Another way to move the eyepiece in and out is to employ a "Helical" focuser.  The draw tube is threaded into the housing, only the pitch of the thread is so small, - that a single turn of the draw tube raises or lowers the draw tube the full length of its travel.  This makes for a very smooth focus. However, these focusers have a very limited travel. A typical travel for a 2" diameter focuser is 1 cm.  This limited travel makes them unable to support a whole line of eyepieces.
The Borg model 7315 is pictured here.  Used in tandem with an inexpensive Rack and Pinion focuser, the helical focuser can provide excellent fine focusing for a telescope for less money than a good Crayford type. The model shown here does not rotate the eyepiece as many other helical focusers do. Helical focusers are not generally suited for Astrophotography through the eyepiece, as they are also limited in the amount of weight they can support.
Helical Focusers with a higher pitch are possible, but they give the focuser much less travel per turn, - making it a pain to move to a focus that is much further along the tube.