Locking apparatus for a string

A locking apparatus (1) for a string of a stringed instrument, which locking apparatus includes a locking cam (20, 23-27) arranged rotatably about a point of rotation and a locking wall, which locking cam and locking wall are positioned relative to each other in such a way that a string can be locked between them. The rotatably arranged cam is an eccentric made from a hard and stiff material, placed in a U-shaped channel of a hard and stiff beam (3). This channel contains the locking wall. A small spring holds the locking cam to the locking wall in case no string is present between the locking wall and the locking cam. The eccentric is shaped in such a way that the locking cam and the locking wall contact at only one point, in case there is no string between them.

FIELD OF INVENTION 
The invention relates to a locking apparatus for a string of a stringed 
instrument, which also includes a tuning mechanism for the string, which 
locking apparatus includes a locking cam arranged rotatably about a point 
of rotation and a locking wall, which locking cam and locking wall are 
positioned relative to each other in such a way, that a string can be 
locked between them. 
BACKGROUND OF THE INVENTION 
Such a locking apparatus for a string of for instance a violin or a guitar 
is known from U.S. Pat. No. 1,732,172. This known apparatus composes a 
joint construction with means to tune the individual strings of the 
stringed instrument. Therefore, the string is guided through an opening in 
a channel of a tuning means. In order to anchor the string to the stringed 
instrument the string is subsequently pulled out of the other side of the 
tuning means. Subsequently, the string is anchored to the locking means. 
The locking apparatus includes a rotatably arranged locking cam and a 
locking wall, between which the end of a string can be guided. Both the 
locking cam and the locking wall in the known apparatus, preferably, 
include a toughened surface. The locking cam and the locking wall are 
shaped in such a way, that, when the string is being pulled in the 
direction of the other end of the string, the locking cam and the locking 
wall seize and lock the string. Thereby the tensioned string is in direct 
physical contact with the locking cam over such an extent of the locking 
cam that the tension of the string provides for a leverage to the 
rotatably arranged locking cam and increases the locking force of the 
locking apparatus. In order to use the tuning means, the locking apparatus 
has to be guided into the channel of the tuning means, while the string 
remains tensioned. After the string is anchored on its other end, the 
locking apparatus can be moved through the channel of the tuning means by 
means of a butterfly nut, in order to regulate the tension in the spring. 
In the known locking apparatus the locking cam is not provided with a 
spring, so that the locking cam, whenever no tensioned spring is inserted, 
will dangle without taking any defined position. Moreover, when a loose 
string is guided between the locking cam and the locking wall, the string 
will not yet be locked. The will only remain locked tight in the locking 
apparatus when the string is tensioned. By insertion of the string into 
the locking apparatus in the channel of the tuning means, therefore, the 
string has to maintain a certain tension, otherwise the string may still 
slip from the locking apparatus. In case the string as yet slips loose, 
the whole procedure has to be repeated, which consumes a lot of time. 
When, for instance during a concert, a string breaks the known apparatus 
is very unpractical, since changing a string takes a lot of time. This is 
certainly true, when the string slips loose from the locking apparatus in 
between. This known apparatus therefore is more suitable for violins, 
where the strings have a lower tension and are made of a rougher material 
than strings for a steel string guitar. 
The object of the locking apparatus according to the present invention is 
to solve the problems mentioned above. 
SUMMARY OF THE INVENTION 
Therefore, the locking apparatus according to the invention is 
characterized in that the rotatably arranged locking cam is an eccentric 
from a hard and stiff material, placed inside a U-shaped channel being 
freely accessible from one side and provided in a hard and stiff beam, 
which channel includes the locking wall, a small spring being provided 
holding the locking cam to the locking wall in case no string is present 
between the locking cam and the locking wall, and the eccentric is shaped 
in such a way that the locking cam and locking wall contact at only one 
point in case there is no string in the channel. 
By using a U-shaped, hard and stiff channel in combination with a hard and 
stiff eccentric which is pushed or pulled towards the locking wall in the 
channel by means of a spring, a locking apparatus of high quality results, 
with which the string can be locked most tightly, and with which the 
string can consequently be tensioned to a very high tension. This locking 
apparatus can be applied at one end of the string, while the tuning means 
is on the other side of the string. Therefore, this locking apparatus is 
highly suitable for use together with a tremolo apparatus, in which the 
tuning means for a string is placed. The U-shape of the channel makes the 
channel easily accessible for a string to be anchored, so that a broken 
string can be substituted quickly and easily during a concert. By 
providing the locking cam with a small spring, the string will also remain 
locked in the locking apparatus in case the string tension is strongly 
decreased. This, for instance, happens when the stringed instrument is 
provided with six strings that are anchored at one side to a tremolo, 
which tremolo is activated after breakage of one of the strings to release 
the tension from the remaining strings in order to attach a new string. 
The springs hold the five strings, which are then unloaded for a short 
period of time, in their original position in the locking apparatus, so 
that they do not need to be tuned again, after the tremolo has returned to 
its neutral position. 
It may be noted that from FIG. 5 from the prior art the application of an 
eccentric is known. However, this is used in combination with a second 
eccentric, which is placed mirrorwise opposite to the first eccentric. 
Both eccentrics can rotate about their own axis, and can lock a string in 
between them. Here also counts, that the string is only locked, when a 
certain tension is applied to the string. If not, then the string will 
slip loose from between the two eccentrics which are not provided with 
springs and consequently have no initial locking load and no well defined 
initial position. As with the above mentioned known locking apparatus this 
locking apparatus is problematic when it has to be inserted in the channel 
of the tuning means, because in that situation the string has to remain 
loaded with a certain tension in order to prevent it from slipping loose 
from the locking apparatus.

DETAILED DESCRIPTION OF THE INVENTION 
In FIG. 1 a locking apparatus 1 according to the invention is shown as a 
beam 3 provided with six U-shaped channels. Each channel is destined to 
hold one string. Of course the invention is not restricted to a beam 3 
having six channels; the invention also relates to beams with n U-shaped 
channels, with n.gtoreq.1. In each channel a locking cam 20, 23-37 is 
located, having the shape of an eccentric and provided with a turning 
point 21 that fits into and interacts with a rotation cavity 5 placed in 
the inside walls 42-47 of each channel. Inside each U-shaped beam, the 
wall opposite to the wall provided with the rotation cavity 5 comprises a 
locking wall (for instance 411 in FIG. 2). Each locking cam is provided 
with an opening 22 through the locking cam. A spring can be placed in 
opening 22, in such a way that the locking cam is pushed or pulled against 
the locking wall in case there is no string present between the locking 
wall and the locking cam. In practice, the top of the beam does not 
provide enough space for such a spring 30. Therefore, beam 3 is provided, 
inside each channel, with slots 31, so that the springs 30 can be attached 
underneath the locking cams 20, 23-27 at the bottom side of beam 3. The 
springs can be attached more or less floating, as shown in the drawing. It 
is also possible to insert a bolt or axle (not shown) to each spring 30, 
around which the spring 30 can rotate. 
Other designs for spring 31 are also possible, as shown in FIGS. 9 and 10. 
Here, the locking cam 20 is provided with an axle 60, that rotates 
together with the locking cam 20 and is inserted in an opening 61 through 
beam 3. At the bottom side of the beam axle 60 is connected to a, 
preferably plastic, retainer 62, in such a way, that the plastic retainer 
62 follows every rotation of locking cam 20. A spring 63 is connected to 
the plastic retainer 62, in such a way that in the initial position 
locking cam 20 is pushed against the locking wall 411. An appropriate 
cavity for plastic retainer 62 and spring 63 may be provided at the bottom 
side of beam 3, so that they do not protrude from the beam 3. In the 
preferred embodiment according to FIG. 9, similar spring designs are 
provided for the remaining locking cams 23-27. An advantage of the spring 
design 60-63 is, that it is easier to assemble than spring construction 
30-31 according to FIG. 2. FIG. 10 shows a cross section along line X--X 
of FIG. 9. 
Between the locking wall 411 and the locking cam 20 a string is installed 
by sliding the string along the bevelled edge 412 of locking wall 411 
between the locking cam and the locking wall, in order to overcome the 
resilience of the small spring 30. The torque of small spring 30 has to be 
of a very low level, otherwise the string will not slide easy enough 
between the locking cam and the locking wall during installation. The 
String will then be locked with an initial clamping stress by spring 30, 
if the spring itself is not yet tensioned. The beam 3 can be mounted to 
the stringed instrument with screws or bolts 52 through mounting holes 6 
and 7. Preferably, the beam is positioned immediately behind the top nut 
50 of the stringed instrument (see FIG. 8). 
The beam 3 is preferably made from quenched quality steel, known under the 
tradename: `RUS`, that can be quenched up to about 60 HRc. The locking cam 
is preferably made from rolled spring steel, class C, with a specific 
hardness of circa 56 HRc. A possible production method is 2D 
electro-erosive metal removing, although this method is rather expensive. 
A cheaper and quicker method is based on laser cutting, which method also 
provides a better controllable surface condition. In principle, it is 
possible also to laser cut the hole 22 for spring 30. Therefore, the 
laser-beam follows the contour of the locking cam and cuts in at slot s1 
(FIG. 3) to make hole 22. This process is rather complex and not fully 
reliable because of the high local heat development. It is easier to 
pierce hole 22 with the laserbeam directly into the locking cam. In that 
case, slot s1, as shown in FIGS. 1,3,5 and 6, is not present at all. 
FIG. 2 shows appropriate dimensions for beam 3. The overall height h1 is, 
for instance, 6 mm, while the height h2 of beam 3 underneath the channels 
is, for instance, 4 mm. The height of the locking cams is designed in such 
a way, that it is a fraction lower than the height of the raised walls 41 
. . . 48 to prevent the cams from being locked in between the beam 3 and a 
cover 51 (FIG. 8) being placed over it. The tolerance between the locking 
cams and the cover 51 has to be small such that even the string with the 
lightest gauge cannot slip underneath a locking cam during the 
installation of the string or afterwards. The width b1 of each channel at 
a locking cam 2 measures approximately 5.5 mm, while the width b2 of a 
complete channel measures 7.5 mm. This width b2 corresponds with the 
standard spacing between two adjacent strings of a steel string guitar. 
When these dimensions are used, which provide beam 3 with sufficient 
stiffness, then a symmetrical design of beam 3, which is not allowed to be 
wider than the neck of the stringed instrument, for instance, an electric 
guitar, would not leave enough space for a channel at one of the sides of 
the beam 3. In order to prevent beam 3 from protruding from the neck of 
the stringed instrument a design is chosen for one of the ends of beam 3, 
as shown in FIG. 1. The locking cams 26, 27 on the right hand side of FIG. 
1 each rotate in their own wall 46, 47 measuring only half of the length 
of the other walls 41-45, 48. It is observed that the right hand side 
locking cam 27 therefore is placed reversely in the channel related to the 
other locking cams 20, 23-26, concerning both the position of the rotation 
point and the position of its top and bottom surface. 
FIG. 3 shows a top view of the locking cam in more detail. The locking cam 
20 can rotate about rotation point 21 having centre 211. The distance 
between centre 211 and the edge of the locking cam with which the string 
is being locked, depends on the angle .phi. and is indicated with 
r(.phi.). Furthermore, the curve of r(.phi.) depends on the type of string 
that has to be locked. The fact is, it is found that the relationship of 
the dimensions of the eccentric for a wound string are preferably chosen 
different from the dimensions for plain strings. When, for instance, in a 
guitar, three plain strings and three wound strings are applied, then 
three eccentrics for plain strings and three eccentrics for wound strings 
are applied. For a wound string the dimension rb, i.e. r(.phi.) with 
.phi.=0, is 5.5 mm. The dimension re, i.e. r(.phi.) for .phi. a little 
over .pi./2, measures in that case 4.0 mm. Slot s1, when present, measures 
less than 0.2 mm. The distance s2 between slot s1 and the centre 211 
measures 3.0 mm. Radius r1 of the rotation point measures 1.0 mm, while 
the width of opening 22 is 0.5 mm. Finally, the dimension of s3 measures 
1.0 mm. These dimensions are applied for wound strings. For a plain 
string, radius re is preferably 4.75 mm, the other dimensions being equal 
to those of the locking cam for wound strings. 
FIG. 5 shows a top view of a locking cam for plain strings, referred to as 
p-type locking cam, while FIG. 6 shows a top view of a locking cam for 
wound strings, referred to as w-type locking cam. The curvature of both 
eccentrics is different, as explained above. In a preferred embodiment of 
both eccentrics the points a to t follow the coordinates, as shown in 
table 1. In both cases point a is chosen as origin. 
FIG. 7 shows how the value of locking force Fk on the string depends on the 
position P where the string S is locked between the locking cam and the 
locking wall. Point O is for instance equal to the centre 211 of rotation 
point 21 of locking cam 20. The line connecting points O and P makes an 
angle .beta. to the line through O perpendicular to the locking wall and, 
consequently, to string S. When the string is loaded with a tension Fs, it 
corresponds with said locking force Fk according to the following formula: 
EQU F.sub.k =F.sub.m /tan .beta. 
By choosing a small angle .beta., the locking force can be increased to an 
extremely high extent. 
FIG. 4 shows a bottom view of beam 3, with the cavities 31 for the springs 
30 and the openings 6 and 7 for the mounting means clearly visible, 
whereas FIG. 8 shows where the locking apparatus according to the 
invention is placed on guitar neck 54. 
FIG. 8 shows the guitar neck 54 with six strings 53. Immediately behind top 
nut 50 the locking apparatus 1 is placed, locking six strings 53. A 
cover-guiding plate 51 is provided to cover locking apparatus 1. Six 
guiding slots 55 are provided in cover-guiding plate 51, one for each 
string 53, and two holes for the mounting screws or bolts 52 are provided, 
with which mounting screws 52 the locking apparatus including the 
cover-guiding plate 51 is fixed to the guitar neck 54. 
TABLE 1 
______________________________________ 
Coordinates of a locking cam type W and type P 
(in mm relative to origin a) 
locking cam locking cam 
type `W`: type `P`: 
X: Y: X: Y: 
______________________________________ 
a 0.0 0.0 0.0 0.0 
b 0.101 0.472 0.061 
0.476 
c 0.241 0.927 0.162 
0.941 
d 0.419 1.361 0.303 
1.391 
e 0.632 1.772 0.482 
1.825 
f 0.878 2.155 0.696 
2.238 
g 1.153 2.510 0.944 
2.628 
h 1.453 2.833 1.223 
2.992 
i 1.777 3.124 1.531 
3.327 
j 2.120 3.380 1.864 
3.632 
k 2.479 3.600 2.221 
3.904 
l 2.850 3.784 2.596 
4.142 
m 3.230 3.932 2.988 
4.345 
n 3.615 4.042 3.393 
4.510 
o 4.000 4.116 3.808 
4.639 
p 4.387 4.153 4.229 
4.730 
q 4.768 4.153 4.653 
4.783 
r 5.139 4.124 5.077 
4.798 
s 5.500 4.060 5.497 
4.777 
t 5.847 3.965 5.911 
4.732 
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FIGS. 11 and 12 show, that the locking cams 20, 23-27 do not necessarily 
have to be positioned parallel to the top plane of beam 3. They may, for 
instance, also be positioned with an angle of about 90.degree. relative to 
beam 3 and, consequently, also relative to neck 54 of a guitar. In this 
case the U-shaped channel in which the locking cam 20 is placed, is 
rotated 90.degree. relative to the designs according to the preceding 
figures. The string s in the design according to FIG. 10, after being 
placed between locking cam 20 and locking wall 411, can be tightened by 
pulling the loose end of the string (in the figure at the right from beam 
3) away from the guitar neck 54. This is also valid for the other strings 
as well. Consequently, the advantage is provided, that a headstock 64 can 
be attached to the guitar neck 54, which is optically preferred above a 
guitar neck without a headstock. With the locking apparatus 1 according to 
the preceding figures the strings have to be tightened by pulling them 
towards the bottom of beam 3, which can be seen referring to FIG. 8. 
Therefore, in that design a headstock is not possible. 
FIG. 12 shows a cross section along the line XII--XII in FIG. 11 alongside 
locking cam 25. Between two adjacent locking cams, 20/23, 24/25, 26/27, 
each time one appropriately wound spring 65 can be provided, which, for 
instance, is anchored to the beam 3 with a bolt 66 and has two loose ends. 
Each end is connected to one locking cam, so that each spring 65 can push 
subsequently two locking cams 20/23, 24/25, 26/27 with an appropriate load 
towards locking wall 411.