Hollow roller bearing preloaded by an interference fit

A hollow roller bearing has inner and outer races with the former provided with opposing axially spaced generally radially outwardly projecting shoulders. Roller bearings, hollow by diameter in the range 55% to 75% are disposed between the shoulders and in loose engagement with the inner and outer races during initial assembly. The bearings are thereafter pressed onto a shaft for preloading and assembly is thus completed. A method for making bearings involves the provision of the bearing as described with a loose assembly of the elements. The mounting method involves the press fitting of the loosely assembled elements on a shaft where upon preloading is achieved.

BACKGROUND OF THE INVENTION 
Hollow roller bearings have heretofore been used successfully but in a 
limited number of applications. The following U.S. Patents are at least 
partially indicative of the state of the art in this area. 
______________________________________ 
Title Inventor 
______________________________________ 
3,765,071 
Method of Forming a 
Willard L. Bowen 
Roller Bearing of 
Superior Runout 
Characteristics 
3,930,693 
Full Complement Bearing 
Willard L. Bowen 
Having Preloaded Hollow 
Rollers 
4,002,380 
Roller Bearing of 
Willard L. Bowen 
Superior Run-Out 
Characteristics 
4,232,914 
Hollow Roller Tapered 
Willard L. Bowen 
Bearing 
______________________________________ 
These patents are at least indicative of the history of development of 
hollow roller bearings and set forth to several advantages associated 
therewith. The hollow roller bearings disclosed by the patents, however, 
are subject to a serious disadvantage in use. The customer must initially 
press the inner race onto the shaft or the like, grind the race to the 
proper diameter for an interference fit, and thereafter the shaft with the 
inner race thereon is assembled within the rollers and the flanged outer 
race to provide a desired preload. This method of assembly is of course 
quite time consuming and cumbersome with the bearings being shipped to the 
customer in separate parts. 
It is the general object of the present invention to provide an improved 
hollow roller bearing and method of making and mounting the same wherein 
the bearings may be shipped and efficiently mounted on a shaft or the like 
as an assembled unit. 
SUMMARY OF THE INVENTION 
In fulfillment of the foregoing object, hollow roller bearings are 
manufactured with inner and outer races and at least the former has 
opposing axially spaced generally radially outwardly projecting shoulders. 
A plurality of hollow roller bearings are provided with the bearings 
arranged in an annular series circumaxially about the inner race and 
within the outer race. The bearings are hollow throughout their length and 
are hollow by diameter in the range 55% to 75%. On assembly within the 
races, a slight internal radial clearance is provided for and the bearing 
is thereafter preloaded as required in the course of mounting the same on 
a shaft or the like. An interference fit between the inner bearing race 
and the shaft or the like serves to radially preload the bearings as 
required for highly efficient and accurate operation as set forth in the 
foregoing patents. 
Preferably, at least two axially adjacent annular series of hollow roller 
bearings are provided in assembled relationship of the bearings and three 
or more such annular series of axially adjacent hollow roller bearings may 
be provided within the contemplation of the invention. 
The outer bearing race may be provided with a constant internal diameter 
throughout its length or may have one or more annular shoulders or other 
features. 
The inner bearing race may have a constant internal diameter or, a tapered 
bearing may be provided in accordance with the invention, wherein the 
inner bearing race is of course provided with a frusto-conical internal 
surface. 
The extent or degree of preload is such that the elastic limit of the 
material of the races is not exceeded and may be on the order of .001 of 
an inch and may even exceed .001 of an inch with the hollow bearings of 
the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS 
Referring to particularly to FIGS. 1 and 2, it will be observed that an 
improved hollow roller bearing constructed in accordance with the present 
invention is indicated generally at 10 and comprises inner and outer races 
12, 14, and a plurality of hollow roller bearings indicated at 16, 16. As 
best illustrated in FIG. 2 the inner race 12 has opposing generally 
radially outwardly projecting shoulders 18, 18 which serve to restrain the 
roller bearings 16, 16 against axial movement. The shoulders 18, 18 are 
formed by similar radially outwardly projecting annular end flanges 20, 
20. 
As will be observed, the roller bearings 16, 16 are hollow throughout their 
length and they may also exhibit other characteristics as set forth in the 
foregoing patents. More particularly, the hollow roller bearings 16, 16 
are preferably hollow by diameter in the range 55% to 75%. 
It is an important feature of the present invention that the hollow roller 
bearings 16, 16 are preloaded when the bearings are in fully assembled 
condition on a shaft or the like and ready for use. Further description in 
this regard may be found in the foregoing patents. Preferably, the preload 
of the hollow bearings is on the order of 0.001 of an inch and may even be 
in excess of 0.001 of an inch. The elastic limit of the material of the 
races is in no event exceeded and all rollers are preloaded equally. 
Despite the necessary preloaded condition of the bearings in their 
operational mode, it is important to note that, in accordance with the 
present invention, the hollow roller bearings are not preloaded in their 
initial assembled condition. That is, the hollow rollers 16, 16 are 
disposed in the bearing between the inner and outer races 12 and 14 in a 
relatively loose arrangement. With a small radial clearance in the 
bearing, the outer race can be readily assembled about the rollers after 
they have been disposed between the shoulders of the inner race. The 
clearance may vary in accordance with the invention but is preferably in 
the range of 0.0001" to 0.001" . 
Prior to entering their operational mode, the assembled bearings of the 
present invention are pressed onto a shaft or the like with a slight 
interference or press fit. This causes preloading of the bearings due to 
the resulting expansion of the inner race and it should be noted at this 
point that the extent or degree of preload is not critical except for the 
aforementioned elastic limit limitation. Considerable flexibility is 
afforded by the hollow rollers without any resulting overstressing of 
bearing parts. Hollow roller bearings of the invention are able to 
withstand an appreciable preload in excess of 0.001 inches without 
detriment to running surfaces of the races or rollers. 
The foregoing is of course not true with regard to solid roller bearings. 
When a solid roller bearing is pressed on to a shaft in order to obtain a 
preload, it is an extremely difficult matter to properly preload the 
bearing without over-stressing the bearing elements. The permissible 
preload with solid roller bearings is on the order of 0.0001 inches to 
0.0002 inches. Preloads in excess of these limits tend to cause high 
contact stresses on running surfaces and may result in spalling and 
premature failure. 
It should also be noted that the loose assembly of the bearing elements in 
accordance with the present invention results in improved shelf life of 
the bearings. The internal radial clearance allows a film of oil to coat 
the bearing race and roller surfaces. If, on the other hand, the bearing 
were first preloaded and then stored, contact corrosion might occur where 
the preloaded rollers and roller races are held in tight engagement. Such 
continuing contact pressure squeezes out the lubricant between the rollers 
and the raceways resulting in metal-to- metal contact. This in turn 
results in fretting corrosion and/or pitting of the roller and raceway 
surfaces. 
In FIG. 3, a bearing constructed in accordance with the present invention 
is indicated generally at 10a and includes inner and outer races 12a and 
14a. Hollow roller bearings 16a, 16a disposed between the inner and outer 
races are arranged in two axially adjacent annular series. The bearing may 
otherwise be regarded as substantially identical with the bearing 10 of 
FIGS. 1 and 2. 
The bearing of FIG. 3 is provided with a small radial clearance during 
initial assembly as in the case of the bearing 10 and is provided with 
shoulders 18a, 18a on its inner race 12a formed by flanges of 20a, 20a as 
in the case of the bearing 10. Thus, the bearing 10a may be assembled with 
the bearings loosely retained in position and thereafter pressed onto a 
shaft or the like to preload the rollers 16a, 16a. All of the advantages 
set forth above are exhibited by the bearing 10a. 
In addition to the foregoing, the multiple annular series of rollers 
improves the runout accuracy of the bearing. The greater number of 
preloaded rollers provides for a greater number of centering forces as 
illustrated by arrows 30, 30 in FIG. 6 with the rollers in circumaxially 
staggered relationship. This arrangement also serves to maintain a more 
constant center of rotation. The radial accuracy and stiffness of the 
bearing is thus substantially enhanced. 
It should also be noted that hollow rollers of limited axial dimension as 
in the bearing 10a are much easier to manufacture. The drilling of a short 
hole is much more readily accomplished than in the case of the relatively 
long central bore in the rollers 16, 16 of FIG. 1. Further, roller size 
and shape is more readily maintained at low tolerances in shorter rollers. 
The increased accuracy of a bearing with multiple series of hollow rollers 
will also be evident. Raceway variations in one annular series of rollers 
will have no effect on the rollers in an adjacent series. The preloading 
results in a sharing of the load by all of the rollers and the greater 
number of rollers provides less variation which might otherwise affect the 
radial runout of the bearing. 
Finally, the multiple annular series of rollers can readily accommodate the 
construction of extremely wide bearings to carry large loads. As many as 
four series of rollers have been employed successfully and it is 
contemplated that even more annular series of rollers could be employed 
due to the fact that preloading allows all rollers to share the load. 
In FIG. 4 a bearing 10b is identical in all respects with the bearing 10a 
of FIG. 3 except for the provision of a conventional tapered bearing 
configuration. That is, the inner diameter of the inner race 12a in FIG. 3 
is constant as illustrated whereas the inner diameter of the inner race 
12b in FIG. 4 is frusto-conical to provide a conventional tapered bearing 
design. Both the method of manufacturing and assembling the bearing as 
well as the mounting method described above are fully applicable to the 
FIG. 4 bearing. 
In FIG. 5 a bearing 10c is provided with three (3) axially adjacent annular 
series of hollow roller bearings 16c, 16c. The bearing is otherwise 
substantially identical with the bearings described above except for the 
provision of spacers 22, 22 between the centrally located series of roller 
bearings and each end series of bearings. It has been found that the 
bearing of FIG. 5 can be provided with or without the spacers 22, 22. 
Substantially identical results in terms of efficiency and accuracy are 
achieved in operation in either event. 
The methods of making and mounting the bearings of the present invention 
will be apparent from the foregoing. As suggested, a first step in the 
method of making a roller bearing in accordance with the present invention 
resides in the manufacture of inner and outer roller bearing races with at 
least the inner race having opposing axially spaced generally radially 
outwardly projecting shoulders. A plurality of roller bearings are 
provided and are hollow throughout their length and hollow by diameter in 
the range 55% to 75%. Initial assembly of the bearing requires that the 
races and the rollers be assembled without a preload and instead with a 
slight internal radial clearance. 
The method of mounting the bearings of the present invention includes the 
foregoing steps and the additional step of assembling a bearing on a shaft 
or the like with a slight interference fit whereby to radially preload the 
bearing. As stated, significant tolerance is permissible in preloading and 
a highly efficient and accurate operation ensues. 
As will be apparent from the foregoing a seemingly minor advance in the 
aforesaid methods and bearing design results in very substantial practical 
improvements and in significant economic advantage. The grinding of inner 
bearing races is achieved with substantial facility and is much more 
readily accomplished than the grinding of channel type outer races. The 
bearings may be readily assembled in loose engagement at the manufacturing 
plant and thereafter preloaded by the customer during mounting of the same 
on a shaft, spindle etc. A simple and expedient procedure is provided and 
yet results in a high degree of efficiency and accuracy inherent with 
hollow roller bearings. A number of other secondary advantages and 
features are also realized as described above.