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Thread: center-lock hubs and wheels

  1. #91
    Why would a precision ground spline have less play than a precision ground drive pin?

    Anyway, I hate forum arguments so I guess I'll just say that if it's such a good idea, why hasn't anybody switched?

    Case and point: http://www.youtube.com/watch?v...U1E&feature=youtu.be

  2. #92
    Originally posted by Canuck Racing:
    Why would a precision ground spline have less play than a precision ground drive pin?
    The angle of the 'teeth'.
    When tightening the wheel, the joint will not bottom out until both sides of a tooth are in contact, thus there is no play. As Z pointed out; this is not quite true for all teeth when there are many splines due to over constraint.

    A pin must be undersized by some amount to allow the wheel to be removed. This under-sizing results in rotational play of the wheel.
    Nathan

    UNSW FSAE 07-09

  3. #93
    Do Force India's wheels satisfy this?


    EX-FSAE

  4. #94
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    ibanezplayer, Hard to tell from the photos, but that looks like a conventional peg/pin drive (effectively an axial spline). The wheel has some extra material machined away, turning the holes into slots, but it looks like all the mating surfaces on the pegs are axial (see below for more explanation). I guess the noses of the pegs are rounded and tapered simply to allow easier and quicker fitting.
    ~~~~~o0o~~~~~

    Originally posted by Canuck Racing:
    Why would a precision ground spline have less play than a precision ground drive pin?
    Canuck, Nathan's got it right.

    An axial spline is a conical spline with half-angle = 0 degrees. This unique angle allows the two parts to slide indefinitely (relative to each other) along the axis, without ever making complete contact. This is USEFUL, for example, in gearboxes where it might be necessary to transmit torque with the sliding part in different positions along the shaft.

    It is USELESS for a completely constrained 6DoF joint. Hence the conventional peg/axial-spline hub joint requires two flanges, perpendicular to the axis, that come into contact to constrain all DoFs. The axial-spline surfaces (pegs) require clearance for easy fitting, so ultimately the joint relies solely on friction between the flanges to control all 6DoFs.

    Canuck, your stressing of the fact that the conventional joint requires a "$30K wrench" to apply the enormous "2000ft/lb" torque in the hope that it "should" work, is proof positive of what a stupid joint it is!

    Incidentally, the P&W aero engine I referred to above initially had HEAVY PRESS FIT axial splines in the built-up crank, and still they didn't work as well as the later face splines.

    ... if it's such a good idea, why hasn't anybody switched?
    Because contrary to the views of those in motorsport, it is actually a minority pastime, peopled mostly by egotistical, brain-dead buffoons, who do NOT have to achieve any level of success, such as winning, to stay indefinitely in the sport, so instead they spend most of their time looking for excuses for why they are losing.

    A typical excuse is "Awww geez..., we can't afford the $30k wrench, so our wheels keep falling off...".

    Z

  5. #95
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    Should anyone want to try a conical spline type wheel attachment, here are some things to consider.

    1. Very important! The nut is only required to provide "force closure". That means it must NOT have a conical seat as is usual, because this seat constrains the wheel in the radial direction, and fights the splines. The nut mating face should be FLAT!

    2. For structural reasons, a nut with large thread diameter with a coarse thread of only a few turns is preferable. A similar sized bolt can also be used, eg. hub threaded internally (inside the splines), and the bolt hollow for less weight.

    3. Cone angle. A face spline with 90 deg half-angle has the machining advantage that both sides of the joint are the same (it is a hermaphrodite, both male and female). A cone half-angle of 45-80 deg (male hub, female wheel) may be better because of greater bearing area on the soft aluminium wheel. Also, if the cone apex lies on the mating plane of the nut, then differential thermal expansion (Al/steel) doesn't effect pre-load.

    4. Number of teeth. One or two DON'T work. Three teeth work exceptionally well, but might be a bit bulky, and so heavy. Four is bad... Ten or more can give a thin lightweight joint, but requires reasonably high accuracy (ie. no good with a hacksaw, or rough castings, but no problem with machine tools).

    5. Fewer, larger teeth are more tolerant of dirt. The pebbles get pushed into the clearance zones (ie. the valleys, see below). Many fine teeth don't like dirt, because the boulders have no place to hide. (This is like coarse vs fine threaded fasteners.)

    6. Tooth profile. Very important! The opposed teeth must CONTACT ON THEIR FLANKS ONLY! No peak to valley contact allowed! So undercut the valleys (generous radius) and chop off the peaks. For a three tooth spline a 60 deg included angle works fine. For many teeth a smaller angle (tall narrow teeth) is good because it allows the teeth to bend slightly for more complete meshing.

    7. (Edit+) The teeth do NOT have to be symmetric or equal sized on the two parts of the joint. For example, narrower steel teeth can mesh with wider aluminium teeth, making their shear strengths more equal. Also the teeth can be "saw-toothed" (/1/1/1/1) if the torque to be transmitted is expected to be asymmetric. However, this probably doesn't make much difference, and symmetric teeth would be the best place to start.

    Z

  6. #96
    Z, is this what you are talking about basically? As you tighten the nut from the left side, the splines mesh up and lock together. And just by looking at the model, it is easy to see that a hack job can work using a 3 spline setup.

    Yes, I know the model is very bad but I just made it very quick so I understand this debate a little better.


  7. #97
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    Originally posted by swong46:
    Z, is this what you are talking about basically?
    Swong,

    Not quite. Your teeth have a rectangular profile, so as you tighten the joint the sides of the teeth remain loose, and only their tops make contact.

    Redraw the teeth with a triangular profile ("A" shaped). Also make them bigger so easier to see. Now when the two parts touch, the contact is at the sides of each tooth (ie. at "/" and "\"). So 3 teeth with 2 contacts each = 6 contact zones, for complete constraint of 6 x DoF.

    I'll try to figure out how to post pictures, but that might take some time. Not much hardware here for getting the pictures into this interweb box.

    Is that "google-whatever" place you use free? Is it easy to use (ie. not too many hurdles)?

    Edit: I went to googleusercontent and "no such place". Can someone please suggest a good image hosting site?

    Z

  8. #98
    Gotcha, I understand now. As for a photo hosting thing, use Picasa. It is included with all gmail accounts.

    Once you log in, click photos on the very top

  9. #99
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    Do you all mind if I ask a really dumb question?

    Why?

    What is the purpose of a centrelock wheel in FSAE? All the professional examples I see above are for racing series where pitstop time is critical. Since when has wheel change been a time-critical issue for us?

    I can quote an example where a centrelock wheel has cost a podium finish, and that was our mob in Detroit 2005. Centrelock came loose, Endurance over...

    I think all the above posts are very interesting and there is some good engineering discussion going on. But I just think it is all about solving a problem that isn't really ours. Can anyone convince me otherwise, I'm certainly willing to listen.
    Geoff Pearson

    RMIT FSAE 02-04
    Monash FSAE 05
    RMIT FSAE 06-07

    Design it. Build it. Break it.

  10. #100
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    Geoff,

    Why centre-locks?

    I'd say the reason FSAE teams use them is that "they are on all the F1 cars" (RMIT'05??? ).

    The main thrust of my ranting is that if a team really wants to use them, then please don't do it like F1. Why?
    I can quote an example where a centrelock wheel has cost a podium finish, and that was our mob in Detroit 2005. Centrelock came loose, Endurance over...
    Geoff, I'm sorry that it was your mob that proves my point!

    Much of what I have written above is stuff I have been pushing for years, but with no results. Just felt I may as well get it out here. There was an article in Race Tech a few years ago about "wheels still falling off...". I wrote to the editor and he replied "Yes, I'd like a short article about these conical splines...". I sent the article ... and ... nothing ...

    The thing is, a well done centre-lock on a conical spline would be a slightly lighter and stronger connection than the standard multi-bolted flange. And possibly more time consuming for the team, although that depends on their facilities.

    It would certainly be educational, given that conical splines are perhaps not well known, and by comparison the normal centre-locks are abominations. Sure, the team wouldn't be solving an important FSAE problem. But solving a problem that is plaguing much of professional motorsport might be fun.

    Anyway, BBolze dragged up this old 2007 thread and asked the question, so I hope we've gone some way to answering it for you...

    (And regarding bearing preload, BBolze, better do it separately to the wheel fixing.)

    Z

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