February 2005 - International Cement Review
Weighing Technology: "Weigh Belt Feeders”

A major cement company uses 20 Thayer Model MH heavy duty weigh feeders to meter various cement ingredients. For more than 30 years this company has been using Thayer heavy duty weigh belts feeders. Feed range from five to 400 TPH depending on material.

1. Slack belt design for more stable accuracy
   Thayer weigh belts are driven from the head pulley, which is located at the outlet end of the feeder. Rubber lagging on the pulley prevents belt slippage and the pulley is crowned to prevent belt tracking problems. The tail pulley, which is located at the inlet end, is an idling pulley. This configuration permits the conveyor to run under slack empty belt conditions, assuring lower belt tensions during operation that any other design. The belt is being “pulled” by the head pulley, rather than “pushed” from the tail pulley, thereby creating sufficient tension for good tracking without the need for a tensioned belt coming into the tail pulley. Low belt tension means better, more stable accuracy. Other weight belt designs go to great pains to maintain consistent belt tension simply because the tension has such a great affect on accuracy. Automatic belt tensioners and automatic belt tracking devices are often used. Thayer’s design eliminates the need for these devices and thus eliminates the maintenance headaches that can accompany them. The belt also lasts longer because it is running under low tension.
2. Speed sensor mounted at idling pulley
   An optical pulser measures speed at the idling tail pulley. This design measures the true speed of the belt via rotation of the idling pulley, not an inferred belt speed based on motor speed. The advantages of this design are that speed is measured accurately over the entire range of operation and belt slippage or breakage are immediately detected because the idling pulley is not rotating.
3. Scale located outside material handling area
   Thayer’s scale is not mounted directly underneath the belt but in a location outside of the material handling area such that an idler supporting the belt (the “weigh idler”) transmits the load to the scale. This design has several benefits. The scale is not prone to damage, is out of the way for cleaning and is not prone to tare build-up because material cannot fall onto the scale. The scale design easily adapts to an automatic test weight calibration mechanism. The test weight is also located outside the material handling area to avoid material build-up that would change the weight, causing incorrect calibration. Thayer’s scale can take high load directed overloads (1000%) that are caused by operating personnel or by the occasional particle pinching that will occur between the conveyor and the skirts.
4. Automatic test weight calibration
   More and more weigh feeders are being used in conjunction with statistical process control where performance records are routinely generated and delivered with the product as required by the customer’s quality assurance program. Such a record should contain a “validation of scale calibration” step to be truly meaningful as a quality assurance tool. Thayer’s automatic test weight mechanism provides a means for applying a known test weight to allow completely automatic calibration. The calibration sequence can be initiated via the weigh belt instrument keypad or via a contact closure. A self-checking software algorithm in the weigh belt instrumentation prevents erroneous calibration. Test weight calibration eliminates the need for test chains.
5. Load call utilization factor > 80%
   Because Thayer manufactures the load cell we can provide a load cell with a capacity that is closely matched to the actual maximum material weight. The tare weight of the weigh idler and belt are mass-counterbalanced. We design our scales such that the maximum material weight is always > 80% of scale capacity. This results in the best possible signal resolution. This flexibility is not possible with strain gage load cells which tend to be available in standard capacities of 10, 50, 100, 200, 500 lbs., etc. Thayer’s rugged load cell design assures reliable operation for the life of the weigh belt. Because load cells never fail, they are not a recommended spare part.
6. Screw adjustable idler alignment
   The idlers adjacent to the weigh idler are mounted on screws for easy alignment in the field. Thayer’s test weight calibration system allows the test weight to be applied, simulating actual belt loading conditions, while idlers are aligned. Idler alignment is critical to minimize transmission of any belt tension force to the scale. All idlers are mounted on individual CEMA brackets permitting removal to either side without major framework disassembly or belt removal.
7. Full length adjustable skirt boards
   The skirt boards on Thayer weigh belts are easily removable for cleaning and are adjustable to accommodate material of varying particle size. Installation of skirt boards along the entire length of the conveyor totally confines the material flow channel which helps to control dust. Skirt boards are tapered and flared fro m the inlet to the discharge to prevent pinching of material between the skirt and the belt. The skirt boards are located inside the belt flanges to provide two barriers to contain material and prevent spillage. Weigh belts that use only a high sidewall belt to contain material lose system integrity when a belt edge rips off. High sidewall belting is also less uniform in weight and more stiff causing greater variation in the weight signal.
8. Welded frame, quick removal access panels
   Thayer weigh belts are of welded, not bolted, construction. This provides a more rigid frame. Frame distortion adversely affects scale measurement performance. The scale needs a rigid mechanical “ground” from which to deflect. Side panels, if included, are removed with quick-release fasteners (no tools required).