Film-engaging sprocket drive

A film-engaging sprocket drive as recited in claim 7, wherein said helical torsion spring is compressed when one sprocket wheel is rotated relative to the other sprocket wheel to increase the pitch of the two teeth in said respective pairs of teeth and expands to rotate one sprocket wheel relative to the other sprocket wheel to decrease the pitch of the two teeth in said respective pairs of teeth.

An edge-perforated filmstrip having successive uniform-pitch film perforations, and a film-engaging sprocket drive including a set of two coaxial sprocket wheels each having a series of uniform-pitch film-engaging teeth that are similarly shaped to engage said filmstrip within said film perforations and that are arranged in successive pairs of one tooth from one sprocket wheel and another tooth from the other sprocket wheel, and said sprocket wheels each are rotatable relative to the other to similarly increase the pitch of the two teeth in said respective pairs of teeth to greater than the pitch of said film perforations and to similarly decrease the pitch of the two teeth in said respective pairs of teeth substantially to the pitch of said film perforations.

A film-engaging sprocket drive for use with an edge-perforated filmstrip having successive uniform-pitch film perforations, said sprocket drive comprising:

• a set of two coaxial sprocket wheels each having a series of at least partially curved face uniform-pitch film-engaging teeth that engage the filmstrip within the film perforations and that are arranged in successive pairs of one tooth from one sprocket wheel and another tooth from the other sprocket wheel, and said sprocket wheels each are rotatable relative to the other to similarly increase the distance between a point on said face of one of the two teeth in said respective pairs of teeth and a corresponding point on said face of the other tooth in the same pair to similarly increase the pitch of the two teeth in said respective pairs of teeth to greater than the pitch of the film perforations;
• and
• a return spring arranged to rotate one sprocket wheel relative to the other sprocket wheel to similarly decrease the distance between the point on said face of one of the two teeth in said respective pairs of teeth and the corresponding point on said face of the other tooth in the same pair to decrease the pitch of the two teeth in said respective pairs of teeth substantially to the pitch of the film perforations.

A method of engaging a sprocket drive with an edge-perforated filmstrip, comprising:

• rotating one of two identical multi-tooth sprocket wheels relative to the other sprocket wheel to position one tooth of the one sprocket wheel in a second available perforation in the filmstrip;
• and
• rotating the other sprocket wheel relative to the one sprocket wheel to position one tooth of the other sprocket wheel in a first available perforation in the filmstrip which precedes the second available perforation.