the present invention pertains to part of a machine for continuously assembling elastic leg disposable diapers . the machine sandwiches a highly absorbent material between a fluid permeable facing sheet and a fluid impervious poly backing sheet . the specific construction of the disposable diaper is the subject of a copending application titled &# 34 ; disposable diaper with elasticised leg openings &# 34 ; by w . sigl and r . frick and assigned to the assignee of the present invention , but the present invention is equally applicable to other disposable diaper designs , including the design disclosed by gore , u . s . pat . no . 4 , 239 , 578 issued dec . 16 , 1980 . the present invention involves the part of the diaper assembly machine that bonds elastic strips to the diaper material for elasticizing the leg openings of the finished diapers . turning now to the drawings , fig1 schematically shows the mechanism whereby differential stretch is imparted to an elastic strip 10 as it is bonded to poly backing sheet 18 . the continuous elastic strip 10 is fed at a generally constant rate v 1 by a fixed stretching roller 12 rotated by shaft 14 . the elastic strip slips off the stretching roller 12 at the tangent point 16 and is picked up by oscillator roller 20 . oscillator roller 20 is journaled for free rotation about shaft 22 secured to a movable oscillator frame 30 . a continuous sheet of poly backing material 18 is also fed around oscillator roller 20 , and receives the elastic at a tangent point 24 . the poly with adhered elastic 28 is then pulled off the oscillator roller 20 for further processing in a diaper assembly line . differential stretch is induced in the elastic strip 10 as it travels over distance x between the tangent points 16 and 24 . the stretch s of the elastic 10 may be defined in terms of the linear density dx / dm of the elastic 10 . with the units of the mass , m , chosen so that the linear density dx / dm of the unstretched elastic 10 is equal to 1 , the elongation e is directly proportional to linear density : where m is the total mass of the elastic strip 10 between the tangent points 16 and 24 . since the process whereby the elastic 10 is carried or taken up by the rollers 12 , 20 is a form of gear engagement , the elongation of the elastic 10 as it is bonded to the poly backing 18 is the same as the elongation of elastic strip 10 between the tangent points . the stretch s may also be defined in terms of the elongation e as : the velocity v 1 of the elastic 10 as it travels around the stretching roller 12 is porportional to the radius and angular velocity of the stretching roller 12 . similarly the velocity v p of the poly backing 18 fed to the oscillator roller 20 is proportional to the radius and the angular velocity of the oscillator roller 20 . the rate at which the elastic 10 between the tangent points 16 , 24 is taken up by the oscillator roller 20 is defined as v 2 , which is fig1 equals v p . v 1 and v 2 are functionally related to the elongation e and stretch s via the mass m of the elastic 10 by the conservation of mass equation : in general , x is not independent of time , and is related to the velocity v s of the tangent point 24 of the oscillator roller 22 with respect to the tangent point 16 of the stretching roller 12 : in fig1 v s = v a , the tangential velocity of the oscillator frame 30 . equations ( 1 ) through ( 4 ) supra completely define the stretch of the elastic strip bonded to the poly 28 . the general solution , however , is non - linear . the solution for de / dt = 0 is of interest since then dx / dt = edm / dt and therefore : or equivalently for fig1 v a = ev 1 - v p . by setting v p = 1 . 5v 1 , for example , v a =- 0 . 5 v 1 for e = 1 and v a =+ 0 . 5 v 1 for e = 2 . the solution for v s = 0 is also of interest since then equation ( 3 ) is linear and has the solutions : ## equ1 ## thus a time constant τ = x / v 2 defines the response of the elongation to changes in v 2 or v 1 . as shown in fig3 if a velocity v which may be either v 1 , v 2 , or v p is switched between two values v l and v h as illustrated by the trapezoidal waveform generally designated 40 , the response of the stretch s as illustrated by the waveform generally designated 50 has a triangular shape caused by a time delay measured by the time constant τ . in practice a stretch s that rapidly changes from a stretched to an unstretched condition is desirable . one method to obtain a rapid change is to reduce the time constant τ by decreasing the distance x . as shown in fig2 the tangent point 16 of the elastic strip 10 with the stretching roller 12 may be made coincident with the tangent of contact 24 with the poly backing 18 by providing the oscillator frame 30 with an arcuate face plate 34 to press the poly backing in contact with the elastic 10 as the elastic slips off the stretching roller 12 . rollers 32 and 36 are provided at the ends of the face plate 34 and journaled to the oscillator frame 30 for free rotation to reduce the sliding friction of the poly backing 18 around the ends of the face plate 34 . the geometry in fig2 indicates that the velocity v 2 at which the elastic 10 is taken up by the poly 18 is the sum of the poly velocity v p and the tangential velocity v a of the oscillator frame 30 . the geometry in fig2 is more complicated than the geometry in fig1 and thus it is desirable to find another way to reduce the delay in the response of the stretch s to changes in velocity v . although the time constant τ is inversely proportional to v 2 , increasing v 2 does not help since the distance on the final diaper product over which the stretch is changing is the result to be minimized and this distance is defined by x and is independent of v 2 . the response of slew rate of the stretch s may , however , be increased by peaking either v 1 or v s while the stretch s is changing , as shown by the waveforms generally designated 60 and 70 in fig3 . the peaking 62 may be adjusted to obtain a desired rate of slew 72 . peaking of v s is easily obtained by varying the tangential velocity v a of the oscillator frame 30 . pursuant to the present invention and as shown in fig4 variations in the tangential velocity v a are generated by the profile of an eccentric cam 102 . a recessed track 107 in the cam 102 is provided to receive a cam follower 106 journaled via pin 110 to bracket 108 fastened to the oscillator frame 30 . the oscillator frame 30 pivots about a shaft 100 affixed to the frame 90 of the diaper making machine . as cam 102 is rotated by a drive shaft 104 of the machine drive 92 , the oscillator frame 30 is tangentially displaced periodically in an upward direction 138 and downward direction 140 through a displacement a . the maximum upward displacement 142 and downward displacement 144 are indicated by phantom lines . in the preferred embodiment , rollers 112 and 120 are journaled to the oscillator frame 30 by shafts 114 and 122 and are positioned about the pivot 100 along a line perpendicular to the longitudinal axis of the oscillator frame , thereby minimizing variations in the feed rate v p caused by variations in the tangential velocity v a of the oscillator frame 30 . the elastic 10 is obtained from a spool 124 which may freely rotate around shaft 126 affixed to the machine frame 90 as the elastic is pulled by a preheat roll 126 driven by a drive shaft 128 of the machine drive 92 . the elastic 10 passes around idler 130 journaled to shaft 132 affixed to the machine frame 90 . the elastic then passes around a stretching chill roller 12 driven by a drive shaft 14 of the machine drive 92 , and glue from a glue gun 134 is applied to the elastic 10 at point 136 . the elastic 10 slips off the stretching chill roller 12 at tangent point 16 . the elastic 10 and poly backing 18 are then both fed to the oscillator roller 20 which is journaled on shaft 22 fixed to the oscillator frame 30 . the elastic 10 , having differential stretch induced by displacement of the oscillator frame 30 , bonds to the poly 18 at the tangent point 24 of the oscillator roller 20 and the poly and bonded elastic 28 are fed around idler roller 116 journaled on shaft 118 fixed to the oscillator frame 30 . from idler roller 116 the poly and bonded elastic 28 pass around roller 120 and exit from the elastic bonding portion of the diaper assembly line . the preferred embodiment of fig4 provides satisfactory differential stretch during the manufacture of diapers even without peaking of the oscillator frame 30 tangential velocity v a . typically the feed rate of the poly backing v p is set about 1 . 5 times the feed rate of the elastic v 1 and a triangular displacement cam profile is used to switch the oscillator frame velocity v a between plus and minus one - half the elastic feed rate v 1 . eighteen inch diapers , for example , may be manufactured using an arc displacement a of three inches , an oscillator frame velocity v a of plus and minus eight feet per minute , an elastic feed rate v 1 of sixteen feet per minute , and a poly feed rate v p of twenty - four feet per minute . the present disclosure includes that contained in the appended claims , as well as that of the foregoing description . although this invention has been described in its preferred form with a certain degree of particularity , it is understood that the present disclosure of the preferred form has been made only by way of examples and that numerous changes in details of construction and the combination and arrangement of parts may be resorted to without departing from the spirit and scope of the invention as hereinafter claimed . in fig4 for example , the elastic 10 is bonded to the poly backing 18 , but in alternative diaper designs the elastic 10 could be bonded to the fluid permeable facing sheet by replacing the poly 18 with the fluid permeable facing sheet . moreover , the adhesive could be applied to the poly backing 18 or fluid permeable sheet before bonding to the elastic 10 without substantially affecting the result . these are just a few of the modifications obvious to persons skilled in the art .
apparatus for inserting elastic strips into elastic leg diapers wherein elastic strips are adhesively secured continuously along the crotch area and in relaxed condition along the waist areas including a mechanism for feeding a continuous strip of elastic at a constant rate , a mechanism for feeding a continuous sheet of diaper material at a constant rate , and an oscillating arm for guiding the sheet of diaper material and inducing periodic variations in the velocity or distance of the diaper material with respect to the elastic feeding mechanism , thereby alternately stretching and relaxing predetermined lengths of continuous elastic strips as they are adhesively secured to the diaper material .