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230 Feeding and Orientation Devices FIGURE 7.3 Plan for automatic weighing machine for granular material. one position above the rotor. This hopper has gate 8 controlled by two electromagnets 9 and 10, which receive commands from control unit 12 connected to force sensors 4. An empty pocket 5 with bowl 6 stops under sleeve 11. At this moment, force sensor 4 produces a signal through control unit 12 which actuates electromagnet 9 to open gate 8. When the weight of the material reaches the value the scale is set for, sensor 4 pro- duces another command to energize electromagnet 10 and close the gate. At this moment the rotor rotates for one pitch, putting the next empty pocket under the hopper. The filled pockets may then be handled and used for specific purposes. We have just considered an interrupted feeding process. Belt conveyors, which are useful for a wide range of capacities, are often used for continuous feeding of granu- lated matter. An effective feeding tool is the vibrating conveyer described in Chapter 6. By changing the vibrational amplitudes or frequency, the feeding speed can be tuned very accurately. The last mechanism we consider for feeding this kind of material is the auger or screw conveyor, a design for which is presented in Figure 7.4. Screw 1 rotates on its FIGURE 7.4 Screw conveyor for feeding granular material. 7.3 Feeding of Strips, Rods, Wires, Ribbons, Etc. 231 shaft 2 which is driven by motor 3 via transmission 4 (here a belt transmission is shown). The screw is located inside tubular housing 5, which has inlet and outlet sleeves 6 and 7, respectively. The material is poured into sleeve 6 and due to rotation of the screw, is led to sleeve 7 where it exits for subsequent use or distribution. Obviously, the speed of the screw's rotation defines the rate of consumption of the material. 7.3 Feeding of Strips, Rods, Wires, Ribbons, Etc. Linear materials are often used in manufacturing. Their advantage is that they are intrinsically oriented. (We will discuss orientation problems later.) Thus, the feeding operation requires relatively simple manipulations. Indeed, in unwinding wire from the coil it is supplied on, only one point on this wire needs to be determined to com- pletely define its position. Thus, an effective technical solution for feeding this kind of material is two rollers gripping the wire (strip, rod, etc.), from two sides and pulling or pushing it by means of the frictional forces developed between them and the mater- ial. We have already used this approach in examples considered in Chapter 2 (for example, Figures 2.2 and 2.4). Continuous rotation of the rollers provides, of course, continuous feeding of the material, which is effective for continuous manufacturing processes. However, for a periodical manufacturing process, feeding must be inter- rupted. One way to do this is based on the use of a separate drive controlled by the main controller of the machine. Such an example was discussed in Chapter 2. When the feeding time is a small fraction of the whole period, this solution is preferable. When the feeding time is close to the period time, the solution presented in Figure 7.5 may be proposed. Here, lower roller 1 is always driven, and upper roller 2 is pressed against roller 1 by force Fto produce the friction required to pull material 3. The force F can be produced by a spring or weight. (The latter needs more room but does not depend on time and maintains a constant force.) Roller 1 has a disc-like cam 4, which protrudes from the roller's surface for a definite angle 0. Thus, during part of the rota- tion of the driving roller 1, i.e., that corresponding to angle 0, upper roller 2 will be dis- connected from the wire (rod, strip, etc.) 3, and the mechanism will therefore stop FIGURE 7.5 Frictional roller device for continuous feeding of wires. 232 Feeding and Orientation Devices pulling or feeding the material. Obviously, other means to disconnect the roller are available; for instance, a mechanism to lift slider 5. Another sort of device for interrupted feeding of materials is also based on creat- ing frictional forces; however, feeding is done by pure pulling and pushing of the mate- rials. Let us consider the scheme in Figure 7.6. Here, lever 1 is pressed by force Q against strip 3 by means of spring 2. Strip 3 is clamped between the lever and surface 4. Due to this pressure, frictional forces F occur at points A and A' (we assume that the net forces acting on the surfaces can be considered at these points). Quantitative relations between the forces are derived from the following equilibrium equations written with respect to lever 1: Here n = frictional coefficient between the materials of the strip and of the lever at point A. We assume that the same condition exists at point A'. The four Equations (7.1) contain four unknown quantities: N, N 0 , F, and F 0 . By substituting Equation 4 into Equa- tion 3 we obtain By substituting Equation (7.2), into the first equation, we obtain From Equations (2) and (4) it follows that The derived results reveal a very important fact: when FIGURE 7.6 Frictional clamping device (lever type). 7.3 Feeding of Strips, Rods, Wires, Ribbons, Etc. 233 no spring (no force Q) is needed—the system is self-locking. The harder we try to pull the strip, the stronger it will be clamped. The force the device applies to the strip equals 2F because there are two contact points A and A' where the strip is caught, and fric- tional forces F affect the strip from both sides. The structure shown in Figure 7.7 works analogously. Here, strip 1 is clamped between surface 2 and roller 3. To produce clamping forces, the roller is pushed by force N c (due to a spring not shown in the figure). The equilibrium equations with respect to the immobile rollers 3 have the following forms: Pay attention to inequalities 3 and 4 in the latter system of equations. The friction force at a point "B" is determined by the pulling force developed by the device, while the friction force at a point "A" fits the equilibrium of all the components of the force. We assume that the frictional coefficients at points A, B, and C are identical. The unknown forces here are F A , N A , F B , and N B . Substituting Equations 3 and 4 into Equa- tions 1 and 2, we obtain From this it follows that and Finally, we have FIGURE 7.7 Frictional clamping device (roller type). 234 Feeding and Orientation Devices Obviously, when self-locking occurs, and no N c force (no spring) is needed to lock the strip, wire. etc. The devices in Figures 7.6 and 7.7 must be designed so that they do not reach the self-locking state, to ensure easy release of the material when the direction of the applied force is changed. Thus, the relations usually should be The principles described above allow an effective feeder to be designed. A possi- ble layout is shown in Figure 7.8. Here, two identical units I and II work in concert so that one (say, I) is immobile and the other carries out reciprocating movement, with the length L of a stroke equal to the length L of the fed section of the strip, etc. Each unit consists of housing 1, two rollers 2 pressed against inclined surfaces inside the housing, and spring 3 exerting force N c . The housings have holes through which the strip, ribbon, etc., passes. How does this device act? First, unit II moves to the right. Then the material is clamped in it due to the direction of the frictional force acting on the rollers, while in unit I the material (for the same reason) stays unlocked and its movement is not restricted. As a result, the material is pulled through unit I while clamped by unit II. Afterwards, unit II moves backward the same distance. This time, the frictional forces are directed so that unit I clamps the material and resists its move- ment to the left. Unit II is now unlocked and slides along the strip as it moves. At the end of the leftward stroke, the device is ready for the next cycle. In the cross section A-A in Figure 7.8 another version of the clamps is shown. Here, instead of two rollers (which are convenient for gripping flat materials), three balls in a cylindrical housing are shown. This solution is used when materials with a circular cross section (wires, rods, etc.) are fed. Finally, we show another strip-feeding device which is suitable when the time r during which the material is stopped is relatively short in comparison to the period T; that is, T»T. The mechanism is shown in Figure 7.9a) and consists of a linkage and 7.4 Feeding of Oriented Parts from Magazines 235 FIGURE 7.9 a) Geared linkage as a drive for roller friction feeder for interrupted feeding; b) Speed and angle changes versus time, with this device. gears. Crank 1 is a geared wheel, rotating around immobile center O^ whose geomet- rical center A serves as a joint for connecting rod 2. The latter drives lever 3. A block of gear wheels 4 and 5 is assembled on joint B. Wheel 5 is engaged with driven wheel 6. The sum of the links' and wheels' rotation speeds (when the tooth numbers are chosen properly) allows this mechanism to have a variable ratio o} G /o) lt which is shown graphically in Figure 7.9b). During rotation interval At, wheel 6 is almost immobile (the backlash that always exists in gear engagement makes this stop practically absolute). Imagine now strip 7 fed by rollers 8 driven by wheel 6, and you have an inter- rupted feeding, although driving link 1 is always rotating. Because of the smooth speed and displacement curves, the dynamics of this mechanism are rather good. 7.4 Feeding of Oriented Parts from Magazines There are essentially two approaches to the parts-feeding problem: first, feeding of previously oriented parts; second, feeding from a bulk supply. We begin with the first: feeding of the previously oriented parts. For this purpose some classical solutions and several subapproaches exist. They will be discussed here on the basis of some practical examples. Example 1 Electronic elements such as resistors, capacitors, and some types of diodes are shaped as shown in Figure 7.10a). To make the feeding of these parts effective, they are 236 Feeding and Orientation Devices FIGURE 7.10 Separate parts arranged for automatic feeding in a band-like form, by means of tapes. assembled into a band by means of tapes or plastic ribbons 1 (Figure 7.10b). The leads 2 of the resistors 3 are glued between two tapes, making a band convenient for storage (wound on a coil), for transportation to the working position of an automatic machine, and for automatic feeding. Obviously, additional orientation of the resistors is unim- portant. It is relatively easy to bring them to the appropriate position accurately enough so that a gripper or other tool can handle them. Example 2 Very often in mass production, parts are stamped out from metal or plastic strips or ribbons. To make them convenient for further processing, the following method can be used. Let us consider a detail made of a thin metal strip, as shown in Figure 7.1 la). It can also be handled in a band form; however, in this case the procedure is simpler because this form can be made directly by stamping a strip (without additional effort). FIGURE 7.11 Stamping sequence to make a product convenient for automatic handling, a) Final product—a contact bar of an electromagnetic relay; b) Intermediate processing stages; c) Cross section of the contact rivets. 7.4 Feeding of Oriented Parts from Magazines 237 Figure 7.1 Ib) shows how this can be done for a contact bar of an electromagnetic relay. Platinum-iridium contacts are riveted in the two small openings in the split end of the bar (see cross section in Figure 7.lie)). This riveting is much more convenient to do while the bars are together in a band-like structure, as in the illustration. Strip 1 is introduced into the stamp. It has a certain width b and is guided into the tool by sup- ports 2. At line A the openings (blackened in the illustration) are cut. In the next step the split end of the bar is shaped and next the lower end is completed. Thus, section LJ is needed to produce the bar. From line B the band-like semiproduct is ready. However, the bars are kept connected by two cross-pieces 3 and 4. The contact is riveted in section L,, either on the same or another machine. An example of this process is explained in Chapter 8. Obviously, in either case no special efforts are needed to bring the bar oriented to the riveting position. When the contact is in its place the bars must be separated. This happens at line C by means of two punches which cut the remain- ing cross-pieces (blackened spots in the illustration). The above examples (Figures 7.10 and 7.11) are typical high-productivity automatic processes, where automatic feeding of parts must be as rapid as possible. Therefore, the contrivances described above are justified. However, often the processing time is relatively long and the automatic operation does not suffer much if feeding is simpli- fied. This brings us to the idea of hoppers or magazines. The classical means of automat- ing industrial processes use a wide range of different kinds of hoppers, some of which are discussed below. Tray hoppers are manually loaded with parts which then slide or roll under the influence of gravity, as shown in Figure 7. 12. A shut-off device is installed at the end of the tray to remove only a single part from the flow of parts on the tray. The design of these devices depends, of course, on the shape of the part they must handle. The rough estimation of the moving time along the inclined tray was considered in Chapter 2, Section 2.1. A phenomenon which must always be taken into account in designing tray hoppers is seizure, which is schematically illustrated in Figure 7.13. To ensure reliable move- ment of the part along the tray, one must keep the seizure angle j as large as possible. This angle depends on the ratio L/D (the length L of the part to its diameter or width FIGURE 7.12 Tray hoppers: a) Usual type; b) Tortuous slot shape for a hopper. 238 Feeding and Orientation Devices FIGURE 7.13 Graphical interpretation of seizure of parts in a tray. D), and values of L/D < 3 are good enough. In practice the clearance A must be chosen correctly to prevent seizure. From Figure 7.13 it follows that which, by substituting yields To avoid seizure in the design shown in the figure, the seizure angle 7 must be larger than the friction angle p, which means Here ju is the factional coefficient between the tray sides and the part. Expressing cos 7 through tgy, we obtain the clearance from Equation (7.14) in the following form: Contrary to case a), case b) in Figure 7.12 is suitable for parts with L/D>3 because, due to the tortuous slot shape, the part cannot fall sideways and achieve dangerous values of angle 7. This design is useful for many other applications in machinery where seizure can take place. The length of the tray depends, obviously, on the processing time and must provide a reasonable amount of parts without frequent human interference. To elongate the tray and increase the number of parts stored in it, zigzag or spiral trays are used (see Figures 7.14a) and b)). The zigzag hopper, in addition, limits the falling speed of parts, which is sometimes important, for instance, when they are made of glass. Tray hoppers are sometimes modified into a vertical sleeve or channel, as shown in Figure 7.15. In case a), hollow cylindrical parts are fed, and in case b), flat parts. Here we see the shut-off mechanisms: a cylindrical pusher in a) and a flat slider in b), which carry out reciprocating motion. The pace of motion is dictated by the control system; however, it must allow the free fall of the parts in the hopper. It may be possible to 7.4 Feeding of Oriented Parts from Magazines 239 FIGURE 7.14 High-volume a) zigzag and b) spiral hoppers. FIGURE 7.15 Examples of vertical sleeve, tube, or channel hopper. drive the parts in the hopper pneumatically or with a spring. The latter is generally used in automatic firearms. To be reliable, cut-off of the fed parts requires a certain degree of accuracy in the mechanism. Thus, the gap A is restricted to a value of about 0.05 to 0.1 mm, the value ^ ~ h - (0.05 to 0.1 mm), and h ^ 0.5 mm. Vertical box hoppers are more compact. Figure 7.16 illustrates several such hoppers. Case a) consists of box 1 in which the blanks are loaded in several layers, tray 2, and shut-off pusher 3 which takes the blanks out of the hopper by pushing along their axis. Viewb) shows the cross section of this hopper, and here agitator mechanism 4 is shown. The purpose of this mechanism is to prevent creation of a bridge of blanks which dis- turbs their free movement towards the outlet. Case c) shows a similar hopper where FIGURE 7.16 Vertical box hopper. [...]... for the shape of the parts handled by the device (see Figure 7 .22 ) To permit free movement of blanks in the slot and optimum feeding and orien- 24 4 Feeding and Orientation Devices FIGURE 7 .21 Sector-type hopper FIGURE 7 .22 Shapes of slots for differently shaped details tation, the following empirical relationships between the dimensions of the parts and the slot parameters are recommended: A very similar... frequency of the feeder depends on its oscillating mass and the stiffness of the springs The device is usually designed so as to be close to resonance conditions However, this makes the feeder very sensitive to minor differences between the excitation and natural frequencies These differences are caused by difficulties in making the actual parameters of the device exactly equal to their calculated... due to inertia; therefore, there is less risk of damage to the parts Constant and uniform speed of the parts is convenient for orientation (see next section) • These devices are relatively simple, having no rotating links, and seizure of parts is less possible • The feeding speed can be easily tuned and controlled The vibrofeeder can be oscillated by an electromagnet (as mentioned above), pneumodrive,... maintaining orientation of the parts), the acceleration S must meet the requirements Vibrofeeders possess certain advantages which explain their widespread use for automatic feeding The advantages are: • Motion of the parts along the tray does not depend on the masses This means that, when the device is tuned appropriately, small and large items move at the same speed 25 2 Feeding and Orientation Devices • •... the degree of stability of an item (see Figure 7 . 29 ) The relative potential energy W of the item when moved from state 1 to state 2 relative to plane 1-1 is described by the expression Here, R = net external force acting on the item (in Figure 7 . 29 , gravitation), T= net torque of external forces acting on the item relative to point A, h = vertical change in the center of gravity, j = the angle of inclination... 25 4 Feeding and Orientation Devices conventional kind Curves 1 belong to conventional design while curves 3 are for the adaptive feeder It is almost independent of voltage changes and of the load in the bowl (curves 2) The energy consumed by a vibrofeeder working permanently at its mechanical resonance frequency is considerably less than that required by a conventional feeder, the energy savings being... shape and other physical properties, an item on a tray may be found in unstable, stable, or indifferent states Transition from some position into the desired position may require several intermediate changes in positions, which can be effected by applying forces to the item The values of these forces depend on the specific shape and state of the item Mathematical criteria are used to describe the degree... the latter feeders can be estimated as shown in Chapter 3, Section 3.1 To provide the required productivity, the length L of the sector or the knife usually has the following relation to the blank's length I: Here / is the length of the blank in the direction of sliding when it is properly oriented The feeding rate of these devices is limited by the acceleration of the knife or sector as it reaches... 24 7 vibrated by electromagnet 7 fastened in the middle of base 4 The electromagnet is made of core 8 and coil 9 To prevent transfer of vibrations to the system or machine on which the feeder is mounted, the latter can be installed on three springs 10, of relatively low stiffness Pin 11 restrains the feeder from moving too much When coil 9 of magnet 7 is energized by alternating current (usually the... otherwise shaped, identical ends (examples 2, 3,7) These parts have only one degree of freedom orientation Indeed, one must bring the symmetry axis of the part in line with one of the coordinate axes II Parts possessing only one axis of symmetry To this class belong cylindrical parts with different ends (examples 1-4), parts with asymmetric necks (example 5), discs with ring-like grooves (example 7) . inlet and outlet sleeves 6 and 7, respectively. The material is poured into sleeve 6 and due to rotation of the screw, is led to sleeve 7 where it exits for subsequent use. automatic processes, where automatic feeding of parts must be as rapid as possible. Therefore, the contrivances described above are justified. However, often the processing time is relatively . The platform is 24 6 Feeding and Orientation Devices FIGURE 7 .24 Vibrofeeder. General view. FIGURE 7 .24 a) General view of a vibrofeeder with its controller. This device is driven