Magnetic record/playback head positioning apparatus

An apparatus for positioning magnetic record/playback heads in a floppy disk data storage and retrieval devices and including a carriage 12 upon which heads 14 and 16 are mounted, and a linear guidance mechanism comprising a fixed guide rod 18, a spring-loaded guide rod 20, and three carriage mounted bearing assemblies 25. The spring-loaded guide rod 20 is biased toward the fixed guide rod 18 and forces two of the bearing assemblies 25 against the fixed guide rod 18 by action through the third bearing assembly. Each bearing assembly 25 comprises two spherical balls 28 and 30 which are mounted in spherical seats 32 and spin in reaction to translation of the carriage 12. Radial positioning of carriage 12 and the attached heads is accomplished by a lead screw 22 driven by a stepper motor 24 which is coupled to the carriage through a zero backlash nut follower 75. Nut follower 75 is mounted in a spherical seat 122 in the carriage to allow rotation about axes perpendicular to the lead screw axis, and is restrained from rotating about the lead screw axis. Two springs 78 and 80 force the nut follower against the spherical seat and apply a torque to the nut follower about an axis perpendicular to the lead screw axis for eliminating clearance between the screw threads of the lead screw and the nut follower. One of the two record/playback heads is attached directly to the carriage, while the other head is attached to a moveable arm portion thereof which is pivotably attached to the carriage using a spring member 52. An adjustment screw acting on the spring member 52 is provided to allow adjustment of the contact force between the heads and the surface of the floppy disk.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to high precision linear 
positioning apparatus and more particularly to such apparatus as utilized 
for the purpose of positioning magnetic record/playback heads used to 
transfer data to and from flexible magnetic disks. 
2. Description of the Prior Art 
Disk drive systems that record data on and read data from flexible magnetic 
disks, or floppy disks as they are commonly known, are composed of several 
elements. The floppy disk itself is a thin mylar or polyester disk that is 
coated with a magnetic material and is enclosed in a protective envelope 
having access holes for spinning the disk and for accessing the magnetic 
surface. Disk spinning is accomplished by a motor driven spindle, while 
access to the magnetically encoded data is provided by magnetic 
record/playback head attached to a carriage that is positioned radially 
along a guide track by a drive mechanism. Digital data is recorded on the 
floppy disk in concentric recording tracks by positioning the 
record/playback head in contact with the spinning magnetic surface at the 
appropriate radial location and by electromagnetically exciting the record 
windings of the head thereby aligning the magnetic particles in the 
underlying surface coating in patterns corresponding to the digital data. 
Playback of the record recorded data is accomplished by sensing the 
electromagnetic response of the playback windings of the head. 
A trend toward higher data storage density has necessitated increasingly 
tighter tolerances on head positioning accuracy to allow closer intertrack 
spacing and, hence, more data storage per floppy disk. Precise head 
positioning throughout the service life of a disk drive, as well as 
accurate head positioning, referenced to a standard to allow floppy disk 
interchangeability among all disk drives, is desired. 
Many disk drives utilize two magnetic record/playback heads and magnetic 
coatings on both surfaces of the floppy disk to increase data storage 
density. To insure the accurate transfer of data to and from the floppy 
disk, the contact force between the heads and the magnetic surface must be 
adjusted to and maintained at an optimum level. If the contact force is 
too high, excessive abrasion of the disk surface and consequently reduced 
service life will result, whereas a contact force that is too low may 
allow head bounce, and loss of data due to variations in disk thickness. 
Provisions must also be made to separate the upper and lower 
record/playback heads to allow for floppy disk loading and unloading. 
What is needed therefore, is a precision record/playback head positioning 
apparatus that will accurately and consistently position the head, or 
heads, throughout the service life of the disk drive. What is additionally 
needed is a mechanism which allows an optimum contact force to exist 
between the record/playback heads and the magnetic surface of the floppy 
disk. 
Another aspect of the present invention relates to devices utilizing screw 
threads for positioning or clamping purposes in which clearances between 
mating screw threads is a concern. A threaded positioning device typically 
employs an externally threaded shaft, or lead screw, and an internally 
threaded follower, or nut, which is translated along the axis of the shaft 
in response to shaft rotation. A clearance between mating screw threads is 
necessary to allow operation of the device without binding. However, this 
clearance also tends to decrease the positioning accuracy of the device. 
Threaded fasteners used for clamping also suffer from thread clearance 
problems. In a situation where a threaded bolt is loaded in tension by a 
nut, a clamping force is generated through compressive loading of those 
screw thread flanks of the bolt that face toward the bolt head and are in 
contact with screw thread flanks of the nut that face away from the bolt 
head. Only one set of the two sets of facing flanks is normally in contact 
due to the clearances between the screw threads. Locking of the nut in 
place is accomplished by a friction force component of the contact force 
existing between contacting flanks, but due to the pitch of the screw, 
this contact force also has a loosening force component which tends to 
loosen the nut. Threaded fasteners used in vibratory situations therefore 
require additional means to insure locking. 
Consequently, what is needed is a method for providing clearance between 
the screw threads of two mating devices so as to allow rotation of one 
device with respect to the other while at the same time maintaining 
accurate axial positioning. What is also needed is a method for locking 
mating screw threads by utilizing both sets of facing screw thread flanks 
to provide a clamping force free from any loosening force components. 
Guidance and drive mechanisms are known in the prior art. For example, U.S. 
Pat. No. 3,946,439 and the "IBM Technical Disclosure Bulletin", Volume 18, 
Number 7, December 1975, pp. 2244-2245, both disclose record/playback head 
positioning apparatus that use parallel rods for lateral guidance, and 
tension band drives for radial positioning. A carriage, upon which a head 
is mounted, is guided laterally by one fixed rod while another fixed rod 
prevents rotation of the carriage. Such guidance mechanisms require 
clearances between the carriage and the rods to allow radial movement of 
the carriage without binding. However, these clearances reduce the 
attainable positioning accuracy of the apparatus. Furthermore, increases 
in clearances due to wear will even further reduce the positioning 
accuracy. 
The tension band drive mechanism consists of a continuous loop, or band, 
that is attached to carriage and loops around a motor driven pulley and a 
tensioning pulley. The carriage is pulled by the tension band, in one 
direction or the other, in response to motor rotation. Inaccuracies due to 
hysteresis in the drive mechanism when reversing directions limits the 
precision of this approach. 
An alternate guidance mechanism including a grooved track and follower is 
shown in U.S. Pat. No. 3,947,886. As disclosed, a carriage is spring 
loaded against a U-shaped track to provide lateral positioning. The 
positioning accuracy of this approach is susceptible to any accumulation 
of dirt in the track area and also to wear of the track or the track 
following pins. 
Other prior art head positioning apparatus utilize motor driven lead screws 
to move and position carriage mounted record/playback heads. See, for 
example, U.S. Pat. Nos. 3,678,481 and 4,030,137. The carriage in each of 
the above mentioned patents translates along the axis of the lead screw in 
response to rotation of the lead screw. A fixed rod, parallel to the axis 
of the lead screw restricts carriage rotation. Running clearance and wear 
between the lead screw and its follower are again limiting factors in the 
positioning accuracy of this type of head positioning apparatus. 
A linear bearing of a type having reduced wear related problems and loss of 
accuracy is disclosed in U.S. Pat. No. 2,952,145. While this linear 
bearing reduces loss of positioning accuracy due to wear of the guidance 
mechanism, no provisions are disclosed for reducing the running clearances 
to those required of a high precision positioning apparatus. 
Spring loaded devices that provide for contact between a record/playback 
head and a floppy disk are disclosed in U.S. Pat. Nos. 3,678,481 and 
3,946,439. The former describes a carriage with a single head and spring 
loaded pressure pad, while the latter shows a carriage with two heads that 
are forced together by two non-adjustable springs. 
Although mechanisms exist in the prior art which tend to improve the 
positioning accuracy of threaded positioning devices by biasing a nut 
follower to eliminate the clearance between one set of facing screw thread 
flanks of mating threads, typically by using a spring to continuously bias 
the follower in one axial direction, long term accuracy is poor due to 
wear of the mating thread flanks. 
SUMMARY OF THE PRESENT INVENTION 
It is therefore a primary object of this invention to provide a high 
precision linear positioning apparatus for accurately positioning a 
magnetic record/playback head for use with a floppy disk drive. 
An additional object of this invention is to provide a high precision 
linear positioning apparatus that minimizes loss of accuracy due to wear. 
A further object of this invention is to provide a lead screw positioning 
apparatus having a nut follower that precisely translates in response to 
lead screw rotation by maintaining a zero clearance between two sets of 
facing screw thread flanks. 
Another object of this invention is to provide a linear guidance mechanism 
which is referenced to a single fixed datum. 
A still further object of this invention is to provide a linear guidance 
mechanism with linear bearings that are high precision, low friction and 
low cost. 
Still another object of this invention is to provide a pivotable 
record/playback head positioning apparatus that allows adjustment of the 
contact force between two sets of facing screw thread flanks. 
These and other objects, which will hereinafter become apparent, are 
accomplished in accordance with the illustrated preferred embodiment of 
this invention by providing a high precision linear positioning apparatus 
for efficiently and accurately positioning two magnetic record/playback 
head with respect to a floppy disk. This apparatus includes a carriage 
upon which the heads are mounted and a linear guidance mechanism 
comprising a fixed guide rod, a spring loaded guide rod and three carriage 
mounted linear bearings. The spring loaded guide rod is biased toward the 
fixed guide rod and forces two of the linear bearings against the fixed 
guide rod by action through the third linear bearing. Each linear bearing 
comprises two spherical balls, mounted in spherical seats, which spin in 
reaction to translation of the carriage. Radial positioning of the 
carriage and attached heads is accomplished by a lead screw driven by a 
stepper motor and coupled to the carriage through a zero backlash nut 
follower. The nut follower is mounted in a spherical seat in the carriage 
to allow slight rotations of the follower about axis perpendicular to the 
lead screw axis, and is restrained froom rotating about the lead screw 
axis. Two springs force the nut follower against the spherical seat and 
provide a torque to the nut follower about an axis perpendicular to the 
lead screw axis for eliminating the clearance between the screw threads of 
the lead screw and the nut follower. One of the two record/playback heads 
is rigidly attached directly to the carriage, while the other head is 
attached to a movable arm that is pivotably attached to the carriage by 
means of a spring mechanism. An adjustment screw acting on the spring is 
provided to adjust the contact force between the heads and the surface of 
the floppy disk. 
In an alternative embodiment of the invention, a self-locking fastener is 
provided which is suitable for use as a part of a nut follower and 
comprises a threaded bolt, a nut, and means for applying a torque to the 
nut about an axis perpendicular to the axis of the bolt. Torque is 
provided by an off-center protuberance on the surface of the nut facing a 
surface to be engaged. Upon engaging such surface, the protuberance causes 
the nut to rotate, or cock, to a locking position whereby axial clearance 
between screw threads is eliminated. 
An advantage of the present invention is that it compensates for wear of 
the positioning apparatus and thereby provides precise record/playback 
head positioning over a long period of use. 
Another advantage of the present invention is that carriage guidance is 
accomplished by the use of only one precision guide rod and with simple, 
low friction linear bearings, thereby minimizing production cost. 
A further advantage of the present invention is that it provides a carriage 
having a simple, pivotable head mounting which allows adjustment of the 
contact force between the heads and the floppy disk surface. 
Other objects and advantages of the present invention will be apparent to 
those skilled in the art after having read the following detailed 
description of the preferred embodiments which are illustrated in the 
attached drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, there is shown a magnetic head carriage and drive 
assembly according to a preferred embodiment of the present invention. 
Specifically, a high precision linear head carriage and positioning 
apparatus 10 is utilized to dynamically position two magnetic 
record/playback heads relative to a magnetic media (not shown) such as a 
floppy disk. The assembly comprises four major subsystems, namely: (1) a 
linear carriage guidance mechanism 13, (2) a bearing assembly 25, (3) a 
carriage drive mechanism 21, and (4) an upper head mount and preload 
mechanism 51. All four of these mechanisms cooperate to provide a means 
for effecting accurate and consistent head positioning. 
A carriage 12 is provided for transporting a lower record/playback head 14 
and an upper record/playback head 16 (see also FIGS. 2 and 3), and is 
guided for movement in a radial direction with respect to a floppy disk 
146 (FIG. 2) by the linear guidance mechanism 13. Three bearing assemblies 
25 provide a low-friction coupling between the carriage 12 and a pair of 
guide rods 18 and 20 which form the linear guidance mechanism 13. The 
radial drive mechanism 21 includes a lead screw 22 and a lead screw 
driving stepper motor 24 for moving the carriage 12 and its associated 
record/playback heads 14 and 16 into alignment with any of a series of 
concentric floppy disk recording tracks under direction from a disk drive 
controller (not shown). 
The upper head pivot and preload mechanism 51 positions the upper 
record/playback head 16 in either an operational position for recording 
and playing back data, as shown in FIGS. 1 and 2, or in a load/unload 
position allowing the insertion and removal of a floppy disk 146 as shown 
in FIG. 3. 
Further details of the apparatus shown in FIG. 1 will now be described by 
additionally referring to other Figs. of the drawing. Note that 
corresponding numbers in the several Figs. refer to corresponding parts of 
the apparatus. 
(1) Linear Guidance Mechanism 13 
FIG. 4 shows generally the layout of the guidance mechanism 13 as viewed 
from below. A smooth surfaced fixed guide rod 18 is attached at each end 
to a disk drive chassis (not shown) as represented schematically at C1 and 
C2, and is utilized as a datum for guiding the carriage movement. A front 
bearing assembly 29 and a rear bearing assembly 73 are carried by carriage 
12 and are provided to engage and guide the carriage along the fixed guide 
rod 18. A third bearing assembly 27 is provided on the opposite side of 
carriage 12 and is positioned approximately midway between the 
longitudinal positions of the other two assemblies. The bearing assemblies 
will be further discussed below. 
On the opposite side of the carriage 12 from the fixed guide rod 18, a 
spring loaded guide rod 20 is mounted on a pair of supports 34 and is 
positioned parallel to guide rod 18. Rod 20 is spring biased toward guide 
rod 18 by a pair of springs 42 and 44 which are eccentrically connected to 
the ends thereof in a manner more fully described below. 
Two guidance functions are provided by the guide rod 20; the first is to 
bias the carriage toward the fixed guide rod 18, thereby forcing the front 
and rear bearing assemblies 29 and 73 into intimate contact with the guide 
rod 18. This biasing force toward guide rod 18 insures that during 
translation, the carriage 12 will utilize guide rod 18 as a datum. 
The second guidance function of guide rod 20 is to provide a third point of 
support for the carriage 12 and at the same time provide a means for 
resisting rotation about the axis of guide rod 18. 
The spring biasing mechanism associated with guide rod 20 is shown in FIGS. 
1 and 5. As illustrated in FIG. 1, two guide rod supports 34 are mounted 
to the disk drive chassis at C3 and C4, and support the ends of the spring 
loaded guide rod 20. Portions of the ends of guide rod 20 are removed to 
form end sections 37 and 39 that are semicircular in cross-section. The 
end sections are then positioned in right angled notches 33 which are 
formed in the supports 34 by vertical walls 48 and horizontal surfaces 46 
(FIG. 5). Two pins 38 and 40 respectively extend from the semicircular 
ends 37 and 39 of guide rod 20 and are located off-center relative to the 
axis of the rod. Two tension springs 42 and 44 are respectively connected 
between the spring pins 38 and 40 and the disk drive chassis at C5 and C6 
to provide downward forces on the pins 38 and 40 that tend to cause rod 20 
to rotate in the notches 36. 
The supports 34 are spaced relative to guide rod 18 such that when carriage 
12 and rod 20 are in position, there is a slight clearance between the 
flat faces 36 of rod 20 and the facing walls 48 of the supports 34. Rod 
ends 37 and 39 rest on horizontal surfaces 46 of the supports 34, and due 
to the moment imposed by springs 44 and 42, the edges 33 are caused to 
contact vertical surfaces 48 and together with the surfaces 46 upon which 
the ends rest serve as pivots about which rod 20 tends to rotate. 
Whereas, rotation of rod 20 is the result of the off-center forces 
generated by the tension springs 42 and 44, the horizontal and vertical 
surfaces 46 and 48 constrain movement of rod 20 to a rolling and laterally 
translated motion toward rod 18 while at the same time maintaining a rod 
orientation which is level with and parallel to the axis of rod 18. This 
enables rod 20 to bias the carriage 12 toward the fixed guide rod 18 while 
simultaneously preventing carriage rotation around rod 18, thereby 
providing accurate linear guidance for the carriage 12 as it is driven by 
drive mechanism 21. 
There are several advantages associated with the use of the guidance 
mechanism 13 described above. One is that since the carriage 12 is biased 
toward the fixed guide rod 18, the position of the carriage 12 is 
referenced to a single datum and, thus, alignment of the carriage with 
respect to the disk drive chassis is simplified. Another is that since 
clearances between the guide rods 18 and 20, and the linear bearings 25 
are eliminated by this mechanism, wear of the guide rods or the bearings 
is compensated for and does not affect the positioning accuracy. 
Additionally, so long as the spring force of springs 42 and 44 exceed the 
maximum rotational torque applied by lead screw 22, the loading of heads 
14 and 16 on the disk 146 (FIG. 2) will remain uniform. 
(2) Linear Bearings Assemblies 25 
In order to provide smooth translation of the carriage 12 along the guide 
rods 18 and 20, bearings 25 are utilized. In FIG. 6, the middle bearing 
assembly 27 is shown in section revealing upper and lower spherical balls 
28 and 30 mounted in spherical bearing seats 32. The two spherical balls 
28 and 30 respectively contact the guide rod 20 at points 91 and 93 which 
lie along straight lines 21 and 23 connecting the ball centers and the 
axis of rod 20. In addition, the balls contact the spherical bearing seats 
32 at 85 and 87 respectively, and each other at 89. The spherical bearing 
seats 32 have a radius larger than the radi of the spherical balls 28 and 
30 so that the balls have ball-to-seat point contacts (at 85 and 87) with 
the spherical bearing seats. 
Contact points 85, 89 and 91 are located such that an axis 90 (FIG. 7) 
passing through the ball-to-seat contact point 85 and the center of the 
spherical ball 28 is a perpendicular bisector of a dashed line connecting 
the ball-to-ball contact point 89 to the ball-to-rod contact point 91. In 
a similar fashion, contact points 87, 89 and 93 are located such that an 
axis 86 passing through the ball-to-seat contact point 87 and the center 
of the ball 30 is a perpendicular bisector of a dashed line connecting the 
ball-to-ball contact point 89 to the ball-to-rod contact point 93. Since 
balls 28 and 30 must rotate relative to each other as well as relative to 
rod 20, and since the resultant rotational vectors will lie along the 
bisecting axis, the balls 28 and 30 will spin on the axes 90 and 86 
respectively, in response to motion of the carriage 12. If, for example, 
the carriage 12 moves in a direction away from one observing FIG. 7, then 
the balls 28 and 30 will rotate about the axes 90 and 86 in directions 
indicated by the arrows 92 and 88 respectively. 
The free-body diagram of FIG. 8 depicts the balance of contact forces 94, 
96 and 98 acting on the ball 28 through the contact points 89, 85 and 91 
respectively. As can be seen, this bearing design permits a simple and 
straight forward calculation of design loads. 
Advantages of the bearing assemblies described above include low 
fabrication cost due to the simple construction and low part count, and 
low resistance to motion due to the minimization of sliding contact. 
Additionally, this bearing configuration compensates for wear when 
utilized with the linear guidance mechanism 13. 
(3) Carriage Drive Mechanism 21 
Radial positioning of the carriage 12, and the record/playback heads 14 and 
16 mounted thereon, relative to a floppy disk engaged thereby, is 
accomplished through the use of a stepper motor controlled by a disk drive 
controller (not shown). The major elements of the drive mechanism 21 are 
shown in FIG. 4 and include the stepper motor 24, a lead screw 22 driven 
by the stepper motor, and a zero backlash nut follower 75 coupled to the 
carriage 12. 
When a lead screw device is utilized for positioning, accuracy is typically 
limited by the amount of clearance, or backlash, between mating threads. 
Although a reduction in the backlash will result in improved positioning 
accuracy, it may result in thread binding during operation. The present 
invention employs a novel technique for the elimination of backlash which 
includes the rotation of the nut follower 75 about an axis normal to the 
plane of FIG. 4 and perpendicular to the axis of the lead screw 22. 
The effect of such rotation is shown in detail in FIG. 9 wherein a torque 
illustrated by the force couple arrows 100 is applied to the nut follower 
75 so as to rotate it until screw thread flanks 108 and 109 of the nut 
follower, and screw thread flanks 106 and and 107 of the lead screw are in 
contact at points 102 and 104. With such engagement, the nut follower 75 
is precisely located along the axis of lead screw 22 because the left 
facing flank 108 of the nut follower is in contact with the right facing 
flank 109 of the lead screw at the point 102, and the right facing flank 
109 of the nut follower is in contact with the left facing flank of the 
lead screw at the point 104. By contacting both flanks 108 and 109, it 
will be appreciated that the axial position of the nut follower 75 is 
solely a function of the angular position of the lead screw 22. The 
magnitude of the torque provided by forces 100 is not large enough to 
cause thread binding since adequate clearances exist away from the contact 
points 102 and 104. 
This zero backlash design is implemented in the preferred embodiment of the 
present invention as shown in FIG. 4, with details thereof being further 
shown in FIGS. 10, 11 and 12. The nut follower 75 includes two torque 
applying arms 74 and 76, which extend laterally of the body 75, and a 
spherical nose 118. Two tension springs 78 and 80, of equal spring rate 
and unstressed length, are respectively connected between the two torque 
applying arms 74 and 76 and two spring mounts 82 and 84 formed on carriage 
12 on opposite sides of screw 22, the mount 84 being positioned slightly 
ahead of mount 82. The force applied by the spring 80 is thus slightly 
greater than the force applied by the spring 78 due to the former's 
greater installed length. The resulting force imbalance thus causes a 
clockwise torque to be applied to the nut follower 75 as viewed in FIG. 4. 
The spherical nose 118 of the nut follower is forced into a spherical seat 
122 on carriage 12 by the combined forces of the springs 78 and 80. These 
combined forces are selected to be greater than the axial force necessary 
to overcome the inertia of the carriage 12 as it follows the rotation of 
the lead screw 22. A support block 120, having the spherical seat 122 
formed therein, receives the nose 118 and is forced against the surface of 
platform 26 by the spring forces 110 and 118 thereby allowing the nut 
follower 75 to seek a preferred position normal to the leadscrew 22 and 
independent of the normal position of platform 26 which comprises the 
lower portion of carrriage 12. Means for preventing the nut follower 75 
from rotating relative to carriage 12 include an upstanding antirotation 
arm 114 (FIG. 10), which is forced against a contact pad 128 by a 
compression spring 124 mounted in a cavity 126, as shown in FIG. 12. This 
arrangement allows nut follower 75 to rotate in the spherical seat 122 and 
about the axis of arm 114 to eliminate backlash while at the same time 
conforming to any runout irregularities in the lead screw 22, but prevents 
rotation of the nut follower about the axis of the lead screw. In this 
manner, the location of the carriage 12 is precisely related to the 
angular position of the lead screw 22. 
FIG. 10 pictorally depicts the nut follower 75 showing application of the 
spring forces 110, 112 and 116 that are applied by the springs 78, 80 and 
124 respectively. 
The above-described drive mechanism 21 has many advantages over other 
mechanisms of similar function. For example, accuracy of positioning the 
carriage 12 with respect to the angular rotation of the lead screw 22 is 
greatly enhanced by the function of the zero backlash nut follower 75. 
Standard screw thread clearances may be used for fabrication of the lead 
PG,20 screw 22 and the nut follower 75, and such clearances will not 
affect the positioning accuracy of the drive mechanism 21. Moreover, wear 
of the mating threaded parts will not decrease positioning accuracy 
because the nut follower 75 will rotate to whatever position is necessary 
to eliminate clearances incurred due to wear. In addition, wear will occur 
on all surfaces simultaneously, thereby cancelling any affect on 
positioning accuracy. 
(4) Upper Head Mount and Preload Mechanism 51 
During operation of a disk drive, the record/playback heads are in a disk 
contact position engaging the surface of the rotating magnetic disk to 
record and playback magnetically encoded information. The upper head must 
be biased toward the disk so as to provide a predetermined constant 
contact force between the heads and the magnetic surface with such force 
being large enough to maintain the desired degree of contact, yet light 
enough so as not to cause excessive wear of the disk and/or heads. 
However, during disk load and unload, the heads must be separated to allow 
for insertion and removal of the disk. The preferred embodiment of a 
carriage in accordance with the present invention includes a mechanism 
which allows the upper record/playback head 16 to be moved between two 
positions, yet be precisely positioned relative to the carriage. 
As depicted in FIG. 1, carriage 12 is comprised of a lower portion forming 
a lower platform 26 carrying the lower head 14 and having the three 
bearing assemblies 27, 29 and 73 (FIG. 4) formed therein, an upper 
platform 50 rigidly affixed by screws 54 and 56 to an upstanding portion 
5l of platform 26, a head load spring 52, a cantilever spring 58, and a 
pivotable arm 62 which carries the upper head 16. 
As is more clearly shown in FIGS. 13a, 13b and 13c, the spring 52 is a bent 
sheet of spring metal tapered toward one end and having two "base end" 
mounting holes 130, two "head end" mounting holes 135, and a bend line 60 
which divides the spring into two portions 131 and 133. A head load 
adjusting tab 134 is formed by a U-shaped cutout in the portion 131. 
As depicted in FIGS. 14a and 14b, the cantilever spring 58 is comprised of 
a tapered sheet of spring metal having mounting holes 132 provided near 
the base end and an upturned flauge 136 at the head end. The base ends of 
springs 52 and 58 are attached to the carriage 12 by clamping them between 
the upstanding portion 51 of platform 26 (FIG. 1) and the upper platform 
50. Two screws 54 and 56 are installed through two counterbored holes 142 
and 144 in platform 50 and through the mounting holes 132 and 130, in the 
springs 52 and 58 respectively, and are threaded into portion 51 (FIG. 1). 
As illustrated more clearly in the exploded view of FIG. 15, the upper 
platform 50 includes a centrally disposed, forwardly extending tab portion 
64 flanked on either side by notched portions forming horizontal stop 
surfaces 68 and 138. Note that the bottom side of tab 64 is notched and 
beveled as indicated at 137. 
The arm 62 has a pair of rearwardly extending tabs 66 and 67 forming a 
fork-like structure for mating with the tab 64. Note that a recess is 
formed between tabs 66 and 67 to provide a stop surface 140 for engaging 
the beveled lower face 137 of tab 64, and the undersides of tabs 66 and 67 
are notched and beveled as indicated at 139. A laterally extending lift 
tab 72 is also provided. 
Two screws 150 (FIG. 2) attach the movable arm 62, with the upper 
record/playback head 16 mounted thereon, to the forward portion 133 of 
spring 52 and at the same time locate arm 62 relative to the upper 
platform 50. Note that the tab 64 extends between the tabs 66 and 67 and 
overlies stop surface 140, and the tabs 66 and 67 extend into the side 
notches formed in member 50 to overlie surfaces 138 and 68 respectively. 
Note also that tabs 64, 66 and 67 all engage the bend line 60 and are held 
in such engagement by the spring force of spring 52. The bend line 60 is 
also engaged by the upturned end of spring 58 which insures that the bend 
line 60 does not move out of engagement with tab 64. 
When the movable arm is rotated about bend line 60 into the operating 
position shown in FIG. 2, the spring 52 takes the shape indicated in FIG. 
13c. Since the base end of the spring 52 is clamped in a horizontal 
disposition to the undersides of platform 50 and arm 62 respectively, the 
spring is caused to bend into the shape shown with the bend line 60 
contacting the underside of both the tab 64 and the tabs 66 and 67. It 
will thus be appreciated that spring 52 will cause arm 62 to be biased to 
rotate downwardly about the bend line 60. To adjust the spring force 
applied to arm 62, and correspondingly, the contact force between the 
upper record/playback head 16 and the disk surface 146, an adjustment 
screw 70 is provided which contacts the load adjust tab 134. Turning the 
adjustment screw 70 so that it moves downward rotates tab 134 downwardly 
thereby applying a small rotational force to the spring 52 about the bend 
line 60 thereby decreasing the resultant contact force applied to the 
upper record/playback head 16. 
To allow for disk insertion and removal, the arm 62 is rotated upwardly to 
a disk load/unload position as indicated in FIG. 3, providing sufficient 
clearance between the upper and lower record/playback heads to allow a 
protective disk envelope 148 to pass therebetween. The lift tab 72 (FIG. 
1) is provided on arm 62 to serve as a means by which a lifting bale or 
other means may lift the movable arm. The load/unload postion of the arm 
62 is defined by the several tabs and stops provided in the structure of 
arm 62 and the upper platform 50. As illustrated in FIG. 3, when arm 62 is 
rotated into its open position, it pivots about the bend line 60 of spring 
52 until surface 137 of tab 64 is engaged by the central stop surface 140 
and the beveled surface 139 engage the two lateral stop surfaces 68 and 
138 of upper platform 50. The movable arm 62 and the attached upper 
record/playback head 16 are positioned both laterally and longitudinally 
by the spring 52 which permits rotation thereof between the load/unload 
positions but always causes the upper head 16 to be returned to the same 
operating position with the same media engaging force. Easily accessible 
adjustment of the contact force between the upper record/playback head 16 
and the disk surface 146 is also provided by means of the screw 70. 
ALTERNATIVE EMBODIMENTS 
An alternative embodiment of the aforementioned linear guidance mechanism 
13 is shown generally in FIG. 16. In this embodiment the biasing function 
of the spring loaded guide rod 20 is alternatively accomplished by a 
spring loaded middle bearing assembly 162 which engages a second fixed 
guide rod 156. The carriage 152 with front and rear bearing assemblies 160 
and 158 provided on opposite sides thereof is guided in linear motion by 
the primary guide rod 154. The middle bearing assembly 162 includes ball 
bearings 164 of the type previously described, a bearing carrier 170, and 
a compression spring 166 mounted in a cavity 168. The middle bearing 
assembly 162 is spring loaded against the secondary guide rod 156 so as to 
urge the front and rear bearing assemblies 160 and 158 into intimate 
contact with the primary guide rod 154. 
An alternative embodiment to the zero backlash nut follower 75 is shown in 
FIG. 17 and includes a pair of fore and aft threaded members 172 and 173 
and a resilient ring 175 disposed therebetween. An offcenter bump 174 is 
formed on one face of the nut 172 for engaging ring 175. A suitable 
upstanding arm 176 similar to the arm 114 shown in FIGS. 10 and 12 couples 
the nut pair to the carriage 12 so as to prevent rotation of the nut in a 
plane transverse to lead screw 22. Nut 172 has a spherical nose 177 which 
engages a spherical seat 178 in carriage 12. Engagement between nose 177 
and seat 178 is maintained by a spring 179, the lateral dimension, i.e., 
transverse to lead screw 22, is reduced at 180 so as to a small amount of 
rotation about the axis of arm 176. 
In use, the nut 172 is threaded onto the lead screw 171 until bump 174 
engages ring 175. With nut 172 held stationary, nut 173 is then further 
rotated relative to nut 172 until a predetermined differential torque is 
reached between the two. The resultant force imparted to the upper sides 
of nuts 172 and 173 will cause both nuts to experience rotational moments, 
as indiacted by the arrows M, having an effect similar to that described 
above relative to the FIG. 9 embodiment. The purpose of ring 175 is to 
insure that as the threads of lead screw 22 and nuts 172 and 173 wear the 
moment forces M will continue to exist. More specifically, by judiciously 
selecting the durometer of ring 175, it will be appreciated that even as 
the engaging lead screw and nut thread faces wear, substantially constant 
rotational forces in the direction of arrows M will be applied to the nuts 
172 and 173. 
A second alternative embodiment of a selfbiasing nut assembly employs a 
resilient washer 184 having an off-center bump or ridge 186 provided on 
one face as shown in FIG. 18. In use, the washer 184 is placed between a 
pair of nuts such as 172 and 173 in FIG. 17, and similarly, when the nut 
173 is tightened relative to nut 172, the bump 186 causes both nuts to 
experience a moment force M about an axis perpendicular to the axis of the 
lead screw 22 which both locks the nuts in place relative to each other 
and accomplishes the purpose stated above. 
An additional embodiment of a resilient washer that may be used in place of 
the embodiment of FIG. 18 is that shown at 190 in FIG. 19. In this 
embodiment the washer 190 has a tapered cross-section which when used in 
place of the washer 184 of FIG. 18, causes substantially the same result 
to be obtained. 
An alternative embodiment of the zero backlash design that incorporates 
rolling surfaces to reduce rotational friction is shown in FIGS. 20, 21a, 
and 21b. Instread of utilizing a spherical bearing to permit rotation of 
the nut follower as previously described, this embodiment utilizes two 
curved surfaces of contact 196 and 198 between the nut follower 200 and 
the platform 26. The first curved surface 196 is situated on the face of 
the nut follower 200 toward the contact point with the platform 26 and is 
radiused about a first axis that is orthogonal to the axis of the lead 
screw 22. The second curved surface 196 is situated on the platform 
adjacent the first curved surface 196 and is radiused about a second axis 
that is orthogonal to both the first and lead screw axes. A flat plate 202 
is positioned between the curved surfaces, thereby separating them and 
permitting independent rolling of the nut follower about the first axis of 
the nut follower and the flat plate in tandem about the second axis in 
response to torque applied to the two torque arms 74 and 76 by the two 
tension springs 78 and 80. 
Another alternative embodiment of the zero backlash design also 
incorporates two curved surfaces 204 and 206 to permit nut follower 
rotation and is shown in FIGS. 22a and 22b. In this instance, the curved 
surfaces 204 and 206 are situated on opposite faces of a spacer 210 that 
is positioned between the nut follower 208 and the platform 26. As above, 
the curved surfaces are radiused about axes that are mutually orthogonal 
to each other and the lead screw axis. The first curved surface 204 
permits rolling of the nut follower about one axis and the second curved 
surface 206 permits rolling of the nut follower and the spacer 210 in 
tandem about the other axis in response to torque applied to the two 
torque arms 74 and 76. 
As will be apparent to those skilled in the art, the above embodiments are 
intended to be illustrative of the several inventive features of the 
present invention and are not intended to be limiting in scope. It will 
also be apparent that various alterations and modifications may be made to 
the disclosed embodiments without departing from the inventive concepts 
thereof. Accordingly, it is intended that the following claims be 
interpreted as covering all alterations and modifications that reasonably 
fall within the true spirit and scope of the invention.