ETD-150 vs 4140PH Nitriding
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  1. #1
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    Default ETD-150 vs 4140PH Nitriding

    Hello all - looking for some process/material advice.

    I'm machining a spline coupler (approx 1" ID, 1.5" OD) for a 220hp electric motor.

    I was planning to use 4140ph, stress-relieve after roughing, finish, then nitride.

    I haven't used ETD-150 before, but thought this might be a good alternative. A few questions:

    1) If we use ETD-150, can we eliminate the stress-relieving step? With standard 4140 I understand this is important to reduce movement during nitriding.
    2) How does ETD-150 nitride? I haven't found much in the way of specs for this material. How much movement should we expect? I'd like to avoid needing to re-finish the splines following the nitriding.

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    Quote Originally Posted by srlake View Post
    How much movement should we expect? I'd like to avoid needing to re-finish the splines following the nitriding.
    You shouldn'gt expect any with any material, that's the whole point of nitriding. It's nowhere near as good as carburizing but due to the low temps involved, doesn't do all those heat-treat distortions.

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    +1 on EGs observation, an example of nitriding well in actual fact Tuffriding, A US company’s proprietary process, the exact process going back to the 70s was applied to finished aluminium extrusion dies. Any distortion was not only unacceptable but from the dies point of view catastrophic, ok the die would require polishing but light hand polish, I beleive thier process involved heating in an inert gas furnace to they’re proprietary temperature, switching to ammonia for however many hours to get required carbon increase dissolved in or whatever depth requirements, purge and cool in argon, my memory ain’t great but either 60 or 70 hours popped in, if the heat treater offers it Ion nitriding might be quicker, but that not that fast, it is however beautiful to look at the part glowing, I’m a sucker for electric blue and violet Corona discharge, way cool
    Mark

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    I'd go with ETD 150. Machine, Nitride, done.

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    Quote Originally Posted by srlake View Post
    Hello all - looking for some process/material advice.

    I'm machining a spline coupler (approx 1" ID, 1.5" OD) for a 220hp electric motor.

    Well, I'd love to see the calculations that show that such a small coupling will live with 220Hp going through it. Is this a typo, or a very high RPM motor? 220hp at 1000RPM means roughly 1150 lbs/ft, which seems a bit much for under a 1.5" pitch diameter and ~1/4" wall.

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    135 modified is made for what you want.

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    Quote Originally Posted by Milland View Post
    Well, I'd love to see the calculations that show that such a small coupling will live with 220Hp going through it. Is this a typo, or a very high RPM motor? 220hp at 1000RPM means roughly 1150 lbs/ft, which seems a bit much for under a 1.5" pitch diameter and ~1/4" wall.
    Consider the torque on the manual 6 speed gear box pilot shaft on a Dodge 3500 PU truck when plugged in the back of a Cummins diesel It is 1.250" diameter.

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    Quote Originally Posted by Milland View Post
    Well, I'd love to see the calculations that show that such a small coupling will live with 220Hp going through it. Is this a typo, or a very high RPM motor? 220hp at 1000RPM means roughly 1150 lbs/ft, which seems a bit much for under a 1.5" pitch diameter and ~1/4" wall.
    For a spline, you don't need to worry too much about bending because the teeth mesh all the way round. Therefore, the calculation you want is ~50% of the shear area around a 1" diameter:

    pi*D/2 = 0.5 in²/in

    4140 quenched and tempered has an estimated (yield strength * 0.58) shear strength of 71x10³ psi.

    Your proposed load of 1150 lbf*ft would load the 0.5 in radius to 27.6x10³ lbf at the roots.

    Shear stress = load/area:

    27.6/A = 71
    A = 0.389 in²
    From above, we determine that a length of 0.777 in will provide this amount of area with no safety factor.

    So a quite reasonable 2" length spline will have a safety factor of approximately 3 at max HP with relatively smooth loading at your proposed 1000 RPM (of course as you suggested it may be higher which would reduce loading further).

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    If it's meant to be the fuse, then fine. But I checked a 200hp motor for specs and it had a 85mm output shaft. Solid, much larger, there's a impedance mismatch here...

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    Quote Originally Posted by srlake View Post
    Hello all - looking for some process/material advice.

    I'm machining a spline coupler (approx 1" ID, 1.5" OD) for a 220hp electric motor.

    I was planning to use 4140ph, stress-relieve after roughing, finish, then nitride.

    I haven't used ETD-150 before, but thought this might be a good alternative. A few questions:

    1) If we use ETD-150, can we eliminate the stress-relieving step? With standard 4140 I understand this is important to reduce movement during nitriding.
    2) How does ETD-150 nitride? I haven't found much in the way of specs for this material. How much movement should we expect? I'd like to avoid needing to re-finish the splines following the nitriding.
    Parts shouldn't move any if at all with Nitride.
    How much of a fit are you allowing between the male and female spline before Nitride?
    Would 8620 carb & hardened be a better choice?

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    Quote Originally Posted by Mtndew View Post
    Would 8620 carb & hardened be a better choice?
    They almost always shrink, but the amount is not closely predictable. Makes it a bitch to get right on just one try. I s'pose you could cut several in .001" increments over wires, if you were doing it on a shaper and not broaching it. Then q&t the whole batch at once, the best fit gets to go.

    I've done that with plastic gears, where the housing is really hard to get into to measure and the plastic was so cheap, easier to make several in graduated sizes than try to figure what it should be. Works okay.

    Nitriding doesn't do that shrink thing, so easier .... altho I think you're right, carburizing makes a better part for power transmission.

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    Quote Originally Posted by Milland View Post
    If it's meant to be the fuse, then fine. But I checked a 200hp motor for specs and it had a 85mm output shaft. Solid, much larger, there's a impedance mismatch here...

    The motor being used has an output shaft with a spline with a .818" pitch diameter, approx 1" of engagement. It is a 10,000 RPM motor with peak torque of 350Nm.

    My calculations are very similar to BoxcarPete below.

    Spline:
    Using an assumption of 25% engagement I get a shear stress of 24 ksi for the external spline (Involute Gear Design Equations and Calculator | Engineers Edge). The ETD-150 shear yield strength should be ~ 0.58* 130 ksi (yeild strength) = 75.4 ksi. So FS around 3.

    Coupling:
    Just to quickly validate, considering the coupler as a tube with OD 1.5" dia and ID 1" dia, I get a shear stress of 6 ksi (Torsion of Solid and Hollow Shafts Calculator), so the coupler will be much stronger than the drivetrain or motor shaft itself in terms of shear (given the polar moment of inertia will dominate with the larger dia/area).

    I took a look at the compressive strength as well using either unhardened, surface hardened (nitride), case hardened, or through-hardened, and it looks like for this application nitriding should be plenty (FS of 10 in compression).

    The spline as drawn is a 92.2M 6h/6H fit, so it's tight enough that if nitriding was going to shrink the part by say 10 thou it would be problematic. It sounds like that won't be the case.

    It sounds like ETD 150 + Nitride should be the way to go. Thank you very much everyone.

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    Quote Originally Posted by srlake View Post
    The motor being used has an output shaft with a spline with a .818" pitch diameter, approx 1" of engagement. It is a 10,000 RPM motor with peak torque of 350Nm.
    OK, so this is an EV-type motor rather than an industrial motor. I take back my concerns.

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    Quote Originally Posted by Milland View Post
    OK, so this is an EV-type motor rather than an industrial motor. I take back my concerns.
    Exactly - application is a electric watercraft.


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