# Yield Point (Joint Analysis)

This strategy evaluates the stiffness of a joint by measuring its yield point.

Parameter | Description
--- | ---
Required tag number | A specific number must be written in the TAG of the end fitting tool.
Name | Name of the tightening program.
Program start | Torque value from which the tightening operation starts. Must be ≥ smartHEAD Min torque value. If Program start = 0, its value is automatically set to the smartHEAD Min torque value.
Torque min | Lower torque limit.
Torque max | Higher torque limit.
Maximum torque limit | If the torque applied reaches this limit, the message Change screw is shown on the wrench display. Must be > Torque max.
Target torque | The torque target.
Trigger torque | Torque value from which the angle measurement starts (usually set to 50% of the Target torque). If Trigger torque = 0, its value is automatically set to the smartHEAD Min torque value.
Angle min | Lower angle limit.
Angle max | Higher angle limit.
Target angle | For joint analysis operation, Target angle is usually set to 0.
Torque units | This parameter is not editable.
Angle limit for rehit | If the Torque min value is reached within the specified angle, the message Joint already tightened is shown on the wrench display.
Stop condition | Select between Torque peak and Angle peak.
Linear slope coefficient | Coefficient that defines the joint stiffness in the linear part of the curve. The coefficient is meant as torque/angle slope and it is measured in Nm/°. Default value: 0.5. Minimum value: 0.1 (also suitable for very soft joints with a ration less than 0.1). If the selected unit of measurement is different from Nm, the Linear slope coefficient must always be converted into Nm/°.
Torque correction coefficient | When extensions are used, the wrench measurement might be compensated to show a more accurate torque value. To calculate the correction coefficient, please refer to How to calculate the Torque Correction Coefficient. The parameter is stored in the RFID TAG of the end fitting tool. When extensions are not used, Torque correction coefficient = 1.
Angle correction | When extensions cause additional wrench bending, the wrench angle measurement can be compensated to show a more accurate angle value. To calculate the correction value, please refer to How to Calculate the Angle Correction. The parameter is stored in the RFID TAG of the end fitting tool. When extensions are not used, Angle correction = 0.
Batch size | Number of tightenings the batch will perform. Maximum value: 250. If Batch size = 0, the number of tightenings is infinite. If Batch size > 1, batch result OK only if all tightenings are OK.
Max consecutive NOK | Number of accepted consecutive NOK to have an OK result.
End cycle time | Starts when the torque goes below the Program start after reaching the 3rd percentage. Default value: 0.1 sec. Minimum value: 0.1 sec. Maximum value: 5 sec.
Ratchet time | Starts when the torque goes below the Program start without reaching the 3rd percentage value. This allows the operator to release the torque for a while and recharge during the tightening operation. Default value: 5 sec. Minimum value: 0.1 sec. Maximum value: 30 sec.

Torque vs. Angle

A | Torque max | C | Program start
B | Torque min | D | Yield point

Define the torque limits to get an OK result.
In joint analysis, instead of setting a narrow range with the torque upper and lower limits, define a wider area to detect the yield point.

Define a wide range for the angle upper and lower limits as well (for example, Angle min set to 0° and Angle max set to 999°).

During the tightening operation, LEDs, buzzer and vibration are activated as follows:

- LED:

  - White LED: activated when the torque value goes over the smartHEAD's Min load value.

  - First, second and third radial gradient LEDs turn green in sequence when the three thresholds defined for the tool in use are reached.
    For further information, refer to Tool LEDs.

  - All three radial gradient LEDs green: torque between Torque min and Torque max.

  - All three radial gradient LEDs red: torque over Torque max.

- Buzzer:

  - The beep starts when the torque goes over the Program start value; the signal increases when the yield point is detected / the tightening process has been completed.

- Vibration:

  - Starts when the yield point is detected.

At the end of the test operation, LEDs, buzzer and vibration are activated as follows:

- LEDs behave according to the configuration of Result Indicator and Duration defined for the tool in use.
  For further information, refer to Tool LEDs.

- Buzzer

  - Two beeps indicate the end of the operation; if the final result is in the red area, the signal in continuous.
    To stop the buzzer, start a new tightening operation or press a button on the tool controller.

- Vibration:

  - Stays active until the torque applied is released.

In the configuration of the Yield Point strategy, there is one advanced parameter: the Linear slope coefficient. This coefficient defines the joint stiffness in the linear part of the curve. The coefficient is meant as torque/angle slope, and it is measured in Nm/°. The minimum value is 0.1, which suits also very soft joints with a ration smaller than 0.1. The default value is 0.5. The Linear slope coefficient may be calculated experimentally for each single joint (only after running a trial test to get data for calculating it) as follows:

Torque vs. Angle

= Linear slope coefficient

If the measuring unit selected for the operation is different from Nm, the Linear slope coefficient must be converted into Nm/°.
For instance, for a joint with a stiffness of 0.6 lbf*ft/°, since 1 lbf*ft = 1.3558 Nm, the Linear slope coefficient must be converted as follow:

0.6 lbf*ft/° → 0.6x1.3558 = 0.81 Nm/°