Weight Plays A Critical Role In Most Industries

By Author: John Tang
Total Articles: 507
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Throughout most industrial environments, weight plays a critical role. Many commodities are purchased by weight. Most mixing, blending, and formulating processes are weight dependent. Shipping and receiving is weight related. And sales are often based on weight.

The type of industrial scale that is used to determine weight is associated with the maximum amount that is being weighed on the scale at any one time. For very small amounts, a laboratory balance would be most suitable. Bench scales would be used for ranges up to about 100 lb. Platform scales are appropriate up to 1000 lb, and large floor scales can have capacities from 2500 lb to more than 30,000 lb.

The most popular range of scale requirements is generally in the area of 500 lb to 1000 lb. Historically this was fulfilled by mechanical beam scales or dial scales. In both cases, a mechanical understructure included a set of pivots and bearings in each corner of the scale platform that transferred the load to a central steelyard rod. The rod ran through the center of the scale post, and connected either to a sliding weight arm, or a spring dial. Typically, ...
... a scale of this type could resolve weight into increments of about ½ lb for a 1000 lb capacity.

There are a number of disadvantages of this type of scale. First of all, there are a significant number of moving parts, including the pivots, bearings, hangers, linkages, lever arms and couplings. All of these parts wear during use and cause inaccuracies. They are also prone to friction errors caused by dirt collecting within the mechanism. Furthermore, shock loading of the scale could cause deformations in the bearing surfaces. Also, mechanical scales are much more prone to user error. It is often difficult to read the weight from a sliding arm. And it is easy to make a mistake when viewing a dial scale.

Electronic scales will not automatically solve all of the problems. Many digital scales are simply a load cell and a digital indicator coupled to an old style mechanical scale. Instead of a dial, the steelyard rod is attached to an S type load cell. The electronic readout is fixed to the top of the scale post. This type of digital scale will not have the same user error issues, as the weight will be unambiguous and easy to read. But all of the mechanical problems noted for beam and dial scales will still be present.

Another approach is to completely eliminate the mechanical lever system, and use a very large bending beam type load cell in the center of the scale platform. That removes all of the issues of mechanical errors form the lever system and bearings. But it limits the size of the scale platform. If the platform is too large, and the load is not exactly centered on the platform, there will be a large twisting, or torsional component of force that will act on the load cell. This will cause accuracy errors, and there will be too much movement in the platform. Even for more moderate sized platforms, the profile of the scale will be fairly high, as the large load cell has significant height. This can make it much more difficult to place the load onto the scale.

A superior approach is used by Arlyn Scales in the design of their platform scales. Instead of a single large load cell, four advanced design shear beam load cells are used. One is placed in each corner of the scale. These load cells can be of much lower profile, as they do not have to resist side loads. For even better performance, the load cells are made out of stainless steel, instead of aluminum that is often used for bending beam load cells. This provides added overload and shock load capability.

The load cell outputs must be carefully matched, so that the same correct reading will be obtained no matter where the load is placed on the platform. One method to accomplish this is to use a summation board, with trimmer resistors used to match the outputs. Another method is to pre-match the load cells for each scale, so they only need to be wired together. This avoids the additional parts and errors that can come from changes in the trimmer adjustment.

The result of this design can be seen in the Arlyn Series 3200 Platform Scales. They are available with capacities of 500 lb and 1000 lb. Platform sizes range from 20 x 27 up to 27 x 60. Even larger custom sizes are manufactured. The platform height is only 1 7/8. The digital display can show increments as small as 1/10 of a lb.