AGMA 91FTM7-1991 pdf free download.Low Cycle and Static Bending Strength of Carburized and High Hardness Through Hardened Gear Teeth.
PINION MATERIAL, MANUPACTURING PROCESS AND MEAT TREATMENT
The 7 tooth output pinion was izade of l7CrNiMo6, a popular European carburizing steel, which has high
hardenability. The manufacturing
process for the pinion consisted of flame cutting the basic profile with an allowance for finishing. The fillets were then bored and the final profile was shaper cut. The pinion was then carburized, hardened and tempered. The actual hardness traverse from a tooth is shown in Pigur. 6.
PINION LOADING, PREDICTED RATING AND smss ANALYSIS
At the maximum required holding load of 1,600,000 lbs. tangential, the AGMA 2001 [9] pinion bending stress was 145 KSI. WjtIa life factor of 2.7 for less than 10 cycles, the predicted Service Factor (SF) is 1.21. The AGMA contact stress was 528 KSI. The calculated contact stress was not considered realistic because there was local yielding of the rack in the contact zone.
The pinion of the jack-up rig was analyzed using the “ANSYS finite element program to calculate the principal stresses at the root of the tooth due to the normal force applied at the highest point of contact when meshing with the rack. Only one tooth and a portion of the adjacent tooth were modeled because of the minimal effects of the other pinion teeth, see Figure 7. The pinion was modeled using the STIF42 – 2D isoparametric solid elements from the ANSYS library. This element is used for two dimensional models of solid structures and can be used either as a biaxial plane element or an axisymmetric ring element. The element is defined by four nodal points and has two degrees of freedom at each node. A two dimensional model with a unit depth was considered adequate as there would be no out of plane effects in the pinion with it’s 8.5 inch face width. Figure 8 presents the calculated principal stress at the root of the pinion tooth.
TRANSVERSE LOAD SHARING
When one pinion tooth is loaded at or near the highest point of single tooth contact, a second tooth pair is very close to contact. The test was run with the pinion and rack indexed relative to each other in such a way that a second pair of teeth had a gap measured between contacting surfaces of .063 in. on one edge of the tooth and .049 in. on the other edge. The pinion of the second tooth pair had two thin foil strips laid parallel to each other near the center of the face width, running from the pitch diameter to the root diameter of the pinion. These strips were connected with wires to a power source and an indicator light so that when the second pair of teeth contacted the foil strips, they would be shorted together completing a series circuit and lighting the indicator lamp. It was anticipated that when the second tooth pair contacted, a significant change in shape of the load versus strain curve would occur as the first tooth pair transferred a portion of the total load to the second tooth pair.
PINION TEST RESULTS
The input torque was applied in increments and complete sets of all available data were recorded. Strain Gauges 2 and 7 , which were at the peak stress points (one on each edge of the pinion tooth), are plotted on Figure 9 against output tangential force. The indicator lamp lit up at 1,000,000 lb. load indicating a second tooth pair contact. Gauge 2 showed a reduction in the slope of strain versus applied load beyond the 1,000,000 lb. load level. Gauge 7 showed less change in slope. This lower slope verified a sharing of load between 2 tooth pairs. The reduction in load of the first pair, due to load sharing, was not as great as was expected. The strain curves continued in a nearly linear fashion up to 1,800,000 lbs. and then showed a slight increase in slope. The load was incremented up to a tangential load of 2,100,000 lbs. and held for 15 minutes at this point. As the highest applied load was approached, there was a cracking noise much like the sound of ice cracking. When the load was released, there was a high amount of residual tensile strain showing on the strain gauges. Gauge 2 showed (1143 microstrain) and gauge 7 showed (1557 niicrostrain). It was hypothesized that yielding of the core occurred which left the case in residual tension.
A second test was run on the same tooth, similar to the one above, except that the initial gap of the second tooth pair was set at .074 in. and .097 in. The load was incremented again and several loud cracking sounds were heard while holding at 1,650,000 lbs. The strain in gauges 2 and 7 went to zero because the cracks occurred right through the gauge length. Within a few seconds the tooth failed catastrophically. The sound was much like a loud gun shot. Sparks flew out of the test fixture. This violent failure was different from what was reported in Ref.8. Moores’s testing indicated a some what gentle parting after the initial cracking. This discrepancy might be a function of the size difference between the test samples. Several other teeth were loaded up to 1,800,000 lb. tangential load several times with no cracking heard or any significant residual strain in the gauges. It was estimated that the low cycle failure load was likely to be around 2,000,000 lbs. based on th. testing done. The maximum strain recorded of 8693 microstrain at this load corresponds to a tensile stress of 261 KSI. Thi5 stress is very close to the estimated tensile strength of 275 KSI for the carburized case material.AGMA 91FTM7 pdf download.