referring now to fig1 and 2 , the mechanism for driving a print head and paper advancing mechanism in a serial printer wherein printing occurs during the reversible rectilinear displacement of the print head is shown . a cylindrical cam 1 having a spiral groove in a closed loop 1a and a cam gear 2 fixedly mounted at one end thereof is supported on a printer frame ( not shown ). a guide shaft 4 is mounted on the printer frame adjacent to and parallel to cylindrical cam 1 and supports a print head 3 slideably mounted thereon . print head 3 includes a projection 5 adapted to engage spiral groove 1a on cylindrical cam 1 in camming like fashion . a motor 8 rotates a motor shaft 8a in one direction only . a motor gear 7 is mounted on motor shaft 8a . a reduction gear 6 is rotatably mounted on the printer frame and engaged with motor gear 7 . a paper feed gear 9 is fixedly supported on a paper feed shaft 10 for advancing a print paper 15 through the printer . the ratchet 11 , a paper feed roller 12 and a torsion spring 13 are mounted on shaft 10 . shaft 10 , ratchet 11 , paper feed roller 12 and torsion spring 13 constitute a spring clutch mechanism for advancing recording paper 15 as a trigger lever 14 engaged with ratchet 11 is intermittently released in a direction f to disengage from ratchet 11 as will be described in more detail below in connection with operation of the drive mechanism . in the drive mechanism illustrated in fig1 and 2 , motor 8 always rotates in the direction of arrow c . rotation in direction of arrow c is transmitted to cam gear 2 by motor gear 7 which also rotates in direction c . reduction gear 6 rotates in an opposite direction to motor gear 7 and cylindrical cam 1 thereby rotates in a direction of arrow d , which in this case is the same direction as arrow c . as cylindrical cam 1 is rotated in arrow direction d , print head 3 is reversibly displaced rectilinearly in arrow directions a and b as projection 5 follows spiral groove 1a in camming like fashion . printing on recording paper 15 occurs as print head 3 is displaced in arrow direction a or b . as print head 3 is displaced , paper feed shaft 10 is rotated in arrow direction e by motor gear 7 engaged with reduction gear 6 , cam gear 2 and paper feed gear 9 . there comes a time when print head 3 is displaced out of a printing range or into the margin of print paper 15 . the printing range is the distance within which print head 3 prints on print paper 15 . after print head 3 is displaced out of the printing range in the direction of either arrow a or b , trigger lever 14 is rotated either mechanically or electrically in the direction of arrow f against a spring 16 . this displacement of trigger lever 14 releases ratchet 11 from engagement with trigger lever 14 thereby permitting paper feed roller 12 to rotate in the direction of arrow e through torsion spring 13 by a given amount until the next tooth of ratchet 11 engages with trigger lever 14 which has been returned to its rest position by the biasing force of spring 16 . during the time paper feed roller 9 has rotated , print paper 15 is advanced . referring now to fig3 and 4 , a drive mechanism for a serial printer wherein a cylindrical cam 17 includes a spiral groove in open looped configuration 17a is shown . a print head 18 includes a projection 18a which engages spiral groove 17a in camming like fashion and in the same manner as print head 3 and projection 5 engage spiral groove 1a in the mechanism illustrated in fig1 and 2 . the remaining component parts in the drive mechanism illustrated in fig3 and 4 remain the same as in the mechanism illustrated in fig1 and 2 . accordingly , like elements are identified by the same reference numerals primed . when a two directional motor 8 &# 39 ; is rotated in the direction of arrow c , cylindrical cam 17 is rotated in the direction of arrow d and print head 18 is displaced in the direction of arrow a as projection 18a follows groove 17a . as print head 18 is displaced , printing occurs on a print medium . when print head 18 is displaced in arrow direction a so as to be out of the printing range , the paper feed mechanism ( not shown ), which may be of the type utilizing a trigger as illustrated in fig1 and 2 , advances the recording paper . after the recording paper has been advanced , the direction of motor 8 &# 39 ; is reversed to direction g and cylindrical cam 17 is rotated in the direction of arrow h . when cylindrical cam 17 is rotated in the direction of arrow h , print head 18 is displaced in the direction of arrow b for printing on the recording medium in this opposed direction . the drive mechanisms of the printers described in fig1 - 4 are not completely satisfactory . firstly , in each case cylindrical cams 1 and 17 are rotated continuously as motors 8 and 8 &# 39 ; are rotated for displacing print heads 3 and 18 in the direction of arrows a or b during the paper advancing operation . for this reason , the loads due to both paper advancing and displacement of print heads are placed on the motors simultaneously , making it difficult to reduce the power consumption for the motor . such operation is often the main reason for problems in connection with paper advancing , such as paper jams and wandering of the paper , and the like . additionally , the distance print heads 3 and 18 are reversibly displaced includes both the printing range and an additional distance into the margin for performing the paper advancing operation . thus , print heads 3 and 18 are required to be displaced a distance greater than merely the printing range . such additional distance prevents additional miniaturization of serial printers constructed and arranged in accordance with these constructions . the second shortcoming of these constructions is the manufacturing cost . a cylindrical cam having a spiral groove of the closed loop type as illustrated in the drive mechanism of the printer in fig1 and 2 is expensive to manufacture . additionally , the drive mechanism of the printer illustrated in fig3 and 4 include a driving circuit which is expensive since the motor is rotated in a forward and reverse direction . both factors are obstacles to reducing the manufacturing costs in either the printer of the type in fig1 and 2 or the printer of the type in fig3 and 4 . additionally , in the drive mechanism of the printer illustrated in fig3 and 4 power consumption is increased when the motor is reversed . further , when using a dc motor , this large power consumption often causes a further problem as it decreases the useful life of the motor brush . referring now to fig5 - 8 , the drive mechanism for a printer constructed and arranged in accordance with a preferred embodiment of the invention which overcomes many of the shortcomings is shown . referring specifically to fig5 and 6 , a cylindrical cam 20 having a spiral groove 20a of the open loop configuration is shown . a cam gear 21 is mounted at one end of cylindrical cam 20 . a print head 22 is slideably mounted on a guide shaft 23 and includes a projection 22a adapted to engage spiral groove 20a . cylindrical cam 20 is driven by a drive shaft 24 which is rotated in one direction only by a drive source ( not shown ). a drive gear 25 is mounted at the drive end of drive shaft 24 and a clutch plate 26 is also fixedly mounted on drive shaft 24 adjacent to drive gear 25 . a change wheel 27 which functions as a rotary member is rotatably mounted on drive shaft 24 and includes a first gear portion 27a and a second gear portion 27b on the periphery thereof with an integral between the gear portions of 180 °. a plurality of pins 28 , 29 , 30 and 31 are mounted on change wheel 27 . a clutch lever 32 including an engaging tooth 32a is mounted on pin 28 . clutch lever 32 is free to be rotated about pin 28 from a first position out of engagement with clutch plate 26 as shown in fig6 and a second engaged position as shown in fig7 . a small planet gear 33 is mounted on pin 29 which is always rotated by engagement with drive gear 25 . a second large planet gear 34 is mounted on pin 30 which is always rotated by its engagement with drive gear 25 . large planet gear 34 is dimensioned to be engageable with cam gear 21 . an intermediate gear 35 is mounted on pin 31 and is always rotated by its engagement with small planet gear 33 which , in turn , is engageable with drive gear 25 . intermediate gear 35 is dimensioned to be engageable with cam gear 21 for driving cylindrical cam 20 . the paper advancing assembly of the drive assembly , illustrated in fig6 - 8 , includes a paper feed gear 36 which is fixedly mounted on a paper feed shaft 38 which is rotatably supported in the printer frame ( not shown ). a paper feed roller 37 is also mounted on paper feed shaft 38 . paper feed roller 37 is intermittently rotated a fixed amount by intermittent engagement with first gear portion 27a and second gear portion 27b of change wheel 27 when change wheel 27 is rotated as will be described in more detail in connection with operation of the drive mechanism . referring now to fig5 an elongated change lever 39 having a first engaging arm 39a and a second engaging arm 39b is arranged so that first and second engaging arms 39a and 39b are engaged by print head 22 as it approaches the end of travel in a print direction . change lever 39 is mounted for displacement in the two directions of travel of print head 22 and is pivotally engaged through a pin 41 to an engagement lever 40 which is free to be rotated about a fulcrum 42 . engagement lever 40 is engageable with clutch lever 32 through a first engagement portion 40a and a second engagement portion 40b . when change lever 39 is displaced by its engagement with print head 22 , engagement between one of the engagement portions 40a or 40b and clutch lever 32 is released so that change wheel 27 is free to rotate until clutch lever 32 engages a further engagement portion thereby stopping rotation of change wheel 25 . operation of the print drive mechanism shown in fig5 - 8 will now be described . referring specifically to fig5 and 6 , rotation of drive shaft 24 which is always rotated in the direction of arrow i is transmitted to cylindrical cam 20 . rotation is transmitted through drive gear 25 , small planet gear 33 rotated in the direction of arrow j , intermediate gear 35 rotated in the direction of arrow k and cam gear 21 fixed on cylindrical cam 20 . cylindrical cam 20 is rotated in the direction of arrow l and print head 22 is displaced along guide shaft 23 in the direction of arrow m as projection 22a follows spiral groove 20a in camming like fashion . as print head 22 is displaced in the direction of arrow m , print head 22 prints on recording paper 44 within the printing range . as print head 22 is displaced in print direction m , clutch lever 32 is held in its first position out of engagement with clutch plate 26 by engagement of engaging portion 40a of engagement lever 40 . when in this configuration , as shown in fig6 rotation of change wheel 27 is suspended . as print head 22 enters out of the print range in print direction m , print head 22 engages first engaging arm 39a of change lever 39 located within the locus of print head 22 . change lever 39 is displaced in arrow m thereby pivoting engagement lever 40 in the direction of arrow n at the same time . when engagement lever 40 is rotated by more than a fixed angle in the direction of arrow n , clutch lever 32 is disengaged from engaging portion 40a of engagement lever 40 . this disengagement permits release of clutch lever 32 which is then rotated about pin 28 in the direction of arrow o by a spring 43 . engaging tooth 32a of clutch lever 32 then engages clutch plate 26 . change wheel 27 is then engaged with drive shaft 24 through clutch plate 26 and change wheel 27 together with clutch lever 32 is rotated in the direction of arrow i . when change wheel 27 begins to rotate in arrow direction i , intermediate gear 35 is disengaged from cam gear 21 and cylindrical cam 20 stops rotating . at this time , print head 22 , change lever 39 and engagement lever 40 are in a stand - by state as illustrated by a two - dot - line in fig5 . change wheel 27 continues to rotate in arrow direction i by drive shaft 24 through operations of clutch plate 26 and clutch lever 32 . as rotation continues , first gear portion 27a of change wheel 27 engages paper feed gear 36 as shown in fig7 and paper feed roller 37 thereby is rotated at a fixed angle for advancing recording paper 44 . as change lever 27 continues rotating for about 180 °, clutch lever 32 is engaged by the other engaging portion 40b of engagement lever 40 and begins to be returned to its first at rest position in arrow direction p against spring 43 . this causes engaging tooth 32a to disengage from clutch plate 26 thereby bringing the members to a halt as shown in fig8 . at this time , change wheel 27 suspends rotation after being rotated through 180 ° from the previously suspended position illustrated in fig5 . at this time , cam gear 21 is engaged by large planet gear 34 as shown in fig8 and the change of the rotating direction of cylindrical cam 20 is completed by operation of the rotary clutch assembly . when the elements are in position as illustrated in fig8 the rotating direction of drive shaft 24 , which is always rotated in the direction of arrow i , is transmitted to cylindrical cam 20 . this rotation in arrow direction i is transmitted through drive gear 25 , large planet gear 34 which is rotated in the direction of arrow r and cam gear 21 . cam gear 21 is rotated in the direction of arrow s , contrary to the rotational direction illustrated in fig6 . thus , print head 22 is displaced along guide shaft 23 in the direction of arrow t for printing on paper 44 within the printing range . when print head 22 is displaced to the end of the printing range , print head 22 engages second engaging arm 39b of change lever 39 which is in the position as illustrated by the two - dot - line in fig5 . at this time , change lever 39 is displaced in the direction of arrow t thereby causing engagement lever 40 to be rotated in the direction of arrow u . when engagement lever 40 is rotated more than the fixed angle , clutch lever 32 is released from the engagement with clutch plate 26 and change wheel 27 begins to rotate in the direction of arrow i in the same manner as described with respect to changeover from rotating cam gear 21 in arrow direction l to rotating cam gear 21 in arrow direction s . as the change wheel 27 begins to rotate further , large planet gear 34 is disengaged from cam gear 21 and cylindrical cam 20 stops rotating . at this time , print head 22 , change lever 39 and engagement lever 40 come into the stand - by state . as change wheel 27 rotates , second gear portion 27b of change wheel 27 engages paper feed gear 36 and paper feed roller 37 is rotated by the fixed angle in the direction of arrow q , as shown in fig7 for advancing paper tape 44 . when change wheel 27 has been rotated an additional 180 ° from the position illustrated in fig8 it comes to rest as shown in fig6 when cam gear 21 is engaged again with intermediate gear 35 and print head 22 commences to be displaced in the direction of arrow m as described above . in a printer including a drive mechanism , as illustrated in fig5 - 8 , print head 22 is displaced reversibly and includes a temporary suspension of displacement at both ends of the printing range . the printing of successive lines on recording paper 44 is made possible by repeating the printing operation which occurs while print head 22 is reversibly displaced with paper advancing occurring during the suspension of displacement of print head 22 . in other words , paper advancing occurs when the direction of displacement of print head 22 is being changed . several advantages are obtained by constructing and arranging the drive mechanism for a printer in accordance with the preferred embodiment just described . first , the print head is forced into rectilinear reciprocating motion with inexpensive component parts without requiring a two speed motor . this reduces the manufacturing cost and extends the lifetime of the motor and the power consumption of the printer is also reduced . second , since the print head displacement is suspended at the time of paper feeding , the load placed on the motor is reduced and the distance which the printer head need travel is also reduced . therefore , there is less power consumed in further miniaturization and a high speed printer is achieved while avoiding the troublesome problems of paper feeding , such as a paper jam or wandering print are avoided . third , as paper advance occurs when the print head is not being displaced , the overall distance of displacement is reduced leading to further reduction in size of the printer . it will thus be seen that the objects set forth above , among those made apparent from the preceding description , are efficiently attained and , since certain changes may be made in the above construction without departing from the spirit and scope of the invention , it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense . it is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described and all statements of the scope of the invention which , as a matter of language , might be said to fall therebetween .