Abstract:
A linear actuator comprises a ball screw carrying an axially restrained rotatable ball nut driven by an electrically energized motor. An enclosing motor housing has front and rear walls with axially aligned openings for passing the ball screw. The motor has a radially inner hollow stator assembly and a generally concentric radially outer rotor assembly. A can member is integrated with the rotor assembly to rotate therewith and has a radial end wall connected to the nut.

Description:
The present invention is directed to ball screw and nut linear actuators and, more particularly, to those which are motor housing contained. 
     BACKGROUND OF THE INVENTION 
     Applicant is aware of Vickers U.S. Pat. No. 2,444,886, wherein a gear driven shaft is operated by the armature of the motor and revolves a nut in an extension of the motor housing which is trapped so that it cannot be moved axially. A screw shaft of limited extensibility operates in conjunction with the nut. In this patent, the exciting field is fixed to the interior of the motor housing and the armature is radially inward of the field and operates the gearing mechanism. The structure disclosed in the Vickers patent is not compact and does not provide a screw which is unlimited by the other mechanism in terms of its permissible axial travel. 
     SUMMARY OF THE PRESENT INVENTION 
     The present invention provides a linear actuator with a ball screw carrying a nut driven by an electrically energized motor which has an enclosing motor housing with both front and rear walls having aligned openings for axially passing the extensible ball screw. The helical grooves and lands formed in the nut and screw provide a helical ball raceway or raceways in which load bearing balls circulate in the usual manner. The electric motor integrated with the housing includes an inner stator assembly and an outer rotor assembly, and a partially enclosing can member integrated with the rotor assembly to rotate therewith. The rotating can has a radial wall with a driving connection to the nut. Two embodiments of the invention are illustrated in which the rotating can radial wall is attached to an end portion of the nut. In one case, the nut is concentrically located with respect to the stator assembly and the rotor assembly and, in the other, the nut is in line with the stator winding assembly. 
     One of the prime objects of the present invention is to provide a compact linear actuator of the type mentioned wherein the screw axial travel is limited only by the length of the screw. 
     A further object of the invention is to provide a linear actuator which is well adapted to use in a wide variety of applications, particularly where space requirements dictate that the linear actuator be compact and fit into restricted spaces. 
     Still another object of the invention is to provide a linear actuator which, in one embodiment, is well suited to accommodating side loads as well as axial loads. 
     A still further object of the invention is to design a linear actuator of relatively simple character which is economical to manufacture and market, and yet is durable in use and requires only a modicum of maintenance attention. 
     Other objects and advantages of the invention will become apparent with reference to the accompanying drawings and the accompanying descriptive matter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The presently preferred embodiments of the invention are disclosed in the following description and in the accompanying drawings, wherein: 
     FIG. 1 is a schematic sectional side elevational view of one embodiment of the motor and actuator assembly; 
     FIG. 2 is a similar view of a further embodiment; 
     FIG. 3 is a schematic exterior side elevation view thereof; and 
     FIG. 4 is a schematic cross-sectional view taken on the line  4 — 4  of FIG.  2 . 
    
    
     DETAILED DESCRIPTION 
     Referring now particularly to the accompanying drawings and, in the first instance to FIG. 1, the linear actuator depicted includes a screw S having the usual helical lands  10  and helical grooves  11 . The screw S may be a hollow, rolled thread ball screw. The fully enclosing motor housing, generally designated  12 , is shown as comprising an annular wall  13 , with an integrated end wall  14 , which provides a cup-shaped motor housing element. Secured to the open end of the cup-shaped member, as with screws or the like, is an end plate  15  which has an axially inwardly extending tubular wall  15   a  forming an annular opening  16 . Opposite the opening  16 , the motor housing wall  14  has an opening  17  and, plainly, the screw S extends freely through the walls  14  and  15 , which do not restrict its axial travel. 
     The ball nut  18 , which similarly has helical lands  19  and helical grooves  20 , conjunctively operates with the screw S to provide a ball circuit raceway R for the circulation of balls B in the usual manner. Typically, the ball nut  18  will have a conventional internal ball return system. This could be of the type disclosed in Babinski U.S. Pat. No. 5,193,409 or consist of essentially a plurality of single turn ball return inserts in which the ball return members may be of the character disclosed in U.S. Pat. No. 5,937,700, for example. Both of these patents are incorporated herein by reference. 
     It will be seen that the nut  18  mounts radial ball bearings  21 , which support the stator assembly, generally designated  22 , concentrically with the axially fixed, but rotatable, nut  18 . The conventional stator assembly will not be particularly described, but includes the exciting field circuitry  22   a  which, when energized, rotates the rotor assembly, generally designated  23 . It is to be understood that rotor assembly comprises the usual electromagnets  23   a  fixed at circumferentially spaced intervals to the interior of an annular can body  24 , which operates as an enclosure for the rotor assembly and has an end radial wall  25 . Wall  25  attaches to the end of the nut  18  as with retention screws  26 . 
     The reversible motor M comprising the stationary stator and the revolvable rotor is a commercially available D.C. motor of the brushless variety wherein commutation is accomplished by an on-board electronic circuit board  26 . The motor used could be a shunt type or a compound type. The motor elements may be said to comprise a D.C. motor of the outside spin variety. The linear actuator nut elements and the motor elements are axially retained within the overall motor housing and provide a compact package to drive a ball screw capable of lifting a variety of typical payloads. 
     It is important to understand that the self-contained actuator eliminates the use of gears, belts, pulleys, etc. and need not provide a drive shaft to drive the nut. The motor provides increased torque, because the greater diameter stator and rotor assemblies are of greater circumference than the prior art motors which typically drive ball nuts, and accommodate a greater number of windings and magnets to provide increased torque. 
     FIGS. 2-4 disclose an embodiment in which like numerals and letters are used to identify like parts. In this embodiment, the nut  18  is in an inline position with respect to the motor stator assembly. Here, the motor housing sleeve  13  is provided with a reduced diameter housing extension  13   a  extending from a shouldered portion  13   b . The end wall  14 , as previously, is provided with the opening  17  to permit the axial passage of the screw S. The motor elements, as previously, include the stator assembly  22  and the rotor assembly  23 . In this version, the stator assembly  22  is fixed on the stationary sleeve portion  15   a  of the housing end plate  15 . 
     As FIGS. 3 and 4 indicate, the stator assembly  22  has a plurality of field or exciter windings  27  mounted on the usual posts  28 . The rotor assembly electromagnets are shown at  24  in the normal circumferentially spaced relation. The nut  18 , which is elongated to receive a plurality of balls circuits of different character, is in line with the stator assembly  22  and, it will be noted, that the can radial wall  25  attaches to its axially inner end wall at  25   a . Provided within the motor housing adjacent shouldered portion or wall  13   b  is an axially restrained ball bearing assembly  29  or roller bearing ring which seats in the recess formed by housing shoulder  13   b  and is spaced from the can wall  25  to provide a space for an optional brake, generally designated  30 , which may be of conventional character. Electrically operating brake  30  may be of any conventional character including spring set solenoid, band style, drum and pressure pad, disc type, and rap spring design, which can be employed for load stoppage or position holding as during a power loss situation. As shown, the brake is of the spring set solenoid variety and incorporates solenoid coils  31 , holding or spacer retainer discs  32  and  33  and a friction disc  34 , which can be moved by the springs into operative engagement with a braking surface  35  provided on the nut  18 . Since any one of a number of different types of brakes can be used, and all are of conventional character, the present brake will not be more particularly described. At its extended end, the housing  13   a  is exteriorly flanged as at  36  to receive needle rollers constituting a roller bearing  37  in circumferentially spaced relationship to provide a needle bearing assembly bearing upon the interior of motor housing wall  13   a.    
     In this embodiment the nut  18  and screw  11  have ball circuits, shown generally at  38  and comprised of an intermediate circuit  39  bounded by end ball circuits  40  and  41 . Circuit  39  is formed between the nut lands and grooves  18   a  and  18   b  and the screw lands and grooves  10  and  11 , respectively, to provide axial thrust capability moving the screw S in an axial direction. As previously, the balls B, which travel in these circuit in the intermediate circuit  39 , are internally returned by internal ball return mechanisms. On either side of the intermediate circuit portion  39  are side load reacting ball circuit portions  40  and  41  wherein the balls B, traveling in these circuits, do not engage in nut groves, but rather bear upon the nut smooth annular surfaces  42  and  43 . 
     Attention is directed to applicant&#39;s assignees U.S. Pat. No. 5,467,662, which is incorporated herein by reference and which, in FIG. 5, discloses the configuration of the nut and screw grooves for the ball circuit or circuits shown in FIG.  1  and in the intermediate portion  39  in FIG.  2 . The screw grooves, of course, will also be capable of absorbing or reacting with side loads when the balls B engage the non-grooved smooth annular walls  42  and  43  of the nut. 
     In FIG. 2, the numeral  45  indicates individual ball return inserts which are of the type used to internally return the balls in the ball circuits of portions  40  and  41 . It is anticipated that these circuits will be of the essentially single-turn variety mentioned previously. 
     The electrical cord, identified at  46 , connects to the stator field windings and leads to a plug  47 . 
     THE OPERATION 
     In both FIG.  1  and FIGS. 2-4, when electric current is supplied to the stator assembly coils, the rotor assemblies are energized to drive the nuts  18 . In each instance, the balls B circulate and are appropriately returned. In the FIG. 2 embodiment, the balls B in intermediate section  39  operate in the same manner as the balls B in FIG. 1 to provide the axial thrust moving the screw S axially in either direction, dependent upon the direction of rotation of the rotor assembly and the nut  18 . It is to be understood that the motors illustrated are of the reversible character wherein nuts  18  can be rotated in a reverse direction also to move the screw S in a reverse direction. 
     In the FIGS. 2-4 embodiment, the balls B in the ball circuitry portions  40  and  41  further react with side loads, which may be imposed on the screw S and provide reacting capability when the side loads applied to the screw S are substantial. In both embodiments of the invention, the screw S may be prevented by the member which it drives from rotating. In FIG. 3, a fitting F can function to attach to the load member being moved. 
     The disclosed embodiment is representative of a presently preferred form of the invention, but is intended to be illustrative rather than definitive thereof. The invention is defined in the claims.