Abstract:
A method and apparatus wherein a travel limit/dampening device is coupled to the end of a screw of a linear ball screw actuator device for limiting the travel of an actuator extension coupled to a nut supported for linear driven displacement in either direction along the screw. The travel limit/dampening device includes a gas shock supported within a double acting reaction device reactive axially between the nut and screw when the extension is fully retracted and extended, respectively, to collapse the shock absorber and thereby dampen the movement of the extension at each end of its stroke.

Description:
This application claims Benefit of Provisional Application Ser. No. 60/014,835, filed Apr. 4, 1996. 
     This invention particularly relates to ball screw and nut assemblies which are useful as linear actuators. 
     BACKGROUND OF THE INVENTION 
     Linear ball screw-type actuators are used in many applications to transmit linear motion in performing such operations as opening and closing, raising and lowering, pushing and pulling, advancing and retracting, and positioning various devices. In these devices a screw is typically supported within a relatively telescoping inner and outer tube for rotation by a motor and gear box assembly. A ball nut is mounted on the screw and coupled to the slidable inner tube, converting the rotary motion of the motor and screw to linear motion of the inner tube. The inner tube carries a clevis or other connector at its free end which is coupled to the device to be actuated. 
     Generally, stop washers or pins are provided on the screw at its opposite ends to halt the travel of the ball nut along the screw, thereby establishing fully extended and retracted inner tube stroke limits. The sudden stoppage of the ball nut as it impacts such stops imparts a jolting shock force to the actuator parts, which can be considerable, particularly when the actuator is under heavy load. 
     SUMMARY OF THE INVENTION AND ADVANTAGES 
     The present invention overcomes the foregoing objections by incorporating cushioning mechanism with the actuator that absorbs the energy of impact as the actuator reaches its fully extended and retracted positions, thus greatly reducing or eliminating the jolting forces applied. 
     One of the prime objects of the invention is to provide a novel method of cushioning the impact of a sudden stop on a payload at either end of an electro-mechanical actuator and collapsing the shock forces imposed, particularly on high speed, high load actuators. 
     Another object of the cushioning device is to obviate the need for the stop washers or stop pins, which cause the sudden impact forces. 
     A further object of the invention is to provide a single cushioning device which serves to cushion the travel of the actuator in both the fully retracted and extended positions. 
     Still another object of the invention is to provide a cushioning device designed to be mounted on the end of the screw for confronting an end wall of the inner tube when fully retracted, and confronting the ball nut when the inner tube is fully extended. 
     Still another object of the invention is to provide a simple, practical and durable cushioning device that can be readily adapted to present actuator designs without significant modification of their construction. 
     Another object of the invention is to provide a cushioning device that is self contained and detachable from the actuator to facilitate repair or replacement, when needed. 
     These and other objects, advantages and features of the present invention will become more readily apparent from the following detailed description when taken together with the accompanying drawings, wherein: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partly sectional, elevational view of a typical ball screw actuator fitted with a travel stop cushioning device which is constructed in accordance with a presently preferred embodiment the invention; 
     FIG. 2 is a partly sectional, exploded view of the components that make up the cushioning device; 
     FIG. 3 is an enlarged cross-sectional view taken along the lines 3--3 of FIG. 1; and 
     FIG. 4 and 5 are enlarged fragmentary sectional views showing the actuator in its fully retracted and extended positions, respectively. 
    
    
     DETAILED DESCRIPTION 
     With reference to FIG. 1 initially, there is illustrated an electro-mechanical linear actuator assembly 10 having an electric motor 12, gearbox 14, and a ball nut and screw mechanism, generally indicated at 16, housed within a fixed outer tube 17 which is secured by nut and bolt fasteners 19 to a base plate 20 of the actuator 10. 
     The mechanism 16 includes a typical ball screw 22 provided with helical ball-accommodating groove portions 22a separated by helical land portions 22b. Coaxially provided on the screw 22, is a ball nut 24 which has matching internal groove and land portions and is provided with an exterior ball return tube 26 to recirculate a train of abutting load bearing balls B to facilitate linear movement of the ball nut 24 along the ball screw 24 in response to rotation of the screw 22. Such a mechanism is disclosed, for example, in U.S. Pat. No. 5,485,760, which is owned by the assignee of the present invention and its disclosure incorporated herein by reference. 
     The nut 24, at the time of fabrication, is provided with an integrated cylindrical extension portion, generally designated 28, which is arranged in concentric spaced relation to the screw 22 to define an annular recess 30 therebetween. An inner extension tube 18 is fixed at its inner end to the extension portion 28 of the nut 24 and extends longitudinally therefrom to an opposite free end on which a clevis 32 or other connector may be mounted. The clevis 32 is shown as having an attachment opening 34 for connection to the device to be actuated (not shown) in the conventional manner. 
     A ball screw mounted travel limit/dampening device constructed in accordance with a presently preferred embodiment of the invention is designated generally at 38 in the drawings and comprises a carrier such as a cylindrical housing, cage, support, or container 38a which is closed at one end by an end wall or activating wall or end part 40 and which is open at its opposite end. 
     A conventional shock absorber or compressible shock absorbing device, generally designated 42, includes a cylinder 44 in which fluid such as a compressible gas is contained, and a relatively slidable plunger 46, which normally is in the extended FIG. 2 position, has relative movement with the cylinder 44 to displace the fluid and absorb an applied force. The plunger 46 is automatically returned outwardly by the usual compression spring in cylinder 44 (not shown) upon removal of the applied force. A fixed, externally threaded adapter 48 forms a part of and is fixed to the interiorly positioned end of the cylinder 44, as illustrated best in FIG. 2. The shock absorber 42 preferably is a self-contained unit available commercially from Endine Company, model number FP3984. The shock absorber or cushioning device 42 of which many forms are available, is housed within a double acting, cylinder carrying reaction carrier or enclosure 49 which operates in conjunction with the extension tube 18, and nut 24 to limit the travel of the nut extension tube 18 while dampening the sudden stoppage of the nut extension tube 18 at each end of its stroke as will be described in greater detail below. 
     A shock absorber guide 50 is coupled to adaptor 48 to function as a part thereof and includes a cylindrical body 50a (FIG. 3) having an outer diameter corresponding generally to that of the fluid cylinder 44, opposing inner and outer end faces 50b, 50c, and an internally threaded, axial bore 50d. A guide part comprising a pair of diametrically opposed wings, ears, or reaction arms 52 project radially outwardly of the body 50a, presenting forward 52a and rearward 52b facing abutment surfaces. The guide 50 is preferably fabricated from a hard, wear-resistant material, such as SAE 4340H, 4140, 4190 or 6150 grades of steel, hardened and tempered typically to Rc 38-40. 
     The shock guide 50 is mounted securely but removably on the end of the shock absorber 42 by threading the externally threaded adapter 48 into the threaded bore 50d of the guide 50, until the inner end face 50b of the guide 50 is brought to bear tightly against the opposing end face 44a of the fluid cylinder 44. Once connected, the shock absorber 42 and guide 50 are inserted as a unit, (plunger 46 first) into the shock absorber carrier or housing 38. 
     The shock housing 38 is formed with a guide part comprising a pair of first open-ended, diametrically opposed longitudinal guide slots 54 that are arranged and dimensioned to slidably accommodate the ears 52 of the shock guide 50, and which terminate within the housing 38 at closed abutment ends 54a. As most clearly seen in FIG. 3, the ears 52 of the shock guide 50 extend radially outwardly beyond the outer peripheral surface of the shock housing 38. The end wall 40 of the housing 38 is also formed on its interior side with a central recessed seat 56 in which the free end 46a of the plunger 46 is accommodated. 
     A guide mount for guide 50, generally designated 58, has a generally cylindrical body 60 formed with a flat inner end face 60a and a transverse bore 60b extending diametrally through the body 60. An externally threaded mounting stud 62 projects from an opposite outer end face 60c of the body 60. The body 60 of the guide mount 58 is received in the open end of the shock housing 38, such that the inner end face 60a confronts the outer end face 50c of the shock guide 50. The guide mount 58 is likewise fabricated of a hard, wear-resistant material and preferably the same materials as those used for the shock guide 50, hardened and tempered typically to Rc 38-40. 
     The guide mount 58 is retained within the housing 38 by means of a hardened, ground dowel pin 64, extending through the opening 60b in the guide mount and having ends 64a thereof projecting outwardly through diametrically opposed second slots 66 formed in the housing 38 (FIG. 3). The structure provides a pin and slot lost motion connection. The slots 66 are preferably 90° offset from the guide slots 54 and extend longitudinally (FIG. 2) between forward 66a and rearward 66b closed marginal slot ends. It should be noted at this time that the relative dimensions of the components are such that the shock absorber 42 must be compressed somewhat during assembly in order to extend the opening 60b of the guide mount 58 sufficiently inward to register it with the rearward ends 66b of the slots 66 to enable insertion of the pin 64 into the opening 60b through one of the slots 66. Once the dowel pin 64 is in place, it is urged by the return force of the shock absorber 42 against the rearward interior ends 66b of the slots 66 so as to retain the components of the device 36 securely but removably in assembled relation as a self-contained unit. 
     As illustrated in FIG. 1, the free end of the ball screw 22 is formed with a threaded axial bore 67. The stud 62 of the guide mount 58 which essentially functions as a part of screw 22 is threaded into the bore 67 until the end face 60c of the guide mount is brought to bear tightly against the end face 22a of the screw 22, thereby mounting the dampening device 36 securely but removably on the screw 22. 
     A pair of wear washers 70 (FIG. 5) are arranged within the inner tube 18, resting against the end face of the extension portion 28. They are freely rotatable relative to the portion 28 and to one another. The washers 70 are preferably fabricated of a hard, wear-resistant material such as SAE 4150H or 6150 steel typically hardened and tempered to Rc 55-60. By use of the wear washers 70, the extension portion 28 can be fabricated of a milder, less costly grade of steel. 
     THE OPERATION 
     The electric motor 12 activates the ball screw 22 through gearing inside the gear box 14 in known manner, causing the ball screw, and hence the dampening device 36, to rotate. U.S. Pat. No. 5,090,513 illustrates typical gearing and is incorporated herein by reference. The ball nut 24 converts the rotary motion of the screw 22 into linear motion of the inner tube 18 to either extend or retract the inner tube. 
     FIG. 1 illustrates the inner tube in a position just prior to full retraction where the dampening device 36 comes into play. As the inner tube 18 is moved inwardly, a clevis wall 43 confronts the end wall 40 of the shock housing 38, and continued inward advancement displaces the shock housing 38 inwardly (to the right in FIGS. 1 and 4) relative to the fluid cylinder 44, shock guide 50, and guide mount 58, bringing the ears 52 and dowel pin 64 relatively forwardly in their associated slots 54, 66. It should be noted at this point that the end wall 40 has a preferably convex outer surface 41 which seats in the corresponding concave recess 43 of the clevis 32 to maintain concentricity. The confronting surfaces of the end wall 40 and clevis 32 are hardened to resist wear. It is advantageous to form the concave portion 43 of the clevis 32 as a separate insert 68 (FIG. 4) of the same material and hardness as that of the end wall 40. The insert 68 may be joined by welding or other suitable securing means to the clevis 32, as illustrated best in FIG. 4. The relatively large radius of the concavity not only provides a greater contact surface, it also performs a centering function to maintain concentricity of the parts as the shock forces are collapsing. 
     The inward displacement of the shock housing 38 forces the plunger 46 further into the cylinder 44, thereby dampening the energy of the impact between the housing 38 and clevis 32. Full cushioned retraction is reached when the base walls 54a of the guide slots 54 bottom out against the forward abutment surfaces 52a of the ears 52, as illustrated in FIG. 4. The cushioning device thus prevents material impact forces from being transmitted to the screw 22 and nut 24. 
     At the other extreme (FIG. 5), driving the inner tube 18 to full extension causes the extension wall portion 28 of the ball nut 24 to move toward the rearward abutment surfaces 52b of the shock guides ears 52. 
     As the wear washers 70 confront the rearward abutment surfaces 52b of the shock guide ears 52, the shock guide 50 is driven outwardly by continued advancement of the nut 24 (to the left in FIG. 5) relative to the shock housing 38, which is held against movement by engagement of the dowel pin 64 with the rearward ends 66b of the slotted openings 66 (see FIG. 5). The advancement of the nut wall portion 28 is accommodated by the accommodation of the rearward end of the shock housing 38 in the recess 30 of the extension 28. The shock of the confronting surfaces is fully cushioned as previously by the shock absorber 42 as the normally projecting, gas pressure retained plunger 46 is displaced relative to the cylinder 44. Full, but cushioned, extension is achieved when the forward abutment surfaces 52a of the ears 52 confront the base walls 54a of the guide slots 54. 
     It is understood that the disclosed embodiment is representative of a presently preferred form of the invention and is intended to be illustrative rather than definitive thereof. Other embodiments which accomplish the same function are contemplated and incorporated herein within the scope of the claims. For example, the invention is not limited to the particular ball screw actuator described nor to the particular screw drive arrangement. Furthermore, while the guide mount 58 is described in the preferred embodiment as being threaded into engagement with the ball screw 22, it will be appreciated that other ways of attaching the dampening device 36 to the screw 22 are contemplated by the invention which include, for example, permanently fixing the guide mount 58 to the screw 22 such as by welding. Moreover, the materials and physical properties of the components described are those presently preferred, but others are contemplated.