Data storage devices of the type known as "Winchester" disc drives are well known in the industry.  Such devices magnetically record digital data on a plurality of circular, concentric data tracks on the surfaces of one or more rigid discs.  The discs are typically mounted for rotation on the hub of a brushless direct current spindle motor.  In disc drives of the current generation, the discs are rotated at speeds of up to 10,000 revolutions per minute. 
Data are recorded to and retrieved from the discs by an array of vertically aligned read/write head assemblies, or heads, which are controllably moved from track to track by an actuator assembly.  Each head typically comprises electromagnetic transducer read and write elements which are carried on an air bearing slider.  The slider acts in a cooperative hydrodynamic relationship with a thin layer of air dragged along by the spinning discs to fly each head in a closely spaced relationship to the disc surface.  In order to maintain the proper flying relationship between the heads and the discs, the heads are attached to and supported by head suspensions or flexures. 
The actuator assembly used to move the heads from track to track has assumed many forms historically, with most disc drives of the current generation incorporating an actuator of the type referred to as a rotary voice coil actuator.  A typical rotary voice coil actuator consists of a pivot shaft fixedly attached to a disc drive housing base member at a location closely adjacent an outer edge of the discs.  The pivot shaft is mounted such that its central axis is normal to the plane of rotation of the discs.  An actuator bearing housing is mounted to the pivot shaft by an arrangement of precision ball bearing assemblies, and supports, in turn, a flat coil which is immersed in a magnetic field of an array of permanent magnets which are fixedly mounted to the disc drive housing base member. 
On the side of the actuator bearing housing opposite to the coil are a plurality of vertically aligned, radially extending actuator head mounting arms to which the head suspensions mentioned above are mounted.  When current is applied to the coil, a magnetic field is formed surrounding the coil which interacts with the magnetic field of the permanent magnets to rotate the actuator bearing housing about the pivot shaft, thereby moving the heads across the disc surfaces. 
Disc drives of the current generation are included in desk-top computer systems for office and home environments, as well as in laptop computers which, because of inherent portability, can be used wherever they can be transported.  Because of this wide range of operating environments, the computer systems, as well as the disc drives incorporated in them, must be capable of reliable operation over a wide range of ambient temperatures. 
Laptop computers can further be subjected to large magnitudes of mechanical shock as a result of handling.  It is common in the industry, therefore, to require disc drives to operate over a wide range of ambient temperatures as well as to be able to withstand substantial mechanical shocks without becoming inoperable. 
One of the areas of disc drive design which is of particular concern when considering ambient temperature variations and mechanical shock resistance is the system used to mount the discs to the spindle motor.  During manufacture the discs are mounted to the spindle motor in a temperature and cleanliness controlled environment.  Once mechanical assembly of the disc drive is completed, special servo-writers are used to prerecord servo information on the discs.  This servo information is used during operation of the disc drive to control the positioning of the actuator used to move the read/write heads to the desired data location in a manner well known in the industry.  Once the servo information has been recorded on the discs, it is essential that the servo information, and all data subsequently recorded, spin in a concentric relation to the spin axis of the spindle motor.  The discs, therefore, must be mounted to the spindle motor in a manner that prevents shifting of the discs relative to the spindle motor when subjected to a mechanical shock. 
Several systems for clamping discs to spindle motor hubs have been disclosed in the prior art, including U.S. Pat.  No.  5,528,434, issued to Bronshvatch et al.  on Jun. 18, 1996; U.S. Pat.  No.  5,517,376, issued to Green on May 14, 1996; U.S. Pat.  No.  5,452,157, issued to Chow et al.  on Sep. 19, 1995; U.S. Pat.  No.  5,333,080, issued to Ridinger et al.  on Jul. 26, 1994; U.S. Pat.  No.  5,274,517, issued to Chen on Dec. 28, 1993; and U.S. Pat.  No.  5,295,030, issued to Tafreshi on Mar. 15, 1994, all assigned to the assignee of the present invention.  In each of these disc clamping systems the spindle motor includes a disc mounting flange that extends radially from a lower end of the spindle motor hub.  A first disc is placed over the hub during assembly and brought to rest on this disc mounting flange.  An arrangement of disc spacers and additional discs are then alternately placed over the hub until the intended "disc stack" is formed.  Finally, a disc clamp of selected construction (such as spring or shrink-fit) is attached to the spindle motor hub to exert an axially directed clamping force against the uppermost disc in the disc stack.  This axial clamping force is passed through the discs and disc spacers and squeezes the disc stack between the disc clamp and the disc mounting flange.  This technique requires that the resulting friction between the clamp and top disc and between the bottom disc and disc mounting flange be sufficient to resist movement of the entire disc pack in response to a shock event. 
With an industry trend toward size reduction in the overall disc drive, the size of various components within the disc drive has necessarily been reduced, including the thickness of the discs.  As the discs have become thinner, the amount of clamping force that can be applied to the discs without causing mechanical distortion of the discs is limited.  That is, variation in the flatness of the disc mounting flange, the discs, and the disc spacers contribute to flatness concerns of the discs relative to the heads.  The elastic modulus of the disc material, too, affects the flatness of the joined assembly providing the disc pack.  Moreover, uneven distribution of the clamping force upon the top most disc of a disc stack can attribute to disc waviness, thus distorting the disc and possibly adjacent heads.  These and other factors limit the axial clamping force that can be applied using presently available techniques. 
With continued demand for ever increasing levels of mechanical shock resistance, there remains a continued need for improvements in the manner in which discs are clamped to the spindle motors of disc drives.  It is to such improvements that the present invention is directed.