the preferred embodiment of a chair tilting mechanism in accordance with the present invention is illustrated in the drawings and designated generally by the reference numeral 10 . as shown , the tilting mechanism includes a spindle support or post 12 . a tilt plate 14 is pivotally connected to the post 12 . the tilt plate includes apertures 16 which serve as attachment points to the underside of a chair or seat ( not shown ). as best seen in fig2 the tilt plate 14 is formed with a centrally disposed , longitudinally extending slot 18 . the forward end of the slot 18 is closed by the cross bar 19 . the cross bar , at its center is provided with a fore and aft extending tubular portion 20 providing the clearance opening 22 ( fig3 and 4 ) having a centerline which is coaxially aligned with the centerline of the longitudinal slot 18 . a return torque imparting component 23 is disposed between the post 12 and the tilt plate 14 , as more fully described below . as best seen in fig1 and 4 , the support spindle or post 12 is trifurcated at its upper end to define outer pivot pin supports 24 and an intermediate pivot pin support arm 26 . each outer pivot pin support trunnion 24 has an aperture 28 formed therein and the intermediate pivot pin support has an aperture 30 formed therein . the tilt plate 14 includes a yoke structure 32 on its underside . the tilt plate yoke 32 includes a pair of spaced depending hinge ears or webs 34 having apertures formed therein . a pivot pin 36 extends through webs 34 and is supported by the outer pivot pin support trunnions 24 and the intermediate pivot pin support arm 26 of the post 12 . the yoke 32 and the tilt plate 14 is thereby pivotally supported on the trifurcated spindle support or post 12 . the support arm 26 is provided with an internally threaded bore 38 adapted to receive a set screw 40 . the set screw 40 locks the pivot pin 36 against both rotational and axial movement with respect to the post 12 . as best seen in fig3 and 4 , the support arm 26 includes an integral , upstanding stop finger 42 . the stop finger extends upwardly through the longitudinal slot 18 of the tilt plate 14 . the rear face of the stop finger is provided with a lower , standard position stop socket 44 and an upper , special , forward tilt or erect position stop socket 46 . the two sockets are separated by a rearwardly extending wall 49 . the forward face of the stop finger 42 includes a centrally disposed , semi - spherical boss or projection 48 . further , the upper portion 50 of the forward face of the stop finger 42 is upwardly and rearwardly inclined or beveled with the bottom of the bevel being approximately at the center of the boss ( fig3 and 4 ). as seen in fig2 the rear end of the longitudinal slot 18 of the tilt plate 14 is formed with semi - cylindrical , latch pin slots 52 and 54 . these slots open through the upper face of the plate . a stop and release latch 56 having pins 58 and 60 is pivotally supported on a tilt plate 14 with the pins seated in the slots 52 and 54 . as a result , the forward end of the latch 56 may be positioned so as to enter either the standard position stop socket 44 or the special position stop socket 46 in the rear face of the stop finger 42 . this is best seen in fig3 and 4 , respectively . as shown , when the stop latch 56 abuts either of the stop sockets 44 or 46 , further counterclockwise rotational or forward pivotal movement of the tilt plate 14 is prevented . as best seen in fig2 and 6 , a slidable latch retainer assembly 108 is secured to the underside of the tilt plate 14 adjacent the rear edge thereof . this latch retainer insures that the latch 56 engages the special socket 46 upon return movement from a tilted position and when shifted permits the latch to pivot under its own weight to the standard position . the retainer assembly includes a slidable member 110 having depending from and rear tabs 112 , 114 , respectively . the member 110 is formed with a centrally disposed , elongated slot 116 . a guide block 118 secured to the tilt plate and having depending sides 126 and 128 prevents sideways movement of the tabbed member 110 . a bolt 120 extending through slot 116 slidably secures the member 110 to the tilt plate . a nut 122 threads to the bolt within countersink 129 . in the alternative , a headed pin could be used to slidably mount the member 110 to the tilt plate . as shown in fig3 when the normal position is desired , the user merely grasps tab 114 and slides the member 110 outwardly , permitting the latch 56 to assume its lower socket engaging position . the latch will rotate to this position under the action of gravity . when the forward tilt position is desired , member 110 is pushed in , as shown in fig4 thereby preventing rotation of the latch 56 . when the latch 56 abuts the standard position stop socket 44 , the tilt plate 14 assumes the rearwardly inclined position illustrated in fig3 . however , when the stop latch 56 engages the special position stop socket 46 , the tilt plate is permitted to rotate forwardly through a greater angle , as shown in fig4 . this dual position feature of the chair tilting mechanism , permits the chair to be readily adapted for special or specific uses as typing , drafting or laboratory use , or any use requiring a person to assume a more erect position . this feature obviates the need for employing different tilt mechanisms in chairs or seats manufactured for such special or specific uses . this feature also permits the same chair to be adapted to both types of uses , those requiring an erect posture and those requiring a tilted posture . the changeover can be made almost instantly . this feature also results in a reduction in manufacturing costs since a manufacturer may employ a modular approach utilizing the same chair tilting mechanism with different chairs and bases . the return torque imparting component 23 employs a coil spring 62 , encapsulated by a plastic material 63 , a spring support block 64 , an adjustment shaft or compression stud 66 , and a knurled , spring preload , adjustment nut 68 . a pair of tracks or guides 70 and 72 are formed in the tilt plate 14 at the forward end of the longitudinally extending slot 18 ( fig2 ). both tracks 70 and 72 have abutment surfaces 74 at each end . the spring support block 64 is generally t - shaped and dimensioned so that the ears 75 and 76 of the block 64 rest on tracks 70 and 72 ( fig5 ). the support block is , therefore , retained against rotational movement about its longitudinal axis by the tilt plate 14 . longitudinal movement is limited by the abutment surfaces 74 . further , the support block 64 is formed with a longitudinally extending aperture 78 in its depending leg . the adjustment shaft 66 includes a threaded portion 82 with a smooth , rounded nose portion 84 at its rearward end . the adjustment shaft 66 is disposed within the clearance opening 22 and passes through the opening 22 . the nose 84 projects rearwardly beyond the support block 64 . a vertical pin 86 ( fig2 and 4 ) secures the support block to the adjustment shaft . this arrangement holds the adjustment shaft against rotation and prevents axial movement of the shaft relative to the support block . the knurled adjustment not 68 is threadably disposed on the adjustment shaft 66 with its forward face seated against the cross bar 19 . as a result , rotation of the adjustment nut 68 is converted into longitudinal movement of the adjustment shaft 66 . this permits preloading of the coil spring 62 since it is confined between the spring support block 64 and the stop finger 42 . the coil spring 62 is of the linear reaction type having flattened ends . it is embedded in a matrix of compressible , fatigue resistant plastic . a suitable plastic for this purpose is a urethane having a durometer of shore a 85 , a 100 % modular at 800 p . s . i ., a 300 % modular at 2000 p . s . i ., an elongation of 570 % and a tensile strength of 6000 p . s . i . the result is a tubular member in which only the coils are enclosed , the center being open . an exemplary spring suitable for use with this invention is one of 0 . 100 inch thick flat wire formed into six active coils of 1 inch o . d . and one - half inch i . d . forming a spring 2 . 45 inches long . the projection 48 of the stop finger is seated in the end of the plastic encapsulated spring 62 and serves as a pivot point about which the spring 62 rocks as the tilt plate is pivoted . the beveled portion 50 of the stop finger 42 provides clearance for the end of the coiled spring 62 during this rocking movement . the initial preload of the plastic encapsulated coil spring 62 may be readily adjusted by rotation of the knurled nut 68 . the construction of this invention permits the preload to be adjusted within the range of 100 - 550 inch pounds . this permits a specific coil spring to be adapted to a wide range of different weights possessed by different people using the seat . by varying the initial preload , many people are able to adjust the chair to provide for them a smooth , comfortable ride on the chair . at full 15 ° tilt the spring can exert a resistance of 400 to 1300 inch pounds . the overall structural arrangement of the tilting mechanism is compact and has a pleasing exterior appearance . due to the fact that the coil spring is encapsulated with a plastic material 63 , the unsightliness of an exposed coil spring is avoided . further , the plastic material 63 forms a smooth surfaced tube which is easily cleaned and is not prone to the collection of dust and dirt as are conventional springs . occupying the space between the coils of the plastic positively prevents anyone from getting his fingers crushed between the coils . the tilting mechanism of the subject invention is readily adaptable to satisfy the requirements of a wide population of people having vastly different physical characteristics , due to the fact that the arrangement readily permits substitution of coil springs having higher or lower spring rates . by tilting the seat and , hence , the pivot plate 14 backwardly and , at the same time , pivoting the stop and release latch so that it is disengaged from the stop sockets , the seat is free to pivot substantially beyond its normal forward position . this changes the effective length between the cross bar 19 and the stop finger 42 . thus , with the latch 56 released and the seat tilted to a forwardly inclined position the spring 23 will normally drop out into the operator &# 39 ; s hand . this provides easy access to the spring by a user permitting ready substitution . conventional coil spring tilting mechanisms , are generally so constructed as to prevent spring substitution at the customer level except with the use of special tools or after dismantling the mechanism . alternately , the entire tilting mechanism is enclosed by a separate housing which does not permit ready access to the coil spring . these problems are eliminated by the present invention . no separate housing is employed . an aesthetically improved appearance is provided . since the encapsulated coil spring 62 seats against the support block 64 , a direct bearing between the adjustment nut 68 and the coil spring is avoided . this feature permits easy preloading of the device by a user without the necessity of special tools since the area of frictional contact between the face of the adjustment nut and the clearance housing 20 is substantially less than would be the case with an arrangement whereby the rotatable adjustment element bears directly on the end of a coil spring as in conventional tilting mechanisms . further , all frictional resistance can be eliminated by releasing the latch , tipping the seat forward and making the adjustment while the spring is loose but still spindled between the boss 48 nd the rear end of the shaft 66 . the tilt plate 14 may be made from various materials such as aluminum . the coil springs are preferably made from flat , steel wire having the ends closed and ground . while a preferred encapsulation material of a resilient plastic material has been described , other encapsulating materials may be employed , such as rubber . whatever material is chosen must have excellent fatigue resistance characteristics , must be capable of adherence to the spring and sufficiently resilient that it will not interfere with the action of the spring . another primary criteria of this material is one which is easily cleaned and which is pleasant in appearance . it has been found to be highly desirable to incorporate an encapsulated coil spring in the tilting mechanism which possesses linear characteristics at the minimum preload conditions , but which possesses an increased spring rate when subjected to maximum preload conditions . such a spring provides the tilting mechanism with the ability to satisfy the need of a wider range of body weights . such characteristics prevent lighter weight people from experiencing a build - up of force as they tilt back in the chair and prevent heavy weight people from experiencing a sinking feeling as they tilt back . this avoids the occurrence of any surprising or fatiguing subjective impressions in a greater range of population . thus , it will be appreciated that the present invention provides a tilting mechanism for a chair possessing the qualities of compact size , low weight , relatively low cost , as well as ease of adjustment and maintenance . it is expressly intended , therefore , that the foregoing description is illustrative of the preferred embodiment only and is not to be considered limiting . the true spirit and scope of the present invention will be determined by reference to the appended claims .
a tilting chair support mechanism is provided including a spindle support , a pivotally mounted tilt plate and an encapsulated spring acting between the support and the tilt plate biasing the plate into a forwardly or return tilted position . the spring is mounted within the side silhouette of the plate . the mechanism is so designed that the plate can be released for full forward pivotal tilting in which position , the spring is released for removal or replacement . means are provided for limiting the forward pivotal movement of the plate when the chair is in normal use . provision is made for adjustment of the preload applied to the encapsulated spring .