for purposes of description herein , the terms “ upper ,” “ lower ,” “ right ,” “ left ,” “ rear ,” “ front ,” “ vertical ,” “ horizontal ,” and derivatives thereof shall relate to the invention as oriented in fig1 . however , it is to be understood that the invention may assume various alternative orientations and step sequences , except where expressly specified to the contrary . it is also to be understood that the specific devices and processes illustrated in the attached drawings and described in the following specification are simply exemplary embodiments of the inventive concepts defined in the appended claims . hence , specific dimensions and other physical characteristics relating to the embodiments disclosed herein are not to be considered as limiting , unless the claims expressly state otherwise . a chair 1 includes a back 2 , a seat 3 , armrests 4 and a base 5 with castors 6 . with further reference to fig2 and 4 , chair 1 includes a four bar linkage comprising a base link 12 that is fixed to the chair base 5 , a seat link 14 , a control link 16 , and a lower link 18 . the links 12 - 16 are interconnected by first , second , third , and fourth pivots 21 , 22 , 23 , and 24 , respectively . a spring assembly 30 generates tension forces f 1 and f 2 acting on points 31 and 32 to thereby bias the four bar linkage 10 into an upright position as shown in fig2 and 3 . it will be understood that the spring and method of the present invention is not limited to coil springs , and also is not limited to spring assemblies generating a tension force . with further reference to fig4 - 6 and fig9 - 10 spring assembly 30 includes a coil spring 34 , an actuator or rod 36 having an opening 52 at end 54 , a housing or enclosure 38 , and an end plug or fitting 40 . the spring assembly 30 also includes washers , 42 a , 42 b and bushings 44 a , 44 b . a pin 46 extends through an opening 50 in end plug or fitting 40 , and a retainer 48 retains the pin 46 in the opening 50 . retainer 48 may comprise a c clip that is received in an annular grove , or it may comprise other suitable retaining devices to retain pin 46 in opening 50 of end plug 40 . with reference to fig5 a , in a preferred embodiment , pin 46 includes a cylindrical portion 46 a having a raised ring - like protrusion or ridge 46 b and a slot 46 c . retainer 48 includes an annular ridge 48 a that snaps over ridge 46 b such that retainer 48 is retained on pin 46 . pin 46 and / or retainer 48 may comprise steel or other suitable material . when assembled , pin 46 and opening 52 at end 54 of rod 36 are utilized to transmit forces through the spring assembly 30 . when spring assembly 30 is assembled , end flanges 56 and 58 of housing 38 retain the spring 34 in a prestressed , compressed condition . as discussed in more detail below , the flange 58 is formed during assembly while measuring a force applied to rod 36 and end plug or fitting 40 , such that the spring 34 has a predefined preload . thus , the magnitude of tension forces f 1 and f 2 applied to rod 36 and end plug 40 , respectively , must exceed a specified preload force amount before the rod 36 and end fitting 40 will move relative to one another from the initial position shown in fig5 . if tension forces f 1 and f 2 exceed the predefined preload force , coil spring 34 will compress further , resulting in movement of rod 36 in the direction of the arrow “ a ” ( 5 ) relative to end plug or fitting 40 . the spring assembly 30 of the present invention can be assembled in a way that ensures that the actual tension forces f 1 and f 2 required to cause movement of rod 36 relative to fitting 40 fall within a predefined acceptable range . with reference to fig8 , housing 38 is initially formed to include an inwardly - extending flange 56 a at first end 62 a of housing 38 . circular edge 66 at second end 62 b of housing 38 is not initially formed to include a flange . during assembly , a first washer 42 a is initially positioned in the inner space 60 of housing 38 in position against flange 56 a at a first end 62 a of housing 38 , and a first bushing 44 a and rod 36 are positioned as shown in fig8 . rod 36 initially includes a reduced diameter end portion 64 that receives second washer 42 b . a coil spring 34 is then positioned in housing 38 , and a second bushing 44 b and second washer 42 b are then positioned in the interior space 60 with second washer 42 b disposed on reduced diameter end portion 64 of rod 36 . the edge 66 of housing 38 is initially not formed to include a flange , such that washers 42 a and 42 b , and bushings 44 a and 44 b can be inserted into interior space 60 of housing 38 . end 64 of rod 36 is then deformed utilizing a known radial riveting process or other suitable process to create an enlarged end portion 65 which securely retains the washer 42 b on end 68 of rod 36 . enlarged end portion 65 is shown in dashed lines in fig8 , and in solid lines in fig9 . this forms a subassembly 30 a as shown in fig8 . subassembly 30 a is then positioned in a fixture 70 ( fig7 ). fixture 70 includes first and second pins 72 and 74 , respectively , that are received in openings 50 and 52 , respectively of spring subassembly 30 a . an end plug or fitting 40 is positioned in second end 62 b of housing 38 . pins 72 and 74 may be inserted utilizing pneumatic or hydraulic cylinders ( not shown ). upper pin 72 fixes the plug 40 relative to upper plate 90 of fixture 70 , and lower pin 74 fixes the rod 36 relative to a vertically adjustable cylinder 84 . cylinder 84 can be driven upwardly by threaded force adjustment device 86 by rotating a hand wheel 88 . a load cell 82 is utilized to measure the force generated by coil spring 34 on pin 74 due to upward movement of cylinder 84 . although a threaded adjustment device 86 and hand wheel 88 are shown in the fixture 70 , the force adjustment device utilized to compress coil spring 34 may comprise a hydraulic cylinder , an electrically - powered actuator , or other suitable powered device . once a predetermined or predefined force is generated as measured by load cell 82 , a forming tool 92 is shifted downwardly into position adjacent second end 62 b of housing 38 utilizing a hydraulic cylinder 80 or other suitable powered actuator . the forming tool 92 then forms edge 66 ( fig8 ) into flanges 56 b ( fig9 ) to retain plug 40 at a position wherein the coil spring 32 is preloaded at the desired level . this process thereby sets the preload force at a desired , predefined level or to a predefined range that includes acceptable tolerance variations . the final spring assembly 30 ( fig9 and 10 ) is then removed from fixture 70 . referring again to fig1 , chair assembly 1 includes two spring assemblies 30 . because the spring assemblies 30 are consistent with respect to the preload or initial force required to initially extend the spring assemblies 30 , a plurality of chairs 1 can be manufactured and the springs 30 do not need to be adjusted at the time of assembly of the chair 1 . furthermore , each chair 1 assembled will have substantially the same tilt characteristics with respect to the amount of force required to tilt the chair back .