Cap screw grade into cast iron?
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    Default Cap screw grade into cast iron?

    I have a repair on my hands.

    A ring gear attached to a cast iron differential housing.

    The 10 M8 X 1.25 x 20 retention cap screws loosened and allowed the ring gear to rotate several degrees with load reversals. Both the threaded holes and the cap screws are truly snookered!

    All in an oil bath, so the ring gear fit to the housing appears to have fared well. ;-)

    I've considered Time-sert thread repair. But have no experience with them in "damaged" cast iron, only aluminum. The threaded hole damage may exceed the the clean up offered by the time-sert. The housing wall thickness is but 24 mm , and likely does not allow other types of solid thread repair inserts..

    But really, my question is, what benefit is offered by "high strength" cap screws onto cast iron in these proportions?

    150ksi is a lot of tensile strength. If I consider "torque to yield" methods, Wouldn't grade 8.8 (110ksi) be a match to the cast iron female thread component?

    N.B. Loctite will be applied what ever the repair ;-)

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    Lots of grades of cast iron, with variations in tensile strength and toughness. In general, I think you won't see much benefit going over a 10.9 class fastener in this application, but fastener geometry and prep of the tapped holes could affect chances of success.

    Does the diff housing have enough meat that you could rotate the gear on the mating diameter and drill/tap new holes? Some pics of the parts will help with giving better advice.

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    What make and model is the diff? Unless they are a special shoulder bolt (some diffs have them) the manufacturer should be able to tell you the required grade. Most I have worked on had the threads in the steel ring gear. Just holes in the carrier casting. Some have had wired bolts, some had washers (one for every two cap screws) that had bend-up lock tabs. I have seen ring gears and pinions with sheared off teeth and the leaf springs warped into S's... but never broken off or wallowed out bolts. I suppose anything is possible though.

    -DU-

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    I would say the only proper way to repair this 'snookered' situation properly would be to remove the carrier and drill and tap it all for .375 fasteners in whatever flavor you desire. Loctite and JB Weld ain't going to 'git er done'!!

    Stuart

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    you can use what ever cap screws you want as long as the torque value specified by the manufacturer of the differential, is applied. So, the ring gear is attached to the differential housing? I have witnessed ring gears that were attached to the carrier with manufactured fitted cap screws.

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    Years ago I worked on a crane that had a poorly designed ring and pinion. It sheared the bolts off the ring gear more than once. The last time I fixed it, using new bolts and cleaning it up real good I Belzona epoxyed the ring to the carrier.
    Didn’t have any problems after that.

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    "But really, my question is, what benefit is offered by "high strength" cap screws onto cast iron in these proportions?"

    i think its simple: you can apply more torque to a hcs hereby stretching it more, so you get a "longer spring" and and higher margin on comming loose. prvided the material has enough shear strenght to support the torque. surface pressure shoud be no problem, as compressive strenght of ci is higher than uts.

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    Quote Originally Posted by Milland View Post
    Does the diff housing have enough meat that you could rotate the gear on the mating diameter and drill/tap new holes? Some pics of the parts will help with giving better advice.
    ^^^^This is a good idea, if possible.^^^^

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    I don't have a repair on my hands.

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    If the ring gear was moving, how flat are the surfaces? The time sert is a bad idea, it will require something of a counterbore not to be proud of the surface. A Helicoil will be stronger than the original and easier to install. The screws are not supposed to be in shear. They are supposed to pull the 2 flat surfaces together tight enough that friction will drive the load. Make sure that the surfaces are flat and clean and dry. Lightly lube the threads and torque to the spec using a star pattern and at least 2 stretches. If the surfaces are not flat then you need to correct that first.
    Last edited by tdmidget; 06-09-2021 at 09:18 PM.

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    Quote Originally Posted by CalG View Post
    The 10 M8 X 1.25 x 20 retention cap screws loosened and allowed the ring gear to rotate several degrees with load reversals. Both the threaded holes and the cap screws are truly snookered! ... But really, my question is, what benefit is offered by "high strength" cap screws onto cast iron in these proportions?

    150ksi is a lot of tensile strength. If I consider "torque to yield" methods, Wouldn't grade 8.8 (110ksi) be a match to the cast iron female thread component?

    N.B. Loctite will be applied what ever the repair ;-)
    Given that the screws loosened and chewed things up, I'd focus on the cast iron material, and not the cap screw material.

    I'd drill out where the M8x1.25 female thread once was, and install a larger cast-iron plug, which plug is drilled and tapped for the M8 cap screws. If there is space, the plug can be tapered and threaded, fitting into a pipe-thread socket in the casting, fixed into place using loctite.

    It may also be a good idea to make the cap screws a size larger, although with ten of them, one would have thought that would work. What is the diameter of the ring gear? Given that the housing walls are 24mm thick, the gear diameter could be large.

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    Picture img_20210610_214521963_hdr.jpg

    Note the special cap screws ;-) replacements in hand

    The caliper is set at 400 thou. the typical damage dimension of the threaded holes.

    Note that small hole between a pair of threaded holes. that's the retainer for the spider gear shaft.

    The ring gear is 10 mm thick, that leaves 10 mm of thread projecting. 8 1/2 good threads at 1.25mm pitch.

    Torque spec is 26-28 pound feet.

    I mis spoke when writing that the housing wall was 24 mm. It's less... :-(

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    That diff housing is pretty simple, I'd be really tempted to remake it in 4140PH and never worry about it again. If that's no bueno, I'd go ahead and rotate 22.5* and D/T new holes, but deeper than you had for longer cap screws. And yes, Loctite them in.

    Another option is drill and plug the bad tapped holes, press in CI plugs (not too hard a press, maybe clean and Loctite plugs in), drill the plugs for dowel pins, drill the gear for those dowel pins and assemble. With the new tapped holes and the dowel pins you'd NEVER-EVER have to worry about it again.

    Let us know what you wind up doing...

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    Quote Originally Posted by Milland View Post
    That diff housing is pretty simple, I'd be really tempted to remake it in 4140PH and never worry about it again. If that's no bueno, I'd go ahead and rotate 22.5* and D/T new holes, but deeper than you had for longer cap screws. And yes, Loctite them in.

    Another option is drill and plug the bad tapped holes, press in CI plugs (not too hard a press, maybe clean and Loctite plugs in), drill the plugs for dowel pins, drill the gear for those dowel pins and assemble. With the new tapped holes and the dowel pins you'd NEVER-EVER have to worry about it again.

    Let us know what you wind up doing...

    Lots of great ideas.

    That hole in the bottom of the housing is a concave bearing surface that a bronze thrust washer rides upon. Tricky and I would need to reverse engineer a lot of unknowns to produce that feature. I don't keep a radius tool just to keep from getting sucked into jobs like this ;-)

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    The picture shows fully threaded fasteners...no shoulders on the bolts and no flange heads. That certainly seems like a poor way to bolt a ring gear to a carrier...no wonder they worked loosed and wiggled.

    Stuart

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    Quote Originally Posted by CalG View Post
    Picture img_20210610_214521963_hdr.jpg

    Note the special cap screws ;-) replacements in hand
    Now that I see the setup, I see that the pipe-tap idea probably won't work, and given the limited space, I think I'd install screw-locking helicoil inserts. This would be the time to increase the screw thread engagement length. Hmm. Already wallowed out to 0.4", or 10.2mm. That's too large for a helicoil to work.

    I also agree with atomarc that use of full-thread fasteners may have been the problem. That setup ought to have worked.

    Perhaps make ten bespoke M8x1.25 cap head screws of 4140, drilling ten new threaded holes in the existing carrier, almost as Milland suggests.

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    I do not see threads at the top of the threaded holes.

    It looks like the "design" may have been to use screws/bolts that are NOT threaded all the way to the head but with a shoulder long enough to go into the part with threads so you have threaded dowell pins in effect.

    There is no shear at a thread riser and the shoulder would have better fit.

    You can make it this way by cleaning up the holes in the threaded part and making a bushing back to same size as gear.

    Get grade 10 fasteners with proper shoulder to reduce and excess room.

    Maybe someone in past replaced wrong fastener?

    Sent from my SAMSUNG-SM-G930A using Tapatalk

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    Quote Originally Posted by Tony Quiring View Post
    I do not see threads at the top of the threaded holes.

    It looks like the "design" may have been to use screws/bolts that are NOT threaded all the way to the head but with a shoulder long enough to go into the part with threads so you have threaded dowell pins in effect.

    There is no shear at a thread riser and the shoulder would have better fit.

    You can make it this way by cleaning up the holes in the threaded part and making a bushing back to same size as gear.

    Get grade 10 fasteners with proper shoulder to reduce and excess room.

    Maybe someone in past replaced wrong fastener?

    Sent from my SAMSUNG-SM-G930A using Tapatalk
    The Mfg. parts list describes the fasteners a M8-1.25 X 20.
    The heads are stamped with a 10. Threads go full length.

    I don't see anything "special about the call out. But certainly there is room for improvement. I plan to add dowel pins, four places.

    The issue is that to clean up the damage requires 10mm and the wall thickness is but 12 mm.

    Leaving 1mm to support the repair bushing. That doesn't fly for me.

    Also, an important detail. None of the fasteners "failed in shear". They loosened, and then rocked to and fro with power transmission reversals.
    I have no idea how long this had been going on. If one of the cap screws had sheared, there would be a lot of parts that would have been damaged. There is no room for stray hardware in this compact differential.

    eta

    I'll get in some of these.

    McMaster-Carr
    They look promising, but... The cap screws should never see shear, so I'm not sure what purpose the shank may serve. I do like the flange head however.

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    That design looks like something out of Bangladesh. No one in their right mind would put a fully threaded fastener in that application, and that's why it failed.
    3/8", in any flavor is the next common size up from 8mm if you're forced to drill and re-tap, which should be done to create a proper repair.

    Stuart

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    Quote Originally Posted by atomarc View Post
    That design looks like something out of Bangladesh. No one in their right mind would put a fully threaded fastener in that application, and that's why it failed.
    3/8", in any flavor is the next common size up from 8mm if you're forced to drill and re-tap, which should be done to create a proper repair.

    Stuart

    It's Austrian, Go figure.

    .375 won't clean up a .400 damaged hole no matter how many times I run the tap in. ;-)

    I might be "screwed" ;-)

    Stuart

    Would you care to add insight into the failure mode of a fully threaded M8 cap screw vs a partially threaded fastener (or better option) of the same grade, in this application.

    The difference I see is that "if" the fasteners fail to hold the required clamping force, then they are placed in both shear and bending. I'm not sure how the added section of an unthreaded portion remedies that.


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