Anthony Romero
Senior Application
Engineer
Machine Tool
The Timken Co.
Torrington, Conn.
Spindle heating can be a problem for
equipment that dry machines
gear teeth. These large-bore hybrid tapered roller spindle bearings
operate at temperatures between 90 and 100°F, significantly cooler than
all-steel bearings. The hybrid bearings can also last significantly
longer than all-steel equivalents under the same operating conditions.
The hybrid bearings come in precision Classes 00 and 000, providing a
rotational accuracy below 1 m (TIR) and nonsynchronous runout of <0.5
m. The latter metric is key to accurate and repeatable cutting
processes. Timken credits tighter bearing tolerances with the improved
rotational accuracy.
Machine tools are one of the most
challenging environments
for bearings. This is particularly true for gear-cutting equipment.
Such machines put significant loads on spindle bearings when they
simultaneously cut multiple gear teeth at high metalremoval rates.
Spindle bearings also need a wide speed range because bevel and hypoid
gear cutters typically run between 200 to 4,500 rpm, depending on the
operation and workpiece size.
There is also a push to eliminate cutting fluids in the interest of
lowering maintenance and disposal costs. Such dry machining is typified
by fast metal-removal rates, which necessitates an extremely rigid
spindle able to handle both high rpm and horsepower. Machining without
cutting fluids can set up large thermal gradients between the spindle
and tooling, making it difficult to hold size on machined parts.
Standard spindle-bearings under these conditions may reach temperatures
above 150°F, exacerbating the problem.
But special hybrid ceramic tapered roller bearings meet all the above
design criteria and, under the same conditions, run at temperatures
between 90 to 100°F. The bearings incorporate ceramic (silicon-nitride)
taperedrolling elements
in a precision-class bearing. Ceramic has a modulus of elasticity 50%
greater than steel, which boosts bearing rigidity. The material also has
a lower coefficient of friction and an extremely fine surface finish
that lessens frictional torque and helps the bearings run cooler.
With optimized races, ribs, rolling elements, and special highspeed
synthetic grease, the hybrid design can outperform conventional bearings
at higher machining speeds, while maintaining a high stiffness and load
capacity at low rpm. A single bearing type generally doesn't work well
in both operating regimes. Ball bearings, for example, tend to run
cooler than other bearings at higher rpm (above 750,000 DN). Tapered
rollers are a better choice when spindle stiffness and load-carrying
capacity are key design considerations.
Compared with other bearings in spindle applications, such as
angular-contact ball bearings (point contact) or cylindrical roller
bearings (line contact), tapered-roller bearings have a significantly
higher radial stiffness and are less susceptible to overload. Hybrid
tapered-roller bearings also simplify spindle design. Spindles need just
two of the bearings — one on each end. For comparison, spindles that
use cylindrical double-row bearings need one at both ends and an axial
angular contact bearing in the middle to achieve similar rigidity
levels.
Tests show tapered-roller bearings (at zero clearance) have a radial
stiffness 4 to 6 that of comparably sized angular-contact ball bearings,
and twice as much as cylindricalroller bearings. Tapered-roller
bearings use angled raceways to carry both radial and thrust loads. As a
rule, a shallow taper is used for heavy radial loads, and a steep
taper, for heavy thrust loads. The
hybrid bearing
is designed with an angularity or K factor specific to the gear-cutting
application. It takes into account preload, external cutting loads, and
operating speed.
Another tapered-roller bearing quality called true rolling motion
further improves bearing performance. Standard-class bearings have
crowned or other profiles on raceways and rollers to minimize contact
stress at the roller ends under heavy loads. Loads on machine tools, in
contrast, are better characterized so the spindle bearings instead use a
straight race profile. Such precision-class bearings exhibit true
rolling motion of rollers on raceways. True rolling motion helps
bearings run cooler and boosts spindle stiffness and accuracy. This
motion is the result of two design features: roller taper; and the
contact between the race rib and the spherical surface ground on the
large end of the rollers.
Rollers are designed such that extensions of the lines along the roller
bodies converge toward the centerline of the bearing and meet at an apex
on this centerline. The result: no relative slip between the rollers
and races. The arrangement also generates a force that seats the roller
spherical end against the race rib. This seating force is a function of
the different angles of the outer and inner races and is desirable
because it helps keep rollers from skewing off apex. A lack of skew
ensures positive roller alignment. This, in turn, boosts stiffness and
accuracy, and extends bearing life.
Analytical models optimize the apex angle so bearings accommodate radial
and axial loads generated during the gear-cutting process. This helps
maintain proper roller alignment at low rpm and controls skidding.
Modifications to the rib/roller interface further help maintain control
at low rpm.
Bearing stiffness also depends on the bearing load zone, which is
directly related to bearing setting and applied loads. Setting in a
tapered-roller bearing system can be defined as the amount of axial
clearance (end play) or axial interference (preload) within a mounted
set of bearings. It is typically measured in the axial direction because
this is the most straightforward way to establish an optimum value.
A conventional tapered-roller bearing with zero endplay has a load zone
close to 180°. The special hybrid bearing, in contrast, has a minimum
200° load zone for added rigidity. Setting is key to the bearing fully
benefiting from the extended load zone. Setting variation caused by
thermal expansion of the spindle-bearing-housing system directly affects
spindle static and dynamic stiffness.
It is generally agreed that the optimum setting is the mounted endplay
that gives maximum bearing life. However, machine tool and other
applications may emphasize system stiffness and heat generation. In this
context, the optimum setting is one that results in minimum deflection,
counterbalanced with maximum bearing life. Often in these applications,
bearing load capacity relative to a fatigue-spall life criterion is
well in excess of the machine design life.
In practice, end users install and preload the bearings to manufacturer
specs. A fixture applies push-pull forces equal to 3 suggested preload
and measures the resulting shaft displacement. The operation typically
takes place in a
temperature-controlled clean room.
This gives total endplay of the bearing system on the spindle. Special
software uses the deflection data to calculate a push-pull constant and a
spacer length that gives the right dimensional preload. Dimensional
preload for taperedroller bearings is defined as the deflection of the
inner race (cone) relative to the outer race (cup) at a specified force.
Users subtract the PPC and add the dimensional preload to an existing
spacer to establish a final setting. The approach gives a bearing
setting that accounts for actual system stiffness and deflections.
ARB Bearings LimitedH-22, Udyog NagarNew Delhi - 110041, Delhi, IndiaPhone: +(91)-(11)-25471274 / 25471255 / 25186300Fax: +(91)-(11)-25475455 / 25470126Email: info@arb-bearings.com, export@arb-bearings.com,sales@arb-bearings.com For Export Enquiry :
Mr. Divay Rathee : +91-9968373086