AU2013101437A4 - A gearbox - Google Patents

Abstract

Abstract A gearbox comprises a housing (2), an input shaft (4) and an output shaft (5). The input shaft (3) has a driving pinion (11) to drive a gearwheel (15) on the output shaft (5). The output shaft (5) has double helix output pinions (19a, 19b) for engaging a double helix gear (7). The output shaft (5) is movable along its longitudinal axis (18) relative to the housing (2). The output shaft (5) drives the double helix ring gear (7) in response to rotation of the input shaft (4). Fig. 1 3 + + Fig.1

Description

- 1 A Gearbox Field of the Invention [0001] The present invention relates to a gearbox and in a particular to a self-aligning single pinion gearbox for use in a mill. The invention has been developed primarily for use as a self-aligning single pinion gearbox in grinding mills and will be described hereinafter by reference to this application. However, it will be appreciated that the invention is not limited to this particular field of use, but also extends to other types of machines and apparatuses that employ gearboxes, such as heavy machinery, mechanical turbines, especially wind turbines. Background of the Invention [0002] The following discussion of the prior art is intended to present the invention in an appropriate technical context and allow its advantages to be properly appreciated. Unless clearly indicated to the contrary, however, reference to any prior art in this specification should not be construed as an express or implied admission that such art is widely known or forms part of common general knowledge in the field. [0003] A grinding mill has a mill chamber and two journal shafts, the journals being mounted upon supports for rotation. A common method of driving the mill rotation is via one or more open gears (also called girth gears or ring gears) mounted on the mill body. The open gears are driven by a gear and pinion arrangement in the simplest form. These open gears for mill drives are probably the single highest expense component of the mill due to their mass and their complexity to manufacture within strict gearing tolerances. Traditionally, open gears were cut with no helix and were essentially spur gears. Spur type open gears had a low overlap ratio between pinion and gear teeth meshing, which meant that their relative torque capacity was low and they were noisy. [0004] Over time open gears were cut with helix angles so as to increase the overlap ratio between teeth, thus allowing a relatively higher torque capacity and lower noise. As the helix angle was increased toward the optimum of about 280 relative to a straight cut (equivalent to a spur gear), the torque capacity increased but so did the resultant axial loads developed by the open gear. This meant that not only was the open gear -2 transferring a rotational moment to the mill journal shaft but also an unwanted axial load. This axial load is quite detrimental so current single helix open gears for a mill utilise a helix angle of only about 70 to balance torque capacity efficiency and axial load. Thus, single helical open gears typically do not take advantage of angles greater than 70, thus limiting their efficiency. [0005] In the early days of gear development, herringbone or double helix gears were proposed since putting two oppositely arranged helixes back to back on a single gear (to make a herringbone pattern) allowed the use of very aggressive helix angles without any resultant axial loads being transferred to the mill. This type of herringbone open gear was very efficient, as it has a high torque capacity relative to its mass to the extent that the torque capacity to mass ratio of a herringbone open gear is greater than that of a single helical open gear or a spur-type open gear. [0006] However, herringbone open gears have some significant fundamental flaws when applied to mill applications. When used with fixed alignment pinions (as is required in mills), the pinion alignment had to be done manually during a shutdown of the mill, resulting in higher maintenance costs and loss of production. Furthermore, herringbone open gears are more expensive on a per kilo basis compared to single helix open gears. [0007] Also, herringbone open gears suffered from the detrimental effects of runout problems more than single helical or spur open gears. Runout in this case refers to the misalignment of the gear. For example, in the case of axial runout there is misalignment of the gear relative to a true disc perpendicular to the mill centre line. In order for a herringbone open gear to be able to operate with axial runout, either the pinion or gear must be able to float axially otherwise load is removed from one helix and transferred to the other. In a mill application it is very difficult to allow a pinion to float axially and still have robust drive connections to the pinion. If the mill open gear floats axially, all the mill related axial process loads have to be restrained by the pinion, which reduces the longevity of the pinion bearings and can cause failure of the gearing. This later arrangement is contrary to modern mineral processing mill design, where the mill body is axially fixed using axial restraint bearings driving against the mill body or the open gear and the driving pinion is arranged based on the fixed axial location of the mill.

-3 [0008] The increased wear with a herringbone open gear also causes greater difficulties in aligning the gear to the driving pinion. New gears are typically readily aligned to a new pinion. However, where the gears are worn, sometimes naturally over time but often because of poor lubrication or contamination entering the system, alignment of the fixed pinion becomes more and more difficult to achieve. For instance, where alignment is good in one part of the gear but not good in other parts, alignment is almost impossible because the alignment involves two helixes, each with independent wear. There is to date no good solution to these problems with herringbone gears. [0009] In some instances self-aligning pinions have been used in order to try to address runout issues of open gears. Self-aligning driving pinions allow swivelling of the pinion about the pinion centre line to allow continuous optimisation of the pinion to the open gear contact, but the designs to date are only useful for spur-type gears as the self aligning mechanism cannot self align to single helix open gears due to load imbalance and they have no axial degree of freedom in order to allow use on a herringbone open gear. [0010] Furthermore, self-aligning driving pinions are larger in diameter than their equivalent fixed pinions (generally having 25 teeth as opposed to 19 teeth in a fixed pinion). This means that the open gear must be larger in diameter to accommodate the greater number of teeth in the self-aligning pinion to achieve the same overall reduction ratio, thus offsetting some of the benefit of having good alignment. [0011] Due to these deficiencies, herringbone open gears are not used in mineral processing mills, and most mill manufacturers use single helical open gears with helix angles of approximately 70. The single helix open gear has the benefit of a good robust arrangement that does not need a self-aligning pinion, unlike a herringbone open gear. The open gear teeth required are normally minimised to reduce the total mass of the open gear but still suit the required overall reduction ratio based on using a pinion of 19 teeth. In addition, since the driving pinion is not self-aligning, the design codes permit some misalignment to be allowed when designing the gear set. However, it is less efficient and more expensive as outlined above. [0012] One mill manufacturer almost exclusively uses self-aligning twin pinion (SATP) gearboxes to drive spur cut open gears. SATP gearboxes have an input shaft with a spur -4 cut pinion to drive the spur cut gearwheel of an intermediate shaft that in turn drives two output shafts. The intermediate shaft has two helically cut pinions that respectively engage a helically cut pinion on each output shaft. The output shafts each have a self aligning spur cut pinion that engages the spur cut open gear of the mill journal shaft. The intermediate shaft is free to float axially and so that when the intermediate shaft floats axially in one direction, one pinion must slow down and the other pinion must speed up due to the helically cut pinions. By floating axially, the pinions speeding up or slowing down results in force balance being maintained automatically. Thus, the intermediate shaft will constantly be moving axially small amounts to ensure pinion load sharing. Thus, in the SATP gearbox, the torque transferred by each pinion is equally shared, good contact of each output pinion on the open or ring gear is ensured and the ring gear width is more than halved as there are two pinions operating on it, not one where those pinions are self-aligning so minimal misalignment needs to be considered in the design. [0013] Unfortunately the spur cut final stage gearing of an SATP gearbox is known to create high vibrations and be very noisy, both of which are detrimental to the operation of the mill. Furthermore, the SATP gearboxes are known for being very heavy thus requiring considerable investment at the time of installing an SATP gearbox in order to ensure the possibility of future maintainability. [0014] It is an object of the present invention to overcome or substantially ameliorate one or more of the disadvantages of prior art, or at least to provide a useful alternative. Summary of the Invention [0015] According to a first aspect of the invention, there is provided a gearbox for driving a double helix gear, comprising: a housing for receiving a first input shaft and an output shaft; said first input shaft comprising a driving pinion for driving said output shaft; said output shaft comprising a gearwheel for engaging said driving pinion of said first input shaft and a double helix output pinion for engaging said double helix gear, wherein said output shaft is movable along its longitudinal axis relative to said housing and said output shaft is operable to drive said double helix ring gear in response to rotation of said first input shaft.

-5 [0016] Unless the context clearly requires otherwise, throughout the description and the claims, the words "comprise", "comprising", and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of "including, but not limited to". [0017] Furthermore, as used herein and unless otherwise specified, the use of the ordinal adjectives "first", "second", "third", etc., to describe a common object, merely indicate that different instances of like objects are being referred to, and are not intended to imply that the objects so described must be in a given sequence, either temporally, spatially, in ranking, or in any other manner. [0018] Preferably, said gearbox comprises a second input shaft operable to drive said first output shaft, said second input shaft comprising an input pinion, and wherein said first input shaft comprises a first gearwheel for engaging said input pinion. In this preferred form of the invention, the first input shaft becomes an intermediate drive shaft between the second input shaft and the output shaft. [0019] Preferably, said output shaft is movable in response to movement of said double helix gear. [0020] Preferably, said input pinion comprises a helical-type pinion and said first gearwheel of said first input shaft comprises a helical-type gearwheel. Alternatively, said input pinion comprises a spur-type pinion and said first gearwheel of said first input shaft comprises a spur-type gearwheel. In either case, it is preferred that one or more axial bearings are located between said intermediate shaft and said housing to restrain movement of said intermediate shaft along its longitudinal axis. [0021] In a further alternative, said input pinion comprises a double helix pinion, said double helix pinion comprising a first helical gear portion and a second helical gear portion. More preferably, said first gearwheel of said first input shaft comprises a first helical-type gearwheel for engaging said first helical gear portion of said second input shaft, a second helical-type gearwheel for engaging said second helical gear portion of said second input shaft. In this further alternative, said first input shaft is movable along its longitudinal axis relative to said housing.

-6 [0022] Preferably, said double helix output pinion is moveable relative to the longitudinal axis of said output shaft. [0023] Preferably, said driving pinion is a spur-type pinion and said gearwheel of said output shaft is a spur-type gearwheel. [0024] Preferably, one or more axial bearings are located between said second input shaft and said housing to restrain movement of said second input shaft along its longitudinal axis. [0025] Preferably, said first input shaft and said output shaft comprises a gear set, and said gearbox further comprises a plurality of said gear sets, wherein said gear sets are operatively connected together. More preferably, said plurality of said gear sets comprise at least a first gear set and a second gear set. [0026] Preferably, said double helix gear is a double helix ring or open gear. More preferably, said double helix ring gear is used to provide rotation to a mill such that said gearbox drives said mill. In one preferred form, said mill is a grinding mill for mineral processing. Alternatively, said double helix ring gear is used to provide rotation to a turbine such that said gearbox drives said turbine. In one preferred form, said turbine is a wind turbine. [0027] Preferably, said double helix gear is attached to an output shaft for driving operation of a machine or apparatus. More preferably, said double helix gear forms part of said gearbox. [0028] In a second aspect of the invention, there is provided a grinding mill for processing minerals, comprising a mill body having a double helix ring gear for driving said mill body, and a gearbox according to the first aspect of the invention for driving said double helix ring gear. Brief Description of the Drawings [0029] Preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: -7 [0030] Figure 1 is a schematic cross-sectional side view of a gearbox according to a first embodiment of the invention; [0031] Figure 2 is a schematic top view of the shaft layout for the gearbox of Figure 1; [0032] Figure 3 is a schematic top view of an alternative shaft layout for the gearbox of Figure 1, and [0033] Figure 4 is a schematic top view of another alternative shaft layout for the gearbox of Figure 1. Preferred Embodiments of the Invention [0034] The preferred embodiments of the invention will now be described in its application to mills, especially grinding mills for mineral processing. However, it will be appreciated that the gearbox is not limited to this use and is readily applicable to other apparatuses and machines having double helix gears, including double helix ring gears, that are typically driven by gearboxes. For example, it is contemplated that the gearbox is readily applicable to heavy machinery, including manufacturing machines, turbines, cement mills and especially wind turbines. [0035] Referring to Figure 1, a gearbox 1 according to one embodiment of the invention comprises a gearbox housing 2 for receiving a gear set that comprises a high speed input shaft 3, an intermediate drive shaft 4, and a low speed output shaft 5. The input shaft 3 is operable to drive the intermediate drive shaft 4, which in turn is operatable to drive the output shaft 5. The output shaft 5 is operable to engage and drive a double helix gear in the form of an open or ring gear 7 of a mill body 8. [0036] Referring to Figure 2, a single helix or helical input pinion 10 is fixedly mounted to the input shaft 3. The input shaft 3 is mounted on axial restraint bearings (not shown) to inhibit or prevent axial displacement of the input shaft, thus avoiding any unwanted axial loads being transferred to the main or drive motor (not shown) driving the input shaft. [0037] The intermediate drive shaft 4 comprises a helical gearwheel 11 for engaging the helical pinion 10 on the input shaft 3. The intermediate drive shaft 4 also comprises a -8 spur-cut driving pinion 12 for driving the output shaft 5. The helical gearwheel 11 and the driving pinion 12 are fixedly mounted to the intermediate drive shaft 4. [0038] The output shaft 5 comprises a spur-cut gearwheel 15 that engages the driving pinion 12 of the intermediate drive shaft 4, and a double helix output pinion 19 having helical portions 19a, 19b for engaging the double helix open gear 7. The double helix output pinion 19 of the output shaft 5 is a self-aligning pinion in that it is mounted on spherical bearings and can pivot or swivel about the pinion centre line or longitudinal axis 18 in response to the forces generated by the double helix open gear 7 during engagement. The self-aligning double helix output pinion 19 works against any possible misalignment induced by twisting of the double helix open gear 7, thus obviates a major problem in implementing a herringbone (double helix) open gear for a mill. [0039] It has also been discovered that using the spur-cut gearwheel 15 on the output shaft 5 advantageously allows for axial movement of the output shaft 5 without adversely affecting gearing timing since the axial movement of the output shaft 5 is substantially perpendicular to the driving action applied by the spur-cut driving pinion 12 on the intermediate drive shaft 4 upon the spur-cut gearwheel 15. [0040] As the input shaft 3 rotates, the helical gearwheel 11 keeps the spur-cut driving pinion 12 hard in mesh with the spur-cut gearwheel 15 (and hence the double helix ring gear 7). The reaction from the helical input pinion 10 pushing against the helical gearwheel 11 generates an axial reaction. The intermediate drive shaft 4 is axially restrained by suitable axial bearings located between the intermediate drive shaft and the housing 2. Thus, the output shaft 5 is able to inhibit axial runout of the double helix open gear 7 by restraining against any axial misalignment that occurs in the double helix open gear. Accordingly, the double helix open gear 7 can be axially fixed by the mill bearing restraint in accordance with standard mill design requirements. Thus, this obviates another one of the major problems involved in implementing a herringbone (double helix) open gear for a mill. [0041] Depending on the gear ratio, the intermediate shaft 4 can be used as the input shaft instead of the input shaft 3. In this case, the helical gearwheel 11 is connected directly to the drive motor. This necessarily removes input shaft 3 and input pinion 10 from the gearbox, thus simplifying the design for suitable gear ratios.

-9 [0042] Another embodiment of the invention is illustrated in Figure 3, where corresponding features have been given the same reference numerals. In this embodiment, the input shaft 3 has a spur-cut type pinion 20 and the intermediate drive shaft 4 has a matching spur-cut type gearwheel 21. In all other respects, this embodiment works in substantially the same way and has the same technical advantages as the embodiment of Figure 2 described above. [0043] Again, the input shaft 3 and input pinion 20 can be omitted for a suitable gear ratio and the intermediate drive shaft 4 acts as an input shaft, whose spur-cut type gearwheel 21 directly engages the drive motor (not shown). [0044] A further embodiment of the invention is illustrated in Figure 4, where corresponding features have been given the same reference numerals. In this embodiment, the input shaft 3 has a double helix input pinion 30 having two helical pinions 30a, 30b or opposing orientation, and the intermediate drive shaft 4 has two helical gearwheels 31a, 31b to respectively engage each helical pinion 30a, 30b, as best shown in Figure 4. [0045] In this form, and unlike the previous embodiments, the intermediate drive shaft 4 is movable along its longitudinal axis 35 relative to the gearbox housing 2, as best shown in Figure 1. When the intermediate drive shaft 4 moves axially the relationship between each helical input pinion 30a, 30b and each helical gearwheel 31a, 31b is "retarded" in that the angular positions of the respective helical input pinion 30a, 30b and helical gearwheel 31a, 31b must change in order to keep them in mesh contact, thus reducing the radial load generated by the driving pinion 12 on the common ring gear 7. In other words, the intermediate drive shaft 4 and the output shaft 5 is each free to move or "float" axially in response to forces exerted during operation of the mill. The intermediate drive shaft 4 moves axially in response to load sharing of the helical gearwheels 31 a, 31 b. The output shaft 5 moves axially to accommodate axial runout of the double helix open gear 7 by working against any axial misalignment that occurs in the double helix open gear. Accordingly, as with the other embodiments, the double helix open gear 7 can be axially fixed by the mill bearing restraint in accordance with standard mill design requirements, and hence obviates a major problem typically expected in implementing a herringbone (double helix) open gear for a mill.

-10 [0046] As noted above, the input shaft 3 and input pinion 30 can be omitted for a suitable gear ratio and the intermediate drive shaft 4 acts as an input shaft, whose double helix gearwheel 31 (or other suitable gearwheel, such as a helical-type or spur-cut type gearwheel) directly engages the drive motor (not shown) [0047] The inventors also contemplate that the bearings for the intermediate drive shaft 4 and the output shaft 5 will be hydrostatic self aligning bearings to facilitate the radial and axial freedom of movement for these shafts. [0048] A comparison of the gearboxes of the preferred embodiments of the invention against prior art gearboxes is set out in the table below, where the power is set at 5400 kW and the mill speed is set at 13.8 rpm. Table I - Comparison of gearbox arrangements Gearbox Number Power per Power Approximate Option pnion per Angle Mass of Description pinions helix open gear Single 540 Current single helical gear 1 5400 0 7.7 39 pinion state of art - base design Single 540 Current single helical rev 1 5400 7.5 32 pinion state of art B fully optimised 270 Double helix, 3A 2x1 5400 0 29 25.5 single pinion, optimised design [0049] It can be seen from Table 1 above that an optimised single helical gear set with a single pinion requires an open gear weighing 32 tonnes whereas a similarly optimised double helical gear set with a single pinion requires an open gear weighing only 25.5 tonnes, representing a significant mass (and hence cost) saving. [0050] In other preferred forms, the double helix gear is located within the gearbox housing 2 instead of being an external double helix ring gear 7. In this case, the output from the gearbox 1 is a single output shaft instead of the mill body 8. This output shaft would be a shaft for operating a suitable machine or apparatus. In this embodiment, the - 11 double helix output pinion 19 does not necessarily need to have self-aligning pinions as the gearbox housing 2 could be made of a sufficiently rigid material so that the now enclosed ring gear 7 has runouts within acceptable running tolerances, thus self alignment of the double helix output pinion 19 to the ring gear 7. Even so, the use of multiple pinions to reduce the mass of the double helix output gear (either as an external ring gear or as a final stage gearwheel - what the ring gear would be called when enclosed in the gearbox housing) remain as advantages of the gearbox according to the invention. [0051] In other embodiments, an additional gearwheel 11 is added and fixed to the intermediate shaft 4 to provide another stage of gear reduction. It will be appreciated that more than one stage of reduction can be added the gearbox 1, as required or desirable. [0052] In some embodiments, the gearbox 1 has multiple gear sets the same as the gear sets described in Figures 1 to 4, with an initial gear set being connected in series to a another gear set, or to two other gear sets in parallel, to form a "stack" or combination of gear sets.. The output shafts of these other gear set(s) would drive the double helix gear 7. It will be appreciated that this stacking of gear sets is not limited to only two or three gear sets, but can be expanded as desired or required. [0053] By providing a gearbox that employs a single pinion, a herringbone or double helix gear can be used for the open or ring gear on a mill without the attendant disadvantages of axial runout, gear wear issues and uneven load sharing on the driving pinions. Thus, the mill can take advantage of the higher torque capacity to mass ratios and greater helix angles of a herringbone open gear, resulting in better efficiency and lower manufacturing costs since the herringbone open gear will have a significantly less mass than other spur-cut or single helix open gears. [0054] The gearbox as described in preferred embodiments of the invention achieves these advantages by providing an output shaft that is able to move axially or "float", mitigating the detrimental effects of axial misalignment of the herringbone ring gear. As a consequence it can be expected the drive will perform with less vibration and have a longer service life. Furthermore, the gearbox of the embodiment also has a pair of self aligning double helix output pinions that work against misalignment induced by twisting of the ring gear. It is also advantageous that the output shaft has spur-cut gearwheels as -12 this permits axial movement of the output shafts without changing gearing timing. In all these respects, the invention represents a practical and commercially significant improvement over the prior art. [0055] Also, while the preferred embodiments of the invention have been described in relation to gearboxes for mills in general, it will be appreciated that each of the aspects of the invention, the corresponding preferred features and their associated advantages described above are also applicable to other types of machines and apparatuses employing a herringbone gear, either as an open/ring gear or as a gear to drive an output shaft, such as mechanical turbines, manufacturing machines and apparatuses. In particular, the inventors contemplate that the gearbox of the invention would provide significant advantages when used with wind turbines, especially the stacked arrangement of Figure 5, since the gearbox would provide high torque capacity in a relatively compact arrangement. [0056] Although the invention has been described with reference to specific examples, it will be appreciated by those skilled in the art that the invention may be embodied in many other forms.

Claims (18)

1. A gearbox for driving a double helix gear, comprising: a housing for receiving a first input shaft and an output shaft; said first input shaft comprising a driving pinion for driving said output shaft; said output shaft comprising a gearwheel for engaging said driving pinion of said first input shaft and a double helix output pinion for engaging said double helix gear, wherein said output shaft is movable along its longitudinal axis relative to said housing and said output shaft is operable to drive said double helix ring gear in response to rotation of said first input shaft.

2. The gearbox of claim 1, wherein said gearbox comprises a second input shaft operable to drive said first output shaft, said second input shaft comprising an input pinion, and wherein said first input shaft comprises a first gearwheel for engaging said input pinion.

3. The gearbox of claim 1 or 2, wherein said output shaft is movable in response to movement of said double helix gear.

4. The gearbox of any one of the preceding claims, wherein said input pinion comprises a helical-type pinion and said first gearwheel of said first input shaft comprises a helical-type gearwheel.

5. The gearbox of any one of claims 1 to 3, wherein said input pinion comprises a spur-type pinion and said first gearwheel of said first input shaft comprises a spur-type gearwheel.

6. The gearbox of claim 4 or 5, wherein one or more axial bearings are located between said intermediate shaft and said housing to restrain movement of said intermediate shaft along its longitudinal axis.

7. The gearbox of any one of claims 1 to 3, wherein said input pinion comprises a double helix pinion having a first helical gear portion and a second helical gear portion, and said first gearwheel of said first input shaft comprises a first helical-type gearwheel for - 14 engaging said first helical gear portion of said second input shaft, a second helical-type gearwheel for engaging said second helical gear portion of said second input shaft.

8. The gearbox of any one of the preceding claims, wherein said double helix output pinion is moveable relative to the longitudinal axis of said output shaft.

9. The gearbox of any one of the preceding claims, wherein said driving pinion is a spur-type pinion and said gearwheel of said output shaft is a spur-type gearwheel.

10. The gearbox of any one of claims 2 to 9, wherein one or more axial bearings are located between said second input shaft and said housing to restrain movement of said second input shaft along its longitudinal axis.

11. The gearbox of any one of the preceding claims, wherein said first input shaft and said output shaft comprises a gear set, and said gearbox further comprises a plurality of said gear sets, wherein said gear sets are operatively connected together.

12. The gearbox of any one of the preceding claims, wherein said double helix gear is a double helix ring or open gear.

13. The gearbox of claim 12, wherein said double helix ring gear is used to provide rotation to a mill such that said gearbox drives said mill.

14. The gearbox of claim 12, wherein said double helix ring gear is used to provide rotation to a turbine such that said gearbox drives said turbine.

15. The gearbox of claim 14, wherein said turbine is a wind turbine.

16. The gearbox of any one of claims 1 to 11, wherein said double helix gear is attached to an output shaft for driving operation of a machine or apparatus.

17. The gearbox of claim 16, wherein said double helix gear forms part of said gearbox. -15

18. A grinding mill for processing minerals, comprising a mill body having a double helix ring gear for driving rotation of said mill body, and a gearbox of any one of the preceding claims for driving said double helix ring gear.