On one part there is an inclined groove, in the other there is an axis moving along the groove. To reduce friction on the axis can be a bearing. The slope of the groove depends on the gear ratio.
On the details of the toothed rails to transmit motion through the gear. If the transmission is not 1:1, the gear unit.
On one part of the rack gear, which is engaged with located coaxially with the other part of the gear. On the second part of the screw passing through the gear and secured from turning. When turning the gear, the screw is screwed, moving the second part.