Abstract:
The present invention relates to a shaft earth and/or sealing assembly for use with housed rotating equipment such as electric motors to earth the shaft and also provide an IP69K-rated seal that is flame and spark resistant.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a shaft earth and/or sealing assembly for use with rotating equipment such as electric motors. 
       BACKGROUND OF THE INVENTION 
       [0002]    Various shaft sealing devices have been utilized to try to protect the integrity of a bearing environment. However, seals that are often used are known to quickly wear to failure and are also known to permit excessive amounts of moisture and other contaminants to immigrate into the oil reservoir of the operating equipment. The problem of seal wear and damage, with respect to electrical motors using variable frequency drives (VFDs) for example, is exacerbated because of the very nature of the control of electricity connected to variable frequency drive (VFD) controlled motors. 
         [0003]    VFDs regulate the speed of a motor by converting sinusoidal line alternating current (AC) voltage to direct current (DC) voltage, then back to a pulse width modulated (PWM) AC voltage of variable frequency. The switching frequency of these pulses ranges from 1 kHz up to 20 kHz and is referred to as the “carrier frequency.” The ratio of change in voltage to the change in time (DELTA V/DELTA T) creates what has been described as a parasitic capacitance between the motor stator and the rotor, which induces a voltage on the rotor shaft. If the voltage induced on the shaft, referred to as “common mode voltage” or “shaft voltage,” builds up to a sufficient level, it can discharge to ground through the bearings. Current that finds its way to ground through the motor bearings in this manner is called “bearing current”. 
         [0004]    There are many causes of bearing current including voltage pulse overshoot in the VFD, non-symmetry of the motor&#39;s magnetic circuit, supply imbalances, and transient conditions, among other causes. Any of these conditions can occur independently or simultaneously to create bearing currents in the motor shaft. 
         [0005]    Shaft voltage accumulates on the rotor until it exceeds the dielectric capacity of the motor bearing lubricant, then the voltage discharges in a short pulse to ground through the bearing. After discharge, the voltage again accumulates on the shaft and the cycle repeats itself. This random and frequent discharging has an electric discharge machining (EDM) effect, causing pitting of the bearing&#39;s rolling elements and raceways. Initially, these discharges create a “frosted” or “sandblasted” effect. Over time, this deterioration causes a groove pattern in the bearing race called “fluting,” which is an indication that the bearing has sustained severe damage. Eventually, the deterioration will lead to complete bearing failure. 
         [0006]    In addition to the need for grounding an electric motor shaft, there is a general need particularly in the mining sector for electric motors to be IP69K-rated and also flame and spark proof. To achieve this rating, the motor must be dust tight and capable of withstanding high-pressure and steam cleaning. 
       SUMMARY OF THE INVENTION 
       [0007]    Therefore in one aspect, the present invention provides an earth assembly for an electric motor having a rotatable drive shaft, a housing and a bearing journalled about said shaft at at least one end thereof, said earth assembly characterised by:
   a bearing cover associated with said bearing and mounted to an end of said motor housing, said bearing cover accommodating a slinger in contact with said shaft, the slinger being rotatable together with said shaft;   a slinger cover disposed on an outer end of said bearing cover, the slinger cover defining a chamber between the slinger cover and the slinger, said chamber including a portion in which an electrically conductive material is accommodated to enable contact between said electrically conductive material and a surface of the slinger.   
 
         [0010]    In an embodiment, said electrically conductive material is a sacrificial portion of an earth brush that is spring biased against said slinger surface. 
         [0011]    In an embodiment, said slinger cover includes a socket extending from an exterior of said slinger cover to inside said chamber, said earth brush being shaped for insertion into said socket and said socket being positioned such that when the earth brush is inserted, the sacrificial portion contacts said slinger surface. 
         [0012]    In an embodiment, said socket includes an internal thread and said earth brush includes a screw head having an external thread configured to engage the internal thread, the sacrificial portion being spaced from the screw head by a compression spring that biases said sacrificial portion towards said surface. 
         [0013]    In an embodiment, the assembly further includes a wear indicator configured to indicate to an operator when the earth brush sacrificial portion is worn to a predetermined level. 
         [0014]    In an embodiment, said electrically conductive material further includes electrically conductive lubricant which fills said chamber. 
         [0015]    In an alternative embodiment, said electrically conductive material is conductive lubricant which fills said chamber. 
         [0016]    In an embodiment, the slinger and slinger cover include axially extending, close tolerance surfaces forming a labyrinth between an exterior of said slinger and bearing covers and inside said chamber. 
         [0017]    In another aspect, the present invention provides an electric motor including an earth assembly as characterised in paras. [007] to [014]. 
         [0018]    In another aspect, the present invention provides a sealing assembly for an electric motor having a rotatable drive shaft, a housing and at least one bearing, said sealing assembly including:
   a bearing cover associated with said at least one bearing and mounted to an end of said motor housing, said bearing cover accommodating a slinger in contact with said shaft, the slinger being rotatable together with said shaft;   a slinger cover disposed on an outer end of said bearing cover, the slinger cover defining a chamber between the slinger cover and the slinger, said slinger cover and slinger including axially extending, close tolerance surfaces forming a labyrinth between an exterior of the slinger and bearing covers and inside said chamber.   
 
         [0021]    In an embodiment, said close tolerance labyrinth surfaces includes an inner surface of the slinger cover adjacent the shaft including an axially extending, annular male portion and two annular female portions on either side thereof. 
         [0022]    In an embodiment, said close tolerance labyrinth surfaces includes an outer surface of the slinger adjacent the shaft including two axially extending male portions and one female portion there between. 
         [0023]    In an embodiment, the male portion of the slinger cover is adapted to extend inside the female portion of the slinger, and the male portions of the slinger extend inside the two female portions of the cover. 
         [0024]    In an embodiment, said chamber includes a portion in which an electrically conductive material is accommodated to enable contact between said electrically conductive material and a surface of the slinger. 
         [0025]    In an embodiment, said electrically conductive material is a sacrificial portion of an earth brush that is spring biased against said slinger surface, the slinger cover including a socket extending from an exterior of said slinger cover to inside said chamber, said earth brush being shaped for insertion into said socket and said socket being positioned such that when the earth brush is inserted, the sacrificial portion contacts one of said slinger male portions. 
         [0026]    In another aspect, the invention provides an electric motor including a sealing assembly as characterised in any one of paras. [016] to [021]. 
         [0027]    In another aspect, the present invention provides an earth and sealing assembly for an electric motor having a rotatable drive shaft, a housing, and a bearing journalled about said shaft at at least one end thereof, said earth assembly characterised by:
   a bearing cover associated with said bearing and mounted to an end of said motor housing, said bearing cover accommodating a slinger in contact with said shaft, the slinger being rotatable together with said shaft;   a slinger cover disposed on an outer end of said bearing cover, the slinger cover defining a chamber between the slinger cover and the slinger, said chamber including a portion in which an electrically conductive material is accommodated to enable contact between said electrically conductive material and a surface of the slinger to thereby ground the shaft; and   wherein said slinger cover and slinger include axially extending, close tolerance surfaces forming a labyrinth between an exterior of said slinger cover and inside said chamber to prevent ingress of fluid or particulates into said chamber.   
 
         [0031]    In another aspect, the invention provides an electric motor including an earth and sealing assembly as characterised in para. [023]. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
         [0032]    The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several implementations of the invention and, together with the description, serve to explain the advantages and principles of the invention. In the drawings: 
           [0033]      FIG. 1  illustrates a front, exploded perspective view of an electric motor and a shaft earth and/or sealing assembly in accordance with an embodiment of the present invention; 
           [0034]      FIG. 2  illustrates a rear, exploded perspective view of the electric motor and shaft earth and/or sealing assembly of  FIG. 1 ; and 
           [0035]      FIG. 3  illustrates an enlarged cross sectional view of a shaft earth and/or sealing assembly according to an embodiment associated with an end of the electric motor, the cross sectional view taken along the motor&#39;s longitudinal axis. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0036]    The following detailed description of the invention refers to the accompanying drawings. Although the description includes exemplary embodiments, other embodiments are possible, and changes may be made to the embodiments described without departing from the spirit and scope of the invention. Wherever possible, the same reference numbers will be used throughout the embodiments and the following description to refer to the same and like parts. 
         [0037]    The present invention relates, in one aspect, to a shaft earth and/or sealing assembly  10  which in an embodiment can be incorporated into an electric motor  12 . The invention relates, in another aspect, to an electric motor  12  incorporating a shaft earth and/or sealing assembly  10 . 
         [0038]    Turning to  FIGS. 1 and 2 , there is shown an electric motor  12  according to an embodiment including a cylindrical outer housing  14  with cooling fins  16 , a rotatable drive shaft  18 , and a shaft earth and/or sealing assembly  10  associated with at least one end of the motor which in  FIGS. 1 and 2  is shown in an exploded form. It is to be understood at the outset that the invention is not intended to be limited to use of only one assembly  10  at one end of the motor  12 . In an embodiment, the electric motor includes a shaft earth and/or sealing assembly  10  associated with bearings  20  at both ends. It is to be further understood that the internal components of the electric motor are considered to be known to those skilled in the art, and so these components are not described in detail herein. 
         [0039]      FIG. 3  illustrates one end of a motor  12  according to an embodiment including the shaft  18 , a roller bearing  20  journalled around the shaft, an end shield  22 , and a roller bearing cap  24 , slinger  26 , and slinger cover  28  which all form part of a shaft earth and/or sealing assembly  10  according to an embodiment. Each of the roller bearing  20 , end shield  22 , roller bearing cap  24 , slinger  26 , and slinger cover  28  include radial centres which are coaxial with the centre of the shaft  18 , but only the slinger  26  rotates therewith as will become apparent. There are various other components which form part of the assembly  10  which are described in more detail below. 
         [0040]    A method of greasing the roller bearing  20  will now be described with reference to  FIG. 3 . The end shield  22  includes an internal cavity  30  which ensures that grease placed into the cavity through capped opening  32  lubricates the roller bearing  20  at all times. The grease path is indicated by arrows  34 , and it can be appreciated that grease also enters the internal cavity  36  of the bearing cap  24  through which it can be discharged via outlet  38 . The bearing cap  24  includes V-ring seals  40  fitted for protection against ingress into the bearing cavity  26  of water, dust, solids, and also excess conductive lubricant which will be described in more detail below. 
         [0041]    Typically, fresh grease will flush old grease from the motor and V-ring seals  40 , preventing dust, water and other contaminants entering the motor. The invention is not intended to be limited to any one means of discharging grease from cavity  36 , for example, one could use an open chamber, a grease exhaust seal cover, a “split rubber” membrane, a grease chute, a grease trap and tray, a pneumatic cylinder discharge pump, or a spring loaded grease valve. 
         [0042]    An advantage afforded by the present invention is that the motor  12  meets the IP-69K rating mentioned in the preamble, as well as being flame and spark proof. Aside from the various O-ring seals that are used, most of which are listed and described below, a mechanism for achieving this rating is the labyrinth created by close tolerance annular surfaces between the slinger  26  and slinger cover  28 . In particular, the inner surface of the slinger cover  28  adjacent the central shaft  18  is shaped such that it includes an axially extending, annular male portion  42  and female portions  44  and  46  on either side thereof. In contrast, the outer surface of the slinger  26  adjacent the central shaft  18  is shaped such that it includes two axially extending, annular male portions  48  and  50  and one female annular portion  52  therebetween. Thus, when the two components are fitted and the respective portions brought together in close tolerance, the male portion  42  of the slinger cover is adapted to extend inside the female portion  52  of the slinger, while the male portions  48  and  50  of the slinger  26  extend inside the two female portions  44  and  46  of the cover. What is thus created is a labyrinth through which water, dust, etc, must travel before it can reach inside a lubricant chamber  54  inside the slinger cover  28 . Of course, the intention is that no water and/or dust particles will be able to travel through this labyrinth into chamber  54 . 
         [0043]    Various seals in addition to those described above are used throughout the system, including: 
         [0044]    Oil seal  56  between the shaft  18  and slinger cover  28 ; 
         [0045]    Shaft seal  58  to provide a first barrier against water and dust ingress; 
         [0046]    O-ring  60  between the slinger  26  and shaft  18 ; 
         [0047]    O-ring  62  between bearing cap  24  and slinger cover  28 ; 
         [0048]    O-rings  64  for bearing cap bolts  66  which fasten the bearing cap  24  to the end shield  22 ; 
         [0049]    O-ring  68  between the end shield  22  and motor housing  14 ; and 
         [0050]    O-ring  70  between bearing cap  24  and end shield  22 . 
         [0051]    Through testing, it has been found that the motor is sealed to an extent that even if an internal explosion occurred (not an uncommon occurrence in mining environments), the motor would still remain substantially intact and so damage to the surroundings would be minimal. 
         [0052]    The assembly  10  is also used to earth or ground the motor shaft  18 . The reader is directed to the enlarged cross sectional view on the upper left of  FIG. 3 . There is shown an earth brush  72  according to an embodiment which is adapted to make contact at all times with a surface of male portion  48  of the slinger  26 . The skilled addressee would realise that the earth brush serves to ground the shaft  18 . In the embodiment shown, the earth brush  72  is biased against the slinger male portion  48  through use of a grub screw  74  and compression spring  76  adapted to provide bias to a brush holder screw  78  at the top of the earth brush  72 . The bias provided by the spring will ensure contact is maintained. The skilled addressee would also realise that the earth brush screw is accessible from outside the motor. Because the slinger  26  is rotating relative to the brush, the brush  72  will become worn and will require replacement at appropriate intervals throughout the life of the motor. 
         [0053]    The slinger cover chamber  54  is adapted to be filled with conductive lubricant via a nipple (not shown). The conductive lubricant provides an electrically conductive path for low voltage currents, to prevent harmful currents from discharging through the bearing to earth. The slinger cover  28  may include a screw or spring loaded lubricant exhaust valve  80  to seal the conductive lubricant exhaust port  82 . 
         [0054]    It is to be understood that the present invention is not intended to be limited to the use of an earth brush to ground the shaft. For example, the earth brush and conductive lubricant used in combination in the embodiment shown could equally well be used independent of each other. For example, a conductive lubricant could be used with the brush. In an alternate embodiment, the spring and grub screw configuration described above could form part of a one-piece elongate insert (not shown) including a housing adapted to hide and protect components such as springs and/or associated components. There are a number of earth brushes available in the market, including conventional friction type brushes to more evolved microfiber brushes, all of which could be implemented within the scope of the present invention. 
         [0055]    In an embodiment, the apparatus  10  includes a wear indicator (not shown) used to warn an operator when the earth brush is almost completely worn. Such an indicator could be in the form of a microswitch (not shown) triggered when the brush is worn to a predetermined level. It is to be understood that the apparatus  10  may include other known components and mechanisms to improve its operation and/or performance. For example, in the embodiment shown, the bearing housing is insulated  84  to isolate the bearing  20  from shaft currents. 
         [0056]    Further advantages and improvements may very well be made to the present invention without deviating from its scope. Although the invention has been shown and described in what is conceived to be the most practical and preferred embodiment, it is recognized that departures may be made therefrom within the scope and spirit of the invention, which is not to be limited to the details disclosed herein but is to be accorded the full scope of the claims so as to embrace any and all equivalent devices and apparatus. 
         [0057]    In the summary of the invention and the claims, except where the context requires otherwise due to express language or necessary implication, the word “comprising” is used in the sense of “including”, i.e. the features specified may be associated with further features in various embodiments of the invention.