US3437001A - Key changer and tremolo for guitar - Google Patents

Description

April 8, 1969 A. A. KRAFT 3,437,0m

KEY CHANGER AND TREMOLO FOR GUITAR Filed April 19, 1966 Sheet of 3 40 7% GD 44? |i. 1

I NVENTOR 49 7/102 4 /(@4F7 April8, 1 969 A. A. KRAFT 3,437,001 KEY CHANGER AND TREMOLO FOR GUITAR Filed April; 19, 1966 Sheet 3 .013'

April 8, 1969 A, KRAFT 3,437,001

KEY CHANGER AND TREMOLO FOR GUITAR Filed April 19, 1966 Sheet 3 of 5 f ZZZ/Z INVENTOR.

427/102 4. KP/IFr United States Patent Oifice 3,437,001 KEY CHANGER AND TREMOLO FOR GUITAR Arthur A. Kraft, Riverside, Califl, assignor of forty-nine percent to George J. Hefiley, Riverside, Calif. Filed Apr. 19, 1966, Ser. No. 543,587 Int. Cl. Gd 3/14 US. Cl. 84-312 10 Claims ABSTRACT OF THE DISCLOSURE Guitar strings are attached at their bottom ends to a rockshaft by adjustable means whereby each string is tangent to a circle concentric with the rockshaft, and the radius of said circle can be increased or decreased by adjustment so that turning the rockshaft causes all strings to maintain correct tonal relationship with one another as their pitch changes. The rockshaft can be oscillated by either a tremolo arm or a key-change control. A second rockshaft is connected to the first by springs, which oppose the torsional effect of string tension, and is rotated simultaneously with the first in the direction to increase spring tension when the opposing spring tension is increased.

The present invention relates to string instruments of the lute class, such as guitors, mandolins, and the like, having a long fretted neck and a plurality of strings that are plucked by the player, while the pitch of the strings is varied by placing the fingertips of one hand at different positions along the neck.

The primary object of the invention is to provide means for raising or lowering the pitch of all strings on an instrument of the class described, while maintaining correct tonal relationships of the strings relative to one another. Thus, all of the strings may be raised or lowered in pitch by one or more frets, without any of the strings getting out of tune with respect to the others. It is desirable to be able to raise or lower the pitch of the strings in this manner, to enable the player to get his instrument in tune with the rest of an orchestra or with another accompanying instrument, where there may be a very slight discrepancy in tune, without individually adjusting all six strings (or whatever number of strings the instrument may have).

It may also be desirable at times to raise or lower the pitch of the instrument by one or more frets toaccommodate the vocal range of a singer, so as to permit playing a number in a sharped or flatted key, using the standard pitch finger position. This eliminates the necessity of barring all six strings with one finger in order to change key, or using a clamp bar, known as a capo. Another reason for changing pitch is that, under certain conditions, an instrument will seem to have better tone quality when the pitch is lowered a fret or two in cases where the instrument is being played by itself. This may be due to the fact that certain strings on the instrument are too tight for best tone when tuned to standard pitch, and the appropriate change of tension will correct this condition by softening the strings.

To change the pitch of the strings, without changing length, requires a change in tension. This may be accomplished by attaching the anchored ends of the strings to a rockshaft, which is turned slightly in one direction or the other, to increase or decrease the tension in all of the strings. If all of the strings are stretched (or relaxed), by the same amount, they will all be raised (or lowered) in pitch, but not by the same amount for each string. This is because the strings are all under different amounts of initial tension, and they are of different diameters and tempers. Thus, an adjustment of the rockshaft to pro duce a one-fret increase in tone in one string, may pro- Patented Apr. 8, 1969 duce a slightly greater or slightly lesser increase in all of the other strings. This slight discrepancy in tone adjustment of the several strings is overcome in the present instrument by having the strings tangent to the rockshaft at different radial distances from the center thereof. Thus, if we assume for the purpose of illustration, that a first string requires one unit increment of stretch to raise its pitch by one fret, and this first string is tangent to the rockshaft at one unit distance from the center thereof; then a second string requiring half a unit increment of stretch to raise its pitch by one fret would be tangent to the rockshaft at half a unit distance from the center thereof.

The concept of using different diameters on a rockshaft, over which the strings are passed, is not entirely new. However, it has been found that strings vary greatly in physical properties such as gage, temper, and metallurgical composition, so that in replacing one string with another supposedly of the same kind, it will be found that the rockshaft diameter that was perfect for the discarded string is no longer perfect for its replacement. As a result, the rockshaft having different fixed diameters is of limited utility, and will perform satisfactorily only where strings are carefully selected for that purpose. Since it is not always possible to select strings from a large assortment, this shortcoming of the rockshaft having different fixed diameters is a distinct handicap.

Another important object of the invention, therefore, is to provide a pitch-changing mechanism for a stringed instrument of the class described, having a rockshaft to which the strings are attached, and also having means for varying the radial distance from the center of the rockshaft to its point of tangency with each of the strings. In this way, the radial distance from the center of the rockshaft to its point of tangency with any one string can be precisely adjusted to give that string the exact amount of stretch required to raise or lower its pitch by one fret when the rockshaft is turned through an angular distance to raise or lower the pitch of the other strings by one fret.

Another object of the invention is to provide a pitchchanging mechanism for a stringed instrument, as described, wherein the strings are trained around and attached to rockshaft, and means are provided for counterbalancing the torsional effect of the string tension on the rockshaft, whereby any pitch adjustment is accomplished without having to overcome the tension in the strings. An added feature of the invention is that the counterbalancing torsional means is adjustable, so that the tension of the springs can be cancelled out to any desired degree. This is important for the tremolo-producing device, which must be relatively effortless to operate.

A further object of the invention is to provide a stringed instrument of the class described, wherein the pitch of the strings can be changed quickly and easily by a simple adjustment of a pitch control member, such as a knob or wheel on the instrument body, without af fecting the adjustment of the tuning pegs of the individual strings.

Another object of the invention is to provide a stringed instrument of the class described, having manually operable means for rapidly and controllably increasing or decreasing the tension of the strings while the instrument is being played, for the purpose of slurring notes or chords up or down from standard pitch, or to produce a tremolo effect. More specifically, it is an object of the invention to provide such a means for varying the tension in the strings while preserving the correct tonal relationship between the several strings, so that if a chord has been struck, it will retain its harmonious quality and will not become discordant during the pitch change.

A further object of the invention is to provide a tremolo-producing device which can be operated selectively to oscillate the pitch of the strings either upwardly from standard pitch, or downwardly therefrom.

Another object of the invention is to provide a stringed instrument of the class described which can be manufactured and sold with the pitch-changing feature but without the tremolo-producing feature, or with the tremolo-producing feature but without the pitch-changing feature, and to which the other feature can be added at a later date with only a relatively minor bit of structural change in the instrument.

FIGURE 1 is a top plan view of a guitar constructed in accordance with my invention;

FIGURE 2 is an enlarged, fragmentary elevational view of the pitch changing mechanism of the present invention, which is partially cut away to reveal some of the underlying structure;

FIGURE 3 is a sectional view of the same, taken at 3-3 in FIGURE 2;

FIGURE 4 is another sectional view, taken at 44 in FIGURE 2;

FIGURE 5 is a sectional view of a detail, taken at 5-5 in FIGURE 2;

FIGURE 6 is a sectional view of another detail, taken at 6-6- in FIGURE 2;

FIGURE 7 is a side elevational view of the pitch changing mechanism, as seen from 77 in FIGURE 2;

FIGURE 8 is an enlarged fragmentary sectional view, taken at 88 in FIGURE 7;

FIGURE 9 is an enlarged, partially cut-away elevational view of the control device for raising or lowering the pitch of the strings;

FIGURE 10 is a sectional view through the same, taken at 10-10 in FIGURE 9;

FIGURE 11 is a perspective view of one of the anchor devices used to connect each of the strings to the rocker bar; and

FIGURE 12 is an enlarged sectional view through the rocker bar of another embodiment of the invention, similar to FIGURE 4, showing another means for attaching the strings to the rocker bar, with means for adjusting the radial distance from the center of the shaft to the point of tangency of the string.

In the drawings, FIGURE 1 shows a typical electric guitar having a body 20, and an elongated neck 22 extending from the upper end thereof. The neck 22 has a plurality of longitudinally spaced, transversely extending frets 23 provided in its top surface, and at the outer end of the neck is a head 24 with tuning pegs 25, to which the customary six strings 26 are attached. The strings 26 extend lengthwise down over the neck 22 in laterally spaced, generally parallel relationship, and pass over an electromagnetic pickup unit 28.

Near the lower end of the guitar, strings 26 are trained over grooved rollers 30 of a roller bridge 31. The rollers 30 are spaced apart laterally and are screw-threaded onto a cross shaft 32, which is supported betwen the upturned ends 34 of a bracket 36 that is attached to the body 20. The rollers 30 are rotatable on the shaft 32 so as to roll freely with the strings as the latter are caused to move lengthwise while being tightened or sla-ckened, as will be described later. In passing over the rollers 30, the strings 26 are bent downwardly at a slight angle, and each string is connected by a link 38 to a rockshaft 40 in a manner whereby the said string is tangent to a circle having its center at the axis of the rockshaft, as best shown in FIGURE 4. The circle referred to is shown in phantom line and is designated by the letter C.

The rockshaft 40 is rotatably supported at its ends in two upstanding support plates 42, which are attached to opposite sides of a bracket 44 that is mounted on the body 20. Extending diametrically through the rockshaft 40 at longitudinally spaced points thereon are six tapped holes 46, and screwed into these holes are adjusting screws 48.

The screws 48 are parallel to one another, and lie in a common plane that is generally perpendicular to the strings 26 for that portion of their length below the roller bridge 31. Each of the screws 48 projects below the bottom surface of the rockshaft 40, and formed in the projecting ends of the screws near their bottom ends are V-shaped, circumferential grooves 50.

Each of the links 38 is preferably stamped out of sheet metal, and is bent to the configuration shown in FIGURE 11. At one end of the link 38 is a hole 52 having a knife edge that seats in the bottom of the groove 50 in the corresponding screw 48. The diameter of hole 52 is sufficiently large to pass the end of the screw 48, and the point of contact where the knife edge of hole 52 touches the bottom of the V-groove 50 constitutes the point of attachment of the string 26 to the rockshaft. At the other end of the link 38 is a second hole 54 of smaller diameter, through which the string 26 passes. The string is wrapped around and attached to a small brass ferrule 56, which prevents the string from being pulled back through the hole 54. The rockshaft 40 is milled ilat at 58 on its underside, to permit the screws 48 to be backed out so as to adjust the links 38 closer to the axis of the rockshaft, than would otherwise be possible if the rockshaft were left round. Each of the screws 48 has a hexagonal socket in its top end to receive an Allen wrench for adjustment purposes.

As mentioned earlier, the strings 26 are attached to the rockshaft 40* by links 38 in such a manner that they are, in effect tangent to a circle having its center at the axis of the rockshaft. The diameter of the circle depends upon the distance of the V-shaped groove 50 from the center of the rockshaft, and this is adjustable over a relatively wide range by advancing or backing out the adjustment screws 48 with respect to the rockshaft. The tension of the strings 26 acting on their respective moment arms exerts a powerful torque on the rockshaft, tending to turn the latter in the counterclockwise direction, as viewed in FIGURE 4. To counteract this torque due to string tension, 1 have provided a counter-balancing arrangement in the form of a plurality of coil springs 62, each of which is connected at one end to the head of a screw 64 that is threaded down into the rockshaft 40 on the top side thereof between the screws 48.

The other ends of the springs 62 are connected to the heads of screws 66, which are threaded into a second rockshaft 68 located above rockshaft 40 and parallel thereto. This second rockshaft 68 is supported at its ends in upturned ears 70 at opposite sides of the bracket 44. Rockshaft 68 lies below the strings 26, and has a plurality of longitudinally spaced grooves, or channels 72, cut into its top side below each string to provide clearance therefor. Screws 64 and 66 have hexagonal sockets 74 formed in their heads, to receive an Allen wrench for adjustment purposes.

Rockshaft 40 projects beyond the right-hand support plate 42 (as viewed in FIGURES 2 and 3), and swingably mounted on the projecting end is a downwardly extending lever arm 76. The lever arm 76 lies flat against the outside surface of the right-hand plate 42 and is constrained to oscillate through a limited angular distance by means of two screws 78, which extend through arcuate slots 80 in the lever arm on opposite sides of the center line thereof. The slots 80 are concentric with the axis of the rockshaft 40, and the heads of the screws 78 overlie the outer surface of the lever arm 76 to hold the latter flat against the support plate 42, while allowing oscillatory movement within the range of the slots.

Laterally outward of the lever arm 76 is a lever arm 82 having a hub 84 which is fixedly mounted on the rock shaft 40. A post 86 is screwed into a tapped hole in the lower end of the lever arm 76, and projects laterall outwardly therefrom, through the horizontal portion 88 of an inverted, L-shaped slot 90 formed in the lower portion of lever arm 82. Post 86 projects beyond the lever arm 82, and swingably mounted on the projecting outer end thereof is a bell crank 92, which is best shown in FIG- URE 7.

One arm 94 of bell crank 92 extends upwardly from post 86, and at its upper end is a pivot screw 96, which is substantially coaxial with rockshaft 40. The pivot screw 96 connects bell crank arm 94 to one end of a link 98, which extends horizontally to the left, as viewed in FIG- URE 7. The other end of link 98 is connected by a pivot screw 100 to the upwardly projecting arm 102, of a bell crank 104 forming the hub portion of the tremolo arm 106.

Tremolo arm 106 is conveniently made of round rod, one end of which is received within a bore in the top end of the vertical bell crank arm 102, and is free to turn therein. The rod forming the tremolo arm 106 extends upwardly, and is bent to extend generally parallel to the top surface of the guitar body 20. Inserted over the outer end of the tremolo arm 106 is a sleeve 108 of soft rubber or the like, which serves as a handle.

Being free to turn within its bore in the vertical bellcrank arm 102, tremolo arm 106 can be swung over to one side and out of the way, when not in use. Then, when the player wishes to use the tremolo, he merely swings the arm 106 inwardly, so that the handle 108 is located at the most comfortable position for vertically oscillating the arm 106 while plucking the strings 26. The tremolo arm is oscillated in the vertical plane by pressing handle 108 downwardly from a normally centered position to raise the pitch of the strings slightly above standard pitch, or by raising the handel 108 to lower the pitch. The means by which this is accomplished will now be described.

Bell crank 92 has a second arm 110 extending downwardly and to the right from post 86, as viewed in FIG- URE 7, and mounted on its outer end is a short, laterally inwardly projecting pivot shaft 112 having a roller 114 journaled thereon. The roller 114 is disposed within the vertical portion of the inverted, L-shaped slot 90, and bears against the left-hand edge 116 thereof (see FIG- URE 6), which lies on a radius extending through the axis of rockshaft 40. The diameter of the roller 114 is a few thousandths of an inch smaller than the width of the vertical portion of slot 90, and therefore the righthand side of the roller clears the right-hand edge of the slot. Thus, the only contact of the roller 114 with lever arm 82 is along edge 116.

The slight counterclockwise torque exerted On rockshaft 40 by the excess of tension in string 26 over the tension of springs 62, causes the lever arm 82 to bear against the roller 114, holding slot side 116 against the outside of the roller. When the bell crank 94 is rocked in the clockwise direction about post 86, roller 114 travels downwardly and to the left (FIGURES 6 and 7) in an arcuate path, pushing the lower end of lever arm 82 to the left. This causes lever arm 82 to rock the rockshaft 40 in the clockwise direction. Clockwise turning of rockshaft 40 causes the strings 26 to be pulled tighter, and this raises the pitch of the strings. Since the bell crank 94 is rocked in the clockwise direction by pressing the tremolo arm 106 downwardly from its normal, springcentered position, it follows that pressing downwardly on the tremolo arm causes the pitch of the strings to be raised.

In the same manner, raising the tremolo arm 106 from its normal, spring-centered position, rocks the bell crank 104 in the counterclockwise direction, pulling link 98 to the left, as viewed in FIGURE 7. This causes the bell crank 94 to be ro-cked in the counterclockwise direction about the post 86, and swings the roller 114 upwardly and to the right, in an arcuate path. Since the lever arm 82 is held against the roller 114 by the spring torque on rock shaft 40, it follows that the rockshaft is rotated in the counterclockwise direction, which has the effect of lowering the pitch of the strings from standard pitch.

The tremolo arm 106 is spring-centered in its normal position of rest by two centering springs 118, and by a rubber mounting 120 for bell crank 104; the latter being shown in cross-section in FIGURE 8. The rubber mounting 120 comprises a sleeve 122 of vulcanized rubber, which is inserted tightly into a bore 124 in the bell crank 104, and inserted tightly into the sleeve 122 is a brass tube 126. Passing through the tube 126 is a screw 128, the inner end of which is threaded into a tapped hole 129 in bracket 44 and is locked against turning by a locknut 130.

The ends of tube 126 project slightly beyond the opposite side faces of hell crank 104, and one of said ends abuts against the outer face of bracket 44, while the other end abuts against the inside surface of a flange 132 on bracket 44. Flange 132 extends parallel to the outer face of bell crank 104, and is the forwardly bent extremity of a flange 134, which projects laterally from the main body of bracket 44, behind the bell crank 104, as best shown in FIGURE 2.

The head of screw 128 is drawn up tightly against the outside of flange 132, thereby clamping the latter tightly against the outer end of tube 126. Tube 126 is thus clamped tightly between the outer face of bracket 44 and flange 132, and is held against turning. When bell crank 104 is rocked in either direction by the tremolo arm 106, the rubber sleeve 122 is stressed in shear, since the outer surface of the sleeve is firmly adhered to the surface of the bore 124 in the oscillatable bell crank, while the inner surface of the sleeve is firmly adhered to the stationary tube 126. Being thus stressed in shear, the rubber sleeve 122 exerts a resilient torsional force on the bell crank tending to return the latter to its initial position. There is no sliding fraction between the bell crank 104 and the stationary tube 126 as a result of their relative rotational movement, and therefore the bell crank is returned all of the way back to its original position by the rubber sleeve when the tremolo arm is released, with no hysteresis lag.

As best shown in FIGURE 7, the two centering springs 118 are connected at one end to a second arm 136 on the bell crank 104, and at the other end to a bracket 138. Bracket 138 has a link 140 connected thereto for vertical swinging movement about a pivot screw 142. Projecting from the top and bottom sides of the link 140 are two screw heads 144, to which the springs 118 attach at one end. The other ends of the springs are attached to screw heads 146, which project from the top and bottom sides of a second link 148 that is connected by a pivot screw 150 to the bifurcated outer end of hell crank arm 136. Link 148 is also swingable vertically in the same plane as link 140, and the two links tend to align themselves parallel to a line connecting the centers of pivot screws 142 and 150.

The pull of springs 118 exerts a yielding spring force on arm 136, tending to pull pivot screw 150 to a position in line with the centers of screws 128 and 142. This is the normal, spring-centered position of the bell crank 104, and any time that the bell crank is deflected from this position, the springs 118 exert a force tending to restore the bell crank to its initial position, aided by the action of the rubber mounting 120.

Means is provided for changing the pitch of the strings 26, so as to raise or lower the pitch of all six strings by one or two frets, as desired. Such pitch-changing means is designated in its entirety by the reference numeral 152, and includes an adjustment lever 154 on the top surface of the body 20 at the upper right-hand corner thereof. Adjustment lever 154 is fixed to the top end of a short shaft 156, which is journaled in aligned bushings 158 and 160, that are pressed into upper and lower plates 162 and 164, respectively. Upper plate 162 is generally rectangular in configuration and is secured to the guitar body 20 by screws 166. Lower plate 164 is considerably smaller than plate 162 and is generally triangular in shape.

7 Plate 164 is connected to plate 162 by three posts 168.

Shaft 156 has an eccentric disk 170 formed thereon at its midpoint, the top side of which lies flat against the underside of a connecting link 172 at one end thereof. The top end of shaft 156 passes through a hole in link 172 and through the upper bushing 158, while the bottom end is received within the lower bushing 160. A ring 174 is secured by screws to the underside of link 172, and this ring encircles the eccentric disk 170 and has an inwardly projecting radial flange that overlies the marginal edge of the disk, whereby the eccentric disk is free to turn within the space defined between the link 172 and ring 174, while the ring follows the orbit of the eccentric disk.

The other end of link 172 is connected by a pivot bolt 176 to a sector 178. Sector 178 is pivotally connected to the top plate 162 by a pivot bolt 180, and is swingable in a plane parallel to plate 162. Sector 178 has two arcuate slots 182 formed therein, which have their centers at the center of pivot bolt 180, and passing through these slots are screws 184, having heads which overlie the bottom surface of the sector along the marginal edges of the slots. Screws 184 hold the sector flat against the underside of plate 162, while permitting the sector to oscillate within the range of the slots 182.

Fixed to the underside of the sector 178 is a radially extending bar 185, the outer end of which projects beyond the arcuate edge of the sector. A screw 186 is screwed into the outer end of the bar 185 on the bottom side thereof, with its head passing downwardly from the underside of the bar. The screw 186 is adapted to be received within a keyhole slot 188 on one end of a push rod 190.

The other end of push rod 190 has another key hole slot 192 formed therein, which receives a screw 194 that is screwed into the lower end of a lever arm 196. Lever arm 196 is fixed to the right-hand end of shaft 68 (as viewed in FIGURE 2), and extends downwardly therefrom. About midway between its ends, the lever arm 196 is connected by a pivot screw 198 to one end of a link 200. The other end of link 200 is connected by post 86 to the lower end of lever arm 76, and controls the swinging movement of the latter. Thus, when push rod 190 is moved to the left (FIGURE 5), lever arm 196 and shaft 68 are rocked in the clockwise direction through a certain angular distance, while lever arm 76 is also rocked in the clockwise direction through an angular distance about half that of lever arm 196. Lever arm 76 is connected to rockshaft 40 in a manner to be described presently, whereby the rockshaft is caused to turn with the lever arm 76 when the latter is swung by link 200, yet the rockshaft is free to turn with respect to the lever arm 76 while the latter is held stationary by link 200.

The differential in angular distance traveled by shaft 68 over that traveled by rockshaft 40, is the result of the connecting linkage that joins them together. The moment arm from the center of shaft 68 to screw 198 is only about half the moment arm to screw 194, whereas the moment arm from the center of rockshaft 40* to post 86 is slightly greater than the moment arm from the center of shaft 68 to screw 194. The result is that when shaft 68 is turned by push rod 190, rockshaft 40 is caused to turn in the same direction but through a smaller angular distance than shaft 68. Thus, when the strings 26 are tightened by turning the rockshaft 40 in the clockwise direction, the springs 62 are correspondingly stretched to increase their tension, due to the greater clockwise rotation of shaft 68, which causes screw heads 66 to move away from screw heads 64. In like manner, when the strings 26 are loosened by turning the rockshaft 40 in the counterclockwise direction, the springs 62 are correspondingly slackened to decrease their tension, due to the greater counterclockwise rotation of shaft 68, which causes screw heads 66 to move toward screw heads 64.

The driving connection between lever arm 76 and rockshaft 40 is as follows: Post 86 is mounted on the lower end of lever arm 76 and moves with the latter in an arouate path, the center of which is located on the axis of rockshaft 40. Bell crank 94 is pivotally mounted on post 86, and therefore swings with the latter in an arcuate path about pivot screw 96, which is coaxial with rockshaft 48, and which is held stationary by link 98. Roller 114 is mounted on bell 94 and moves therewith in an arcuate path having its center at the axis of pivot screw 96. Since roller 114 is confined within the vertical portion of L- shaped slot 90, it follows that lever arm 82 and rockshaft 40, to which it is fixedly attached, are caused to rock through the same angular distance and in the same direction as lever arm 76.

It will be noted in FIGURES 5 and 9 that the large ends of the keyhole slots 188 and 192 are at the outer ends of the push rod 190, and that the smaller ends of the slots extend inwardly toward one another. This is because the push rod is maintained under compression at all times by the torque on shaft 68 caused by the tension of springs 62. As described earlier, the counterclockwise torque on rockshaft 40 caused by the tension of strings 26 is opposed and substantially cancelled out by the clockwise torque of springs 62. Thus, lever arm 76 has only an insignificant amount of torque exerted on it by post 86. However, lever arm 76 is connected by link 200 to lever arm 196, and the latter is fixedly mounted on shaft 68, which has a considerable counterclockwise torque exerted on it by springs 62. As a consequence, lever arm 196 exerts a force on push rod 190, tending to push the latter to the right, as viewed in FIGURE 5.

The thrust force on push rod 190 toward the right (FIGURE 5) due to the counterclockwise torque of shaft 68 is opposed by a substantially equal and opposite force exerted on section 178 by two springs 202 and 204. Springs 202 and 204 are attached at one end to pins 206 fixed to the underside of plate 162, and at the other end to pins 208 fixed to the bar 185. These springs exert a counterclockwise torque on sector 178, which is applied to the right-hand end of push rod 190 as a thrust force directed to the left.

The operation of the invention is as follows: The strings 26 of the guitar are first tuned to standard pitch, using the tuning pegs 25. At this time, the pitch-changing lever 154 is turned to an intermediate, standard pitch position. When the strings are tuned to standard pitch, they produce the following tones, reading from left to right in FIGURE 1: E, A, D, G, B, and E. To lower the pitch by one fret (i.e., one half-note), the pitch-changing lever 154 is turned counterclockwise to an appropriate index mark on plate 162, as illustrated in FIGURE 1. This counterclockwise turning of lever 154 causes the push rod 190 to be moved to the right (FIGURE 9) which turns the rockshaft 40 in the counterclockwise direction (FIG- URE 5), thereby slackening the strings. The strings should now produce the following tones: E-fiat, A-flat, D-flat, G-flat, B-flat and E-flat. If they do not, the adjustment screws 48 are turned in the appropriate direction to bring the strings into tune. For example, if one of the strings is slightly higher in pitch than it should be, after turning lever 154, the corresponding screw 48 would be turned in the direction to increase the distance from the center of the rockshaft 40 to the point of contact of link 38 with the bottom of groove 50. In like manner, if another of the strings is slightly lower in pitch than it should be, the corresponding screw 48 would be turned in the direction to decrease the distance from the center of the rockshaft to the point of contact of link 38 with the bottom of the groove 50.

When all of the strings have been re-tuned with the adjustment screws 48, the lever arm 154 is turned to the next index mark in the counterclockwise direction, which drops the pitch of the strings by two frets (i.e., one full note). The strings should now produce the following tones: D, G, C, F, A, and D. If any of the strings are out of tune at this second position of lever arm 154, they 9 are again adjusted by turning the appropriate adjustment screws 48.

The lever arm 154 is then returned to its standard pitch position, and the strings are again tuned by using the tuning pegs 25. Usually, this will sufiice for tuning and adjusting the guitar, although it may be necessary to repeat the process while turning the lever arm 154 first to the one-fret-above-standard-pitch position, in which the strings will sound: F, A-sharp, D-sharp, G-sharp, C and F, and then to the two-frets-above-standard-pitch position, in which the strings will sound: F-sharp, B, E, A, C-sharp and F-sharp.

When the guitar has been finally tuned and adjusted properly, the points of contact of links 38 with the bottoms of grooves 50 will be at various distances from the center of the rockshaft 40. As a result, when the rockshaft is turned through a given angular distance, each of the strings is tightened or slackened by an amount directly proportional to the distance from the axis of rockshaft 40 to the point of contact of the link 38 with the bottom of groove 50 in the adjusting screw 48. This amount can be adjusted so that for a given angular increment of rockshaft 40, each of the strings 26 will be raised or lowered in pitch by exactly a half-note or whole-note, depending upon whether the adjustment lever has been turned to one index mark or the other. The amount of stretch required to change the pitch of a string by a half-note or wholenote varies with the diameter of the string, its temper, and the tension of the string when tuned to standard pitch. Even two strings that are nominally the same pitch and supposedly identical, will require different amounts of stretch to increase the pitch by a half-note or whole note, and these varying requirements are met by the adjustment screws 48, which enable the stretch to be matched to the string.

When the tremolo arm 106 is oscillated in the vertical plane by the player, bell crank 92 is oscillated about post 86 by link 98, and this causes roller 114 to oscillate in an arcuate path. Lever arm 82 follows roller 114 and is oscillated thereby, causing rockshaft 40 to oscillate rapidly, which produces the rapid raising and lowering of pitch of the strings 26, which is characteristic of the tremolo effect. The tremolo effect can be obtained by raising the pitch of the strings slightly from the playing key, which is done by pressing the tremolo arm downwardly from its spring-centered position, or by lowering the pitch from the playing key, which is done by raising the tremolo arm away from the guitar body 20.

One feature of the invention is that the guitar can be sold with the tremolo feature but without the pitch-changing feature, or vice versa, and the missing feature can be added at a later date. For example, the pitch-changing device 152 could be omitted, and the lever arm 196 locked in a fixed position. In this case, the lever arm 76 would be immobilized by the fixed lever arm 196, and the rockshaft 40 would be oscillated only by the tremolo arm 106, acting through link 98, bell crank 94, roller 114 and lever arm 82. At any later date, the owner of the guitar could add the unit 152, at which time lever arm 196 would be released and push rod 190 connected thereto.

On the other hand, if the purchaser wished to buy the guitar with the pitch-changing unit 152 but without the tremolo arm 106, the arm 106, bell crank 104, and centering springs 118 could be omitted from the guitar, and lever arm 82 connected directly to post 86, using a filler block to fill the L-shaped slot 90. As thus constructed, the lever arm 76 would act on lever arm 82 through post 86, causing the two lever arms to move as one. Then, at any later date, the owner could add the tremolo-producing unit.

Another embodiment of the invention is shown in FIGURE 12, to which attention is now directed. In this embodiment, parts which are identical in function to those previously described are given the same reference numerals with the prime sufiix.

The chief difference between this embodiment and the one described above is in the means for connecting each of the strings 26 to the rockshaft 40', whereby the distance from the center of the rockshaft to the point of tangency of the string may be adjusted to obtain the exact amount of tightening or slackening required to raise or lower the pitch of the string by the desired amount.

In this case, the rockshaft 40' is provided with sloping, upwardly converging sides 210, and slidably engaging these sloping sides are the inwardly turned ends 212 of six generally circular collars 214. Each of the collars 214 is mounted on the underside of the rockshaft 40' in line with its respective string 26', and the latter is trained around the bottom side of the collar and then up to an attachment plate 216. Attachment plate 216 is an elongated plate which is secured by screws 218 to the top side of the rockshaft. The plate 216 is preferably seated on top of washers 220, and the latter bear on a flat surface 222 milled into the top of the rockshaft. The rear edge of the plate 216 has slots 224 formed therein, through which the wires 26 are led. The ferrules 56 seat on the top side of the plate 216, at the junction of the main body of the plate with an upwardly turned edge portion. The forward edge of the plate 216 is bent upwardly to form a flange 226, having holes formed therein, into which the ends of springs 62 are hooked.

The adjustment screw for each of the collars 214 is designated by the reference numeral 230, and extends diametrically through rockshaft 40. The lower end of screw 230 projects beyond a flat surface 232 milled into the bottom side of the rockshaft, and its extremity is provided with a shoulder flange 234 and reduced-diameter tip end portion 236. Flange 234 bears downwardly against the top side of an inverted U-shaped clip 238, while tip end portion 236 is seated in a hole 240 in the clip. The sides of the clip 238 extend downwardly on opposite sides of the collar 214, and provide side flanges which keep the string 26' from running off the collar 214 on either side thereof.

The upper end of the adjusting screw 230 has a hexagonal socket 242 provided therein, into which an Allen wrench may be inserted. Plate 216 also has holes 244 provided therein directly over each of the adjusting screws 230, so that the screws are accessible for adjustment purposes without removing the plate.

To increase the radial distance from the center of the rockshaft 40' to the point where string 26 is tangent to collar 214, the screw 230 is turned with a wrench to advance it downwardly into the rockshaft. The bottom end of the screw 230 pushes the collar 214 ahead of it, and as the collar moves downwardly, the ends 212 are spread apart by the diverging side walls 210. Spreading the ends 212 apart has the effect of increasing the diameter of the collar 214 to match its distance from the axis of the rockshaft. The adjustment and manner of using this embodiment of the invention are substantially the same as in the preceding embodiment.

I claim:

1. A key changing device for a musical instrument of the lute class having a body, a fretted neck extending from one end of said body, and a plurality of strings connected to tuning pegs at the outer end portion of said neck, said strings extending longitudinally above said neck and over said body, said device comprising:

a rockshaft mounted on said body near the lower end thereof, said rockshaft being disposed transverse to said strings;

means for attaching the other end of each of said strings to said rockshaft in a manner whereby the said string is tangent to a circle having its center at the center of said rockshaft;

means for adjustably varying the distance from the center of said rockshaft to the point of tangency of each of said strings; and

means for turning said rockshaft so as to tighten or slacken all of said strings simultaneously and thereby increase or decrease the pitch of said strings;

the adjusted distance from the center of said rockshaft to the point of tangency of each of said strings being such that for any given increment of angular distance through which said rockshaft is turned, each string is tightened or slackened by the exact amount required to keep said string in tune with the other strings as their pitch is changed.

2. A key changing device for a stringed instrument as set forth in claim 1, wherein said means for adjustably varying the distance from the center of said rockshaft to the point of tangency of each of said strings comprises a plurality of adjusting screws extending diametrically through said rockshaft in threaded engagement therewith, each of said screws having means at one end to receive an adjusting tool, and the other end of said screw projecting beyond said rockshaft, each of said means for attaching one of said strings to said rockshaft being cooperatively associated with said other end of its associated adjusting screw.

3. A key changing device for a stringed instrument as set forth in claim 1, wherein spring means is connected to said rockshaft to exert a torsional force thereon substantially equal and opposite in direction to the torsional force exerted on the rockshaft by said strings, and means for varying the tension in said spring means in substantially direct proportion to the change in tension of said strings as the latter are tightened or slackened by rotating said rockshaft.

4. A key changing device for a stringed instrument as set forth in claim 3, wherein said means for varying the tension in said spring means comprises a second rockshaft disposed parallel to said first-named rockshaft, said spring means comprising at least one coil spring attached at one end to said first-named rockshaft and at the other end to said second rockshaft, each of said points of attachment being spaced radially from the center of its respective rockshaft in the direction generally perpendicular to the line of pull of said coil spring, whereby tension of said coil spring exerts a torsional force on each of said rockshafts, said means for varying the tension in said spring means comprising a driving connection between said first-named rockshaft and said second rockshaft whereby said second rockshaft is rotated in a direction and through an angular distance to cause said coil spring to be stretched when said first-named rockshaft is rotated in the direction to tighten said strings, and to cause said coil spring to be slackened when said first-named rockshaft is rotated in the direction to slacken said strings.

5. A key changing device for a stringed instrument as set forth in claim 1, wherein said rockshaft has a plurality of adjusting screws extending diametrically through it in threaded engagement therewith, there being one screw for each string and said screws being generally perpendicular to said strings and projecting at one end beyond said rockshaft, and link means connecting each of said strings to said projecting one end of its associated adjusting screw, the radial distance between the center of said rockshaft and the point of contact of said link means with said screw being adjustable by turning said screw within said rockshaft.

6. A key changing device for a stringed instrument as set forth in claim 1, wherein said means for turning said rockshaft comprises a lever fixed to said rockshaft at one end thereof, a manually adjustable pitch control member on said body movable between a standard pitch position and at least one other position to change the pitch of the instrument to another key above or below said standard pitch, and force-transmitting means connecting said pitch control member to said lever arm whereby 12 moving said pitch control member from one of said positions to the other causes said rockshaft to be rotated, thereby tightening or slackening said strings to raise or lower their pitch. I

7. A key changing device for a stringed instrument as set forth in claim 6, including a tremolo arm supported for oscillatory movement, linkage means connecting said tremolo arm to said lever arm whereby said rockshaft may be oscillated by the tremolo arm independently of said pitch control means, and spring means connected to said tremolo arm and exerting a force thereon tending to return the same to its initial position when said tremolo arm has been deflected therefrom.

8. A key changing device for a stringed instrument as set forth in claim 6, including a tremolo arm supported for oscillatory movement, a second lever arm swingably supported on said rockshaft closely adjacent said firstnamed lever arm, a link connecting said second lever arm to said pitch control member, a bell crank pivotally supported on said second lever arm at a point spaced radially from the axis of said rockshaft, means connecting one leg of said bell crank to said first-named lever arm whereby angular movement of said bell crank about its pivot axis causes said first-named lever arm to swing through an angular distance, thereby rocking said rockshaft, the other leg of said bell crank extending toward the axis of said rockshaft, and a second link connecting said tremolo arm to said other leg of said bell crank substantially on the axis of said rockshaft, said connection between said second link and said other leg of said bell crank constituting a second pivot axis for said bell crank coaxial with said rockshaft, whereby swinging movement of said second lever arm responsive to movement of said manually operable pitch control member causes said bell crank to swing with said second lever arm about said second pivot axis, thereby swinging said first-named lever arm with it to rock said rockshaft.

9. A key changing device for a stringed instrument as set forth in claim 1, wherein said rockshaft has a plurality of generally circular collars mounted thereon in line with said strings, each of said strings being trained over its respective collar and attached to said rockshaft, and means for adjusting the distance from the axis of said rockshaft to each of said collars at the point where the string is tangent to the collar.

10. A key changing device for a stringed instrument as set forth in claim 9, wherein each of said collars is split, said rockshaft having angularly related flat sides provided thereon which diverge in the direction toward said collar, the ends of said collar bearing on said diverging sides, and adjustment screws extending diametrically through said rockshaft in threaded engagement therewith, each of said screws projecting beyond said rockshaft and bearing against the inner surface of said collar approximately midway between the ends thereof, said diverging sides causing the ends of said split collar to spread apart as said collar is moved away from the axis of said rockshaft, thereby increasing the radius of curvature of said collar to approximate the curvature of a circle concentric with said rockshaft and tangent to said string.

References Cited UNITED STATES PATENTS 10/1953 Barr 84--312 3/1966 Moseley 843l3 US. or. X.R. 84--313

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