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    Discussion in 'The main mechanical design forum' started by Goldenwheatfield, Dec 9, 2014.

    1. Goldenwheatfield

      Goldenwheatfield Member

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      Hello all, I am a freshman enrolled in Mechanical engineering.
      Growing up, I have always been fascinated by the LEGO technic toys and the real-life machines and wondered how do engineers design these machines and mechanisms.

      When someone design a complex machine or a mechanism, how do they figure out the size of each components(gears, axle, pulleyes...etc) inside the machine? And how do they decide on what machine elements they are going to use and place them in which places inside the machine..etc ? Decisions on shape of each elements..etc
      They must have the ability to see the big picture in the back of their mind and visualize the the whole mechanism working to achieve the desired functions.

      As above, I want to know more on this area. But I do not know where to look for.
      I asked a professor and he told me to look into synthesis or machinery.

      Help would be greatly appreciated!

      Sincerely,
       
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    3. Lochnagar

      Lochnagar Well-Known Member

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      Hi Goldenwheatfield,

      I see you are from Canada - and from your pseudonym - I am going to take a guess - you are probably somewhere near the Canadian Prairies - which is an area that is a great producer of wheat - and lots of farm machinery.

      I was brought up on a small farm - and I found farm machinery very interesting - because you have a wide variety of mechanical and electrical systems, like:-
      Engines, gearboxes, hydro-static transmissions, clutches, hydraulics, sometimes pneumatic's, chain drives, belt drives, shaft drives, mechanisms, electrics, electronics, and now software too.

      So I would try and get a holiday job on a farm - and I am sure you would learn a lot from the wide variety of machinery you would come across - and realizing its limitations - and why it has these limitations.

      When you design something - it is a combination of what you have seen before - together with appropriate analysis - to make sure it is fit for purpose - and as you get more experienced over time - you will perhaps combine this with some lateral thinking - to incrementally move the design forward.

      I think however in the first instance - you need to get that practical experience I have suggested above.

      Hope this helps.
       
    4. Goldenwheatfield

      Goldenwheatfield Member

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      Thank you Lochnagar!
      i am actually from the west coast and I have only visited the prairies once ;)

      I agree with you that I need the practical experience. Also you mentioned that with the experience and the appropriate analysis, I would have a better foundation to design. If I may, could you elaborate on "the appropriate analysis"?
       
    5. Lochnagar

      Lochnagar Well-Known Member

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      Hi Goldenwheatfield,

      I think on the West Coast of Canada you have lots of tree felling and logging - and the machines you get in that industry are just as interesting as farming machinery. You have what are called;
      1) Feller bunchers
      2) Forestry harvesters
      3) Forestry forwarders
      See pictures below - (I think you may need to click on image in the last link below to see it full size). Maybe you could go onto Youtube and see these machines in action - and you will see how harsh the conditions are.

      https://www.deere.com/en_US/products/equipment/feller_bunchers/feller_bunchers.page
      http://commons.wikimedia.org/wiki/File:Harvester_John_Deere_1170E.JPG
      http://www.komatsuforest.com/default.aspx?id=2413&mode=gallery

      I think you can see that these machines have to endure very harsh conditions, and are frequently coming into contact with tree stumps, and boulders - and the operators sometimes give these machines some abuse too - and if you look at them - you will quite often see parts that have plastically deformed, and sometimes fatigue cracks where it has been welded.

      However, the simplest machine you could study - is your bicycle - which will possibly have an aluminium frame. If we take the frame for example - we tend to break the loads down into different load cases, and also load combinations - so in simple terms we might have:-
      1) The frame has to support your weight through the reactions coming from the two wheels. Bear in mind that the front forks are subject to axial load - and they are slender - so there needs to be a buckling check done here too.
      2) The frame has to support your weight as you stand on the pedals trying to impart maximum torque through the chain drive to the rear wheel. Bear in mind that one pedal will take more load than the other - so the problem becomes a three dimensional load on the bottom bracket/crank case.
      3) The frame has to take the reaction force from heavy breaking as you are going down a steep hill - which is most severe on the front forks - as you have more load on the front wheels due to the slope.
      4) The frame has to be able to take cyclic loading - coming from you pedaling, as well as impact loads from the road - from undulations and pot holes.
      5) Etc.

      So in simple terms you need to do a free body diagram - for every load case - and analyse the frame for that load case. Then there are usually - dynamic factors applied to the static loads - and safety factors applied to the yield stress. As I said above, you may well also consider load combinations - and check the suitability of those to.

      So the analysis involves hand calculations and finite element analysis - from the above design criteria.

      You may well find that your aluminium bike fails at the welds - you will see cracks - since the fatigue strength of welded aluminium is not very good by comparison to steel - see SN curve in link below.

      http://en.wikipedia.org/wiki/Fatigue_limit#mediaviewer/File:S-N_curves.PNG

      http://forums.mtbr.com/general-discussion/frame-crack-just-paint-crack-863153.html
      http://www.tetongravity.com/forums/showthread.php/192277-Welding-An-Aluminum-Bike-Frame
      http://forums.mtbr.com/general-discussion/how-many-frames-have-you-broken-725307.html

      Hope this helps.
       
    6. thebestjake

      thebestjake Member

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      Lochnager gives some good advice.

      What you are asking isn't really a field of study. (They don't offer a college degree in it) you are talking about design engineering. If you are interested in designing machines them a BSME is a good degree.

      I am a design engineer. How did I learn to size components? By sizing components. I just started designing and building things. Used them. Broke them. Redesigned. Broke them more. Redesigned.

      A BSME will give you all the analysis tools you need, but the designing is up to you. Working under other design engineers is helpful. After your sophomore year you can start to do internships. DO AN INTERNSHIP. actually as many as you can. You get paid pretty well. Get experience to set yourself apart when you graduate. Be very picky about what you get. A lot of MEs go to manufacturing but you want to try to get design experience.

      Go to the career office at your school they should be able to help you start the process of finding an internship.
       
    7. Goldenwheatfield

      Goldenwheatfield Member

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      Thank you so much Lochnagar and thebestjake!! Thank you!
       
    8. Goldenwheatfield

      Goldenwheatfield Member

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      Is BSME stand for bachelor of science in mechanical engineering?
       
    9. Goldenwheatfield

      Goldenwheatfield Member

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      From where I study, we have B.eng mechanical engineering which stands for bachelor of engineering in mechanical engineering. I wonder if there are any difference between BSME and B.Eng mechanical engineering...

      thank you!
       
    10. thebestjake

      thebestjake Member

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      Yes BSME is Bachelor of Science Mechanical Engineering. In the States we have BS and BA (Bachelor of Arts) degrees. I can't say for sure but I would guess they would be comparable.
       

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