Mechanical design systems

PROBLEM STATEMENT:
The design is to engineer a drive system to operate two extrusion rolls in opposite directions to compress the caramel. The drive system consists of a flexible drive system that operates a spur gear drive, which in turn operates the extrusion rolls at equal and opposite speeds. The power source to this design is a five horse power normal torque AC electric motor, operating at 1160 rpm. The system must be designed to run 24 hours per day, 3 days per week. There will be 4 shafts is the drive system. The shaft that is being driven by the flexible drive system directly is to be called shaft A, for design reference. The extrusion rolls shafts are to be called shaft B1 and B2, for design reference. The last shaft C is in the system only to reverse the direction of rotation of one of the extrusion roll shafts. The speed of shaft A is to be determined by the designer. The speed of the extrusion rolls is to be 200 rpm. When designing this drive system calculate all forces, life expectancies and stresses for both systems. The centerline distance from motor shaft to shaft A is to be 72 inches.




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DESIGN DECISIONS:
While designing this multiple drive system there are many decisions to be made in order to successfully design the system according to the problem statement. The first is deciding whether to use a belt drive or a chain drive; one would realize that the system is operating at fairly low speed so a chain would be ideal. The next step to designing this system is to consider and analyze a speed for shaft A. Factors must be considered while deciding the speed of shaft A. The factors are that the speed of shaft A should be high enough so that the speed ratio of the gears is large enough, so that the gear sizes make a large enough distance between extrusion rolls. When designing this system one would choose around 480 rpm. Using the rpm out of the flexible drive one would select the appropriate gears for the application. Which the designer will find is a difficult task considering contact stress. When choosing the proper chain size and sprockets one will find that a number 40, 19 tooth “n1” and a 45 tooth “n2” are optimal stock components and get you fairly close to the desired 480 rpm. When designing the gear drive system one must understand the configuration of the gears. The chain drive operates shaft A, shaft A has a sprocket and a gear on the opposing ends (sprocket= N2) (gear= G1A). The first decision that has to be made when designing a gear drive is what size pitch is right for the design. When deciding this one would find you can either use an 8 pitch or a 6 pitch gear. One would realize during calculations that the 8 pitch gears don’t work so well for this gear design due to high contact stress which would be high material cost. The designer would see that a 6 pitch gear set yields extremely high horse power ratings but, is still the best selection. Gear G1A is a 15 tooth gear and drives two shafts, shaft B1 and C. Shaft B1 is one of the extrusion roll shafts and has power transferred to it through a 36 tooth G2A. Shaft C is in the system to change the direction of rotation of shaft B2. Shaft C has a gear identical to shaft A but is called G1B for reference. Shaft B2 is the other extrusion roll shafts that is equipped with a gear identical to shaft B1 but, labeled G2B. Shaft B1 and B2 are rotating at the speeds the rolls rotate in opposite directions so the caramel is drawn though the rolls and compressed. When designing this system one would realize that this system is compressing the caramel and when compressing something that is going into another process one would always want to change the amount of compression that the system is applying on the caramel to make sure it is just the way they want it when going into the next process. One would realize there is only one way to do this. That is to make the gear G1B and G2B adjust. Making G1B adjust on an arc that’s radius is equal to the center distance between shaft A and shaft C as shown on Drawing #2. Shaft C is only held in place by the control arms, stops have to be made so that gear G2B can not come in contact with the pinion G1A. Gear G2B shaft would be mounted on pillow blocks that are mounted on slides that would adjust on a straight line level with shafts A and B1. To keep the centerline distances perfect for the gear sets that are adjustable. Control arms must be made which will operate the amount of compression that is exerted on the rolls. Two control arms must be made for
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between shafts A and C, for between shaft C and shaft B2 and for between shaft B2 and the hydraulic cylinder that will operate the Compression. So, when the hydraulic cylinder extends shaft B1 moves closer to Shaft A but keeps required centerline distance due to the control arms and shaft C moves downward along its arc to reduce the distance between the extrusion rolls. While designing this adjustment system for the gears one would realize the control arms should be on both side of the gears so the gear teeth mesh perfectly. One would also realize that a lubrication method must be chosen for the chain drive system. While designing this system one would realize that an oil drip method of lubrication is sufficient being that its life expectancy is 37,000 hours. The last decision to be made is material selection. One would choose a Grade 1 flame hardened 50 HRC steel for the pinion (gears; G1A and G1B). One would decide that a ductile (nodular) iron ASTM A536 120-90-02 quenched and tempered gear is needed for the driven gears (gears; G2A and G2B). The driven gears will be moderately expensive but compared to an 8 pitch pinion that is made of Nitrided, 2.5% chrome it is much cheaper.

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DESIGN CONCLUSIONS:
When designing this chain drive system one would realize that a no. 40 chain is the best option. In conclusion from engineering this drive system one would realize that using an 8 pitch gear set, they would run into a problem with material selection due to the extremely high contact stress. When designing this system one would realize that a 6 pitch gear seems excessive but the calculations prove that it is needed to obtain a low contact stress so that the gears can be made of a cheaper material and still be able to transmit more power than the 8 pitch gear set would have been able to using high quality steel gears that costs much more. Being that this a molten caramel extrusion process and one would conclude that at the end of each operation sequence more than likely this machine will have to be cleaned. The only thing is that you would not want the gears to be washed or even get wet. So, the solution to that is to make a water tight cover out of light steel where the shaft B1 and B2 exit the cover, put sealed bearings so that water can not enter to closed drive system environment. On the opposite side of the extrusion rolls of the over make a hinged seal door for maintenance. The only other things that will stick out of the cover are the hydraulic hoses that operating the compression cylinder. The hydraulic tension lever should be mounted in such an arrangement that the operating can reach easily for adjustment. The system requires no chain guard because the entire drive system is completely enclosed.