Magnetic tape scanning assembly for use in video tape recorder and playback apparatus

Guiding drums for a magnetic tape scanning assembly for use in a video tape recorder and playback apparatus are made of a specific aluminum-silicon alloy consisting essentially of 8 to 15% by weight of silicon, 1 to 4% by weight of copper, 0.05 to 0.6% by weight of magnesium, and the balance being aluminum. A mean grain size of silicon crystal in eutectic structure is controlled to be 5 .mu.m or less. The drums are provided with a diagonal or helical guiding path on the peripheries thereof in which there are formed on the guiding path generally continuous, fine machining lines having a roughness (H max) of 1 to 6 .mu.m, preferably 2 to 5 .mu.m, in a direction parallel with sliding travel of a magnetic tape.

BACKGROUND OF THE INVENTION 
The present invention relates to a magnetic tape scanning assembly for use 
in a video tape recorder and playback apparatus, and more particularly to 
a magnetic tape scanning assembly having a guiding path that has desirable 
surface characteristics. 
U.S. Pat. No. 3,955,215 discloses apparatus and method for forming a head 
drum assembly for a video tape recorder and playback apparatus in which a 
rotatable magnetic head assembly is interposed between a pair of 
stationary drums. A pair of magnetic heads fixed to the head assembly are 
protruded from the peripheries of the drums so that a magnetic tape which 
passes a guiding path diagonally formed on the peripleries of the drums is 
magnetically scanned with the magnetic heads as the magnetic tape travel 
with guiding path. Although various types of loading and unloading 
mechanisms and systems are known and disclosed as in U.S. Pat. No. 
3,979,772, and U.S. Pat. No. 3,333,753, explanations about tape loading 
and unloading mechanisms and systems are omitted from the specification 
for simplicity, as such the explanations are not concerned with the 
present invention. 
U.S. Pat. No. 3,333,753 discloses another type of a magnetic tape scanning 
assembly in which a rotatable drum having a pair of magnetic heads is 
supported on a shaft, and a stationary drum is coaxially supported about 
the shaft. A diagonal or helical guiding path is formed on the peripheries 
of the drums. These two types of magnetic tape scanning assemblies are on 
practical use. 
In the conventional magnetic tape scanning assemblies, guiding means 
including a cyindrical stationary drum and a rotatable magnetic head 
assembly are of extreme importance. As a magnetic tape under tension 
slides the guiding path formed on the stationary drum, the drum must have 
a small coefficient of sliding friction and a good wear resistance. If a 
coefficient of sliding friction is large, jitters may appear on the screen 
of a cathode ray tube. Further, if wear resistance is poor, the surface of 
the guiding path will become like a mirror surface resulting that the 
magnetic tape is adsorbed and sticked on such the surface of the guiding 
path. Such the adsorption of the magnetic tape will make the scanning 
assembly inoperable. 
In order to make small a cofficient of sliding friction of the guiding path 
of the drums which have heretofore been made of aluminum material or 
aluminum-silicon alloy material, various measures have been employed one 
of which was surface treatment with, such as, ceramic coating, alumite 
coating, metal plating, etc. These conventional surface treatments were 
not staisfied in wear resistance and coefficient of sliding friction and 
were expensive because of an increase in manufacturing steps. 
As a demand for high quality magnetic tapes with highly smoothed surfaces 
increases, the requirements i,e, low coefficient of sliding friction and 
wear resistance are becoming severer and severer. In recent color video 
tape recorder and playback apparatus, the following characteristics are 
required for magnetic tape guiding assemblies. 
(1) Small coefficient of sliding friction--less than 0.3 when measured 
under a sliding test which will be described hereinafter. 
(2) Good wear resistance--a wear amount (.mu.m) in terms of decrease in 
peak height of the guiding path is less than 0.7 when measured under a 
wear test which will be described hereinafter. 
(3) Magnetic tape surface is not injured by scraching with the guiding 
path. 
(4) Good surface condition of the guiding path--the surface should maintain 
a good surface condition for a long period of time. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide a magnetic 
tape scanning assembly for use in a video tape recorder and playback 
apparatus, which has an improved guiding path along which a magnetic tape 
travels slidably. 
It is another object of the present invention to provide a magnetic tape 
scanning assembly including an improved stationary drum for slidably 
guiding a magnetic tape. 
It is still another object to provide a magnetic tape scanning assembly 
including a stationary and rotatable drums for slidably guiding a magnetic 
tape. 
It is further an object to provide a magnetic tape scanning assembly with 
improved operating characteristics, whereby excellent video recording and 
playback are established for a long period of time. 
In order to accomplish the objects of the present invention, there is 
provided a magnetic tape scanning assembly having a guiding path whose 
coefficient of sliding friction is sufficiently small, and generally 
continuous, fine guiding lines having a suitable roughness are formed on 
the surface of the guiding path in a direction of travel of the mangetic 
tape. More specifically, the coefficient is less than 0.3 over a load 
range of 10 to 100 grams and the guiding lines have a roughness of 1 to 6 
.mu.m. In order to obtain the guiding path, the present invention employed 
an aluminum-silicon alloy material which consists essentially of 8 to 15% 
by weight of silicon, 1 to 4% by weight of copper, 0.05 to 0.6% by weight 
of magnesium, the balance being aluminum, wherein a mean grain size of 
silicon crystal in eutectic structure is less than 6 .mu.m, more 
specifically less than 4 .mu.m. A preferable composition is: Si 9.5 to 
11.5%, Cu 2 to 3%, Mg 0.1 to 0.5%, Al bal. 
Other objects and features and advantages of the present invention will be 
apparent from the following detailed description taken in conjunction with 
the accompanying drawings.

DETAILED DESCRIPTION OF EMBODIMENTS 
As shown in FIG. 1, a magnetic tape scanning assembly of one type generally 
comprises a stationary cylindrical upper drum 1, a stationary cylindrical 
lower drum 2, and a rotatable magnetic head assembly 3, which is 
interposed between the upper and lower drums 1, 2 with a small clearances 
4. The drums 1, 2 and the magnetic head assembly 3 shown as a disc shape 
in FIGS. 1 and 2 are coaxially mounted around a shaft 5. The upper and 
lower drums 1, 2 are fixedly connected by means of a connector 6 and bolts 
6'. The magnetic head assembly 3 is rotatably supported on the shaft 5 by 
means of ball hearing assemblies 11, while the upper drums 1, 2 are 
fixedly mounted on a base plate 18 by means of bolts 10 and sub-base 
plates 17, as shown in FIG. 2. 
The magnetic head assembly 3 is provided with one or more magnetic heads 7 
that are slightly protruded to the extent of 0.05 to 0.3 mm in the radial 
direction of the drums 1, 2 and disc 3. A guiding path 8 is diagonally or 
helically formed on the drums 1, 2 and the head assembly 3. The guiding 
path 8 is slightly recessed from the peripheries of the drums 1, 2 so that 
a stable guide for the magnetic tape is established. Preferably, the depth 
of the guiding path is controlled to be 0.05 to 0.3 mm. 
The lower drum 2 has a sleeve portion 9 extending downwardly around the 
shaft 5 as shown in FIGS. 1 and 2. In this embodiment, the drums 1, 2 and 
magnetic assembly 3 other than magnetic heads 7 are made of a specific 
aluminum-silicon alloy material. The composition of the aluminum-silicon 
alloy is as follows: 
Si: 11% by weight 
Cu: 2.5% by weight 
Mg: 0.3% by weight 
Al: balance 
The alloy material was prepared by a continuous casting method at a cooling 
rate of 50.degree. C./sec., whereby tabular or flaky silicon crystals were 
crystallized in eutectic structure. A mean width of the tabular or flaky 
silicon crystals was 4 .mu.m. The so prepared cast product was subjected 
to 80% plastic working, whereby the tabular or flaky silicon crystals were 
finely divided. Then, the plastic worked product was heat-treated at 
420.degree. C. to 450.degree. C. for 2 hours to effect a solution 
treatment. Thereafter, a hard againg treatment was carried out at 
170.degree. C. for 10 hours. As a result, in the alloy structure there 
were formed generally round silicon crystals in eutectic structure having 
a mean grain size of 1.7 .mu.m. Several drums shown in FIGS. 1 and 2 were 
cut out from the resulting alloy material. Then, the guiding paths were 
formed by machining. The roughness (H max) of the guiding paths was 
controlled to be 3 to 4 .mu.m. When the surfaces of the guiding paths were 
observed through an electron microscope, it was found that generally 
continuous, fine machinning lines were formed. The machinning lines were 
arranged in the direction of travel of a magnetic tape. 
The roughness of the surface of the guiding path 8 plays a very important 
role. When the roughness is too large, the magnetic tape will be injured 
during travel and a coefficient of sliding friction will inveitably 
increase. According to the experiments, the guiding path should have a 
coefficient of sliding friction of less than 0.3, preferably 0.28 or less. 
In order to obtain the coefficient of less than 0.3, the roughness (H max) 
should be 1 to 6 .mu.m, preferably 2 to 5 .mu.m. 
When the roughness is too small, or the surface of the guiding path is too 
smooth like a mirror surface, a magnetic tape will be adsorbed on the 
surface so that the tape will be sticked on the guiding path. Such 
adsorption phenomenon will make the scanning assembly inoperable. 
Therefore, the roughness should be 1 .mu.m or more, preferably 2 .mu.m or 
more. In most cases the roughness is controlled to be 3 to 4 .mu.m. 
The magnetic head assembly 3 is provided with a rotary transformer 12. The 
sleeve 9 has a brush 13 for earthing. The shaft 5 is rotated by turning a 
pulley 14 which is driven by a belt 15. The shaft 5 is fixed to the pulley 
14 by means of a bolt 16. In this embodiment, the connector 6 and pulley 
14 were made of a conventional aluminum die-cast material. 
In FIG. 3 there is shown another type of a scanning assembly in which an 
upper drum 20 is rotatable, while a lower drum 21 is stational. In this 
embodiment, therefore, no rotatable disc like disc 3 in FIGS. 1 and 2 is 
disposed. The upper drum 20 is provided with a pair of magnetic heads 22 
that are slightly protruded from the guiding path 8 as in FIGS. 1 and 2. A 
shaft 5 is fixedly connected to the upper drum 20. The drum 20 is also 
provided with a rotary transformer 12. The shaft 5 is rotated in the same 
manner as described in connection with FIGS. 1 and 2. 
In this embodiment, the upper drum 20 and lower drum 21 were made of an 
aluminum-silicon alloy material. The composition of the alloy consists of 
11% by weight of silicon, 2.5% by weight of copper, 0.3% by weight of 
magnesium, the balance being aluminum, and a mean grain size of generally 
round silicon crystals in eutectic structure is 1.8 .mu.m. A roughness (H 
max) of the surface of the guiding path was 3 .mu.m and a coefficient of 
sliding friction was 0.24 over a load range of 20 to 100 grams. The 
scanning assembly exhibited satisfactory characteristics for a long period 
of time. 
FIG. 4 shows a method of measuring a coefficient of sliding friction and 
FIG. 5 shows the measuring results of coefficients. A commercially 
available magnetic tape 31 for video tape recorder and playback apparatus 
(CrO.sub.2 tape, manufactured by Sony Corp., Japan) is placed on a solid 
rod 30 having a diameter d of 5 mm under tension exerted by a weight 32. 
An angle .theta. is adjusted to 90 degrees. The surface condition of the 
rod 30 is conditioned to be as the same as that of the guiding path 8. The 
magnetic tape thus placed is pulled through a spring gauge 33, so that a 
force F.sub.2 is measured as a sliding friction. In order to measure 
coefficients of sliding friction under different forces F.sub.1, the 
weight 32 was changed over the range of from 10 grams to 100 grams. Since 
there is the following relation, a coefficient (.mu.) can be calculated: 
EQU F.sub.2 =F.sub.1 e.sup..mu..theta. 
The results are shown in FIG. 5 in which a curve A of coefficients of 
sliding friction in the case of the aluminum-silicon alloy material 
mentioned hereinbefore is under 0.3. Since a magnetic tape is under about 
30 to 50 grams during normal recording and playback operations of video 
tape apparatus, the drums made of the specific aluminum-silicon alloy 
material are particularly suitable due to low coefficients of sliding 
friction (0.24 to 0.25). 
A curve D shows coefficients of sliding friction for an alumina rod. It is 
apparent that the rod made of the specific aluminum-silicon alloy material 
has a smaller coeficient than that of ceramic rod. 
A curve B shows coefficients of sliding friction for an as cast aluminum 
material consisting essentially of 0.6% by weight of silicon, 0.5% by 
weight of iron, 1.5% by weight of magnesium, 4% by weight of copper, 2% by 
weight of nickel and the balance being aluminum. The rod was cut out of 
the as cast aluminum material. The surface of the rod was machined to have 
a roughness of 3 to 4 .mu.m. A curve C shows coefficients of sliding 
friction for an as cast aluminum-silicon alloy consisting essentially of 
10.5% by weight of silicon, 3% by weight of copper, 1.2% by weight of 
magnesium, 1.1% by weight of nickel, 0.5% by weight of iron and the 
balance being aluminum. From the curves B and C it is apparent that the 
rods made of these conventional materials have coefficients of sliding 
friction of 0.31 to 0.27. These conventional materials have 0.27 to 0.26 
of coefficients under weight of 30 to 40 grams. Drums made of these 
conventional materials tend to produce jitters on the screen. 
It has been confirmed that the coefficients thus measured represent 
coefficients of the actual drums. 
Surface conditions or maching lines on the surface of guiding paths were 
observed through an electron microscope. FIGS. 6(a ) to 6(c) show the 
surface conditions. A magnitude of the photographs is 2000. FIG. 6(a) 
shows the surface of the guiding path of the present invention; FIG. 6(b) 
the surface of the guiding path of the as cast aluminum material; and FIG. 
6(c) the surface of the guiding path of the as cast aluminum-silicon alloy 
material. The roughness (H max) of the surfaces were all controlled to 3 
to 4 .mu.m. As is seen from FIGS. 6(a) to 6(c), the machinning lines of 
the guiding path of the present invention are continuous and smooth in a 
direction of travel of a magnetic tape, while in the case of the 
conventional drums, the machining lines are neither continuous nor smooth. 
In particular several large concaves are found in the photograph of FIG. 
6(c). The inventors believe that these concaves were formed by plucking 
silicon crystals off from the matrix during machinning. 
It is another important advantage of the drums of the present invention 
that the guiding path has excellent wear resistance. FIGS. 7(a) to 7(c) 
are results of a wear test using actual drums. The wear test was continued 
for 600 to 700 hours. A magnetic tape used in this test was the same as 
used in measuring coefficients of sliding friction. FIG. 7(a) shows the 
result of the drum made of the specific aluminum-silicon alloy material; 
FIG. 7(b) the result of the drum made of the as cast aluminum material; 
and FIG. 7(c) the result of the drum made of the as cast aluminum-silicon 
alloy material. 
Wear amounts in FIGS. 7(a) to 7(c) are represented by decrease in roughness 
(H max). For the purpose of comparison there are shown in left hands in 
FIGS. 7(a) to 7(c) surface curves of not-tested regions of the guiding 
path. It should be noted, however, that the curves of FIGS. 7(a) to 7(c) 
do not show the actual surface contuors at all, because they are only 
recorded diagrams. 
It is apparent from FIGS. 7(a) to 7(c) that a wear amount of the drums of 
the invention is only 0.5 .mu.m, while wear amounts of the conventional 
drums are 0.8 to 1 .mu.m and 0.7 .mu.m, respectively. Therefore, the 
guiding path of the drum of the present invention can keep desired surface 
conditions for a long period of time. According to the calculation of 
service life of the drums, it is speculated that the drums of the present 
invention has about three times the service life of the conventional 
drums. 
The present inventors further tested different drums made of the 
aluminum-silicon alloy materials having the following compositions and 
mean grain size of silicon crystals in eutectic structure. A casting 
method, plastic working, and heat treatment are also shown below. 
__________________________________________________________________________ 
Mean 
Cooling 
Compositions 
grain 
speed 
Working 
(% by weight) 
size 
of ingot 
ratio 
Solution 
Hard 
Si Cu 
Mg Al (.mu. m) 
(.degree. C./sec) 
(%) treatment 
aging 
__________________________________________________________________________ 
9.6 
2.5 
0.1 
bal. 
1.7 30 80 450.degree. C. 
170.degree. C. 
.times. 3 hours 
.times. 10 hours 
10.5 
2.1 
0.3 
" 2.1 30 85 " " 
11.4 
1.5 
0.2 
" 1.6 55 90 " " 
13.6 
2.8 
0.5 
" 1.7 60 50 " " 
14.0 
1.5 
0.1 
" 2.8 50 40 " " 
__________________________________________________________________________ 
The drums made of these alloy materials were found to have desired 
characteristics. On the other hand, when the mean grain size of silicon 
crystals in eutectic structure is larger than 5 .mu.m, it will be 
difficult to give the surface of the guiding path the desired surface 
conditions by machinning. Further, when an amount of silicon is less than 
8%, a wear amount of the drum increases so that a service life of such the 
drum will be short. On the other hand, when an amount of silicon exceeds 
15%, a large amount of primary silicon crystal which can not be divided by 
plastic working will be crystalized in the matrix so that a magnetic tape 
may be injured by the large primary silicon crystal grains during travel. 
Magnesium and copper function as to strengthen the aluminum-silicon alloy 
material. An excessive addition or short addition of these elements may 
degrade physical properties of the alloy material. Although other elements 
such as iron, chromium, manganese, nickel, zirconium and titanium may be 
present in the alloy material. In the case of iron, the maximum amount 
should be controlled to be less than 0.7% by weight. In the case of 
chromium, manganese, nickel, zirconium or titanium, amounts thereof should 
be controlled to 0.15% by weight or less. 
Although a method of preparing the aluminum-silicon alloy material has been 
described in brief, a detailed description is found in U.S. patent 
application Ser. No. 567,009, filed Apr. 10, 1975, now U.S. Pat. No. 
4,077,810, titled "Aluminum Alloys Having Improved Mechanical Properties 
and Workability And Method of Making Same."