Pros and Cons of jackshaft power.

Discussion in 'Metal Boat Building' started by Capt Sport, Apr 8, 2007.

  1. lazeyjack

    lazeyjack Guest

    well down here we call a shaft driven off qnd running parellel to the main shaft, a layshaft, we call a jackshaft a filler shaft between inboard and outdrive
    i guess we all call a prop shaft, just that. allthough some use it for cars, God knows why
    I do know that on newbuilds, layshafts belted off the front end of the crankSHAFT, are seriously considered my the Engine makes, because of the torsional stress, , if you do this, then you must try have an equal and opposite force on the other side Cummin Cat give a CLOCK diagram that assists in setting up auxillary loads around the front of the crank
     
  2. alan white
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    alan white Senior Member

    Okay, Billy, I've got the V drive scanned, but I don't know how to put it up here. Never did it before. any suggestions? I got lost real quick. Web ain't my thing...

    Thanks, Alan
     
  3. alan white
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    alan white Senior Member

    V-drive
     
    Last edited: May 2, 2007
  4. rayk
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    rayk Senior Member

    Very clever alan white,

    sincerely rayk.
     
  5. TerryKing
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    TerryKing On The Water SOON

    V-Drive

    VERY interesting.

    Slightly like a mechanism I once saw with a horizontally-sliding plate with two vertical slots in it. There were two shafts with offset cranks that fit the slots. Kind of like a locomotive with side-rods.

    A question: There is quite a bit of reciprocating mass, right? It would have some vibrational force, I think. Of course, the oscillating section could continue past the pivot point and have a counterweight balance, right?

    This maybe brings up another possible configuration (I'm always looking for ways to re-use existing cheap junkyard parts that are hard to make yourself). OK, What If:

    - We took a standard automotive/truck "rear end" with it's typical ring gear and pinion gear.
    - Built a new housing with just the ring gear and it's bearing points, and
    - TWO pinion gears together in the same housing, angled as closely as possible together.

    So, there is this gearbox with two automotive type 'driveshafts' in a Vee.
    All the expensive parts like bearings, gears, splines, universal joint adapters etc. are junkyard parts and easily available. Only the housing is new.

    Truck rearends can handle large amounts of horsepower continuously. In a boat you'd want to build a water jacket into the gear housing if you were running 200 HP or so..

    Anyone seen such an approach??

    I'll have to look at a few existing rearends to see what the minimum angle could be....

    I'm determined to come up with a Marine drive system that is mostly standard automotive parts...
     
  6. alan white
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    alan white Senior Member

    A question: There is quite a bit of reciprocating mass, right? It would have some vibrational force, I think. Of course, the oscillating section could continue past the pivot point and have a counterweight balance, right?

    Note the offset counterweight on one of the shafts---um---the left one, cause it was just a sketch and I drew more detail on one side.

    a.
     
  7. Frosty

    Frosty Previous Member

    what the hell is that? ---Thats not a v drive

    This is a V drive, and the bit in the middle is a " jack shaft"
     

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  8. alan white
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    alan white Senior Member

    You mentioned the need to cool a gearbox, Terry. That is horsepower going down the drain, of course. That was why I worked out the V drive with dual cranks and needle bearings.
    I don't know, but I'd guess V drive manufacturers recommend a 10% or so increase in engine horsepower unless the V drive is supplied at engine RPM (meaning the drive provides the speed reduction instead of a gearbox mounted on the engine).
    I've worked with reduction drives too. My own approach used a trochoidal race, but unlike the ones out there already, which use ball bearings or parallel roller bearings, mine used tapered rollers and a wobbling stator. Picture 10 radially oriented tapered rollers in a cage. Now picture them lying on a round plate with a ring under the rollers. Then imagine the ring is shaped like the top surface of a pie-edge like mom used to make with her fingers--- a serpentine surface on the topface of the ring.
    Now see the serpentine surface as a sine wave. Further, give it 9 or 11 humps to the roller cage's 10.
    The roller cage now falls into 100% contact with the wobble plate, each roller in a slightly (1/10) different position relative to the humps and valleys of the wobble plate.
    Take your imaginary hand and wobble the roller plate without rotating it. seen edge-on, any point on the roller cage is seen to revolve in a perfect circle, but the circles do not progress left or right. Many small cranks limit the movement.
    Instead, the wobble plate rotates one revolution for every 10 revolutions of the cranks.
    All of this contact (50% of the rollers push, 50% "ride the backside") is rolling and not sliding contact. The 10:1 reduction is achieved at perhaps 99% efficiency. Compare this to a worm drive at 60% efficiency, or a spur gear drive at 88% efficiency.
    The drive also does not depend on two or three driving teeth that, in order to all settle into contact, require a certain backlash to develop. The power is transmitted instead at many points at once. The power driven through a common U-joint is tremedous relative to its size. Imagine a transmission that size putting out almost no heat!
    These kinds of transmission devices become more viable where power is limited and silence and lack of vibration (related problems) are required, like on a nuclear sub or a space station. Even in a steam turbine at a power plant (although heat can be recycled somewhat there), a ten to fifteen percent savings in energy is achieved in switching to a trochoidal-race drive.
    Another advantage of the wobble plate type is that it can be adjusted for wear if ever need be by moving the rollers inwardly. The other type (which I think is used (secretly?) on nuke subs for quiet operation) is not adjustable. Its rollers are parallel on the outside of a plate with a sinoid rim surface. As soon as I'd progressed to that design, I checked the existing patents, and saw that the US Navy had bought the identical patent in the early eighties. I saved the patent page, which I'll post (along with my identical design) when I figure out how (I emailed the last drawing to a friend in Holland, and she, god bless her, posted it for me).

    a.
    Alan
     
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  9. alan white
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    alan white Senior Member

    One note. The drawing shown actually requires an additional bearing between U joint and cranks which allows the crank socket (pie shape) to twist, something I noticed this early drawing doesn't show. Also can be done by allowing the fixed bearings of the U joint to revolve about an offset (as if the bearings were also cranking in addition to spinning).
     
  10. alan white
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    alan white Senior Member

    Ohhh. But wouldn't the boat go backwards? Does the whole boat have to be reversed in its position on the floor during installation? Isn't that somewhat cumbersome? And even then, the drive would stick out of the bow, which would be a lot of weight up there, not to mention dangerous when anchoring?

    Alan
     
  11. BillyDoc
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    BillyDoc Senior Member


    VERY interesting Alan! But, if I am understanding the drawing isn't that a "U" joint on the bottom with bearings oriented horizontally and parallel to the page as well as perpendicular to the page? If so, and the input shaft, the output shaft, and both of the "crank" shafts are all axial with the point where the two axis of the "U" joint bearings cross (which looks like how you have drawn it), then I would think no further equipment is required except the missing counterweight! I love it!

    BillyDoc
     
  12. alan white
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    alan white Senior Member

    Imagining this mechanism is a mind-f%&k, and I'm glad you see what is there so far and appreciate it.
    The best way to see why another element of movement is required is to imagine the cranks further apart. What if they were 180 apart? See how at that orientation, the fixed U-joint pivot is axial to the two shafts if centered between them as it is in the drawing.
    The vertical movement from BDC to TDC is impossible because now the fixed bearings of the U-joint don't do anything relative to the input and output shafts.
    This is a lockup situation. While it is possible to run the shafts inline (at 180), they would also have to be indexed 180 apart. But inline of course needs no more than shafting.
    This gets into an area that describes how my trochoidal wobble drive works in a torque multiplier (speed reducer).
    You see, a crankpin rotates at a slower speed relative to the other as it gets further away from right angles to either shaft because the fixed U-joint bearings allow the pie-shaped arm to make the greatest arc when the fixed U-joint axis is closest to right angles to a given shaft.
    It's counterintuitive, I know, and seems very puzzling. The idea, however, is to get the pie-arm to wobble like a pie-shaped portion of a spun penny would. Then any number of cranks could circle the U-joint, each a bit further around than the last, and the pie piece would become a whole wobbling pie.
    Allowing the fixed U-joint bearings to ocillate on their own cranks makes for a control element (this would just be a bearing inside of a round cam shape with another bearing outside of that).

    Fun, huh?

    Alan
     
  13. alan white
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    alan white Senior Member

    Imagine this: A spherical explosive mine (the kind with spikes sticking out all over) is fitted with pencils facing out instead of spikes. This assembly is fitted into an even larger sphere which just touches the tip of every pencil.
    One grasps the outer sphere anywhere and rotates that chosen fixed point in a circle. What have the rest if the pencils drawn inside the outer sphere?
    A bunch more circles? How many other circles could be drawn by the "slave" pencils?
    Heh heh.

    A.
     
  14. BillyDoc
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    BillyDoc Senior Member

    Not sure that "fun" is the right word . . . but certainly intriguing!

    I'm still missing something here. If I'm reading your drawing correctly the "U" joint shafts cross at a common point, and the bearings shown near the center-bottom are oriented on a shaft perpendicular to the page and the other bearings on the "U" joint are aligned horizontally. So, the motion of the "pie" is spherical around that crossing common point and a simple ball centered on that point would also work . . . except that it would have another degree of freedom and allow the "pie" to rotate around it's long axis. Not a particularly good thing in this case. Perhaps a CV joint would be better!

    I can certainly see where the 180 degree case would not work, but this seems to me to be a special case and not applicable where the angle between input and output shafts plus the additional angle from the offset crank is less than say, 30 degrees (whatever the normal angle a "U" joint can usually accommodate is) and a line bisecting the axes of input and output shafts is perpendicular to both bearing sets on the "U" joint. In this other special case it seems to me that an input shaft with an axis aligned with the crossing point of the two "U" joint bearings, and with a crank pin also so aligned and not so far from the input shaft axis to exceed the normal range of the "U" joint, would rotate very nicely and simply impose a "wobble" on the "pie." The output shaft would be similarly constructed and would follow this "wobble" in the same fashion. (I'm assuming normal bearings on input and output shafts.)

    So, what did I get wrong?

    BillyDoc

    BillyDoc
     

  15. alan white
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    alan white Senior Member

    You got it but for one important detail. The same detail causes two u-joints to be required any time shafts are not exactly inline on a rear wheel drive car. Otherwise, the driven shaft would speed up and slow down relative to the driving shaft.
    Remember too that a V drive eats power, so we want a better V drive to offer the oppertunity to downsize the engine with no real power loss at the prop. The smaller engine saves the weight of extra fuel, and its own weight is less, and so even if a 100 hp engine at 88% transfer efficiency were putting out a true 88 hp at the prop, an 88 hp engine at a 98% efficiency is putting out about 86 hp driving a lighter boat, meaning the power to weight ratio is higher and the boat will actually do the same thing with maybe 84 hp!
    That amounts to a 16% real difference.

    Getting back to the improved V drive, you have noticed that a 180 degree offset wouldn't be parallel any longer. The clue is in the fact that it WILL work at 180 if the arm (now a wobbling plate) is down on one end and up on the other.
    This cannot be said, however, for any other positioning without another element of movement and control.
    I mentioned the sphere within a sphere because it helps visualizing this. Those pencils could describe two side-by-side circles, but if one of them is a perfect circle, the other will be ever so slightly elliptical. Remember that to make a single circle, it is guaranteed that a spot exactly across the sphere will make a perfect circle too, but starting at the bottom instead of the top.
    A second prayed for perfect circle next to it (say 10 degrees away) would make (if it could) an upside down circle opposite its location also. Now you have something interesting. The first (really perfect) circle drawn isn't exactly across from the upside down circle of the (wishfully perfect) circle 10 degrees away. There are four circles now, but two of them aren't perfect circles. In fact, if a ring of (dreamed for) circles were drawn around the whole thing, they would progressively become ellipses, then at 90 degrees straight lines, and then ellipses drawn upside down, and finally, a perfect upside down-drawn circle at 180.
    A way to defeat this paradox is to place the only true circle invisibly between the two shafts. Each shaft speeds up on the longer part of the ellipse and slows down on the shorter part.
    The eccentric around the fixed bearings does this. You can imagine this with effort. The two cranks both describe slight ellipses simultaneously, neither of them taking the perfect circle position. The ellipse isn't on the drawing, or relative to the gearbox. It is what would happen on a piece of paper if you drew a circle at a constant speed and someone moved the paper up and down under your pencil while you drew twice for each revolution. You would "speed up" across the paper sometimes and "slow down" at other times. So the ellipse lives in an imaginary world where that picture is the being drawn in accelleration and decelleration curves by an index line on a point somewhere on the outer face of a shaft crankpin. It could be shown by a pencil attached to the pie-part facing a wheel on either shaft that had paper on it. Remember, the pie part is tilting, speeding and slowing that pencil even while the shaft spins at a constant rate.



    Alan
     
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