Retainer for an adjustable balance ring in a disc clamp assembly

RELATED APPLICATIONS

[0001] This application is a divisional application of U.S. patent application Ser. No. 09/934,164 filed on Aug. 21, 2001, which claims priority of U.S. provisional application Serial No. 60/227,616, filed Aug. 23, 2000.

FIELD OF THE INVENTION

[0002] This application relates generally to magnetic disc drives and more particularly to a disc drive disc clamp assembly having a base, retainer and balance ring.

BACKGROUND OF THE INVENTION

[0003] Disc drives are data storage devices that store digital data in magnetic form on a storage medium on a rotating information storage disc. Modern disc drives include one or more rigid discs that are coated with a magnetizable medium and mounted on the hub of a spindle motor for rotation at a constant high speed. Information is stored on the discs in a plurality of concentric circular tracks typically by an array of transducers (“heads”) mounted to a radial actuator for movement of the heads relative to the discs. Each of the concentric tracks is generally divided into a plurality of separately addressable data sectors. The read/write transducer, e.g., a magnetoresistive read/write head, is used to transfer data between a desired track and an external environment. During a write operation, data is written onto the disc track and during a read operation the head senses the data previously written on the disc track and transfers the information to the external environment.

[0004] The heads are mounted via flexures at the ends of a plurality of actuator arms that project radially outward from the actuator body. The actuator body pivots about a shaft mounted to the disc drive housing at a position closely adjacent the outer circumference of the information storage discs. The pivot shaft is parallel with the axis of rotation of the spindle motor and the information storage discs, so that when the actuator arms are pivoted, the heads move in an arc across the surfaces of the information storage discs.

[0005] As briefly noted above, modem disc drives include one or more information storage discs mounted on the spindle motor. Spacers are typically mounted on the spindle motor between information storage discs to provide adequate separation between the information storage discs for allowing the actuator arms and the attached flexures to position two heads against adjacent disc surfaces. The information storage discs and spacers collectively form a disc stack, or disc pack, that is mounted on the spindle motor hub and held together with a disc clamp to form the spindle motor hub assembly.

[0006] It is critical in the disc drive industry, especially in light of increased spindle motor speeds, to not only have the disc stack secured to the spindle hub, but to have an overall balanced spindle hub assembly, i.e., where the center of gravity of the spindle hub assembly is positioned at or close to the central axis of rotation of the spindle hub. Imbalance within the spindle hub assembly may cause undue and uneven wear on the spindle motor and may result in rotational vibrations within the disc drive. Rotational vibrations may cause undesired variations in the read/write signals detected by the read/write heads and may result in unwanted noise emissions from the disc drive.

[0007] The overall balance of a spindle hub assembly results from imbalances contributed by the information storage discs, the disc spacers, the hub itself, the disc clamp and the clamping force applied by the disc clamp, or any combination of the above imbalances of the above. Improved disc clamp design has helped improve the overall balance of the spindle hub assembly, especially with regard to manufacturing of disc clamps that provide a more uniform vertical clamping force. Additionally, recent aluminum disc clamp designs have introduced a balancing structure that modifies the disc stack center of gravity to be positioned toward the central axis of spindle hub rotation. However, these newer disc clamp designs often require complex and costly fabrication methods.

[0008] Accordingly, there is a need in the art for a relatively inexpensive disc clamp having a balancing structure for balancing the spindle hub assembly.

SUMMARY OF THE INVENTION

[0009] Against this backdrop the present invention has been developed. In one embodiment, the invention is a disc clamp assembly for exerting a clamping force on a disc stack to secure the disc stack to a spindle hub, where the disc clamp assembly, disc stack, and spindle hub combine to form a spindle hub assembly. The disc clamp assembly modifies the center of gravity of the spindle hub assembly toward a central axis of rotation of the spindle hub, and includes a disc shaped base piece having a top surface and a bottom surface, the bottom surface for contacting a top surface of the spindle hub. The base piece also includes a peripherally located annular ring for contacting a top information storage disc of the disc stack.

[0010] The disc clamp assembly also includes a disc shaped retainer piece engaged to the top surface of the base piece, the retainer piece having a peripheral balance ring receiving portion for receiving a balance ring. The balance ring having a first end and a second end with a gap formed therebetween, where the predetermined mass of the balancing ring is varied dependent on the length of the balance ring. The balance ring is secured within the peripheral balance ring receiving portion of the retainer piece to modify the center of gravity of the spindle hub assembly toward the central axis of the spindle hub.

[0011] In another embodiment, the invention is a balance ring retainer for balancing a spindle hub assembly, and includes an annular shaped central web portion for operative attachment to the spindle hub and a peripheral balance ring accepting portion for accepting a balancing ring. The balance ring has a first end and a second end a gap therebetween. The length of the balance ring determines the mass of the balance ring so that a balance ring is selected to improve the balance of the spindle hub assembly.

[0012] Finally, an embodiment of the invention is a method of balancing a spindle hub assembly within a disc drive. The method includes the steps of assembling a disc stack on a spindle hub of a disc drive spindle motor; assembling a disc clamp assembly having a base piece and a balance ring retainer piece; positioning the disc clamp assembly on the spindle hub so that the base piece engages both the spindle hub and a top surface of an information storage disc in the disc stack; determining the center of gravity of a spindle hub assembly comprising the assembly of the spindle hub, the disc stack, and the disc clamp assembly; and positioning a balance ring having a predetermined mass in the balance ring retainer piece of the disc clamp of the disc clamp assembly to move the center of gravity of the spindle hub assembly toward the central axis of rotation for the spindle hub assembly.

[0013] These and various other features as well as advantages which characterize the present invention will be apparent from a reading of the following detailed description and a review of the associated drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a plan view of a disc drive incorporating a preferred embodiment of the present invention and illustrating primary internal components of the disc drive.

[0015]FIG. 2 is a representative cross sectional view of a disc stack illustrating a disc clamp assembly in accordance with a preferred embodiment of the present invention.

[0016]FIG. 3 is an enlarged isometric view of a base portion of the disc clamp assembly shown in FIG. 2.

[0017]FIG. 4 is a cross-sectional view through the base portion of the disc clamp taken along line <b>4</b>-<b>4</b> in FIG. 3.

[0018]FIG. 5 is an enlarged isometric view of a retainer portion of the disc clamp assembly shown in FIG. 2.

[0019]FIG. 6 is an exploded view of the base portion, the retainer portion, and a balance ring of the disc clamp assembly shown in FIG. 2.

[0020]FIG. 7 is an isometric view of the disc clamp assembly shown in FIG. 2.

[0021]FIG. 8 is a cross-sectional view through the disc clamp assembly taken along line <b>8</b>-<b>8</b> in FIG. 7.

[0022]FIG. 9 is a process flow diagram for balancing a spindle hub assembly in conformity with one embodiment of the present invention.

DETAILED DESCRIPTION

[0023] A disc drive <b>100</b> constructed in accordance with a preferred embodiment of the present invention is shown in FIG. 1. The disc drive <b>100</b> includes a base plate <b>102</b> to which various components of the disc drive <b>100</b> are mounted. A top cover <b>104</b>, shown partially cut away, cooperates with the base plate <b>102</b> to form an internal, sealed environment for the disc drive in a conventional manner. The components include a spindle motor <b>106</b> which rotates one or more discs <b>108</b> at a constant high speed. Information is written to and read from tracks on the discs <b>108</b> through the use of an actuator assembly <b>110</b>, which rotates during a seek operation about a bearing shaft assembly <b>112</b> positioned adjacent the discs <b>108</b>. The actuator assembly <b>110</b> includes a plurality of actuator arms <b>114</b> which extend towards the discs <b>108</b>, with one or more flexures <b>116</b> extending from each of the actuator arms <b>114</b>. Mounted at the distal end of each of the flexures <b>116</b> is a head <b>118</b> which includes an air bearing slider (not shown) enabling the head <b>118</b> to fly in close proximity above the corresponding surface of the associated disc <b>108</b>.

[0024] During a seek operation, the track position of the heads <b>118</b> is controlled through the use of a voice coil motor (VCM) <b>124</b>, which typically includes a coil <b>126</b> attached to the actuator assembly <b>110</b>, as well as one or more permanent magnets <b>128</b> which establish a magnetic field in which the coil <b>126</b> is immersed. The controlled application of current to the coil <b>126</b> causes magnetic interaction between the permanent magnets <b>128</b> and the coil <b>126</b> so that the coil <b>126</b> moves in accordance with the well known Lorentz relationship. As the coil <b>126</b> moves, the actuator assembly <b>110</b> pivots about the bearing shaft assembly <b>112</b>, and the heads <b>118</b> are caused to move across the surfaces of the discs <b>108</b>.

[0025] The spindle motor <b>106</b> is typically de-energized when the disc drive <b>100</b> is not in use for extended periods of time. The heads <b>118</b> are moved over park zones <b>120</b> near the inner diameter of the discs <b>108</b> when the drive motor is de-energized. The heads <b>118</b> are secured over the park zones <b>120</b> through the use of an actuator latch arrangement, which prevents inadvertent rotation of the actuator assembly <b>110</b> when the heads <b>118</b> are parked.

[0026] A flex assembly <b>130</b> provides the requisite electrical connection paths for the actuator assembly <b>110</b> while allowing pivotal movement of the actuator assembly <b>110</b> during operation. The flex assembly includes a printed circuit board <b>132</b> to which head wires (not shown) are connected; the head wires being routed along the actuator arms <b>114</b> and the flexures <b>116</b> to the heads <b>118</b>. The printed circuit board <b>132</b> typically includes circuitry for controlling the write currents applied to the heads <b>118</b> during a write operation and a preamplifier for amplifying read signals generated by the heads <b>118</b> during a read operation. The flex assembly <b>130</b> terminates at a flex bracket <b>134</b> for communication through the base plate <b>102</b> to a disc drive printed circuit board (not shown) mounted to the bottom side of the disc drive <b>100</b>.

[0027] One or more information storage discs <b>108</b> are mounted on a spindle hub <b>136</b> as shown in FIG. 2. A typical spindle hub assembly <b>138</b> has a rotating hub <b>136</b> journaled to a non-rotating spindle shaft <b>140</b> where the spindle shaft <b>140</b> is attached to the base plate <b>102</b> and anchors the rotating hub <b>136</b> within the disc drive <b>100</b>. Extending from a lower end of the rotating hub <b>136</b> is an annular flange <b>144</b> having a loading surface <b>146</b> for receiving a bottom information storage disc <b>148</b>. An annular disc spacer <b>150</b> is stacked on the bottom information storage disc <b>148</b>. A top information storage disc <b>152</b> is stacked on the disc spacer <b>150</b>. The combination of information storage discs and disc spacers is referred to as a disc stack <b>151</b>.

[0028] A disc clamp assembly <b>154</b> in accordance with a preferred embodiment of the present invention is secured to the spindle hub <b>136</b> and exerts a downward force on the information storage discs <b>108</b> and disc spacers <b>150</b>. Note that while two discs <b>148</b> and <b>152</b> are discussed for purposes of FIG. 2, it is envisioned that one or more discs and corresponding spacer(s) may be used with the current invention as long as the number of discs and spacers fits within the tolerance limits of the particular disc drive.

[0029] One critical aspect to proper disc drive operation is that the spindle hub <b>136</b>, information storage discs <b>108</b>, disc spacers <b>150</b> and disc clamp assembly <b>154</b>, i.e., the spindle hub assembly <b>138</b>, have a cylindrical symmetry about the central axis of rotation <b>156</b> of the spindle hub <b>136</b>. Cylindrical symmetry of the spindle hub assembly is the result of having a balanced spindle hub assembly <b>138</b>, i.e., the center of gravity of the spindle hub assembly <b>138</b> is essentially positioned at the central axis of rotation <b>156</b> of the spindle hub <b>136</b>. However, imperfections and cost considerations in design, manufacture and the assembly of the spindle hub assembly often result in variations in the center of gravity for the spindle hub assembly, thus resulting in an off-center center of gravity for the assembly.

[0030] The present invention provides a device and method for balancing the spindle hub assembly <b>138</b>, and is implemented through embodiments of a disc clamp assembly <b>154</b> that includes a balance feature for the spindle hub assembly <b>138</b>. Embodiments of the disc clamp assembly <b>154</b> of the present invention provide a simple and inexpensive solution for both securing the disc stack <b>151</b> to the spindle hub <b>136</b> and balancing, i.e., aligning the center of gravity and central axis of rotation <b>156</b> of the spindle hub assembly <b>138</b>, the spindle hub assembly.

[0031] Referring again to FIG. 2, a spindle hub assembly <b>138</b> is shown having a balancing feature in accordance with an embodiment of the present invention. A disc clamp assembly <b>154</b> having a base piece <b>158</b>, balance ring retainer piece <b>160</b> and balance ring <b>162</b> are shown, where the balancing ring <b>162</b> modifies the center of gravity of the spindle hub assembly <b>138</b> toward the central axis of rotation <b>156</b> of the spindle hub <b>136</b>. When the center of gravity and central axis of rotation <b>156</b> of the spindle hub <b>136</b> are substantially aligned, rotational vibrations are minimized within the spindle hub assembly <b>138</b>, thus reducing acoustic emissions and read/write disturbances.

[0032] Preferred embodiments of the disc clamp assembly <b>154</b> of the present invention have a three piece design as shown in FIGS. <b>6</b>-<b>8</b>: a bottom hub-engaging base piece <b>158</b> is secured to the spindle hub <b>136</b> and exerts a downward force on the disc stack <b>151</b>; an upper balance ring retainer piece <b>160</b> attaches to the base piece <b>158</b> for receiving and securing a balance ring <b>162</b>; and a balance ring <b>162</b> is received in the retainer piece <b>160</b> for modifying the balance of the spindle hub assembly <b>138</b>. A mass of the balance ring <b>162</b>, as determined by the total length of the balance ring <b>162</b>, is selected for each particular spindle hub assembly <b>138</b> so as to modify the particular center of gravity for the spindle hub assembly <b>138</b> toward the central axis of rotation <b>156</b> of the spindle hub, as is discussed in greater detail below.

[0033] One embodiment of the base piece <b>158</b> of the disc clamp assembly <b>154</b> is shown in FIGS. 3 and 4. The base piece <b>158</b> is generally a disc having a central aperture <b>164</b> for mounting on the spindle shaft <b>140</b> and includes an annular hub engagement portion <b>166</b> and a peripheral annular rim <b>168</b>.

[0034] The annular hub engagement portion <b>166</b> of the base piece <b>158</b> has flat top <b>170</b> and bottom surfaces <b>172</b>, and is shaped and sized to mimic the shape and size of the top surface <b>174</b> of the spindle hub <b>136</b> (see FIG. 2). Six screw holes <b>176</b> are spaced an equal distance from each other in the annular hub engagement portion <b>166</b> around the central aperture <b>164</b>, and equally spaced between every two screw holes <b>176</b> is a slot or opening <b>178</b> for engagement with the balance ring retainer <b>160</b> which is described in greater detail below. Six screw holes <b>176</b> and corresponding slots <b>178</b> are shown for illustrative purposes, although other numbers of screw holes <b>176</b> and slots <b>178</b> may be used in the present invention, where the greater number of screw holes provides for a more even load being applied by the base piece on the top surface <b>174</b> of the spindle hub <b>136</b>.

[0035] A series of semi-circular shaped rims <b>180</b> (FIGS. 3 and 4) extend above the top surface <b>170</b> of the hub engagement portion <b>166</b> of the base piece <b>158</b> to partially surround each screw hole <b>176</b>. Each raised rim <b>180</b> extends around an inner portion of the screw hole <b>176</b> and is positioned between the central aperture <b>164</b> and the peripheral rim <b>168</b>. Each raised rim <b>180</b> provides a shoulder for directly receiving/contacting a disc clamp retaining screw <b>182</b> used to secure the disc clamp base piece <b>158</b> to the top surface <b>174</b> of the spindle hub <b>136</b> as shown in FIG. 3. The raised rims <b>180</b> minimize pressure hotspots on the information storage disc formed by the downward torque of the clamp retaining screws <b>182</b>, as is described and discussed in greater detail below.

[0036] The bottom surface <b>172</b> of the annular hub engagement portion <b>166</b> is smooth and flat for engagement with the top surface <b>174</b> of the spindle hub <b>136</b>. Six semi-circular shaped indentations <b>184</b>, positionally in-line with the raised rims <b>180</b> on the top surface <b>174</b> of the hub engagement portion <b>166</b>, are defined around the inside half of each screw hole <b>176</b>. The indentations result from formation of the raised rims <b>180</b>.

[0037] Peripheral to the hub engagement portion <b>166</b> of the base piece <b>158</b> is the annular shaped rim portion <b>168</b>. When positioned on the spindle hub <b>136</b>, the rim portion <b>168</b> sits beyond the outer surface <b>186</b> of the spindle hub <b>136</b> and is axially aligned with the inner circumference <b>188</b> (FIG. 1) of the top information storage disc <b>152</b>. (FIG. 2) As shown in FIG. 4, the peripheral rim <b>168</b> has a generally U-shaped cross section, where the bottom of the U defines an information storage disc engagement surface <b>190</b> and the peripheral arm of the U forms an upwardly extending ring of pressure lobes <b>192</b>. When the base piece <b>158</b> of the disc clamp assembly <b>154</b> is loaded on the spindle hub <b>138</b>, the ring-shaped information storage disc engagement surface <b>190</b> extends below the hub engagement portion <b>166</b> of the base piece <b>158</b> and is coplanar with the top surface of the top information storage disc <b>152</b> to uniformly engage the top information storage disc <b>152</b>. The downward torque of the clamp retaining screws <b>182</b> into the top surface <b>174</b> of the spindle hub <b>136</b> is uniformly translated to a downward axial force through the engagement surface <b>190</b>.

[0038] The pressure lobes <b>192</b> of the rim portion <b>168</b> are six uniformly shaped upwardly curved extensions, where the curve peak <b>196</b> of each lobe <b>192</b> is positioned laterally in-line with the center of each of the six screw holes <b>176</b>, and the curve minimums <b>198</b> on either side of a curve maximum <b>196</b> are positioned laterally in-line with each slot <b>178</b>. Each pressure lobe <b>192</b> spreads and distributes the force being translated from the torqued clamp retainer screw <b>182</b>.

[0039] The base piece <b>158</b> of the disc clamp assembly <b>154</b> is preferably made of aluminum, stainless steel, or another suitable malleable metal. In preferred embodiments, the base piece <b>158</b> is stamped or formed from stainless steel sheet metal.

[0040] One embodiment of the balance ring retainer piece <b>160</b> of the disc clamp assembly <b>154</b> of the present invention is shown in perspective view in FIG. 5. The balance ring retainer piece <b>160</b> has an overall circular shape with a central aperture <b>200</b> and includes a central web portion <b>202</b> spaced from a peripheral balance ring accepting portion <b>204</b> by a series of radial ribs or spokes <b>206</b>.

[0041] The central web portion <b>202</b> of the balance ring retainer piece <b>160</b> is a generally circular shape ring that surrounds the central aperture <b>200</b>. The central aperture <b>200</b> is of such size and shape so as to be snugly received by the spindle shaft <b>140</b> of the spindle hub assembly <b>138</b> and in general has a similar shape and size as the central aperture <b>164</b> in the base piece.

[0042] In the preferred embodiment shown in FIG. 5, six equally spaced apart radially extending ribs <b>206</b> extend from the central web portion <b>202</b> to the peripherally located balance ring accepting portion <b>204</b> of the balance ring retainer <b>160</b>. The ribs are narrow strips, having flat top and bottom surfaces in-line with the central web portion <b>202</b> and the peripheral balance ring accepting portion <b>204</b>.

[0043] Defined between each set of two ribs <b>206</b> in the central web portion <b>202</b> is an arcuate shaped cut-out <b>208</b>. Each cut-out <b>208</b> provides access for one of the partially raised rims <b>180</b> that extend from the base piece <b>158</b> of the disc clamp assembly <b>154</b> when the base piece and balance ring retainer piece are engaged. The cut-outs <b>208</b> assists in aligning the base piece <b>158</b> with the balance ring retainer piece <b>160</b>, as shown in FIG. 7.

[0044] Three male snaps <b>210</b>, equally spaced from each other in the central web portion <b>202</b> of the balance ring retainer piece <b>160</b>, protrude downwardly from the bottom surface of the central web portion of the retainer piece <b>160</b>. Each snap <b>210</b> has an opening <b>212</b> and two opposed downwardly extending protrusions <b>214</b>, each protrusion <b>214</b> of a snap <b>210</b> being outwardly biased where one protrusion extends from one half of the opening <b>212</b> and the second protrusion extends from the other half of the opening <b>212</b>. In use, each snap <b>210</b> inserts into one of the axially aligned receiving slots <b>178</b> in the base piece <b>158</b> of the disc clamp assembly <b>154</b> and thereby aligns and snaps together the base piece <b>158</b> to the balance ring retainer piece <b>160</b> of the disc clamp assembly. The snap protrusions <b>214</b> frictionally engage the inner surface of the slots <b>178</b> in the base piece <b>158</b> to hold the two pieces of the disc clamp assembly <b>154</b> together. The engagement is sufficient to keep the parts together during shipment, and assembly of the disc clamp assembly <b>154</b> onto the spindle hub <b>136</b>. Note that other attachment means, including but not limited to adhesives, are envisioned to be within the scope of the present invention, as long as the attachment is sufficient to hold the retainer piece <b>160</b> together with the base piece <b>158</b> during shipment and presentation to the spindle hub <b>136</b>.

[0045] The peripheral balance ring accepting portion <b>204</b> of the balance ring retainer piece <b>160</b> has an annular shape upwardly bent rim <b>216</b> that defines a groove <b>218</b> for receiving and retaining a balance ring <b>162</b>.

[0046] The balance ring retainer <b>160</b> may be formed from a metal such as aluminum or stainless steel, or may be formed from a plastic material. Preferred embodiments of the balance ring retainer <b>160</b> are stamped from stainless steel sheet metal or are formed from injected molded plastic. Preferable plastics for use with the present invention do not outgas and include, but are not limited to, polycarbonate, PPS, and the like.

[0047] The balance ring <b>162</b> itself is preferably a flexible piece of spring wire having a first end <b>215</b> and a second end <b>217</b>. The spring wire length is less than the inside perimeter length of the peripheral balance ring accepting portion <b>204</b>. Spring wire length for use with a particular balance ring retainer piece <b>160</b> is determined so as to modify the center of gravity for the spindle hub assembly <b>138</b> toward the central axis of rotation <b>156</b> of the spindle hub <b>138</b>. Placement of the balance ring <b>162</b> in the balance ring retainer piece <b>160</b> entails compressing the balance ring <b>162</b> to a diameter smaller than the diameter of the balance ring retainer piece <b>160</b> and releasing the balance ring <b>162</b> in the groove <b>218</b> to outwardly press against the upwardly bent rim for frictional engagement with the balance ring retainer piece. Note that the length of the gap between the first end <b>215</b> and the second end <b>217</b> of the balance ring <b>162</b> determines the mass of the balance ring.

[0048] With reference to FIGS. <b>6</b>-<b>8</b>, the balance ring retainer piece <b>160</b> and the base piece <b>158</b> of the disc clamp assembly <b>154</b> align with each other so that the bottom surface <b>220</b> of the central web portion <b>202</b> of the balance ring retainer <b>160</b> sits on the top surface <b>170</b> of the central hub engagement portion <b>166</b> of the base piece <b>158</b>. The diameter of the base piece <b>158</b> is smaller than the diameter of the balance ring retainer piece <b>160</b> so that the upwardly extending pressure lobes <b>192</b> at the periphery of the base piece <b>158</b> extend upwardly into a space <b>222</b> between the central web portion <b>202</b> and peripheral balance ring retaining portion <b>204</b> of the balance ring retainer <b>160</b>. The maximum peak <b>196</b> of each pressure lobe <b>192</b> sits equally between any two radial ribs <b>206</b> that connect the central web portion <b>202</b> and peripheral balance ring accepting portions <b>204</b> of the balance ring retainer <b>160</b>. Additionally, as noted above, each raised rim <b>180</b> of the base piece <b>158</b> extends into the cut-out <b>208</b> defined in the central web portion <b>202</b> of the balance ring retainer piece <b>160</b>. Once the base piece <b>158</b> and balance ring retainer piece <b>160</b> are engaged, each raised rim <b>180</b> extends to substantially the same height as the thickness of the central web portion <b>202</b> of the balance ring retainer <b>160</b> so that a top surface of the rims <b>180</b> are flush with the top surface of the central web portion <b>202</b>.

[0049] In use, the base piece <b>158</b> and balance ring retainer piece <b>160</b> are snapped together and the central apertures <b>164</b>, <b>200</b> for each piece are positioned around the spindle shaft <b>140</b>. The base piece <b>158</b> sits on the top surface <b>174</b> of the spindle hub <b>136</b> so that the screw holes <b>176</b> of the base piece <b>158</b> are axially aligned with corresponding screw holes <b>224</b> (see FIG. 2) in the spindle hub <b>136</b>. Each screw <b>182</b> is torqued into the spindle hub <b>136</b> until the screw <b>182</b> contacts the area <b>226</b> adjacent the cut-out <b>208</b> of the central web portion <b>202</b> of the balance ring retainer piece <b>160</b>, compressing the central web portion <b>202</b> downward onto the top surface <b>170</b> of the central hub engagement portion <b>166</b> of the base piece <b>158</b>. Each screw <b>182</b> is torqued into the spindle hub <b>136</b> until the screw head <b>228</b> comes to rest on the raised rim <b>180</b> of the base piece <b>158</b> thereby directly applying a downward force on the base piece <b>158</b> that is translated to the information storage disc engagement edge <b>190</b> as an axial downward clamping force on the disc stack. Note that the raised rim <b>180</b> also evens out and distributes the force exerted by the screw <b>182</b> onto the base piece <b>158</b> thereby providing a more uniform downward clamping force at the periphery of the base piece of the disc clamp assembly. The raised rim feature <b>180</b>, in association with the pressure lobe feature <b>192</b>, remove any pressure “hotspots” caused by the torqued screws and provide for a substantially flush connection between the base piece <b>158</b> and the top surface <b>170</b> of the hub, and hence a more uniform downward clamping force on the top information storage disc <b>108</b>. The uniform clamping force on the disc prevents distortion during temperature changes within the disc drive and is therefore advantageous to the operation of the disc drive <b>100</b>.

[0050] Once the base piece <b>158</b> and balance ring retainer piece <b>160</b> are secured to the spindle hub <b>138</b>, the center of gravity for that particular spindle hub assembly can be determined by means well known within the art. For instance, the disc stack may be spun, and the resulting forces measured with an accelerometer coordinated with an index mark on the ring retainer. The appropriate spring wire is selected from the information provided by the accelerometer, i.e., the correct mass/length of spring wire is selected, compressed into a curved ring and aligned over the balance ring retainer <b>160</b> so as to align the gap <b>230</b> in the balance ring <b>162</b> in such a manner as to move the center of gravity of the spindle hub assembly toward the center of rotation. The curved spring wire is compressed to a smaller diameter than the balance ring retainer piece <b>160</b> diameter and released into proper position in the peripheral ring portion <b>204</b> where the spring wire is frictionally engaged within the groove <b>218</b>.

[0051] One method for balancing a spindle hub assembly <b>138</b> in conformity with the present invention is shown in FIG. 9. In operation <b>900</b>, a disc stack <b>151</b> (information storage discs and associated disc spacers) is assembled on the spindle hub <b>136</b> of the disc drive spindle motor <b>106</b>. In operation <b>902</b>, the base piece <b>158</b> and balance ring retainer piece <b>160</b> of the disc clamp assembly <b>154</b> are attached together through the male snaps <b>210</b> on the retainer piece being frictionally engaged by the slots <b>178</b> in the base piece. In operation <b>904</b>, the assembly of the base piece and the balance ring retainer piece is positioned over the spindle shaft <b>140</b> and on the top surface <b>174</b> of the spindle hub <b>136</b> so that the peripheral disc engagement surface <b>190</b> is positioned against the top information storage disc <b>152</b> mounted on the spindle hub. In operation <b>906</b>, the appropriate number of screws <b>182</b> are used to secure the spindle hub <b>136</b> to the base piece <b>158</b> and balance ring retainer piece <b>160</b>. In operation <b>908</b>, the center of gravity for the spindle hub assembly <b>138</b>, i.e., the spindle hub, disc stack, base piece and retainer piece, is determined. In operation <b>910</b>, a balance ring <b>162</b> having the correct mass is selected for balancing the spindle hub assembly. In operation <b>912</b>, the appropriate balance ring <b>162</b> is positioned in the balance ring retainer <b>160</b> to modify the center of gravity of the spindle hub assembly toward the central axis of rotation of the spindle hub. Finally, the assembly may go through a balance verification for process control.

[0052] In summary, the preferred embodiment of the invention described herein is directed to a disc clamp assembly (such as <b>154</b>) for exerting a clamping force on a disc stack (such as <b>151</b>) to secure the disc stack to a spindle hub (such as <b>136</b>) and form a spindle hub assembly (such as <b>138</b>). The disc clamp assembly (such as <b>154</b>) modifies the center of gravity of the spindle hub assembly toward a central axis of rotation (such as <b>156</b>) of the spindle hub (such as <b>136</b>). The disc clamp assembly (such as <b>154</b>) includes a disc shaped base piece (such as <b>158</b>) having a top surface (such as <b>170</b>) and a bottom surface (such as <b>172</b>), the bottom surface for contacting a top surface (such as <b>174</b>) of the spindle hub (such as <b>136</b>), a periphery of the base piece (such as <b>158</b>) having a downward extending annular ring (such as <b>168</b>) for contacting a top information storage disc (such as <b>108</b>) of the disc stack (such as <b>151</b>). The disc clamp assembly also includes a disc shaped retainer piece (such as <b>160</b>) engaged to the top surface (such as <b>170</b>) of the base piece (such as <b>158</b>), the retainer piece having a peripheral balance ring receiving portion (such as <b>204</b>) for receiving a balance ring (such as <b>162</b>). The balance ring has a first end (such as <b>215</b>) and a second end (such as <b>217</b>) and an intervening gap therebetween. The balance ring (such as <b>162</b>) has a predetermined mass and is secured within the peripheral balance ring receiving portion (such as <b>204</b>) of the retainer piece (such as <b>160</b>) to modify the center of gravity of the spindle hub assembly toward the central axis of rotation of the spindle hub.

[0053] In other preferred embodiments of the present invention, the base piece (such as <b>158</b>) or retainer piece (such as <b>160</b>) may be stamped from stainless steel sheet metal. In an alternative embodiment, the retainer piece may be plastic.

[0054] In another preferred embodiment of the invention, the balance ring (such as <b>162</b>) is a spring wire, the spring wire is curved and compressed to frictionally sit within the peripheral balance ring receiving portion (such as <b>204</b>) of the retainer piece (such as <b>160</b>). In a further embodiment, the predetermined mass of the balance ring (such as <b>162</b>) is adjusted by varying the length of the spring wire.

[0055] In another preferred embodiment of the invention, the base piece (such as <b>158</b>) includes a series of equally spaced apart holes (such as <b>176</b>) for receiving screws (such as <b>182</b>) to secure the base piece (such as <b>158</b>) to the top surface (such as <b>174</b>) of the spindle hub (such as <b>136</b>); and a raised rim (such as <b>180</b>) partially surrounds each hole (such as <b>176</b>) to contact a head of the screw received within the hole. In another embodiment, the retainer piece (such as <b>160</b>) has a central web portion (such as <b>202</b>) defining a series of cut-outs (such as <b>208</b>), each cut-out receiving a raised rim (such as <b>180</b>) from the base piece (such as <b>158</b>) so that a top surface of the retaining piece (such as <b>160</b>) is substantially flush with a top surface of the raised rim (such as <b>180</b>) to allow the screw head to simultaneously secure both the base piece (such as <b>158</b>) and the retainer piece to the top surface (such as <b>174</b>) of the spindle hub (such as <b>136</b>). In anther embodiment, the base piece (such as <b>158</b>) may include a series of upwardly extending pressure lobes (such as <b>192</b>) located about the periphery of the base piece, the pressure lobes adjacent each spaced apart screw hole (such as <b>176</b>) to help distribute the downward force applied by the screws to the base piece. In another embodiment, the central web portion (such as <b>202</b>) of the retainer piece (such as <b>160</b>) defines a series of equally spaced apart downwardly extending snaps (such as <b>210</b>), each snap (such as <b>210</b>) is frictionally received by a corresponding slot (such as <b>178</b>) formed in the central hub engagement portion (such as <b>166</b>) of the base piece.

[0056] A further preferred embodiment of the invention described herein is directed to a balance ring retainer (such as <b>160</b>) for balancing a spindle hub assembly (such as <b>138</b>). The balance ring retainer has an annular shaped central web portion (such as <b>202</b>) for operative attachment to the spindle hub (such as <b>136</b>) and a peripheral balance ring accepting portion (such as <b>204</b>) for accepting a balance ring (such as <b>162</b>). The balance ring has a first end (such as <b>215</b>) and a second end (such as <b>217</b>), where the length of distance between the two ends of the balance ring determines the mass of the balance ring. The mass of the balance ring (such as <b>162</b>) being selected to improve the balance of the spindle hub assembly (such as <b>138</b>).

[0057] In another embodiment of the invention, the annular shaped central web portion (such as <b>202</b>) defines a series of equally spaced apart cut-outs (such as <b>208</b>), each cut-out receiving a screw for operatively attaching the balance ring retainer (such as <b>160</b>) to the spindle hub assembly (such as <b>138</b>). Note also that the balance ring retainer (such as <b>160</b>) may have a groove (such as <b>218</b>) for frictionally the balance ring (such as <b>162</b>). Finally, the annular shaped web portion (such as <b>202</b>) of the balance ring retainer (such as <b>162</b>) may have a plurality of snaps (such as <b>210</b>) for engaging a series of corresponding holes or slots (<b>178</b>) on the disc clamp (such as <b>158</b>) so as to attach the balance ring retainer to the disc clamp.

[0058] A further preferred embodiment of the invention described herein is directed to a method of balancing a spindle hub assembly (such as <b>138</b>) within a disc drive (such as <b>100</b>). The method includes the steps of assembling a disc stack on a spindle hub of a disc drive spindle motor (such as in operation <b>900</b>); assembling a disc clamp assembly having a base piece and a balance ring retainer piece (such as in operation <b>902</b>); positioning the disc clamp assembly on the spindle hub so that the base piece engages both the spindle hub and a top surface of an information storage disc in the disc stack (such as in operation <b>904</b>); determining the center of gravity of a spindle hub assembly where the assembly includes the spindle hub, disc stack and disc clamp assembly (such as in operation <b>908</b>); and positioning a balance ring having a predetermined mass in the balance ring retainer piece of the disc clamp assembly to move the center of gravity of the spindle hub assembly toward a central axis of rotation for the spindle hub assembly (such as in operation <b>912</b>).

[0059] It will be clear that the present invention is well adapted to attain the ends and advantages mentioned as well as those inherent therein. While a presently preferred embodiment has been described for purposes of this disclosure, various changes and modifications may be made which are well within the scope of the present invention. Numerous other changes may be made which will readily suggest themselves to those skilled in the art and which are encompassed in the spirit of the invention disclosed and as defined in the appended claims.