History of Belt Weighing
Preface
This is an introduction to a series of largely anecdotal comments concerning beltweighers (or belt-conveyor scales in US terminology). Be aware that facts and opinions may be freely intermingled. Many of the comments rely on personal experience and are expresses in first person format as opposed to the third person conventional for technical presentation.
A little autobiographical information that may provide some insight into the mindset involved in the development of this discussion series. Although I was technically too young to be employed in the mines, I did some unofficial work for a small mining company during World War II. No pay, but I learned to weld. During high school I built an electric motor with nails and cotton covered wire (wish I still had it). After completing high school I attended a local junior college for two years with the intent of pursuing electrical engineering. Switched to metallurgical engineering with the intent of working in the iron mining industry. After graduation a couple years in the military teaching electronics. A little later a couple years in graduate school studying physics.
I went to work for a mining company in Northern Minnesota, assigned to develop process control techniques for mineral processing. The pilot plant included several Merrick Model E belt conveyor scales used to totalize the feed rate to the process.
That was the beginning of a relationship with beltweighers that has now extended for many years.
Prologue
Memories of the Merrick E (1960-1970)
The mineral processing plant where I was employed starting in 1959 had three parallel grinding lines. Each rod mill was fed by a belt containing a Merrick Model E belt conveyor scale. As used in this application, the beltweighers were calibrated using roller chains. No material tests were ever performed.
The processing lines operated continuously 24hours/day, 7days/week with an interruption of a few minutes every few days to add grinding rods. About each month, the beltweighers were tested with the roller chains.
The Merrick belt-conveyor scale was invented about 1908 and was a well thought-out invention. Most of the principles that were included in that design are still in use in beltweighers today. There were probably a number of different variations of this scale system but the most common was the Model E.
A common configuration used 4 idlers supported by two frames that were in turn suspended from a housing mounted above the belt. Each frame supporting the idlers was pivoted on one end using knife edges with the other end supported by a tension rod (“steelyard rod” in other scales) that was in turn connected to the weight sensing and integrating mechanism located above the belt. The two frames, each supporting two idlers, were arranged with the pivots on the ends and the weight sensing in the center. This arrangement was called “approach-retreat” and that term may have been copyrighted by Merrick.
A displacement float in a pool of mercury provided the weight sensing mechanism. As more weight was applied to the idlers and the sum transmitted to the lever system in the weigh housing, the displacement float would sink into the mercury pool until weight balance was achieved. This is a true mass-sensing device and not a force sensing device such as a strain-gage load cell.
A bend pulley mounted in contact with the belt conveyor on the return side sensed the belt travel. A belt about 20mm wide was driven by the bend pulley and the drive belt was in contact with the disk described below.
The mathematics, multiplication of mass times distance, was carried out using a disk about 200mm in diameter free to rotate on its own axis. The periphery of the disk contained a number of rollers about 10-12mm in diameter. The belt travel drive belt passed over these rollers. When the scale was in zero balance, the plane of the disk was normal to the motion of the drive belt so the rollers rotated on their bearings, but did not cause the disk to rotate. As increased mass on the conveyor belt caused the displacement float to be further immersed in the mercury pool, it caused the disk to tilt and therefore turn because of the angle between the drive belt and the rollers.
The Merrick E scale suspended the weight bearing part of the frame on knife edges. The knife edges were used on inclined belts as well as on horizontal belts.. The box containing the integration mechanics was always mounted horizontally above the belt and contained the mercury pool and displacement float for the weight balancing.
The rollers around the edge of the integrating disc needed periodic maintenance to keep the bearings in condition for free rotation of those rollers. This maintenance was performed annually on the scales in this facility and required about one day for each scale.