Short then longer rotation from single motor input

[TheFocusDev]

HI there! 

I'"m working on a Technic/Mindstorms/Lego System hybrid MOC, more specifically a realistic passenger lift/elevator. On the lift cabin, there's an Ev3 medium motor to move the sliding door, with a rack and pinion configuration. I need a way to somehow delay opening the door mechanically, so I can engage a so-called "skate" or "vane" mechanism, thats job is to extend and thus push a wheel on the floor, unlocking the latch of the shaft door. I based my implementation on real lift doors, with a slight modification due to space constrains. The skate on my lego lift cabin is stationary, mounted right beside the door. I need some kind of mechanism that can do the following:

Rotate the skate to the extended position (quite small rotation), then slide the door open (longer rotation). On closing, first close the sliding door, then rotate the skate back to retracted position, causing it to relock the shaft door. so I need an A->B->B-A type of sequenced motion.
Here's a video how real elevators do this 

The difficult part is that I use a fixed skate thus it is not mounted on the door panel, and I use a rack and pinion type sliding door, and I need to get the same sequence

Here's my skate

And here's how it is mounted


 

Edited December 16, 2025 by TheFocusDev

[Berthil]

I haven't looked in detail what you want but sounds like a Cardan Lift mechanism might be of use to you. Here you see one in a GBC. Unfortunately these mechanisms use space, something you might not have a lot in a lift.

 

[TheFocusDev]

39 minutes ago, Berthil said:

I haven't looked in detail what you want but sounds like a Cardan Lift mechanism might be of use to you. Here you see one in a GBC. Unfortunately these mechanisms use space, something you might not have a lot in a lift.

 

Yes space is quite limited, and I need a very small motion and a longer one, turning therack and pinion, and both have physical end stops, also oscillating motion is out of the question.

Edited December 16, 2025 by TheFocusDev

[Stereo]

Maybe a spring-loaded addition to the door slider?

Then the rack can withdraw an extra stud (or however far necessary) to engage the skate without the door being able to move, once it moves out of that space it can slide the door.

Edited December 16, 2025 by Stereo

[TheFocusDev]

2 minutes ago, Stereo said:

Maybe a spring-loaded addition to the door slider?

Then the rack can withdraw an extra stud (or however far necessary) to engage the skate, once it moves out of that space it can slide the door.

Yes I thought of something like this but I can't just modify it like this because it is a 2-speed telescopic door with 2 panels moving in the same direction and one moves half the speed as the other. See the first picture (so the one right after the yt video), there's an 8 and 16 tooth gear moving the 2 panels
 

[Lok24]

1 hour ago, TheFocusDev said:

Yes space is quite limited, and I need a very small motion and a longer one 

Why dont't you program the complete process with the EV3?

[TheFocusDev]

6 minutes ago, Lok24 said:

Why dont't you program the complete process with the EV3?

I can't fit 2 motors on top of the elevator cabin to do this, also most real elevators accomplish this mechanically, with a single motor. Oh and to clarify, I need 2 axles to be moved with a single motor in a specific sequence, and not a single one a bit, then same one for longer

Edited December 16, 2025 by TheFocusDev

[Stereo]

2 hours ago, TheFocusDev said:

Yes I thought of something like this but I can't just modify it like this because it is a 2-speed telescopic door with 2 panels moving in the same direction and one moves half the speed as the other. See the first picture (so the one right after the yt video), there's an 8 and 16 tooth gear moving the 2 panels
 

Hmm, maybe you could build it into the geartrain then.  Like use 2L beams (axle+pin) with an elastic and towballs.  So normally the towballs touch the other beam, but when the gear rack hits its end, the motor can keep turning about a quarter turn.  Looks like you have ~7.5 teeth of sliding so a bit less than 360 degrees of output, so this does take up 3 studs of vertical space if it's geared 1:1 to the gear racks.

Edited December 16, 2025 by Stereo

[Plumber]

How about something similar to the rack card 'programmable' crane in the back of the lego ideas book 8888?

 

[TheFocusDev]

On 12/16/2025 at 7:32 PM, Stereo said:

Hmm, maybe you could build it into the geartrain then.  Like use 2L beams (axle+pin) with an elastic and towballs.  So normally the towballs touch the other beam, but when the gear rack hits its end, the motor can keep turning about a quarter turn.  Looks like you have ~7.5 teeth of sliding so a bit less than 360 degrees of output, so this does take up 3 studs of vertical space if it's geared 1:1 to the gear racks.

This could be useful, but I see it as just part of the solution. So to reiterate, Small motion should come first, which makes the skate extend (hence pushing a wheel fixed to the current floor's wall with a simple linkage to turn it into upward motion, that raises the door latch. This latch part is working well). Then when nothing blocks the door from sliding, it should rotate the rack and pinion, and the door is fully opened. And the closing sequence should start with the rack and pinion, and finalise with the skate retracting, thus letting go of the wheel. So A->B on opening, then  B->A on closing. A is the skate, B is the door

Edited December 19, 2025 by TheFocusDev

[Divitis]

Can building around a differential help? First motion happens until the door is unlocked, than that becomes the path of least resistance.

 

 

[TheFocusDev]

17 minutes ago, Divitis said:

Can building around a differential help? First motion happens until the door is unlocked, than that becomes the path of least resistance.

 

 

The sliding door relies on the Ev3 medium motor and it's rotation encoder to stop where it needs to. If I introduce a differential then if something blocks the door halfway then it would stay open and software wise it would look as if it closed perfectly, which isn't ideal