fig3 shows an embodiment of this invention . the drawing shows a drive fitting yoke 1 and a shaft member 2 coupled to the yoke 1 by a joint member j 1 . these elements constitute a universal joint and provide one half , serving as a drive assembly , of the drive shaft 3 . the drawing further shows a driven fitting yoke 4 and a shaft member 5 coupled to the yoke 4 by means of a joint member j 2 . these members constitute a universal joint and provide the other half , serving as a driven assembly , of the drive shaft 3 . the pair of shaft members 2 and 5 are each in the form of a sleeve and are splined to each other as at 6 for the transmission of torque and for an axial sliding movement , whereby the halves provide an extensible drive shaft . the drive fitting yoke 1 and driven fitting yoke 4 fit around a drive shaft element 7 and a driven shaft element 8 at fitting portions 9 and 10 respectively where each of the yokes is made rotatable with the corresponding shaft element in the direction of the rotation of the shaft element by well known means . the distance from the base portion of the drive shaft element 7 to the base portion of the driven shaft element 8 in definite and invariable as indicated at l . the drive shaft 3 , more specifically the driven fitting yoke 4 , is engageable with and disengageable from the driven shaft element 8 by the collapsing and extension of the drive shaft 3 due to the sliding movement of the shaft members 2 and 5 relative to each other . unillustrated support means has a shifter 11 which clamps and pulls the driven fitting yoke 4 toward the drive assembly to disconnect the drive shaft 3 from the driven shaft element 8 . the driven fitting yoke 4 is disengageable from the driven shaft element 8 by the leftward movement of the support means with its shifter 11 fitting around the fitting yoke 4 . while the drive shaft 3 is connected to the driven shaft element 8 as well as during the rotation of the drive shaft 3 , the shifter 11 is held out of engagement with the fitting yoke 4 . the pair of shaft members 2 and 5 coupled together are formed with a hollow chamber 12 extending coaxially therethrough and accommodating a spring 13 biasing the shaft members 2 and 5 away from each other . the spring 13 is compressible to such an extent as to permit the sliding movement of the pair of shaft members 2 and 5 required for the disengagement of the fitting yoke 1 , 4 from the shaft element 7 , 8 . the spring 13 is further such that while the driven fitting yoke 4 is fitted around and connected to the driven shaft element 8 , the spring 13 has its maximum extended length or is prevented from further extension while exterting a suitable biasing force on the pair of the shaft members 2 and 5 . accordingly , even when the fitting yoke 1 or 4 is subjected to an action which tends to slip off the yoke from the drive shaft element 7 or from the driven shaft element 8 owing to rotational oscillation or like cause during the rotation of the drive shaft , the displacement of the fitting yoke 1 or 4 from its proper fitting position produces forced compression of the spring 13 in proportion to the amount of the displacement , so that the resulting counteraction hampers the displacement . thus , the spring 13 effectively prevents the removal of the fitting yoke 1 or 4 from the shaft element 7 or 8 or such displacement of the yoke that will adversely affect the function of the drive shaft , against rotational oscillation or like cause produced during the rotation of the drive shaft . the provision of the spring 13 in the hollow chamber 12 formed in the shaft members 2 and 5 centrally thereof permits the spline coupling portion 6 to have an increased diameter without increasing the diameters of the shaft members 2 and 5 , thus lessening the circumferential surface pressure acting on the spline coupling portion 6 per pline tooth and greatly mitigating the frictional resistance against the sliding movement involved . the free extended length of the spring 13 is further so limited that the spring 13 will produce little or no spring action on the pair of the shaft members 2 and 5 while the drive fitting yoke 1 is in fitting connection with the drive shaft element 7 with the driven fitting yoke 4 connected to the driven shaft element 8 . consequently , while the drive shaft is mounted in place , the spring 13 produces little or no action to press the fitting yokes 1 and 4 against the drive shaft element 7 and the driven shaft element 8 axially thereof . however , when oscillation or like cause involved in the rotation acts to slip off the fitting yoke 1 or 4 from the drive shaft element 7 or driven shaft element 8 , the displacement if any of the yoke 1 or 4 from its proper fitting position will create a counteraction resulting from the forced compression of the spring 13 proportional to the displacement , with the result that the displacement can be hindered as already stated . with the drive shaft 3 in its fitting position , the spring 13 is therefore nearly in its free state almost free of any compression even when subjected to pressure acting axially in alternating directions . the spring 13 is accordingly almost free of any fatigue . furthermore since the closed hollow chamber 12 accommodating the spring 13 is defined by the inner cylindrical surfaces of the shaft members 2 and 5 and is in communication with the spline coupling portion 6 , the lubricant applied to the spline coupling portion 6 through an oil port 14 ingresses into the chamber 12 , lubricating the spring 13 to prevent the corrosion shaft member 5 or the like . as a result , the structure described renders the spring 13 serviceable properly over a prolonged period of time , retaining the parts in position with stability over a prolonged time period . to disconnect the drive shaft 3 from the drive shaft element 7 or driven shaft element 8 , the drive shaft 3 must be collapsed against the action of the spring 13 . since the spring 13 is almost in its free state while the drive shaft 3 is in its fitting position , the drive shaft 3 can be forced to collapse with ease and smoothness by sliding the shaft members 2 and 5 relative to each other against the spring 13 . springs seats 15 , 16 are attached to the shaft members 2 and 5 respectively at the opposite ends of the hollow chamber 12 defined by the shaft members . the spring 13 is provided between the spring seats 15 and 16 . in the driven assembly , the outer peripheral portion of the spring 13 is supported by the inner wall 5a of the shaft member 5 defining the hollow chamber 12 . in the drive assembly , the inner peripheral portion of the spring 13 is supported by a guide bar 17 extending from the spring seat 15 secured to the shaft member 2 , the guide bar 17 projecting into the hollow space defined by the inner wall 5a . the inner wall 5a and the guide bar 17 prevent the spring 13 from buckling . a cover 18 is secured at its base end to the outer periphery of the shaft member 5 and has a forward end slidably fitting around the shaft member 2 . the cover 18 is provided on the inner periphery of its front end with a projection 18a engageable with a projection 2a on the outer periphery of the inner end of the shaft member 2 . the engagement between the projections 18a and 2a holds the shaft members 2 and 5 together against inadvertent disconnection when the drive shaft 3 is brought out of place as disconnected from the drive shaft element 7 or driven shaft element 8 . the engagement between the projections 18a and 2a serves to prevent the drive shaft 3 in its fitting position from extending to a length greater than is required . the spring 13 may be so adapted as to exert no biasing force on the shaft members when the drive shaft 3 is mounted in place . the cover 18 also serves to protect the spline coupling portion 6 from dust . a seal can be provided in the clearance between the cover 18 and the shaft member 2 . the maximum extended length of the spring 13 can be limited by the engagement between the projections 18a and 2a . with the drive shaft 3 fitted and connected to the drive shaft 7 and to the driven shaft 8 , the spring 13 may bias the shaft members 2 and 5 away from each other , but the spring force is received by the shaft members 2 and 5 and is prevented from being delivered to the joint members j 1 , j 2 , fitting yokes 1 , 4 , drive shaft element 7 and driven shaft element 8 . state more specifically , the cover 18 limits the maximum distance over which the shaft members 2 and 5 are movable away from each other under the action of the spring 13 to a predetermined distance by virtue of the engagement between the projections 18a and 2a , thus limiting the range of the action of the spring 13 by which the shaft members 2 and 5 are movable away from each other . as already mentioned , the distance from the base portion of the drive shaft element 7 to the base portion of the driven shaft element 8 is limited to a definite distance l in fig3 . it is further supposed that the distance from the position of the engagement between the projections 18a and 2a to the outer end of the drive fitting yoke 1 is l 1 , and that the distance from that position to the outer end of the driven fitting yoke 4 is l 2 . the above - mentioned maximum distance is so limited that l ≧ l 1 + l 2 , the distance l 1 + l 2 being the entire length of the drive shaft 3 . thus , axial clearances s 1 and s 2 are provided between the opposite axial ends of the driven fitting yoke 4 and the corresponding ends of the driven shaft element 8 respectively . alternatively , such clearances can be eliminated where the yoke exerts no axial pressure on the driven shaft element 8 . fig4 shows another embodiment of this invention in which the spring 13 in the foregoing embodiment is divided into two segments 13a and 13b with a spacer ring 19 provided therebetween . this makes it possible to prevent the buckling of the spring 13 more effectively and renders the spring easy to make .