Outside the box -- sort of

Discussion in 'Propulsion' started by Wingandaprayer, Dec 13, 2011.

  1. Wingandaprayer
    Joined: Oct 2011
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    Wingandaprayer Junior Member

    Hi guys, I need help finding a resource(s) for the dimensions of off-the-shelf (automobile) CV axle assemblies. Every online parts house in the world will sell me an exact OEM replacement (cheap) if I will only tell them what car I have. :confused: Help! I haven't even named my boat yet, let alone given it a make or model number. :D

    Extra points if you're an (ex) car mechanic with vast knowledge of the subject.
     
  2. gonzo
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    gonzo Senior Member

    What is the application? You have to decide the CV joint specifications. With that, you can choose among the available models. They make them really small for a sub-compact car to huge industrial sizes.
     
  3. Petros
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    Petros Senior Member

    I have lots of car repair experience. Contact the manufacturers of the CV axles, most of them will have dimensioned drawings. Failing that, I would go to a wrecking yard with a measuring tape and find an axle about the size you need (choose a common car from your area), and than design the installation around the specifications of the one you choose.

    The CV axles are a great way to make a smooth running and durable drive shaft, because of the numbers produced they are far less costly than most other ways to make a drive shaft. Best is to just design the installation you are ding around the axle you choose.
     
  4. gonzo
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    gonzo Senior Member

    CV joints are not designed to take thrust. They will fail in short time if you don't use a thrust bearing to take the load.
     
  5. Landlubber
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    Landlubber Senior Member

    CV joints, cardan shafts are actually NOT constant at all, the Thompson coupling however is, try their site for some real enlightenment.
    http://www.thompsoncouplings.com/
     
  6. Wingandaprayer
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    Wingandaprayer Junior Member

    Thanks for the responses.

    However,
    Gonzo said: "You have to decide the CV joint specifications." but that is like creating a hypothesis before you gather evidence. Not a good way to do science, or design. Also, "CV joints are not designed to take thrust.", but of course that's what wheel hubs and bearings are for. Oh yeah, the application is to push a boat, but I suspect you knew that.

    Petros, two things: One, I'm in the Caribbean and junk yards are a bit scarce. Two, I do intend to design the actual install around what already exists. That's why I need numbers (specs). Electronic data is much lighter than sample CV axles to manipulate, and somebody will have already done the measuring for me, I hope.:)

    Landlubber: Thompson couplings is completely out of the small CV joint business, having chosen to focus on industrial strength applications exclusively. Too bad, but I probably couldn't have afforded them anyway.:(

    Here's the thing. I have a blank piece of paper and lots of degrees of freedom. Of course there are some constraints, but they are all reasonable and not really an obstacle. What I need is hard data. Surely someone has collected either manufacturer's specs for lots of different models or otherwise created a spreadsheet for car parts dimensions, including CV axles. I can't be the only person who wants the overview of the field first, from which to make my decisions. :idea:

    Of course one option is to buy a couple of joints and have an appropriate axle modified or fabricated, but that seems expensive and energy consuming since it is likely that what I want is already made in the millions of units. I just need a way of identifying it without spending more than the large number of hours already spent searching the web.

    Thanks for any help.
     
  7. Boston

    Boston Previous Member

    you might think of cutting the bottom part off an old junker outboard and just replacing some bearings, maintenance, and then V belt a couple pulley's to your engine, bigger engine = more belts to a point, after which you might consider a motorcycle chain and a few sprockets.
     
  8. DCockey
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    DCockey Senior Member

    In auto industry terminology a CV joint is different than a cardan joint. Cardan joints are the simple U-joints commonly used in drive shafts from the transmission to rear axle in cars and trucks.

    There are a number of designs of CV joints currently in use in autos, some are true constant velocity, others are essentially constant velocity at small angles and have some variation at large angles. Probably the most common use of CV joints is in the outer joints of auto/truck front axle half-shafts.

    Automakers generally don't make drawings of parts of their vehicles available to the public. CV joints are made by a supplier and I doubt it would be easy even if possible to get a drawing of a joint from a supplier. The other challenge would be finding out the suppliers identification for the joints in a particular vehicle.
     
  9. CDK
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    CDK retired engineer

    All CV and universal joint shafts used in road vehicles are made from carbon steel, limiting the application to dry areas only.

    Because the prop shaft must be corrosion resistant, if the power source cannot be lined up with the prop shaft, the way to go is use a stainless shaft, a thrust bearing and a pair of universal joints or a double one. Joints like Spicer 1310 or 1350 are readily available in the US for very reasonable prices.
     
  10. Wingandaprayer
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    Wingandaprayer Junior Member

    DCockey:

    Thanks for the info about auto industry practices. What I am looking for is the longest half axle between the inner and outer joints that I can find, up to about 46", LOA. I tried looking at a few off-road vehicle blogs, but everyone seems to be interested in specific modifications of a limited number of parts. Not much real info about CV joints/axles there. Any other ideas where else I could look?

    CDK:
    "Experimental engineer" sounds like me. I like it. Any idea how they protect those carbon steel axles against road salt and constant wet weather conditions? Any ideas about how I might do that?

    I suppose I should explain that the lash up I am designing is intended to spend most of its life out of the water (though not out of salt spray) as the prop and shaft will only be immersed when motoring. Otherwise they will ride tucked up against the underside of the bridge deck on my catamaran. One of the CV joints rides constantly just above the water, protected from floating logs etc. by a small cowling just in front of the joint. The other CV joint is inboard and dry, connected to the output shaft of the transmission. On the end of the outside CV joint I will mount a wheel hub with sealed bearings. It will transmit the thrust from the prop shaft to the boat and (with a custom made bracket to mount it in) allow me to pivot the shaft and prop up or down as needed. I need the hub, CV joints and shaft to handle about 60 HP at a max speed of 800~1100 rpm. As mentioned above, I need an overall length (joint to joint) of 32" to 46", or so.

    I can do research on the web but have already spent a lot of time and come up nearly dry. I considered two right-angle gear boxes per drive, but they are horribly expensive, especially compared to CV axle assemblies.Anyone got a specific suggestion where I might look for half axles with approximately such specifications?
     
  11. DCockey
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    DCockey Senior Member

    Road vehicle CV joints are exposed to salt water spray during the winter months if driven in the northern US states and most of Canada. Salt is used on the roads for ice control. The salt melts the ice and a concentrated brine solution results. It's not uncommon for parts of the undersides of cars and trucks to be white with salt build-up. CV joints in road vehicles are protected by flexible rubber points and well greased.

    What joint angles are you planned for the CV joints? Vehicle front half-shafts typically have a jointed limited to rather low angles inboard bacause the inboard joints only need to accomodate shaft angle variation due to suspension travel. Outboard joints in the front are designed for large angles because they must also accomodate the front wheels turning for steering.

    I'm not aware of any front half-shafts which would be 32" joint to joint other than possbily a Hummer, usually they are considerably shorter. However depending on the operating speeds and torque it may be possible to lengthen the shaft.
     
  12. DCockey
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    DCockey Senior Member

    Also, while the front half-shaft outer joints are designed to operate at relatively large angles, they are not designed to operate at large angles continuously.
     
  13. Wingandaprayer
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    Wingandaprayer Junior Member

    That's what I like about this site. Bam! Somebody replies.

    I'm not sure exactly how to describe the angles so I'll tell you what the design program says. Keep in mind that some parts can be moved around, some more than others, and the angles are in part determined by the shaft lengths.

    From a side view: the prop shaft is at a 15 degree down angle when it enters the CV joint. That is with a 1.7m shaft in order to keep the 18" prop far enough below the surface. Still from a side view, the angle of the shaft between the joints is only about 2 or 3 degrees.

    Overhead (plan) view: The prop shaft is parallel with the hull and the shaft between the joints is at an angle of about 37 degrees relative to the prop shaft. Again the angle of the half axle shaft is about 7 degrees off from the end of the transmission output shaft. That angle can actually be reduced to 0 fairly easily.

    I believe the angle for the inboard joints are probably within spec, but I am not sure about the outboard joint. Advice much appreciated.

    Front (body) view: The half axle shaft is at a 14 degree down angle from the transmission output shaft. I have not made any allowance for the angle available from a transmission with an 8 or 15 degree down angle output,which of course will reduce the angle of the half shaft. Since the prop shaft is parallel to the hull, there is no lateral angle when seen from the front.

    One more thing, as the CV shaft passes through the side of the boat, I can place a pillow block or other support bearing at that point, or simply make a gasketed hole to keep the water out and the dry in.

    I found one half-axle assembly with a spec of 31" from the outside of the outer joint to the innermost side of the inner joint, but I could really use a few more inches (no not those kind of inches);).
     
  14. gonzo
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    gonzo Senior Member

    "but that is like creating a hypothesis before you gather evidence." Hypothesis are postulated and then the evidence is gathered to either prove it or disprove it.
    However, what I said has nothing to do with hypothesis. You are creating a design, or so you claim, then you have to specify what the parts are. That is from size to working parameters. Then you can either find a ready made part, or specify custom parts. Also, without a thrust bearing the CV joint will not last long. Pillow bearings do not keep water from entering the hull. You need a stuffing box or a seal for that. Another part you will need is a cutlass bearing next to the propeller.
     

  15. Wingandaprayer
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    Wingandaprayer Junior Member

    Sorry Gonzo, you said:

    "An Hypothesis is postulated and then the evidence is gathered to either prove it or disprove it."

    You cannot have an hypothesis about anything without first having an observation of the thing. That observation is called evidence. Usually, a curious observation causes one to observe more and more closely (more evidence). Finally, one creates a hypothesis to explain what one has observed. With the hypothesis, you can set up an experiment to gather a different kind of evidence which will prove or disprove your hypothesis.

    Same with design. first I want to do something. Then I think about the possible ways to do it. Next I investigate what might exist that will do it best (however one defines that). Finally, I create a design with specifications and THEN I look to see what's available. It might be that the method selected is no good or the parts needed are only made from unobtanium, so you have to back up a bit and try a different approach. Well, at least that's how I design things since I don't have a large research budget or infinite knowledge about a subject when I start. I suspect that just like with observations, you actually follow a similar path for design, but the two are actually very much related.
     
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