I don't own a 42009 set (MK II crane). But I have modelled it in LDCad in order to understand how the set works. There is something I don't understand related to the actuators - as I don't have two pieces either I have come here to ask.

The 61927 actuator The 61927 actuator.

I see the two pieces of 61927 actuators attached to a gearing comming from one L motor, the gearing seeming to eventually produce a rotation for the two actuators in opposite direction. So, I expect them to be antagonic, meaning one extending and one contracting at the same time. What is wrong in my assumption? Where the rotation is synchronized for the two actuators?

The gearing, stripped The gearing, stripped.

So, why do they work in the same direction while they are turned in different directions at the same time? Or aren't they?

1 Answer 1


THe actuators both turn in the same direction, as there is a gearing “bridge” between the two axles driving the actuators that consists of 3 interlocking gears. 3 gears that interlock cause the 2 outermost gears to spin in the same direction. You can easily validate this with a small test setup using a Technic brick with holes, a small axl and a few gears and axle pins.

EDIT: For clarity, this picture shows how each of the key gears spin in this setup.

enter image description here

The two flat bevel gears marked with #1 spin in opposite directions from eachother, but this is possible because even though they are on the same axle (#2), they are not attached to this axle as the gears have a pinhole, not an axle hole, and hence they can spin freely (see part 87407 on Bricklink). It is likely gear #3 (marked in yellow) that is the gear that is driven, and its power and rotation is transfered to its twin gear which drives the other actuator (and spins in the same direction).

  • Thanks, I believe it is, I just fail to see it. Where is that bridge? If you discuss the 3 ones with a circular hole, I agree. From my point of view, the tan gears before the actuators turn the same way, but after, the gray ones, at 90 degrees, mounted as opposite, make the actuators turn differently.
    – mike
    Jun 8, 2019 at 12:20
  • Those tan gears with the gray in the center form a similar bridge, just under an angle. This is similar to how a differential works. In the end, the two tan gears within the bridge turn in opposite directions, but this happens equally on both sides, so both actuators still activate in the same direction.
    – Phil B.
    Jun 9, 2019 at 1:16
  • Exactly, this is the question, why do they work in the same direction while they are turned in different directions at the same time?
    – mike
    Jun 9, 2019 at 8:05
  • Added a picture and some further clarifications, hope this makes it clearer.
    – Phil B.
    Jun 10, 2019 at 4:37
  • Excellent, thank you, I think the position of the last gray gears does not matter, if they are on the left or on the right side, isn't it?
    – mike
    Jun 10, 2019 at 8:42

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