Strand Tension Indicator

Abstract

A hand-held device for determining the tension in a flexible strand such as a tennis racquet string. One end of a pivoting device clamps around the string and bends it under spring pressure. The tension can be read directly on a scale, since the bending deflection is a function of the tension on the string.

Description

This invention relates to devices for measurement of tension forces within flexible strands, and more particularly to those which are inexpensive and direct-reading.

Variations in the tension of strings in a tennis racquet can be responsible for poor control on the part of the player. Yet there is no commercially available method of establishing this tension except while the racquet is being strung. Once the strings are cut and tied off, the tension changes and becomes an unknown. It is also important to be able to check the string tensions immediately prior to a match, so that adjustments can be made to the playing techniques. Selection of racquets is also aided by this information.

A primary object of my invention is to provide a method of determining the tension of the strings in a tennis racquet.

A further object is to provide an indicator in which the force range can be varied.

Other objects and advantages of the present invention will become apparent from the following detailed description of specific embodiments thereof, when read in connection with the accompanying drawings.

Briefly, the preferred embodiment of the present invention comprises a pair of pivoting members, one of which supports the flexible strand to be measured at two points, and the other of which engages the opposite side of the strand between the two points. A compression spring between the two members causes bending of the flexible strand, by forcing the end of one member to pass between the support points of the other member. The strand is thus deflected in the form of a vee, with the depth at the apex varying inversely with the tension in the strand. Numbered lines, or indicia, are scribed along the edge of one member, and the tension is readable directly as the line opposite the edge of the other member. This intersection occurs at the apex of the included angle between the members, which angle is held to less than 20.degree. to create an expanded length of travel of the apex along the indicia.

Referring now to the drawings,

FIG. 1 is a partially sectioned side view showing the device engaging a strand, and in the fully compressed position.

FIG. 2 is a perspective view of the indicator in the released position.

FIG. 3 is an alternate sectioned side view in the released position, showing two methods of varying the force range of the spring.

Referring again to FIG. 1, upper member 4 is pivotally mounted to lower member 5 at pivot 2. Flexible strand 1 is engaged between pin 10, attached to member 4, and edges 11 and 12 which are parts of member 5. Spring 3 is held in position by recess 13 in member 4, and forces the longer portions of members 4 and 5 apart.

As shown in FIG. 2, separation of the longer portions of members 4 and 5 causes pin 10 to press the center portion of strand 1 down between edges 11 and 12, forming strand 1 into a vee. Angle A is the deflection angle of strand 1. Indicia 6 are scribed along the upper edge of member 5, so that lower edge 14 of member 4 causes apex 15, of the included angle between members 4 and 5, to indicate the tension in strand 1. Corner 9 of member 4 limits the angle at apex 15 to 20.degree..

In FIG. 3 slide 16 is added with guide 19 attached to the upper surface and fitting inside spring 3. Left end 18 of slide 16 is turned up to ride in recess 17 of member 4. Recess 13 has been widened so that spring 3 can move away from pivot 2 as the longer portion of member 4 is raised. The bottom of spring 3 is located by guide 19 and moves to the right when member 4 is raised as shown, moving is perpendicular to the spring axis. This increase in moment arm about pivot 2 increases the force on strand 1 at maximum deflection, allowing more movement of member 4 for the same change in tension in strand 1. To further augment this effect, slide 16 can have end portion 7 sloped to ride over protrusion 8 on member 5. Spring 3 is further compressed, since the bottom portion is raised at the point of maximum deflection of strand 1.

When the preferred embodiment shown in FIGS. 1 and 2 is to be designed for a specific purpose, criteria can be applied as in the following example:

For use with tennis racquet strings, the device must be narrow enough to fit between cross-strings, or about half an inch. The maximum closing force at the long ends of the pivoting members should be about 5 pounds for closing between two fingers. The length should be under 4 inches to fit into the user's pocket.

Using 0.20 inches for half-width between pin 10 and edge 11, an angle A of 30.degree. causes 0.116" deflection at pin 10. Applying this deflection at a handle motion of 20.degree. maximum gives 1.27 inches travel at the handle end for an 11:1 ratio. If the longer portions of members 4 and 5 are taken at 3 1/2 inches, strand 1 should be located 0.32 inches ahead of pivot 2. A 5 pound maximum force at the handle end thus produces a 55 pound force at the strand.

Assuming a minimum tension of 20 pounds at 30.degree. deflection, the vertical force is:

vertical force = tension .times. 2 sin A = 20 .times. 2 (0.50) = 20 pounds

Since the vertical force is caused by a spring, it must vary linearly between 20 and 55 pounds.

The change of vertical force per inch becomes:

55 - 20 / 0.116 = 300 lbs./in.

Tension at other angles of deflection A are calculated by:

Tension = vertical force / 2 sin A

Thus at 20.degree. deflection, the vertical force is:

55 - 300 (0.20) 0.364 = 33 lbs.

and tension = 33/2(0.364) = 45 lbs.

If the compression spring is located at 1.28 inches behind the pivot, the vertical force becomes one-quarter as much at the spring. Maximum force on the spring is one-fourth of 55 lbs. or 14 lbs. The deflection at the spring is four times as great as at the strand, so the spring rate becomes one-sixteenth of the vertical force rate, or 19 lbs. per inch. After choosing 3/8 inches O.D., the proper spring is found to have 10 active coils of 0.047 inch diameter wire, calculated by standard spring design methods.

Location of the indicia scale is most easily done empirically. A typical strand is clamped at the top and a series of known weights hung from the bottom. With each weight, the indicator is placed around the strand and the apex of the angle between members marked on the lower member. It will be found that the apex travels faster for small angles A. The tension also changes more rapidly here, so the two effects tend to counteract each other, allowing more evenly spaced indicia.

Although the preferred embodiment is one in which the pivotal axis is parallel to the strand being measured, an axis perpendicular to the strand can also be used. The pivot can also be one of the strand contacting points. Instead of being pivotally mounted, the two members can also slide parallel to each other.

Although only a preferred embodiment has been shown and described, it is to be understood that the foregoing disclosure is given as an example only, rather than by way of limitation, and that without departing from the invention, the details may be varied within the scope of the appended claims.

Claims

I claim:

1. A device for insertion between the cross strings of a tennis racquet to determine the tension in a string of said tennis racquet comprising:

A first member adapted to support one side of a string under tension at two points, a second member including a string-engaging portion adapted to engage the opposite side of said string between said two points, indicia placed on at least one of said members in such a way that the tension in said string can be determined directly by reference to said indicia, a pivot connecting said first and second members at a point between said string-engaging portion and said indicia, a compression spring having two ends and acting between said members to cause bending of said string, and said first and second members having edges which cooperate to form an included angle of less than 20.degree. in the area of said indicia, the edge of one of said members acting as a reference point for interpreting the indicia on the other of said members.

2. The invention in claim 1, further including means for moving said compression spring in a direction away from said pivot as said edges of said first and second members are separated.

3. The invention in claim 2, wherein said means for moving said spring include a slide having two end portions, one of said end portions engaging one of said members and the other of said end portions engaging one of said ends of said compression spring in such a way that motion of said members about said pivot causes said one of said ends of said compression spring to move in a direction substantially perpendicular to its axis.

4. The invention in claim 3, further including a protrusion on the other of said members, said protrusion adapted to contact said other of said end portions of said slide in such a way that motion of said members about said pivot causes said one end of said spring to move in a direction along the axis of said spring.