in fig1 there is shown a front elevation view of a powered rotary screed 10 of the present invention . in this preferred embodiment , the rotary screed is comprised of four modules 12 separated by structural plates 14 . a pair of drive tubes 16 ( only one drive tube shown ) and a strike tube 18 are rotatably connected to the plates 14 . at one end of the rotary screed a power supply 20 and hydraulic pump 22 are mounted onto a platform 24 . the hydraulic pump supplies hydraulic pressure to hydraulic motors 26 which power the drive tubes 16 and motors 27 which rotate the strike tubes 18 . at the other end of the rotary screed there is located an operator station having a seat 28 , control station 30 and foot rest 32 . as stated , the rotary screed 10 is preferably comprised of four modules 12 . in the preferred embodiment , the modules comprise drive modules 34 and 36 and two screed modules 38 . all the modules 12 share the common characteristic of a frame section having a ridge stringer 40 whose ends are connected to the top portions of the structural plates 14 and two base stringers 42 connected to the bottom portions of the plates 14 . the ridge stringers and base stringers are connected by metal sheets 44 for rigidity . the drive modules 34 and 36 are preferably a standard length of approximately four feet . the drive modules 34 , 36 have a frame including the stringers 40 and 42 . at their outermost , distal ends , the stringers 40 , 42 are connected to a distal plate 46 by a fastening system 50 . at their other end , the stringers 40 , 42 are connected to an intermediate plate 48 . as can be seen in greater detail in fig2 the fastening system 50 comprises a tenon 52 that is bolted to the distal plate 46 by a bolt 54 . the tenon is received within a socket 56 that is welded to the distal plate 46 . the base stringer 42 fits over an end of the tenon 52 and slides toward the distal plate 46 until it abuts shoulders 58 on the tenon 52 . holes are provided through the base stringer 42 and tenon 52 to receive bolts 60 for securing the stringer and , in turn , the distal plate 46 . in a like manner , all of the stringers are connected to the structural plates 14 with the exception of the medial connections of the ridge stringer 40 , which connection will be described separately below . both drive modules 34 , 36 include one drive tube 16 that spans between the distal plate 46 and the intermediate plate 48 . attached to each drive tube is a hydraulic motor 26 that rotates the drive tube , which in turn propels the rotary screed 10 . the drive tubes attach to plates 46 , 48 by means of rotary thrust bearings as described below in connection with the strike tube 18 . the drive tubes will preferably have a high friction surface such as rubber or similar material . the screed modules 38 are preferably available in a plurality of lengths at four - foot increments . for example , screed modules may be available at lengths of 4 , 8 , 12 , 16 feet , etc . the four - foot increments of the screed modules coincide with the preferred embodiment of the four - foot length for the drive modules so that the rotary screed can always be configured for the proper width of a screed job . as noted above , screeds ride on guide rails ( not shown ). by coordinating the length increments of the screed modules to the preferred length of the drive modules , the rotary screed 10 can always be configured so that the drive rollers will ride on guide rails regardless of their separation distance . each screed module 38 connects at one end to one intermediate plate 48 and connects at its other end to a medial plate 62 . as stated above , the screed modules 38 have the ridge stringer 40 and two base stringers 42 that are interconnected by the structural skin 44 . the ridge stringers of the screed modules 38 connect at the medial plate 62 by means of an expansion member 64 . as seen most clearly in fig3 the expansion member 64 comprises two power screws 66 that thread into collars 68 that are welded to the ridge stringers 40 . each power screw 66 is provided with a threaded portion 70 , a shoulder nut 72 and a smooth surface stub 74 . the stub 74 is received within a receptacle 76 that is welded to the medial plate 62 . a hole is provided through receptacle 76 and stub 74 to receive a pin 78 for securing the power screw 66 . because the stub is smooth - surfaced , it is easily removed from , and inserted into , the receptacles 76 , without turning the power screws 66 or unscrewing them from the ridge stringer 40 . the purpose of the expansion member 64 is to adjust the effective length of the ridge stringers 40 . when one or both of the screws 66 are rotated to increase the effective length of the ridge stringers 40 , top portions of the plates 46 , 48 on one side of the medial plate are moved away from top portions of the corresponding plates 46 , 48 on the other side of the medial plate , thereby causing the medial plate 62 to move upward raising medial ends 80 of the strike tubes . in this manner , the rotary screed 10 can be configured for long lengths because any sag that occurs in which the medial ends 80 of the strike tubes dip below distal ends 82 can be adjusted by turning the power screws 66 . a platform 24 is mounted onto the top of the drive module 34 for supporting a power supply 20 and pump 22 . preferably , the power supply 20 is a gasoline - powered engine that is coupled to a hydraulic pump 22 for generating hydraulic pressures for running hydraulic motors 26 and 27 . alternatively , the power supply 20 could be a gasoline - powered generator generating electricity to drive an electric hydraulic pump that would supply hydraulic pressure to run the hydraulic motors . alternatively , the power supply could be an electric generator supplying electricity to electric motors , instead of the hydraulic system with hydraulic motors 26 , 27 . on top of the drive module 36 is an operator station having seat 28 and control panel 30 . preferably , the operator will have controls for controlling the speed and direction of each drive tube 16 individually as well as for controlling the direction and rotary speed of the strike tubes 18 . as noted , the rotary screed 10 preferably comprises two drive tubes 16 and two strike tubes 18 . each drive tube 16 extends between one distal plate 46 and one intermediate plate 48 . each strike tube 18 extends from one distal plate 46 to the medial plate 62 , passing beneath the intermediate plates 48 . hydraulic motors 26 drive the drive tubes 18 . hydraulic motor 27 drives the strike tube 18 . preferably , the strike tube will be driven in a rotational direction opposite to that of the drive tubes for optimal screeding of the cementious material . in the preferred embodiment , the drive tubes will be capable of propelling the screed 10 at speeds up to 45 ft / min ., and the strike tubes can be rotated at speeds up to 200 rpm . as best shown in fig2 the tubes 18 are connected to the plates 14 by thrust bearings 86 that are bolted to the plates . where the tubes connect to a hydraulic motor 27 , a splined and threaded shaft 88 is provided wherein the splined portion 90 passes through the plate and connects to a coupler 92 , which in turn connects to the hydraulic motor 27 . on a threaded portion 94 of the shaft 88 are two nuts : jam nut 96 and lock nut 98 . with reference to fig2 it can be seen that turning jam nut 96 so that it progresses along the threaded portion 94 in the direction of the hydraulic motor 27 will urge the strike tube 18 away from the distal plate 46 and toward the medial plate 62 . the adjustment of the strike tube 18 in the direction of the medial plate 62 is resisted by a compression spring 103 ( fig3 ). after the jam nut 96 has been properly snugged against the thrust bearing 86 so that the strike tube 18 is properly mounted between the distal plate 46 and the medial plate 62 , then the lock nut 98 can be tightened against the jam nut 96 to lock it into position . drive tubes 16 are likewise rotatably connected to plates 48 and hydraulic motors 26 . the tubes 18 connect to the medial plate 62 in a similar fashion . in fig3 it is seen that the strike tubes 18 are fixedly connected to a splined shaft 100 that is connected to the thrust bearings 86 by set screws in collars 101 . springs 103 encircle the shaft 100 and are compressed between the strike tubes 18 and the medial plate 62 for urging the strike tubes toward the distal plates 46 so the strike tubes will not rub against the medial plate . fig4 and 5 show cross - sections taken along lines 4 -- 4 and 5 -- 5 of fig1 respectively . in fig4 there can be seen the hydraulic pump 22 resting atop platform 24 mounted onto a top surface of the distal plate 46 . ridge stringer 40 and base stringers 42 are connected by the structural skin 44 . drive tube 16 and strike tube 18 are shown connected to lower portions of the distal plates 46 . in fig5 there is shown a further cross - section through the rotary screed 10 looking towards the medial plate 62 . fig6 - 8 show the configurations of the distal plate 46 , intermediate plate 48 and medial plate 62 , respectively . the distal plate 46 contains holes , 102 , 104 near its lower region for receiving the mounting splines of the strike tube and drive tube , respectively . holes 106 are provided for connection to the stringers 40 and 42 . the intermediate plate 48 has a hole 108 to receive the connecting spline for the drive tube , but is cut out to permit the strike tube 18 to pass beneath the intermediate plate 48 . medial plate 62 has a hole 110 for connecting to the strike tubes 18 . all of the plates 46 , 48 , and 62 are provided with a cutout 112 in order to reduce weight . in view of these and the wide variety of other embodiments to which the principals of the invention can be applied , the illustrated embodiments should be considered exemplary only and not as limiting the scope of the invention . i claim as the invention all such modifications as may come within the scope and spirit of the following claims and equivalents thereto .