Tighten the joints properly on a production line is a fundamental step to ensure the quality on the assembly process.
The selection and use of wrenches, electronic or mechanical, is not always clear to those who are managing the tightening strategies to be adopted in line.
There are different kinds of wrenches: click wrenches, slip wrenches, analogic and electronic torque wrenches with torque or torque/angle measurement.
CLICK WRENCHES
Click wrenches apply torque and click at a certain threshold, typically adjustable.
The click can be a few degrees (with high risk of over tightening) up to a few tens of degrees.
Pro:
- Light weight
- Cheap
Cons:
- After the click point, torque is applies again, and typically producing an over tightening, depending on the operator. Therefore, someone use to set a lower target torque, but the final torque becomes a direct result of the action of the operator instead of the wrench calibration.
- The torque applied is influenced by the wrench handling. Incorrect handling alters the torque applied.
- The tightening is performed without rotation control (the operator may tighten twice the same screw, or the bolt may be locked in the thread giving the false impression of being locked)
- The torque is not measured and traced
SLIP WRENCHES
Slip wrenches apply torque and slip at a certain threshold, usually adjustable. After slipping, the torque is released completely and, continuing the rotation, the torque is applied again.
Pro:
- Light weight
- Cheap
- Over torque risk eliminated
Cons:
- Used only at low torque. There are some models up to 100 Nm or slightly beyond, torque values that can already give problems to the operator when the torque is suddenly released
- Each slip point has a torque value different from the others, since the wrench works on different mechanical teeth
- The torque applied is influenced by the wrench handling. Incorrect handling alters the torque applied
- The tightening is performed without rotation control (the operator may tighten twice the same screw, or the bolt may be locked in the thread giving the false impression of being locked)
- The torque is not measured and traced
ANALOGIC DIAL WRENCHES
Analogic dial wrenches provides real time torque reading.
Pro:
- Light weight
- Cheap
- A estimated torque is provided
Cons:
- The final torque depends on where the operator stops. The repeatability of the tightening is therefore not guaranteed, even if the wrench is perfectly calibrated.
- The torque applied is influenced by the handling. Incorrect handling alters the torque applied.
- The wrench cannot be programmed to perform the tightening and cannot guide the operator to a correct tightening operation
- The tightening is performed without rotation control (the operator may tighten twice the same screw, or the bolt may be locked in the thread giving the false impression of being locked)
- The torque is not measured and traced
DIGITAL TORQUE WRENCHES
Modern electronic torque wrenches provide a reading of the torque and, if equipped with gyro, the angle of rotation.
Pro:
- Torque and angle control for a “certificated” error proof tightening
- Torque and parameters displayed on the wrench
- Tightening result is shown in real time to the operator
- Programming of the tightening strategy
- Production tightening and quality control test (control of residual torque)
- Ability to manage extensions for tightening in uncomfortable positions
- Torque measurement does not depend from the handling (if the wrench is equipped with a double bridge transducer)
- Data traceability
- The wrench can be used as a backup of a tightening tool on the assembly line
Cons:
- Higher cost than a normal torque wrench
- Weight probably higher than a normal torque wrench
HOW TO CHOOSE A DIGITAL TORQUE WRENCH FOR PRODUCTION OR FOR QUALITY CONTROL
The quality control wrench must be equipped with:
- Torque sensor and gyroscope
- Large display that displays tightening parameters
- Traces on display
To perform production tightening operations, the same wrench used for quality control can be used (if equipped with production tightening strategies), or a slimmer digital torque wrench with only the tube + LED to guide the operator in tightening, with a remote control panel:
The production digital torque wrench must be equipped with:
- Torque sensor and gyroscope
- LED / buzzer / vibration device for real-time feedback to the operator
- Association of the test with the serial number of the piece in production
- Communication with software and database for data traceability
