Optimal Design Of Mechanism For Stirrup Making Machine - A Computer Approach

Transcription

1 Optimal Design Of Mechanism For Stirrup Making Machine - A Computer Approach A.V. Vanalkar and P.M. Padole Dept. of Mechanical Engineering K.D.K. College of Engineering Nagpur Dept. of Mechanical Engineering Visvesvaraya National Institute of Technology Nagpur, INDIA. ABSTRACT Stirrup is an important reinforced element which acts as a shear reinforcement. Presently, stirrups are made manually, which suffers from many drawbacks like lack of accuracy, low productivity and resulting into severe fatigue in the operator. In manual stirrup making process, operators not only subjecting their hands to hours of repetitive motion, but in many occasions it results into several musculoskeletal disorders (MSDs). Considering these factors authors have designed and developed mechanical systems for stirrup making []*,[]. However, this system have some limitations. Optimal design of mechanism using computer simulation is carried out and it is used in development of the improved stirrup making machine which is presented in this paper. INTRODUCTION Stirrups or lateral-ties are rectangular, square or circular shaped reinforced element made out of 6 mm, 8 mm or 0 mm bar either in plain mild steel or torr steel. In order to safeguard the structure against failure by diagonal tension, reinforcement is required. This reinforcement is called Shear reinforcement which is provided by the stirrups. The important functions of a stirrup are - to hold and support horizontal and vertical mild steel plain bar or torr-steel. to provide reinforcement and rigidity to column and beams. * Figure in [ ] bracket indicate references at the end. to take shear force in horizontal beams structures as well as vertical columns. to avoid buckling of long slender column or avoid sagging of horizontal beam. to provide proper anchorage which in turn safeguard the structure against horizontal forces occuring due to wind, earthquake etc.

2 Presently, stirrups are made manually. In this process initially, as per the required size of the stirrup i.e.perimeter, stirrup wire or M.S. round bar of 6 mm or 8 mm are cut. The operator uses a wooden block as a platform for bending on which three nails are fixed, around which the bending takes place. The stirrup wire is passed horizontally between the nails up to the chalk mark and then the operator bends the bar using a lever like hand-tool 'daag', about the nail through an angle of 90 degree. The chalk marks are made as per the size of the stirrups. After completing one bend the operator lifts and repositions the bar for the next bending. In this way the operator makes all five bends for making one stirrup. The manual stirrup making process suffers from the drawbacks like lack of accuracy, low productivity and resulting into severe fatigue in the operator. The construction worker not only subject their hands to hours of repetitive motion (i.e. performing bending operation to produce stirrup) but also sometimes suffers internal injury to his body organ i.e. disorder carpal tunnel syndrome CTS, slipped disc problem etc. BRIEF IDEA ABOUT EARLIER WORK ON STIRRUP MACHINE WITH SINGLE WORK STATION. Considering above limitations, in the recent past authors have developed a Stirrup Making Machine with single work station. Fig. shows stirrup making machine with single workstation. The workstation consists of steady-shaft (), gear (), 'L' shaped follower () and cam (4). The steady shaft is provided with helical groove in order to ensure contact between follower and stirrup wire. As the surface of follower is plane (i.e. groove provided on steady shaft instead of follower), hence releasing of stirrup wire after completion of bend through 90 degree is easier. A stirrup wire is placed between guides (5) provided over top surface plate (6) of the machine. The wire is fed to the workstation through guide- ways. When the follower rotates about the fixed cam, it also oscillates about hinge. The cam is of rise-dwell-return type with 65-degree SHM rise, 95-degree dwell and 0 degree return. During 95 degree rotation the follower remains in its topmost position and bends the stirrup wire effectively through 90 degree. After completing the bend the follower suddenly falls and releases the stirrup wire. To ensure the sudden fall, springs (7) are used. The follower rises to topmost position when gear rotates through 65 degree. During this period, the stirrup wire is pushed forward and positioned for the next bend. The top surface plate (6) is provided with tapered surface made out of wooden plates (8) near guide ways. This surface ensures the overlap of the extreme ends of the stirrup which is the requirement of stirrup as per IS code 456: 978. Thus all five bends of the stirrup are completed in five rotations of the gear in this stirrup making machine.

3 5 6 5? C CCC 7 4 Steady shaft 4 Cylindrical cam 6 Surface plate Gear box 5 Guides 7 Spring 'L' shapped follower Fig. Top view and front view of stirrup making machine with single work station. However, this sytem has some limitations like - It is desired that the follower should fall down suddenly after the completion of the bend through 90 degree. To ensure this, springs are used as shown in fig.. However, it is observed that due to variation in the frictional force between the follower and the stirrup wire, sometimes follower does not fall down even after completion of the bend. This results into excessive undesired bending of the wire. This can also cause an accident. The springs loose its stiffness after about 500 to 600 cycles of operation. Thus the springs need to be replaced frequently. The follower moves in the top position over the cam profile. This movement is against the spring force. Therefore, if the spring stiffness is increased, then follower develops scratches on the cam surface and thus reduces its life. To avoid this, cam needs the surface treatment, which increases the cost.

4 Sudden fall of the follower results in an impact and heavy vibrations in the machine. This affect the quality of the stirrup. The tapered surface of wooden plate provided for achieving overlapping ends, causes uncontrolled non-coplaner bending of stirrup wire, which is not desirable. These limitations further encouraged the authors for the development of the new mechanism for stirrup making. MECHANISM SYNTHESIS OF STIRRUP MAKING MACHINE WITH SINGLE WORK STATION USING COMPUTER SIMULATION. Considering the limitations of stirrup making machine described above, a thought is given for new mechanism for stirrup making machine so as to remove problem of uncertainty in releasing the stirrup wire by follower. In the begining, it is decided that the path of follower or bending pin should be such that, after completion of bending, the contact between the bending pin and the stirrup wire should be ceased automatically. Simulation using elliptical path An elliptical path of the bending pin is initially considered for the operation. Simulation of the motion is made as shown in fig.. A B A B C D C Fig. Simulation using elliptical path Fig. Simulation using circular path of pin rotating in clockwise direction. Stirrup wire Elliptical path of bending pin Guide plates The stirrup wire () is held between two guide plates () as shown in fig.. The bending pin traces the elliptical path () in clockwise direction. The pin comes in contact with wire at point 'A' and starts bending, the bending continues till pin reaches to point 'C' through 'B'. During the travel ABC, the wire is bent through 90 degree. The pin looses the contact with wire at point 'C' automatically.

5 Thus, from the simulation it is noticed that, releasing of stirrup wire is certain. Although, the releasing of stirrup wire by follower is positive, but fabrication for generating elliptical path is difficult. Simulation using circular path of pin rotating in clockwise direction In order to simplify the construction of the system, it is decided to rotate the pin in a circular path () and guide plates holding stirrup wire be off-set by some distance 'e' from the center of rotation bending pin. Simulation of the motion is made as shown in fig.. Here, the bending pin traverses in a circular path () in clockwise direction. The pin comes in contact with stirrup wire () at position 'A'. The bending of stirrup wire continues while the pin moves along the path ABCD. Thus, during approximately 80 deg. rotation the pin completes the bending of stirrup wire through 90 deg. From point 'D' onwards the pin starts loosing its contact with stirrup wire. Thus, from this simulation it is noticed that - Stirrup wire is positively released by bending pin By varying off-set distance 'e', spring back effect can also be controlled by bending wire through more than 90. Inspite of these advantages it is observed that approximately 80 rotation of bending pin is required to complete a bend of 90. Thus, only 80 deg. angular displacement of the pin, is available for feeding of the stirrup wire for next bending. This put limitations on feeding mechanism. Simulation using circular path of pin rotating in anticlockwise direction In order to overcome the above limitations, it is decided to rotate the bending pin in anticlockwise direction. The simulation of the motion is as shown in fig. 4. D A B C Fig.4 Simulation using circular path of pin rotating in anti-clockwise direction.

6 Here, the bending pin is allowed to rotate in circular path () in anticlockwise direction. The pin comes in contact with stirrup wire () at position 'A', The bending of the stirrup wire starts and continues as the pin traverses a path ABC during 60 deg. rotation of the bending pin. During 0 deg.rotation of the bending pin i.e. from A-B, bending of the stirrup wire is completed. From point 'B' onward, the pin starts loosing its contact with stirrup wire and at point 'C' it completely ceases the contact with wire. Thus, from the above simulation it is noticed that in addition to advantages discussed early, this simulation has following benefits - Bending of stirrup wire through 90 to 95 is completed only by 0 rotation of the bending pin and further rotation through approximately 0 ceases the contact with wire completely. Thus, for remaining 00 angular displacement of pin, feeding and repositioning of the stirrup wire for next bending is possible. As the bending pin comes in contact with stirrup wire very close to guide plates () (point 'A') it is possible to bend stirrup wire through small length even 0 mm. Thus, using this mechanism a smallest bend approximately of 0 mm length can be made which can save the material. Simulation of the motions discussed above are studied carefully and it is observed that the motion simulated in the third option can be used very effectively in developing the stirrup making machine with single work station. The detailed description of the machine is explained in the next section. PRESENT MACHINE WITH SINGLE WORK STATION The stirrup making machine is as shown in fig. 5. A 440-rpm geared-motor (9) is mounted on a structure (). The output speed of the geared-motor is 0 rpm. A specially designed arm (6) is keyed on the output shaft of the geared-motor. A bending pin (5) is inserted in a bush provided at the other end of the arm. The pin and the bush has loose running fit, which enables vertical movement of the pin within the bush. The vertical movement of the pin is controlled by a surface cam (7) which secured to the geared-motor below the arm. Thus, when arm rotates about the gear shaft, pin also rotates and reciprocates vertically within the bush and comes up and down from a slot () provided on the structure. The surface cam is of Rise-Dwell-Return-Dwell type having 00 SHM rise, 60 Dwell, 60 SHM Return and 40 Dwell. The entire assembly is referred as workstation. The stirrup wire placed between guide-plates () provided over the top supporting plate and held firmly by two rollers (4). One of the roller is a live roller and other is an idler. The live roller is rotated by a gearised d.c. motor (8) whose motion is controlled through an electronic circuit (0). This is referred as feeding system [] whose function is to position and move the stirrup wire in forward direction. The guide-ways, guiding stirrup wire, is off-set by distance 'e' as shown in fig.6 from the centre of rotation of the gear so that the stirrup wire is bent 5 more to compensate for the spring back effect.

7 Top supporting plate 5 Bending pin 9 Geared motor Guide plate 6 Arm 0 Electronic circuit Slot 7 Surface cam Structure 4 Roller 8 Gearised d.c. motor Fig.5 Front view and top view of present stirrup making machine using single work station with feeding system e E D F A B C Fig. 6 Schematic of bending operation perform by pin.

8 As shown in fig.6 when the stirrup wire is positioned, the pin comes in contact with it at point 'A' and starts bending as the pin rotates in anticlockwise direction. During this period the pin is at the topmost position. The bending of the stirrup wire continues until the pin reaches to the point 'B' where bending of the stirrup wire is completed through 95. However, during this period pin rotates through 0 only i.e. from point 'A' to point 'B'. Further rotation of the pin (after point B) in anticlockwise direction automatically ceases the contact with the stirrup wire at point 'C'. From point 'C' to 'D' the pin moves downward and reaches to bottom-most position at point 'D'. From point 'D' to 'E' the pin remains in the bottom-most position. From point 'E' to 'F' the pin rises slowly in the upward direction. At point 'F' the pin reaches to its topmost position. From point 'F' to 'C' the pin remains in topmost position. This vertical movement is controlled by surface cam which is located below the pin. After completing the bending through 95, the stirrup wire is fed in forward direction (during the period when pin is in the downmost position) and positioned it for next bending operation. This rotation is about 00 and therefore sufficient time is available to feed the stirrup wire. Photo fig. 7 shows the latest machine. Photo fig. 7 Latest stirrup making machine.

9 CONCLUSION In latest attempt a successful solution for the manual stirrup making is obtained. In the present machine, stirrup of varying sizes (04 x 04 to 65 x 65 mm) can be made. Use of cylindrical cam, spring (to cease the contact of follower with stirrup wire). 'L' shape follower etc. used in the earlier version are avoided in the latest stirrup making machine. In this system bending of stirrup wire having minimum 5 mm extended length against workstation is possible. The desired bend radius of stirrup can also be obtained. The rate of production is around 800 to 850 stirrups in 7 hours of effective working per day. The system is light, portable and can be handled by any operator. Cost of the machine is Rs. 40,000/- with minimum pay back period of 8 months. REFERENCES. Vanalkar A.V., et. al., Design Development and Fabrication of Stirrup Making Machine, Proceeding, 9th National Conference on Machines and Mechanisms, NACOMM 999, I.I.T., Powai, Mumbai, India, pp. 4-5, December Vanalkar A.V., et. al., Mechanism Synthesis for Stirrup Making Machine, Proceeding, 4th International Conference on Mechanical Engineering, ICME-00, BUET, Dhaka, BANGLADESH, pp. 7, December-00.. Padole P.M., et. at., Feeding system using Gearised D.C. motor for stirrup making, Proceeding, 0th National Conference on Machines and Mechanisms, NACOMM 00, I.I.T., Kharagpur, India, pp , December 00.