Hi all! I am going to design a double reduction speed reducer. using spur gears. There are a few basic layout that I can choose. And which layout would be the best? Below is my consideration: All the design have the same number of bearings, so I think that the efficiency of the bearing is not a key. I can't see any big difference between design 1 and design 3 except that design 3 can save more space. In terms of efficieny, they should be the same, isn't it? Also, the axis of input an output shaft are aligned differently in design 1 &3, that should have some drastic difference, right? But what? I am not quite sure. Comparing design 1 and design 2, the main difference is that design 2 utilized an additional pair of gear. That reduce the load on driving gears and hence a longer life could be achieved, right? In terms of efficieny, the intuitive feeling tells me that the efficiency of having an additional inefficient (not 100% efficiency) gears would be reduced. But I work out some analogue and find that the efficieny should be the same due to the reduced load on each pair of gears. Am I correct? So what do you think about the disadvantages and advantages of each design? By the way, the speed reducer is going to be used in a tractor.