Reliable two-way 90° stepper




In pursuit of a reliable two-way 90° stepper that is capable of handling high torque without slipping, I came up with this solution. It has been specifically designed for use with a PF Servomotor, that is for 90° input rotations, clockwise and counterclockwise.

How does it work?
While idle, the differential housing can't rotate, because it's locked by two ratchets (which lock in one direction and slip in the other direction). The ratchets are tied together with a single white silicon band, only one stud distant from their rotation points (not visible in video). When the input engages, one ratchet opens, but the other ratchet prevents the differential from rotating along with the input axle. The result is that the input flows through the bevel gears inside the differential housing, and outputs with a 1:1 ratio. When the input returns, the 90° limiter prevents the output from returning, forcing the differential to slip over the one ratchet that is still engaged. After returning completely, the open ratchet re-engages and the differential housing is completely locked again.

When you listen carefully, the return gives three clicks. The differential housing returns with half the speed of the input axle. This means it will rotate backwards 45°, which covers exactly 3 teeth of the 24-teeth side of the differential.

It all comes very precise. Here are a few requirements that needed to be met:

1.) After a return, when the differential is idle and locked by both ratchets, the exact position where the ratchets hook on to the differential should be such that one tooth of the 24t side of the differential is pointing straight up. This is needed to get equal starting points for shifts in both directions. The previous shift should not put one direction in favor of the other by leaving a slighlty off-center differential. The 12t gears hook on to the differential at exactly the right height to obtain a properly centered differential after each shift.
2.) When the input returns, the open ratchet needs to re-engage with the 24t side of the differential only after the 3rd tooth passes the other ratchet. If the open ratchet re-engages too early, the differential will not return the required 45 degree (3 teeth). To prevent the open ratchet from re-engaging too early, the orange (0X)-piece which controls the ratchets needs to sit tight against the ratchets, even in its centered (idle) position. That way the ratchet will only be fully closed once the (0X)-piece points straight up again. I used the reddish brown 3L axles with stop to bring the ratchet close enough to the (0X)-piece.
3.) When the output is blocked, for instance in case of a 4th-to-1st gear block in a sequential gearbox, the ratchets should not slip, not even when a Servo-motor is used as input. This means an engaged ratchet should firmly hook on to the differential to keep it put. The 12t gears do a very good job in that respect.

I also tried with a 28t differential and an 8t gear as ratchet, but a 45 degree return would translate into 3.5 teeth, which would practically result in a 3 or 4 teeth return and thus a biased starting point for the next shift.
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Name Reliable two-way 90° stepper
Designer Didumos
Designed 2019
Inventory 72 parts
Theme Technic
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