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Automotive One piece or multi piece crankshaft

  1. Oct 25, 2016 #1
    Just curious to know what difference does it make while using multi piece crankshaft instead of single piece crankshaft?
     
  2. jcsd
  3. Oct 25, 2016 #2

    Nidum

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    In what application ?
     
  4. Oct 25, 2016 #3
    2/4 stroke engines.
     
  5. Oct 25, 2016 #4
    One-piece cranks are mainly found on older American-made bicycles, and children's bicycles made for the U.S. market. Most one-piece cranks use a single metal forging as left crank, right crank and bottom bracket axle.

    Bicycle bottom bracket (crank hanger) shells fall, in general, into two groups, threaded and threadless. One-piece cranks fit only the unthreaded shells 51.3 mm (2.02") in diameter.

    One-piece cranks are the easiest type to service, and require no special tools. All you need is a large adjustable wrench and a screwdriver.

    The types of engine that commonly use this method of construction are either single-cylinder four-stroke engines such as we find in motocross machinery, or two-stroke engines. Two-stroke engines find common use in motorcycle racing - although sadly even the 125 cc Grand Prix class is soon to disappear - and karting.

    In conventional two-stroke race engines, crankcase lubrication is looked after by oil that is either pre-mixed with the fuel or metered in some proportion to the fuel by a pump from a tank. After it has fulfilled its lubrication task, the oil passes into the combustion chamber where it is combusted with the fuel. Given the marginal lubrication, the needle roller is a natural choice for such engines.

    The crankshaft for a single-cylinder engine can typically be made in two or three pieces. The two-piece crankshaft has one main bearing, a crank 'cheek' and the crankpin as a single piece, and a second piece which is a main bearing journal and a crank cheek. The crankpin is a heavy interference fit into the cheek of the second piece. A three-piece single-cylinder crank has a separate crankpin.
     
  6. Oct 25, 2016 #5
    It was helpfull.
    But my question remains unanswered!
     
  7. Oct 26, 2016 #6

    Ranger Mike

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    Lets look at the traditional methods to manufacture crank shafts

    Starting with cast cranks, there are three options in increasing order of strength:

    Cast iron, , Nodular cast iron, Cast steel

    Cast iron is not pure iron at all. Pure iron is too soft to carry the load so we have to add graphite for strength.

    In regular cast iron, the graphite particles are flakes. In nodular cast iron, they are spherical nodules. These nodules gives the cast iron more strength and flexibility.

    Cast steel is stronger than “ Iron” simple because steel is much stronger than iron. As with iron...different alloys of steel will give a crank different levels of strength. To make a cast crankshaft, we heat iron or steel to its melting point then pour it into a mold. The up side is that this process really cuts down machining costs compared to the machine cost of a billet crank shaft.

    Forged steel cranks begin with a metal bar that is heated until it is soft enough to pounded into the rough shape of the crankshaft. Then it is machined into the final shape.

    The billet steel crankshaft starts with a large bar of steel is forged into a cylinder shape as large as the finished crank shaft.

    I could go into which is strongest and why ..but that is another post. Lets just says the forged billet is the top of the line and the cast iron crank is the weakest.


    When we talk about multi piece crank shafts, I guess they would be ok but introducing the welding process can introduce a lot of unwanted stress points, really messes up grain structures, undetected stress cracks, hidden grind burns. A lot of hot rodders made their own Stroker crankshafts back in the day but the horsepower was way under 500 hp back then.

    Yes you can weld up a crank shaft and it will probably work if you don’t go too big on the torque and horsepower.
     

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  8. Oct 26, 2016 #7
    The main problem with a multi piece Crankshaft...is lost motion...hence newton's cradle....whenever there is two much play in the crank bearings...Motion is being transferred to the wrong areas, instead of being transferred to the critical mass thats being accelerated. Don't get me wrong a two piece Crankshaft may be the answer to get more power in a over-square engine design.
     
  9. Oct 26, 2016 #8

    JBA

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    Having once been required to design a multi-piece welded design crankshaft for the production of a three cylinder positive pressure pump I can verify that all of the fabrication and welding issues iterated above by Ranger mike are absolutely true.
     
  10. Oct 26, 2016 #9
    That was well elaborated perspective. Thank you.
    But the type of crankshaft I am dealing with, doesn't consist of welded joint. Instead, crankpins are press fitted in the crank webs.
    But yes the power ratings with which I am working is certainly not at all as high as what you have mentioned. So what we can say is, multi piece can be a better option considering low power rating with sufficient strength and ease of manufacturing.
     
  11. Oct 27, 2016 #10

    Randy Beikmann

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    A major reason I've seen for multi-piece cranks is to allow roller/ball bearings instead of (2-piece) journal bearings. I think early Porsches used them - they were bolt-together. But one-piece cranks are stronger, and I'm sure easier to machine to maintain alignment of all the main bearings (unless it's machined after assembly).

    How large is this engine? Is there a specific reason for considering an assembled crank?
     
  12. Oct 27, 2016 #11
    Really cant reveal the engine details, but what can I tell you is, it is basically a low cc engine with low rpm range. I have just started working on this, so I myself is trying to figure out many HOW? & WHY? regarding the topic. As per my knowledge, journal bearings works efficiently only at higher RPM, whereas roller/ball bearing works efficiently at low RPM. So further what we can conclude more in addition to my reply to @Ranger Mike is that low rpm engine can efficiently run with roller/ball bearing, and so as to allow usage of these types of bearings multipiece crankshaft should be employed.
     
  13. Oct 28, 2016 #12

    Ranger Mike

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    Ifin you are gong to make a low rpm engine with mild horsepower / torque you had better study up on what works and what has been tried and don’t work.
    Save your self a lot of grief.
    Question – how is the crank going to get lubrication? pressure oil feed or splash oiling?
    Question – How is any load going to be hooked up to the crank? clutch? Centrifugal clutch (go kart type)?
    What is frequency of the load applied?
     
  14. Oct 29, 2016 #13
    When engineering a motor like you described I would start from a base point. That base point is the expansion-contraction aspects of the type of energy that you are going to use to drive the motor .The next part is storing and recovering lost motion. It may not even be necessary to use a crank or even bearings. The least amount of moving parts you can get by with would even be better. Then I would build it in my mind's-eye before I built it in real time... I can see Ranger Mike's point too... Also... it can be a lot of fun figuring out the... HOW's & WHY's .
     
  15. Oct 31, 2016 #14

    Ranger Mike

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    this is a teaching forum for the student as well as a forum for those who wish to improve their knowledge. Correct terminology is a must. Motors are electric. Engines are internal combustion (IC) and 2 or four cycle in nature..just a point. I got no idea about contraction of energy nor motion recovery..maybe some one can enlighten me, please.
     
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