Low friction backup system for magnetic bearings

An assembly in which a rotating shaft is supported by a magnetic bearing has a back-up bearing to support the shaft in the event of failure of the magnetic bearing, the back-up bearing comprising one or more coaxial bearing parts each including cylindrical rolling elements supported on a single raceway, or each part comprising a ball race with radially inner and outer raceways. If a radially outer raceway is provided it is secured to an anchorage, such as a housing. A layer of dry bearing material is secured to rotate with each raceway to be rotated by the shaft, the dry bearing material having a surface opposite to the shaft. After any failure of the magnetic bearing the shaft is contacted by the layer of dry bearing material, and the rolling elements are brought up to speed at a slower rate than otherwise would be the case, because of slipping of the shaft relative to the dry bearing material. By the time the back-up bearing is rotating at the same speed as the shaft, the back-up bearing is rotating at a speed not significantly greater than the maximum design speed for the rolling elements.

This invention relates to an assembly in which a rotating shaft is to be 
supported by a magnetic bearing, but is to be supported by a backup 
bearing in the event of failure of the magnetic bearing. A magnetic 
bearing may be provided for a shaft having high rotational energy, and it 
is especially important in such a case to provide a satisfactory backup 
bearing for the shaft. 
The backup bearing is required to be of a form to support the shaft in a 
satisfactory manner, and to enable the shaft to be brought to rest, after 
any failure in the operation of the associated magnetic bearing. While the 
shaft is being brought to rest, it is required that the shaft, any part of 
the assembly rotating with the shaft, and the backup bearing, do not 
become damaged, or worn. 
The shaft is brought to rest by the rotational energy of the shaft being 
converted into heat by the shaft contacting the backup bearing, and then 
dissipating the heat, and/or by the rotational energy of the shaft's being 
absorbed by the system associated with the shaft doing work. If a 
substantial amount of the rotational energy of the shaft is to be 
converted into heat by the shaft's contacting the backup bearing, there is 
to be a high coefficient of friction associated with the backup bearing. 
Alternatively, if a substantial amount of the rotational energy of the 
shaft is to be absorbed by the system associated with the shaft, there is 
to be a low coefficient of friction associated with the backup bearing. 
If the shaft is to be brought to rest in a short period, the rate of 
conversion into heat of the rotational energy of the shaft by the shaft 
contacting the backup bearing, and/or the rate of absorption of the 
rotational energy by the system associated with the shaft, is required to 
be high. 
It is known to have a backup bearing having at least one constituent part 
with rolling elements, for example, each constituent part of the backup 
bearing comprising cylinders supported on a raceway; or comprising a ball 
race with radially inner and outer raceways; and possibly without the 
provision of any form of lubrication. The sole raceway, or the radially 
inner of two raceways, of each constituent part of the backup bearing is 
to rotate with the shaft when the magnetic bearing fails. If a radially 
outer raceway is provided it is secured to a suitable anchorage. 
Usually the sole, or the radially inner, raceway is provided with a sleeve 
of brass, or hard bronze, to facilitate the rotation thereof when 
contacted by the rotating shaft. Any such construction for a backup 
bearing having rolling elements has a low coefficient of friction 
associated therewith, say of the order of 0.001. However, immediately 
after any failure of the magnetic bearing the shaft may be rotating at a 
speed greater than the maximum design speed limit of the rolling elements 
of such a back-up bearing. Thus, after any failure of the magnetic bearing 
it is common for a backup bearing with rolling elements to fail. However, 
it is sometimes desirable to employ a backup bearing having rolling 
elements, for example, when the clearance between the anchorage for the 
backup bearing and the shaft is small. 
Plain bearings are known, each of which includes a layer of a dry bearing 
material. Such bearings usually fail by seizure of the otherwise 
relatively rotating, constituent parts of the bearing, due to expansion of 
the bearing material because of high temperatures being generated within 
the bearing. Such high temperatures are generated because the rate of 
conversion of rotational energy into heat by the bearing is greater than 
the rate of dissipation of heat therefrom. It will be understood that the 
rate of dissipation of heat varies with the temperature of the bearing, 
and the bearing fails, with the temperature at the value at which the 
constituent parts of the bearing seize by being mechanically welded 
together, when the rate of conversion of rotational energy into heat is 
greater than the rate of dissipation of the heat. 
For convenience, hereinafter in this specification, and the accompanying 
claims, the term dry bearing material is used to refer to a known 
composition of a dry layer suitable to be included in a plain bearing. A 
surface provided by such a material has a low coefficient of friction 
associated therewith. 
It has been proposed in our co-pending UK patent application No. 8917875.0 
to provide, with a magnetic bearing, a backup bearing comprising a layer 
of dry bearing material co-operating with a layer of wear-resistant 
material, one such layer rotating with the shaft, and the other layer 
being on a stationary part of the assembly, and the materials of the 
layers are selected so that the rate of dissipation of heat generated 
within the backup bearing is greater than the rate the heat is generated, 
under normally encountered operating conditions for the assembly, and the 
shaft does not become seized with the backup bearing. 
It is an object of the present invention to provide a novel and 
advantageous arrangement for a back-up bearing having rolling elements for 
an assembly in which a rotating shaft is to be supported by a magnetic 
bearing, the backup bearing having a construction such that it does not 
tend to be damaged, or worn, when the shaft is being brought to rest in 
the event of failure of the magnetic bearing, and when the shaft initially 
is rotating at a speed greater than the maximum design speed limit for the 
rolling elements of the backup bearing. 
In accordance with the present invention an assembly in which a rotating 
shaft is to be supported by a magnetic bearing includes a backup bearing 
fixed to an anchorage and having rolling elements supported on at least 
one raceway to be rotated by the shaft; and a layer of a dry bearing 
material, secured for rotation with said at least one raceway, and 
providing a surface opposite to the shaft, in the operation of the 
assembly, the clearance between the opposing surfaces of the dry bearing 
material and the shaft is arranged to be less than the clearance between 
the rotating shaft and the magnetic bearing. 
The cylindrical part of the rotating shaft opposite to, and to co-operate 
with, the layer of dry bearing material of the backup bearing also may be 
of dry bearing material and/or it may be of a wear resistant material. For 
convenience, in this specification and the accompanying claims, reference 
only is made to the layer of dry bearing material supported on the raceway 
being contacted by the rotating shaft unless otherwise appropriate, any 
layer on the shaft being considered to be part of the shaft. 
Thus, the backup bearing includes at least part of a plain bearing of a 
conventional form, with the, or one, constituent layer of the plain 
bearing being secured to said at least one raceway for the rolling 
elements to be rotated by the shaft. Because of the presence of at least 
part of a plain bearing, when the backup bearing is required to be in 
operation, the rolling elements are brought up to speed at a slower rate 
than otherwise would be the case, because of slipping of the shaft within 
the dry bearing material. It is arranged, by selecting the appropriate 
coefficient of friction between the shaft and the layer of dry bearing 
material, that, by the time the backup bearing is rotating at the same 
speed as the shaft, inevitably the shaft is rotating at a speed at most 
not significantly greater than the maximum design speed for the rolling 
elements. 
Further, the dry bearing material of the backup bearing is selected such 
that there is a desired coefficient of friction with the shaft, taking 
into account the rate at which heat can be dissipated from the assembly 
before the seizure of the shaft with the backup bearing occurs. 
The coefficient of friction between the dry bearing material of the backup 
bearing and the shaft is required to be greater than that associated with 
the remainder of the backup bearing. 
Hence, the backup bearing easily can be arranged so that the backup 
bearing, including the dry bearing material; and the shaft; do not become 
damaged, or worn, whilst the shaft is being brought to rest after any 
failure of the magnetic bearing. 
Also for convenience in this specification, and the accompanying claims, 
reference is made to the coefficient of friction between the dry bearing 
material of the backup bearing and the shaft being relatively low when it 
is less than 0.2; and reference is made to this coefficient of friction 
being relatively high when it is greater than 0.2. 
The dry bearing material may comprise a dry lubricant, such as graphite, 
held within a matrix of a sintered powder, say, of tin bronze, inevitably 
there being a relatively low coefficient of friction associated therewith. 
Alternatively, the dry bearing material may comprise a textile material, 
say, of asbestos or glass fiber, impregnated with a phenolic resin, and 
having a relatively high coefficient of friction associated therewith.

DETAILED DESCRIPTION OF THE DRAWING 
As shown in the accompanying drawing, an assembly includes a rotating shaft 
10 mounted within a conventional magnetic bearing, indicated generally at 
11, the gap between the magnetic bearing and the shaft being indicated at 
12. 
One end of the shaft 10 is illustrated, and this end protrudes through an 
aperture 14 in a apart of a housing 16 for the assembly, only this part of 
the housing being shown. Within the aperture 14 in an anchorage comprising 
the housing 16 in which is provided a backup bearing for the assembly, the 
backup bearing being indicated at 18. 
In accordance with the present invention, the backup bearing 18 includes 
rolling elements 20 supported in radially inner and outer raceways. In the 
illustrated arrangement there are two substantially conventional ball 
races, and each ball race is fabricated in a very precise manner, and no 
lubricant is provided therein. The two ball racers are axially separated 
by two spacers 21. Each radially outer raceway 22 is secured to the 
housing 16 by being an interference fit within the aperture 14. A layer of 
dry bearing material 24 is secured to each radially inner raceway 26. 
Opposite to the layer of dry bearing material 24 is a layer of material 
28, secured to the shaft 10 by being a press fit on a reduced diameter 
cylindrical part thereof. The layer of material 28 is considered to be 
part of the shaft 10. In particular, the radially inner surface 30 of the 
layer 24 of dry bearing material is opposite to the radially outer surface 
32 provided by a wear-resistant surface layer portion 34 of the layer 28. 
In one example of the assembly, and with the assembly in operation, the 
clearance between the magnetic bearing 11 and the shaft 10, comprising the 
gap 12, is 0.5 millimeter; and the clearance between the opposing surfaces 
30 and 32, respectively, of the backup bearing 18, and the shaft, is 0.25 
millimeter. 
It is required that the shaft 10 is brought to rest after any failure in 
the operation of the magnetic bearing 11, and before the shaft, any part 
of the assembly rotating with the shaft, and the backup bearing, becomes 
damaged, or worn; and it is required that the shaft and the backup bearing 
18 do not become seized, because of temperature rises within the backup 
bearing, and the shaft. 
Heat is generated by contact of the shaft with the backup bearing, and is 
dissipated substantially by conduction through the shaft 10; by conduction 
to any part of the assembly secured to the shaft; and by conduction to the 
stationary part 16 in which the backup bearing 18 is provided. Usually the 
shaft 10 is at least substantially of steel, and has a high coefficient of 
thermal conductivity. 
If the arrangement is such that heat can be dissipated only at a low rate 
before the backup bearing 18 and the shaft 10 become seized together, the 
coefficient of friction between the layers 24 and 34 has to be relatively 
low. Hence, each radially inner raceway 26 takes a relatively long time to 
be brought to the speed of the shaft. 
Alternatively, if the arrangement is such that heat can be dissipated at a 
high rate before the backup bearing 18 and the shaft become seized 
together, the coefficient of friction between the layers 24 and 34 may be 
higher than for the arrangement described above. Consequently, each 
radially inner raceway 26 may take only a short time to be brought to the 
speed of the shaft. It is required to ensure that the rate of conversion 
of the energy of the rotating shaft to heat by contact of the shaft with 
the backup bearing is less than the rate of dissipation of heat at 
expected normally encountered temperatures of the backup bearing. 
The layer 24 of dry bearing material of the backup bearing 18 comprises a 
dry lubricant, such as graphite, or molybdenum disulphide, or tungsten 
disulphide, or polyethylenetetrafluoride, held by a matrix of a sintered 
powder, say, of tin bronze, if there can be only a relatively low 
coefficient of friction between the layers 24 and 34. 
Alternatively, the layer 24 of dry bearing material of the backup bearing 
18 is of a textile material, say, of asbestos or glass fiber, impregnated 
with a phenolic resin, if there can be a relatively high coefficient of 
friction between the layers 24 and 34. 
When there is a relatively high coefficient of friction between the layers 
24 and 34, conveniently, as shown, the layer 28 comprises a composite 
layer, the wear-resistant layer 34 being on a layer 36 comprising a sink 
for the heat generated between the layers 24 and 34. In the illustrated 
embodiment the layer 28 has a surface layer portion 34 of hard chromium, 
and providing the wear-resistant surface 32, plated onto a layer 36 of an 
alloy of copper and chromium, having a Vickers hardness value of 130, and 
having a thermal conductivity of 0.95 calories per sec cm .degree.C. 
Whether there is required to be a relatively low, or a relatively high, 
coefficient of friction between the layers 24 and 34, it is required that 
these layers have compositions which are not likely to become seized 
together, by becoming mechanically welded to each other, at temperatures 
expected to be generated in the operation of the backup bearing. 
However, further, and in particular for an arrangement in accordance with 
the present invention, it is required that the backup bearing does not 
become damaged, or significantly worn, by the rolling elements 20 rotating 
at a speed greater than the maximum design speed for the backup bearing 
18, when the shaft 10 is rotating at a speed greater than this value in 
operation. Hence, it is required to be arranged that, when the magnetic 
bearing 11 fails, and the shaft 10 first contacts the layer of dry bearing 
material 24 of the backup bearing, the backup bearing 18 is brought up to 
speed at a slower rate than otherwise would be the case, because of 
slipping of the shaft relative to the layer of dry bearing material 24. In 
addition, the assembly is arranged to be such that, under normally 
encountered operating conditions, by the time the backup bearing 18 is 
rotating at the same speed as the shaft, inevitably the backup bearing is 
rotating at a speed at most not significantly greater than the maximum 
design speed for the rolling elements 20. This criterion is obtained by a 
suitable selection of the coefficient of friction between the shaft 10 and 
the layer 24 of dry bearing material, commensurate with the shaft and the 
layer of dry bearing material not becoming seized together, as referred to 
above. 
Modifactions to the illustrated assembly are possible. 
The layer 28 on the shaft may not include a layer 36 of a material of high 
thermal conductivity. 
It is not essential that a layer of wear-resistant material 34 is provided 
on the shaft, and instead, the layer 34 is of a dry bearing material as 
referred to above; or the layer 34 is omitted, and the layer of dry 
bearing material 24 of the backup bearing directly contacts the shaft 10. 
The stationary part 16 of the assembly may not comprise a housing. 
Any required number of ball races may be provided. 
Any suitable form of backup bearing with rolling elements may be provided, 
for example, comprising at least one bearing having cylindrical rolling 
elements supported in a single raceway to rotate with the shaft.