dc.description.abstract | Comparing with bevel gears, conical gear drives, which are paired with a shaped conical gear and a spur gear, have some significant advantages, such as lower assembly sensitivity, low complexity in geometry of tooth surface for ease of design, and lower manufacturing cost of molded gears (e.g. powder metallurgy gears). The conical gear drives have thus a potential to replace bevel gears in the application of mass production and transmission with a large shaft angle. But the bearing contact during gear meshing is similar to spur gear pairs that the load sharing is discontinuous. This phenomenon can cause an impact on tooth pairs and concentrated contact stresses on flanks at contact begin or end and. Abrasive wear or pitting damage on tooth flanks, and even breakage of teeth could occur. In order to expand the application of conical gears, the load capacity of the gear drives must be able to calculate to fulfill the design requirements, especially for application of powder metallurgy gear pair widely used by confirm modified flanks.
The aim of this thesis is to propose a parabolic profile crowning approach for conical gears based on the developed design approach for nonstandard conical gears haiving a small gear ratio [4], as to improve a smooth variation of the load sharing of the contact tooth pair. A Loaded and an unloaded tooth contact analysis approach for conical gear pairs are also proposed respectively to verify the feasibility of the proposed approach for flank modification. Finally, test gears are manufactured to measure the tooth contact patterns considering assembly errors. A fatigue test is also conducted on a close-loop test rig to validate the surface durability.
The specific sliding is at first introduced in the design approach as an additional design criterion to reduce the wear on flanks. In order to to simplify the solving process for the positions of contact points, the proposed tooth contact analysis (TCA) model is developed based on the geometric characteristics of involute. The influences of the flank modification parameters on the contact positions and the transmission errors of the gear pair under different assembly errors are analyzed by using the TCA model. The results show that:(1) The shift distance of contact positions from the middle of the face-width to the heel (the toe) at the contact begin (end) is larger with increased modification amounts; (2) The unloaded contact ratio of non-modified flanks of conical gear drives under assembly errors is inversely proportional to the working pressure angle due to shifting of contact position; (3) The unloaded transmission error (UTE) performs similar to parabolic curve in the presence of assembly errors. But, the UTE will be discontinuous at contact begin if the contact position is shifted to the heel
On the other hand, a loaded tooth contact analysis (LTCA) model based on influence coefficient method is also developed to confirm the load capacity of the modified conical gear drives. The tooth contact stress, the load sharing and the loaded transmission errors during gear meshing are analyzed. The results for gear drives under ideal condition show that:(1) The load sharing during gear meshing becomes continuous and no load exists at contact begin or end; (2) A concentrated contact stress on a small area occurs at the contact end due to contact with the tip edage of the pinion; (3) The loaded transmission error (LTE) is also similar to parabolic curve. The change of the amplitude value is lower with the increased load. The results for gear drives under assembly errors show that:(1) The load at contact end or begin doesn’t disappear and is higher with the increased shift distance; (2) The variation of the load sharing and the LTE become discontinuous at the position from single to double tooth pair contact, and vice versa.
The TCA model is also validated by measuring the contact patterns of test gears and interference analysis in a CAD software under assembly errors. The results show that the TCA model is in good agreement with the mentioned methods.
A overloaded fatigue test is conducted on a power close-loop test rig with two types of PM conical gears, each with non-modified and modified flank. The experimental results show that the pitting damage on modified flanks is far less than non-modified flanks under the same conditions. Finally, the analysis results of the measured vibration data show that the vibration amplitudes of the gear pairs with flank modification are lower than those of the gear pairs without modification. Especially the reduction of the vibration amplitudes at the 1st gear mesh frequency is affected strongly by the improvement of the load sharing of the gear drives due to flank modification.
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