Matt Morrissey, Product Manager, Weighing Technology What drives you? Matching the right gearbox with the right weighfeeder Conveyors, although long considered to be the most basic of options for processing bulk solids, are actually very complex systems. Not only do they require a considerable amount of planning to ensure that the belts keep the material on them moving consistently, they must also be designed to endure in some of the most challenging environments in the industry. Conveyor systems are comprised of many individual parts that are interconnected in such a way that changing one could ultimately affect any one of the others. With over forty years experience with weighing technology, Siemens knows this better than most, and it shows in their integrated design of two important components of conveyor systems: weighfeeders and gearboxes. Weighfeeders A weighfeeder is a self-contained conveyor that controls the flow of material into or out of a process. Siemens produces many different types of weighfeeders, from light-duty units that use 10 cm (4") pulleys and are only 1 m (3 ft) long, to heavy-duty units as long as 20 m (65 ft), that use 40 cm (16 ) pulleys. There are two main differences between weighfeeders and conveyors: 1. Control A weighfeeder can provide a set material flow based on the motor speed and belt speed indication, as well as the load from the weighing components. They almost always have a flat belt, as this reduces the tensions and provide more reliable weighing repeatability. 2. Shearing When material is fed to a conveyor belt via an adjustable shear gate, a specific material profile and bed depth is created. Such action generates forces that are not usually present in a normal conveyor application, and must be considered when values for torque and power are calculated in order to ensure proper selection of the drive unit. 1
What drives you? Matching the right gearbox with the right weighfeeder Weighfeeders are very versatile devices. They are ideally suited for the batching or recipe control of bulk solids material beacuse a high accuracy of +/-0.5% can be achieved. In addition, production changes and different recipe ratios can easily be acheived through a change in belt speed or in bed depth (as set by the shear gate). What's more multiple units can all be controlled by a master recipe, or a master integrator, thus ensuring that quality control of end product is optimized and costs are controlled. Weighfeeders are driven by motors that are in turn regulated by gearboxes, a crucial component requiring the same attention to detail as high precision weighing components. Gearboxes A gearbox takes the higher revolutions from the drive motor and reduces them to an appropriate speed and torque. Often, the gearbox is a stand-alone unit, but when it is combined with a motor, it is called a gear-motor. When Siemens first started producing weighfeeders over forty years ago, the company borrowed from the accepted practice of the day by using an inline helical style gearbox mounted to the top of the unit and coupled through a chain to the drive shaft. As safety standards became more and more important, and guarding these chains (while still allowing for easy maintenance) became troublesome, a natural transition to a shaft mount version took place. This transition was facilitated in part by the fact that this kind of gearbox needed only a minor amount of additional space, had fewer pinch points, and required less maintenance. There are two types of these shaft mounted gearboxes: the helical worm and the helical bevel, both of which can be mounted on the foot, flange, or torque arm. The last kind of gearbox that is available but is very seldom used because of the amount of space needed is the parallel shaft unit. It is supplied on a customer requirement basis. Although the gearboxes listed here are different in the way they convert the drive power, they all have something in common in that they can all be either hollow bore or output shaft style. Hollow bore and output shaft style gearboxes These two styles are exactly as their names suggest. The hollowbore gearbox has a cylinder that fits directly over the drive shaft of the motor, and the output shaft gearbox is a metal rod that is directly connected to the drive shaft with a coupling. The output shaft option is ideal in situations where the pulley or bearings require frequent replacement. However, in most cases, good quality conveyor components will not fail that often, and the added cost as well as the need for increased adjustment have driven most customers to the hollow bore mount style. Mounting the hollow bore gearbox on the drive shaft provides the support it needs, as well as allowing for very simple installation and removal. A retaining ring inside the bore aligns the shaft, and a bolt with a large washer keeps the gearbox from walking off the shaft during rotation. Options for anti-rotation also make it very easy to design for this style or retrofit in an existing application. When faced with so many factors to consider, Siemens engineers use their extensive knowledge and expertise to examine the mechanical design along with the installation requirements, and match the right weighfeeder to the right gearbox. Helical worm unit Inline helical unit 2
The gearbox is a crucial component requiring the same attention to detail as high precision weighing components. Selecting the right gearbox When configuring the ideal weighfeeder and gearbox combination, engineers consider the following factors: 1. Application details such as: minimum torque (for normal running conditions as well as a cold start under full load conditions) flow rate belt width pulley size belt tension infeed opening material bed depth shear forces speed 2. Reduction ratios A gearbox takes the actual speed of the drive motor and converts it to the speed and torque values that are required by the application using a mathematical formula known as a reduction ratio. Since there are many different sizes of gearboxes available, it stands to reason that each one will require a different reduction ratio. For the record, the actual derived torque value should exceed the required value by a factor of 1.2 or 2, depending on the customers need or the internal design guidelines, with 1.2 being the accepted industry minimum. 3. Shaft size A weighfeeder's shaft is exposed to a number of different kinds of stress, so the required shaft size calculations must consider: the bending and tortional tensions on the shaft the stress concentrations on the varied shaft diameters the overhung load of the gearbox 4. Input The input is the size of the motor adaptors and hp ratings. These will determine the size of the gearbox that is needed. 5. Flange type The flange type on the gearbox must match the motor flange size and take into consideration whether it is NEMA or IEC based. 6. Mounting position The mounting position will affect the amount of oil required in the box and where the breather will be located. Parallel shaft unit Helical bevel unit 3
What drives you? Matching the right gearbox with the right weighfeeder Factory photo of weighfeeder gearbox prior to delivery to end user 7. Finish If the application environment is caustic, a special epoxy paint can be applied to the finish. Food grade options are also available. 8. Safety Safe use and protection are also aspects that must be included in the supply of a gearbox, as rotating parts and shafts need to be isolated from personnel in order to reduce the chance of entanglement. Caps and shaft guards are also considered, and many gearbox options can include a cover for the non-drive side of a hollow bore. Customer considerations The customer is responsible for a couple of factors, such as: 1. Motor and gearbox integration Although Siemens weighfeeders normally ship with an individuated gearbox and motor, there is room for customization as integrating the motor with the gearbox does have some advantages. For one, when combined, the size and weight of the gearbox is reduced, and can also be equipped with any of a number of options such as speed encoders and thermistors. In addition, a self-contained gear-motor can carry hazardous approvals for use in dangerous environments. When considering such a customization however, it is important to remember that there is also a disadvantage to using a combined unit: if the motor itself has to be changed, then the entire gear-motor will have to be replaced. This wouldn't be an issue with a separate gearbox with an input flange. 2. Maintenance Gearboxes have a small break-in period, and after 250 hours of operation the oil should be drained and re-filled. In weighfeeder applications, the oil will generally last for 2,500 hours in normal conditions (and even as long as 25,000 hours in ideal conditions), but this period will be reduced in situations where the temperature exceeds 70 C (158 F). The oil should be checked periodically to ensure it is not blackening prematurely, and is still at the proper fill level. Fresh oil is clear to the eye, and has a typical smell and a specific product color. Watch for these changes in the oil: Clouding or a flocculent appearance indicates water and / or contamination. A dark or black color indicates residue, serious thermal decomposition or contamination. Selecting the right supplier Almost as significant as choosing the right gearbox and weighfeeder combination for your application is finding the right supplier. It's common sense to look for a supplier who provides excellent support, offers the latest and most innovative technology, boasts a large distributor network, and supplies a comprehesive warranty. But, perhaps more importantly, you need a supplier who can expertly consider the options detailed in this article as they relate to your application needs, anticipate the long-term challenges, and deliver a complete integrated package that it fully compatible with your existing system. For example, consider the following example: A number of years back, a coal company in the United States approched Siemens in search of a weighing solution. At their coal plant in Carolina, they needed to accurately measure ambient temperature coal that was being sheared out of a bin at a size of ¼", with a bulk density of 50 lb/ft 3, and a 4
very low flow rate of only 4 MTPH (4 tph). This kind of environment required a robust solution with a modular design, and one that could accomodate the relatively slow flow rate. Ultimately, Siemens engineers installed six Sitrans WW310 weighfeeders that were each 480 cm (189") long, with a belt speed set (through the use of an appropriately configured gearbox) at a moderate 4 m/min (13 ft/min). That was seven years ago. Because of the attention to detail paid to the gearbox and how it interacted with all of the weighfeeder components, those six units have been performing so well that routine maintenance has been the only order of duty, and there have been no failures of any kind. Now, that's the kind of service and results that every company should expect. Summary Conveyor systems are anything but simple. There are a number of factors to take into consideration when designing, installing and maintaining one, especially when it comes to the all-important weighfeeder and gearbox combination. If customers choose their supplier carefully and select one that offers a complete weighing solution, then they can rest easy knowing that the many aspects listed in this article are being handled, and can instead focus on other aspects of their enterprise. Siemens AG Industry Sector Sensors and Communications 76181 KARLSRUHE GERMANY Subject to change without notice Available as pdf only Siemens AG 2014 The information provided in this article contains merely general descriptions or characteristics of performance which in case of actual use do not always apply as described or which may change as a result of further development of the products. An obligation to provide the respective characteristics shall only exist if expressly agreed in the terms of the contract. All product designations may be trademarks or product names of Siemens AG or supplier companies whose use by third parties for their own purposes violate the rights of the owners. 5