Suchy_CH11.qxd 11/08/05 11:11 AM Page 503 DIE PROCESS QUALITY AND AUTOMATION, DIE MAINTENANCE DIE PROCESS QUALITY AND AUTOMATION, DIE MAINTENANCE 503 FIGURE 11-9 Schematics of the automatic hardware insertion (Reprinted with permission from PEM Fastening Systems, a PennEngineering Company, Danboro, Pennsylvania.) multiple hardware insertion and where the need for still more hardware arises, another wagon can be rolled to the production line A feeding schematics is included in Fig 11-9 There are many variations to automatic hardware insertion As shown previously in Fig 11-6, a computer-driven robotic arm can be used to place the part into the die for hardware insertion Computerized memory tells the robot exactly how to handle the part, as well as where to place it and when to it We all know, that there is no way the machine will ever forget or neglect this task, even at the end of a long and tiresome shift Further synchronizing of the robotics with a press can be used with many other types and forms of fasteners 11-2-2 In-Die Staking Staking of any hardware is another such operation that could only benefit from automation Manual staking, similarly to hardware insertion, is cumbersome and slow when done in a separate assembly operation The inserted hardware is not always large enough for the operator’s fingers to handle, and may often fall down, or be inserted the wrong way, and this way both the sheet-metal part and the hardware may end up in the scrap bin In-die staking utilizes a standard bowl feeding equipment as well, along with a customized transfer mechanism The delivery of parts into the die is done via compressed air A dual escapement bowl feeder can be used when placing two kinds of hardware at a time The bowl feeder and its PLC controls are positioned on a portable cart, which allows for mobility from press to press Designed for a quick change, standardized locators are utilized to attach the portable cart to the press, with quick disconnects for stud insertion and PLC controls To control the process and to monitor the quality of the parts, sensors are being used in the die, as shown in Fig 11-10 The sensors monitor whether or not • • • • The material was properly fed The studs are present after staking The alignment of the studs is correct The part is properly ejected Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website Suchy_CH11.qxd 11/08/05 11:11 AM Page 504 DIE PROCESS QUALITY AND AUTOMATION, DIE MAINTENANCE 504 CHAPTER ELEVEN FIGURE 11-10 In-die staking: Sensors are monitoring the automatic placement of hardware (Reprinted with permission from GR Spring & Stamping, Grand Rapids, MI.) In this case, proximity sensors are implemented to detect the stud presence; photoelectric through beam sensors are there to verify the stock has fed properly Another photoelectric sensor oversees the parts’ ejection And all sensors are integrated with the press controls, to prevent any problems during production 11-2-3 In-Die Tapping In-die tapping, not long ago considered impossible to achieve, is quickly becoming an industry standard (see Figs 11-11 and 11-12) So far, the on-going research came up with three different types of tapping systems: • Tapping with an external lead screw • Tapping with an internal lead screw • Tapping with a rack and pinion system External lead screw systems use a series of gears, which are driven by a helix lead screw on descent of the press ram The lead screw does not rotate; it only drives the gear assembly to generate and transfer the motion necessary for a tap cartridge to produce the thread The length of the travel of the tap cartridge with respect to the ram travel is adjusted by changing the gear ratio The gears are further adjustable to accommodate for a different thread pitch; they can tap downward or upward, vertically, horizontally, or under any angle A pitch multiplier allows for tapping of multiple holes in one operation, often varying the pitch from hole to hole Where the press travel is too long, shock absorbers can be utilized to activate the tap cartridge only partially during the press stroke For the opposite situation, where short press travel exists, a stroke reducer doubling the length of the tap path may be utilized Internal lead screw systems depend on a cam for transfer of the ram travel into tapping of openings to specified depths Here the lead screw rotates when driven by the roller nut on its way down The system can be designed as vertical or horizontal, with dependence on the preferences of the user Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website Suchy_CH11.qxd 11/08/05 11:11 AM Page 505 DIE PROCESS QUALITY AND AUTOMATION, DIE MAINTENANCE FIGURE 11-11 In-die tapping units: a For a hydraulic press; b For a mechanical press (Reprinted with permission from Danly IEM, Cleveland, OH.) 505 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website Suchy_CH11.qxd 11/08/05 11:11 AM Page 506 DIE PROCESS QUALITY AND AUTOMATION, DIE MAINTENANCE 506 CHAPTER ELEVEN FIGURE 11-12 Self contained in-die tapping assembly (Reprinted with permission from Danly IEM, Cleveland, OH.) The lead screw and the roller nut are internally positioned because of precise mounting and gearing requirements The lead screw rotates at high speeds, transferring its motion to the roll-forming tapping unit Cams as a source of driving power have a definite advantage over gear assemblies, as their profiles can be developed in such a way that they bring the tapping unit to speed with no dependence on the ram acceleration The change in pitch is possible too by swapping the tapping inserts Rack and pinion system of in-die tapping is similar to the external lead screw system, the difference being in a rack and pinion replacing the helical lead screw Multiple tapping units can be attached with chain drives to the main drive system The design of a die that is expected to contain the tapping unit must consider this inclusion already in the first stages of planning To retrofit existing dies will most often fare poorly, as the requirements for the inclusion will be difficult to meet Already the fact that one rotation of the lead screw needs a sizeable portion of the ram’s travel can disqualify many existing dies The stripper’s length of travel must be at least equal to the tapping stroke Additionally, the height of the die must not accommodate only the tapping unit itself; it must further allow for an easy access for the purpose of lubricating and for the exchange of tapping inserts The tapping inserts produce the thread by roll-forming the material Such a process generates a considerable amount of heat, for which reason the need for tapping fluid may be considerable The size of the opening to be tapped must be per recommended diameter— here the designers should not forget that there are different diametral tap drill sizes recommended for a cut tap and for that which is roll-formed Naturally, for such an accuracy sensitive operation, the strip must be well guided through the die, with proper piloting at proper places It is pertinent that at the engagement Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website Suchy_CH11.qxd 11/08/05 11:11 AM Page 507 DIE PROCESS QUALITY AND AUTOMATION, DIE MAINTENANCE DIE PROCESS QUALITY AND AUTOMATION, DIE MAINTENANCE 507 of the tapping unit, the opening to be tapped will be exactly where it should be and will not be swayed aside by strip buckling, defects in strip positioning, or other variables A proper supervisory method of such in-die process via sensors is a must 11-2-4 In-Die Welding In-die resistance welding has lately achieved a large popularity Years ago, nobody even dared to think about attaching a spot welder to the progressive die and produce welded assemblies right there, automatically But then, we must realize that years ago, sensors were not as common as they are nowadays, and without sensors in-die welding may not be possible Sensors in the in-die welding process are necessary to ensure a total protection to the die A thorough monitoring of parts’ feed length, die components’ position, scrap removal, and the overall die function as combined with the control of the moving strip, is essential The welded-on objects must be monitored for their proper positioning within the die to make sure the welding electrode will engage the material right where it was planned and exactly the way it was planned The amount of pressure the upper electrode exerts toward the assembly-to-be-welded must be carefully monitored as well, and this information must be reported back to the PLC controller This pressure is necessary not only to hold the parts in place, but to provide for a firm contact of the two, so that welding can occur (see Fig 11-13) Without a positive contact of the components, a resistance weld is very difficult to produce As can be easily imagined, oil, grease, or dirt on the surfaces may impair the weld quality Timing of the welding operation and that of the application of electric current should be developed and tested offline A timing chart (see Fig 11-14) shows the typical weld cycle’s timing FIGURE 11-13 Welding of two nuts, in-die, top view (Reprinted with permission from GR Spring & Stamping, Grand Rapids, MI.) Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website Suchy_CH11.qxd 11/08/05 11:11 AM Page 508 DIE PROCESS QUALITY AND AUTOMATION, DIE MAINTENANCE 508 CHAPTER ELEVEN FIGURE 11-14 Timing of a resistance weld (Reprinted with permission from GR Spring & Stamping, Grand Rapids, MI.) The pressure of the welding unit must be constant, which is not all that easy when depending on the periodic movement of the ram of a mechanical press Because of such type of an equipment, the amount of pressure reaches its greatest values near the bottom dead center and immediately drops down to zero in accordance with the ram’s descend and ascend To overcome this drawback, cams can be installed within the ram, and with the aid of linkage mechanism the press movement can be translated to suit the pressure distribution pattern needed for the welding head Resistance welding occurs easily when the two parts’ surfaces are in close contact, pressed together However, some parts are not quite flat, others are slightly twisted, and for these reasons, components to be attached by welding are sometimes provided with small projections to achieve a positive contact of the two The projections are located on that side which will be in contact with the material, to which the other item will be welded As can be seen in Fig 1-55 previously, the first nut shown there has three welded projections on its bottom surface, whereas the second nut contains a round ridge, which is another way of providing a positive touch-contact with the substrate material In automatic in-die welding (see Figs 11-15 and 11-16), sensors detecting a misfed item must be in place, as well as those that will monitor the electric current delivered to the FIGURE 11-15 Samples of in-die welded nuts (Reprinted with permission from GR Spring & Stamping, Grand Rapids, MI.) Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website Suchy_CH11.qxd 11/08/05 11:11 AM Page 509 DIE PROCESS QUALITY AND AUTOMATION, DIE MAINTENANCE DIE PROCESS QUALITY AND AUTOMATION, DIE MAINTENANCE 509 FIGURE 11-16 Sample of in-die welding (Reprinted with permission from GR Spring & Stamping, Grand Rapids, MI.) welder Monitoring the amount of current that flows through the two materials during welding operation can be utilized as an in-die weld inspection This can be automated to the point where the data reported by the sensors is compared to given parameters of acceptancy by the PLC controller, and on application of tolerance ranges, nonqualifying weldments will be disposed off into the scrap bin right on exiting the die Surprisingly, the actual welding time is very short, often measured in milliseconds, which should theoretically allow for a maximum of 600 welds per minute This can be considered true only where the material of the strip and that of the component to be welded to it can be delivered into the die and properly positioned in such a short time (see Fig 11-14) The actual delivery of parts into the welding station can be achieved via vibratory bowl in the case of hardware Where two sheet-metal parts are to be attached by welding, one of the strips can be fed under an angle, joining the second part right in the welding station The exact placement and its monitoring is naturally of great importance The separation of welded assemblies from the strip can be achieved via either cutting the parts free, or via their breakage off the strip, or via any other method of choice When breaking parts off the strip, minute amounts of material are being left in the corners for their attachment (see Fig 11-17) This method is called shake-and-break in sheet-metal fabricating and the width of the joining strip is often dependent on testing This is a similar method to that called cut-and-carry in diework, with the only difference being in the thickness of the web Additionally, cut-and-carry parts have to be separated by a final blanking punch, whereas shake-and-break parts separate on shaking the strip or sheet, or on slightly hitting its surface Of course, minute burrs may often be left where the metal bridges where positioned For in-die welding, a standalone cart can be utilized on which all the welding equipment is positioned (see Fig 11-18) The cart can be rolled to any suitable press and the welding station implemented into the die Of course, the die has to be designed with this inclusion in mind, as already mentioned with other in-die processes FIGURE 11-17 Shake-and-break method of parts’ separation off the strip Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website Suchy_CH11.qxd 11/08/05 11:11 AM Page 510 DIE PROCESS QUALITY AND AUTOMATION, DIE MAINTENANCE 510 CHAPTER ELEVEN FIGURE 11-18 Cart with equipment for in-die resistance welding (Reprinted with permission from GR Spring & Stamping, Grand Rapids, MI.) 11-2-5 Linear and Radial NC Multicenters These unique machines were developed by Otto Bihler Maschinenfabrik, GmbH & Co, in Germany They are complex assemblies of stations, either linearly or radially positioned around the machine board, which stands vertical (Figs 11-19 and 11-20) Directed by the CAD/CAM software, with their components adjustable per the given task, the multicenters are capable of cutting and forming the components from either a single or multiple strips of material, assembling them together, attaching hardware, and welding where necessary As an example, a folded rectangular sleeve with a screw inserted through the joint surfaces (Fig 11-21a) is produced in such a way that the part is cut from a strip and folded by an action of permanent cams By permanent is meant that these cams are permanently included within the system, and can be adjusted to fit each new arrangement of components The screw is fed through a tubing, it is inserted and tightened afterwards The whole assembly may be ejected from the machine by sliding down a round rod, around which it is enwrapped A similar assembly shown in Fig 11-21b is produced along the same lines, with the exception of another component, made in another die segment, using a different strip material, being added to the original part Again, there is the cam action, the assembly, and the final fastener attached at the end An interlocking sleeve is produced from a single strip, retained by a centrally located bridge, formed closed, and cut off (see Fig 11-22) Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website Suchy_CH11.qxd 11/08/05 11:11 AM Page 511 DIE PROCESS QUALITY AND AUTOMATION, DIE MAINTENANCE FIGURE 11-19 Linear NC multicenter (Reprinted with permission from Bihler of America, Inc., Alpha, NJ.) FIGURE 11-20 Radial NC multicenter (Reprinted with permission from Bihler of America, Inc., Alpha, NJ.) 511 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website Suchy_CH11.qxd 11/08/05 11:11 AM Page 512 DIE PROCESS QUALITY AND AUTOMATION, DIE MAINTENANCE 512 CHAPTER ELEVEN FIGURE 11-21 a Folded rectangular sleeve; b Assembly of several components (Reprinted with permission from Bihler of America, Inc., Alpha, NJ.) The principles of linear and radial approach behind these ideas are shown in Figs 11-23 and 11-24 11-2-6 Quick Die Change With all the increased production demands of present times, the manufacturers not only depend on a quick assembly of all die components and enhancements and on a quick turnaround of the dies in the press, but, for that purpose, on a quick way to change the dies Hilma Co came up with several products that can assist considerably with the quick die changing First, when the die is delivered to the press, their die cart’s upper surface consists of heavy duty roller bars, which ease the movement of heavy dies in and out of the press Actually, a single operator can slide a bulky die in, effortlessly Where needed, the press can be equipped with an out-sticking carrying consoles (either swiveling or fixed and supported), over which the die can be slid in and out Again, the consoles are topped with rollers, over which the die slides The press bed, provided with hydraulically adjustable roller bars (Fig 11-25a), makes moving of the heavy die easy to accomplish, especially where such is guided to its destination by additional side rollers All clamping, changeover, and unclamping are monitored by inductive proximity switches, which are tied directly to the press controls Once the die is positioned, swing clamps can locate and hold the upper section of the tool to the press ram (Fig 11-25b) The bottom shoe can be retained similarly, or by using hollow piston cylinder clamps (Fig 11-25c), or similar clamping arrangement from their assortment of retaining devices The clamps slide easily into the T-slot bolsters and rams, retracted by springs during the die switchover This way, the whole procedure of die change takes minutes, where hours were spent previously on tightening nuts and bolts, aligning the die elements, die tryouts, and similar tasks Figure 11-26 shows clamping technique for forming dies Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website Suchy_CH12.qxd 11/08/05 11:14 AM Page 560 SPRINGS, THEIR DESIGN AND CALCULATIONS 560 CHAPTER TWELVE Based on Bernoulli-Euler beam theory for bending of beams, the maximum stress can be calculated as S= Mc I (12-40) where c = distance from neutral axis to outside or one-half of material thickness M = moment, amounting to distance from support times the load, or M = PL I = moment of inertia For a rectangular section, moment of inertia can be calculated: I= bh3 12 (12-41) Combining the above values into the single equation, we get the maximum stress expressed as S= PL bh (12-42) where P = load on the spring, lb L = length of lever arm, in The load value may be calculated by using a formula P= fEbh3 L3 (12-43) where E = modulus of elasticity f = deflection, in For flat springs, where the width to thickness ratio is relatively small, the maximum stress and deflection formulas are reasonably accurate Higher width to thickness ratio increases the flexural rigidity of the spring, resulting in the modulus of elasticity E being replaced by E¢ as follows: E′ = E 1− µ2 (12-44) where m = Poisson’s ratio The deflection can be calculated using a standard formula f = PL3 3EI (12-45) where E = modulus of elasticity I = moment of inertia; for a rectangular section the value can be calculated with Eq (12-41) This formula, when applied to a rectangular section, changes to fRT = PL3 Ebh3 or SL2 3Eh (12-46) where S = stress Since L and h values are raised to the third power, accurate measurements are vital Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website Suchy_CH12.qxd 11/08/05 11:14 AM Page 561 SPRINGS, THEIR DESIGN AND CALCULATIONS SPRINGS, THEIR DESIGN AND CALCULATIONS 561 All these equations were proved satisfactory where the ratio of deflection to cantilever length F/L was less than 0.3 For larger deflections, E should be replaced by E¢, as given by Eq (12-44) 12-9 GAS AND AIR SPRINGS AND THEIR APPLICATIONS Most probably, forced by an unending and never-resting competition, the industry had to come up with a different type of springs, to ease their installation, improve their function, and remove the “gray areas” of preload from the spring usage dictionary With the wound springs, special spring pockets had to be milled into the blocks; the correct spring height was an unending problem; and the force buildup, also called preload, was always somewhat a mystery During the operation, the force of wound springs started from zero and progressed upwards, sometimes becoming unpredictable and often even excessive Gas springs are different They are capable of delivering much more force in lesser area than ordinary wound springs They generate pressure on contact, eliminating the need for preload This way the pressure pad can be smaller, the amount of cylinders diminished, the stroke shorter, while the force produced by springs is constant and unwavering alongside the stroke of a press Their travel to length ratio is much larger and their pressure can be easily monitored Gas springs are also more balanced Whereas in an assembly of several wound springs some may be cracked and the rest may not produce the pressure needed, gas springs are always there, always working Should their pressure drop somehow, the gas springs can reclaim the gas needed and prop up the pressure to the demanded levels Out of all gasses, nitrogen springs gained the ground across the board One of the reasons may be the low cost of nitrogen gas, but nitrogen is also nonflammable, inert, and tonnage resistant This means that as the pressure against such spring rises, the force of its output increases in proportion to the volume of gas that was compressed Nitrogen springs should never be preloaded, and, actually, manufacturers caution against preload with determination But at the same token, nitrogen charge should not be lowered in anticipation of extending the life expectancy of the seal Such a precaution may actually harm the spring, as the modern seals are designed to operate at the full nitrogen charge Their loading is of concern though, as they are not to be used at the operating pressure exceeding 90 percent of their recommended maximum 12-9-1 Nitrogen Springs and Their Types There are several types of nitrogen spring systems available on the market today, the difference between each group being provided by the method of attachment and gas distribution These types are as described below: 12-9-1-1 Manifold System Manifold system is a closed system, embedded in a metal plate, which is cross-drilled to allow for the nitrogen gas distribution The spring cylinders are attached to the channels through the tapped holes and may be positioned where needed The whole assembly is connected to the control panel, which directs the volume of gas within the system (See Fig 12-22) Manifold systems require clearance between the die and the end of the cylinder rod, so that the rod does not touch the plate of the opposite die half (either upper or lower plate, with dependence on the type of mounting) The clearance is necessary for the piston to come to a full die-open position Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website Suchy_CH12.qxd 11/08/05 11:14 AM Page 562 SPRINGS, THEIR DESIGN AND CALCULATIONS 562 CHAPTER TWELVE FIGURE 12-22 Nitro Dyne® XP manifold system of gas springs (Reprinted with permission from HysonTM Products, Brecksville, OH.) Nitrogen reservoirs can also be added to the bottom die shoe (see Fig 12-23), if the shut height of the press is too limited The details of such an arrangement are the same as those of the regular manifold system The nitrogen reservoir interconnects the cylinders via the holes drilled through the shoe As can be expected, a demand such as this will weaken the die shoe somewhat The compression tank retains the excessive nitrogen, which the springs leak when being pressed down To determine the tank size, the amount of excessive nitrogen (also called “swept volume”) has to be determined first This can be calculated as: VSW = AP × LWK × No of Cylinders (12-47) where VSW = swept volume AP = area, piston LWK = working stroke From the result, the volume of the tank can be determined as: VT = VSW (100 : RP) (12-48) where VT is the required volume of the tank, and RP is the percentage of desired pressure rise, or increase FIGURE 12-23 Nitrogen cylinders as installed in the die shoe (Reprinted with permission from HysonTM Products, Brecksville, OH.) Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website Suchy_CH12.qxd 11/08/05 11:14 AM Page 563 SPRINGS, THEIR DESIGN AND CALCULATIONS SPRINGS, THEIR DESIGN AND CALCULATIONS 563 Pressure increase, also called pressure rise, is generally recommended at 15–20 percent for draw dies and 30–40 percent for strippers, form pads, and cam returns 12-9-1-2 Hose and Tank System In this design, a reservoir tank is connected with cylinders via high-pressure hoses The whole assembly is wired to the control panel for the balance of pressure between cylinders There is no fixed mounting and the cylinders can be bolted exactly where needed, as shown in Fig 12-24 12-9-1-3 Self-Contained Cylinders Self-contained cylinders (see Fig 12-25) are isolated springs, which already contain the amount nitrogen needed for their function and not need any additional supply of it Where balanced force is necessary, several cylinders can be connected together with pressure hoses, as shown in Fig 12-25 As with all other springs/cylinders, self-contained cylinders should be protected from the contact with any fluids, be it die lubricants, cleaners, water, or any other liquids For this reason, their retaining pockets should be provided with adequate draining channels The spring should always be attached to the bottom surface of its retaining pocket with bolts This precaution not only prevents the cylinder from being swayed aside during the die function; it also does not allow for a gap to retain metal chips, lubricants, grime, and other debris underneath it The piston contact surface should be straight and perpendicular to the die surface Where a slanted contact surface may be used, side loading will result, which may sway the piston aside and eventually ruin it Same with surfaces containing pockets or screw heads— these may produce an uneven pattern of wear on the piston rod (see Fig 12-26) The disadvantage of single, self-contained cylinders is their height: these types of gas springs are always higher than other cylinder types (see Fig 12-27) 12-9-1-4 Spring Cushions These are small assemblies of cylinders under a common pressure pad as shown in Fig 12-28 As these are very powerful devices, such cushions are useful in aiding the press function and can be installed either attached to the ram, or under FIGURE 12-24 Hose and tank system (Reprinted with permission from HysonTM Products, Brecksville, OH.) Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website Suchy_CH12.qxd 11/08/05 11:14 AM Page 564 SPRINGS, THEIR DESIGN AND CALCULATIONS 564 CHAPTER TWELVE FIGURE 12-25 Super Tanker® Cylinders (Reprinted with permission from HysonTM Products, Brecksville, OH.) the bolster, or just about everywhere The advantage is in their nearly constant force throughout the stroke Self-contained pressure pads are also used as cam-driving devices, in which case they can be provided with a cam-driving block or with a roller Stripper springs for the return pressure must be used in conjunction with the cam pads The cam-driving cushions also serve well in delayed piercing FIGURE 12-26 Inclined or uneven surface produces damage to the piston rod (Reprinted with permission from HysonTM Products, Brecksville, OH.) Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website Suchy_CH12.qxd 11/08/05 11:14 AM Page 565 SPRINGS, THEIR DESIGN AND CALCULATIONS SPRINGS, THEIR DESIGN AND CALCULATIONS 565 FIGURE 12-27 Height of cylinders (Reprinted with permission from HysonTM Products, Brecksville, OH.) 12-9-2 Air and Hydraulic Cylinders Air springs, also called pneumatic cylinders (or air pistons) are preferred where long press strokes and adjustable forces of their application are required Often, these types of springs contain a hollow cylinder, which at the same time acts as a surge tanks for air The pistons operate on shop air, the pressure of which can be increased at the die closure by implementing one-way check lines at the air inlet FIGURE 12-28 Spring cushion (Reprinted with permission from HysonTM Products, Brecksville, OH.) Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website Suchy_CH12.qxd 11/08/05 11:14 AM Page 566 SPRINGS, THEIR DESIGN AND CALCULATIONS 566 CHAPTER TWELVE Hydraulic cylinders are slower in response, for which reason they are not used in dies too often However, their usage with some forming applications brings about definitive advantages First of all, by adding an accumulator of fluid, their force can be adjustable A press operator can monitor this force by himself or herself and change it on demand, either up or down Additionally, a typical lifetime of a hydraulic cylinder system is up to 2.5 million cycles, with dependence on other variables, such as the severity of manufacturing operation 12-9-3 Calculation of Resistant Tonnage for Nitrogen Springs The tonnage, also called the resistant tonnage, is an important element in the selection of proper nitrogen springs Resistant tonnage is the force needed to maintain the given pad pressure The calculation of tonnage, meaning any tonnage at all, is mostly an open guess and subject to variations due to friction, heat, galling, to name but few invasive effects on the metal stamping field that not hesitate to exert their influence when the least appropriate The resistant tonnage calculation of nitrogen springs is probably bound to change over time, as new formulas come up quickly and the old ones not die easily But the basic formula for the drawing tonnage is similar to the calculation of the blanking or piercing pressure, and it can be calculated as follows: VTON = L ◊ t ◊ c ◊ Fs (12-49) where L = length of the line, linear t = material thickness FS = safety factor, 1.2 to 1.25 (i.e., 20 to 25%) c = coefficient, per values below The values of the above coefficient c are approximately 23 = for cold rolled steel 18–20 = for aluminum 28–35 = for stainless steel 12-9-4 Comparison of Different Types of Springs Each of the spring systems described earlier has its advantages and disadvantages For example, coil springs may still be needed in great quantities for shorter die runs, or where the life of a spring is of no concern The cost of wound springs is low and their exchange is most often quick and cost-efficient, when not counting the time a die needs for its removal from the press Air, gas, and pneumatic springs’ performance suffers where a short stroke, high-speed application with many million of cycles are expected At the same time, longer spring travels with high speed applications can incredibly weaken wound springs A nitrogen cushion does not require a compressor; an air spring does need one The nitrogen spring cushion may cost half of the air cushion’s price tag The force exerted by nitrogen cushion is constant, unwavering An air cushion’s force varies, since the build up of pressure is controlled by the air valves, expansion tanks, and compressors, all attached to the spring The cost of maintenance for the nitrogen spring is also lower If the parts not come out from the die as they should and a wrong amount of spring force is suspected, it is quite a task to determine the spring pressure received from wound springs Where an increase in force is needed, wound spring’s pressure cannot be easily adjusted Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website Suchy_CH12.qxd 11/08/05 11:14 AM Page 567 SPRINGS, THEIR DESIGN AND CALCULATIONS SPRINGS, THEIR DESIGN AND CALCULATIONS 567 Nitrogen gas is nontoxic, colorless, odorless, and inert It will not ignite under high pressures or in the close proximity of an open flame Nitrogen gas additionally does not support combustion Air and other gases may be flammable in some environments For this reason, their application with dies producing heat, should be limited Their maximum operating temperature is approximately 170°F, and unless the heat-producing dies are cooled, coil springs should be opted for Hose-and-tank system of nitrogen springs is more costly, but it allows for controlling and adjusting the spring pad balance Their safety factor is greater than that of other spring types As seen earlier, each type of a spring has its application range and subsequently, its usage The proper decision depends on each particular situation, on the die function, and on the environment where it operates Naturally, the cost factor exerts its influence here as well Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website Suchy_CH12.qxd 11/08/05 11:14 AM Page 568 SPRINGS, THEIR DESIGN AND CALCULATIONS Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website Suchy_CH13.qxd 11/08/05 11:16 AM Page 569 Source: HANDBOOK OF DIE DESIGN CHAPTER 13 SPRING WASHERS Spring washers, even though small in size, may sometimes outperform much larger springs They are used in offset situations, to provide tension in bolted assemblies, or to furnish the recoil action of springs Even though their span of deflection is limited because of their size and especially because of their height, a somewhat improved performance may be expected where coupling several washers together Their stackability, along with their compactibility and versatility, makes spring washers quite advantageous where used in confined spaces or where a stabilizing function is needed In bolted assemblies, spring washers are capable of keeping all parts under tension, preventing threaded items from rotating and loosening up Spring washers can negate the effects of vibration, they can diminish the side-acting force and control the pressure in vibration mounts, aside from many other applications, where their usage is often taken for granted Basically, there are three types of spring washers: • Cylindrically curved washers • Wave washers • Conical disks, or Belleville washers These three basic variations are capable of covering a wide range of loading applications Where cylindrically curved washers will sustain a loading of several ounces, the sturdiness of Belleville washers allows for loads ranging within tons (Table 13-1) The effect of loading force is localized in spring washers, which causes a stress response within a small area surrounding the inside diameter of the part The subsequent deformation tends to increase the affected area in size, which can never be large enough to influence the height of the washer Conical washers can take up to 200,000 lb/in.2 loading, which is the value of their maximum stress in load cycles of 500,000 With more cycles, loading limits must be reassessed on the basis of fatigue testing of actual washers However, some conical washers will tolerate stresses in the range of two or three times the maximum permissible value All spring washers are usually made of spring steel, with some marginal use of spring brass, beryllium copper, phosphor bronze, and other materials Hardness of the material does not influence the spring rate of the washer in any way as some may have believed Corrosion resistance is ensured by application of coatings, which may include electrogalplating, electro-plating cadmium plating, black oxide, nickel and chromium plating, etc The actual usefulness of each washer varies along with its shape, making each of them restricted in application to specific situations, with their interchangeability outright impossible 569 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website Suchy_CH13.qxd 11/08/05 11:16 AM Page 570 SPRING WASHERS 570 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website Suchy_CH13.qxd 11/08/05 11:16 AM Page 571 SPRING WASHERS SPRING WASHERS 571 13-1 CYLINDRICALLY CURVED WASHERS This type of washer demonstrates a considerable uniformity of spring constant over a wide range of deflections Cylindrically curved washers are suitable for application of light loads or where repeated cycling with varied range of motion is involved The recommended range of their maximum height is limited to less than one-half of their outer diameter Cylindrically curved washers (Fig 13-1) may be used where tightening of assemblies is needed, to protect them from looseness and lack of stability The functionability of these washers should not be hampered by installing them in restricted or confined spaces, as they need room for diametral expansion under loads The condition and hardness of their bearing surface is of importance as well, since it must allow for easy sliding of edges during expansion, with no subsequent digging into the material To calculate the values of cylindrical washers, the following formulas may be used: P= Ef bt D3 K (13-1) where P = applied load, lb E = modulus of elasticity, lb/in.2 K = empirical stress-correction factor per Fig 13-2 D = outside diameter, in f = deflection, in b = radial width of the material, or (D − d)/2 t = material thickness FIGURE 13-1 Typical curved spring washer (From “Design Handbook,” 1987 Reprinted with permission from Associated Spring, Barnes Group, Inc., Dallas, TX.) Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website Suchy_CH13.qxd 11/08/05 11:16 AM Page 572 SPRING WASHERS 572 CHAPTER THIRTEEN FIGURE 13-2 Empirical stress-correction factor K for cylindrically curved spring washers (From “Design Handbook,” 1987 Reprinted with permission from Associated Spring, Barnes Group, Inc., Dallas, TX.) The maximum induced stress S will be Eft D2 (13-2a) 1.5P K t2 (13-2b) S= or S= where K = empirical correction factor per Fig 13-2 These equations are valid for deflections of up to 80 percent of the washer’s height h, where the actual amount of deflection f is smaller than 1/3D Beyond these ranges, the spring rate, which so far was found linear, will begin to rise in value, becoming higher than calculated The radius of curvature R may be figured as follows: R= ( D/2)2 + h 2h (13-3) Equation (13-3) may be used to evaluate the height of the washer h, h = R − R2 − ( D/2)2 (13-4a) D = 2hR − h (13-4b) or the outside diameter Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website Suchy_CH13.qxd 11/08/05 11:16 AM Page 573 SPRING WASHERS SPRING WASHERS 573 Spring rate k may be obtained from the formula k= P 41.75Ebt = f D+d (13-5a) where b = radial width of the material, or (D - d)/2 k = spring rate, lb/in 13-2 WAVE WASHERS Wave washers may be used for small to moderate static loads, ranging from a few pounds up to hundreds They are an excellent choice for mounting within tight or restrained areas, as their outside diameter increases in size only very slightly under a load (Fig 13-3) Wave washers are often utilized in situations where some amount of cushioning is required, to offset components of shaft assemblies, or to prevent loosening of parts due to vibration These washers may be obtained in a wide range of sizes, but for the best balance between their flexibility and load-carrying capacity, the ratio of mean diameter Dm to the radial width of the washer material b should be kept at the numerical value of 8, or Dm =8 b Ratios smaller than generate a discrepancy between the calculated and actual values of load and stress Such an impediment to the washer’s performance is caused by the inability of waves to assume their previous shape after deflection Where the Dm /b ratio should fall considerably below 8, a replacement with a Belleville washer is recommended The number of waves must be three or more, with the most commonly used washers having three, four, or six waves By increasing the number of waves, the washer’s thickness may need to be reduced for a required load, with a subsequent decrease in allowable FIGURE 13-3 Typical wave spring washer (From “Design Handbook,” 1987 Reprinted with permission from Associated Spring, Barnes Group, Inc., Dallas, TX.) Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website Suchy_CH13.qxd 11/08/05 11:16 AM Page 574 SPRING WASHERS 574 CHAPTER THIRTEEN deflection The number of waves is based on the desirable spring rate and may be calculated by using the formula k= P Ebt Na D = f 2.40 Dm d (13-5b) Dimensional uniformity of waves is important, as the actual load deflection rate takes effect only after all waves are equally loaded For this reason, the load deflection rate should always be verified against the initial preload With evenly loaded waves, the spring constant is expressed by a linear segment, especially within the range of 20 to 80 percent of the total available deflection of the washer The point at which the spring rate begins to deviate from its linear representation differs with various types of washers Its occurrence should be prevented by making the washer height equal to twice the amount of deflection Stress range of wave washers (Table 13-2) may be calculated as follows: S= 0.75πP( D + d ) Na t b (13-6a) or 48E tf N a π2 S= (D + d) (13-6b) where Na = number of waves Material thickness t may be related to the following values: t= 0.635( D + d )( P/bf )1/ E1/ Na / (13-7) where E = modulus of elasticity, lb/in.2 P = applied load, lb b = radial width of material, or (D − d)/2 f = deflection, in TABLE 13-2 Maximum Recommended Operating Stress Levels for Cylindrically Curved and Wave Washers Made of Steel in Cyclic Applications Percent of tensile strength Life (cycles) Maximum stress 104 105 106 80 53 50 This information is based on the following conditions: ambient environment, free from sharp bends, burrs, and other stress concentrations, AISI 1075 Source: Design Handbook, 1987 Reprinted with permission from Associated Spring, Barnes Group, Inc., Dallas, TX Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website ... 106 8. 43 8. 25 8. 14 8. 83 8. 46 8. 86 8. 53 8. 75 8. 26 8. 53 7 .81 8. 03 7.92 7 .86 7 .86 7 .86 7 .86 7 .86 7 .86 g/cm3 (0.304) (0.2 98) (0.294) (0.319) (0.306) (0.320) (0.3 08) (0.316) (0.2 98) (0.3 08) (0. 282 )... (11.5) 8. 86 8. 26 7 .81 7.92 7 .86 7 .86 7 .86 7 .86 7 .86 (0.320) (0.2 98) (0. 282 ) (0. 286 ) (0. 284 ) (0. 284 ) (0. 284 ) (0. 284 ) (0. 284 ) 0. 08 0. 08 0. 08 15 21 0. 08 0.25 0. 08 0. 08 0. 08 0.10 7 7 (0.003) (0.003)... (0. 282 ) (0.290) (0. 286 ) (0. 284 ) (0. 284 ) (0. 284 ) (0. 284 ) (0. 284 ) (0. 284 ) (lb/in3) Densitya 1.5 1.6 3.5 15 12 21 17 2 7 7 0.10 0.10 0.10 0.05 0.05 0.10 0.10 0.10 0. 08 0.10 0. 08 0.40 0.13 0.50 0.50