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Apparatus is provided for mounting a rotatable member such as a machine tool spindle within a bore provided in a housing, the rotatable member being journaled in a number of bearings. A bearing support sleeve disposed within the bore to support the bearings includes a first portion tightly fitted in

Apparatus is provided for mounting a rotatable member such as a machine tool spindle within a bore provided in a housing, the rotatable member being journaled in a number of bearings. A bearing support sleeve disposed within the bore to support the bearings includes a first portion tightly fitted in the bore, a second portion slip-fitted in the bore and positioned to apply a pre-load force to the bearings, and a yieldable member joining the first and second portions. The pressure in a fluid system associated with the yieldable member is increased to deform the yieldable member and to thereby urge the second portion axially to apply a pre-load force to the bearings which corresponds to the pressure increase.

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1. A mechanism of a type comprising a bearing (18) for rotatably mounting a first machine tool member (14) and means (22,28) for pre-loading the bearing (18) and supporting the bearing (18) and first machine tool member (14) with respect to a second machine tool member (10), said means characterized

1. A mechanism of a type comprising a bearing (18) for rotatably mounting a first machine tool member (14) and means (22,28) for pre-loading the bearing (18) and supporting the bearing (18) and first machine tool member (14) with respect to a second machine tool member (10), said means characterized by: a sleeve (22) having a first portion (22a) held in immovable relationship by said second member (10), a second portion (22b) carrying said bearing (18) and displaceable along the axis of said sleeve, and a portion (22c) interconnecting said first and second sleeve portions and displaceable along the axis of said sleeve; and means (28) for controllably urging said interconnecting portion (22c), said second portion (22b) and said bearing (18) away from said first portion (22a) to selectively increase the pre-load applied to said bearing (18). 2. The mechanism of claim 1 wherein: said interconnecting portion (22c) is spaced apart from said first sleeve portion (22a) to form a reservoir (24) in said sleeve (22) for a fluid; said mechanism includes means (48) for resisting displacement of said bearing (18) along the axis of said sleeve (22) and away from said first sleeve portion (22a); and said urging means comprises a fluid contained in said reservoir (24) in said sleeve, means (36,38) for sealing said fluid in said reservoir, and means (28) for increasing the pressure of said fluid in said reservoir (24) to urge said bearing (18) away from said first sleeve portion (22a) and against said resisting means (48) to increase the pre-load applied to said bearing (18) as a function of said increased pressure. 3. The mechanism of claim 2 wherein: said interconnecting portion (22c) comprises a wall member (22c) spaced apart from said first sleeve portion (22a) by a first annular groove (24) formed in said sleeve (22), and from said second sleeve portion (22c) by a second annular groove (26) formed in said sleeve (22), said first annular groove (24) comprising said reservoir and containing said fluid and said second annular groove (26) containing air at atmospheric pressure; and said fluid comprises an incompressible fluid. 4. The mechanism of claim 3 wherein: said first (24) and second (26) annular grooves open in opposite directions. 5. The mechanism of claim 3 wherein: said second machine tool member (10) comprises a housing (10) provided with a bore (8) for receiving said sleeve (22), said bearing (18) and said first member (14); said first annular groove (24) opens toward the wall of said bore (8); and said sealing means comprises first (36) and second (38) O-rings positioned between said wall of said bore and said sleeve on opposing sides of said first annular groove (24). 6. The mechanism of claim 5 wherein: said means for increasing fluid pressure comprises a source (28) of hydraulic pressure in communication with fluid in said first annular groove (24) through a passage (30) formed in said housing (10). 7. The mechanism of claim 5 wherein: said first sleeve portion (22a) is tightly fitted in said bore to prevent axial and radial movement of said first sleeve portion (22a) with respect to said bore; and said second sleeve portion is fitted in said bore (8) to allow axial displacement of said second sleeve portion (22b) with respect to said bore (8). 8. Apparatus for mounting a machine tool spindle in a bore provided in a housing, said apparatus comprising: a bearing support sleeve disposed within said housing which comprises a first portion tightly fitted within said bore, second and third portions disposed for axial displacement within said bore, and first and second yieldable members respectively joining said first portion and said second portion, and said first portion and said third portion; sensing means abutting said third portion for taking up an axial force urging said third portion toward said sensing means, and for generating a signal representing said axial force; first and second bearings having their inner races locked to said spindle for axial displacement therewith, the outer race of said first bearing positioned to be urged axially by said second portion and the outer race of said second bearing positioned to urge said third portion axially against said sensing means, said first and second bearings each being of a type wherein an axial force applied to one of the bearing races is transmitted to the other; and a system of fluid under pressure for selectively deforming said first yieldable member to urge said second portion and the outer race of said first bearing axially, and thus pre-load said first and second bearings against said sensing means and through said spindle and said third portion. 9. The apparatus of claim 8 wherein: said second portion comprises a rearward portion of said sleeve, and said third portion comprises a forward portion of said sleeve; and said forward portion includes a flange having an edge tightly fitted within said bore, a relief being placed in said flange to form a third yieldable member. 10. The apparatus of claim 9 wherein: a first annular groove containing fluid of said system is formed in said sleeve between said first yieldable member and said first portion of said sleeve, an annular seat being formed in said first annular groove; a second annular groove at atmospheric pressure is formed in said sleeve between said first yieldable member and said rearward portion of said sleeve; said fluid system includes means for selectively increasing the pressure of said fluid contained in said annular groove to apply a pre-load to said bearings corresponding to said pressure increase; a split ring of specified width is located on said seat to apply a static pre-load to said bearings corresponding to said width; and a key and corresponding keyway are positioned between said sleeve and said housing to take up back thrust applied to said spindle, and to prevent forward movement of said first portion within said bore.