scale actually translates into substantial error at the low range of the tester.

Case 2 - Assume you have a 100 ft. lb. tester (maximum), and that the stated accuracy is +/- 0.5% of indicated value.

      at 100 ft. lb. - .5% error .5 ft. lb.
      at 50 ft. lb. - .5 % error .25 ft. lb.
      at 10 ft. lb. - .5% error .05 ft. lb.

As can be seen by the above examples, error as related to full-scale value increases significantly as you go lower in the range, while error as related to indicated value stays constant throughout the useful range of the tester.

2 Range: Generally when manufacturers advertise % error of full-scale, their useful ranges will be advertised from zero to full-scale. That is, +/- 0.5% accurate (full-scale) from 0-100 ft. lb. This is interesting because at zero ft. lb., the system is only accurate to within +/- 0.5 ft. lb. Basically, error goes to infinity at zero.

Furthermore, transducers which are used to convert the mechanical torque into an electrical signal may become inconsistent below 10% of full-scale deflection.

It is for the above stated reason that systems which have accuracy as related to indicated value should state the useful range to be 10% to 100% of the tester range.

Therefore, if a tester has 100 ft. lb. maximum range, it should not be used at less than 10 ft. lbs. if the desired accuracy is needed.

It is CDI’s belief that in order to be completely honest to the customer, accuracy should always be stated as a percent of indicated value and the useful range should correspond to that stated accuracy. This will prevent the user from having to calculate what the
real error is at any given range.

3 Circuitry: There are two basic ways of measuring the output of a torque transducer.
  1. Analog (non-microprocessor based pure analog)

  2. Digital (microprocessor based plus analog input)

Without in-depth explanations of these two systems, the following advantages of having digital circuitry are well known throughout the electronics industry.

  1. Digital systems are economical, flexible and compact.

  2. Digital systems improve reliability in the face of hardware imperfections.

  3. Digital systems allow the ability to make logical decisions, carry out digital computations (unlimited unit conversion) and store the results in memory.

Basically, full digital systems are computer controlled. It is important that the terms “digital display” or “digital memory” do not necessarily mean that the system has full digital circuitry.


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Bolt Torque Specifications

Bolt Torque Charts
These charts show suggested maximum torque values for threaded products and are intended only as a guide. Always refer to the manufacturers recommended torque values if possible. CDI Torque Products is not responsible for any application of torque or it's consequences as a result of using this chart. Use at your own risk!
Bolt Size
18-8
    Stainless
    Steel 
Brass
Aluminum
    2024-T4
316
    Stainless
   Steel
Nylon
INCH POUNDS
2 - 56
2.5
2.0
1.4
2.6
0.44
4 - 40
5.2
4.3
2.9
5.5
1.19
4 - 48
6.6
5.4
3.6
6.9
 
6 - 32
9.6
7.9
5.3
10.1
2.14
6 - 40
12.1
9.9
6.6
12.7
 
8 - 32
19.8
16.2
10.8
20.7
4.30
8 - 36
22.0
18.0
12.0
23.0
 
10 - 24
22.8
18.6
13.8
23.8
6.61
10 - 32
31.7
25.9
19.2
33.1
8.20
1/4" - 20
75.2
61.5
45.6
78.8
16.00
1/4" - 28
94.0
77.0
57.0
99.0
20.80
5/16" - 18
132.0
107.0
80.0
138.0
34.90
5/16" - 24
142.0
116.0
86.0
147.0
 
3/8" - 16
236.0
192.0
143.0
247.0
 
3-8" - 24
259.0
212.0
157.0
271.0
 
7/16" - 14
376.0
317.0
228.0
393.0
 
7/16" - 20
400.0
357.0
242.0
418.0
 
1/2" - 13
517.0
422.0
313.0
542.0
 
1/2" - 20
541.0
443.0
328.0
565.0
 
9/16" - 12
682.0
558.0
413.0
713.0
 
9/16" - 18
752.0
615.0
456.0
787.0
 
5/8" - 11
1110.0
907.0
715.0
1160.0
 
5/8" - 18
1244.0
1016.0
798.0
1301.0
 
3/4" - 10
1530.0
1249.0
980.0
1582.0
 
3/4" - 16
1490.0
1220.0
958.0
1558.0
 
7/8" - 9
2328.0
1905.0
1495.0
2430.0
 
7/8" - 14
2318.0
1895.0
1490.0
2420.0
 
1" - 8
3440.0
2815.0
2205.0
3595.0
 
1"- 14
3110.0
2545.0
1995.0
3250.0
 

Bolt
    Size
    Inches
Coarse
    Thread/
    inch
SAE 0-1-2
     74,000 psi
     Low Carbon
    Steel
SAE Grade 3
    100,000 psi
    Med Carbon
    Steel
SAE Grade 5
     120,000 psi
    Med. Carbon
    Heat T. Steel
SAE Grade 6
    133,000 psi
    Med. Carbon
    Temp. Steel
SAE Grade 7
    133,000 psi
    Med. Carbon
    Alloy Steel
SAE Grade 8
    150,000 psi
    Med Carbon
    Alloy Steel
FOOT POUNDS
1/4
20
6
9
10
12.5
13
14
5/16
18
12
17
19
24
25
29
3/8
16
20
30
33
43
44
47
7/16
14
32
47
54
69
71
78
1/2
13
47
69
78
106
110
119
9/16
12
69
103
114
150
154
169
5/8
11
96
145
154
209
215
230
3/4
10
155
234
257
350
360
380
7/8
9
206
372
382
550
570
600
1
8
310
551
587
825
840
700
1-1/8
7
480
872
794
1304
1325
1430
1-1/4
7
375
1211
1105
1815
1825
1975
1-3/8
6
900
1624
1500
2434
2500
2650
1-1/2
6
1100
1943
1775
2913
3000
3200
1-5/8
5.5
1470
2660
2425
3985
4000
4400
1-3/4
5
1900
3463
3150
5189
5300
5650
1-7/8
5
2360
4695
4200
6980
7000
7600
2
4.5
2750
5427
4550
7491
7500
8200