Abstract: A thread-cutting screwed dowel has an essentially cylindrical, sleevelike dowel body with an axial bore and with a load engagement device disposed in the interior of the dowel body, for example with a female thread. On its outer face, the dowel body is equipped with at least one cutting edge, which extends helically along a majority of the outer face of the dowel body and protrudes radially past it. The helical cutting edge is embodied as a primary cutting edge and wraps around the dowel body over its length in at least three windings. Besides the load engagement device, an additional engagement is provided for transmitting a torque in the interior of the dowel body.

Abstract: A front mounted retaining mechanism including a retention device and a captivatable device. The retention device has a head and a hollow shank. The head or the shank may include inwardly directed threads. The shank may include outwardly directed threads to facilitate mounting within a structure. The captivatable device includes a head and a shank, with the shank including a portion having a greater diameter than the remainder of the shank. That portion may be threads or a retainer ring. The captivatable device is threaded into the retention device in such a way as to allow its captivation therein. Then the first structure is aligned with a second structure into which the captivatable device is mounted.

Abstract: A helical thread insert having a removable tang attached to the front end of the insert, a retention device, and a removal notch located along the rear portion of the insert. The removable tang is oriented radially inward and includes a “V” shaped notch oriented along a bottom of a thread to detach the tang at a specified point. The retention device is located at a rear end of the thread insert and prevents counterclockwise, or removal rotation of the insert by engaging female threads of the bore in which the helical thread insert is disposed. The removal notch is formed along an inner surface of the insert proximal to the retention device to allow a tool to actuate the retention device inward, generally away from the female threads of a parent material, to allow relatively free counterclockwise rotation and removal of the thread insert.

Abstract: Described herein are fastener inserts formed from a titanium alloy which reduce corrosion in a fastener assembly. In addition to reduced corrosion, particularly galvanic corrosion, the fastener insert is lightweight and provides improved strength to weight ratios as compared to stainless steel fastener inserts.

Abstract: A threaded insert for providing stronger threads to a parent material. The threaded insert includes a bushing portion having male threads on the external surface and female threads within an internal bore. The threaded insert also includes keys which lock the threaded insert into a threaded hole within the parent material. The keys are positioned within slots along the external surface of the bushing portion of the threaded insert. The slots include two substantially parallel walls which are substantially perpendicular to a bottom surface of the slots. A method for assembling the threaded insert reduces the tendency of the keys to be installed offset from the bottom surface of the slots.

Abstract: The present invention includes a device for providing a blind hole for making a removable connection to an enclosure. The device has a cylindrical shaft with a proximal portion and a distal portion. A head is formed at the distal portion of the shaft. The proximal portion of the shaft has external threads, which may by threaded into a hole of the enclosure. The proximal portion also has a tapped bore, axially formed in the shaft, which has internal threads to receive another threaded device. The tapped bore is closed at the distal portion, such that when it receives the other threaded device, the other device does not intrude into the enclosure.

Abstract: An improved fastener for boarding, particularly to a fastener adopted for use on fragile boards and panels such as gypsum boards mainly comprises a shank, a flange formed on one end of the shank, a drill bit formed on another end of the shank, a screw thread section formed on the peripheral surface of the shank and a center hole formed in the center of the shank for accommodating a screw nail. The shank has at least two debris troughs. Each debris trough is extended diagonally from a blade nib of the drill bit to a front section of the screw thread section. The debris troughs have a through hole formed on a rear section.

Abstract: A fastener includes a main body having a number of spaced ridges formed on an outer periphery thereof. The ridges are arranged along a helix. At least one of the ridges has a retainer extending in a direction that is at an angle with an extending direction of the ridge. In another embodiment, a fastener includes a main body having a number of spaced ridges formed on an outer periphery thereof. Each ridge extends along a direction that is parallel to a longitudinal direction of the main body. At least one of the ridges has a retainer extending in a direction that is at an angle with an extending direction of the ridge.

Abstract: A threaded insert for providing stronger threads to a parent material. The threaded insert includes a bushing portion having male threads on the external surface and female threads within an internal bore. The threaded insert also includes keys which lock the threaded insert into a threaded hole within the parent material. The keys are positioned within slots along the external surface of the bushing portion of the threaded insert. The slots include two substantially parallel walls which are substantially perpendicular to a bottom surface of the slots. A method for assembling the threaded insert reduces the tendency of the keys to be installed offset from the bottom surface of the slots.

Abstract: A thread insert and method to replace the damaged threads and tapered seat in a spark plug bore of an internal combustion engine allows for the continued use of the original factory specified spark plugs where the original threads in the spark plug bore have been damaged by stripping or cross threading. The thread insert's inner bore is designed to replace the original threads and tapered seat in the cylinder head. The thread insert may be adapted to fit any internal combustion engine using tapered seat spark plugs, and is particularly useful in deep spark plug bores with limited access as found in the aluminum heads in Ford modular engines. The insert includes a flange head that determines how far into the head the insert can extend and a recess below the flange to collect any bonding agent that may be squeezed from the threads during installation of the insert. Special tools make the installation of the insert easy and accurate.

Abstract: A device for assembling two components (1,2) includes a lateral positioning set (3). The device includes a fixing screw (10), in particular threaded on the right, and capable of moving freely in a smooth through hole (13) provided in the first component (1), a spacer (20) bearing an external thread (23), in particular on the left, capable of co-operating with a tapped through hole (23), in particular on the left, provided on the second component (2) and an internal tapping (21), in particular on the right capable of co-operating with the fixing screw (10), the screw (10)/spacer (20) friction coefficient, directly or indirectly, being greater than the spacer (20)/tapped hole (23) friction coefficient, directly or indirectly.

Abstract: A device is provided which enables an object such as a window protection device to be secured to a structure, where the device passes through a hole in one or more structures located between the object, and the structure to which attachment is desired. The device can be screwed, or otherwise attached, to wood structural framing which is concealed beneath veneer, such as bricks and other masonry. The other end of the device receives a tool which allows such an installation, and also the removal of the device. The other end also receives a fastener, such as a bolt, which secures the material or the track to the end of the device, the end of the device being substantially flush with the exterior veneer surface. The end of the device is flared to seal the hole in the veneer and increase the stability of the device in the hole.

Abstract: A threaded insert for providing stronger threads to a parent material. The threaded insert includes a bushing portion having male threads on the external surface and female threads within an internal bore. The threaded insert also includes keys which lock the threaded insert into a threaded hole within the parent material. The keys are positioned within slots along the external surface of the bushing portion of the threaded insert. The slots include two substantially parallel walls which are substantially perpendicular to a bottom surface of the slots. A method for assembling the threaded insert reduces the tendency of the keys to be installed offset from the bottom surface of the slots.

Abstract: Disclosed is a method for preventing galvanic corrosion in fastener assemblies employing plated fastener inserts wherein the plating of the insert has an emf potential which is similar to that of the substrate within which the insert is disposed.

Abstract: A self-tapping threaded insert (20) for threadedly mounting another component by a threaded central connector (50). An assembly (24) of the insert (20) and a mounting member (22) for threadedly mounting the insert are assembled by a method for mounting the self-tapping threaded insert. The insert (20) has a threaded shank (26) that is threaded into a partially tapped hole and has a shank end with flutes (32) of a construction that provides self-tapping. Upon final torquing, a locking flange (42) of the insert (20) locks against a surface (46) of the member (22) as threads of the insert shank (26) are completely engaged with the hole.

Abstract: A wood plate connecting element is disclosed. The connecting element has an outer periphery which is slightly larger than a diameter of a positioning hole of a wood to be combined thereto. An outer thread section is formed at an outer surface of connecting element. The connecting element has a hollow channel. A rear section of the hollow channel of the connecting element is a hexagonal screw hole. A front section of the hollow channel of the connecting element being formed with an inner thread which is engageable with a stud. An embedding positioning seat is installed at a front end of the connecting element; an interior of the embedding positioning seat has a reduced shape. A foremost of the embedding positioning seat has an embedding rings.

Abstract: A barrel nut for securing furniture panels is comprised of a cylindrical body with an external thread extending between its opposite ends. The cylindrical body is arranged for being positioned in a snug fitting hole in a furniture panel. A transverse hole is positioned completely through the cylindrical body and has an internal thread extending between its opposite ends. The transverse hole is arranged for receiving a bolt extending from an adjacent furniture panel. A tool socket is arranged at an outer end of the cylindrical body for receiving a turning tool which can screw the cylindrical body into the snug fitting hole. An alignment mark aligned with the transverse hole is provided on the outer end of the cylindrical body to indicate the position of the transverse hole for facilitating alignment with the bolt.

Abstract: An assembly and method for mounting two panels where a first member is mounted to, into or from a support, a second member is threaded to said mounted first member, and a third member is threaded to said second member when it is already threadingly engaged to said mounted first member, wherein said second member has a portion extending through a hole in a first panel and into said first member so as to retain between the first and second member the first panel and wherein the third member has a portion extending through a hole in a first panel and into said second member so as to retain between the first and second member the second panel.

Abstract: An improved bolt/nut structure capable of preventing from dead locking and loosening and being vibration resistive, wherein, an internal threaded lining and an external threaded sleeve are made from helix springs with the cross section of each ring thereof in the shape of “]” and are used as an external thread and an internal thread for the bolt and the nut respectively to provide the function of dead locking and loosening and of vibration resisting. A plurality of expansion slits are provided on the internal and external peripheries of the helix springs, the springs with the expansion slits are suitable for use on the surface with collapsing of screw threads to suitably expand, and to confine the screw threads respectively. Moreover, a single or a plurality of helix springs are provided to form the internal threaded lining or the external threaded sleeve of one of various pitch gauges, thereby, the bolt structure has the capability of changing for the coarse and fine threads.

Abstract: The present invention provides for fastener inserts and methods of manufacturing the same. The fastener inserts include selectively removable tangs which can be broken away more cleanly than with other commercially available fastener inserts. Further, the methods of manufacture offer a significant improvement in the production process.

Abstract: A threaded fastener which includes a shank, a radially enlarged head at one end, an external thread on at least a part of the shank, and a bore which extends through the head and into the threaded part of the shank. At least the threaded part of the shank is radially expandable into the threaded engagement with the wall of a suitable unthreaded bore in a workpiece in which the threaded part has been inserted. The fastener thereafter is removable by unscrewing it from the workpiece bore. The expandable part of the shank is provided with a plurality of radially extending lugs or other protrusions provided on the threaded part of the shank thereby to resist unscrewing of the fastener from the workpiece bore.

Abstract: An insert for use in allowing spark plugs, fasteners, couplings for hydraulic fittings and pipe threads of a given dimension to be used in a material.

Abstract: An insert for thread repair having an interior thread and a specially formed exterior thread which both precludes upheaval between sides of the cracked material on opposite sides of the crack and draws the crack together, radially inward towards the center of the insert. The insert may include a driving head, either solid or having a central hollow and may include a frangible neck to dissociate the head from the insert.

Abstract: A self-tapping threaded insert (20) for threadedly mounting another component by a threaded central connector (50). An assembly (24) of the insert (20) and a mounting member (22) for threadedly mounting the insert are assembled by a method for mounting the self-tapping threaded insert. The insert (20) has a threaded shank (26) that is threaded into a partially tapped hole and has a shank end with flutes (32) of a construction that provides self-tapping. Upon final torquing, a locking flange (42) of the insert (20) locks against a surface (46) of the member (22) as threads of the insert shank (26) are completely engaged with the hole.

Abstract: Device for rapidly fixing cross-pieces to pairs of uprights, in particular pairs of legs of chairs, tables and the like, in which the cross-piece have inserted within a pair of cylindrical elements which each have a first thread in opposite directions to each other for insertion, axially, into the opposite ends of the cross-piece and a second thread in the same direction as the respective first thread for insertion into a corresponding seat of the uprights, the rotation of the cross-piece in the same direction causing the simultaneous pulling of the two opposite uprights until they come into stable contact against the opposite ends of the cross-piece.

Abstract: A wall anchor for use in a friable material, composed of a generally cylindrical body having a flanged end, a central portion, and a drill end. The drill end has a generally flat blade which includes a first portion and a second portion. The first portion has a first angled cutting edge and a first side cutting edge, and the second portion has a second angled cutting edge and a second side cutting edge. These portions are formed on a common line and are offset relative to each other along a plane passing through a diameter of the cylindrical body, and these portions together form a point. The central portion and the flange end have a combined axial length, and the drill end has an axial length which is shorter than that of the central portion and the flange end.

Abstract: The present invention relates to a system and device which allows the replacement of a damaged thread in an object with a thread identical to the damaged thread by enlarging the original threaded hole and inserting a thread-insert in a newly threaded hole in the object which has an internally threaded hole identical to the original threaded hole. The thread-insert is fastened into the enlarged hole using a thread that securely holds it in place. When the thread-insert is threaded into the enlarged threaded hole, a pocket formed below the drive head of the thread-insert causes metal flow into the pocket to create a seal between the thread-insert and the object.

Abstract: Fasteners and devices and methods for placing fasteners in a workpiece are disclosed. More particularly, non-removable permanently-placeable fasteners for placement in a workpiece to hold elements thereof together, permanently-placeable but removable fasteners for placement in a workpiece to hold elements thereof together, headless removable one-way fastener inserter or heads by which the fasteners are placed, tools for permanently placing such fasteners, tools for removing certain of the fasteners and methods by which such fasteners are placed in, and in some cases removed from a workpiece.

Abstract: An annular turbine frame includes a first ring such as a radially outer casing disposed coaxially about an axial centerline axis and having a plurality of circumferentially spaced apart first ports. A plurality of circumferentially spaced apart hollow struts are joined radially to the first ring by a corresponding plurality of collars. Each strut has radially opposite first and second ends, and a through channel extending therebetween. Each of the collars is disposed between a respective one of the strut first ends and the first ring in alignment with a respective one of the first ports for removably joining the struts to the first ring. Each of the collars includes a base disposed against the first ring and has a plurality of mounting holes for receiving mounting bolts therethrough to removably join the base to the first ring. The base has a central aperture aligned with the first port. Each collar mounting hole has a hole counterbore through a radially outer portion of the collar mounting hole.

Abstract: An electrically insulated anchor device is described, comprising a rigid elongated metal sleeve including an external surface that leads from an open top end to a bottom end. A longitudinal internal bore is formed within the sleeve with a bore wall spaced from the external surface by a sleeve wall thickness dimension. An electrically insulative polymer insert with an external surface engages the bore wall within the sleeve. The insert further includes a fastener receiving bore defined by a fastener receiving wall spaced from the external insert surface by an insert wall thickness dimension. The sleeve wall thickness dimension is substantially greater than the insert wall thickness dimension.

Abstract: The present invention is for a fastener device used to releasably fasten a first object and a second object. The fastener device is self-contained and includes a unitary housing that is releasably fastened to the first object. The housing has an annular wall that defines an exterior surface, an interior surface, and an upper and lower opening. Within the housing, extending radially inwardly from the interior surface is a housing stop shoulder. A retaining cover is threadingly attached to the housing at the lower end of the housing. The retaining cover is retained within the housing by a retainer ring. The fastener device also includes a threaded stud that is located within the housing. The stud includes an enlarged section which mates with the housing stop shoulder when the fastener is in a fully engaged position. The engagement of the enlarged section and the housing stop shoulder serves to retain the stud within the housing when the fastener is in a fully engaged position.

Abstract: A device is provided which enables one or more objects to be secured to a structure, where the device passes through a hole in one or more structures located between the object, and the structure to which attachment is desired. The device is particularly useful for attaching window protection devices, such as rolldown shutter tracks and sheets of plywood. The device can be screwed, or otherwise attached, to wood structural framing which are concealed beneath veneer, such as bricks and other masonry. The other end of the device receives a tool which allows such an installation, and also the removal of the device. The other end also receives a fastener, such as a bolt, which secures the material or the track to the end of the device, the end of the device being substantially flush with the exterior veneer surface.

Abstract: A device for attaching an elongated handle to any one of a variety of cleaning implements, regardless of whether the cleaning implements are provided with a socket or a protuberance for purposes of attaching an elongated handle thereto. The device preferably comprises an externally threaded protuberance and an internally threaded socket, where the threads of the externally threaded protuberance and the internally threaded socket are non-complementary to one another, the externally threaded protuberance is adapted for mating with a socket having complementary internal threads, and the internally threaded socket is adapted for mating with a protuberance having complementary external threads, whereby the device can be joined with another device comprising either one of the socket having complementary internal threads and the protuberance having complementary external threads.

Abstract: An insert for thread repair having an interior thread and a specially formed exterior thread which both precludes upheaval between sides of the cracked material on opposite sides of the crack and draws the crack together, radially inward towards the center of the insert. The insert may include a driving head, either solid or having a central hollow and may include a frangible neck to dissociate the head from the insert.

Abstract: Conductive devices and tools to provide electrical conduction with a generally planar conductor which is covered with an insulation substance such as paint, rust or dust, by scraping a portion of the conductor and removing the insulation substance. A conductive device includes a conductive metal serration means and a tool utilizes such a serration device. Such a serration device can be a bolt device, nut device or washer device, or washer-incorporated threaded device. A connector device is also disclosed to provide electrical conduction between a conductor and a conductor plate covered with an insulation substance.

Abstract: An insert for thread repair having an interior thread and a specially formed exterior thread which both precludes upheaval between sides of the cracked material on opposite sides of the crack and draws the crack together, radially inward towards the center of the insert. The insert may include a driving head, either solid or having a central hollow and may include a frangible neck to dissociate the head from the insert.

Abstract: Disclosed is a method for preventing galvanic corrosion in fastener assemblies by coating fastener inserts with a resin bonded fluoropolymer composition having an average viscosity of between about 20 to 30 seconds at 25° C. Also disclosed is a method for providing coated inserts as well as the coated inserts produced.

Abstract: The thread insert of the present invention comprises a helical body having a front end portion (3) and a back end portion (6), and a detachable tongue (4) connected to the front end portion (3) of the helical body, and a pair of engaging notches (10) are formed in the back end portion (6). A female thread (2) is formed on an inner surface of the helical body, a male thread (1) is formed on an outer surface of the helical body. The detachable tongue (4) has a circular portion (3a) having an outer diameter which is smaller than that of the helical body. A cutting notch (11) is formed between the front end portion (3) and the circular portion (3a) on at least one of a surface facing backward and a inner surface of the helical body.

Abstract: The invention relates to a wire-type screw thread insert for insertion into a tapped hole of a workpiece. A cylindrical body comprising a plurality of helically wound coils is provided with at least one insertion zone which is bent radially inwards from the outer diameter of the cylindrical body and extends about a circumferential angle of about 360.degree. to 720.degree.. This facilitates insertion of the insert into a tapped hole of the workpiece so that special tools required hitherto may be dispensed with because the wire-type screw thread insert of the invention may be readily introduced into the tapped hole and may be threaded thereinto by a simple tool.

Abstract: The anchor bolt comprises a generally hollow body (1) with a drilling end (4), a supporting collar (5) at the other end and an external screw thread (6). The collar (5) forms two openings (10, 11), diametrically opposed, for receiving an anti-rotation pin (8, 9) removably fixed to the collar (5) and movable in the opening (10, 11). The pins (8, 9) are intended to anchor both in the collar (5) and in the receiving material, in order to prevent rotation of the bolt.

Abstract: A sleeve nut includes a nut (1) and a screw like sleeve (2) screwed into that nut, thereby fixing the sleeve nut to a component (7) of e.g. an aircraft, a car or the like. Between the component (7) and the nut (1) a socle (3) is inserted having at least one upright tongue (22). This socle (3) is locked against rotation with respect to the component (7). Furthermore the nut (1) is positively locked against rotation at the socle (3) by flats (21) or edges there-between respectively with the tongue (22) of the socle (3).

Abstract: A thread insert comprises an externally threaded housing with an internally threaded floating nut therein. Diametrically spaced ears on the nut are engaged in diametrically spaced grooves in the housing, respectively, to limit relative rotation between the nut and the housing yet permit radial movement of the nut relative to the housing. A single split ring locks the nut into the housing.

Abstract: An interchangeable fan blade system for a ceiling fan, made up of a set of adapters having a plurality of elongated attachment apertures that allow the adapter to be threadably fitted to virtually any type of fan support arm, and a complimentary set of interchangeable fan blades. The adapters include one of a variety of connectors for attaching one end of the fan blades to the support arm. The one end of the fan blade is configured and adapted, or otherwise provided with mating connector elements adapted to engage the connectors of the adapters, thereby allowing the fan blades to be removably attached to the adapters. A second embodiment of the interchangeable fan blade system is a set of interchangeable fan blades having a plurality of elongated attachment apertures which allow the fan blades to be secured to virtually any type of fan arm, by threading a plurality of shoulder cap screws and a plurality of male-female connectors though the plurality of attachment apertures.

Abstract: A locking nut for use with a bolt for connecting members together. The bolt has a head and a shank. The members are disposed between the head and the nut when the locking nut connects the members together. At least a portion of the shank has external threads formed thereon. The locking nut includes a shell having a bore extending therethrough for receiving the bolt. At least a portion of a wall of the bore has internal threads formed thereon. The shell is sized and shaped to permit rotation of the nut on the bolt for tightening the nut on the bolt. The nut further includes a generally tubular insert sized for being received in the bore of the shell. The insert has a threaded portion having an outer surface with external threads for threaded engagement with the internal threads of the shell, and an inner surface with internal threads for threaded engagement with the external threads of the bolt. The insert includes a locking portion constructed for interlocking the insert with the shell.

Abstract: A bushing adapted to be secured within a circular aperture of a structural channel support member has a head, and a shank portion having an external male machine thread. A centered bore having an internal machine thread penetrates the head and shank portion. The head has a flat inside shoulder adapted to abut against the channel member.

Abstract: An improved threaded joint for connecting a first tubular member, for example a valve housing, with an outer end having interior threads with pitch W and terminating in a shoulder. A second tubular member has an exterior threaded end and a flanged end. A shoulder extends outwardly of the second tubular member intermediate the threaded and flanged ends. A flange at the flanged end has N equally circumferentially spaced connectors. One of the first shoulder and second shoulder includes a portion that is elastically deformable over a distance at least equal to W/N so that connectors in the flange may be aligned with connectors in a connecting flange of a pipe line. A side entry cap is disposed in the valve assembly. The side entry cap includes a threaded portion and a sealing portion inward of the threaded portion, the threaded portion being cylindrical and the sealing portion having an exterior surface forming a conical section increasing in diameter outward.

Abstract: A threaded self-locking fastener, having a hollow body open at both ends and an elongated central through bore with a slot extending the length of the hollow body.Version 1 has sharp edges on the slot that dig into the bore of the work piece to prevent rotation. The fastener is used in conjunction with an internal driver. The driver located in the bore of the fastener engages detents to produce rotary drive motion. The driver also has a means of manipulating the first and second edges to facilitate assembly and disassembly.Version 2 has curved or chamfered edges in the slot and inward facing lobes also used in association with a driver. Rotation of the driver against the lobes causes the tubular fastener to be reduced in diameter against spring tension thereby allowing the fastener to freely rotate in the threaded bore of the work piece. By releasing the driver, the fastener expands and self locks in the threaded bore.

Abstract: A tool having a profile for the forming or shaping of a complementary thread in and/or on a fastener which can be utilized with advantage as a bone screw or another osteosynthetic force transmitting member, instrument or accessory. The thread which is formed by the profile of the tool has two flanks each having a first section adjacent the crest and a second section adjacent the root of the thread. The second sections jointly include or consist of at least four concave portions having different radii of curvature and gradually merging into each other. The first sections make angles of less than 40.degree. with a plane which is normal to the longitudinal axis of the thread. The first and second sections of each flank are separated from each other by an imaginary cylinder having a diameter which matches the pitch diameter of the thread and a longitudinal axis which coincides with or is at least close to the longitudinal axis of the thread.

Abstract: An orbital threaded fastener assembly includes an orbital nut or bolt and a split thread ring. Each of the nut or bolt and the thread ring has formed thereon a series of alternating, coaxial, circumferential grooves and teeth sized to mateably engage each other. These grooves and teeth can be formed on either the internal surface of the nut and the external surface of the ring or the external surface of the bolt and the internal surface of the ring. In use, the diameter of the ring is resiliently expanded or contracted to permit it to be disposed at a desired axial position along the nut or bolt to engage each other, thereby preventing relative axial movement of the parts while accomodating relative rotational movement thereof. The surface of the ring opposite that carrying the grooves and teeth is threaded to permit threaded coupling to an associated threaded component.

Abstract: A drink container holder is mounted to a generally upright seat standard having at least one seat bracket to support a seat back in a generally upright position. The seat back is mounted to the seat bracket with a bolt and nut. The container holder includes an open top receptacle and a pair of bosses that have threaded metal inserts that receive the seat fasteners so the fasteners can be tightened into the inserts. When tightened, the seat bolts and container holder are securely held in place. A support flange attached to the container holder braces the container holder against the seat backs for stability.