~038433 This invention is direct:ed to an improved bearing structure and to a large rotary machine such as a grinding mill embodying the structure.
In the provision of large machines particularly in grinding mills there is a trand towards ever increasing sizes of mill drum. Thus, the provision of mill drums of very great weight produces an ever increasing problem in maintain-ing satisfactory support bearings. This is a problem which may not readily be solved, as in the past, by over-design, becausa the economic limit of such a structure has been reached so that optimized performance of the mill support ; bearings is fast becoming of critical importance.
In the past, considerable success has been enjoyed in the u-Qe of dynamically lubricated bearings, wherein the radial clearances of the bearing are such that a dynamic ~; film of lubricant i8 established and maintained between the - sliding surfaces of the bearing, due to the relative motion therebetween.
However, it has been found that the function of such bsarings i8 heavily dependant upon maintaining the radial film between limiting upper and lower values. One such value, which shall not be construed as limiting the scope of the present disclosure, due to the many, varied and complex factors governing this relationship, is a maximum upper limiting value of 10 thousandths of an inch (0~010") and a coxresponding lower limiting value of 2 thousandths (0.002"). Thus, for a given lubricant under given conditions of viscosity, temperature and loa~, satisfactory load transfer between the two relatively sliding bearing surfaces can be maintained with lubricant film thickness established by the relative sliding motion and extending for values in abo~t the range of 2 thou.
(thousandth of an inch) to lO thou.. One prior arrangement provided multiple slipper bearings having provision for load sharing between the slippers.
It has b~en found, however, that owing to the ; large loads and dimensions involved there i6 a tendency to generate deformations in the mill and bearing structure such that with prior art arrangements the limiting radial clear-ance cannot be maintained, and lubricant film break down occurs. This problem is further aggravated by the fact that increases in mill sizes leads to mill support structures that no longer maintain effectively their design shape. Thus it has been discovered under the loads imposed by self weight, together with the weight of the charge, mill bearing components go out-of-round by flexing to a flattened condition. -In the case of an autogenous mill, for instance, having a drum in the order of thirty feet diameter and of an economic design the sag of the drum to a flattened oval condition, by flexing, can cause the drum diameter to go out of round by as much as 25 thou. In the case where slipper bearings are used in conjunction with a contact tire mounted on the drum, or forming a portion of the periphery of the drum, the effect of such deformation, added to deformation of the support bearing slipper may readily deform the related bearing surfaces to the extent that limiting film clearance cannot be maintained and hence dynamic film lubrication can no longer be maintained.
Similarly in the case of trunnions attached to the drum end shells, the load deflections can similarly lead to structure distortion sufficient to cause brsak-down of dy-namic film lubrication.
The present invention provides a bearing slipper or shoe wherein the bearing support structure in combination with
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~038433 the shoe provides a predetermined pattern of deflection in the working load range such that the dynamic deflection of the mating drum structure is accommodated to, so as to maintain the radial film clearance for a dynamic film of lubricant within predetermined values.
In providing compensation for structure deformations it will be understood that such compensation probably includes profiling the initial unloaded surface of the support bearing to a non-circular profile.
~hus there is provided a bearing construction for use as a support bearing of a large, heavy rotary drum ~tructure, the drum structure having an annular support portion . . .
~ located coaxially with the main axis of the drum, to receive - at least one arcuate bearing surface means in supporting relation therewith, the drum support portion including a radially outer bearing surfacs, being subject to significant deformation out-of-round under load conditions, the bearing constrcution including a support structure having an attachment portion for mounting the bearing in supporting relation with the drum, with support pad means attached in mounted relation . as the support structure, the pad means including an arcuate member having a bearing surface to receive a lubricant film for supporting the drum bearing surface thereon, the pad support means having an arcuate stiffness related to the . structural stiffness of the drum bearing surface whereby in use in a loaded condition the arcuate deformation of the bearing pad beneath the drum bearing substantially matches the effective profile of curvature of the pad bearing surface to the profile of curvature of the contacting portion of the drum bearing so that over a major portion of the pad bearing area the radial clearance between the two surfaces satisfies predetermined lubricant dynamic film thickness .; ~. : :
' ~ V38433 criteria, to maintain a lubricant dynamic film in effective load transfer relation between the two surfaces in bearing.
` Certain embodiments of the invention are described, reference being made to the accompanying drawings, whsrein;
- Figure 1 is a side elevation in diametrical section showing an autogenouC mill incorporating journal bearings in accordance with the present invention;
Figure 2 is a section of a bearing taken at 2-2 of ; Fig. l;
Figure 3 iæ a view similar to Fig. 1 of a ball mill incorporateing slipper bearings in accordance with the present invention, and Figure 4 is a section of slipper bearings at 4-4 of Fig, 3.
In Figs. 1 and 2, the mill 10 has a drum or shell 11, with end heads 12, 14 to which are attached support trunnions 16, 18. Access to the inside of the mill is provided through ;~ the centre of the hollow trunnions 16, 18, through which ' material is admitted and removed. No drive system components ; 20 are shown.
' The supporting bearings 20, 22 in the form of half~
; shells are mounted on stools 24, 26~
~he bearing 20 comprises a semi-cylindrical shell 30 -having a radially inner surface 32 against which the radially outer surface 34 of the trunnions 16 is supported. The trunnion 16 is illustrated in a theoretical circular condition in order to bring out the "oval" shape of the bearing surface 32, to which the trunnion 16 is conformed in usual operating conditions. Whilst not illustrated, it will be understood that a aynamic film of lubricant should at all times of operation be interposed between the surfaces 32, 34. This lubricant, usually an oil, is forced between ., ,~ .
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~0389~33 the bearing surfaceæ 32, 34 by way of a jacking pump (not shown), as is well known in the art, prior to startup of the mill. This has the effect of floating the drum from off its bearings, so that metal to metal contact at the bearings is terminated prior to the commencement of rotation.
In accordance with the invention, the range of deformation of the trunnions 16, 18, from a no-load condition of the mill to a mill fully-loaded condition is determined, from which may ~e obtained the film pressure necessary to support the trunnion. The effective area and arcuate profile of the bearing shell 30 is then arranged such that in the mill operating weight range the generation of film pressure of the determined value will produce deformation of the shell 30 to bring the arcuate profiles into correspondence such that the radial clearance between the bearing surfaces 32 and 34 occupied by the oil film lies in a predetermined range.
~he problem resides generally in maintaining the radial clear-ance substantially that of a predetermined profile and for ~ a predetermined minimum area of the bearing surface 32, - 20 such that a dynamic film of lubricant will be maintained.
The thickness of this film generally lies between 2 thou. and 10 thou. (thousandths of an inch) but the actual limiting values will be functions of rotational speed, acceptable economic design and the characteristics of the lubricant.
` It will be understood that in addition to out-of-round daformation, the trunnions also can deform longitudinally, as well as tilt in response to loads of sub-weight and the weight of the working charge. The general capability of axial conformation is illustrated diagrammatically in the stools 24, 26 where deformable web sections 34, 36 are shown, which permit the respective bearing surfa~es 32 of the support bearings to level themselves transversely in accordance . " . . , :. : . :- : .
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~U38433 with longitudinal inclination of the contact surface of the trunnions from a horizontal position.
In the case of Fig. 3 arrangement, a trunnion-less ball mill 40 is mounted for rotation, having a pair of axially spaced annular tires of which only one, tire 42 is shown supported on slipper bearing 46, 48, 49. Each bearing (Fig. 4) has a support pedestal 50 with a spherical pivot piece 52 mounted thereon. A socket portion 54 of a bearing pad 58, 59, 60 is mounted on the pivot 52. It will be noted ~, .
that the pads 58, 59, 60 are shown as being non-symmetrical, and may differ significantly from each other, due to inherently different loading produced by the direction of rotation of the drum and the location of the centre of gravity of the loaded drum load closer to one pad than the other. It should be further noted that bearing pads 58, 59, 60 may be preset in a radial sense to selectively control the distribution of the load between the pads, when the mill is loaded thereon.
This is illustrated schematically by the provision of removable pacXing pieces 53, which permit accurate shimming '`r"' 20 of the bearing to a predetermined radial relation relative to the mill axis to achieve a desired pre-load relation.
While the spherical bearing provided by pivot 52 and socket 54 permits alignment of the surface 64 of the respective pad with the surface 66 of the tire 42, this does not necessarily of itself maintain the desired film geometry at the bearing surfaces. Accordingly, the profile of the .
pad, both in an arcuate sense and in a transverse or axial sense is usually adjusted initially to compensate for a "flattening" effect in the same sense, in both directions tending to cause the film gap to open up at the edges of the bearing contact surfaces, thereby diminishing the effective pad area by which the lubricant film dynamic load - . . , , , . ~ . . ~ ., . -. :
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~ .03~433 is transferred from the tire surface 66 to the support structure 50.
It has been observed in practice that in a mill of about 30 feet diameter the diametrical vertical contraction is about 25 thou., with an approximately corresponding increase in the horizontal diametrical dimension. ~his ovality of the drum 40, as seen from the ends of the mill is substantially reflected in the profile of the tire 420 However, the local profile of the tire 42 in sliding over the bearing surfaces 64 of the two illustrated slippers is not that of a modified but constant arc, as local flexures occur due to the effect of the local loads applied to the tire 42 by the pad surfaces 64, by transferrence of the force through the pressure of *he lubricant film interposed therebetween.
It will be understood that in an unloaded condition the self weight of the mill is sufficient to produce sign-ificant deformation of trunnion or tier bearing surfaces from their initially machined circular profile. The variable factors introduced by addition of the working load to the ~ 20 mill drum, the fact that the bad can vary and that the ; effective position of the load within the drum also is .,;
variable maXe it generally impossible to profile the mating bearing surfaces under all conditions to ensure precise -~
desired profile of the lubricant filmO However, with due -~
-` consideration, the profiling corrections and stiffness factor applied to the stationary bearing surface of pad 58 or journal 30 can maintain the conformity between the bearing opposed surfaces wi*hin desired limits of film thickness. Typical desired values of film thickness would lie ideally between 30 2 and 6 thou., but an upper limit of up to 10 thou. is normally acceptable. These values are primarily illustrative, .
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