A torque wrench is a device which indicates to a mechanic when he has exerted the desired amount of torque on a fastener. A reading is made directly in pounds-foot or pounds-inch. However, when accessories which add lever length are used with torque wrenches, special corrections must be made.
Adaptors or extensions are attached to the square drive of a torque wrench...
1. to attach the torque wrench to various types of fasteners or fittings so that torque may be applied.
2. to gain access to locations which cannot be reached with the torque wrench alone.
HOW ADAPTORS AFFECT TORQUE
To understand why some extensions and adaptors change or alter the torque at a fastener, a brief explanation of what is meant by "torque" and how it is measured is helpful. Theoretically, torque is the force on an object which tends to produce twist or rotation. A wrench acts as a lever when force is applied, and the amount of torque produced upon the fastener is dependent upon the length of the wrench and the amount of force applied. In Figure 1, the lever length of the wrench from the center of the fastener to the center of the hand applying the force is represented by L. The applied force is indicated by F. Since torque T is the product of the applied force multiplied by the effective lever length L, it can be calculated by using the basic formula: T = F x L. Because the applied force usually is measured in pounds, while the lever length is measured in inches or feet, the resulting torque is measured in "pounds-inch" or "pounds-foot". Thus, if F is 30 pounds and L is one foot, T becomes 30 pounds-foot. As shown in Figure 1, F must be applied at a 90° angle to the lever. When the force is applied in any other direction, a lesser torque than that calculated is exerted by the wrench.
An examination of the formula and Figure 1 brings out the following points:
a) Increasing the lever length L increases the torque if F is constant. If L is changed to 2 feet, then T becomes 60 pounds-foot, even though F remains at 30 pounds.
b) Decreasing the lever length L decreases the torque if F is the same. If L is changed to 1/2 foot, then T becomes 15 pounds-foot, even though F remains at 30 pounds.
c) The torque T is directly proportional to F, if L remains unchanged. Thus, if F is 30 pounds while the torque is 30 pounds-foot and then F is increased to 60 Pounds, torque or T then becomes 60 pounds-foot.
HOW TO COMPUTE TORQUE WHEN USING ADAPTORS
If an adaptor is attached to the square drive of a dial-type or a click-type torque wrench, adding to the length from the square drive to the fastener (parallel to the wrench), then the applied torque will be greater than the dial reading or the pre-set torque. The following formula can be used to find what the dial should read or what the pre-set torque should be in order to obtain the correct applied torque:
Dial Reading or Pre-Set Torque = (Torque Wrench Length x Torque Desired) / (Torque Wrench Length Adaptor Length)
This becomes: RS = (L x T) / (L A) when:
RS = Dial reading or torque setting of the torque wrench.
L = Distance from the center of the square drive of the torque wrench to the center of the handle grip.
A = Length of the adaptor from the center of the square drive to the center of the fastener. Use only the length which is parallel to the handle. See figures 2 and 3.
T = Torque desired. This is the actual torque applied to the fastener.
Here is a typical problem: What should the dial read or the setting be when L is 12", A is 6" and T is 30 lbs. ft. ?
RS = (L x T) / (L plus A) or (12 x 30) / (12 plus 6) = 360 / 18 = 20 lbs. ft.
Therefore 30 pounds-foot of Torque will be applied at the fastener when RS is 20 pounds-foot.
Note: If the torque wrench RS reads in pounds-foot, then T must also be in pounds-foot. Also, L and A must be the same unit of measurement.
Types of Adaptors
The problem just explained gives a very simple conversion using a straight adaptor. However, there are many types of adaptors and extensions. A few are shown in figures 4, 5, 6, and 7; but in each one, note that only the added distance from the square drive of the torque wrench increasing its length will change the readings. Note that an extension which is perpendicular to the torque wrench, regardless of length, does not affect the readings. The effective torque will be about 7% lower than the setting or dial when flex-type torque wrenches are used in the fully-flexed position.
The adaptor in Figure 4 changes the length and leverage of the torque wrench. The dial will read only a portion of the torque. A correction must be made.
Neither the extension nor the adapter in Figure 5 affect torque because neither increases the length of the torque wrench. Factor A is not involved, therefore no correction is necessary. When adaptor length A is 0, the formula RS = (L x T) / (L plus A) becomes: RS = L x T / L = T. In other words: Dial Reading or Pre-Set Torque = Torque Desired
The adaptor in Figure 6 affects the dial reading. Factor A is added and therefore a correction is necessary. Note that A is not the length of the adaptor; but only the increase in length parallel to the torque wrench. If the adapter were rotated on the square drive, its effective length would change, changing the calculation for RS.
The adaptor in Figure 7 adds length. Note that only the distance parallel to the torque wrench is used to get factor A; not the offset, or height.
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