The present invention relates to a method of manufacture of a gearbox and a gearbox made by the method.
Small gearboxes are used in many applications; one example is in seat moving mechanisms in automobiles. The efficiency of such gearboxes is greatly affected by alignment of components in the gearboxes and reduction of tolerances and the surface finish and metallurgy of bearing surfaces, this also has a great effect on the noise emitted by the gearbox during use and wear of gearbox components and hence gearbox life.
In a first aspect the present invention provides a method of manufacture of a gearbox comprising:
forming a plurality of gearbox casing components each with an end stop feature;
in a pre-assembly stage, bringing the formed gearbox casing components together prior to final assembly with the end stop features of the casing components together defining an end stop internal to the casing;
applying pressure on the end stop to deform the end stop;
disassembling the gearbox casing components;
assembling the gearbox components around an end of a shaft, the shaft having a gear mounted thereon for rotation therewith and the assembled gearbox casing components encasing the gear; and
bringing the end of the shaft into abutment with the end stop either directly or via one or more spacer element(s) wherein:
deformation of the end stop in the pre-assembly stage sets a distance between the end stop and a surface of the gearbox casing which in the assembled gearbox faces a side surface of the gear, the said gear side surface facing away from the end stop.
In a second aspect the present invention provides a method of manufacture of a gearbox comprising:
forming a plurality of gearbox casing components which when assembled together provide a cylindrical bearing surface for a shaft;
in a pre-assembly stage forcing the gearbox casing components together around a former which has rollers mounted therein which engage the bearing surface of the gearbox casing and indent the bearing surface as the casing components are forced together;
rotating the former to roll the rollers around the bearing surface to deform the bearing surface;
disassembling the gearbox casing components from around the former; and
assembling the gearbox components around an end of a shaft, the shaft having a gear mounted thereon for rotation therewith and the assembled gearbox casing components encasing the gear.
In a third aspect the present invention provides a method of manufacture of a gearbox comprising:
forming a plurality of gearbox casing components, a first of the gearbox casing components having spigots and a second of the gearbox components having matching sockets;
in a pre-assembly stage forcing the spigots of the first gearbox components into the sockets of the second gearbox component and in doing so deforming the spigots to fit into and match in shape with the sockets;
disassembling the gearbox casing components; and
assembling the gearbox components around an end of a shaft, the shaft having a gear mounted thereon for rotation therewith and the assembled gearbox casing components encasing the gear.
In a fourth aspect the present invention provides a method of manufacture comprising:
moulding a toothed gear wheel from plastic having a central aperture and slots in an annular surface defining the central aperture;
fashioning a pair of metal load-bearing elements each having a central aperture therethrough and, a load-bearing surface;
mounting the gear wheel and the bearing elements on a metal shaft, with the load-bearing elements sandwiching the plastic gear wheel and with a part of at least one of the load-bearing elements extending through the central aperture in the gear wheel to abut the other load-bearing element;
deforming the metal of the shaft to form a pair of annular shoulders on the shaft which engage the load-bearing surfaces of the load-bearing elements, with the forces applied to the load-bearing surfaces being transmitted through directly abutting faces of the load-bearing elements; and
allowing axial movement of the gear wheel and load-bearing elements as the shoulders are formed so that the shoulders fix the gear wheel in position axially on the shaft.
In a fifth aspect the present invention provides a gearbox comprising:
a plurality of metal casing components which together define both an internal end stop and a cylindrical burnished bearing surface for engaging a shaft, a first of the casing components having a plurality of spigots and a second of the casing components having has a plurality of matching sockets;
a metal shaft having mounted thereon for rotation therewith a toothed gear, the toothed gear comprising a plastic toothed gear wheel secured between a pair of metal bearing elements which are in turn engaged by a pair of shoulders formed integrally in the metal shaft; and
a worm gear; wherein:
the metal casing components encase and secure both the metal shaft with the toothed gear mounted thereon and also the worm gear, with the worm gear meshing with the toothed gear and with an axis of rotation of the worm gear being spaced apart from the shaft and perpendicular to a plane which includes an axis of rotation of the shaft;
the shaft is secured axially in the gearbox casing between the end stop, which faces an end of the shaft, and a gearbox casing surface which faces a bearing surface of one of the bearing elements, said bearing surface facing away from the end stop; and
a cylindrical portion of the shaft is surrounded by the cylindrical bearing surface formed by the assembled casing components.
Preferred embodiments of the present invention will now be described with reference to the accompanying drawings, in which:
The plastic gear wheel 16 has three slots 22, 23, 24 (see
An inner part of the thrust face (e.g. 25) of each washer 17, 18 is chamfered to provide a conical surface which has a plurality of notches defined therein. As an example, notches 30, 31, 32 are shown in
During assembly the injection moulded plastic gear wheel 16 is sandwiched between the two metal washers 17, 18, with the teeth of the washers 17, 18 extending through the slots 22, 23, 24 of the plastic gear wheel 16. This leaves a component as shown in
The gear wheel 16 is next fixed axially in place on the shaft 12 by a “shoulder rolling” process, as previously described in EP 1000686. In this process metal flows from the regions 33, 34 to form annular shoulders 35, 36. In the process the material flow to form shoulder 35 and the material flow to form shoulder 36 causes movement of the gear 16 to locate the gear 16 exactly in a desired axial location on the shaft 12. The flowing metal flows into the notches 30, 31 and 32 described previously. The shoulders 35, 36 securely locate the gear 16 on the shaft 12 in an axial direction and also lock the gear 16 to rotate with the shaft 12. The notches 30, 31, 32 help in securing the gear 16 to rotate with the shaft 12. The metal bearing components 17, 18 react loading applied on the gear 16 during the forming of the shoulders 35, 36. They thereby enable the use of a gear 16 which is predominantly injection moulded plastic to be located on the shaft 12 with a “shoulder rolling” process. An injection moulded plastic gear itself would not be capable of withstanding the loading and for this reason the thrust bearing components 17, 18 are essential.
The shoulder rolling process also causes the diameter of the shaft between the shoulders to increase, to take up any clearance between the shaft and adjacent metal bearing components 17,18. Thus, while prior to shoulder rolling there is a clearance fit between the metals bearing components 17,18 and the shaft, after shoulder rolling there is a “size and size” (i.e. matched diameters) fit, which prevents the gear from rocking on the shaft. The gearbox of the invention can be used in a seat adjustment mechanism of a vehicle and thus must be able to withstand crash loads. Such crash loading is transmitted from the metal of the shaft through the metal of the bearing components to the metal of the gearbox casing, without the plastic gear having to transmit such high forces.
The exact concentricity and exact perpendicularity of the shoulder rolling dies are transmitted to the gear and shaft sub-assembly during the shoulder rolling process. At the front end of the shaft 12 there is formed by a drilling operation a closed bore 37 having a conical end face 38 (see
In a parallel manufacturing process the gearbox casing is formed. An important feature of the gearbox casing is a deformable end stop 40 (see
During formation of the gearbox, the gearbox casing components are initially brought together around a forming mandrel 1000 (see
The displacement of the rod 1010 will be carefully controlled, to account for the fact that the end stop 40 deforms both elastically and plastically. The control unit for the movement of the rod 1010 will be encoded to give a displacement which will give a final position of the end stop (after the end stop has expanded elastically on removal of pressure from the end stop) equivalent to that desired.
Also seen in
In
In manufacture of the gearbox the casing parts 120, 121 (like the casing part 14 and its counterpart) are brought together around a mandrel, as described above, prior to final assembly. The spigots (e.g. 116, 117, 118, 119) are forced into the corresponding sockets (e.g. 130, 131, 132, 133). The spigots (116,117,118,119) are deformed by their insertion with the corresponding sockets (130,131,132,133). The spigots (116,117,118,119) are initially sized such that the leading edges of the spigots engage the socket surface 1 mm from the end of the matching frusto-conical surface. The plunger forcing the casing components together is then advanced a further pre-selected distance so that the spigots deform to take up a typical 0.1 mm clearance left between the spigot and socket at the initial point of contact. The material of the spigots will flow most where there is least resistance. This improves the concentricity of fit of the spigot in socket. The spigot material deforms plastically whilst the socket material remains in an elastic state. Once the spigots have been fully advanced then the casing components are kept together as the metal of the spigots and the metal of the sockets relax, the relaxation of the socket metal in its elastic state typically giving rise to further plastic deformation of the spigots.
In the manner described above the alignment of the casing parts is set and the end stop suitably deformed while the parts are held in place around a mandrel. Once the deformation processes described above are completed then a bearing surface finishing operation can be carried out, as will now be described.
The mandrel 1000 used in the forming process is also provided with rollers 1004, 1005, 1006, 1007 (see
Instead of roller burnishing the bearing surfaces as described above, the surfaces can be finished by a swaging process. In such a process the rollers 1004, 1005, 1006 and 1007 will indent the casing metal deeper than in the burnishing process and will displace metal circumferentially around the bearing surfaces as the rollers are rotated through 90 degrees. Typically a pair of undercuts or slots will be cast in the bearing surface to provide voids into which metal can be displaced during the swaging process. The greater deformation of the bearing surfaces by swaging can give a better surface finish than burnishing and a tighter tolerance to the formed cylindrical bearing surfaces and the alignment between them.
After a suitable dwell period following completion of the processes described above, the two casing parts are then separated from each other.
Once the casing components have been through the pre-assembly stage and the end stop (40 in
On assembly the shaft 12 and gear 11 are located between the pre-deformed casing components (e.g. 14), with the insertion of a hardened spacer 50 between the end of shaft 12 and the end stop 40, the ball bearing 39 engaging the spacer 50. The worm gear 60 is mounted cross-axially to the axis of shaft 12 and meshed with gear 11. Two “top hat” zinc (or zinc alloy) bearing caps 52 and 53 (see
The casing parts (e.g. 14) are held together in final assembly by rivets or screws securing spigots 45, 46, 47, 48 in the matching sockets. The pre-assembly operations performed on the casing components ensure a good and exact fit of all components in the final assembly.
In a similar fashion the components of the
A fully assembled gearbox as shown in
The principles behind the present invention are now further explained with reference to a third embodiment of gearbox illustrated in
The metal bearing washer 2006 has a first side shown in
The moulded gear 2003 is secured between the bearing washer 2006 and a fine blank mild steel washer 2012 seen in
The keying features 2010 are depicted in
The plane shaft 2000 provides the basis for the shoulder rolling of the gear 2003 into position. The bearing washer 2006 is assembled to a first side of the moulded gear locating the mating key features 2002, 2005. The fine blank bearing washer 2012 is assembled to the gear 2003 on the other side. Next, the sub-assembly of moulded gear 2003 and bearing washers 2006 and 2012 is located onto the shaft 2000 mating the inner diameter of the bore in the bearing washer 2006 with the outer diameter of the plane shaft 2000. The shaft 2000 is then loaded into a shoulder roll machine. In
The load transmitted by the shoulder rolling process is resisted by the thrust face 2009 of the bearing washer 2006 and the blank washer 2012, thus preventing damage to the moulded gear 2003.
As a result of the loading from the shoulder rolling process, the gear washer assembly 2006, 2012 is compressed onto the shaft, thereby increasing the ability to transmit torque. Advantageously, shoulder rolling also increases the diameter of the shaft in the region where the inner diameter of the bore in the bearing washer 2006 mates with the outer diameter of the plane shaft 2000, thereby firmly fitting the shaft to the washer and preventing rocking of the gear on the shaft by achieving a very high order of concentricity of shaft to bore.
The high levels of concentricity and perpendicularity of the shoulder rolling dies are imparted to the sub-assembly of gear and shaft during the shoulder rolling process.
As mentioned above, the gearbox can be used in vehicle seat position adjustment mechanisms and thus be subject to high loading during a vehicle impact. Such high loading is transmitted from the shaft 2000 to the gearbox casing via the washer 2006 or washer 2012, bypassing the plastic gear 2003.
The casing consists of 2 casing halves provided by full zinc die cast bodies (2050 and 2051, shown in FIGS. 21,22) formed on one central mandrel assembly (a roller burnishing tool 2060 shown in
There are two sets of four rollers, such as 2061 and 2062 of a first set and 2063 and 2064 of a second set seen in
Four spigots 2052, 2053, 2054 and 2055 (also called pins) are formed on one casing half 2050 and four corresponding sockets 2056, 2057, 2058 and 2059 (also called buckets) on the opposite casing half 2051. The casting halves 2050 and 2051 are aligned and the pins 2052, 2053, 2054 and 2055 are deformed by forcing them into the buckets 2056, 2057, 2058 and 2059, in order to provide an accurate positioning of the two casting halves relative to each other when they are subsequently dis-assembled and re-assembled and also to ensure that there is no movement in the casting during operation.
Rear end stop features 2070, 2071, 2072, 2073 are provided on the castings and are deformed to a known distance so that accurate axial location of the shaft can be achieved once castings are assembled around it.
The casing halves 2050,2051 are assembled over the precision roller burnishing tool 2060 with the rollers (e.g. 2061, 2062, 2063, 2064) on the tool located in the v-grooves (2080-2083 and 2084-2087) in the casing halves 2050 and 2051. This creates a v-block effect to centre the casings both over the tool and accurately aligns the two casing halves 2050,2051. When the clamping force is applied to the two casing halves then the rollers (e.g. 2061, 2062, 2063, 2064) indent into the grooves and this aligns the casing halves in two directions and fixes them from rotational motion about the mandrel axis. The tooling must be lubricated on the mandrel to ensure that the frictional force is minimised during roller burnishing. A clamping force is applied to the casing halves 2050,2051 to ensure that there is no axial movement of the halves relative to each other during the deformation process, plus a force must be applied independently to the each casting pushing them both against the front thrust face 2162 on the roller burnishing tool 2060. The rear end stop features 2070-2073 are then deformed by a central pin 2069 of the roller burnishing tool 2060 and this ensures that the datum bearing faces of both casing halves 2050,2051 and the end stop features are positioned accurately relative to each other. A closed loop controlled hydraulic actuator 4000 is incorporated in the roller burnishing tool to facilitate this, as can be seen in
While maintaining the axial force, leaving the two casing halves in tension between the end stop feature and the thrust face, the four spigots 2052-2055 are then deformed simultaneously by forcing them into the buckets 2056-2059 in the opposite casing half and to provide accurate casing location on subsequent disassembly and reassembly. This is achieved by clamping together the two casing halves 2050 and 2051 around the burnishing tool 2060 with the rollers, e.g. 2061-2064, of the burnishing tool 2060 indenting the abutting journal surfaces of the casing by up to 0.1 mm. The pins/spigots 2052-2055 are deformed through the elastic region of their material properties and into the plastic region, thus they maintain a constant deformed state even when force is withdrawn. Doing so with four pins/spigots eliminates any freeplay between the two halves and ensures they are both aligned about the front thrust face 2162. Preferably, the pins/spigots are designed to be “long and slender”, i.e. with an axial length exceeding diameter, the length being preferably at least a 1.1 multiple of diameter, or a 1.25 multiple or a 1.5 multiple or higher. This permits a significant degree of deformation to ensure good alignment of the finally assembled casing halves.
Once deformation is complete, then the pins/spigots 2052-2055 are removed partially from the buckets 2056-2059 and the axial force on the end stop is relaxed to allow the rotation of the roller burnishing tool. This is rotated 90° to provide an accurate bore diameter and improved surface finish. The final operation before dis-assembly is to ball burnish worm journal holes, e.g. 2100, for a worm gear. Whilst the gearbox is still aligned a carbide ball is pushed through the worm holes improving surface finish and improving the accuracy relative to each other. This process is indicated by the arrows 4002 in
The worm gear sub-assembly of the gearbox will now be described with reference to
The worm gear 2200 has a gear roll-formed on it to ensure accurate tooth profile and finish. The worm has turned journal diameters 2201 and 2202 provided at both ends, with a circumferentially extending groove 2203, 2204. Mild steel worm thrust washers such as washer 2205 of
The thrust washers, e.g. 2205, are assembled onto the worm gear 2200 with their shoulders, e.g. 2207, facing away from the gear form. The assembly of worm gear 2200 and washers, e.g. 2205, is then loaded into a shoulder roll machine.
The high levels of concentricity and perpendicularity of the shoulder rolling dies tools are imparted to the sub-assembly of worm gear 2200 and thrust washers (e.g. 2205) by the shoulder rolling process. Also the spacing between the thrust faces of the thrust washers is set to a high degree of accuracy.
Once all sub-assemblies have been assembled and the forming processes described above have been completed then components can be assembled together to form a completed gearbox. The spindle and gear sub-assembly, shown in
As depicted in
Then the roller burnishing tool 2060 is placed again the first casing half, and the second casing half is placed upon both the roller burnishing tool 2060 and the first casing half, in a jig as seen in
Advantageously, the rollers 2061-2064 engage the parallel grooves 2080-2087 to align and centre the cast bodies 2050, 2051 along the roller burnishing tool 2060 axis.
The jig further comprises adjustable sprung grub screws 5008, depicted in
Then, four plungers 5006, seen in
The casing halves 2050, 2051 are thereby accurately constrained in all directions by the eight support pins 5002, 5006, the grub screws 5008 and thrust face 2162 of the roller burnishing tool 2060.
Next, accurate alignment of the axial position of the end stop features 2070-2073 is precisely defined by applying a force with the hydraulic actuator 4000 between thrust face 2162 and the end surface 2069 of the roller burnishing tool 2060. Since the hydraulic actuator can be accurately controlled, it is possible to precisely determine the movement of the end face 2069 relative to the thrust face 2162, and therefore determine the precise deformation of the end stop features 2070-2073.
Once the end stop has been deformed, and while the mandrel is kept in place by the hydraulic actuator to keep the casing halves in tension between the end stop and the opposed thrust face of the casing, the plungers 5006 are advanced to force the pins/spigots into the sockets/brackets (as previously described).
The plungers 5006 are next removed and, whilst the mandrel is still held in its position against the end stop, the rollers, which are located in a roller burnishing tool rotationally mounted as an inner mandrel element, are rotated to roller burnish or swage the bearing sections of the casing, as described above. The carbide ball bearing will then be pushed through the worm bores to provide a good surface finish.
Number | Date | Country | Kind |
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0722199.7 | Nov 2007 | GB | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/GB2008/003797 | 11/12/2008 | WO | 00 | 8/25/2010 |