Abstract:
A vibration damping nock for crossbow arrows includes an insert to absorb bow string slap, thereby to prevent damage to the nock during crossbow firing.

Description:
FIELD OF THE INVENTION 
     This invention relates to nock constructions for use with crossbows and more particularly to a vibration damping insert for reinforced nocks to absorb bow string slap. 
     BACKGROUND OF THE INVENTION 
     As shown in U.S. Patents Applications 61/748,526 filed Jan. 3, 2003; 61/621,221 filed Apr. 6, 2012 and Ser. No. 13/785,862 filed Mar. 5, 2013 nocks usable with cross bows have been reinforced utilizing a metal support structure which surrounds a portion of a nock and a portion of the cross bow bolt to attempt to prevent fracture of the nock when the bolt is fired from the cross bow. It is noted that all of these patent applications are incorporated in their entirety by reference. 
     Whether the cross bow nock is lighted or unlighted in general cross bows have a significant safety problem in that cross bows are designed such that the string has some slight separation from the projectile prior to firing of the projectile upon release of the bow string. From a physics perspective the string travels forward and actually impacts or slaps the nock rather than pushing on the nock. 
     Nocks in general are plastic and existing plastic nock systems are problematic if the nock breaks. This can result in what is called a dry fire with the string moving forward without pushing on the projectile because the nock has broken or fractured. The result is that the string slides over the projectile. When this happens there is nothing to absorb all of the stored energy. Thus when the string is released all of the energy reverberates back into the bow which can cause damage to the bow itself. 
     As will be appreciated, in a dry fire situation in which the nock is fractured the energy is not put into the projectile but rather is put back into the bow where it can actually cause portions of the bow to break and detach, becoming a serious safety problem for the hunter or archer. 
     Metal nocks are known in the industry, although not used as commonly as plastic nocks. However, the metal nocks are solid and have no ability to be lighted. Lighting of nocks has proven to be a valuable means for the hunter or archer to easily track the trajectory of the projectile to correct shooting errors, and to locate the projectile after shooting. Additionally, the solid metal nocks do not have the ability to reduce the impact from the bow string, and can therefore cause unwanted vibration in the crossbow. 
     As a result and for cross bows in particular there is a significant need to be able to provide a plastic nock that is reinforced with either metal, a ceramic or an advanced composite that has the structural strength and ability to absorb the impact of the bow string. As mentioned above there are metal support structures that cooperate with the plastic nocks that to a certain extent limit the fracture or damage of the nock during cross bow firing. It will be appreciated that the amount of stress produced in the nock from the energy in the crossbow is over 7,000 psi. 
     Should the nock break or fracture not only is the bow string released with no retarding force such as would be associated with the bolt or projectile, the arrow itself can fly off at any angle thus potentially causing injury to the hunter or those nearby. 
     It is therefore important to be able to provide a nock structure capable of withstanding tremendous forces associated with the release of a crossbow string, the need being both for unlighted nocks and lighted nocks alike. 
     It will be appreciated that lighted nocks are activated when the bow string presses on a plunger which in turn presses on an internal light emitting diode assembly to close a switch between the light emitting diode and a battery pack contained within the bolt or arrow shaft. When the bow string is released the plunger is pushed in and the internal light is activated to provide a lighted nock that is used by the hunter to trace the path of the arrow and also to be able to find the arrow if it has missed its target. This in turn permits retrieval of the arrow for a missed shot. 
     In the case of lighted nocks a clear plastic is utilized for the nock construction so that light that is generated internal to the bolt or arrow shaft is radiated out from the lighted nock. It is therefore important to provide a lighted nock which is capable of sustaining the tremendous forces associated with the release of a crossbow bow string. 
     Not only is a fracture resistant nock important for lighted nocks it is likewise important for unlighted nocks. In addition to the reasons stated above, it is beneficial to have a shock absorbing elastomeric material as part of the construction of any nock, lighted or unlighted, to reduce vibration in the crossbow and bolt. 
     SUMMARY OF INVENTION 
     In order to prevent fracture of a nock, lighted or not, in the subject invention the distal portion of the nock is provided with a shock absorber insert that in essence absorbs the impact forces so that the nock will not shatter due to the slap of the bow string against the nock. An additional benefit of the system is the overall reduction in vibration in the system which tends to increase accuracy, reduce noise and improve overall shooting enjoyment from a smoother feel to the shooter. 
     In a preferred embodiment the nock is encased in the aforementioned metal support structure. However the distal end of the nock is provided with the shock absorbing material, in one case TPU or thermo polymer urethane or thermo plastic urethane as it is sometimes called. In one embodiment, the TPU shock absorber is injection molded into an aluminum housing and absorbs the impact to prevent the nock from breaking or shattering during firing, especially when there is a space between the bow string and the distal end of the nock causing a high impact slap against the nock that otherwise might cause the nock to fracture. 
     The preferred material for the shock absorber at the distal end of the nock is clear TPU. From a structural perspective the TPU allows some resilience and therefore vibration damping. As a result the slap from the string will be damped. It is noted that urethane has extremely good impact absorption characteristics, and is a material commonly used for skate wheels. It also has good absorption resistance as well as good impact absorption characteristics Since the TPU is preferably clear, it allows a lighted nock to not only have the structural benefits from this insert but will also allow a light from a light assembly to exit to the rear of the bolt or arrow shaft when a battery and LED assembly is located at the proximal portion of the TPU insert. 
     Moreover, when the TPU insert is impacted by the bow string it moves slightly forward in the structural housing such that rather than having to utilize a plunger or pin to push the LED light emitting unit forward to make switch contact, the TPU insert itself forms a plunger like function that moves upon impact to push the end of a dome-shaped LED forward in the bolt or arrow shaft, whereupon traditional switch contact is made to illuminate the LED. 
     It is preferable to use injection moldable urethane as opposed to a castable urethane or a two part urethane. This is important because injection moldable TPU urethanes are stronger and more impact resistant than castable urethanes. Note first and foremost TPU must have the requisite strength. Secondly, it must have resilience or ability to absorb energy without permanent deformation. Thirdly, it must have good spring back characteristics after it has been pushed out of its shape so that it will spring back to its original shape without permanent deformation. Fourthly, it must have good vibration damping and the requisite have toughness as well as abrasion resistance. The above characteristics are best embodied in the TPU material which allows one to build the insert as a mechanical button comprising a molded piece of clear urethane. As the string moves forward it pushes the clear TPU forward to close a switch in the lighted nock assembly. 
     Note that there are a few alternate materials to TPU, but if so, they must be optically as clear as possible and must transmit a large portion of the light out the distal end of the nock. Other exemplary materials that could be used would be commonly referred to as thermo plastic elastomers (TPEs) or simply rubber materials. While rubber could not be used in a lighted nock, it would be sufficient in an unlighted application. 
     The TPU insert in the distal end of the nock may either have a notch or half-moon configuration to control the string motion appropriately to keep it from slipping off the back of the projectile. In another embodiment the TPU insert may be a flat disk button which is contacted by the bow string. 
     In summary, a shock absorbing insert is placed at the distal end of a nock, lighted or not, in which the insert serves as a shock absorber to prevent fracture or damage to the nock during crossbow firing, thus to eliminate safety problems associated with crossbow string slap. An additional benefit is the overall reduction in vibration throughout the crossbow and projectile system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features of the subject invention will be better understood in connection with the Detailed Description, in conjunction with the Drawings, of which: 
         FIG. 1  is a diagrammatic illustration of a crossbow showing the separation between the bow string and the end of a typical nock at the distal end of a bolt, also showing the result of fracturing the nock during firing causing the bow string to be unloaded, also causing the arrow to move out of the cross bow chamber in an uncontrolled fashion; 
         FIG. 2  is a diagrammatic illustration showing the spacing of a crossbow bow string from the distal end of the nock, showing the spacing over which bow string slap is operative; 
         FIG. 3  is a diagrammatic illustration of a dry fire situation in which the unloaded bow string moves in a forward direction, causing the arms of the crossbow to snap or otherwise be damaged; 
         FIG. 4  is a diagrammatic illustration of the TPU shock absorber insert into a metal support structure which shows the motion of the TPU insert forward against an illumination source connected to a battery within the bolt or arrow shaft to activate the illumination source for providing an illuminated nock while at the same time absorbing the high loads due to bow string slap during crossbow operation; 
         FIG. 5  is a diagrammatic illustration of a typical compound crossbow arrangement showing the mechanical advantage cams; 
         FIG. 6  is a diagrammatic illustration of one embodiment of the subject shock absorber which is impacted by the bow string, with the shock absorber shown as an insert to a metal retaining cylinder at the distal end of a crossbow bolt; 
         FIG. 7  is a diagrammatic illustration of the force imparted to the TPU insert of the nock in  FIG. 6  illustrating the force concentration against the distal end of the insert followed by a focusing of the force to the center of the insert; 
         FIG. 8  is a diagrammatic illustration of the insert of  FIG. 7  showing the movement of the proximal end of the insert so as to activate an internal lighting structure; 
         FIG. 9  is a detailed diagrammatic illustration of the resilient shock absorber insert into a metal reinforcing structure showing the resilient shock absorber at the distal end of the nock; 
         FIG. 10  is a diagrammatic illustration of one embodiment of the resilient shock absorber illustrating a bow string notch and a central protruding rib adapted to be contacted by the crossbow bow string; 
         FIG. 11  is a further detailed diagrammatic illustration of the TPU resilient material insert surrounded by a metal reinforcing structure; and, 
         FIG. 12  is a diagrammatic illustration of the resilient injection molded insert to be inserted into the metal support structure of  FIG. 11 . 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to  FIG. 1 , a simplified crossbow  10  is provided with limbs  14  having a bow string  16  attached to the distal ends  18  of the limbs. A bolt  20  is inserted into the breach  22  of the crossbow in which bolt  20  has a nock  24  generally made of plastic which is adapted to be struck by bow string  16  when bow string  16  is released by trigger mechanism  26 , thus to project the bolt forward upon bow string release. 
     The problem with such a nock construction is that the nock may fracture as illustrated at  30  with the slap of bow string  16  against the distal end of the nock. Not only does the fracturing of the nock eliminate all loading on the bow string as it is released which can cause fracture it also can cause the bolt shown at  20 ′ to move off axis as illustrated by arrow  32  which can impact hunters or other people nearby, a clear safety problem. 
     Referring to  FIG. 2 , the problem with cross bows is that there is often a small but significant offset distance indicated by arrow  34  from the distal end  36  of nock  24  such that upon release of the bow string, the bow string rather than pushing against the nock impacts the nock in a slapping motion causing tremendous forces to be imparted to the nock which can cause nock failure and even dry fire. 
     Referring to  FIG. 3 , the dry fire situation is indicated in which a fractured nock  30  no longer provides a load on bow string  16  such that arms  14  of the crossbow may fracture as illustrated at  38 , again resulting in projectiles directed back at the hunter or archer or to individuals who may be in the immediate vicinity of the hunter. 
     Referring now to  FIG. 4 , in one embodiment a cylindrical nock support structure  40  is utilized to house a shock absorbing insert  42 . Shock absorbing insert  42  in one embodiment is an injected moldable urethane in the form of a thermo polymer urethane or a thermo plastic urethane. Upon slap of the bow string a force  44  is imparted to the distal end  46  of the insert which causes the insert to slightly deform as well as move as illustrated by arrow  48  in the direction of a light assembly  50  causing the light assembly to move in the direction of arrow  52  for activating a switch utilized to power the light assembly. 
     It has been found that injection molded TPU is not permanently deformable but rather has a memory such that after impact of the bow string it moves back to its original position, in one embodiment having actuated an internally carried light source. Further it is noted that support structure  40  which in one case is metal and preferably aluminum is inserted into a channel  54  in the distal end of a bolt here shown at  56  such that a unitary structure is provided with the metal support structure being inserted into channel  54  and extending aft to receive the injection molded TPU shock absorbing insert. 
     Typically a crossbow  10  shown in  FIG. 5  incorporates the mechanical advantage of a compound bow structure  60  to deliver a stress in the nock from the impact in excess of 7000 psi to the distal end of the bolt. This compound bow bowstring structure is generally indicated at  62  and is not described further other than to say that the amount of energy deliverable by the bow string of such an assembly is more than that necessary to fracture the traditional nock at the end of a bolt. 
     Referring now to  FIG. 6 , what is shown is a shock absorber  70  inserted into a cylindrical metal support structure  72  which is in turn inserted into a channel  74  in the bolt, with the bow string  76  adapted to contact an internal bow string receiving structure  78  to propel the bolt as a projectile in a forward direction when the bow string is released. 
     As illustrated in  FIG. 7 , the injection molded portion  70  is shown having a cylindrical forward structure  80  which has projections  82  utilized to join this insert to the metalized support structure  72  of  FIG. 6  by insertion into orifices  73  in the support structure. 
     As illustrated, the force imparted by the slap of the bow string is illustrated at  84  in terms of the arrows which impact first a transverse rib  86  which forms part of the shock absorber insert, with the force then tending towards the center of the insert as illustrated by arrows  88 . 
     Referring to  FIG. 8 , the interior of the insert moves as illustrated by double ended arrow  90  to act as a shock absorber as well as in one embodiment to activate an internally carried nock light assembly. In  FIG. 9  it can be seen that insert  70  is housed within metal support  72  such that it is able to move within this housing to provide the shock absorbing characteristics due to a flexible narrowed portion  75 . Thus the shock absorbing insert is surrounded by a metal support structure to increase the structural rigidity and strength of the crossbow bolt nock. 
     Referring to  FIG. 9 , a more detailed view of the insert and nock structure is shown in which shock absorber  70  is shown carried by a metal support  72  which is inserted into a channel in bolt  20 , whereas in  FIG. 10  the resilient shock absorber  70  is shown having an overall nock structure shown by notch  96  which has internal to the notch a transverse rib  78  adapted to be struck by the bow string. 
     Referring to  FIG. 11 , the assembled structure with the resilient shock absorber insert and the metal support  72  is illustrated in which as illustrated in  FIG. 12  the resilient shock absorber insert  70  to be placed into a metal structure  72  has the aforementioned projections  82  which are adapted to lock into metal support  72 . 
     While the present invention has been described in connection with the preferred embodiments of the various figures, it is to be understood that other similar embodiments may be used or modifications or additions may be made to the described embodiment for performing the same function of the present invention without deviating therefrom. Therefore, the present invention should not be limited to any single embodiment, but rather construed in breadth and scope in accordance with the recitation of the appended claims.