Abstract:
A ball bearing ( 1 ) has an inner race ( 2 ) and an outer race ( 3 ). In order to prevent the rotating parts from being damaged when the bearing assembly fails, the bearing has emergency bearing surfaces ( 14, 15 ) which are concentric to the rotational axis ( 6 ) and of which one is a part of the rotating bearing race and the other is a part of the fixed bearing race. During normal operation, the emergency bearing surfaces ( 14, 15 ) are situated opposite one another with a relatively narrow gap ( 24 ) therebetween. But, in the event of failure, the surfaces ( 14, 15 ) engage and function as emergency bearing surfaces.

Description:
[0001]    This application is a divisional of U.S. application Ser. No. 10/536,473, filed May 25, 2005. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to a ball bearing having an inner race and an outer race. Moreover, the present invention relates to a vacuum pump, preferably a turbomolecular vacuum pump equipped with a ball bearing of this type. 
         [0003]    Ball bearings of the type stated serve the purpose of holding and guiding rotatable machinery components, generally shafts. The outer race—in the instance of inverted bearings also the inner race—is supported by a fixed component (bearing disk, housing or alike). Generally bearings of this type are oil- or grease-lubricated bearings. The present invention may also be applied to grease-free bearings. Equally the present invention is independent of whether the bearings are implemented with or without a cage. 
         [0004]    It is the task of the present invention to implement a ball bearing of the aforementioned kind such that in the event of a failure of the bearing and guidance of the rotating component damage to, respectively within the machine is avoided. 
       SUMMARY OF THE INVENTION 
       [0005]    This task is solved by the present invention through the characterising features and measures of the patent claims. 
         [0006]    In that the gap between surfaces which oppose each other is relatively small, these surfaces assume in the instance of uncontrolled movements of the rotating unit the function of emergency bearing surfaces. The rotating unit is guided in a single emergency rundown to standstill without the occurrence of a rotor crash. The friction produced during an emergency rundown is so great that the installed drive power will no longer suffice. The converter of the drive unit switches to failure so that standstill is attained rapidly. 
         [0007]    Still further advantages of the present invention will be appreciated to those of ordinary skill in the art upon reading and understanding the following detailed description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating the preferred embodiments and are not to be construed as limiting the invention. 
           [0009]      FIGS. 1 to 4  depict cross sections of ball bearings with differently designed emergency bearing surfaces and 
           [0010]      FIG. 5  depicts a molecular drag vacuum pump equipped with emergency bearing surfaces in accordance with any one of  FIGS. 1-4 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0011]    The bearings  1  depicted in the drawing  FIGS. 1 to 4  exhibit each an inner bearing race  2 , an outer bearing race  3 , balls  4  and a cage  5 . The axis of the bearing  1  is in each case designated as  6 . In the axial direction (in the drawing  FIGS. 1 to 4  at the top in each instance) the inner chamber  7  of the ball bearing  1  is substantially sealed by a bearing cover  8  and specifically employing a snap ring  10  which is clamped within an inner groove  11  in the outer bearing race  3 . Commonly bearing covers of this type are provided to both sides of the balls  4 . 
         [0012]    In order to form the emergency bearing surfaces  14 ,  15  in accordance with the present invention one or both bearing races  2 ,  3  are equipped with ring-shaped projections which—when arranged on the side opposing the bearing cover  8  simultaneously provide the function of a second bearing cover  8 . In the solution in accordance with  FIG. 1  the outer bearing race  3  is provided on its side opposing the bearing cover  8  with a projection  16  extending in the direction of the inner race  2 . The inner surface of said outer bearing race  3  forms with reference to the axis  6  the cylindrical emergency bearing surface  14 . The section of the outer surface of the inner race  2  opposing said surface  14  is the second emergency bearing surface  15 . 
         [0013]    In the solution in accordance with  FIG. 2 , the inner race  2  is equipped with a projection  17  extending radially towards the outside. The outer surface of the inner race  2  and a part of the inner surface of outer race  3  also form cylindrical emergency bearing surfaces  14 ,  15 . 
         [0014]    In the solutions in accordance with  FIGS. 3 and 4  the inner bearing race  2  and the outer bearing race  3  are equipped with projections  18 ,  19  respectively  21 ,  22 . The emergency bearing surfaces  14 ,  15  opposing each other exhibit a stepped cross-section ( FIG. 3 ) respectively form with the axis  6  the angle α. In this manner emergency bearing surfaces are created which not only become effective in the instance of a failure of the radial guidance of the rotating system by the bearings but also in the instance of an axial failure. 
         [0015]    The size of the gap  24  between the emergency bearing surfaces  14 ,  15  should be as small as possible. However, the size of said the gap must not fall below the permissible bearing tolerances. The fact that the bearing tolerances are frequently different in the radial and the axial direction needs to be taken into account when selecting the gap size. 
         [0016]      FIG. 5  depicts as an example for a molecular drag vacuum pump a turbomolecular pump  25  the stator of which is designated as  26  and the rotor of which is designated as  27 . Said pump is designed by way of a compound pump and is equipped with a turbomolecular pumping stage  28  equipped with blades and a molecular pumping stage  29  equipped with a thread. The rotor  27  is partly of a bell-shaped design. Within, respectively slightly below the space  31  encompassed by the bell, the rotor is supported rotatably through the shaft  34  in the bearings  35  and  36 . Moreover, there is accommodated within the space  31  the electric drive motor, its stator pack which is designated as  37  and the rotor pack which is designated as  38 . The bearings  35 ,  36  and the rotor stator  37  are supported by a sleeve-like carrier  39 . 
         [0017]    For the purpose of supplying the bearings  35  and  36  with a lubricant, a vessel  41  filled with oil  40  is affixed underneath the turbomolecular pump  25 . The drive shaft  34 , the lower end of which is immersed in the oil exhibits an inner coaxial bore  42  which owing to the conically expanding bottom section  43  effects pumping of the lubricating oil towards the top. Through cross bores  44  the oil first arrives at the upper bearing  35  and there flows, due to the effect of gravity, through the bottom bearing  36  back into the oil vessel. 
         [0018]    Through the forevacuum port  45  and the line  46  the turbomolecular pump  25  is connected to the forevacuum pumping facility  47 . Since there exists between the motor/bearing chamber  31  and the forevacuum port  45  a connection, there also prevails in space  31  the necessary forevacuum pressure needed to operate the turbomolecular pump. In order to prevent corrosive gases being pumped by the turbomolecular pump from entering into the bearing chamber  31 , a purge gas facility is provided which initially comprises the gas admission pipe  48  opening out into the bearing chamber. For the purpose of admitting the purge gas in a controlled manner said gas inlet pipe  48  exhibits a valve  50 . The purge gas (N 2  for example) entering into the motor/bearing chamber  31  flows through the motor as well as the upper bearing  35  and passes outside the bearing carrier  39  to the discharge port  45 . Thus the entry of corrosive gases, which are being pumped by the turbomolecular pump  25 , into the motor/bearing chamber  31  is prevented. 
         [0019]    Within the scope of the present invention one bearing or both bearings  35 ,  36  has/have been designed (not depicted in detail) as depicted in one of the  FIGS. 1 to 4 . An advantage of this measure is that in the instance of a failing bearing, the active pumping surfaces (blades of the rotor/stator thread) are not damaged. The gap  24  between the emergency bearing surfaces  14 ,  15  defines in the instance of a failed bearing the maximum deflection of the rotor  27  from its nominal position. Correspondingly narrow also the distances between the active pumping surfaces can be selected. The smaller these distances, the better the properties of the pump. Moreover, the fact, that between the bearing races  2 ,  3  at least for bearing  35  there exists a narrow relatively long gap  24 , offers the advantage of a considerable reduction in the rate of the purge gas flowing through the bearing. Finally, the projections at the bearing races  2 ,  3  permit larger contact surfaces which effect an improvement in the dissipation of heat from the bearing. 
         [0020]    The gap  24  needs to be selected corresponding to the bearing tolerances. In the instance of pumps of the described kind, the gap width is expediently less than 0.1 preferably less than 0.05 mm. The size of the emergency bearing surfaces is defined through the axial extension of the gap. Said extension should not drop below 1.5 mm, in the case of oblique or stepped emergency bearing surfaces correspondingly larger. 
         [0021]    It is of importance that in the instance of a failed bearing the friction produced by the emergency bearing surfaces  14 ,  15  is high so that the drive for the rotating system can switch to failure. The friction characteristic of the emergency bearing surfaces  14 ,  15  depends on the material (expediently hardened rolling bearing steel). By coating one or both emergency bearing surfaces (with MOS 2 , Teflon, for example) it is not only possible to increase the amount of friction but also reduce the tendency of seizing for the given pair of materials. 
         [0022]    The invention has been described with reference to the preferred embodiments. Modifications and alterations may occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be constructed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof. 
         [0023]    Having thus described the preferred embodiments, the invention is now claimed to be: