How do the 61927 actuators work in MK II crane set (42009)? 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.

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.

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



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).