NTN - Precision Rolling Bearings

Technical Data
3 Adjustable preload bearing unit
A recent trend in the machine tool industry is a steady
increase of operating speeds. The maximum dmn value
(pitch circle diameter across rolling elements dmmm
multiplied by speed nmin -1 ) reached by main
spindles with air-oil lubricated lubrication can be as high
as 2.5 to 3.8 10 6 . At the same time, main spindles are
requiring increased rigidity. Therefore, main spindle
bearings must be capable of both high-speed operation
and high rigidity. This can be achieved through optimal
preloading.
A fixed preload (spring preload) system is usually
employed to satisfy both these high speed and high
rigidity requirements. A spindle unit with fixed-position
preload that is adjustable for different speed conditions
is advantageous for optimizing the rigidity of the unit.
The NTN Adjustable Preload Bearing Unit is a highspeed,
high-rigidity unit that features fixed position
preload that can be adjusted for different speed
conditions.
The NTN Adjustable Preload Bearing Unit is
illustrated in Fig. 5.1. Hydraulic pressure is used to shift
the position of the adjustable preload sleeve situated in
the rear bearing section of the unit. This changes the
preload on the bearings.
A spindle incorporating a 3-step adjustable preload
bearing unit is illustrated in Fig. 5.2. The sleeve in the
adjustable preload section is comprised of two hydraulic
pressure chambers, A and B, as well as a spiral groove
for sliding motion. The preload can be adjusted to one
of three settings by changing the hydraulic pressure in
each of the chambers. To achieve instantaneous and
reliable adjustment, high-pressure oil (at the same
pressure as in the hydraulic chambers) is supplied to
the spiral groove on the outside of the sleeve. This oil
provides lubrication so that the sleeve can move
smoothly.
Angular contact ball bearing
with ceramic balls
Adjustable preload section
Adjustable
Hydraulic pressure chamber A
preload sleeve Hydraulic pressure chamber B
Hydraulic pressure in spiral
groove (for sliding motion)
Adjustable
preload unit
Work piece
side
Locating side bearing
Preload adjustment side bearing
Drive
side
Bearing box
Main spindle
Angular contact ball
bearing with ceramic
balls
Fig. 5.1 Adjustable preload bearing unit
Low speed range
(high preload)
Medium speed range
(medium preload)
High speed range
(low preload)
5
2
4
1 3
Spiral groove
Hydraulic pressure ON
(Positive sign means clearance, while negative sign means preload.)
Hydraulic pressure ON
(Positive sign means clearance, while negative sign means preload.)
Hydraulic pressure OFF
(Positive sign means clearance,
while negative sign means
preload.)
L2 L1
Fig. 5.2 Typical spindle configuration incorporating
3-step Adjustable Preload Type Bearing Unit
Operating mechanism
Fig. 5.3 shows the hydraulic operation of the unit for three
preloading conditions as well as the associated motion of the
adjustable preload sleeve.
Low speed operation(high preload): Chamber A is
pressurized.
Component 1 moves to the right by a preset clearance L1
and contacts component 3. The axial clearance is 1.
Medium speed operation(medium preload):Chamber B
is pressurized.
Components 1 and 2 move to the right by a preset
clearance L2, causing Component 2 to contact Component
4. The axial clearance is2.
High speed operation(low preload):Chambers A and B
are not pressurized.
Components 1 and 2 return to the left due to the reaction
force on the bearing. This causes Component 2 to contact
Component 5, thereby returning the axial clearance to the
initial setting of3.
NOTE: The return motion of the components 1 and 2 is achieved by
the reaction force of bearing or a separately provided spring.
Fig. 5.3 Operating mechanism of Adjustable preload
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