Torque Limited Male Screw Base for Lampholder Fitting

This invention relates to lampholder fittings that are adapted to screw into the Edison base lampholder sockets commonly used for supporting and operating light bulbs in lighting fixtures and portable lamps, and more particularly to the male screw bases that are used to permit the use of fluorescent lampholder fittings and other electrical devices in ordinary incandescent lamp sockets.

There are a number of lampholder fittings currently available which are adapted to fit Edison base lampholders (commonly called female sockets) and to provide both mechanical support and electrical input power for various devices including fluorescent lamp systems. Since most of the devices are substantially larger than the incandescent light bulbs for which the lampholder was originally designed, there is a known safety hazard that may occur from over-torqueing the lampholder fitting into the lampholder. In such cases, the ability to grip a lampholder more firmly than is possible with an ordinary light bulb creates excessive torque to the degree that the lampholder is damaged. Further, an overtightened lampholder fitting tends to freeze in place to the extent that the lampholder may be damaged when the lampholder fitting is unscrewed.

The magnitude of the described problem is great enough that certain safety testing agencies have placed limitations on the type of device that can be installed in an Edison base lampholder.

The object of this invention is to provide a male screw base that cannot apply any excessive torque load on a lampholder, either during installation or removal of the lampholder fitting to which the screw base is attached.

The features of the preferred embodiments of the invention are illustrated in the drawings in which:

FIG. 1 is a side elevation view of the male screw base which is adapted for attachment to a lampholder fitting.

FIG. 2 is a cross-sectional view of the screw base of FIG. 1, taken along section line 2--2.

FIG. 3 is a cross-sectional view of the screw base of FIG. 2, taken along section line 3--3.

FIG. 4 is a side elevation view of the screw base of FIG. 1, shown under maximum torque condition within a lampholder.

FIG. 5 is a cross-sectional view of a three-way screw base adapted for use in a conventional three-way lampholder.

In FIG. 1 a male screw base 1 is shown having a generally cylindrical insulative body 2. A helical groove 3 extends along substantially the full length of the cylindrical body 2, and disposed within the helical groove 3 is a conductive helical spring 4. The groove 3 has a pitch to match the thread pitch of a female Edison base lampholder, so that the helical spring 4 disposed within the groove 3 will be adapted to engage the female threads of a lampholder. Groove 3 is of sufficient depth to receive substantially all of spring 4, and is of sufficient width to loosely restrain spring 4 in the axial direction. A first end 5 of spring 4 is bent inwards into a slot 6 in the body 2, and a second end 7 of spring 4 is also bent inwards into a slot 8 in body 2. In a normal free position spring end 5 is in contact with the end of slot 6 nearest the helical part of the spring, shown as position "A", and spring end 7 is also in contact with the end of slot 8 nearest the helical part of the spring, shown as position "B"; whereby in the free position spring 4 is limited to little or no rotational movement about the body 2. One end of body 2 is closed be a generally conical wall 9 in which a center contact 10 is coaxially disposed. The opposite end of body 2 is provided with a mounting means 11 containing a plurality of fastener holes 12 suitable for the use of fasteners to mount the male screw base to a lampholder fitting.

In FIG. 2 a cross-sectional view of the male screw base of FIG. 1 is shown taken along section line 2--2 and in which body 2 is shown having a hollow cavity 12. Center contact 10 has an insulated electrical conductor 13 connected thereto and extending through cavity 12 to provide input power to an attached lampholder fitting which is not shown in the interest of clarity. End 5 of spring 4 is shown protruding through slot 6 and into cavity 12, and terminating at an electrical terminal means 14 to which a second insulated conductor 15 is attached and extends through cavity 12 to provide a second power connection for a lampholder fitting.

In FIG. 3 a cross-sectional view of the male screw base is shown taken along section line 3--3 of FIG. 2. The depth of helical groove 3 may be seen in body 2, said depth being sufficient to permit spring 4 to substantially retract within the outer diameter of body 2 under high applied "wind-up" torque. End 5 of spring 4 is shown resting against the end of slot 6 at position "A" where end 5 is adapted to secure terminal 14.

In FIG. 4 the screw base 1 is shown being screwed into a female thread in an Edison base lampholder 16, shown in phantom. The screw base 1 has in the condition shown been subjected to high installation torque as contact 10 bears against the center contact 18 of lampholder 16. As the screw base 1 is torqued further clockwise, end 5 of spring 4 is pulled against the end of slot 6 at position "A" which causes the spring 4 to be wound up to a smaller diameter, pulling the outside diamater of spring 4 away from the female threads 17 of lampholder 16, causing thread disengagement and relief of the excessive torque without structural damage to lampholder 16. As the diameter of spring 4 is reduced, the trailing end 7 of spring 4 moves radially in slot 8 from the free position "B" to the overtorque position "C".

If the torque is applied in the counter-clockwise direction, body 2 first rotates without movement of spring 4. The first movement of the helical groove 3 on spring 4 produces an axial component of the radial movement which relieves the force against contact 18 by contact 10. Further radial movement in the counter-clockwise direction moves slot 8 so that its trailng edge rotates from position "B" to position "C", and slot 6 also moves so its leading edge is spaced away from position "A". Spring 4 is then wound up to a reduced diameter by the further movement of the trailing edge of slot 8 against end 7 of spring 4, while end 5 of spring 4 floats freely within slot 6.

In FIG. 5 another preferred embodiment of the male screw base 21 is shown in a cross-sectional view taken generally along the central axis, wherein an intermediate ring contact 18 supported on conical surface 10 of body 2. Ring contact 18 is so positioned as to engage an intermediate contact within a conventional three-way lampholder commonly used on portable lamps. Ring contact 18 is provided with an electrical attachment 20 to an insulated conductor 19 extending through cavity 12 of body 2, whereby in combination with conductor 13 from center conact 10 and conductor 15 from terminal 14 three way operation of an attached lampholder fitting is accomplished.