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    2. ASME B18.2.2 Hex Jam Nut Standard

    ASME B18.2.2 Hex Jam Nut Standard

    ASME B18.2.2 Hex Jam Nut specifications provide detailed dimensional criteria for regular hex jam nuts, commonly utilized as anti-vibration lock nuts in dynamic systems.
    In dynamic mechanical assemblies subjected to high-frequency vibration, severe thermal cycling, or continuous rotational stress, a solitary standard hex nut is inherently susceptible to slowly backing off its threads, resulting in a critical and potentially catastrophic loss of joint clamping force.
    To counteract this physical phenomenon without resorting to chemical thread-locking compounds, mechanical engineers frequently employ the hex jam nut—a secondary, low-profile fastener detailed under the ASME B18.2.2 standard.
    By design, a standard hex jam nut possesses identical Width Across Flats (WAF) dimensions to its standard full-form counterpart, ensuring the same wrenches can be used during assembly.
    However, it features a significantly reduced vertical thickness, generally constituting about two-thirds the height of a standard full-form nut.
    The mechanical efficacy of the jam nut relies entirely on exact dimensional tolerances and, crucially, the proper physical installation sequence.
    In correct engineering practice, the thin jam nut is actually threaded onto the bolt first and tightened against the bearing surface to a fraction (typically 25% to 50%) of the final required torque.
    The thicker, primary hex nut is then threaded onto the bolt and torqued down entirely against the jam nut.
    As the primary, full-height nut is tightened, it pulls the bolt shank outward away from the joint.
    This stretching action physically unloads the thread flanks of the jam nut and forces them aggressively onto the opposing flanks of the bolt threads.
    This creates an intense, localized frictional locking interface that binds the two nuts together, effectively resisting vibratory decoupling even under severe cyclic loading.
    Because of their intentionally reduced profile and smaller thread shear area, jam nuts are mechanically incapable of sustaining the massive proof loads characteristic of full nuts.
    For instance, low-carbon steel jam nuts exhibit a proof load of only 54,000 psi for coarse threads and 48,000 psi for fine threads, compared to the 90,000 psi rating of standard nuts.
    Consequently, if a jam nut is improperly placed on top of the primary nut (which is a notoriously common field error), the thin threads will rapidly shear out under the full tensile load of the tightened assembly.

    ASME B18.2.2 Hex Jam Nuts

    ASME B18.2.2 Hex Jam Nuts Dimensions (mm)

    Nominal Size Basic Major Diameter of Thread Width Across Flats, F Width Across Corners, G Thickness, H1 Runout
    Min Max Min Max Min Max Max
    1/4 6.350 10.871 11.125 12.395 12.827 3.810 4.140 0.381
    5/16 7.938 12.421 12.700 14.148 14.656 4.572 4.953 0.406
    3/8 9.525 13.995 14.300 15.951 16.510 5.334 5.766 0.432
    7/16 11.113 17.145 17.475 19.507 20.168 6.096 6.604 0.457
    1/2 12.700 18.694 19.050 21.336 21.996 7.671 8.204 0.483
    9/16 14.288 21.869 22.225 24.943 25.654 7.645 8.230 0.508
    5/8 15.875 23.419 23.825 26.695 27.508 9.220 9.830 0.533
    3/4 19.050 27.635 28.575 31.496 32.995 10.109 11.328 0.584
    7/8 22.225 32.233 33.325 36.754 38.506 11.633 12.954 0.635
    1 25.400 36.830 38.100 41.986 43.993 13.183 14.605 0.686
    1 1/8 28.575 41.427 42.875 47.219 49.505 14.707 16.231 0.762
    1 1/4 31.750 46.025 47.625 52.476 54.991 17.450 19.075 0.838
    1 3/8 34.925 50.648 52.375 57.734 60.503 18.974 20.701 0.914
    1 1/2 38.100 55.245 57.150 62.992 65.989 20.523 22.352 0.991
    1 5/8 41.275 59.690 61.722 68.047 71.247 22.047 23.978 1.118
    1 3/4 44.450 64.465 66.675 73.482 76.987 23.597 25.629 1.219
    1 7/8 47.625 69.139 71.450 78.816 82.474 25.121 27.254 1.295
    2 50.800 73.660 76.200 83.972 87.986 26.670 28.905 1.397
    2 1/4 57.150 82.880 85.725 94.463 98.984 29.337 32.182 1.549
    2 1/2 63.500 92.075 95.250 104.978 109.982 35.585 36.246 1.727
    2 3/4 69.850 101.295 104.775 115.468 120.980 38.659 39.522 1.880
    3 76.200 110.490 114.300 125.959 131.978 41.732 42.799 2.057
    3 1/4 82.550 119.710 123.825 136.474 142.977 44.399 46.076 2.210
    3 1/2 88.900 128.905 133.350 146.964 153.975 47.498 49.352 2.388
    3 3/4 95.250 138.125 142.875 157.455 164.973 50.546 52.629 2.540
    4 101.600 147.320 152.400 167.945 175.971 53.645 55.905 2.718

    ASME B18.2.2 Hex Jam Nuts Dimensions (Inch)

    Nominal Size Basic Major Diameter of Thread Width Across Flats, F Width Across Corners, G Thickness, H1 Runout
    Min Max Min Max Min Max Max
    1/4 0.2500 0.428 0.438 0.488 0.505 0.150 0.163 0.015
    5/16 0.3125 0.489 0.500 0.557 0.577 0.180 0.195 0.016
    3/8 0.3750 0.551 0.563 0.628 0.650 0.210 0.227 0.017
    7/16 0.4375 0.675 0.688 0.768 0.794 0.240 0.260 0.018
    1/2 0.5000 0.736 0.750 0.840 0.866 0.302 0.323 0.019
    9/16 0.5625 0.861 0.875 0.982 1.010 0.301 0.324 0.020
    5/8 0.6250 0.922 0.938 1.051 1.083 0.363 0.387 0.021
    3/4 0.7500 1.088 1.125 1.240 1.299 0.398 0.446 0.023
    7/8 0.8750 1.269 1.312 1.447 1.516 0.458 0.510 0.025
    1 1.0000 1.450 1.500 1.653 1.732 0.519 0.575 0.027
    1 1/8 1.1250 1.631 1.688 1.859 1.949 0.579 0.639 0.030
    1 1/4 1.2500 1.812 1.875 2.066 2.165 0.687 0.751 0.033
    1 3/8 1.3750 1.994 2.062 2.273 2.382 0.747 0.815 0.036
    1 1/2 1.5000 2.175 2.250 2.480 2.598 0.808 0.880 0.039
    1 5/8 1.6250 2.35 2.43 2.679 2.805 0.868 0.944 0.044
    1 3/4 1.7500 2.538 2.625 2.893 3.031 0.929 1.009 0.048
    1 7/8 1.8750 2.722 2.813 3.103 3.247 0.989 1.073 0.051
    2 2.0000 2.900 3.000 3.306 3.464 1.050 1.138 0.055
    2 1/4 2.2500 3.263 3.375 3.719 3.897 1.155 1.267 0.061
    2 1/2 2.5000 3.625 3.750 4.133 4.330 1.401 1.427 0.068
    2 3/4 2.7500 3.988 4.125 4.546 4.763 1.522 1.556 0.074
    3 3.0000 4.350 4.500 4.959 5.196 1.643 1.685 0.081
    3 1/4 3.2500 4.713 4.875 5.373 5.629 1.748 1.814 0.087
    3 1/2 3.5000 5.075 5.250 5.786 6.062 1.870 1.943 0.094
    3 3/4 3.7500 5.438 5.625 6.199 6.495 1.990 2.072 0.100
    4 4.0000 5.800 6.000 6.612 6.928 2.112 2.201 0.107
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