Abstract:
An actuator for engaging and disengaging parts together which, during actuation may incur an obstacle to engagement, the actuator including a mechanism for incurring the obstacle and automatically stopping actuation, a mechanism for removing the obstacle to engagement and for automatically restarting actuation upon removal of the obstacle thereby to achieve engagement.

Description:
This invention generally relates to actuators for bringing gears into meshing engagement. More particularly, this invention relates to such actuators which find particular, but not exclusive, utility as part of a power takeoff device attachable to the transmission of a vehicle. 
     BACKGROUND OF THE INVENTION 
     The mechanical arts have universally had to face the problem of how to successfully bring two rotatable gears, selectively, into and out of gear meshing engagement, such that both gears when engaged will then operate (rotate) together when desired, to do some form of useful work, normally one gear driving the other. One environment in which this problem has had to be faced and solved if the device is to be commercially acceptable and safe to employ, is in the well known art of power takeoff devices (herein referred to by their industry acronym “PTO&#39;s”). PTO&#39;s have found commercial uses in a wide variety of areas, such as in the vehicular arts where the PTO is attached to the transmission of the vehicle for translating the power of the engine into the rotation of a shaft for doing useful work auxiliary to the vehicle itself (e.g. operating a hydraulic pump for raising and lowering a dump truck bed, compacting a garbage compactor, and the like). 
     In order to perform their assigned tasks PTO&#39;s are generally formed of an arrangement of gears associated with an output shaft and a mechanism for selectively engaging and disengaging one of the gears in the PTO with another gear in the PTO which is constantly engaged with a corresponding gear in, for example, the transmission of the vehicle. Since it is highly undesirable in most instances to have the PTO output shaft constantly rotating, various devices have been developed in the art for effecting, selectively, the aforesaid engagement and disengagement, whether the PTO be attached to a vehicle (truck, tractor, etc.) or to some other power driven device (e.g. stationary engine rig). 
     In the normal situation experienced in the PTO art, meshing engagement and/or disengagement of the gear(s) of the PTO is generally achieved by axially displacing the operative PTO gear with respect to its input gear. Axial displacement for meshing engagement or disengagement (with the gears at rest, i.e., without synchronization) is usually achieved by means-of mechanical actuators, which may be pneumatically or electrically (and less often manually) operated. 
     There are certain known drawbacks to the actuators currently in use. For example, certain electrically operated actuators employ solenoids which are rather large and space consuming and provide a limited amount of actuation travel. Certain other electrical actuators use electric motors which actuate screw transmissions having or requiring bearing recirculation and/or which are operated by complex electronic control circuits. For example, to mesh the gears requires a “dosed” sequential execution having a very precise succession of operative phases (or steps). In certain prior art devices, furthermore, the axially movable gear (wheel) must be brought into contact with an axially fixed gear and thereafter maintained in pressed contact against the fixed gear until, by the effect of relative rotation, the gear teeth of the two gear wheels are caused to coincide (i.e., enter into the inter-gear spaces of the other) thus achieving operative meshing engagement. Moreover, in order to carry out the above-described phases (steps), special operations have to be put in place. For example, the use of a screw transmission with,bearing circulation may have to be employed, as well as a fine and sophisticated adjustment of the electric motor through the use of a complex and expensive control circuit employed to actuate it. 
     In another known type of actuator, an electric motor is employed to draw in, by rotation and usually with an axially slidable, geared coupling, an axially-slidable intermediate rotating body. In such a device a nonrotatable screw coupled with a lead screw bored coaxially into the intermediate body, functions as the moving part of the actuator. In this configuration, the positioning of the intermediate body in the direction of mesh is not rigid. Rather it is governed or controlled by an intermediately located, specially calibrated spring. If meshing is accomplished, the screw in the intermediate body becomes immediately locked and the body acts rigidly on a slidable rod to directly command an appropriate “endrun” switch to actuate and stop the motor. If meshing is not achieved (i.e., is messed), the intermediate body is caused to retreat from the meshed position. When and if complete meshing is achieved, the screw head retreats with respect to the intermediate body, eventually terminating travel by acting directly on another “endrun” switch which, again, deactivates the electric motor. 
     The drawbacks to these known devices are known and generally include, as their main drawback, the fact that the alignment of the parts for actuating the switches must be of such preciseness that repeatability becomes unreliable. 
     It is apparent from the above that there exists a need in the art for an actuator which overcomes or at least minimizes the above-described drawbacks endemic to prior actuators, as above-described. Preferably, such an actuator would also be simple to operate, economical in its simplicity of parts, and yet reliable in its operation. It is a purpose of this invention to fulfill this and other needs in the art apparent to the skilled artisan once given the following disclosure. 
     SUMMARY OF THE INVENTION 
     Generally speaking this invention fulfills the above needs in the art by providing: 
     an actuator comprising a frame and a rotatable intermediate body freely rotatable with respect to the frame and provided with an externally toothed gear wheel meshingly engaged with a pinion fixed to an end of a shaft of an electric motor, the rotatable intermediate body being further provided with a lead screw area extending therewithin and arranged coaxially thereto; the intermediate body being capable of moving axially in either direction over a predetermined distance which distance is sufficiently limited in length so as to maintain the meshing engagement between the externally toothed wheel and the pinion; the actuator further comprising a spring operatively located coaxially between the frame and the intermediate body, a screw shaft member coaxially and screwably coupled with the lead screw area and which has a first end connected to the frame by a coupling member which prevents relative rotation while allowing free axial movement, a switch means for operating the electric motor, the switch means being actuatable by the intermediate body upon relative movement of the intermediate body. In preferred embodiments the intermediate body is not mechanically rigidly connected to the switch means. 
     In certain particularly preferred embodiments of this invention, the aforesaid actuator is operatively coupled to a power takeoff device for selectively engaging and disengaging the input gear of the power takeoff device with its output shaft. 
    
    
     This invention will now be described with respect to certain embodiments thereof as illustrated in the following drawings wherein: 
     IN THE DRAWINGS 
     FIG. 1 is a side sectional view of an embodiment of an actuator according to this invention when coupled to a conventional PTO in its disengaged position; the PTO in turn being coupled via its input gear to the output gear of a conventional transmission. 
     FIG. 2 is a top plan, sectional view of the actuator of FIG.  1 . 
     FIG. 3 is a top plan, sectional view of another embodiment of this invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to the drawings, and initially with particular reference to FIG. 1, there is presented an actuator  1 , which constitutes an embodiment of this invention, connected to a conventional power takeoff (PTO) device  22  which in turn is connected to a typical transmission T (e.g. of a truck). It will be understood in this respect that the figures are not dimensionally accurate with respect to the relative size of actuator  1  as compared to PTO  22 , and that only the operative output gear Tg of transmission T (and only a portion of T&#39;s housing) is shown for convenience. Actuator  1  has been enlarged in FIG. 1 so that its internal mechanisms can be more clearly illustrated. In actual practice it will be made as small as feasible to conserve space and for economy of cost. Moreover, it is also understood that the PTO illustrated is only representative of a wide variety of types and sizes of PTO&#39;s to which the actuators of this invention may be operatively applied. In this respect, PTO  22  as illustrated is a conventional PTO well known in the art. Schematically and only partially illustrated, conventional transmission “T” is likewise presented for illustrative purposes only. Other types of transmissions or indeed, nontransmission driving mechanisms are contemplated within the scope of this invention. In addition, the actuators of this invention find utility in the gear meshing art generally and are not limited to their use in association with PTO&#39;s. However, because of the high utility of the actuators of this invention for use in operating PTO&#39;s in the truck art (e.g. operating hydraulic pumps associated with dump trucks, garbage compactor trucks, snowplows, spreaders, and the like) such an environment is a preferred one for the purposes of this invention. 
     To best explain the embodiment of this invention as illustrated in FIG. 1, attention is first directed to PTO  22 , a conventional type of PTO used, for example, throughout the trucking industry. PTO  22  includes the usual housing “H” which, in a known manner, is connected to transmission T through window “W” in the transmission housing and which is provided by the transmission manufacturer for that purpose. Characteristically, the window, when opened, exposes the requisite transmission gear Tg for operating PTO  22 . 
     PTO  22  is provided with an input gear mechanism generally shown at  21   a  which includes laterally spaced cojoined gear (toothed) wheels  21   b  and  21   c , respectively, and which are rotatable about or with common shaft S. PTO  22  further includes output shaft  100 , which is the shaft when rotated that performs the intended auxiliary work (e.g. operates a hydraulic pump for raising a dump bed of a truck, etc.). As is known in the art, it is highly undesirable in most PTO operations, particularly in the truck art, for shaft  100  to be continuously rotating for lengthy periods of time. This in turn gives rise to the need for an actuator which can effectively engage and disengage shaft  100  for rotation or nonrotation as desired. This invention fulfills this need as now demonstrated with reference to the embodiment of FIGS. 1-2. 
     With specific reference to FIG. 1, conventional gear teeth  21   c  of PTO  22  are in continuous meshing engagement with the teeth of gear Tg of a transmission T (e.g. of a truck engine) and thus gear  21   a  (as well as gear teeth  21   b ) will rotate whenever transmission gear Tg rotates (which in the usually truck environment is whenever, for example, the truck engine is operating and the clutch is not depressed). To prevent unwanted rotation of output shaft  100 , there is provided on shaft  100  (e.g. as by splinning  105 ) a gear (toothed) wheel  23  having teeth  21   d  which teeth are designed to be capable of meshing engagement with teeth  21   b . Wheel  23  is further provided with a slot (circumferential groove)  103  for accommodating one end of actuator arm  20 . This end of actuator arm  20  is designed along with slot  103  to allow wheel  23  to freely rotate without rotating actuator arm  20 , but to allow the end of actuator arm  20  located in slot  103  to alternately be brought into abutting engagement with one of the lateral walls of the slot (i.e. those perpendicular to shaft  100 ) for axial shifting of gear teeth  21   d  into and out of meshing engagement with teeth  21   b  of input gear wheel  21   a . In this respect, such axial shifting of wheel  23  along shaft  100  is accomplished by appropriately designed glide splines  105 . When not in an axial shifting mode, arm  20 , as shown in FIG. 1, preferably does not touch any of the walls, bottom or sides, its dimensions in this respect being less than those of the slot. 
     The other end of actuator arm  20  is rigidly connected to end  28  of shaft  101  which is normally biased by concentric coil spring  29  to the nonengaging position of PTO  22  (as shown in FIG.  1 ). In this respect, shaft  101  is conveniently formed of a large head end  10   a  from which extends a section  28  of lesser diameter to which arm  20  is connected and which finally terminates in a section  28   a  of still smaller diameter. Extending in the path of end section  28   a  and secured in orifice  109  of PTO  22  is a striker (stop mechanism)  107  for stopping the advancement of shaft  101 . Orifice  109  enlarges at  111  to accommodate the striker  107 . Orifice  109 , as well as, has provided in it a positive indicator switch (not shown), threadedly engaged therein, which is activated by end section  28   a  contacting striker  107  thereby to turn on warning light  113  when the PTO is in its operating (engaged) mode. In the truck arts this warning light is normally located in the cab of the vehicle in close proximity to the switch or lever (not shown) in the cab which actuates motor  2 . 
     Enlarged head end  101   a  is held in sliding engagement in its PTO housing orifice  101   b  which includes an O-ring seal  115 . Striker  107  is retained in housing orifice  111  sealed by a sealing ring  117  provided in the housing, sealing the interface between the housing and shaft end  28   a . In conventional fashion, as shown, shaft  100  of the PTO is also provided with conventional seals and bearings as illustrated. 
     Attention is now directed to actuator  1  shown in FIG. 1 which, as aforesaid, is on embodiment of an actuator as contemplated by this invention. Actuator  1  includes a frame or housing “F” which houses the various components of the actuator and provides various orifices for the moving parts as well as abutting stop walls for the biasing coil springs, all as illustrated in FIG.  1 . 
     Housing F has located therein a conventional dc electric motor  2  provided with a shaft  2   a , which has mounted thereon toothed gear pinion  3  to be rotated by shaft  2   a  when motor  2  is operated. 
     An externally crowned (i.e., gear toothed) wheel  5  is provided at the innermost end of (and as an integral part of) intermediate rotatable body  4 . The teeth of wheel  5  are meshingly engaged with their counterpart gear teeth in pinion  3 . Rotatable body  4  is freely supported for rotation in housing F by roller bearings  11 . Body  4  and its wheel  5 , in this respect, are arranged for coaxial rotation about their common centerline “C”. Internal wall portion  6  of body  4  is provided with a lead screw thread area into which shaft  8  having a complementary screw thread on its external surface is threadably engaged and retained thereby for axial extension and retraction depending on the rotational direction of shaft  2   a . Stated another way, shaft  8 , through the aforesaid screw thread arrangement will move axially in one or the other direction when motor  2  is operated in one or the other direction respectively. 
     Coil spring  7 , of predetermined calibration, is provided coaxially about the inner walls of body  4  and shaft  8  so as to extend and create a biasing force between thrust bearing  12  and thrust washer  12   a  and the end of body  4  opposite thrust bearing  12 . At this opposite end of body  4  there is provided further thrust bearings  13  such that the effect of coil spring  7  is to resiliently retain body  4  in its illustrated position so as to insure axial displacement of shaft  8  whenever motor  2  is operated. Retaining spring  7  is provided at one of its ends with an “S” shaped cup member  12   b  whose innermost horizontal flange terminates in housing orifice  119  having an internal wall  10 . 
     Screw shaft  8  is provided at one end thereof with a hexagonal head  18  located internally of sliding guide wall  10 , which, of course, is or may at least be considered part of housing F. Head  18  is designed to allow enough clearance between it and wall  10  of orifice  119  for ease of sliding as well as rotation, but not so much clearance as to allow for undue vibration or radial oscillation of shaft  8 . The lower horizontal end of S-shaped cup member  12   b  terminates well prior to hex head  18  so as not to interfere with any portion of the full, preselected amount of axial travel of shaft  8 . In other words, shaft  8  should be free to move axially (i.e. rotatably retract and extend via its screw threads) in order to properly and effectively actuate PTO  22  by complete or substantially complete meshing engagement of teeth  21   d  with teeth  21   b , but at the same time, not be so loose radially as to allow undue oscillation within orifice  119  which might injure or deteriorate the screw thread connection between shaft  8  and internal wall  6 . 
     At the end of hex head  18  opposite that end connected to shaft  8 , there is located a coaxial coil spring  39  having substantially the same outer diameter as (i.e., slightly lesser than) the internal diameter of housing orifice  119 . Coil spring  39  is compressed between hex head  18  and lower flange  40   c  of activator element  40 . In order to actuate motor  2 , T-shaped switch activator  40  (i.e. an endrun pivot or piston) is located concentrically in orifice  119  within coil spring  39 . Activator  40  is preferably formed of an enlarged head end  40   a  and a shaft portion  40   b  ending as aforesaid in outwardly extending flange  40   c . Striker  42  is provided in housing wall F onto which flange  40   c  is biased by coil spring  18 . Between striker  42  and enlarged head end  40   a  there is located a second biasing coil spring  41 . 
     Electric motor  2  is operated by switch  15  which in turn is operated remotely from, for example, the cab of a vehicle (not shown) through control wires  127  and operating control plug  129 . 
     To engage PTO shaft  100  for rotation when the device is in its disengaged mode as shown in FIG. 1, rotation of gear Tg (if it is rotating) is stopped (e.g. as by depressing the clutch in a non-automatic transmission of a vehicle). Electric switch mechanism  129  is then operated to signal motor  2  to rotate in the engage direction. Shaft  2   a  then operates to drive gear pinion  3  which in turn rotates intermediate body  4 . This causes shaft  8  to unscrew and contact head end  101   a  of shaft  101 . Shaft  101  then moves against the bias of coil spring  29 , moving actuator arm  20  into engagement with a sidewall of slot  103  in wheel  23 . 
     Wheel  23  is then caused to advance along splines  105  on output shaft  100  until teeth  21   d  are brought into meshing engagement with teeth  21   b  of PTO input gear mechanism  21   a . When the clutch is then released, or Tg gear is again otherwise allowed to rotate (by whatever mechanism) output shaft  100  rotates, and is available for useful work. Striker portion  107 , now contacted by the end of portion  28   a  of shaft  101  has been caused at this point through a conventional spring mechanism (not shown) to activate positive indicator switch  111 , turning on warning light  113  to indicate that PTO  22  is engaged. 
     The extent of travel of shaft  8  and thus shaft  101  during the “engage” mode is limited by the actuation of an endrun switch  9  (shown in FIG. 2) which turns motor  2  off. This is accomplished by the use of a predetermined compressed position of spring  7 . As can be seen in FIG. 2, body  4  acts only indirectly on switch  9  through an elastically deformable cushioning mechanism moved in a parallel direction to the axis of rotation of body  4  itself. This mechanism comprises a mobile elastomeric spacer  34 , a first elastically deformable element or spring  35 , an endrun pivot (i.e., piston)  36  which eventually is brought into direct contact with switch  9 , and a second elastically deformable element or counteracting spring  37 . Each of these elements, as illustrated, is aligned and slidable axially in housing F. 
     Spacer  34  is provided with an end which is aligned so as to come into direct contact with the forward end of body  4 . Spring  35  is located to operate between spacer  34  and endrun pivot  36 . Counteracting spring  37  is located to operate between endrun pivot  36  and striker  38  which is fixed to housing F. Spring  35  is more rigid (i.e. stiffer) than counteracting spring  37  and is coaxial therewith. As can be seen endrun switch  9  will be activated (to turn motor  2  “off”) each time (due to movement of body  4 ) that spring  35  pushes pivot  36  against switch  9 , overcoming the opposite biasing force of spring  37 . In this way the action of endrun pivot  36  on switch  9  is both gentle and gradual (i.e., cushioned) and is not adversely affected by the inertia of the system which inevitably permits neither instantaneous arrest of the rotation of body  4  nor a precise axial repositioning thereof at every cycle. In short, the presence of the cushioning mechanism assures that these potentially adverse effects are “absorbed” without them being transmitted to endrun pivot  36 , thus protecting switch  9 . 
     In this way then, the advance of shaft  8  and thus shaft  101  is accurately controlled. As shaft  8  advances in one direction to actuate PTO  22 , intermediate body  4  advances (i.e., retreats) in the opposite direction against the biasing force of spring  7  until the end “E” of body  4  contacts spacer  34  and retreats far enough to cause pivot (plunger)  36  to contact and activate switch  9 , turning off motor  2  (and thus stopping the advance of shafts  8  and  101 ). The system is designed so that at the point motor  2  is turned off, PTO  22  is effectively operating and all relevant, intended gears are in full or substantially full engagement, effectively retained as such by the mechanics of the system as described. In addition, end  28   a  has contacted striker  107  and warning light  113  has been activated. 
     Disengagement of PTO  22  now requires a reverse operation. Again, Tg gear is caused to stop rotating (e.g. by depressing the clutch of the vehicle). The “disengage” lever or button (not shown) is then activated to cause motor  2  to rotate in the opposite direction. Shaft  8  begins to retract (via the screw thread mechanism at  6  between shaft  8  and body  4 ) while body  4  moves in the opposite direction away from contact with spacer  34 . Plunger  36  moves out of contact with switch  9 , while hex head end  18  of shaft  8  via the cushioning mechanism of coil springs  39 ,  41 , causes activator element (plunger)  40 , to eventually contact and activate endrun switch  15  which stops motor  2 , thus stopping the retraction of screw shaft  8  (designed so it is fully retracted when motor  2  is stopped). By the time, however, that switch  15  is contacted (FIGS.  1  and  2 ), PTO  22  has become deactivated (see FIG. 1) because actuator arm  20 , via the uncoiling biasing force of coil spring  29  (compressed when PTO  22  was engaged) has moved in the opposite direction to contact the opposite sidewall of slot  103  and move (retract) gear wheel  23  along splines  105  until teeth  21   d  are fully retracted from, i.e., fully out of meshing engagement with, gear teeth  21   b  (as shown in FIG.  1 ). Rotation of gear Tg such as by release of the clutch will then not cause rotation of PTO output shaft  100 . 
     In order to better describe the cushioning effect upon contact with switch  15  which is achieved when shaft  8  is retracted, and as shown in FIGS. 1-2, a cushioning mechanism is provided, and which is aligned and guided to slide axially one element with the other, the mechanism comprising an elastically deformable element or spring  39 , an endrun pivot  40  and an elastically deformable element or counteracting spring  41 . Counteracting spring  41  is located to operate between endrun pivot  40  and striker  42  (FIG. 2) attached to frame F. Similarly, as described above with respect to the elements which cushion the contact force experienced by switch  9 , these similar elements associated with the end of shaft  8  serve to cushion the contact force experienced by switch  15 . In short, the presence of spring  39  enables the adverse effects described above that otherwise would be transmitted to switch  15  during contact, to be “absorbed” and thus to protect switch  15  in a similar way that switch  9  was protected. 
     The “engage” or “disengage” mode of operation may be effected by any convenient technique such as by a switch, lever, or button mechanism conveniently located, such as in the cab of a dump truck in close proximity to the driver. The warning light  113  is also locatable in any visible, convenient location which will be noticeable by the driver such as on a conventional console near his operating hand or on the dashboard panel. 
     As can be seen, if during the engagement operation or mode, the gear teeth on wheel  23  are precisely (perfectly) synchronized with respect to the inter-gear teeth gaps on wheel  24 , meshing engagement occurs without any problem. On the other hand, in the statistically rather frequent situation where the teeth of wheel  23  are not in perfect synchronization with the inter-gear gaps of the teeth of wheel  24 , meshing engagement will not be accomplished. In such an instance axial advancement of screw shaft  8  stops and body  4  continues to rotate and move with respect to screw shaft  8 , thus compressing coil spring  7 . This, in turn, brings end E of body  4  into contact with spacer  34 , which forces pivot plunger  36  into contact with switch  9 , stopping motor  2 . 
     This activation of switch  9  when meshing engagement is not achieved does not, however, necessarily produce an instantaneous lock (blockage) of the whole kinematic chain of the mechanism. Instantaneous stoppage of movement of spacer  34  in the same position is not possible. An overrun does occur. The interposition of spring  35  as illustrated allows spacer  34  to stop anywhere without rigidly acting on endrun pivot (plunger or piston)  36 . This, then safeguards switch  9 . In other words, the inevitable overrun is “absorbed” by the elastic deformation of spring  35 , with no other consequences as previously described. 
     There, of course, exists the need when meshing engagement has not been achieved and the above mechanism activated so that motor  2  was automatically turned off, to eventually achieve the desired meshing engagement. This is rather easily accomplished because when motor  2  has been automatically turned off, screw shaft  8 , due to the screw threads, maintains actuator arm  20  in its position and thus keeps gear wheel  23  against gear wheel  24  with a maximum force determined by the axial biasing force of coil spring  7 . This keeps the system under a predetermined axial force without there being any dissipation of energy due to the motor being shut off. 
     To achieve full meshing engagement with the system in this position, then, the operator need only activate a small (short) rotation of gear wheel  24  with respect to gear wheel  23  until precise synchronization (i.e. whatever degree of synchronization is needed to achieve meshing engagement) is realized. Wheel  23  is, at this time being biased in the direction of meshing engagement by spring  7 . When the degree of synchronization needed to achieve meshing engagement is realized, spring  7 &#39;s expansion causes engagement to occur. This action also withdraws end “E” of body  4  from spacer  34  causing, via springs  35 ,  37 , plunger (endrun pivot)  36  to retract from switch  9 , turning motor  2  on to complete the meshing operation. Engagement (shaft  8  extension) terminates when end portion  28   a  of shaft  101  contacts stop striker  107  secured in the PTO wall, which stops further movement of shaft  101  and causes body  4  to move axially until the end “E” of body  4  once again comes in contact with spacer  34  and pushes it far enough so that piston  36  contacts switch  9  to shut off motor  2 . At this point engagement of PTO  22  is now complete and PTO output shaft  100  may be rotated to do useful work (i.e., by reactivating the rotation of gear Tg). Disengagement when desired is achieved by stopping the Tg gear, activating motor  2  in the “disengage” direction, whereby disengagement occurs as aforesaid until switch  15  is activated by actuator element (piston)  40  to turn motor  2  off. 
     It is worth noting at this point that the system is a flexible one in that the positioning of the end  28   a  of shaft  101  and switch  9  may be varied or adjusted so that actuator  1  can accommodate various power takeoffs having different lengths (distances) of meshing engagement as well as for those specific applications other than PTO&#39;s for which an actuator is required which automatically stops its own action when a resistance to further actuation is experienced, then to resume its action, also automatically when that resistance no longer is present. 
     By the appropriate choice of spring  7 &#39;s characteristics, establishment of the desired axial forces for a given system on the basis of optimal operation can be achieved and is well within the skill of the ordinary artisan using no more than simple routine calculation or experimentation, once given this disclosure. In this respect it is here again pointed out that during disengagement, by effect of the reverse direction of rotation of motor  2 , screw shaft  8  retreats, gradually disengaging wheel  23  from wheel  24 . This retreat occurs until the head of screw shaft  8  via spring  39  forces endrun piston (pivot)  40  to activate switch  15  to stop motor  2 . Springs  39  (and  41 ) will be designed accordingly, of course, to achieve this desired afore-described purposes, which includes, of course, the achievement of the aforesaid cushioning (absorbing) effect to protect against the overrun of head end  18  that may inevitably occur. 
     Turning now to FIG. 3, a second embodiment of this invention is illustrated. Here, in place of switches  9  and  15  in the first embodiment (FIGS.  1 - 2 ), magnetically activated switches  9 ′ and  15 ′ are provided. Here, also, to the ends of spacer  34 ′ and head end  18 ′ (corresponding to spacer  34  and  18 , respectively, in the embodiment of FIGS. 1-2) are attached actuation elements  29  and  45 , such as, for example, comprised of small magnets. Elements  29  and  45  are arranged so as to be freely movable in a direction parallel to the actuator&#39;s axis C′. Such elements may be located in various positions, in this respect, relative to the switches  9 ′ and  15 ′ preferably so that the interaction is of an electromagnetic nature only rather than of a physical contact. 
     Once given the above disclosure many other features, modifications and improvements will become apparent to the skilled artisan. For example, apart from its use as an actuator for PTO&#39;s, the actuators of this invention can be used in a wide variety of applications. Illustrative of this, is where the actuators herein may be used to actuate mechanisms for opening or closing gates of many types such as where the opening and closing motion must be stopped from time to time (from only occasionally to always) when the operation encounters a force which is superior to a predetermined limit value. Such other features, modifications and improvements are considered to be a part of this invention, the scope of which is to be determined by the following claims: