Setting up SM differential
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Thread: Setting up SM differential

  1. #1
    Fellow Frogger! citroenthusiast's Avatar
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    Default Setting up SM differential

    I am working on getting the adjustments correct for my SM differential. The ring and pinion already have some 50,000 miles on them. I set up the pinion depth and backlash using a dial indicator as shown in 581, but when I ran a pattern, I get some very strange shapes. The drive side looks well centered on the tooth, but somewhat narrow. The coast side is very oddly shaped, sort of a flag-shaped pattern. Has anyone had much experience verifying the setup of a spiral bevel final drive using gear marking compound? Does this look like an acceptable set of patterns? Thanks


    Setting up SM differential-160620-coast-side.jpgSetting up SM differential-160620-drive-side.jpg

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    Cheers,
    John T.

    54 11BL; 61DS19 LHM (son's); 71DS21 BVH; 73SM 3.0; 73SM 2.7EFI; 73SM 3.0 (other son's); 74 Maserati Merak

  2. #2
    1000+ Posts gerrypro's Avatar
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    It appears to me that you have excess tangential play ( Backlash). This allows the over run side to contact further out on the tooth flank and to make point contact instead of a full, wide, well centred pattern. It is very bad news for the longevity of the mesh and would soon cause erosion of the case hardening as the edge of the pinion tooth gouges into the crown wheel tooth. For DS and Traction the backlash should be between .19mm and .22mm. I always try to err on the lower side of this range.
    It appears that the pinion depth setting in relation to the crown wheel centre line is very close to correct, possibly just a little tight as your drive side flank marks are just a tad more towards the centre line of the crown wheel than I would like.
    To set your mind at ease be assured that my experience has come via many rebuilds of traction boxes and that the averade time I spend on a TA box is around 9 1/2 hours! The easiest box I have ever done is that from a C-Matic CX
    Cheers Gerry

  3. #3
    Fellow Frogger! citroenthusiast's Avatar
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    Thanks for that. 581 says the backlash should be .16 to .24. I think that the backlash is pretty close to .21mm as measured by the dial indicator. I will see about running it closer to .16 if I can.

    Good news that the pinion depth looks good (I measured it about 20 times). Pretty much have to disassemble the whole box to change the pinion shim. thanks for the help.
    Cheers,
    John T.

    54 11BL; 61DS19 LHM (son's); 71DS21 BVH; 73SM 3.0; 73SM 2.7EFI; 73SM 3.0 (other son's); 74 Maserati Merak

  4. #4
    Real cars have hydraulics DoubleChevron's Avatar
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    The guru of DS/SM gearboxes is a member here. Have you tried contacting Harry Martens. Not only will he be able to help, he'll probably have any shims or bits you require to get it right!

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    seeya,
    Shane L.
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  5. #5
    Fellow Frogger! citroenthusiast's Avatar
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    Harry says he doesn't use gear marking compound. He just uses a dial indicator. I set everything up with a dial indicator, but wanted to use gear marking compound as a final check since I don't do this every day.

    (note the Quaife LSD sourced from Harry)
    Cheers,
    John T.

    54 11BL; 61DS19 LHM (son's); 71DS21 BVH; 73SM 3.0; 73SM 2.7EFI; 73SM 3.0 (other son's); 74 Maserati Merak

  6. #6
    Fellow Frogger! citroenthusiast's Avatar
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    By the way, does it make any difference if you run the pattern by turning the input shaft versus turning the ring gear? It is easier for me to shift the transmission into reverse (to provide a load) then spin the output shafts to get a pattern. My instinct tells me that it shouldn't make any difference because you are measuring contact area of two otherwise immovable objects, but I have been wrong before...
    Cheers,
    John T.

    54 11BL; 61DS19 LHM (son's); 71DS21 BVH; 73SM 3.0; 73SM 2.7EFI; 73SM 3.0 (other son's); 74 Maserati Merak

  7. #7
    Fellow Frogger! badabec's Avatar
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    I really like these masterclasses. How can you deduce all that from witness marks in the marking compound?

  8. #8
    Fellow Frogger! citroenthusiast's Avatar
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    Here is a short video, not much on theory, but good practical advice

    https://www.youtube.com/watch?v=nVb5WRUfM7Y
    Cheers,
    John T.

    54 11BL; 61DS19 LHM (son's); 71DS21 BVH; 73SM 3.0; 73SM 2.7EFI; 73SM 3.0 (other son's); 74 Maserati Merak

  9. #9
    Fellow Frogger! badabec's Avatar
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    Excellent, thanks

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    1000+ Posts Wildebeest's Avatar
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    Not Citroen related but back in my working life at a Volvo dealers I was given the diff jobs, never touched one before I started here!
    Where was this tutorial video when I needed it?
    Fortunately we had all the manuals so I just followed these. Apart from the clinical conditions and some of the tools shown, over the years I managed.
    The Volvo diffs shown were similar, although they didn't have the adjustable rings for backlash adjustment. Calculating the shims was necessary for both backlash and pre-load. Pinion height and pre-load the same. Later model Volvos had the crushable pre-load making things a bit simpler.
    Although I initially did crown wheel markings [with bearing blue] we were told it wasn't necessary providing the pinion height and backlash were within spec.
    During one years model [240] a problem occurred in that the diffs made what Volvo described as a "helicopter noise" ? The side gears spacer/thrust washers were collapsing. Very few if any caused damage to the diff.
    A lot of warranty work involved. We were instructed by Volvo to ignore previous settings other than noting the pinion height, marked on the pinion nose. The c/w back-lash was also noted and we were just to observe these on re-assembly. All this was successful and no noises returned.
    The above made me wonder at the time just how close tolerances were on assembly at the factory. It certainly reduced the time spent on warranty.
    I also had some experience fitting the Dana limited slip centres on customer cars. During the run up to the 1979 Repco Trial I had to install these in some of the Volvo Team cars. Imagine my dissapointment when they had diff troubles, the crown wheel bolts were breaking! All new too. After the event a bulletin arrived from Volvo Sweden advising the fitting of improved bolts!!

    Just to lighten this story. I had to o/haul a diff on a 164 model, advise was to replace everything. About to go on road test accompanied by the service "mangler" and the State engineer. Got up to speed on the freeway when wow- wow- wow, [diff noise impression]. I reached out of the window and grabbed the tubular roof rack. No more noise, let it go, noise came back.
    Red face time. Although I have to say that the Volvo did come on the back of a truck from Geraldton so unable to initially road test.

    Wildebeest

  11. #11
    Real cars have hydraulics DoubleChevron's Avatar
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    Quote Originally Posted by citroenthusiast View Post
    Here is a short video, not much on theory, but good practical advice

    https://www.youtube.com/watch?v=nVb5WRUfM7Y
    brilliant. thanks for that. It's like a black magic until someone explains it!

    seeya,
    Shane L.
    'Cit' homepage:
    Citroen Workshop
    Proper cars--
    '85 Series II CX2500 GTi Turbo I
    '63 ID19 http://www.aussiefrogs.com/forum/showthread.php?t=90325
    '72 DS21 ie 5spd pallas (last looked at ... about 15years ago)
    '78 GS1220 pallas
    '92 Range Rover Classic ... 5spd manual.

    Yay ... No Slugomatics


    Modern Junk:
    '07 Poogoe 407 HDi 6spd manual

  12. #12
    Fellow Frogger!
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    A common enough pattern for the first runup. Done a hell of a lot of diffs and then some in my day and I would say your pinion is out too far. Move it in and possibly move the C/W away if the backlash is too close. Remember it's a used pair so focus on getting the correct amount of bearing preload first [are they new or reused ] and then set up the C/W as required by the contact marking, ignoring all else.
    I always used red lead as a diff marker, bearing blue is too vague under loadup. Marking should be checked by turning the crownwheel never the pinion. But with some sort of loading placed on the pinion.
    cheers.

  13. #13
    Fellow Frogger! citroenthusiast's Avatar
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    Adjusted the backlash per gerrypro's recommendations. The SM box, like the DS, requires you to swap shims so as to keep the .30mm preload on the bearings constant while moving the crown wheel toward and away from the pinion. Carrier bearings are new SKF. With the dial indicator showing .19mm of backlash, I get these patterns. As you can see, the drive side contact has moved from almost a line contact (bad) to more of a smear, offset toward the toe, which I believe is an improvement. The coast side is also a smear, offset somewhat toward the heel.

    As a bit of background, this is the original CWP to this box and I have already moved the pinion away from the crown wheel by .02mm (2.63mm shim in place of original 2.65mm shim). I think (hope) this is an acceptable pattern since it is reasonably well centered from root to tip and at least the drive side is offset toward the toe, but will wait for further comments.

    Setting up SM differential-coast-side-160622-.jpgSetting up SM differential-drive-side-160622-.jpg
    Cheers,
    John T.

    54 11BL; 61DS19 LHM (son's); 71DS21 BVH; 73SM 3.0; 73SM 2.7EFI; 73SM 3.0 (other son's); 74 Maserati Merak

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