Method to Reduce Outer Arm Gram Load Bias in Hard Disk Drives
Original Publication Date: 2001-Aug-01
Included in the Prior Art Database: 2003-Jun-20
Hard disk drives requiring multiple heads have multiple actuator "arms" which support one or two heads. Outer arms support one head which flys over the top or bottom of the disk stack while inner arms support two heads, one which interfaces with the bottom of the disk above it and another which interfaces with the top of the disk below it. The gram load (i.e., the force which the actuator assembly exerts on the head in the direction perpendicular to the plane of the disk when the drive is in use) is one factor which determines the flyheight of the head. After assembly outer heads often display lower gram load values than inner heads due to the attachment (swage) process and the fact that there is no opposing force applied by another head on the same arm, which causes the outer arm to bend away from the disk and decrease the gram load of the outer head. This invention compensates for the inner versus outer arm gram load difference. It enables populations of head gimbal assemblies with similar gram load properties to retain similar gram load values after integration into the head stack assembly regardless of whether they are attached to inner or outer arms. This reduces the gram load range across all actuator heads which results in greater manufacturing yield and product reliability. The invention also reduces the need for gram load adjust processes, enabling companies to operate more efficient manufacturing lines which require less capital investment. The invention reduces the inner head versus outer head gram load difference by utilizing the spring properties of the head stack assembly's arms. To negate the inner/outer gram load bias, the free state locations of the outer heads (i.e., the location of the heads when no force is applied to them) are displaced along the axis perpendicular to the disk surfaces but retain the same orientation as the free state inner heads. This is done by moving the mount surface of the HGA on the arm tip closer to the disk for the outer arms. The amount to move can be determined by experiment or by analysis. The same effect could be achieved by adding material to part or parts of the outer arm HGAs or thickening the disks which interface with the outer heads.