EP0021623B1

EP0021623B1 - Worm screw jack apparatus and method of making same

Info

Publication number: EP0021623B1
Application number: EP19800301809
Authority: EP
Inventors: Alan C/O Fortune Engineering Limited Fortune
Original assignee: FORTUNE ENGINEERING Ltd
Current assignee: FORTUNE ENGINEERING Ltd
Priority date: 1979-05-31
Filing date: 1980-05-30
Publication date: 1987-03-04
Also published as: DE3071916D1; EP0021623A1

Description

This invention relates to a worm screw jack.
Known worm screw jacks generally have a housing in which is provided a worm rotatively arranged to be driven by means outside the housing, the worm being connected to meshingly engage with a worm gear which is constrained against axial movement within the housing and which, in turn, meshingly engages with a lifting screw. In operation the lifting screw is normally secured to a load so that the lifting screw is unable to rotate and rotation of the worm causes rotation of the worm gear which, since it is constrained against axial movement raises the lifting screw and hence the load.
In use, with such a screw jack, wear results between the meshing threads of the worm and the lifting screw so that backlash occurs when changing the mode of use from lifting to lowering of the lifting screw, or vice versa.
One known attempt at overcoming the problem of backlash is described in British Patent 1,345,136 in which the worm gear is provided with an integral sleeve portion which is radially spaced from the threads of the lifting screw. In the annular space between the sleeve portion is an anti-backlash nut which meshes with the lifting screw. The lower end of the anti-backlash nut is initially spaced from an internal shoulder of the worm gear by a predetermined distance so as to form a wear gap which enables the anti-backlash nut to move downwardly with the load relative to the worm gear as the upper surface of the threads of the worm gear wear away. The lower portion of the anti-backlash nut is externally spined and mates with internal splines on the lower portion of the sleeve portion of the worm gear. The upper portion of the anti-backlash nut is externally smooth and makes a sliding fit with the internally smooth surface of an annular takeup nut which occupies the annular space between the anti-backlash nut and the sleeve portion of the worm gear. The takeup nut is externally threaded in mesh with the internal threads on the upper portion of the sleeve portion and a shoulder, radial, projection on the anti-backlash nut bears against the lower end of the takeup nut. Thus wear between the worm gear and lifting screw is taken up by screwing down the take-up nut which bears against the shoulder of the anti-backlash nut thereby forcing the threads of the lifting screw into firm engagement with the internal threads on the worm gear.
Such a known arrangement had the disadvantage that drive between the worm gear and the anti-backlash nut is performed by splines of relatively narrow cross sections so that the safety factor of the screw jack is detrimentally affected. However a more serious problem with such a known screw jack is that in service, once the predetermined gap between the anti-backlash nut and the worm gear has been taken up by screwing down the take-up nut when the meshing threads between the lifting screw and worm gear have been worn away then it has been known for users to dismantle the jack and raise the anti-backlash nut by one pitch of the lifting screw thread to thereby reinstate a gap between the anti-backlash nut and the worm gear and then to re-assemble the screw jack. It will be apparent that the meshing threads of the screw jack have already reached a severe state of wear when the predetermined gap between the anti-backlash nut and the worm gear is taken up on a first occasion but by re-assembling the components the meshing threads become dangerously thin.
It is an object of the present invention to overcome the foregoing defects.
According to one aspect of this invention there is provided a worm screw jack including a worm meshing with a worm gear which is drivinglyÍ connected to a lifting screw, an anti-backlash means including a sleeve meshingly engaged with the lifting screw and axially spaced from the worm gear, the sleeve being engaged with the worm gear to permit axial relative movement therebetween but preventing relative rotational movement therebetween, characterised in that the anti-backlash means comprises a shoulder member integrally formed on the worm gear and the sleeve, said shoulder member having driving surfaces with an axial length less than the pitch of the lifting screw and which mate with a correspondingly shaped recess in the sleeve, the sleeve being axially movable in relation to the shoulder member.
By using the worm screw jack as defined in the characterising portion of claim 1 of the present invention not only are fewer parts employed leading to a less expensive assembly but a greater surface area of contact is provided between the worm gear and sleeve so that a screw jack having an increased safety factor is produced. More importantly, it is not possible to disassemble the screw jack and reinstate the gap between the sleeve and worm gear since the mating surfaces between the shoulder and recess have an axial length which is too short to enable re-engagement when re-assembled with the shoulder member being moved upwardly along the lifting screw by the pitch of the lifting screw thread.
The axial length of the shoulder is preferably 8% less than the pitch of the lifting screw.
Conveniently, the shoulder member is a quadrilateral protrusion diametrically extending across the lifting screw axis formed by substantially parallel flat surfaces the height thereof extending in the plane of the lifting screw axis, with partially circular end faces, and the sleeve recess is arranged to mate with the quadrilateral protrusion.
Advantageously, a circularly cross-sectioned extension is provided on the shoulder member, having a smaller diameter than that of the partially circular end faces, which is arranged to be an interference fit inside a bore of the sleeve thereby assisting to maintain the worm gear and sleeve in axial alignment.
Preferably the sleeve and worm gear are predeterminably spaced in manufacture by a distance of 0.25 x screw pitch.
According to a further aspect of this invention there is provided a method of making a screw jack having a worm meshing with a worm gear which is drivingly connected to a lifting screw, an anti-backlash means including a sleeve meshingly engaged with the lifting screw and axially spaced from the worm gear, the sleeve being engaged with the worm gear to permit axial relative movement therebetween but preventing relative rotational movement therebetween, characterised by the anti-backlash means comprising a shoulder member integrally formed on the gear and the sleeve, said shoulder member having driving surfaces with an axial length less than the pitch of the lifting screw and which mate with a correspondingly shaped recess in the sleeve, the sleeve being axially movable relative to the shoulder member said method including the step of axially aligning the worm gear and sleeve a predetermined distance apart and cutting an internal thread through the worm gear and sleeve combination.
In a preferred embodiment the predetermined distance is provided by a split spacer of the thickness of 0.25 x screw pitch which is secured between adjacent faces of the worm gear and sleeve.
Preferably, the method includes the further steps of mounting the assembly of worm gear, split spacer and the sleeve on the lifting screw, removing the split spacer, engaging the worm gear with the worm and loading the sleeve so that the internal threads of the sleeve and worm gear apply opposing axial forces on the lifting screw threads.
In use wear will inevitably result between the meshing threads of the sleeve/lifting screw and worm gear/lifting screw and advantageously the loading applied to the sleeve is increased to take up the wear and conveniently the loading is applied by screwing down a shell cap known per se.
The invention will now be described with reference to the accompanying drawings in which:

Figure 1 shows a partly broken away view of a partially sectioned worm screw jack in accordance with this invention,
Figure 2 and Figure 3 are mutually orthogonal views of a worm gear used in the jack of Figure 1,

In the Figures like reference numerals denote like parts.
The worm screw jack shown in Figure 1 has a housing 1 in which is a worm 2 mounted on a shaft 3, the shaft 3 being supported within the housing 1 by bearings (not shown). The worm 2 engages concave external worm threads of a worm gear 4 which is rotatable upon a lower thrust bearing 5 comprising bearing elements caged in any suitable manner, the bearing being restrained within an annular recess in a base portion of the housing 1. The worm gear 4 is drivingly connected to a sleeve 6 which is mounted in an upper thrust bearing 7 which is similar to the lower thrust bearing 5 and is restrained within an annular recess in a shell cap 8, the shell cap 8 being threadably engaged within a top, neck portion of the housing 1. Upon assembly at least there is a gap later defined herein between adjacent faces 41, 61 of the worm gear and sleeve respectively. Extending perpendicularly to the axes of the worm 2 and worm gear 4 is a lifting screw 9, having a trapezoidal thread, which meshingly engages with an internal thread in both the worm gear 4 and sleeve 6. The lifting screw 9, at its upper end, as shown in Fig. 1, has a screw threaded portion 12 of smaller pitch circle diameter than the screw thread engaging with the lifting screw and worm gear which portion 12 is normally screwed and fixed to a plate member (not shown) for supporting a load. The housing additionally has an aperture in the base portion through which the lifting screw is able to move in a vertical direction and a tubular extension 10 of the housing (shown broken away) shields the lifting screw in its lowered position. Two mounting apertures 11 are provided in the base portion of the housing 1.
The worm gear 4 is shown in detail in Figures 2 and 3 and from these Figures it will be seen that the axially upper part of the worm gear has a shoulder 20 formed from a quadrilateral protrusion which extends diametrically across the lifting screw axis, the quadrilateral being shaped by substantially parallel flat surfaces 21 in the plane of the lifting screw axis and with partially circular end faces 22. The axial length of the shoulder 20 is arranged to be less than the pitch of the lifting screw and is preferably 8% less. A circularly cross-sectioned extension 23 is provided on the shoulder 20 having a smaller diameter than that of the partially circular end faces 22.
The detail of the sleeve 6 is shown in Figures 4 and 5 and this sleeve, which is made from phosphor bronze, has a stepped outside circular cross-section dimensioned so that the upper thrust bearing is fitted around the upper, smaller diameter of the sleeve and abuts the step in the sleeve. The sleeve has a coaxial recess 30 in the larger diameter portion which is of similar shape to the quadrilateral shoulder 20 of the worm gear and is arranged to be an interference fit with the shoulder 20. A bore 31, coaxial with the recess 30 is also provided in the sleeve and the bore 31 is an interference fit with the circularly cross-sectioned extension 23 of the worm gear, the purpose of the bore 31 and extension 23 being to assist the alignment of the worm gear 4 inside the sleeve 6.
In manufacture the worm gear and sleeve are made separately until the internal screw thread for mating with the lifting screw is provided. The worm gear and sleeve are then axially aligned and spaced apart by a split spacer having a thickness of 0.25 x screw pitch. The combination of worm gear, split spacer and sleeve are then machined with an internal screw thread which will mesh with the trapezoidal screw thread of the lifting screw 9. The machined combination is then assembled on the lifting screw, the split spacer is removed and the assembly together with the worm and shaft 2, 3 and the bearings 5, 7 is mounted in the housing and the shell cap 8 screwed into place. By screwing the shell cap into the housing a load is applied to the upper thrust bearing 7 which forces the sleeve downwardly against the screw threads of the lifting screw with the result that a corresponding upward force is applied by the internal screw threads of the worm gear 4 upon the lifting screw. Thus, the lifting screwthreads have opposite forces applied by the sleeve 6 and worm gear 4 so that backlash is at least upon assembly, substantially eliminated.
When the desired amount of loading is provided by the shell cap holes are drilled through the shell cap into the housing 1 and these holes are then tapped to receive a grub screw for preventing relative movement between the shell cap 8 and housing 1.
In operation of the screw jack, the plate fixed to the threaded portion 12 is secured to a load and since the load does not rotate there is no rotational movement between the housing 1 and the lifting screw 9 and so rotation of the worm 2 causes rotation of the worm gear 4 and sleeve 6 combination thereby resulting in vertical drive to the lifting screw 9.
With use, wear will inevitably result between the meshing threads of the sleeve/lifting screw and worm gear/lifting screw and this is taken up by screwing down the shell cap 8 until there is no longer a gap between adjacent faces 41 and 61 of the worm gear and sleeve respectively; the initial distance between the faces 41 and 61 therefore determines the maximum permitted amount of wear with the threads of the lifting screw 9.
The quadrilaterally shaped protrusion on the shoulder of the worm gear 4 and mating recess in the sleeve 6 has the advantage that a substantial driving area of contact is provided. Furthermore, by machining the internal trapezoidal screw thread on the worm gear 4 and sleeve 6 with these two members combined together it is ensured that the angular relationship between these members is maintained and that the only manner by which backlash may be taken up is by screwing down the shell cap 8, it being remembered that the length of the shoulder 20 is preferably less than the screw pitch. Thus the present invention has the great safety advantage in that the maximum amount of wear permitted with the screw threads of the lifting screw which may be eliminated is governed by the initial distance between the faces 41, 61, i.e. 0.25 screw pitch.

Claims

1. A worm screw jack including a worm (2) meshing with a worm gear (4) which is drivingly connected to a lifting screw (9), an anti-backlash means including a sleeve (6) meshingly engaged with the lifting screw (9) and axially spaced from the worm gear (4), the sleeve (6) being engaged with the worm gear (4) to permit axial relative movement therebetween but preventing relative rotational movement therebetween, characterised in that the anti-backlash means comprises a shoulder member (20) integrally formed on the worm gear (4) and the sleeve (6), said shoulder member having driving surfaces (21) with an axial length less than the pitch of the lifting screw and which mate with a correspondingly shaped recess (30) in the sleeve (6), the sleeve (6) being axially movable in relation to the shoulder member (20).

2. A worm screw jack according to claim 1 characterised in that the axial length of the shoulder is 8% less than the pitch of the lifting screw.

3. A worm screw jack according to any preceding claim characterised in that the sleeve and worm gear are predeterminably spaced in manufacture by a distance of 0.25 x screw pitch.

4. A worm screw jack according to claim 3 characterised in that the shoulder member (20) is a quadrilateral protrusion diametrically extending across the lifting screw axis formed by substantially parallel flat surfaces (21) with partially circular end faces (22), and the sleeve recess (30) is arranged to mate with the quadrilateral protrusion.

5. A worm screw jack according to claim 4 characterised in that a circularly cross-sectioned extension (23) is provided on the shoulder member (20), having a smaller diameter than that of the partially circular end faces (22), which is arranged to be an interference fit inside a bore (31) of the sleeve thereby assisting to maintain the worm gear and sleeve in axial alignment.

6. A method of making a screw jack having a worm (2) meshing with a worm gear (4) which is drivingly connected to a lifting screw (9), an anti-backlash means including a sleeve (6) meshingly engaged with the lifting screw (9) and axially spaced from the worm gear (4), the sleeve (6) being engaged with the worm gear (4) to permit axial relative movement therebetween but preventing relative rotational movement therebetween, characterised by the anti-backlash means comprising a shoulder member (20) integrally formed on the gear (4) and the sleeve (6), said shoulder member having driving surfaces (21) with an axial length less than the pitch of the lifting screw and which mate with a correspondingly shaped recess (30) in the sleeve (6), the sleeve (6) being axially movable relative to the shoulder member (20) said method including the step of axially aligning the worm gear (4) and sleeve (6) a predetermined distance apart and cutting an internal thread through the worm gear (4) and sleeve (6) combination.

7. A method according to claim 6 characterised in that the predetermined distance is provided by a split spacer of a thickness of 0.25 x screw pitch which is secured between adjacent faces of the worm gear and sleeve.

8. A method according to claim 7 characterised by the further steps of mounting the assembly of worm gear, split spacer and the sleeve on the lifting screw, removing the split spacer, engaging the worm gear with the worm and loading the sleeve so that the internal threads of the sleeve and worm gear apply opposing axial forces on the lifting screw threads.

9. A method according to any of claims 6 to 8 characterised in that the loading applied to the sleeve is increased to take up the wear.

10. A method according to claim 9 characterised in that the loading is applied by screwing down a shell cap (8).