SM0486
(1) When you study the schematic diagram of Figure 11 and compare this
diagram with the torque application system of Figure 10, you should understand the
relationships of the bridges illustrated in Figure 11 and the operation of the
measuring system. One of the bridges (external) is located in the SR-4 cell, while
the other (internal) is located in the instrument proper.
The external-bridge
resistors are actually four SR-4 strain gages, each consisting of a length of very
fine wire arranged in the form of a grid and bonded to a resin carrier.
Physically, the four strain gages are cemented to the surface of the SR-4 cell.
(2) When you activate the power switch identified in Figure 9, power (12
volts DC) is applied continuously to the two series-opposed bridges. When there is
no torque exerted on the SR-4 cell, the error voltage is zero as the outputs from
the two opposing bridges balance each other.
However, as soon as a force is
exerted on the ST-4 cell, an error voltage proportional to the strain exerted on
the cell is directed to the amplifier, due to a change in bridge resistance. The
error signal is amplified to drive the motor which is connected to the indicator
dial and slide wire assembly. The slide wire is driven to change instrument bridge
resistance so as to wipe out the error signal and stop motor operation.
During
this operation, the dial (mechanically connected to the motor) is continuously
indicating the value of the error voltage in either inch-pounds or foot-pounds.
d. Torque calibrator application and operation.
We feel that the knowledge
you have already gained can be reinforced if we include a simplified operating
procedure for the torque calibrator.
You should not attempt to memorize the
procedure.
On the other hand, you should examine each step to see if you can
recognize a principle in the step or a reason for the step. The entire procedure
should be preceded by a check to see if your air supply is developing the required
80 psig.
(1) Table 2 is included so that you can understand the function of each
control mentioned in the operating procedures which follows the table. You should
refer to Table 2 and Figures 9, 10, and 11 as you study the steps in the operating
procedure.
Prior to the test of a torque wrench, you should prepare the torque
calibrator as indicated in the steps which follow.
a. Place the range switch to the proper range position determined by
the size of torque wrench being tested. (Are you sure you understand the operation
of the range switch? If you are not, read the function in Table 2).
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