1.  Field of the Invention
This invention relates to a device for expanding a fixing sleeve by the use of a hydraulic pressure to impart a predetermined pressure to a bearing and fix the bearing, and for releasing said hydraulic pressure in a state in which a required pressure force is caused to act axially, thereby mounting the bearing in a state in which a preload is applied thereto. 
2.  Description of the Prior Art
Heretofore, where a double row angular ball bearing provided with a seat is to be mounted, for example, on a spindle unit by applying a preload to the bearing, the bearing has been fixed onto the spindle with the inner race thereof being tightened by rotatively displacing a nut threadably engaged with a shaft.  However, when the inner race is tightened by the nut, a high load of 3 to 4 tons is immediately created and one could not know by what degree of force the inner race is actually tightened. 
Also, it has been thought that there is little or no variation in the dimensions of the inner race spacer inserted between the inner races of the double row angular ball bearing provided with the spacer, but it has been found that the dimensions of the spacer are actually varied by the deformation of the spacer, the destruction of the contact surface or the like when a great load is applied thereto during the preload of the bearing. 
Accordingly, even when a preset preload appropriate to the bearing has presumably been applied it has sometimes been the case that a preload two to three times the preset preload is actually applied. 
Even when a preload greater than the preset preload has been applied as described above, the influence thereof is small if the rotation of the spindle is not so fast, that is, the rotational speed of the bearing is in a low or medium speed range (500,000 dmn or less).  However, when the rotational speed of the bearing is 600,000 dmn or more as has been experienced recently, the great temperature rise by an excessively great preload and further the possibility of seizure or the like have offered problems and stable highspeed rotation could not be obtained.