With some type of couplings whose flexible element is visible while in operation, non-intrusive inspection methods may be possible. The use of strobe lights during operation can help identify abnormal deflections of the flexible elements. As an example, Photographs 4a and 4b show a flexible disc pack in operation. Torsional vibration can cause the individual discs to separate and
buckle. Coupling manufacturers can provide guidelines for acceptable limits of disc spreading. Note that shaft misalignment and installation errors can also create this occurrence. Torque-related problems typically cause buckling to occur at every other link between bolted connections. This is because the disc packs are alternately bolted to the driving and driven side of the coupling.
Photographs 4a and b: Inspection of coupling element (disc pack style) showing minor distortion due to slight
overload (left) and severe buckling of discs (right). Note that deformation/buckling occurs in the centre of the link between alternating bolted connections.
Elastomeric couplings are often installed to increase system damping, and hence reduce or restrict torsional vibration. These couplings utilize rubber in compression or rubber in shear as the flexible elements. Compressed rubber will tend to absorb the torsional vibration energy in the blocks where it is converted to heat. Excessive vibrations will melt the rubber material from the inside of the blocks which won’t be visible until complete failure occurs or if the blocks are sectioned for inspection when they are replaced. Rubber in shear couplings will often show signs of degradation at the interface between the rubber and metallic material which can be visually inspected.
In the event that failure does occur, the fracture surface should be inspected as it can help distinguish between different potential failure sources – a one-time overload from a process upset, or torsional vibrations which are inherent to the drivetrain design or operation, requiring more extensive corrective actions.
A metallurgical analysis performed by a competent test laboratory will provide the most comprehensive results, but it can be time-consuming and doesn’t typically provide insight into potential design or operational modifications necessary to correct the problem. One key piece of information that can be gained from the fracture surface is the crack initiation point. Simply knowing where the crack started can determine if design changes should be considered, which might be as simple as increasing a radius, shot-peeing the surface for fatigue resistance, or recognizing the need for improved manufacturing and quality processes. Most fracture surfaces from fatigue failures will exhibit beach- marks (striations), which 8 of 19 resemble the tidemarks on a beach. The beach-marks expand outward from the crack initiation point in a circular or semi-circular pattern and are followed by a region of fast fracture. Multiple initiation points can be present and are usually evidenced by what are called ratchet marks, which occur when separate patterns of beach-marks intersect.
Photographs 5a and b: Fatigue failure surfaces of shear pin subjected to cyclic stress below the prescribed overload limit but above the material endurance (left) and fatigue fracture of a crowned gear tooth from repeated impacting (right).
THE REPORT | SEP 2024 | ISSUE 109 | 67
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112 |
Page 113 |
Page 114 |
Page 115 |
Page 116 |
Page 117 |
Page 118 |
Page 119 |
Page 120 |
Page 121 |
Page 122 |
Page 123 |
Page 124 |
Page 125 |
Page 126 |
Page 127 |
Page 128 |
Page 129 |
Page 130 |
Page 131 |
Page 132 |
Page 133 |
Page 134 |
Page 135 |
Page 136 |
Page 137 |
Page 138 |
Page 139 |
Page 140 |
Page 141 |
Page 142 |
Page 143 |
Page 144 |
Page 145 |
Page 146 |
Page 147 |
Page 148
