Abstract:
A device having an allowance for creeping is provided. The device includes: a base member having a receiving element; and an overlaying member, subject to creeping, overlaying on top of the base member, the overlaying member having a region at one end thereof, wherein a tip member is free from restriction or force exerted thereon due to creeping deformation thereof.

Description:
REFERENCE TO PROVISIONAL APPLICATION 
   This application claims an invention which was disclosed in Provisional Application No. 60/498,172, filed Aug. 27, 2003, entitled “Creep Locking Retention Mechanism (Clip)”. The benefit under 35 USC §119(e) of the United States provisional application is hereby claimed, and the aforementioned application is hereby incorporated herein by reference. 

   FIELD OF THE INVENTION 
   The invention pertains to the field of assembly of many plastic parts to other parts. More particularly, the invention pertains to a creep locking retention mechanism (clip). 
   BACKGROUND OF THE INVENTION 
   Published WIPO patent application, WO03062668A1, entitled SNAP-FIT CHAIN GUIDE teaches a chain guide including a bracket adapted for being secured to an engine. The bracket includes a support surface including a leading end and a trailing end, and further includes a first saw-tooth fixing element. A guide shoe includes: (i) a shoe leading end; (ii) a shoe trailing end spaced from the shoe leading end in a chain movement direction; (iii) a chain guide surface extending between the shoe leading end and the shoe trailing end and adapted for slidably supporting an associated chain moving in the chain movement direction; and, (iv) an inner surface positioned adjacent the support surface of said bracket. The guide shoe further includes a second saw-tooth fixing element that mates with the first saw-tooth fixing element of the bracket to inhibit disconnection of the guide shoe and the bracket. 
   U.S. Pat. No. 6,302,816 entitled CHAIN GUIDE OR TENSIONER ARM WITH SHEET METAL BRACKET AND ALTERNATING TABS by Wigsten, Mark and assigned to BorgWarner Inc teaches a guide or tensioner arm including a bracket made from a formed sheet metal stamping. The bracket is formed of a body portion with a series of tabs formed along a bottom edge. The tabs are bent perpendicular to the body portion of the bracket in an alternating fashion. This forms a bracket having a T-shape providing a flat base for mounting a plastic shoe. Since the body portion is centered with respect to the alternating tabs a strong bracket for holding the plastic shoe is provided to guide or tension an associated strand of chain in a power transmission system. 
   Assembly of many plastic parts to other parts entail the use of a dart-type clip whereby a one-way tapered clip on the plastic part cantilevers into a locking position over a protrusion of the base part. The process of the cantilever spring portion of the clip bending to allow a taper to snap over the protrusion, places a tensile beam stress on the cantilevered spring. At times, the stress induced to the cantilevered spring necessary to clear the height of the protrusion can put high stresses on the cantilevered spring. Exposure to high stress can lead to reduced fatigue life of a part. 
   SUMMARY OF THE INVENTION 
   In a device having dart-type snap on clips without subjected to creeping, an apparatus and method are provided to improve upon the existing technology by eliminating or reducing the stress induced to the cantilever spring portion during the assembly process. 
   Accordingly, a device having an allowance for creeping is provided. The device includes: a base member having a receiving element; and an overlaying member, subject to creeping, overlaying on top of the base member, the overlaying member having a region at one end thereof, wherein a tip member is free from restriction or force exerted thereon due to creeping deformation thereof. 
   Accordingly, a method for reducing the deformation of a device due to creeping is provided. The method includes the steps of: providing a base member having a receiving element; providing an overlaying member, subject to creeping, overlaying on top of the base member, the overlaying member having a region at one end thereof; and allowing room for deformation due to creeping between the base member and the overlaying member within the region. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
       FIG. 1  shows an example of the present invention. 
       FIG. 1A  shows a perspective view of the top portion or wearing element of  FIG. 1 . 
       FIG. 2  shows a device having dart-type snap on clips without creeping. 
       FIG. 2A  shows a device having dart-type snap on clips with creeping. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   This section includes the descriptions of the present invention including the preferred embodiment of the present invention for the understanding of the same. It is noted that the embodiments are merely describing the invention. The claims section of the present invention defines the boundaries of the property right conferred by law. 
   A part made from plastic or composite material will deform due to creep effect when loaded with a long-term static load or a cyclic repeating load. Creep is commonly known as the deformation on the part due to long term applied load or cyclic load. Creep behavior is especially pronounced while the part is subjected to elevated temperatures relative to the material properties of the part. Materials undergoing creep deformation will take a set relaxing in the direction of an applied load. The relaxation effect of the part caused by the creep effect eventually becomes a permanent deformation upon the part, and the part remains deformed in the direction of the applied load even after the load has been removed. The tendency of a material to deform due to creep is more pronounced with plastic polymer having no fillings (particularly including, but not limited to, glass fillings as the filler material). 
   The present invention takes advantage of the formally negative of the deformation of a part due to creep in a positive way. In other words, the formally negative tendency of a part to deform in creep is employed, in a positive way, to engage the clip locking mechanism on the part. 
   An additional improvement applies to a special case whereby the clip is designed to completely clear the protrusion of the mating part during the assembly process. As the part deforms due to creep under operation, the hook moves into place to engage the protrusion, resulting in a positive lock with the mating part. In this special case, the leading edge of the hooked clip does not need to be tapered thus saving space. In many design conditions, space is not readily available for tight packaging reasons. 
   The following figures show an example of applying the present invention. 
   Referring to  FIG. 1 , a part  10  is shown. A structure element  12  forms the base of the 2-shot molded part  10 . Structure element  12  has a generally elongated shape and includes a top wearing surface  14  along the longitudal length of the same. Structure element  12  may be made of glass-filled plastic or polymer structure. Alternatively, structure element  12  may be a metal bracket. A connecting member  16  having an annular opening for pivotal coupling to a stationary member (not shown) which is stationary in relation to a chain (also not shown) is provided at a first end  15  of structure element  12 . A suitable connecting member (not shown) such as a bolt or dowel pin may be transposed through connecting member  16  for coupling to the stationary member. Along a second end  17  of structure element  12 , recesses  18  are formed on both sides of the structure element  12  (only one side shown). 
   A wearing element  20  formed of a material having more elasticity or less rigidity as compared with structure element  12  is provided. Wearing element  20  is also known as a face or a shoe: Wearing element  20  is generally made of polymer material such as plastic without filler reinforcement. Wearing element  20  has a shape that traces or follows the generally elongated shape of structure element  12 . Wearing element  20  further includes a first surface  22  and a second surface  24 . First surface  22  is disposed to be in contact with a chain (not shown) thereby keeping the chain at a suitable tension or guide the chain in a predetermined matter. Second surface  24  is disposed to be in physical contact with top surface  14  of structure element  12 . It is pointed out that there is no chemical bonding between wearing element  20  and structure element  12  even if both elements ( 12 ,  20 ) are made of polymer materials. Wearing element  20  is divided into a first end  25  and a second end  27 . First end  25  structurally corresponds to the first end  15  of structure element  12 . Second end  27  structurally corresponds to the second end  17  of structure element  12 . 
   A set of connecting elements  26  is formed on both the first end  15  and the second end  17  of wearing element  20 . Elements  26  may be clips which are formed along with wearing element  20 . It is noted that top surface  14  possesses a suitable receiving elements for connecting elements  26  to element  12  thereby physically engaging or connecting wearing element  20  and structure element  12  together. Wearing element  20  further has a dart-type snap  30  for engaging or connecting structure element  12  thereto. 
   It is noted that  FIGS. 1 and 1A  are merely an example of the present invention. Other type of devices having dart-type snap on clips is contemplated by the present invention as well. Furthermore, structure element  12  may have different elements formed in location other than dart-type snap  30  area. Also, structure element  12  may be part of the tensioner arm or part of a chain guide. 
     FIGS. 2 and 2A  show more detailed depiction of the dart-type snap  30  as applied to the present invention. 
   Referring to  FIG. 2 , a first view  30   a  wherein a device having dart-type snap on clips without creeping is shown. A one to one correspondence of elements with  FIGS. 1 and 1A  can be shown. However,  FIG. 2  is generally applicable to devices having dart-type snap on clips. Second end  17  of structure element  12  possesses a retaining element  17   a  which is formed through an integral extension of second end  17 . In other words, retaining element  17   a  is a suitable protrusion from second end  17 . Retaining element  17   a  is formed to stop the movement of another member the details of the stopping is disclosed in  FIG. 2A  infra. An elongated member  20 , such as the face or shoe of a tensioner arm or chain guide, overlaying end  17  of the structure element  12  is provided. At a tip or end of the elongated member  20 , a tip element  20   a  is formed in which element  20   a  is an integral part or an integral extension of elongated member  20 . Tip element  20   a  is suitably formed to be in contact with retaining element  17   a  is a desirous way. In addition, tip element  20   a  may not span the whole width of top surface  22  of the elongated member  20 . For example, second end  27  of the elongated member may branch into two sections at its tip in area  30 . The two sections may include a first section with tip element  20   a , a second section without the tip element  20   a . Opening  18  is provided for a suitable structural formation of the structure element  12 . 
   Referring to  FIG. 2A , a second view  30   b  wherein a devices having dart-type snap on clips with creeping is shown. As can be appreciated, due to the creeping effect the elongated member  20  which is typically made of polymer materials together with some kind of filler material tends to deform. The creep may be caused by wear of a chain onto surface  22 , the ambient temperature around the device, or a combination of the former. The elongated member  20  is typically made of polymer materials together with some kind of filler material. 
   As can be appreciated, the present invention improves upon the existing technology for dart-type snap on clips by eliminating or reducing the stress induced to the cantilever spring portion such as region  30  of the clip during the assembly process. Reducing the stress induced during assembly will help improve durability of the part by subjecting the clip free from an initial high stress cycle during the assembly process. The hooked portion or tip  20   a  of the clip is designed to either clear the protrusion of the mating part such as by leaving a desire gap  2017 , or to reduce the maximum stress induced to the clip. 
   The present invention provides an apparatus, such as the shoe or face on a tensioner arm, having no force impeding its deformation due to creeping. By way of examples, gap  2017  is provided such that when member  20  deforms due to creeping, no force or other members will stand in its way because of the existence of gap  2017 . 
   As can be appreciated, the present invention functions much like the way a fishhook works. 
   Accordingly, it is to be understood that the embodiments of the invention herein described are merely illustrative of the application of the principles of the invention. Reference herein to details of the illustrated embodiments are not intended to limit the scope of the claims, which themselves recite those features regarded as essential to the invention.