Sunday, August 21, 2016

Load Cell Alignment Plugs Proper Use

Morehouse load cell alignment plugs are used to help center the load cell in calibrating machines. They are used in combination with the other adapters below.   More information on load cell adapters and the mounting accessories can be found here.

Morehouse Reference Standard Mounting Kits for Universal Calibrating Machines


Thread is past flush and into the cell.  


When using your alignment plugs that thread into the bottom of your load cells, make sure they are threaded flush to the bottom of the cell.  Once they are flush, thread the adapter an extra turn into the cell.  You want to make sure that none of the threads are exposed below the base of the cell.   If there is a thread or more exposed, the load will be generated through the internal threads of the cell and not its base.  This will result in an additional calibration error of about 0.012 %.  It will often result in damage to the alignment plug.


Alignment plug is not threaded past the base of the cell.

Written by Henry Zumbrun with help from the calibration lab technicians,  who have received numerous load cells with damaged alignment plugs.


Monday, August 1, 2016

How to Lose a lot of Money on Annual Load Cell Calibrations

Purchasing the "right" load cell system is critical for the application, and obtaining reliable results. Purchasing a system merely based on cost and not looking at the right specifications may result in a complete waste of capital. If the system chosen does not perform well in the environment it is being used, the resulting measurements may not be meaningful.   


Questions to ask when purchasing a load cell system:

·  Does it meet my mechanical and environmental requirements?
·  What is the cost?
·  How easy is it to use?
·  How accurate is it? 


Morehouse load cell systems consists of  one or more Morehouse load cells, a digital indicator, a cable, possible adapters and a quality shipping case with custom cut high density foam.

When a buyer evaluates a system based on meeting requirements such as:  cost, ease of use and accuracy, they often forget to look at what really matters. Buyers often get caught in marketing claims and overlook the fine print. Things that are commonly missed are environmental, side load sensitivity and stability. All of these can have a significant impact on the load cells performance and make the original accuracy claim, you know the one you read in the marketing literature, false! If the load cell is not temperature compensated, the expected performance may decrease by about 0.015 % per degree C, resulting in a decrease in accuracy. In fact, a few degrees C deviation from the calibration temperature may cause more error than the originally stated accuracy in the specs sheet.  

Side load sensitivity (eccentric loading) may have the most significant impact on the expected performance of the system. Morehouse has done several tests and demonstrated side load sensitivity can vary from 0.002 % on a Morehouse load cell system up to 0.75 % of full scale on some other load cells. Depending on the load cell application and mechanical design of the system, off-center loading can simply happen even if all loading points and adapters seem to be in line. If you are buying a load cell with a 0.5 % accuracy specification, do you think it is really accurate when the slightest bit of off-center loading, produces a 0.75 % error?  

The final specification, often neglected is stability. Stability will have a significant impact on your cost of ownership. If the system is not stable, it will require more frequent calibration. More frequent calibration means higher lifetime cost of ownership. It is not uncommon to compare a $ 1500.00 system with a $ 3,000.00 system and have the lifetime cost of ownership end up being closer to $ 10,000.00 versus $ 5000.00.   


The only way to address these issues is to educate the customers. In an effort to educate the end buyer, we have developed videos, training classes, webinar and often do several speaking engagements per year. The videos and training material can be found on our website www.mhforce.com. Inform the buyer on what matters by educating them. Have them start asking, how well does it actually accomplish the measurement for my application? Try to persuade the buyer to purchase based on overall value and not the upfront cost alone.         



Written by Henry Zumbrun

        

Thursday, July 14, 2016

FORCE SAFETY - FORCE SAFETY GUIDE WHEN USING FORCE MEASURING EQUIPMENT IN VARIOUS APPLICATIONS

FORCE SAFETY GUIDE WHEN USING FORCE MEASURING EQUIPMENT

            To help avoid situations that may cause personal injury or equipment damage when using Load Cells, Proving Rings, Force Gauges and other types of instruments to which forces are applied, this post was created.  Our goal is to help anyone making force measurements, be safe and keep their equipment functioning properly.  These are common mistakes  and there are several more not covered in this post. In the future, we hope to continue this series of posts for both force and torque measurement safety tips.   



When loading thru a steel ball, the instrument should have a conical ball seat of the recommended size (see figure 2 below), for the size of ball being used and the opposing surface should have a similar ball seat.  Alternately, a soft steel pad of sufficient size and thickness may be used in place of a ball seat in the opposite surface for capacities of 200,000 lbs and less.  The force must be applied axially within one degree.  The bottom boss of the load cell or other instrument must bear against a flat hardened steel surface, and if possible, should be restrained.  See figure 1 above.



When loading a load cell or other force measuring instrument thru a steel ball, be certain that the ball is made of hardened chrome alloy steel, and that it is the recommended size (see figure 2) to withstand the force applied.  Never use a carbide ball... carbide is brittle and will shatter under load



Do not load between unstable surfaces.  Under load, the instrument could be spewed from the machine with tremendous force.  Never use a set-up where there are two spherical surfaces opposing another without making the appropriate adapters to contain the instrument.  Morehouse has developed special adapters for this type of loading.  Without the appropriate adapters the instrument could be thrown from the machine with tremendous force.   See figure 3 above for an example of what not to do.





Do not load between two steel balls unless the loading components are mechanically restrained to prevent any possible lateral movement when loaded.  Additionally, the surfaces of the components must have properly sized ball seats, the ball seats of the components must be axially aligned, and ball retainer clips should be used.  See figure 4 






When using tension member assemblies having mating spherical surfaces, be certain that they are properly installed. See figure 5.  Morehouse Quick Change Tension members (pictured below) are designed to help eliminate eccentric loads, resulting in a more accurate and safer force application.

Morehouse UCM with Load Cell in Tension using Quick Change Tension Members

Do not load between surfaces that exceed one degree deviation from parallel.  See figure 6 below.



MOST IMPORTANT...Any adapter or accessory you may design, make or purchase for use with a calibrating or force measuring instrument must be of proper design and made from steel of the proper strength to withstand the forces to which it is subjected.  It is most important that adapters and accessories be test loaded under safe conditions prior to actual use with an instrument.  Equipment should not be used beyond its maximum rated capacity.  Failure to use the proper strength material may result in serious injury or death.

Read and understand all instructions and precautions applicable to the use of the instrument and/or machines being used to apply the force.

If there are questions or doubts at any time about the use of Load Cells, Proving Rings or other force measuring instruments, contact us by telephone 717-843-0081 or email: info@mhforce.com



written by Henry Zumbrun





Saturday, July 2, 2016

Load Cell Measurement Error - Top Block Adapter Hardness and Flatness


Can a load cell adapter plate or block be interchanged without introducing additional error?  

This blog will attempt to answer this question.   The answer is maybe.   If the new adapter consists of the same material and has identical characteristics, then it may be able to be substituted, without introducing additional error.   Though, some material will harden with repeated use and there could be a difference.    It is important the material of the top block be softer, has a lower hardness, than the load cell it is used with.  If a top block is replaced, the recommendation is to have the force measuring equipment checked, or calibrated to ensure errors are accounted for.   



·         Using a top adapter with a different hardness value may affect the strain level in the load cell column or web; and therefore, result in different measurement outputs.  We have observed errors of up to 0.15 % from varying the material on top compression pads.  We highly recommend the end user send us the top adapter they are using with the load cell, and even load cell bases.  If either adapter is not ground flat, additional errors could result.   We have conducted several tests and have found repeatability errors to be about 3 times higher, when the compression pads or load cell base is not flat. Morehouse has a full machine shop and can grind top adapters for a nominal fee (typically $30 to $40 per block).


Real World Example:  A customer brought in a 1,000,000 LBF load cell for calibration.   Morehouse performed a calibration.  The output of the load cell was recorded as 1,500 LBF higher than the previous calibration for a force applied 1,000,000 LBF. 


Is this a stability issue, or an adaptor issue?  

After calling the customer, we were informed a new top loading block was supplied with this load cell for the current calibration.   When we told them what was happening, they sent the original top loading block.  When tested, the original block resulted in an output of 1,000,180 LBF when loaded to 1,000,000 LBF.


When using the new adaptor and figuring the measurement error between the different top blocks (adaptors), Expanded Uncertainty would have increased from 269 LBF with original top adaptor to  1,490 LBF using the newly fabricated adaptor.   The individual contribution to the overall measurement uncertainty was dominant.    More information on this error and other common measurement errors, can be downloaded here.




written by Henry Zumbrun







Wednesday, June 1, 2016

Tips From The Cal Lab - Setup Reduction With Morehouse Quick Change Tension Members and Adapters

Tips from the Cal Lab – Setup Reduction with Morehouse Quick Change Adapters


Morehouse employees went through a lean manufacturing course. Part of this course emphasized reducing cycle time, by reducing the amount of time it would take to set up equipment for a load cell calibration. During the class, the concept “quick changeover” was discussed and applied to a single piece flow.  We had the idea to apply some of these setup reduction concepts to our force calibration lab, since the lab workflow mimics that of single piece flow production. We discovered that a set of universal tension members in our calibration machines could be used along with a few mating adapters, and this reduced three shelves full of various adapters down to half a shelf.   Soon after building these for our lab, visiting customers started asking for their own sets, and we decided to start offering Quick Change Tension Member Assemblies.  Do you want to improve your measurement process, save space, and reduce long term cost of purchasing many adapters for each instrument you need to calibrate? 





Quick Change Tension Members and Adapters help with the following:

  • Reduce Tension Changeover Time
  • Improve Alignment by using a Spherical Radius Contact as Defined by ISO 376  (note below)
  • Improve Alignment and better Repeatability 
  • Reduce Overall Calibration Cycle time
  • Simplify Setups by Using One Tension Member with Several Adapters
  • Standardize Production Flow
  • Available for capacities up to 500kN or 112,000 lbf. 


 Quick Change Adapter in Morehouse UCM


Reducing the changeover time allows for a more efficient calibration process that does not sacrifice quality, while improving cycle time. Moreover, shorter cycle time allows for improvements regarding on-time delivery, lowered cost from excess inventory, and increase in machine capacity levels.  

These adapters simplified our laboratory setups, improved our performance with better alignment, and were cost-effective compared to the alternative of multiple pieces of threaded rod or multiple rod end type setups. As a result of optimizing the calibration setup,, we were able to standardize production flow, provide better measurements, and free additional floor space for future expansion. 
Full Product Guide on Quick Change Tension Members can be downloaded here 


ISO 376 Note:


Morehouse Quick Change Drawing
ISO 376 recognizes the importance of adapters in reproducibility conditions of the measurement.   Proper adapter use in accordance with ISO 376 Annex A, helps ensure the reliability of reported measurements.  Note:  Annex A is not a requirement for labs to adhere to.



A.4 Loading fittings

A.4.1 General




Loading fittings should be designed in such a way that the line of force application is not distorted. As a rule, tensile force transducers should be fitted with two ball nuts, two ball cups and, if necessary, with two intermediate rings, while compression force transducers should be fitted with one or two compression pads.

Morehouse Design Notes:

1.  Roark's formula for stress and strain concentration was used to verify FEA results.  
2.  Design used spherical radii called out in the ISO 376:2011 standard.
3.  Safety factor of 2:1 implemented in design

Stress Analysis Plot of Quick Change Tension Member


written by Henry Zumbrun


 

Monday, May 16, 2016

Universal Calibrating Machines versus Universal Testing Machines - These machines are different

Universal Calibrating Machines versus Universal Testing Machines


Morehouse UCM 


Universal Calibrating Machine - a versatile compression and tension machine used for the calibration of load cells, force gauges, crane scales and other force measuring instruments for the purpose of calibrating or characterizing the devices for use as secondary standards, working standards, or to make field measurements.  

Accuracy -  The machine matches the accuracy of the calibration standard with which it is used.   Most machines are aligned well enough that the maximum amount of side load is less than 0.0625 inches.  On a shear web load cell this amounts to a possible error source of 0.003 %.

CMC -  Calibration and Measurement capability is typically 0.02 % or better.  A Morehouse UCM with two to three load cell standards is capable of achieving CMC's  of better than 0.01 % throughout the loading range.   A spreadsheet template to help figure out CMC's for calibrating machines can be at http://www.mhforce.com/s/CMC-CALCULATIONS-FOR-FORCE-MEASUREMENTS.xlsx

Range of Use - Typically a Morehouse UCM can be used as low as 1 % of full scale capacity.    



Universal Testing Machine

Universal Testing Machine - also known as a  universal tester, materials testing machine or materials test frame, is used to test the tensile strength and compressive strength of materials. 

Accuracy -  Most machines are between 0.3 % through 1 %.   Errors less than 0.5 % are not uncommon. Very few labs can calibrate with devices sufficiently more accurate than 0.25 %. 

CMC -  Calibration and Measurement Capability is typically above 1 %.     Note:  A UTM may be used as a means to apply a force if a well defined reference standard, calibrated by primary standards is used in combination with the load cell in the frame (for control only), a CMC of 0.05 % may be achievable.   

Range of Use - Varies depending on the manufacturer.  


ASTM Measurement Pyramid








Written by Henry Zumbrun


Thursday, May 12, 2016

Load Cell Measurement Error - If a load cell is to be used to make descending measurements, it must be calibrated with a descending range

If a load cell is to be used to make descending measurements, it must be calibrated with a descending range



The difference in output on an ascending curve versus a descending curve can be quite significant.  A very good 100K load cell had an output of -2.03040 on the ascending curve and -2.03126 on the descending curve.  Using the ascending only curve would result in an additional error of 0.042 %. 

The common term to describe this result is hysteresis.  

Hysteresis is defined as the algebraic difference between the output at a given load descending from maximum load and the output at the same load ascending from minimum load.  This is normally expressed as a % of full scale output.  In this post, we are only looking at the percentage difference between the same force point, ascending versus descending.  If someone were to use the ascending calibration curve to make descending measurements, the difference between the ascending and descending points would be significant measurement error.


Morehouse sampled several instruments and recorded the following differences.   




Load Cells from five different manufacturers were sampled and recorded the results.  The numbers varied from 0.007 % (shear web type cell) to 0.120 %.  On average, the difference was approximately 0.06 %.  Six of the seven tests were performed using dead weight primary standards known to be accurate within 0.0016 % of applied force.   

The conclusion from these tests is clear:  If a load cell is going to be used to calibrate both ascending and descending forces, it must be calibrated in both modes.


Additional comments:  When the load cell is used to calibrate decreasing forces, it must be loaded to the maximum force at which it was calibrated to ensure repeatable results.   

Ascending and descending calibration is typically required for low cycle fatigue machines, nuclear requirements, and universities conducting a lot of research and development.

If a load cell is calibrated in accordance with the ASTM E74 standard and a combined curve is used, the end user could use the load cell anywhere in the loading range.  The downside to this method is that the combined curve will produce a Lower Limit Factor large enough to encompass any point within the loading range.  If the end user cannot always load the reference standard to capacity and wants a smaller LLF, they will need to have the load cell tested with several hysteresis loops for every capacity they wish to calibrate.

Direct text from ASTM E74-13a
NOTE 6—For any force-measuring instrument, the errors observed at corresponding forces taken first by increasing the force to any given test force and then by decreasing the force to that test force may not agree. Force-measuring instruments are usually used under increasing forces, but if a force-measuring instrument is to be used under decreasing force, it should be calibrated under decreasing forces as well as under increasing force. Use the procedures for calibration and analysis of data given in Sections 7 and 8 except where otherwise noted. When a force measuring device is calibrated with both increasing and decreasing forces, it is recommended that the same force increments be applied, but that separate calibration equations be developed. 


Direct text from ASTM E4-15
NOTE 4—For any testing machine the errors observed at corresponding forces taken first by increasing the force to any given test force and then by decreasing the force to that test force, may not agree. Testing machines are usually used under increasing forces, but if a testing machine is to be used under decreasing forcesit should be calibrated under decreasing forces as well as under increasing forces. 


Example:  On our 1,000,000 lbf standard calibrated by N.I.S.T, the ascending LLF is around 11 lbf and the descending LLF is around 12 lbf.  Since we cannot always descend from 1,000,000 lbf, we must use the combined curve that has an LLF of around 58 lbf.    


Page from N.I.S.T. Calibration Report for our 1,000,000 lbf load cell
Dropping off our reference 1,000,000 lbf load cell on May 10, 2016