Developing device brought into contact with an electrostatic latent image support member

An image forming apparatus comprising:

• an electrostatic latent image support member;
• a developer support member which is brought into contact with said electrostatic latent image support member;
• wherein at least one of said electrostatic latent image support member and said developer support member has a flexible surface;
• an electric field forming means for forming an alternating electric field such that an attractive force capable of bringing said electrostatic latent image support member and said developer support member into contact with each other is applied between said electrostatic latent image support member and said developer support member;
• and
• a changeover means for changing over said electric field forming means to a connective state or a nonconnective state.

A developing device comprising:

• a drive roller which is driven so as to be rotated;
• a developing sleeve which is formed by a thin film and has a peripheral length slightly larger than that of said drive roller so as to be loosely mounted around said drive roller;
• a press means which presses said developing sleeve against said drive roller at one side of said drive roller so as to form a sag portion of said developing sleeve such that said sag portion is brought into contact with an electrostatic latent image support member;
• an electric field forming means for forming alternating electric field such that an attractive force for bringing said developing sleeve and said electrostatic latent image support member into close contact with each other is applied between said developing sleeve and said electrostatic latent image support member;
• and
• a changeover means for effecting changeover between a first state in which said developing sleeve and said electrostatic latent image support member are held in close contact with each other and a second state in which said developing sleeve and said electrostatic latent image support member are held out of close contact with each other.

A developing device comprising:

• a drive roller;
• a developing sleeve which has an inside diameter slightly larger than an outside diameter of said drive roller so as to be loosely mounted around said drive roller;
• and
• a press member for pressing said developing sleeve against said drive roller at one side of said drive roller such that a sag portion of said developing sleeve formed at the other side of said drive roller is brought into contact with an electrostatic latent image support member;
• wherein supposing that character .alpha. denotes an angle formed between a line connecting a center of said electrostatic latent image support member and a center of said drive roller and a line connecting the center of said drive roller and a radial end of said press member disposed, in a rotational direction of said developing sleeve, upstream of a location where said sag portion is held in contact with said electrostatic latent image support member and character .DELTA.L denotes a difference between an outside diameter of said developing sleeve and that of said drive roller, the equation:
• .DELTA.L.ltoreq.(117/.alpha.)-0.7
• is satisfied.