Gear-driven bearing unit

Abstract

A gear-driven bearing unit comprising a housing, a drive gear and a rolling element bearing The rolling element bearing comprising: an outer member with teeth for meshing with drive gear teeth, an inner member and rolling elements between the inner and outer members. At one axial side, the housing is connected to the inner member enclosing the outer member. A first radial gap extends between an outer member radially inner surface and an inner member radially outer surface. A second radial gap exists between an outer member radially outer surface and a housing radially inner surface. The radial gaps are sealed by seals, each comprising radial sealing lips. The unit is lubricated with a grease comprising a calcium-based thickener or calcium salt additives and the teeth on the outer member are made of a hardening treated bearing-grade steel which diffuses nitrogen into the surface of the teeth.

Claims

1. A gear-driven bearing unit comprising a housing, a drive gear and a rolling element bearing, the rolling element bearing comprising: an outer member provided with teeth for meshing with teeth of the drive gear, an inner member and one or more rows of rolling elements disposed between the inner and outer members, whereby at one axial side of the gear-driven bearing unit a first radial gap exists between a radially inner surface of the outer member and a radially outer surface of the inner member; and a second radial gap exists between a radially outer surface of the outer member and a radially inner surface of the housing, the gear-driven bearing unit being at least partly filled with a grease lubricant, which is retained within the gear-driven bearing unit by a first seal provided in the first radial gap and by a second seal provided in the second radial gap, wherein: the radially inner surface of the housing comprises a first axially extending surface, a radially extending groove, and a finger having a rectangular-shaped cross-section, the first axially extending surface radially overlaps the one or more rows of rolling elements, the radially extending groove extends radially outwardly from an axial end of the first axially extending surface and is axially delimited by the axial end of the first axially extending surface and the finger, the finger located on an opposite side of the radially extending groove from the axially extending surface such that the finger extends radially inwardly with respect to a bottom surface of the radially extending groove and has a minimum radius greater than a radius of the first axially extending surface, one of the first and second seals comprises a first radial lip and a second radial lip that bear against a radially oriented counterface, the first seal comprises a first mounting part that has a rectangular cross-section and is inserted in a groove located in the radially outer surface of the inner member, the second seal comprises a second mounting part that has a U-shaped cross-section, the second mounting part being configured to receive the finger therein such that a portion of the second mounting part is located in the radially extending groove and such that the finger is contacted on each axial side by the second mounting part; and in that the grease lubricant comprises a calcium-based thickener or comprises calcium salt additives; the teeth of the drive gear are made of a bearing-grade steel; and in that the teeth on the outer member are made of a bearing-grade steel that has been subjected to a hardening treatment that diffuses nitrogen into a surface of the teeth.

2. The gear-driven bearing unit according to claim 1, wherein the first radial lip is closer to a radial centreline of the gear-driven bearing unit than the second radial lip, and wherein the first radial lip extends towards the radial centreline and the second radial lip extends away from the radial centreline.

3. The gear-driven bearing unit according to claim 2, wherein an axially outer surface of the second radial lip comprises a thermally reflective material or has a thermally reflective colour.

4. The gear-driven bearing unit according to claim 3, wherein the axially outer surface of the second radial lip is non-smooth, so as to have an increased surface area relative to a smooth surface.

5. The gear-driven bearing unit according to claim 1, wherein the first and second radial lips extend from one of the first mounting part and the second mounting part.

6. The gear-driven bearing unit according to claim 5, wherein the first and second radial lips extend from the second mounting part that has a U-shaped cross-section such that the second mounting part, the first radial lip, and the second radial lip together form an X-shape.

7. The gear-driven bearing unit according to claim 1, wherein the housing comprises a thermally reflective outer surface.

8. The gear-driven bearing unit according to claim 1, wherein the grease lubricant comprises a synthetic mineral oil and a complex calcium suphonate thickener.

9. The gear-driven bearing unit according to claim 1, wherein the teeth provided on the outer member are machined directly into the outer member.

10. The gear-driven bearing unit according to claim 1, wherein the teeth of the drive gear are subjected to a hardening treatment that diffuses nitrogen into a surface of the teeth.

11. The gear-driven bearing unit according to claim 1, wherein the rolling element bearing is a double-row angular contact bearing and the inner member comprises a first inner ring and a second inner ring.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a cross-sectional side view of a gear-driven bearing unit according to the invention;

(2) FIG. 2 shows a cross-sectional plan view of the same bearing unit;

(3) FIG. 3 is a plot of friction test results from three material combinations in sliding contact, lubricated by a Grease A.

(4) FIG. 4 is a plot of friction test results from hardened bearing steel in sliding contact with nitrided bearing steel, lubricated by Grease A and a Grease B;

(5) FIG. 5a, 5b, 5c are plots of friction test results from hardened bearing steel in fretting contact with hardened bearing steel, lubricated by: Grease A (FIG. 5a); Grease C (FIG. 5b); and Grease B and Grease D (FIG. 5c).

DETAILED DESCRIPTION

(6) An example of a gear-driven bearing unit according to the invention is shown in FIGS. 1 and 2. A common application of such a gear-driven bearing unit is to rotatably support an array of solar panels on a pedestal. The array of panels is attached to a rotatable part of the bearing unit, while a stationary part of the bearing is mounted to the pedestal. Thus, the array of panels can be rotated from east to west as the sun's relative position in the sky changes, ensuring that the panels remain facing towards the sun for optimum electricity generation.

(7) The unit 100 comprises a housing 110, a drive gear 120 and a driven gear, which is supported by a rolling element bearing. In the depicted example, the rolling element bearing is a double-row angular contact bearing. This type of bearing is particularly advantageous as it is able to take up high axial loads and tilting moments which are exerted on the unit by e.g. a solar panel array. Other suitable types of bearing include single and double-row slewing bearings or 4-point contact ball bearings.

(8) For optimal radial compactness, the driven gear also serves as the outer ring of the bearing. Accordingly, a radially outer side of the outer ring 130 is provided with gear teeth 135, and a radially inner side is provided with first and second angular raceways 131, 132. The bearing further comprises a first inner ring 141 and a second inner ring 142, with corresponding inner raceways for a first row 151 and a second row 152 of rolling elements. The first and second inner rings are joined together by means of a plurality of first screw connections 161. The first screw connections also set the initial bearing preload. Additionally, the first and second bearings rings are attached to the housing 110, by means of a plurality of second screw connections 162.

(9) The drive gear 120 in this example is a worm gear that is mounted tangentially to the toothed bearing ring 130. It is also possible to use a spur gear. The drive gear comprises a gear shaft 122 with an input end 125 that is connectible to a motor, for providing the drive. The gear shaft is supported relative to the housing 110 by a first tapered roller bearing 171 and a second tapered roller bearing 172. The worm gear 120 is made of bearing steel and in this example the teeth of the gear are case carburized. In a particularly preferred embodiment, the gear teeth on the drive gear are made of bearing steel and subjected to a hardening treatment that diffuses nitrogen into the gear surfaces.

(10) The housing 110 defines a gear chamber 127 for the drive gear 120, as well as enclosing the outer ring 130 and the first inner ring 141 at a first axial side. Further, the gear chamber 127 is in open connection with a bearing cavity of the double-row angular contact bearing, via an axial gap between the housing and a side face of the outer ring 130. This means that the same lubricant is used to lubricate the gear contacts and the rolling contacts of the double-row angular contact bearing and of the first 171 and second 172 taper roller bearings. A grease lubricant is used, given that grease acts as a barrier for contaminants and is less prone to leakage. A grease is selected that provides good lubrication under high load and at slow rotational speeds. For example, when the unit is employed for solar tracking purposes, the rotational speed typically varies from 0.001 rpm up to 0.01 rpm. According to the invention, a grease comprising calcium is used. An example of a preferred grease is a synthetic mineral oil-based grease with a complex calcium sulphonate thickener, such as LGWM 2 from SKF. The calcium in the grease reacts with the steel contacting surfaces, to form a protective reaction layer that protects the contacting surfaces from wear and provides low friction over a long period. This will be discussed in more detail further on.

(11) The load to be rotated (e.g. solar panel array) is attached at a second axial side of the unit, via a plurality of mounting holes 136 in the toothed outer ring. To allow for rotation of the outer ring 130, a first radial gap exists between the second inner ring 142 and the outer ring 130 and a second radial gap exists between the outer ring and the housing 110. The unit is therefore provided with a first seal 180 and a second seal 190, for enclosing the first and second gaps respectively and preventing leakage and the ingress of contaminants. In a gear-driven bearing unit according to the invention, the first and second seals comprise two radial sealing lips that bear against a radially oriented counterface. The present inventors have found that this considerably extends the relubrication interval of the unit.

(12) The first seal 180 may be made from an elastomeric polyurethane such as H-ECOPUR and has a first lip 181 and a second lip 182 which bear against a first radially oriented counterface on a cylindrical inner surface 137 of the toothed outer ring 130. The first lip is predominantly responsible for retaining lubricant within the unit. The second lip 182 excludes contaminants and further shields the first lip 181 from contact with environmental factors, such as sunlight, that would cause aging of the first lip. This helps ensure that the first lip retains optimal sealing function. As shown in FIG. 1, the first lip 181 is preferably angled towards a radial centreline 105 of the unit, while the second lip 182 is angled away from the radial centreline. Thus, internal pressure within the unit acting on the first lip 181 urges this lip towards the counterface 137, which further helps ensure that the first lip 181 retains optimal sealing function. Furthermore, external pressure from outside the unit acting on the second lip 182 will urge this lip towards the counterface 173, thereby enhancing the second lip's ability to exclude contaminants.

(13) The first seal further has a mounting part 185, which in the depicted example is simply a ring of rectangular cross-section. To receive the mounting part 185, the second inner ring 142 is provided with a groove 145 in its radially outer surface. Alternatively, the groove may be provided in the cylindrical inner surface 137 and the first radially oriented counterface may be provided on a cylindrical outer surface of the second inner ring 142. Both the inner ring and the outer ring are made of hardened bearing steel, which is extremely suitable as a counterface for elastomeric lip seals.

(14) The second seal 190 may also be made from an elastomeric polyurethane material and comprises a first radial lip 191, a second radial lip 192 and a mounting part 195. The first and second lips of the second seal 190 bear against a second radially oriented counterface 138, and are angled in the same way as described for the first seal 180. The mounting part 195 of the second seal differs from that of the first seal and has U-shaped cross-section. Thus, the mounting part 195 is adapted to surround a protrusion in a radial surface. In the depicted example, the housing 110 comprises a finger 112 for engagement with the U-shaped cross-section of the mounting part 195. The radial counterface for the second seal is then provided on a cylindrical outer surface 138 of the outer ring 130, which in use of the unit is in sliding contact with the first and second radial lips 191, 192.

(15) The second counterface could also be provided on the housing, but is preferably provided on the toothed outer ring. As mentioned, the outer ring 130 is made of a bearing-grade steel. The housing is cast from a material such as ductile cast iron, which is less suitable as a counterface. Also, the provision of a cylindrical surface on the housing would require relatively more material than the depicted finger 112.

(16) Thus, it is an advantage of the finger that the housing, at the location of the second seal 190, can be relatively thin. In addition to saving material, this also provides a little more mounting space, which can be important depending on the design of the component that is attached to the outer ring via the mounting holes 136. As will be understood, however, the housing can easily be provided with a groove, such as the groove 145 shown in FIG. 1. The second seal may thus have an attachment part with a rectangular shaped cross-section or a U-shaped cross-section. Similarly, the first seal may have an attachment part with a U-shaped cross-section. With reference to FIG. 1, the groove 145 in the second inner ring 142 would then receive the axially inner part of the U-shaped cross-section.

(17) The gear-driven bearing unit according to the invention is adapted for supporting a compressive load or a suspended load. Especially in the case of a suspended load, when the unit is mounted with a side plate of the housing facing upwards, leakage of lubricant must be prevented. The present inventors have found that by means of first and second seals with two radial lips, leakage is minimized and the relubrication interval for the unit can be extended by many years. In effect, the unit is greased for life.

(18) In addition to being greased for life, maintenance-free operation requires that the grease used is able to provide satisfactory lubrication that prevents wear of the contacting surfaces for the service life of the unit. This is achieved by means of the grease selection in combination with the materials used in the contacting surfaces of the unit.

(19) The unit has two types of contacting surfaces: the sliding contacts between the gear teeth on the worm drive 120 and the teeth 135 on the outer ring 130; and the rolling contacts between the rolling elements and the bearing raceways.

(20) The rolling elements 151, 152 and bearing raceways are made of hardened bearing steel. As mentioned, the raceways 131, 132 on the outer member are induction hardened. The inner raceways may also be produced via induction hardening or the inner rings may be through-hardened. According to the invention, the teeth 35 on the outer ring 130 are made of the same bearing steel as the outer raceways 131, 132, but the surface of the teeth is nitrided or undergoes ferritic nitrocarburizing or other treatment which diffuses nitrogen into the surface. The calcium in the grease used according to the invention is thought to initiate a reaction with the steel surfaces, forming a reaction layer that protects the surfaces from wear. When the surface of the teeth on the outer ring has been diffused with nitrogen, the formed reaction layer not only provides wear protection, but also particularly low friction.

(21) This may be seen from the plot in FIG. 3. Using a Cameron-Plint test rig for measuring sliding friction, a hardened steel ball was linearly reciprocated on a plate made from three different materials under the following conditions:

(22) Maximum Herzian contact pressure: 500 MPa

(23) Sliding amplitude: 4 mm;

(24) Frequency: 0.034 Hz;

(25) Lubricant: SKF grease LGWM2, which will be designated as Grease A.

(26) In a first test, the plate was made of nitrided bearing steel according to the invention; in a second test, the plate was made of cast iron; and in a third test, the plate was made of mild steel. The plots of the friction results from the first, second and third tests are respectively shown by lines 31, 32 and 33 in FIG. 3. As may be seen, the test using the plate made of nitrided bearing steel produced the lowest friction. Consequently, the drive gear in a bearing unit according to the invention may also advantageously be made of bearing steel that is subjected to a treatment that diffuses nitrogen into the surface.

(27) A comparison test was conducted using the combination: hardened steel-nitrided steel, under the same conditions described above, but using a different grease, Grease B, which is a grease employed in a commercially available worm drive unit. Grease B does not contain calcium.

(28) In FIG. 4, the results from the first test (using Grease A) are plotted together with the results from the comparison test. As may be seen, Grease A provides a lower and highly stable friction, while Grease B exhibits a higher friction level which increased after approximately 3000 cycles. Furthermore, capacitance was measured during the first test and during the comparison test, using LubCheck equipment from SKF. The capacitance measurement showed that Grease A formed a reaction layer that remained in tact for the duration of the test. Grease B was not able to form a reaction layer and, although the friction was reasonably low, a visible wear track could be observed on the nitrided plate after the test ended.

(29) Grease A and other calcium containing greases also form a good reaction layer on hardened bearing steel. With regard to the rolling contact surfaces of the bearing unit 100, fretting is thought to be the main failure mechanism. The reaction layer also provides wear protection and exhibits low friction under fretting conditions.

(30) The Cameron Plint test rig was used to perform fretting friction tests. A hardened steel ball was linearly reciprocated on a plate made from hardened bearing steel under the following conditions:

(31) Maximum Herzian contact pressure: 6 MPa

(32) Sliding amplitude: 100 microns;

(33) Frequency: 20 Hz;

(34) The following greases were used in the fretting friction tests: Grease A Grease B Grease C: SKF grease LGHB2 having a complex calcium sulphonate thickener; Grease D: SKF grease LGAF 3E, which is a lithium based grease comprising calcium hydroxide additives.

(35) The fretting friction results for Grease A are shown in the plot of FIG. 5a; the results for Grease C are shown in the plot of FIG. 5b and the results for Grease B and Grease D are shown in FIG. 5c.

(36) Grease A, Grease C, and Grease D all exhibit a stable low-friction performance. Furthermore, capacitance measurements using LubCheck also show that each of these greases forms a reaction layer that remains in tact throughout the friction test. Grease B does not form a reaction layer on the hardened bearing steel.

(37) Consequently, the bearing surfaces and the gear surfaces in a unit according to the invention can be protected from wear for the service life of the unit, due to the combination of the grease and materials used. Furthermore, the improved sealing ensures that sufficient grease is always present in the unit, such that the reaction layer can be continuously formed. A gear gear-driven bearing unit according to the invention is thus maintenance free.

(38) A number of aspects/embodiments of the invention have been described. It is to be understood that each aspect/embodiment may be combined with any other aspect/embodiment. The invention may thus be varied within the scope of the accompanying patent claims.