Abstract:
Example embodiments of the present invention relate to a torque-limiting tool which includes a central cam having a receiver component, at least one cam holder component surrounding the central cam, propelled by a driving member or a tool handle, wherein the cam holder component includes first portions that substantially complement an external surface of the central cam and at least two second portions providing space for rotation of the cam holder component relative to the central cam, and at least one connection controlling element around the cam holder component having a bending strength to enable the cam holder component positioned to interact with the movement of the cam holder component.

Description:
FIELD OF THE INVENTION 
   The present invention generally relates to tools and fastening devices. More particularly, the present invention pertains to a device for limiting an amount of torque applied by the device. 
   BACKGROUND OF THE INVENTION 
   In various manufacturing, construction, and medical industries, fasteners are utilized that are threaded or screwed into place. These fasteners may require a predetermined amount of torque that has been determined to be optimal for a given fastening situation. In addition, the fastener may identify or stipulate a predetermined amount of torque that has been determined to fatigue or break the fastener. Often, these predetermined torque values are determined by the manufacturer or by a testing facility. In use, a technician or user may employ a device such as a torque wrench to set the fastener according to the predetermined amount of torque. In a particular example, bone screws may be employed by surgeons to reconstruct bones or attach reconstructive components to bones of patients. In such circumstances, applying a proper amount of torque may be critically important. 
   Conventional torque wrenches utilize forces from coil springs and spring washers along with friction to limit the amount of torque applied. Unfortunately, as components within these conventional torque wrenches slide by one another, wear may alter the torque setting of the conventional torque wrench. As such, conventional torque wrenches often need to be re-calibrated to maintain their torque limit range, typically every six months. 
   In a typical example, conventional torque wrenches will utilize Belleville washers, which are slightly convex or domed, and flex under pressure. The precision of the force of these Belleville washers may be limited by their dimensional tolerances and their reported spring rates, which may vary from batch to batch. Because they may exhibit a high force value over a small travel distance, the use of many washers may be required to reach a travel distance necessary for the assembly to cycle. Thus, the washers may be stacked up along a rod and pressed together with a screw plunger. In some cases, the washers are alternatively flipped, back-to-back or front-to-front. The net result is a tool that may have many small, imprecisely-aligned pieces, wherein each part may wear and flex at different rates. 
   In many conventional or state of the art torque tools that can be classified as “screwdriver” type tools, i.e. a handle connected to a shaft, the torque-limiting cycle may be controlled by a “one way dog clutch”. This type of device is composed of two components, both aligned along a common axis. One component is stationary and the other is free to rotate about and slide along the axis. A series of radial ramps on each face align and lock the faces together. The rotation of the free component causes the two faces to alternately align and then slip over each other to the next alignment. During the rotation cycle, the faces separate by displacing a spring designed to apply a predetermined amount of torque resistance to the cam face engagement. 
   When the faces realign they do so rapidly with the force of the spring driving them together. This snapping together action is known to cause one or the other clutch component to split in half, thereby, breaking the clutch mechanism, additionally the spring washer stack is in constant compression and is continually pressed against a flat washer that in turn presses upon the dog clutch. 
   In the case of the wrench-style tool, the spring stack presses upon a plunger which, in turn, presses against one of the faces of a rotating cam. The plunger, in turn, pushes the cam into the opposing housing wall, thereby, creating the friction or resistance to rotation. 
   In axial handled torque-limiting tools, torque limiting occurs when the cam or clutch rotates by pushing the spring washers rearward a sufficient distance to allow rotation. The cam faces rub against each other with every rotation. When the cam rotates, the spring washers are compressed and the cam faces move apart due to the angular inclination of the ramped surfaces. Peak torque is reached when the cam faces are at their maximum separation. When the cam faces realign, they snap together into the next low torque position, and thus, no additional torque may be applied to the fastener. 
   A major disadvantage of conventional axial handled torque-limiting tools is that the spring washers are constantly under a compression load, even when the tool is at rest. Parts under load tend to fatigue more quickly than parts at rest; as a result, these types of tools require frequent recalibration, usually every six months. Furthermore, each snap of the cams coming together sends a shock wave down the tool shaft and into the fastener, thereby, transmitting an unnecessary force through the tool shaft to the fastener, this action, if of sufficient strength, could potentially damage the fastener. 
   In the conventional torque wrench type tool, a handle is attached to a tool head, which contains a hexagonal cam. In use, a tool shaft is inserted and locked into the center of the cam. The handle extending from the head contains a piston and spring washers or compression springs, which press the piston against one of the cam faces. The cam face is in turn pressed against the inner wall of the tool head. As with the axial handled torque-limiting tools, when the fastener begins to resist rotation and this resistive force exceeds the predetermined torque force of the springs, the shaft and cam may no longer be driven by the piston. Continued rotation of the handle may cause the piston to compress the springs, permitting the piston to slip over the cam peak to the next cam face. When the piston is located at the cam peak maximum torque is achieved and no further force may be applied to the fastener. With this design as well, the springs are under constant compression load leading to increased susceptibility to wear and breakage. The piston pressure causes the cam to rub and chafe against the opposite inner wall of the tool head during its rotation. Thus, heavy lubrication may be required. Also, the cam is not centered in the head and moves freely when the piston pressure is removed. 
   When conventional torque-limiting mechanisms fail they bind or lock-up, thereby losing the torque-limiting effect and essentially, converting the tool into a rigid, non-limiting tool, wherein the torque is regulated by the user&#39;s ability to discern torque forces by hand. This could lead to over-torqueing, which is an unsafe condition especially in the medical context. 
   Accordingly, it is desirable to provide a device that may be capable of overcoming at least to some extent the disadvantages of wear and breakage described herein. 
   SUMMARY OF THE INVENTION 
   The foregoing needs are met, to a great extent, by the present invention, wherein in one aspect an apparatus is disclosed providing a torque-limiting mechanism that uses a snap ring or spring coil to create a precise torque-resisting arrangement. 
   Example embodiments of the present invention relate to a torque-limiting tool which includes a central cam having a receiver component, at least one cam holder component surrounding the central cam, propelled by a driving member or a tool handle, wherein the cam holder component includes first portions that substantially complement an external surface of the central cam and at least two second portions providing for space for rotation of the cam holder component relative to the central cam, and at least one connection controlling element positioned to interact with the movement of the cam holder component. The central cam may exhibit a multifaceted or multi-lobed external configuration. In preferred embodiments of the present invention, the central cam may be a hex cam, cylindrical, triangular, or shaped as any polygon. Accordingly, the inner surfaces of the cam holder components may be substantially complementary in shape to the exterior shape of the central cam. The connection controlling element will be composed of a resilient, spring-like material to functionally limit the movement of the cam holder component. 
   The receiver component may be shaped to receive a connective end of a tool bit. In example embodiments of the present invention, the cam holder component may include a ring spreader bisected through the center of the central cam. In preferred embodiments of the present invention, bearings center the central cam in the base housing. The connection controlling element may include a snap ring or similar ring-type spring to control the torque of the tool shaft. Other example embodiments include a pivot pin about which the cam holder component pivots. 
   In example embodiments of the present invention, a torque-limiting tool may include a cam having a tool bit receiver, a pair of cam holder contained within a housing and driven by a tool handle, wherein the cam holder components interact with the cam, and at least one connection controlling element limiting the movement of the pair of cam holder components having a tensile strength equivalent to a predetermined torque limit and enabling the cam holder components to move into a cam expansion space. 
   In example embodiments of the present invention, a torque-limiting device for a tool may include a central cam element disposed about an axis of rotation and defining a receiving space for receiving a body, such as a tool bit body, a pair of cam holder components disposed around the central cam element within a housing. At least one connection controlling element may be disposed around the pair of cam holder components to bias the pair of cam holder components about the central cam element to enable rotation of the central cam element. The pair of cam holder components may be disposed within the housing to spread and rotate around the central cam element in response to an external torque applied to the central cam element by the tool shaft in response to the resistance of the fastener when said resistance exceeds a predetermined torque limit. 
   There has thus been outlined, rather broadly, certain embodiments of the invention in order that the detailed description thereof herein may be better understood, and in order that the present contribution to the art may be better appreciated. There are, of course, additional embodiments of the invention that will be described below and which will form the subject matter of the claims appended hereto. 
   In this respect, before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of embodiments in addition to those described and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein, as well as the abstract, are for the purpose of description and should not be regarded as limiting. 
   As such, those skilled in the art will appreciate that the conception upon which this disclosure is based may readily be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a view of the torque-limiting device with the handle, according to an embodiment of the invention. 
       FIG. 2  is a sectional view of a split, expandable chuck assembly within the torque-limiting device of  FIG. 1 . 
       FIG. 3  is a partial sectional view of the split, expandable chuck assembly within the torque-limiting device of  FIG. 1 . 
       FIG. 4  is an angled side view of the split, expandable chuck assembly without the housing and handle of the torque-limiting device. 
       FIG. 5  is an angled side view of the split, expandable chuck assembly dismantled and without the cover and handle of the torque-limiting device. 
       FIG. 6  is an exploded view of the individual components of a torque-limiting device, according to another embodiment of the invention. 
       FIG. 7  is a frontal view of the tool bit receiver end of the central cam and the cam holders disposed about a pivot pin. 
   

   DETAILED DESCRIPTION 
   The invention will now be described with reference to the drawing figures, in which like reference numerals refer to like parts throughout. An embodiment in accordance with the present invention provides a mechanism for limiting an amount of torque applied by a device using a torque-limiting chuck assembly. Example embodiments of the present invention include a snap ring or similar ring-type spring to control torque. Example embodiments of the present invention potentially allow for a less expensive, simpler assembly of a torque-limiting wrench having fewer parts to assemble than the conventional torque-limiting tools, which typically require many compressed spring washers. Example embodiments of the present invention provide a torque-limiting device that may be less susceptible to wear or breakage because the mechanism does not cause its components to be constantly under compression load. In one example embodiment, the torque-limiting device includes two bearings to center the cam in the wrench head. 
   Turning now to the figures,  FIG. 1  shows a view of the torque-limiting device with the handle according to an embodiment of the invention. In example embodiments of the present invention, a torque wrench or torque-limiting chuck assembly  101  includes a receiver component or a tool bit receiver  110 , which may accept and retain a tool bit (not shown) in its receiver section. The end of the tool bit conforms to the geometry of the tool bit receiver  110 , which is the central cam opening. The tool bit is received therein. Typically, a spring loaded detent retains the tool bit in the cam opening  110  in a manner similar to current socket wrench products in the market today. A tool handle  105 , which is the driving member, is connected to the torque-limiting chuck assembly  101  and is used to rotate the torque-limiting chuck assembly  101  and a retained tool bit (not shown) in a prescribed direction. 
     FIG. 2  is a sectional view of a split, expandable chuck assembly within the torque-limiting device of  FIG. 1 . In one of several example embodiments of the present invention, a central cam  215  with a hexagonal shaped external configuration and a hexagonal shaped lumen or center opening designed to be a tool bit receiver  210 , is centered in the torque-limiting chuck assembly  201 . The hex cam  215  describes a form with a hexagonal exterior configuration that is in mating contact with at least two ring spreading cam holders  220 , individually  220 A and  220 B, which enclose the external sides of the hex cam  215 . The inner surfaces of the cam holders  220  are substantially complementary to the outer surfaces of the hex cam  215  sufficient to retain the hex cam  215  in a holding position but are not complementary where at least two radial spaces  225  exist between the cam holders  220  and the hex cam  215 . In an embodiment of the present invention, the at least two radial spaces  225  permit the hex cam  215  to rotate in the direction “C” by forcing the cam holders  220  to spread along the arrows “A” and “B”, as shown in  FIG. 2 . The space created when the cam holders  220  are pushed apart is the cam holder expansion space  230 . In a preferred embodiment of the present invention, as shown in  FIG. 2 , two radial spaces  225  exist between each cam holder  220  and the hex cam  215 . It is understood that the cam holders  220  could function together or have a structure encompassing a single cam holder element having at least one opening point, wherein the single cam holder element would enclose the hex cam  215 , disposed within the housing to disengage from the central cam  215  upon rotation of the cam holders  220  relative to the central cam element  215  in response to an external torque applied to the tool bit exceeding a predetermined torque-limit. 
     FIG. 3  is a partial sectional view of the split, expandable chuck assembly within the torque-limiting device of  FIG. 1 . In example embodiments of the present invention, at least one connection controlling element  235  holds the cam holders  220  around the hex cam  215 , limiting the ability of the cam holders  220  to expand and rotate around the hex cam  215  and its central rotational axis “X”. As used herein, “connection controlling element” can be a snap ring, a garter spring, or a coil spring, or similar spring-like device. In the embodiment shown in FIG.  3 , they are disclosed as snap rings  235 . As the torque applied at the tool bit receiver  210  increases, the hex cam  215  forces the cam holders  220  apart and stretches the snap rings  235 . When this occurs, the cam holders  220  are free to expand and separate and thereby rotate around the hex cam  215 . The force to open a snap ring  235 , is regulated to the desired torque limit. If the external torque manifested by the fastener remains below the proscribed torque limit, the cam holders  220  will not separate. By varying the size or stiffness of snap ring  235 , the applied torque may be modulated. The snap rings  235  remain unflexed or unstressed when the tool is not in use because force is exerted upon the snap rings only when the cam holders  220  expand due to an applied torque from the central cam. Bearings  240  may be used to center the hex cam  215  in the housing, which is comprised of a base  250  and a cover  245 . 
     FIG. 4  is an angled side view of the split, expandable chuck assembly without the housing and handle of the torque-limiting device. A multifaceted relationship between the cam holders  220  and the hex cam  215  offers a start/stop cycle to control torque. The hex cam  215  has a tool bit receiver  210 , which is a rotating member that can be coupled to a tool bit shaft. A snap ring  235  maintains a controllable connection between the chuck assembly and said tool bit shaft. The snap rings  235  rotationally couple the chuck assembly to the tool bit shaft up to a predetermined point of applied external torque. Beyond that point, the chuck assembly no longer drives the shaft. The torque-limit will be met and reset several times per handle/chuck assembly rotation depending on the number of holding engagements designed into the cam and can holder relationship. The difference in applied torque is sufficiently pronounced to clearly indicate the change to a user. Once the chuck assembly slips to the next position on the central cam the user knows that the predetermined torque limit has been reached and any further rotation will not increase the torque. The mutual configurational relationship between the cam holders  220  and the hex cam  215  must not only allow the holders to engage and drive the central cam but the relationship must also provide sufficient opening clearance to permit the cam holders  220  to rotate relative to the cam  215  when the torque limit is reached. The hex cam  215  always remains in a fixed position, relative to the fastener. 
     FIG. 5  is an angled side view of the split, expandable chuck assembly dismantled and without the housing and handle of the torque-limiting device of  FIG. 2 . The cam holders  220  enclosing the hex cam  215  form a substantially hexagonal enclosure, formed such that four of the inner surfaces are complementary to the hexagonal portion of the hex cam and four smaller surfaces have a curved radial edge  255 . The radial edge  255  allows for a radial space  225 , which provides room for rotation of the cam holder(s) relative to the central cam, as discussed with regard to  FIG. 2 . 
     FIG. 6  is an exploded view of the individual components of another embodiment of the torque-limiting device of the present invention. A hex cam  615  may have cylindrical portions  615 A that connect to bearings  640 . The hexagonal portion  615 B of the hex cam  615  is enclosed by the cam holders  620 , individually  620 A and  620 B. In example embodiments of the present invention, one snap ring  635  would surround the cam holders  620 . In a preferred embodiment of the present invention, the cam holder  620  can have round exterior surfaces. Because the outer surfaces of the cam holders  620  are curved, in this example embodiment, the base  650  and cover  645  are rounded to conform to the cam holders  620 . 
     FIG. 7  is a frontal view of another embodiment of the present invention showing the tool bit receiver end of a split, expandable chuck, which includes a pivot pin. In an example embodiment, two cam holders  720 , individually  720 A and  720 B, may have curved outer surfaces and inner surfaces forming a substantially hexagonal enclosure around the hex shaped cam  715 . The cam holders  720  may interface at a common opening point  770  such that when the two cam holders  720  open, they each pivot around a commonly attached pivot pin  760 . In a preferred embodiment, the cam holders  720  are notched, to engage the pivot pin  760 . The pivot pin  760  may have a narrow portion  760 B to engage a tool cover (not shown) and a cylindrical portion  760   a  to receive the cam holders and maintain the pivot pin  760  within a base. A rounded base (not shown) having a pin pocket may be used to accommodate the pivot pin  760 . The rounded base additionally may have base guide slots to accommodate cam holder guide pins  765 . 
   In other example embodiments, the cam may alternatively be a cylindrical cam surrounded by cam holders having curved surfaces where they contact the cylindrical cam so that the motion of the cam holders around the cylindrical cam is a smooth rotation and torque limiting is controlled by friction rather than geometric configuration. Accordingly, the cam holder(s) may be split with a single separation or segmented with several separations. The clamped or released distance may be very short, as small as a few thousands of an inch. The force limit attainment may only be signaled by a slight change in rotational force. Similarly, a cam having a square, triangular or even-sided polygonal shape may be substituted for the hex cam, as long as a cam holder having a complementary inner shape is used and a space for cam rotation is provided. If triangular cam and cam holders are used, the cam holder may need to be trisected at the three apexes of the triangle. In this embodiment, the clamped or released distance would be the difference between the triangle center to apex distance minus the center to base distance. A square cam may be used if the cam holder is at least bisected or quartered. An even-sided polygonal cam may be used if the cam holder is at least bisected through the center at corners of the polygonal cam. 
   The many features and advantages of the invention are apparent from the detailed specification, and thus, it is intended by the appended claims to cover all such features and advantages of the invention which fall within the true spirit and scope of the invention. Further, since numerous modifications and variations will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation illustrated and described, and accordingly, all suitable modifications and equivalents may be resorted to, falling within the scope of the invention.