Combining motors

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Part 2: Connecting two motors together

In this section, I present two methods for connecting two motors. 

  1. Late Junction method

The first, called Late Junction method, connects the two motors after speed of each has been reduced. This method is illustrated in the following figure, with 4 sets of 24:8 reduction gears. The motors have thus a speed reduced by 81 times.

Using techniques presented in the previous section, I measured the torque of the assembly. Torque is 200 bricks X 8 bumps, or 1600 bricks X bumps. It is less than expected since each motor individually has a torque of 880 bricks X bumps. So we expected 1760 bricks X bump with two motors. The lost is 160 bricks X bumps, which is substantial.

The next method is designed to reduce the lost.


  1. Early Junction method

The method proposed here connects the motors before their speeds are reduced. As a side advantage, the number of gears needed is smaller. Next figure illustrates the assembly, with 4 sets of 24:8 gears.

The final speed of the motors is the same as in the Late Junction method, however, torque differs. I measured a torque of 220 bricks X 8 bumps, that is, 1760 bricks X bumps. This value indicates that the Early Method has no loss of power since the total torque equals the sum of the individual torque.

The Early Junction method is to be preferred in every situation where you need extra power. It can also be adapted to more than two motors by adding another layer on top of motor 1 and motor 2.


  1. A comment on the above methods

It might seems at first sight that it is useless to measure torque. However, I disagree. If you want to make a walking robot for example, you will notice it is to weak to move rapidly. Recall that one Lego motor (with speed reduced 81 times) has a torque of 110 bricks X 8 bumps. If you add another motor to your robot, with its gears, you add a certain mass to your robot. I estimated the weight of a motor and a few extra piece to 25 bricks. Therefore, the effectiveness of your second motor is reduced by the extra weight it adds.

Suppose you use an inadequate method for connecting your motors (such as the Late Junction method). Suppose further that the legs of your robots are 16 bumps in length. With one motor, you had a torque of 55 bricks X 16 bumps. With two motors (Late Junction method), you now have 100 bricks X 16 bumps AND an extra weight of 25 (the second motors). Your final torque is thus 75 bricks X 16 bumps. The second motors adds only 20 bricks X 16 bumps of power to your robot. An increase of 50%! Overall, you might notice no difference at all!

With the Early Junction method, odds are a bit better. Two motors with the Early Junction method provides 110 bricks X 16 bumps of power. If I remove the extra weight added by the second motor, the resulting torque is 85 bricks X 16 bumps. Now the increase in power is 70% (We get from 50 to 85 bricks X 16 bumps). Although not a miracle, the increase should now be visible.

The two methods presented here, Early and Late Junction methods, are the only two I tried. If you have any other method to suggest, please, measure its performance, and send me a e-mail. I will be pleased to add your contribution to this page.