A Web Tension Load Cell is a specialized sensor used to accurately measure the longitudinal force (tension) exerted by a continuous sheet material (the “web”) as it moves through a machine.

• Primary Function: They ensure the web is running at the precise tension required for optimal product quality, consistent printing, slitting, laminating, or winding.
• Monitored Materials:
  • > Paper and cardboard
  • > Films, plastics, and foils
  • > Textiles and non-wovens
  • > Metal strips and wire
  • > Rubber and flexible packaging materials

Web tension load cells are designed for highly sensitive measurement and are generally integrated into the machine structure at key locations:

• > Installation: They are mounted on the machine frame to support the bearings of one or more idler rolls (tension sensing rolls).
• > Measurement Principle: As the material (web) passes over the idler roll, the tension in the web creates a resultant horizontal force vector on the roll’s axis. The load cells measure the vertical or horizontal components of this resultant force, which is directly proportional to the web tension.
• > Configuration: They are almost always used in pairs (one on each end of the sensing roll) and wired to a specialized tension controller or amplifier.

These terms primarily refer to the physical mounting style required for different machine designs:

The choice depends on the available space, the orientation of the machine frame, and the total load/speed requirements of the application.

A specialized tension controller or amplifier is mandatory because web load cells are designed to measure tension using the F = T x factor principle (where F is the measured force and T is the actual web tension).

• > Sensitivity and Calibration: The amplifier allows the system to be precisely calibrated based on the specific wrap angle of the web around the sensing roll.
• > Dynamics: It provides the necessary high-speed signal processing, filtering, and stability control required to handle the fast-changing dynamics of a high-speed web line (e.g., during acceleration or deceleration).
• > Output: The controller converts the raw millivolt output from the pair of load cells into a usable signal (e.g., 4-20 mA or digital) that the machine’s PLC uses to adjust brakes, clutches, or motors.

If web tension readings are erratic or incorrect, check these common issues:

• > Wrap Angle: Ensure the wrap angle (the angle of contact the web makes with the sensing roll) used in the amplifier calibration matches the actual physical installation angle. The tension reading is highly dependent on this angle.
• > Friction: Check the idler roll bearings. Excessive friction in the sensing roll bearings will be measured as “false tension,” leading to unstable readings. Use high-quality, low-friction bearings.
• > Alignment: Ensure the web and the sensing roll are perfectly parallel and square to the machine frame. Misalignment causes uneven tension across the web width, leading to poor product quality and inaccurate total load readings.