Linear ratchet mechanism 1 

Yellow slider can be positioned at various positions during going up thanks to pink pawl and the gravity.
Blue spring forces the pawl towards white runway.
At highest position of the yellow slider the pawl is separated from the runway by a wedge. The upper pin (fixed to violet slider) enters the pawl slot thus maintains the separation. Red spring forces the pin on the pawl. The pawl does not contact the runway when going down.
At lowest position of the yellow slider the violet slider is pushed up, the upper violet pin releases the pawl which now comes into contact with the runway.
So the engagement of the yellow slider with the runway is controlled automatically.
To get higher position of the yellow slider: move it up.
To get lower position of the yellow slider: move it as high as possible, then move it down as low as possible and raise it to desired position.
Instead of this automatic function pull the pawl and move the slider to desired position.
This mechanism can be used for adjustable backrest of a chair. The backrest is fixed to the yellow slider.
STEP files of this video:
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Inventor files of this video:
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