| dc.description.abstract | While designing a compact gearbox with a high speed ratio, the multi-stage tandem planetary gear drive, which is common in the practice, could not effectively reduce the size of the gearbox. In order to obtain the planetary gearboxes with high speed ratio and small size, two-stage compound planetary gear train is designed and tested in this study where two types of gearboxes, 3K-type and differential type gearbox are considered repectively. Since these gearboxes are subjected to variable load conditions with maximum input speed of 3000 rpm and maximum torque of 50 Nm, and with a specific load spectrum, performance and fatigue tests were conducted to verify the transmission performance of the gearbox prototypes.
The design and analysis of the compound two-stage planetary gear drives include the analysis of transmission performance, gear strength calculation, and the design of gearbox prototypes. At first, a set of matrixes for calculation of the speed, torque, and efficiency of planetary gearboxes is established with which the relation of the speed ratio with the tooth numbers of the gears, the torque distribution between the two stages and the power flow within the gearbox. Afterwards preliminary design parameter can be determined for the required speed ratio of the gearbox from these analysis results.
While in the analysis of transmission efficiency and internal power flow of the planetary gearbox, a part of the power flow circulates within the 3K-type paradox planetary gearbox, and it results that the overall efficiency of the gearbox is very sensitive to the mesh efficiency of the gears, and this mechanism might be even self-locked in oprating as a speed increaser. While in designing the two-stage differential type planetary gearbox, the torque distribution between the differential stage and the deceleration stage is designed as 25% and 75% respectively, which is based on the relation of the power and speed ratio of each planetary stage. As a result, a smaller-sized gearbox can be obtained to meet the limited space requirements due to the reduction of the maxmum torque in the deceleration stage.
While designing gearbox prototypes, the tooth profile of the differential type gearbox is based on the standard profile with profile-shifting. The 3K-type paradox planetary gearbox, on the other hand, was designed with asymmetric tooth profile to enhance the tooth strength by considering that the gearbox is loaded under a specific direction of rotation. However, because the planet gears engage two diffrernt ring gears with the same center distance, the direct gear design method is used to avoid the complex design process as using the standard tooth profile. And, the addendum and the tooth thickness are selected, and a design chart for selection of appropriate profile parameters is established based on the equations for gear meshing and geometrical limits.
In general, the load capacity calculation of gears in the standard, ISO-6336, is based on the verified results of gears with module greater than 2 mm. The size factor is thus modified for the strength calculation of the proposed gearboxes with the module 0.5 mm. From the analysis results of the gear strength, it was found that the tooth surface durability of both type of the gearbox is close to the maximum limit of load capacity.
It is shown from the results of performance test that the transmission efficiency of the differential type planetary gearboxes is between 70% and 80% , and the efficiency of 3K-type gearbox is only 40% to 50%. Because the transmission efficiency is the major evaluating criterion for gearbox performance and the slef-locking of the 3K-type gearbox can not be avoided, the 3K-type gearbox is not used in the fatigue test.
The results of fatigue test show that the gearbox sample 1 and 2 of the 2nd version of differential type planetary gearbox are failed due to the broken planet pins, but the teeth of the gears remain good condition. the sample 3 were damaged severely due to the over-loading and unstable loding conditions of the test bench. After discussion with the experts, the designed tooth strength of the gearbox prototypes is enough to meet the required specificatiom. The subsequent test will proceed by controlling carefully the surface treatment and hardness of the teeth and the calibration and setting of the fatigue test machine.
The research results show that the direct gear design method of 3K-type paradox planetary gear drive and the transmission analysis method of the differential type planetary gear drive can be used as a reference for designers when designing compound planetary gear drives. From the gearbox performance test results, it can be confirmed that the efficiency performance of the 3K-type paradox planetary gear drive has the properties of low efficiency and self-locking for speed increasing, and the differential type planetary gear drive has the higher transmission efficiency as the theoretical analysis results show. The damage of the samples of the fatigue life test shows that the tooth profile modification and parameter design of the second version of the differential type gearboxes can meet the requirements of the critical strength under the limited life design conditions of the gear transmission. | en_US |