Machinery: Industrial Machinery News

Precision Weigh Scale load cells must be designed in a rugged manner to withstand the normal abuses of daily operation in sometimes less than desirable situations, while maintaining a high degree of reliability and conformance to strict specifications. Normally, load cells falling into this category must adhere to performance criteria determined by the U.S. Weights and Measures and the OIML.

Universal Load Cells are the most common in industry, and can be found working in industrial environments ranging from the laboratory to harsh production line application. Universal load cells are designed to tolerate extremes of temperature, off-axis loading and dirty environments that often occur during the working lifetime of the transducer. Their force capacities range from a few pounds in the smallest to a few million pounds measuring force in the largest. As with any transducer, the most important criteria for proper usage and to guarantee the longest possible working life, proper attention must be given to working capacity and loading.

A good rule of practice to determine proper working capacity is to predetermine the maximum expected force that the load cell will be subjected to, including all vibration induced spikes and keep that total force within the published working capacity of the load cell. Any electrical overload capability of the load cell should be reserved as protection against those excess forces acting upon the load cell that were initially unforeseen.

Proper loading is important not only to the life of the load cell, but also to the integrity of the measurement. Structural crosstalk of varying degrees of severity is the normal result of improper loading. As with any mechanical arrangement, there should never be more or never less, than 2 planes of axial freedom in measuring plane.

With load cells this is easily accomplished by the use of self-aligning swivel bearing for tension applications and load buttons or expansion plates in compression applications. Because of the relative stiffness a strain gage load cell has, consideration must also be made for inadvertent loading due to the thermal expansion of the surrounding loading fixtures. Once again, proper coupling procedure can eliminate this problem.

Fatigue Rated Load Cell
The fatigue rated load cell is a special structure designed to withstand millions of cycles while maintaining its integrity and accuracy. The structural design typically consists of a shear beam or columnar design. A fatigue rated load cell is also extremely resistant to extraneous bending and side loading forces.

These structures virtually eliminate bending strains at the strain gage, minimizing the primary cause of load cell failure. A wide range of capacities can be handled by the Fatigue Rated load cell family and have now become the standard in the high frequency load testing industry.

Since these load cells are often used in long term Fatigue tests, they also are available with Dual Bridges as a backup or used as a second primary bridge.

Selecting The Proper Load Cell

A) To determine the proper capacity:

1) What is the maximum force value
2) What are the dynamics of the system, i.e., frequency response
3) What effect will placing the transducer in the force path have
4) What are the maximum extraneous loads that the load cell will see

B) Evaluate the system in which the load cell is to be placed:
1)Will the load cell be in the primary load path or will the load cell see the forces indirectly
2) Are there physical constraints that should be met for size and mounting

C) What accuracy is required to meet the goals of the test

D) What is the environment the load cell will be in and will this cause special problems? Keeping in mind the above suggestions, choosing the proper load cell should be made easier. All Load Cells are calibrated at Lebow and are traceable to the National Bureau of Standards. This calibration is accomplished in two manners:

1) Deadweight, all deadweight calibrations are performed in English force units to 5000 lbs.

2) Computer Calibration, all computer calibrations are done in conjunction with a reference load cell. Most calibrations are done in English force units; however, Metric Ton calibrations are available on selected Load Cells upon request.

Output At Rated Capacity Considerations
Load cell output is specified in millivolts per volt at rated capacity. A typical Lebow load cell will have an output specification of 2 millivolts per volt nominal. Certain model load cells have full scale output trimmed to within a specified tolerance limit, for example: 2 +/- 0.25% Mv/V. This tolerance applies to the output in the primary direction of the load cell.

For Universal Load cells listed in this catalog, tension will be considered the primary axis. A specified output tolerance in a non-primary direction can be met on individual request. Safety Considerations It would be unsafe to operate Lebow Torque Sensors and Load Cells beyond Static Overload or Ultimate Extraneous Load limits as defined in the Glossary of Terms or when applicable, higher than maximum speed. When in doubt consult the factory. Lebow Products is not responsible for any property damage or personal injury which may result because of the misapplication of the Lebow Transducers.

Dimensions And Specifications Metric dimensions and specifications are purely mathematical calculations from standard english dimension control drawings. Request certified drawings before designing mountings or fixtures. Dimensions and specifications are subject to change without notice.

Types And Applications For Weighing
Checkweigh, parts counting, and other platform type weigh scales demand precision measurements with reliable daily performance. Typical problems solved include moisture, shock or impact loading, off-axis loading and repeated static overloads in tension or compression. Lebow designs precision weigh scale load cells to meet the rugged requirements of Handbook 44 and OIML.

Tank weighing can easily be performed with simple-to-maintain electronic instrumentation and precision load cells. Large tanks may be weighed in either tension or compression modes and, depending upon the application, 1, 2, 3, or 4 load cells can be used for precise weigh information. Load cells used in parallel multiples can be calibrated and summed as a single load cell thereby requiring only a single signal conditioning instrument.

Conversions of existing mechanical scales are easily and inexpensively made with Lebow precision load cells. Steelyard rod mechanical scales, often found in factory shipping and receiving docks, can be upgraded to meet the requirements of Handbook 44 and OIML without interfering with the mechanical weighing components of the scale.
For Industry

Among the most common industrial testing applications is the load cell placed in series with a standard hydraulic actuator. The work performed by the actuator can then be monitored and controlled by a closed-loop servo system using precise force measurement output from the load cell.

Fatigue testing material specimens in a precise, controlled manner is an often cited application for load cells. The Lebow complete line of fatigue rated transducers assures the integrity of the test information from the first cycle to the last. True fatigue application demands full tension/compression operation, thru zero, with full signal outputs and high reliability against costly breakage during the test cycle.

Components testing is a must with today’s emphasis on quality control and product safety. Lebow load cells, with over 30 years of design and application history, can be utilized in an almost endless variety of configurations and capacities. Our standard off-the-shelf model probably will fit your special, one-of-a-kind application.

For Special Purpose
Life cycle testing of mechanical systems can be precisely controlled with the use of load cells. Mechanically integrated into the stress path, a Lebow fatigue rated transducer will enable the user to control and document the forces involved in the life cycle test of a product or mechanism.

Special applications comprise a significant portion of Lebow transducer files. Cutting tool forces involving up to 3 components are just one area where the solution to your special measurement problem may have already been designed and can be tailored to meet your criteria.

Load cells mounted between ground and the casing of a trunnion bearing motoring/absorbing dynamometer have been a fundamental measurement technique for years. Lebow manufactures a complete line of general purpose/fatigue load cells to suit the adverse conditions inherent in dyno lab testing. Most load cells can also be fitted with a patented overload protector for added safety and load cell overranging protection.