Differential ball screw and nut assembly and method of obtaining relative linear motion differentially

A method of operating a differential ball nut and screw assembly incorporating an axially and rotatable fixed nut;

• a tubular drive screw having an externally helical groove providing a first raceway with said nut and a generally co-extensive internal helical groove;
• a rotatably fixed inner actuator screw received in said tubular drive screw and having an external helical groove forming a second raceway with said internal groove of the drive screw;
• and a first ball train in said first raceway and a second ball train in said second raceway comprising:
• a. providing said tubular screw with external and internal helical grooves of the same hand but different lead, with the lead of the internal groove being reduced to provide a lead differential between said external and internal grooves;
• b. moving said tubular drive screw in one axial direction by revolving it relative to said axially and rotatably fixed ball nut;
• c. providing said actuator screw external groove with a reduced lead corresponding to said reduced lead of the internal groove of the tubular drive screw;
• d. internally carrying said actuator screw axially along with the tubular screw;
• and
• e. utilizing a recirculating ball train between said internal groove of the tubular screw and exterior groove of the inner actuator screw to differentially change axial movement of said inner actuator screw relative to the axial movement of said tubular screw and move said inner screw an axial distance corresponding only to the number of rotations of the tubular screw times the lead differential.

A dual screw differential ball screw and nut assembly comprising:

• a. an axially and rotatably fixed nut having an internal helical groove with a first lead;
• b. a first tubular ball screw with an external helical groove having a corresponding lead cooperating with said nut groove to provide a first raceway;
• c. a train of balls in said first raceway;
• d. said tubular ball screw having an internal helical groove generally coextending with and of the same hand as said external helical groove, with a second different lead relative to said first lead;
• e. a second rotatably fixed, radially inner screw having an external helical groove with a lead corresponding to said second lead and cooperating with said internal groove of said tubular screw to provide a second raceway;
• and
• f. a second train of load bearing balls received in said second raceway to transmit load to move said screws axially in the same direction differentially.

A method of making a differential ball screw and nut assembly comprising the steps of:

• a. providing a tubular ball screw having generally co-axial differential lead helical groove portions of the same hand externally and internally, the lead of the internal groove being reduced relative to the external groove;
• b. inserting an inner screw, having an external helical groove with a lead corresponding to said reduced lead of said internal helical groove portion of the tubular screw to form an inner raceway, into said tubular screw;
• c. loading a train of abutting recirculating load transfer balls to said inner raceway;
• d. providing a fixed ball nut having an internal helical groove corresponding to and forming an outer raceway with said external groove portion on said tubular screw and loading a train of abutting recirculating load transfer balls to said outer raceway;
• and
• e. fixing said inner screw from rotating while permitting it to translate.