Mounting of low-profile load cells without base

Achieving precise and reliable force measurements with your Interface LowProfile load cell depends on the replication of the conditions under which it was carefully calibrated.

When integrating a low-profile cell directly onto a customer-supplied mounting surface without using a factory-supplied base, strict adherence to certain criteria is recommended, which are essential to maintaining the sensor's inherent accuracy. Deviations from these guidelines may key performance features such as linearity, repeatability and hysteresis, resulting in unreliable data.

Interface load cells respond to forces acting along the axis perpendicular to the mounting surface. The load cell's response to an off-axis force is proportional to that force multiplied by the cosine of the angle it forms with the load axis.

Tip: Interface strongly recommends using a factory-supplied base for your LowProfile. We offer bases for most Low-profile universal, tension and pure pressure sensors Interface also offers Load cell base sets as part of our accessories.

The following reference, as it appears in our Load Cell Field Guide is described, describes, How to properly mount a low-profile load cell without a base. It combines the instructions with the underlying benefits and important tips derived from Interface's expert recommendations.

The golden rule is to reproduce the calibration conditions

One fundamental principle is central to accurate load cell installation: The direct mounting of a low-profile load cell to a structure should accurately replicate the mounting used during calibration. This ensures that the sensor operates under the same mechanical constraints and material interactions for which its performance specifications were validated.

Five pillars of correct direct assembly

If you mount your LowProfile directly onto your mounting surface, consider the following five critical criteria:

#1 – Harmony of material and hardness: Coordinating thermal expansion and hardness

• Instructions: Choose a material for the mounting surface that reflects the thermal expansion coefficient and hardness of your specific LowProfile.
• Advantages: This crucial step minimizes stress and potential measurement errors caused by differential expansion or contraction due to temperature fluctuations. A surface with similar hardness ensures proper load transfer and prevents local deformation of the direct mounting interface.
• Reference: For low-profile load cells with a load capacity of up to 2000 lbf, 2024 aluminum should be used. For all larger-capacity cells, the mounting surface should be made of 4041 steel, hardened to a Rockwell C (RC) hardness of 33 to 37. This ensures compatibility with the load cell material in your direct-mount configuration.

#2 – Thickness is crucial: Ensuring the dimensional stability of your interface

• Instruction: Your mounting surface should be at least as thick as the factory-supplied base typically used with your specific Interface LowProfile load cells.
• Advantages: While the cell may physically function with a thinner direct mount, it may compromise its dimensional stability under load. This can directly impact the sensor's ability to meet linearity, repeatability, and hysteresis specifications. Adequate thickness provides the necessary rigidity for optimal direct mount performance.
• Note: Remember that following these thickness guidelines will help ensure your LowProfile® cell meets the specified performance specifications when mounted directly to your structure.

#3 – The imperative of flatness: Optimal contact during direct mounting

• Instruction: The mounting surface must be ground to a flatness of 0,0002″ TIR (Total Indicator Reading).
• Advantages: A flat surface ensures consistent contact between the load cell and its mounting structure. This even force distribution prevents stress concentrations that can distort readings and negatively impact the accuracy and durability of the cell when mounted directly.
• Note: If your mounting plate will be heat treated after the initial sanding, it is advisable to perform a final light sanding to ensure the required flatness for the direct interface.

#4 – The strength of mounting screws: Selecting the right fasteners for direct fastening

• Instructions: Use Grade 8 mounting bolts to directly attach your LowProfile.
• Advantages: Grade 8 bolts provide the high tensile strength required to securely attach the load cell without yielding or loosening under load. This ensures a stable and reliable direct connection, which is critical for accurate measurements.
• Reference: Interface offers these high-quality screws for direct ordering, eliminating the need for local sourcing and ensuring the correct specification for your direct mount. For cells with countersunk mounting holes, use cylinder head screws for a flush fit. For all other low-profile cells, hex head screws are the recommended choice for direct mounting. Do not use washers under the screw heads for this direct mount.

#5 – The Art of Torque: A Step-by-Step and Sequential Approach to Direct Mounting

• Instruction: Use a three-step, sequential tightening procedure for the mounting screws when directly mounting the load cell.
• Advantages: This methodical approach ensures an even distribution of the clamping force across the direct mounting interface, prevents deformation of the load cell and guarantees a safe and stable installation.
• Note: The recommended tightening sequence is as follows:
  • First tighten all screws to 60% of the specified torque.
  • Then increase the torque to 90% of the specified value.
  • Finally, tighten to the full torque of 100% of the specified value. For Interface LowProfile load cells with four mounting holes, follow the tightening pattern for the first four holes in a standard 8-hole pattern for direct mounting.