Helicoid structure used to drive a lens barrel in a vari-focal apparatus

A helicoid structure for driving a lens barrel in a vari-focal apparatus, said helicoid structure comprising:

• a male helicoid formed around an inner barrel, said male helicoid including long male crests and short male crests;
• and
• a female helicoid formed on an inner peripheral surface of an outer barrel, said female helicoid including:
• a plurality of long female crests having engagement ends at which said male helicoid initially engages said female helicoid and which are positioned near a forward end of the outer barrel,
• a plurality of short female crests having engagement ends which are positioned further from the forward end of the outer barrel than the engagement ends of said long female crests, wherein said long female crests and said short female crests are alternately arranged on the outer barrel, and
• a plurality of guiding crests connecting the engagement ends of at least some of said short female crests to an adjacent one of said long female crests, said guiding crests being formed along a plane extending orthogonally to an axis of the outer barrel,
• wherein said long male crests have engagement ends at which said male helicoid initially engages said female helicoid and which are positioned near a rear end of the inner barrel, said long male crests being formed so as to cooperate with troughs respectively defined between pairs of those short female crests having said guiding crests connected thereto and adjacent long female crests having no guiding crest connected thereto, and
• said short male crests have engagement ends which are positioned further from the rear end of the inner barrel and which are formed so as to cooperate with troughs respectively defined between pairs of long and short female crests that have no guiding crest connected thereto,
• whereby, with the engagement ends of said long male crests bearing against said guiding crests, the inner barrel is adapted to rotate relative to the outer barrel so that said male helicoid initially engages said female helicoid upon disengagement of said long male crests from said guiding crests.