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  1. #1
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    Default Rethreading stub axle.

    I am rebuilding a 1959 Morgan and the one of the front stub axles has the thread stripped.
    It is 5/8” BSF currently.
    I am thinking of filing it to its root dia which should be about 13.55mm.
    Retapping it Metric 14 x 1.5mm.
    A chart on the net shows a min dia for this at 13.73mm.
    Any thoughts on this.
    A new one would be about $1k. as it’s part of the sliding pillar suspension.
    H.
    Jimcracks for the rich and/or wealthy. (aka GKB '88)

  2. #2
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    Default

    I suppose this is a difficult part to hold to machine and assuming the stub axle has enough diameter I think a better way around this might be to make a drilling jig and loctite a stud into it.
    Cut the stripped stub off the end of the stub axle and file it flat to the stub axle.
    Bore a peice of 2 inch long bar stock to slide nicely on the axle to about 1 inch, the other end of this drilling jig you would drill a suitable pilot drill hole, drill that into the stub axle then drill out the pilot to the tapping size of the thread to be used for the stud and drill the stub axle with that hole, then drill the hole in the jig to the OD of the tap and use it as a tap guide.
    Dont break a drill off in it.....
    Cheers, shed

  3. #3
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    Default

    Are you talking about the thread for the nut that keeps the front wheel and brake and bearings on the car? If so I would be more inclined to build up the area with weld and recut the proper thread. A photo would help.

  4. #4
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    Default Sliding pillar hub.

    Thanks for the replies.
    Here’s a pic from the spares listing.
    Its all one piece, don’t think it would fit in a 4 jaw chuck on the Macson and I don’t have a mill.
    H.
    Attached Images Attached Images
    Jimcracks for the rich and/or wealthy. (aka GKB '88)

  5. #5
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    Default

    You should have a centre drill hole in the end of the spindle and a matching hole in the back of the casting left over from when the spindle was first made, it was most likely turned between centres. You can duplicate that process to turn the built up area back to size and re-cut the thread. The trick is to find a lathe with a big enough swing to accommodate the part.

    For something important there is no way I would be going the filing and die cutting route. It might not hurt to have the part magna fluxed before you expend too much effort on it.

  6. #6
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    Henry,
    A couple of things to consider:

    The axle may have been heat treated after machining, possibly case hardened, so reducing the diameter may result in a softer part of the axle now taking the load.

    What manufacturing method was used for the original thread? Was it rolled? Or cut with a controlled lead out to reduce any stress concentration at the end of the thread? Both these details would have been taken into consideration by the original designer when he/she chose the part dimensions, and any new thread may not duplicate these exactly.

    Reducing the diameter from 16mm to 13.5mm will decrease the bending strength of the axle at that location by close to 50% !! This is because the strength of a round part like this changes with the forth power of diameter. The shear strength, and straight tensile strength will change by ~15%, still significant. The load on a stub axle is a combination of bending, tensile, and shear - they all add up to a total stress on the axle.

    If it was me, I'd fork out for the replacement part. The thought of a wheel coming off scares the hell out of me, especially if this is one of those three wheeled Morgans.

    Graham

  7. #7
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    As said above definitely have magnafluxed also if you are doing a true "restoration" the thread should be as original'

  8. #8
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    Default

    Quote Originally Posted by Oldbikerider View Post
    Henry,
    A couple of things to consider:

    The axle may have been heat treated after machining, possibly case hardened, so reducing the diameter may result in a softer part of the axle now taking the load.

    What manufacturing method was used for the original thread? Was it rolled? Or cut with a controlled lead out to reduce any stress concentration at the end of the thread? Both these details would have been taken into consideration by the original designer when he/she chose the part dimensions, and any new thread may not duplicate these exactly.

    Reducing the diameter from 16mm to 13.5mm will decrease the bending strength of the axle at that location by close to 50% !! This is because the strength of a round part like this changes with the forth power of diameter. The shear strength, and straight tensile strength will change by ~15%, still significant. The load on a stub axle is a combination of bending, tensile, and shear - they all add up to a total stress on the axle.

    If it was me, I'd fork out for the replacement part. The thought of a wheel coming off scares the hell out of me, especially if this is one of those three wheeled Morgans.

    Graham
    As a retired Rigger and Scaffolder I'm not sure my qualifications count, but my gut feeling tells me that there isn't going to be too much of a bending moment at the threaded end of a stub axle. of course good engineering practice should be followed, and if for example a thread runout groove is used, it would be better IMO if the groove had a rounded form rather than square sharp corners. I wouldn't have thought that the part would be case hardened, but could well be of something like the British equivalent of 4140 with or without heat treatment, so any welding would need to be done with due consideration. The figure of 13.73 mm for the minor diameter of a 14 x 1.5mm thread is incorrect, the Black Book figure being 12.16mm. Considering the weight of a Morgan, and the power of its drivetrain, I think it most likely that it would still be ok, but if it were me deciding, I think that I would have it welded, then machined down to original spec. Magnafluxing or any other suitable non destructive testing would be a good idea too when the job is done and even before starting as has already been stated, in case there are other, so far undetected flaws in the part.

  9. #9
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    Default

    Quote Originally Posted by Oldbikerider View Post
    The load on a stub axle is a combination of bending, tensile, and shear - they all add up to a total stress on the axle.
    OK, but I agree with Ropetangler. At the threaded end I reckon tensile is the only one of any significance.
    Chris

  10. #10
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    Default

    There's a complex loading on the threaded end of a stub axle. The bearing inner race gets an offset axial load on it due to bulk axle deflection, which is taken by the nut and of course the thread on the end of the axle. The bearing centre line is also offset (by at least half the bearing width) from the end of the stub axle taper, which places a direct bending load on the axle at that location. But yes, the tensile and shear stresses will be larger - I never said they were not.

    The small weight and power of the Morgan is not relevant if the designer took this into account with the original design, which they hopefully did. What matters is the reduction in axle cross section compared to the original design resulting from this modification, and the resulting increased stresses.

    The 13.73mm dimension was stated by the OP as the min OD of a 14mm thread, not the minor diameter.

    Graham.

  11. #11
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    Quote Originally Posted by Oldbikerider View Post
    The bearing centre line is also offset (by at least half the bearing width) from the end of the stub axle taper, which places a direct bending load on the axle at that location.
    Thanks Graham. The way I’m reading the OP, he is only talking about reducing the diameter of the treaded portion of the axle. The parallel part of the axle that the outer bearing rides on is unchanged.
    Chris

  12. #12
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    Yes, you're correct. I was confusing general stub axle design issues, with the particular changes proposed here. I've had tens of thousands of my design stub axles manufactured, and some failures, and this thread brought up old thought processes.

    I guess the general point I was trying to make is that the small end of a stub axle is not a trivial part of the design, and if the original design was efficient, ie. made down to a size and a cost, then it shouldn't be altered.

    Graham.

  13. #13
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    Fair enough Graham, what I was referring to when I said that the low power and weight of the Morgan was unlikely to over stress the part was a stub axle nut of 14mm on such a small unit when cars such as Morris Minors and Renault R8s or 10s would have similar sized stub axles, so I don't think a 14mm thread would be a problem on a Morgan. If the part was made to be just strong enough, then the original material would have been pretty soft I think, mild steel perhaps??

  14. #14
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    Dont forget that when cornering the only thing that keeps the wheel (and the brakes) on is that nut.

    Roger

  15. #15
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    Hi guys,
    By all means, weld the thread and re-machine but be aware of the best procedure.

    If one were to weld radially around the circumference if is possible to induce a cooling stress fracture as the metal cools right at the smooth machining to thread interface.

    A microcrack can develop from that technique.

    To ensure this won't happen I would advise longitudinal beads -parallel to the axis

    Dribble the start and finish over the ends and you will be able to machine clean weld metal. LH 2.6mm electrodes will machine better and be quite bit stronger if you have them.

    It is just a belt and braces approach. I have seen this happen in steam turbine where the weld-repaired thread broke right on that smooth to threaded interface line.

    Yes, yes, I know the dynamic stresses are different but this failure is possible, so I thought I would post this.

    Grahame
    Last edited by Grahame Collins; 12th Oct 2018 at 10:37 PM. Reason: spelling and mis keying

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