the present invention provides a semi automatic torque limiting device and method . in one example of a preferred embodiment , the torque limiting device includes a disposable , torque limiting , insert . the disposable insert includes a predetermined number of segments . during use , a segment of the insert is sheared off in response to a predetermined amount of torque being applied to a handle of the torque limiting device . the sheared segment may be stored in the handle . in various embodiments , the torque limiting device may include any suitable material , such as , plastic , polymer , resin , metal , or the like . in a preferred example , the torque limiting device includes materials that may be sterilized for use in an operating theater . for example , autoclaveable polymers and corrosion resistant - type metals may be utilized to fabricated the torque limiting device and / or the insert . the insert may include a rod or bar with a number of annular grooves that correspond to the number of segments . material characteristics of the insert and the cross sectional area at the groove may be utilized to determine a torque value for the torque limiting device . it is an advantage of embodiments of the invention that the torque limiting device substantially and / or continually maintains a pre - set torque limit that is governed by the insert . it is another advantage that this pre - set torque limit is essentially unaffected by frictional wear . as such , the need for testing and recalibration of the torque limiting device may be eliminated . preferred embodiments of the invention will now be described with reference to the drawing figures , in which like reference numerals refer to like parts throughout . as shown in fig1 , a torque limiting device 10 may be utilized to install a fastener 12 in a substrate 14 . the torque limiting device 10 includes a handle 16 for the user , technician , or surgeon to grasp . the torque limiting device 10 further includes a shaft 18 connecting the handle 16 to a head 20 . the head 20 is configured to engage the fastener 12 . in various embodiments , the head 20 and shaft 18 may be essentially a single component or the head 20 may be releasably attached to the shaft 18 to facilitate exchanging the head 20 with another head 20 . in general , the torque limiting device 10 is configured such that in response to turning the handle 16 about an axis a ( as shown in fig1 ), an applied torque will urge the shaft 18 to rotate in a similar fashion . the torque limiting device 10 is configured such that at a threshold or max torque , the shaft 18 ceases to rotate about the axis a in response to rotating the handle 16 about the axis a . in conventional devices , the limit at this threshold torque is affected by frictional forces of one member sliding against another . unfortunately , this friction results in mechanical wear that may alter the characteristics of the torque applied through the device . it is an advantage of various embodiments that friction is a substantially negligible component of the applied torque . as such , the embodiments are essentially unaffected by frictional or mechanical wear . in addition , while not explicitly shown in fig1 , the torque limiting device 10 may include a display to display the amount of torque being applied to the fastener 12 . furthermore , the torque limiting device 10 may include a ratcheting mechanism to provide unidirectional or selectable unidirectional rotation of the shaft 18 . this is an optional feature so that the operator does not have to change or reset the hand position on the tool . fig2 is an exploded view of the torque limiting device of fig1 . on a driver - end of the device 10 there is the shear component 22 . on the proximal end of the shear component 22 there are two holes : a center hole 24 and a shearing hole 26 a . the center hole 24 receives a shoulder screw 28 that connects the rotating handle 16 to the driver - end 18 . the shearing hole 26 a carries a spring - loaded ejector pin 30 and a spring 32 whose travel is limited by the mating of an axial slot 34 in the ejector pin 30 and a limit pin 36 . the handle 16 includes a plurality of holes or bores that may be selectively aligned with the shearing hole 26 a . for example , as shown in fig2 , the handle 16 includes an insert loading bore 26 b and a segment collecting bore 26 c . the insert loading bore 26 b may further include a bushing 26 bb to facilitate shearing the torque limiting insert 40 . in addition , the torque limiting device 10 includes one or more torque limiting disposable inserts or inserts 40 , springs 42 to advance the inserts 40 and a cap 44 to retain the inserts 40 and a spring assembly 46 . fig3 a is a side view of the disposable insert 40 suitable for use with the torque limiting device 10 . the inserts may be made from any suitable material . suitable materials include plastic or polymers , metals , and the like . in a particular example , the inserts include a segmented plastic length of material . as shown in fig3 a , the insert 40 includes segments 50 a to 50 n that are delineated by annular grooves 52 a to 52 n . the annular grooves 52 a to 52 n serve to concentrate shear stress between adjacent segments 50 a to 50 n . in this manner , the annular grooves 52 a to 52 n facilitate a clean and / or repeatable shearing action . because the shearing hole 26 a is rotationally offset from the central axis a , torque applied to the handle 16 generates a corresponding force at the shearing hole 26 a and a shearing force upon the torque limiting insert 40 . in particular , the depth of the shearing hole 26 a and length of the segments 50 a to 50 n are matched such that the shearing force is exerted , each in its turn , upon the annular grooves 52 a to 52 n . the cross sectional area and material characteristics of the torque limiting insert 40 at the annular grooves 52 a to 52 n is configure to shear in response to a predetermined amount of shear force . the amount of shear force each of the annular grooves 52 a to 52 n can absorb before being sheared or severed is a “ shear limit .” the shear limit is defined as the amount of shear stress or strain sufficient to result in the partial or complete fracture or breaking of the torque limiting insert 40 . one or more of the segments 52 a - 52 n may be sheared in the torque limiting device 10 or reasonable facsimile thereof to test or calibrate the insert 40 . the insert 40 further includes a base or flanged insert portion such as a flange 54 shown in fig3 a . in various embodiments , the shear limit of the torque limiting insert 40 may include any suitable value . for example , various standard fasteners may have a manufacturer &# 39 ; s recommended installation torque of less than 1 newton meter ( nm ) to greater than 10 nm . accordingly , the torque limiting insert 40 may include a shear limit of 1 nm , 2 nm , 4 nm , 6 nm , and the like . to differentiate the torque limiting insert 40 having one shear limit from another torque limiting insert 40 having another shear limit , the torque limiting insert 40 may include an indicator value , differentiating color , size , shape , or the like . in a particular example , the torque limiting insert 40 may include a particular shape for each shear limit . that is , while the torque limiting insert 40 shown in fig3 a is depicted as generally cylindrical , the torque limiting insert 40 having a different shear limit may be , for example , an elongated 3 , 4 , or multi - sided prism . according to an embodiment , the insert loading hole 26 b and / or bushing 26 bb may be configured to accept only a certain shaped torque limiting insert 40 . fig3 b is a side view of the disposable insert following a calibration procedure . once the testing or calibration is performed a “ working ” length segmented insert 40 is left with three torque - limiting segments 50 c to 50 n remaining . however , in other embodiments , more or less than three segments 50 a - 50 n may be retained . fig3 c is an exploded view illustrating insertion of the disposable insert 40 into the torque limiting device 10 . loading the torque limiting device 10 is simple and a thrust bearing 56 , for example , may be included on the backside of the spring assembly 46 to reduce assembly friction . fig4 a is a top view of the torque limiting device 10 in a loaded position . the handle 16 is rotated to a “ load ” position . at this point an insert 40 will be aligned with the eject pin 30 . fig4 b is a cross - sectional view a - a of the torque limiting device 10 in the loaded position . in the transparent view above , the loaded positions are shown . the handle 16 is turned till one of the insert 40 , hereafter referred to as the “ first insert 40 ,” is loaded into the hole 26 in the shear component 22 . there are springs 42 and 32 behind the first insert 40 and the ejector pin 30 . the larger spring 42 behind the first insert 40 has a higher spring rate than the ejector pin spring 32 , thus over - powering the ejector spring 32 and the first insert 40 advances the ejector pin 30 until its movement is limited by the pin - slot combination . when the first insert 40 is fully advanced to the stop the insert 40 is correctly positioned for the “ torque - limiting ” step . fig5 a is a top view of the torque limiting device 10 in a shear position . when the first insert 40 shears at the pre - determined torque value , the handle 16 will rotate relative to the shear component 22 to an in - between position between “ load ” and “ eject ”. fig5 b is a cross - sectional view b - b of the torque limiting device 10 in the shear position . in the above view the front segment 50 c of the first insert 40 is shown sheared and retained inside the shearing hole 26 a of the shear component 22 . because there is only the one shearing hole 26 a on the shear component 22 that is on the same bolt circle as the first insert 40 , the first insert 40 will not advance to the next position until the shearing hole 26 a is emptied . fig5 c is an end view of the torque limiting device in the shear position . as shown in fig5 c , the section line c - c is disposed at an angle to pass through the loading bores 26 b . fig5 d is a cross - sectional view c - c of the torque limiting device 10 in the shear position . as shown in fig5 d , the cross - sectional view c - c transects the loading bore 26 b and illustrates how both the first insert 40 and a second insert 40 are essentially prevented from advancing while the shear component 22 is positioned between “ load ” and “ eject ” position . fig6 a is a top view of the torque limiting device 10 in an eject position . as shown in fig6 a , the handle 16 is rotated into an “ eject ” position . fig6 b is a cross - sectional view c - c of the torque limiting device 10 in the eject position . as shown in the cross - sectional view , the segment 50 c of the first insert 40 that has been sheared off will now advance into ejection chamber of the handle 16 . because of the limit slot in the ejector pin 30 it will not advance into the ejection chamber . a cleaning port or clearing port 58 is optionally included to augment ejection . for example , if included , a wire or pin may be inserted to augment the action of the spring 32 . fig6 c is an end view of the torque limiting device 10 in the eject position . as shown in fig6 c , the section line e - e is disposed at an angle to pass through the loading bores 26 b . fig6 d is a cross - sectional view e - e of the torque limiting device 10 in the eject position . as shown in fig6 d , the cross - sectional view e - e transects the loading bore 26 b and illustrates how both the first insert 40 and second insert 40 are essentially prevented from advancing while the shear component 22 is in the eject position . fig7 a is a top view of the torque limiting device 10 in a second loaded position . the handle 16 is now rotated to the next “ load ” position . fig7 b is a cross - sectional view f - f of the torque limiting device 10 in the second loaded position . as can be seen in the cross - sectional view , the first segment 50 c of a second insert 40 advances into the shear component 22 , pushing the ejector pin 30 until it stops . note that the first segment 50 c will not advance until it aligns with the hole 26 a in the shear component 22 . fig8 a is a top view of the torque limiting device 10 in a second shear position . when the second insert 40 shears at the pre - determined torque value , the handle 16 will be in an in - between position of “ load ” and “ eject ”. fig8 b is a cross - sectional view g - g of the torque limiting device 10 in the second shear position . in the above view the front segment 50 c of the second insert 40 is shown sheared and retained inside the hole 26 a of the shear component 22 . because there is only the one hole on the shear component 22 that is on the same bolt circle as the second insert 40 , the second insert 40 will not advance to the next position . fig8 c is an end view of the torque limiting device 10 in the eject position . as shown in fig8 c , the section line h - h is disposed at an angle to pass through the loading bores 26 b . fig8 d is a cross - sectional view h - h of the torque limiting device 10 in the eject position . as shown in fig8 d , the cross - sectional view h - h transects the loading bore 26 b and illustrates how both the first insert 40 and second insert 40 are essentially prevented from advancing while the shear component 22 is between the load eject positions . fig9 a is a top view of the torque limiting device 10 in a second eject position . in response to rotating the handle 16 the torque limiting device 10 advances to an “ eject ” position . fig9 b is a cross - sectional view i - i of the torque limiting device 10 in the second eject position . as shown in the transparent view above , the segment 50 c of the second insert 40 that has been sheared off will now advance into ejection chamber of the handle 16 . because of the limit slot 34 in the ejector pin 30 it will not advance into the ejection chamber . note that there are now two separate insert 40 ( e . g ., the first and second insert 40 ) segments in two separate ejection chambers . fig9 c is an end view of the torque limiting device 10 in the second eject position . as shown in fig9 c , the section line j - j is disposed at an angle to pass through the loading bores 26 b . fig9 d is a cross - sectional view j - j of the torque limiting device 10 in the second eject position . as shown in fig9 d , the cross - sectional view j - j transects the loading bore 26 b and illustrates how both the first insert 40 and second insert 40 are essentially prevented from advancing while the shear component 22 is in the eject position . fig1 a is a cross - sectional view of the torque limiting device 10 in a final eject position . as shown in fig1 a , the segments 50 c - 50 e are retained within the collecting bores 26 c fig1 b is a cross - sectional top view of the torque limiting device 10 in the final eject position . as shown in fig1 b , the flange 54 is configured to engage a lip or otherwise essentially prevent the insert 40 from progressing further than the final segment . fig1 c is a cross - sectional top view of the torque limiting device 10 in the final eject position according to another embodiment . as you can see from the cross - sectional views above , the torque limiting device 10 according to this particular embodiment is capable of six torque - limiting uses . in other embodiments , the torque limiting device 10 and / or the insert 40 may be configured for more or fewer torque - limiting uses . once the segments 50 c - 50 n are all sheared off the flange 54 will prevent the insert 40 from advancing further . at this point the device 10 will only spin . the device 10 is also designed to hold all six segments 50 a - 50 n . fig1 is an exploded view illustrating removal of the used insert 40 from the torque limiting device 10 of fig1 . fig1 is a flow diagram illustrating a method 60 of installing a fastener according to an embodiment of the invention . as shown in fig1 , the method 60 may be initiated at step 62 in response to installing a torque limiting insert such as the insert 40 in the torque limiting device 10 . to initiate installation , loading sleeves or bores may be accessed . for example , the retaining cap 44 may be unscrewed or otherwise removed along with the spring 42 and spring assembly 46 to expose the loading bore 26 b and the collecting bore 26 c . thereafter , the insert 40 may be positioned in the loading bore 26 b and the torque limiting device reassembled . of note , the insert 40 installed in the torque limiting device 10 may be selected based upon a predetermined torque limit for a particular fastener that is to be installed . for example , a fastener such as a bone screw may have a recommended installation torque of 4 newton meters ( nm ). the insert 40 selected for installation may include a predetermined shear strength that corresponds to 4 nm of torque . at step 64 , the fastener 12 may be installed in the substrate 14 . for example , a bone screw may be screwed into the bone of a patient . more particularly , the fastener 12 may be coupled to the torque limiting device 10 by a bit and the handle 16 may be rotated until the insert 40 is sheared as shown in fig5 a and 5b . at step 66 , a severed segment may be ejected from the head 20 . for example , as shown in fig6 a and 6b , the shearing hole 26 a may be aligned with the collecting bore 26 c . in response , the segment 50 c , for example , may be urged by the spring loaded ejector pin 30 and spring 32 into the collecting bore 26 c . at step 68 , it may be determined if additional fasteners are to be installed . for example , if an ongoing operational procedure indicates addition fasteners may be utilized , it may be determined if the additional fasteners include essentially the same torque indication at step 70 . if no additional fastener installation is indicated , the torque limiting device 10 may be cleaned at step 78 . at step 70 , it may be determined if essentially the same torque limit is indicated . for example , essentially the same torque limit may be indicated in situations utilizing the same type of bone screw being installed into similar boney tissue . if it is determined that essentially the same torque limit is indicated , it may be determined if additional segments 50 c - 50 n remain attached to the insert 40 . if it is determined that a different torque limit is indicated , the insert 40 may be exchanged at step 74 . at step 72 , it may be determined if more segments remain . for example , if upon rotating the handle 16 relative to the head 20 one full rotation , no segment 50 c - 50 n is loaded into the shearing hole 26 a , then it may be determined that no segments remain and the insert 40 may be replaced at step 74 . if segments 50 c - 50 n do remain , a next segment may be loaded if not already done so at step 76 . at step 74 , the insert 40 may be replaced . for example , the retaining cap 44 may be unscrewed or otherwise removed along with the spring 42 and spring assembly 46 to expose the loading bore 26 b and the collecting bore 26 c . as shown in fig1 , the insert 40 and any spent segments 50 c - 50 n may be removed via the assistance of gravity by tilting the torque limiting 10 . thereafter , the insert 40 having a torque limit corresponding to a next fastener to be installed in the loading bore 26 b as shown in fig3 c and the torque limiting device reassembled . at step 76 , a next segment 50 c - 50 n may be loaded . for example , if a segment has not already been loaded , the head 20 may be rotated relative to the handle 16 until the load position is obtained as shown in fig7 a and 7b . following loading , a fastener may be installed at step 64 . at step 78 , the torque limiting device 10 may be cleaned . for example , the torque limiting device 10 may be disassembled as shown in fig1 and the torque limiting device 10 may be washed . once emptied of inserts 40 , the torque limiting device may be reassembled and / or sterilized . in a particular example , the torque limiting device 10 may be autoclaved . 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 .
in an apparatus that includes a head , spindle , and a handle , the handle is rotatably fastened to the head by the spindle . the handle and head mate at an interface . the apparatus further includes a head axial bore offset from the spindle and configured to occupy a portion of the head and a handle axial bore offset from the spindle and configured to occupy a portion of the handle , the handle axial bore being configured to receive a torque limiting insert , wherein in response to the head being at a first rotational conformation relative to the handle , the head axial bore and handle axial bore is disposed in cooperative alignment to allow the torque limiting insert to transect the interface and enter the head axial bore . the head and the handle are rotationally coupled via the torque limiting insert . the head and the handle are rotationally uncoupled in response to a shear force being applied across the interface that exceeds a shear limit for the torque limiting insert .