Cam-driven linear actuator apparatus

Abstract: A precision, high load capacity linear actuator apparatus (20) incorporating a rotary-actuated cam (48) is disclosed. A moveable load carriage assembly (40) containing a cam member (48) mounted on bearings (50) therein is free to ride linearly on stationary tracks (34a,34b). The cam member (48) engages statio­ nary cam followers (38) mounted on the staionary tracks' support (32,36). In the preferred embodiment, a linear ball-slide mecha­ nism (49) supported in bearings at each end of the stationary tracks (34a,34b) passes trough the cam member (48), with its linear slide member (56) attached to the cam's center bore (58). Rotary actuation of the linear ball-slide shaft (52) causes ro­ tation of the cam member (48) which is converted into a linear motion of the load carriage (40) along the stationary tracks (34a,34b). Thus, linear positioning of the load carriage (40) is effected through rotary positioning of the ball-slide shaft (52) through associated motor means (22). In an alternate embodiment, to be utilized where extrem­ ely long linear strokes are required, the drive motor assembly (22) is mounted directly to and travels with the moveable load carriage assembly (40) so as to directly drive the cam member (48).

Description: Field of the Invention This invention relates to cam-driven linear actuator mechanisms, and more particularly, to a linear actuator where an aligned series of roller follower members are positionally fixed, while the rotating drive cam is caused to move linearly along an axis parallel to the aligned follower members. Background of the Invention The uses for point-to-point linear transfer devices are numerous, i.e., where it is desired to convert rotary motion to linear motion on an economical basis. Typical applications for such linear transfer devices include loaders and unloaders for machine presses, transfer devices for automated machine tools and welding lines, and the like. In such devices the rotating drive cams or screws are held stationary, while the cam followers are mounted on a moveable carriage. Thus, with such prior devices, the cam would have to run the full length of the linear slide mechanism. This would necessitate expensive elongated cam structure; further, such elongated cams inherently had a high moment of inertia requiring large drive motors in order to actuate them. Several prior mechanisms have been utilized to effect such linear movements. One type prior art linear drive actuator utilized a conventional acme screw mechanism, i.e., a large diameter lead drive screw running the full length of the actuator. Another prior device was the so-called ball screw mechanism, where again the shaft was maintained stationary, and the ball screw slide would move linearly relative to the shaft. However, such ball screw mechanisms have substantial speed limitations, i.e., typically in the range of 500 r.p.m. or less. Also, such ball screw type linear actuators have a relatively low load capacity. Another type prior device was the so-called band-type linear actuator, where a flexible band member, connected at each end to a tooling slide plate, was entrained about two or more drive wheels. One significant disadvantage with most prior cam-driven linear slide devices was that, because they utilized a relatively few number of cam roller followers, which were in continuous engagement with the drive cam, the life expectancy of such roller followers was very limited. Additionally, with most prior cam-driven linear actuator designs, it was often required to virtually dissemble the moveable load carriage assembly when a cam follower needed to be replaced, as the same were typically carried within the load carriage. Further, most acme or ball screw type prior devices inherently required a high reduction ratio; thus, they had to run at very high speeds or for excessive rotations just to produce the desired linear actuation motion. There is disclosed in FR-A-2 535 426 a linear actuator according to the precharacterizing portions of independent claim 1. The known actuator has a drive cam driven by a motor by means of an intermediate synchronous belt. A plurality of pairs of cam follower members are spaced from one another along the length of an elongated track assembly for a load carriage means. The pairs of follower members are spaced a substantial distance from one another and during a substantial portion of the travel of the load carriage and cam along the elongated track assembly the cam is engaged at one location only by a single pair of cam follower members. Reference is also made to US-A-3 798 984 which discloses a manually driven rotary clothes hoist comprising a cam member having a helical groove driven by a rotatable shaft. The cam member is disposed in a tubular housing carrying two sets of aligned pins extending into the helical groove. When the cam is driven by the shaft the pins cause simultaneous translation of the cam member in the tubular housing to thereby rotate and vertically move a post carried by the cam member. Summary of the Invention The object of the invention is to provide an improved linear actuator apparatus having a low moment of inertia to permit use of a relative small drive motor. According to the invention there is provided a linear actuator apparatus comprising a drive motor assembly, a drive cam member operatively coupled to said drive motor assembly to be driven thereby and provided with a helical cam lobe having a plurality of convolutions, a plurality of aligned cam follower members each positionally fixed and respectively engageable with said cam drive member, and load carriage means journalled for sliding movement on an elongated track assembly along a path parallel to said plurality of aligned cam follower members, said load carriage means being journalably supported relative to said drive cam member, whereby rotation of said drive motor assembly causes rotation of said drive cam member whereupon said engagement of said positionally fixed cam follower members with said drive cam member causes said drive cam member and said load carriage means to move linearly, characterized by comprising a slide mechanism means driven by said drive motor assembly, said slide mechanism means comprising an elongated shaft having its ends rotatably supported in said elongated track assembly and a slide means disposed about said elongated shaft and being rotatably driven thereby and axially slidable therealong, said drive motor assembly being stationarily mounted at one end of said elongated track assembly and having its output member directly drivingly coupled to said elongated shaft, said drive cam member being securely mounted to said slide means to rotate and linearly slide therewith and said drive cam member being pre-loaded against said cam follower members so as to prevent backlash therebetween, and said aligned cam follower members being equally spaced from one another with the spacing between the centerlines of the cam follower members being equal to the pitch dimension of the helical cam lobe. This unique arrangement allows the drive cam to be of a relatively short overall length thereby providing a low moment of inertia and permitting use of a small drive motor, while providing contact with a plurality of cam roller followers at axially spaced locations of the cam at any given instant, and yet retaining a preload therewith to prevent backlash. The means by which the foregoing and other advantages of the present invention are accomplished and the manner of their accomplishment will be readily understood from the following specification upon reference to the company drawings, in which: Fig. 1 is a front elevation view of the cam-driven linear actuator apparatus of one embodiment of the present invention, and showing certain components broken away for better viewing; Fig. 2 is an end elevation view, taken along lines 2-2 of Fig. 1, of the linear actuator thereof; Fig. 3 is a section view, taken along lines 3-3 of Fig. 1, showing a cam roller follower as used in the present invention; Fig. 4 is an enlarged front elevation view, partially in section, of the load carriage assembly of the present invention and showing a flanged mounting arrangement for the drive cam; and Fig. 5 is another enlarged front elevation view like Fig. 4 but showing an alternate mounting arrangement for the drive cam. Description of the Preferred Embodiment Having reference to the drawings, wherein like reference numerals indicate corresponding elements, there is shown in Fig. 1 an illustration of the preferred embodiment of the cam-driven linear actuator apparatus of the present invention, generally denoted by reference numeral 20. The linear actuator 20 comprises a motor 22 connected by an adaptor 24 to a reducer 26 which is mounted at one end of a linear track assembly, generally denoted by reference numeral 28. The reducer is preferably of the minimum backlash type. The track assembly 28 includes end plates 30A, 30B and a base plate 32. The base plate 32 carries adjacent its one edge (see Fig. 2) a linear guide track 34A, and adjacent its other edge, as mounted upon a track support 36 in an elevated manner, a linear guide track 34B. Stationary cam roller followers, generally denoted by reference numeral 38, are each affixed to and supported by the track support 36 in an aligned series (see Figs. 1 and 2). A load carriage assembly, generally denoted by reference numeral 40, is slidably mounted by bearings 42 upon the guide tracks 34A, 34B for linear movement therealong; however, the load carriage 40 cannot angularly rotate. The housing for the load carriage assembly 40 comprises a lower housing wall 44 and an upper tooling mounting plate 46. Preferably, the bearings 42 are so-called Thompson linear roller bearings, but any suitable type bearing can be used. A barrel-type drive cam, generally denoted by reference numeral 48, is mounted (either directly or indirectly, as discussed below in relation to Figs. 4 and 5) by bearings 50 to the carriage housing walls 44, such that it is rotatable within the load carriage 40. In the preferred embodiment, the drive cam 48 comprises a constant lead-type barrel cam. However, depending upon the type of slide movement desired in any given linear actuator application, and depending further upon the type of drive motor 22 that is used and how it is controlled, different type cams can be used. For example, an index cam, variable velocity cam, or an oscillating cam could be used for drive cam 48. Also, it will be understood that the drive cam 48 is formed as a relatively short cam member. That is, in the preferred embodiment, the drive cam could be from 10 to 25 cm (four to ten inches) in length, for example, yet provide a linear stroke of several feet or more. This is contrasted to the prior art devices where the drive cam necessarily extended the entire length of the desired linear stroke. Thus, it was not uncommon for such prior devices' cams to be several meters (feet) or more in length. It will be understood that the bearings 50 can be roller bearings, ball bearings, tapered roller bearings, or any other type bearing which gives proper rotational support on the carriage wall 44 to the drive cam 48. The drive cam 48 is rotatably driven by a ball slide mechanism, generally denoted by reference numeral 49, which mechanism also permits the cam 48 to move linearly along the cam's axis of rotation (which as seen in Fig. 1 is parallel to the aligned series of cam followers 38). In the preferred embodiment there is mounted at each end of linear actuator 20, within suitable roller bearings (not shown) which are carried by the respective end plates 30A, 30B, a rotatable ball spline shaft 52. The left end of ball spline shaft 52, as shown in Fig. 1, is connected to and rotatably driven by the output (not shown) of the reducer 26. Preferably, the ball spline shaft 52 is of the type having radially-protruding and longitudinally-extending drive ribs 54 (see Figs. 1 and 2). The other half of the ball slide mechanism comprises a flanged spline nut 56. As best seen in Fig. 4, the spline nut 56 comprises two separate halves 56A, 56B. In the preferred embodiment of Figs. 1-4, the ball slide mechanism 49 is a commercially available ball spline shaft mechanism sold under the THK brand. In any event, as best seen in Figs. 1, 2, and 4, the ball spline shaft 52, along with the flanged spline nuts 56A, 56B, extend through the central bore 58 of drive cam 48. In the particular mounting arrangement of Fig. 4, the flanged spline nuts 56A, 56B are journalled directly by the bearings 50 and are fastened via threaded fasteners 60 and dowel pin 61 to the drive cam 48, such that rotation of the flanged spline nut 56 effects rotation of the drive cam 48 and vice versa. In a well-known manner, the recirculating drive balls 62 of the ball slide mechanism 49 are retained by a retainer ring 64 on the interior of the spline nut 56. So as to prevent unwanted backlash for actuator 20, the drive balls 62 are pre-loaded during assembly against the respective drive ribs 54 of ball spline shaft 52. Also it will be understood that other configurations for the ball slide mechanism 49 will work in the present invention, as long as both rotary motion and linear sliding movement is provided by the particular mechanism's slide member to the drive cam 48. As best seen in Fig. 3, each of the cam roller followers 38 comprises a threaded stud portion 66 which extends through an appropriate opening 68 of the track support 36. The threaded stud 66 terminates at one end in an enlarged head portion 70 against which is seated the cam roller 72 and the needle bearings 74; these are retained by a retainer ring 76. The other end of the stud 66 is retained against the track support 36 by a fastener 78. The cam followers 38, thus, provide high load-carrying capacity for the linear slide actuator 20. Also, so as to prevent unwanted backlash during operation, the drive cam is pre-loaded as is well-known against the cam followers 38 during assembly of the linear actuator 20. Each of the aligned and equally-spaced cam roller followers 38 is able to engage the respective convolutions of helical cam lobe 80 of the rotating drive cam 48. The spacing between the centerlines of the followers 38 is equal to the pitch dimension of the helical cam lobe 80 The followers 38 are each positionally affixed to track support 36 and thus, do not move relative to the slidable load carriage assembly 40. That is, the cam 48 moves along the line of roller followers 38, not vice versa. Figure 5 depicts an alternate arrangement for attaching the slide member of the ball slide mechanism 49 to the drive cam 48. That is, rather than using a flanged spline nut 56 (see Fig. 4), Fig. 5 depicts a modified spline nut 82 comprising respective spline nut halves 82A, 82B, which are separated by a spacer 84. In this alternate mounting arrangement, the spline nut halves 82A, 82B are respectively secured to the drive cam 48 by retainer keys 88 fitted in mating keyways (see keyway 89 in Fig. 2). The recirculating drive balls 62 are retained to the spline nut halves 82A, 82B by retainer rings 86. Further, the spline nut halves 82A, 82B are retained axially relative to the drive cam 48 by an end retainer plate 90 (only one shown). However, in this alternate mounting arrangement of Fig. 5, the keyed combination of drive cam 48 and modified spline nut 82 is again rotatably journalled on the lower housing wall 44 by roller bearing 50. Yet, in this alternate mounting arrangement, the drive cam 48 is directly journalled on the housing wall 44 by bearings 50, rather than being indirectly journalled (via flanged spline nut 56) in the preferred mounting arrangement of Fig. 4. Nevertheless, it will be understood that, other than the specific difference regarding the particular arrangement for journalling the joined combination of the drive cam 48 and the respective spline nuts 56, 82, the two embodiments depicted in Figs. 4 and 5 operate and function in the same manner, as will now be discussed. The motor 22 and reducer 26 operate to drivably rotate the ball spline shaft 52 in either direction, and for a specific number or degrees of rotation(s). The motor 22 can be a so-called stepping motor, whereupon by selectively turning on and off the motor 22 for desired periods via control means (not shown), a known rotation for ball spline shaft 52 can be achieved. Preferably, such a stepping motor would be combined with a drive cam 48 of the so-called lead-cam or constant velocity design. Another motor usable with such cams is a so-called servo motor. Alternatively, the drive cam 48 could be an indexing-type cam, of a barrel cam design. In that case, the motor 22 preferably would be a constant rotation type motor. This specific combination would result in a step-wise movement, i.e., a fixed incremental indexing movement, for the ball spline shaft 52. Yet another type motor 22 that could be used with the present invention is an alternating current type motor, with an accompanying clutch brake. In any event, regardless what specific motor is used to drive the spline shaft 52, any rotation thereof will likewise cause the associated spline nut 56 to rotate, due to the angularly-constrained recirculating drive balls 62. Moreover, since the drive cam 48 is affixed by fasteners 60 (or alternatively by keys 88) to the spline nut 56 (or alternatively the nut 82), the drive cam 48 also is caused to rotate. Due to the presence of the aligned series of stationary cam roller followers 38, at least a few of which are always in engagement with the cam lobes 80 of drive cam 48, any rotation of the cam 48 causes it to be linearly moved. That is, rotation of cam 48 causes the engaged roller followers 38 to create a linear thrust motion on the cam lobes 80, whereby the drive cam 48 (and the affixed spline nut 56 or 82) is caused to be linearly driven along the ball spline shaft 52. This linear action will occur because the recirculating drive balls 62 not only transfer the rotary drive motion of the shaft 52 and its drive ribs 54 to the spline nut 56, but also permit linear movement of the spline nut 56 (and the affixed drive cam 48) relative to the ball spline shaft 52. Further, since the carriage housing walls 44, 46 are rotatably journalled to and carried by the joined combination of drive cam 48 and spline nut 56, via journal bearings 50, any linear movement induced into drive cam 48 (by its rotation against the followers 38) necessarily causes the load carriage assembly 40 to also move linearly, as journalled along the guide tracks 34A, 34B. In sum, controlled rotary positioning of the ball spline shaft 52 effects the desired linear positioning of the load carriage assembly 40. Depending upon the type of motor 22 and how it is controlled, and also depending upon the type of cam selected for drive cam 48, any number of different linear actuation motions can be achieved for the load carriage assembly 40 and any tooling or workpiece gripper devices associated therewith. Due to the fact that the present invention permits the use of a relatively short drive cam member 48, several significant advantages are achieved. First, the manufacturing costs of the drive cam 48 itself are reduced in view of its shortened length. Second, the cam's short length permits it to be easily moved out of the way when a particular cam roller follower element 38 becomes worn and needs replacement, i.e., easy accessibility. Third, the cam's short length inherently results in relatively low moments of inertia as seen by the reducer 26 and drive motor 22, thereby permitting use of small capacity motor drive assembly components, yet without compromising the ultimate output load capability of the linear actuator. Fourth, the drive cam 48 need not be custom built for each different required slide movement. Instead, one drive cam 48 (and the associated load carriage assembly 40) can work for many different lengths of required linear stroke, and only the length of the aligned series of cam followers need be changed in any given application. And fifth, since the cam 48 is relatively short and only a given number of followers 38 are engaged with the cam at any one time, the longevity of the followers 38 is greatly enhanced.

Claims: Linear actuator apparatus comprising: a drive motor assembly; a drive cam member (48) operatively coupled to said drive motor assembly to be driven thereby and provided with a helical cam lobe (80) having a plurality of convolutions; a plurality of aligned cam follower members (38) each positionally fixed and respectively engageable with said cam drive member (48); and load carriage means (40) journalled for sliding movement on an elongated track assembly (28) along a path parallel to said plurality of aligned cam follower members (38), said load carriage means (40) being journalably supported relative to said drive cam member (48), whereby rotation of said drive motor assembly causes rotation of said drive cam member (48) whereupon said engagement of said positionally fixed cam follower members (38) with said drive cam member (48) causes said drive cam member (48) and said load carriage means (40) to move linearly, characterized by comprising a slide mechanism means (49) driven by said drive motor assembly, said slide mechanism means (49) comprising an elongated shaft (52) having its ends rotatably supported in said elongated track assembly (28) and a slide means disposed about said elongated shaft (52) and being rotatably driven thereby and axially slidable therealong, said drive motor assembly being stationarily mounted at one end of said elongated track assembly (28) and having its output member directly drivingly coupled to said elongated shaft (52), said drive cam member (48) being securely mounted to said slide means to rotate and linearly slide therewith and said drive cam member (48) being pre-loaded against said cam follower members (38) so as to prevent backlash therebetween, and said aligned cam follower members (38) being equally spaced from one another with the spacing between the centerlines of the cam follower members (38) being equal to the pitch dimension of the helical cam lobe (80). Linear actuator according to claim 1, characterized in that said drive shaft (52) has radially protruding drive ribs (54) and said slide means comprises a ball slide spline nut (56, 82) carrying a recirculating series of drive balls (62) engageable with said drive ribs (54). Linear actuator according to claim 2, characterized in that said drive ribs (54) extend longitudinally of said elongated drive shaft (52). Linear actuator according to claim 1, characterized in that said load carriage means (40) is journalled directly on said drive cam member (48).

Inventors: GALLOWAY PETER EDMUNDS, GALLOWAY, PETER EDMUNDS
Applicants: EMERSON ELECTRIC CO, EMERSON ELECTRIC CO.
Application year: 1989
Main classification: F16H25
IPC codes: B23Q5, B30B1, F16H25