Vibra Screw has been designing batch and continuous gravimetric control systems for decades. We were quick to adopt useful technology by incorporating digital and graphics displays, as well as an ever-growing number of I/O interfaces into our products. As expected, customer response to these improvements was overwhelmingly favorable.
However, all gravimetric control systems, no matter how fancy the user interface, measure weight using a transducer called a load cell. (See Figure 1) The load cell converts mechanical weight into an electrical signal. The most common type of load cell employs a strain gauge transducer (See Figure 2) in a Wheatstone bridge and can be found in devices ranging from deli scales to systems designed to process solid rocket fuel.
While computer technology has enabled high tech gravimetric control systems to evolve significantly over the last 30 years, a load cell is still the basic workhorse at their core. And, though it may be hard to believe, the strain gauge load cell technology got its start in the late 1800’s. In 1843 Sir Charles Wheatstone perfected and popularized his Wheatstone bridge. (See Figure 3) A resistance change in the bridge produces a proportional change in voltage across the bridge. Then almost 100 years later, in 1930’s, Edward E. Simmons and Arthur C. Ruge invented a device that could measure strain in an object.
It wasn’t long before the strain gauge and Wheatstone bridge technologies were combined and the “load cell” was born. This transducer changes mechanical energy into a proportional electrical signal. Even with the invention of the transistor following World War II, the load cell still found it’s primary application in laboratory test instrumentation. The small signal created by the load cell wasn’t ready for commercial/process gravimetric control systems. Lever scale and balance remained the preferred method of measuring weight.
Finally, in the computer age of the late 20th century, electronic technology had advanced enough to accurately measure small signal changes and the load cell started to replace the balance and mechanical lever scales used in processing industry. Even in today’s cutting edge technology, over 170 years later, the strain gauge load cell continues to be the preferred transducer used for process weighing and gravimetric control and the one used by Vibra Screw.
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For over a half-century, Vibra Screw has been a major supplier of hoppering and feeding equipment used to make much of the United States solid rocket fuel and ordnance. Vibra Screw vibrating systems, without mechanical agitation or conveying, are ideal for safely discharging and moving solid rocket propellants and explosives. Vibra Screw’s Bin Activator’s and Feeders and over 60 years of bulk material handling experience provide proven solutions.
Hercules Aerospace, now Alliant Techsystems [ATK], constructed a solid rocket fuel manufacturing facility in Utah to process solid fuel for the Space Shuttle and nearly all the nation’s other launch vehicles and strategic missiles. It required precise weighing of the ingredients that make up the fuel. Design criteria included batch accuracies of 0.05%, safety factors that permit remote operation and do not require periodic recalibration and self-cleaning characteristics that permit total elimination of residue between cycles. Vibra Screw was awarded the contract to provide the turnkey system. Based entirely on scale mounted Bin Activated Bins and Vibrating Tube conveyors to deliver precise weighed batches of propellant ingredients, the design was the first time screw-less conveyors had been used for these materials, creating a much simpler and safer operation.
Thiokol Corporation, also now part of ATK, has used Vibra Screw equipment continuously since the 1970’s for similar propellant production. The unusually sticky nature of some of their materials led to Vibra Screw developing a cleaner profile Bin Activator, without internal cross bracing, that greatly enhanced flow from storage while reducing welds in the material contact areas.
The nation’s major ammunition depots have also used Vibra Screw equipment for decades. Again, the simplicity of vibration-only movement of dangerous material provided many safety benefits when handling explosive powders.
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You can’t drive your car forever with the same oil, brakes, muffler and tires. Periodically it will need to be serviced. Solids processing equipment is no different. It needs to be properly maintained in order to perform within its design parameters. And, eventually certain parts will need to be replaced. Maintaining a spare parts inventory is often considered a luxury you can’t afford. But stop to consider how much downtown you can afford if a critical part fails. There is no perfect answer but here are some insights into some different types of equipment and parts.
In the best of conditions, solids processing equipment like vibrating bins, bin activators, vibrating conveyors and screw feeders, are subject to stresses that will, over time, cause critical part failure. Even if the part in question is a standard item and readily available, you need to gauge the consequence of the downtime resulting from ordering, shipping and installing that part. What if that part requires custom manufacturing? How much downtown is too much? At what point should you consider having a replacement in your spare parts inventory. It is a question you should be asking before a part fails.
Bin Activators have two critical parts that will need to be replaced and we recommend you have spares.
- Elastomer seals between the bin and the bin activators
- Isolators for your bin activator hangers
After two years of service these parts should be carefully inspected. This inspection should then be repeated every six months. Obvious signs of degradation are checking and cracking. Also examine your isolators for sag. If signs of fatigue or wear are detected, these parts should be replaced. If you have spare parts on hand, be sure to inspect them carefully before installing them. Elastomer parts in extended storage can be compromised. Nevertheless, the cost of the part vs. the cost of the downtime suggests having spares is a wise decision.
You might think that the Gyrators on vibratory equipment are subject to a high rate of failure due to what is perceived as their high-stress function. The fact is, in continuous use situations, they are subject to very little stress. Gyrators are oil lubricated and with proper maintenance they can last years. Simply conduct the recommended inspections every 1000 hours of operation and change the oil every 6000 hours. Like a well maintained car, you can get years of trouble-free service following a simple maintenance routine. When processing is not continuous and Gyrators are frequently started and stopped, stresses on the Gyrators are increased and these inspections become more important.
There are also parts that are custom engineered and built specifically for the equipment they help operate. For example, most feeder screws are unique to the feeder they are shipped with. If the equipment is operated according to manufacturer’s guidelines, the screw can last longer than the feeder. But in real world situations, screw failure is all too common and when they break, there is no quick fix. Again, you have to weigh the cost of having a spare on hand vs. the downtime you will experience having to order a custom-engineered part.
So…to borrow a tried and true motto, be prepared. Consider what equipment you are operating, how it is being used and your tolerance for downtime. Refer to your manufacturer’s recommended spare parts list. If you have questions consult your equipment manufacturers and get their recommendations to help find the balance between the investment you are able to make in spare parts and the risk you are willing to take. Our experience shows that you should have some spare parts on site. It will pay off some day.
Picture what happens when you pour a mixture of stone and sand on the ground. A rough cone forms, with stones mainly around the outside and sand mainly in the center. The stones tend to roll down the surface of the cone as it is formed, while the sand sifts down closer to the center. Segregation has taken place.
The same thing can happen in storage bins with a material mixture of differing particle sizes. In a conventional bin [Fig. 1], materials separate as it is loaded. On discharge, finer material emerges from the center of the bin first, down a pipe and coarse material last. In a production process requiring a uniform blend, product quality suffers and the downtime from rejected batches can be costly.
When storage bins are equipped with Bin Activators, the segregation problem can be eliminated. [Fig. 2]. Vibration keeps material mobile and free-flowing, particularly at points of potential bridging. An integral baffle directly over the outlet prevents material from emptying down a pipe in the middle first and then off the walls as in conventional bins. Because of the baffle, the pipe never gets a chance to form. Instead material draws evenly across the bin cross section. And the final discharge is a combination or remix of materials from the center and those from nearer the walls. This flow pattern is called mass flow.
With Bin Activators, larger quantities can be bulk stored because the risk of segregation no longer limits capacity. Management gains more flexibility in plant design for example with fewer bins and conveyors required. That can lead to improved production and important cost savings.
Non-free-flowing dry solid materials are a challenge to feed consistently and uniformly in volumetric & gravimetric screw feeders. Materials that tend to be readily adhesive (TiO2, foundry sand, pigments) or become easily fluidized (hydrated lime, cement, talcum powder, confectionary sugar, fly ash) can be quite difficult to feed. The simple and proven solution is the introduction of “controlled vibration” as a flow aid. Vibrating screw feeders are able to condition difficult materials to a uniform density and thereby ensure smooth movement and positive discharge throughout the feed process.
The operating principle of vibrated screw feeder is comparable to the repetitive filling and emptying of cups:
1) The most accurate filling occurs when the cup is filled with material.
2) When vibration is introduced, the material in the cup reaches a uniform density.
3) The excess material is stuck off.
4) While still vibrating, a precise amount of material is completely released from the cup.
At a much larger scale, the same process takes place in a vibrated screw feeder.
1) Material fills the screw flights in the trough area.
2) It is vibrated to a uniform density.
3) The material is struck off as it enters the metering tube.
4) Continued vibration of the metering tube and screw ensures complete release of the material at discharge.
It is extremely important to work with an experienced and knowledgeable manufacturer of Screw Feeders who will take into account the unique characteristics of your material to design and build equipment that will meet the critical requirements of your application.
Since its introduction over 50 years ago, the Bin Activator has become the most widely used bin discharge device in the process industries. It can be used on new or existing bins, and it will discharge virtually any dry material. In most cases, a successful Bin Activator installation is simply a matter of picking the right size Activator for a given bin diameter.
There are over 100,000 Bin Activators in service worldwide and we have a tested materials data base of 8000 dry solids. This experience enables us to properly size Bin Activators by measuring a few basic characteristics. If we know bulk density, moisture content, slide angle, particle size, compressibility, and cohesiveness, we can place the material in one of four general classifications. Then with bin dimensions we can select the proper size Bin Activator.
Class I materials are granular and would normally flow unassisted. But with changes in moisture or temperature they may become flow resistant and require a small Bin Activator. Granular salt, whole grains and plastic pellets fall into this category. Bin Activators should be 1/4 to 1/3 of the bin diameter.
Class II materials are the sluggish powders which would not normally flow by gravity alone. Examples would be flour, starch, powdered sugar, and most plastic powders. Bin Activators should be 1/2 to 2/3 the diameter of the bin.
Class III materials are some of the most challenging to discharge. They can be either readily adhesive or easily fluidized. Sometimes, in the case of cement or hydrated lime they can act either way. A product that that is too easily fluidized will be nearly impossible to control after it leaves the bin. The job of the Bin Activator, then, is to overcome the strong tendency of this material to bridge and pack while at the same time de-aerating it on discharge. Bin Activators for Class III materials should be ½ to 2/3 the bin diameter.
Class IV materials are fibrous or flaky with a relatively low bulk density. They typically have a large particle size and tend to interlock and absorb vibration. Examples of Class IV materials would be wood chips, shavings, plastic scrap and chopped fiberglass. Bin Activators should be as large as the bin will accept.
This information is summarized in the table below. The table also indicates expected discharge rates for a given outlet size with each of the 4 material classifications.
So if you know the bin diameter and the material class for the product you are storing, you can easily and confidently select the proper size Bin Activator for your application.
Vibra Screw Inc. has substantially expanded its vibratory feeder and conveyor lines through an exclusive license agreement with Mavi Maquinas Vibratorias Ltda, Brazil. The agreement, covering North America, gives Vibra Screw access to Mavi’s 50 years of experience which has made it the largest supplier of vibrating feeders and conveyors in South America.
Using the latest resonant conveying technology, Vibra Screw can now offer vibrating conveyors up to 50′ long and rates up to 8000 ft3/hr. The agreement also expands Vibra Screw’s range of vibrating spiral elevators, vibrating pan and tube feeder, and vibrating screens. With the right to manufacture all Mavi products in the USA, Vibra Screw will be able to offer their proven technology in U.S. made products.
|Bin Activators are one of the most successful devices for ensuring flow of bulk material from storage. They can range in diameter from a couple of feet to 18 feet. Bin Activators can be extremely heavy, and while being subjected to vibration, must support the head load created by additional tons of material in the silo. Making a mistake in the design or material used in the hangers that support them can have catastrophic consequences.
Hangers perform two critical functions. First, they must safely carry the Activator and silo head load. Second, they must provide flexibility of movement under vibration and prevent transmission of that vibration to the stationery silo structure. Accordingly, they must be incredibly strong in tension yet free to move horizontally. Our experience has proven that forged steel hangers combined with isolator bushings provide the best uniform tensile properties and strength. At Vibra Screw we produced both with our own dies and molds.
Smaller Activators use smaller forged hangers combined with a lower durometer isolator. Larger activators use larger forged hangers and progressively higher durometer isolators. The critical calculation is to design for each specific load situation with the right choice of forging size and isolator stiffness. The correct design will maximize strength while minimizing vibration transmission. It may sound simple but for Activators to operate within optimal parameters hangers are a critical component.
When applications require even more load carrying ability, simply adding additional standard hangers is not always possible. Our R&D engineers have designed a cable hanger similar in design to the cables used on the world’s largest suspension bridges. The cable hanger provides excellent horizontal flexibility and vibration isolation, and up to 10 times the load capacity of our strongest regular hangers.
Our experience has shown the hanger designs we’ve described here are far superior to those using ordinary bar stock, threaded rod or even castings for vertical support combined with rudimentary off-the-shelf isolators.
Beyond the nuts and bolts of installing a Bin Activator, a carefully designed hanger system is essential. Vibra Screw’s 60 years of experience, from invention of the Bin Activator to development of its present design, ensures the safety and integrity of each system.
If you’re a process engineer involved in handling dry solid materials, this blog is for you. Here at Vibra Screw, we have over 60 years experience in designing and manufacturing high quality equipment to store, discharge, meter, convey, size and blend. In that time, we’ve learned quite a lot about this business. We’ve learned the best way to move materials from storage to process and we’ve learned about all the other ways. Now we’ve created this blog to share some of what we’ve learned along with some interesting things that are happening here at Vibra Screw. We invite you to stop back frequently as we will be sharing useful and interesting information.
We’re also pleased to let you know we now have social media presence on Linkedin, Facebook, Twitter, GooglePlus and YouTube. This blog will be fed to our social media profiles so you’ll be able to read it where it’s most convenient for you. Please feel free to share with your colleagues whatever you find of interest or value.