Confirm Formula Please (total load on load cell)

In summary, your equation is correct if you torque the bolt first (to create the preload) and then add the static live load.
  • #1
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I am using the following formula to determine the lb load against the bottom of a load cell from tightening the bottom anchor bolt nut (which anchor bolt goes through the center of the load cell structure and anchors to a bottom base plate) to a certain torque force, plus adding a live static load from above, creating a total load on the load cell.
Will this formula give me the correct total load ?

FOMULA: T=(K)(D)(P)
T= Torque in inch lbs (divide by 12 to get ft lbs )
K= Coefficient of friction ( nut to dry bolt threads) ( range of .35 to .40 from standard table )
D= Bolt diameter in inches
P= Bolt's desired tensile load in lbs ( generally 75% of yield strength )
Then add to P the static live load to get the TOTAL LOAD applied to the load cell.

I tried to upload a sketch of above and this system would not accept it ( to large). <<Mentor note: Personal identifying information removed>>
Thank you very much!
 
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  • #2
It depends.

If you torque your bolt first to create the preload and then add the static load, the load cell will read the highest of the preload OR the static load.

If you put the static load first and set the preload with the static load on the load cell, the load cell will read the preload PLUS the static load. And if you take the static load off, the preload on the bolt will increase by an amount equivalent to the static load, maybe leading to failure.

I came to this conclusion by modeling your load cell as a spring. If you preload it with, say, 100 lb, it will deflect by a certain amount. Adding 50 lb on the spring will not deflect it more because whatever deflection increase would the static load impose, it would also remove the preload; so everything will balance out and stay constant. If you add 150 lb, the spring will deflect more, completely releasing the preload, such that the spring will only «feel» the 150 lb.

If you put the load on it first, say 50 lb, the spring will deflect. By adding the preload to the bolt, say 100 lb, the spring will deflect even more, to an equivalent 150 lb. If you remove the static load, then the spring will push 150 lb against the bolt.
 
  • #3
Thanks for your reply.
I understand your concerns. Let's see if this helps.
Once the bottom nut is tightened or torqued and the preload is set (and before any static live load is imposed from the top). The load cell is then calibrated to zero by it's indicator brain box. Then the live load from above is imposed and the display window, after pressing the appropriate button, shows the live weight only on the display window. So at that point the load cell is only reading the live load applied after is was re-calibrated to zero, after the preload was imposed.
Assuming that is all good, I am really double checking if the formula I used to calculate the preload was a correct one to use.
Thanks...
 

Related to Confirm Formula Please (total load on load cell)

1. What is a load cell?

A load cell is a type of transducer that is used to measure force or weight. It typically consists of a metal body with strain gauges attached to it, which convert the force being applied to the load cell into an electrical signal.

2. How does a load cell work?

A load cell works by converting the force being applied to it into an electrical signal. This is done through the use of strain gauges, which change their resistance when a force is applied. The change in resistance is then converted into an electrical signal that can be measured and interpreted.

3. What is the formula for calculating the total load on a load cell?

The formula for calculating the total load on a load cell is: Total Load = (Output Voltage / Sensitivity) x Calibration Factor. The output voltage and sensitivity can be obtained from the load cell's datasheet, while the calibration factor is determined during the calibration process.

4. Why is it important to confirm the formula for total load on a load cell?

Confirming the formula for total load on a load cell is important because it ensures that accurate measurements are being taken. If the formula is incorrect, the readings from the load cell may be incorrect as well, which can lead to inaccurate data and potentially harmful consequences.

5. How can I confirm the formula for total load on a load cell?

To confirm the formula for total load on a load cell, you can perform a calibration process. This involves applying known weights to the load cell and recording the corresponding output voltage. From this, you can calculate the sensitivity and calibration factor, and then use the formula mentioned earlier to confirm the total load calculation.

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