Strain measurements are to be made at the root of the gear tooth while the pump is operating. The fillet radius at the tooth root is 0.125 in (or about 3 mm) and test temperatures are expected to range from 0 deg to +180 deg F (-20 deg to +80 deg C).

Gage Length     A gage length which is small with respect to the fillet radius should be specified for this application. A length of 0.015 in (0.38 mm) is preferable, but reference to Standard Gage Series table, used in conjunction with Catalog 500, Micro-Measurements Precision Strain Gages , indicates that such a choice severely limits the available gage patterns and grid alloys. Anticipating problems which would otherwise be encountered in the following two steps, a gage length of 0.031 in (0.8 mm) is selected.

Gage Pattern     Because the gear is a spur gear, the directions of the principal axes are known, and single-grid gages can be employed. A gage pattern with both solder tabs at the same end should be selected so that leadwire connections can be located in the clearance area along the root circle between adjacent teeth. In the light of these considerations, the 031CF pattern is chosen for the task.

Gage Series     Low strain levels are expected in this application; and, furthermore, the strain signals must be transmitted through slip rings or through a telemetry system to get from the rotating component to the stationary instrumentation. Isoelastic (D alloy) is preferred for its higher gage factor (nominally 3.2, in contrast to 2.1 for A and K alloys). Because the gage must be very flexible to conform to the small fillet radius, the E backing is the most suitable choice. The maximum test temperature is not a consideration in this case, since it is well within the recommended temperature range for any of the standard backings. The combination of the E backing and the D alloy defines the ED gage series .

Options     For protection of the gage grid in the test environment, Option E , encapsulation, should be specified. Because of the limited clearance between the outside diameter of one gear and the root circle of the mating gear, a particularly thin gage installation must be made; and very small leadwires will be attached to the gage tabs at 90 deg to the grid direction, and run over the sides of the gear for connection to larger wires. This requirement necessitates attachment of the small leadwires after gage bonding, and prevents the use of preattached leads.

Resistance     In the ED-Series version of the 031CF gage pattern, Catalog 500 lists the resistance as 350 ohms. The higher resistance should usually be selected whenever the choice exists, and will be advantageous in this instance in improving the signal-to-noise ratio when slip rings are used.

S-T-C Number     D alloy is not subject to self-temperature compensation , nor is compensation needed for these tests since only dynamic strain is to be measured. In the ED-Series designation the two-digit S-T-C number is replaced by the letters DY for "dynamic."

Combining the results of the above selection procedure, the gage to be employed is:

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