Screw drive bearings have evolved to meet the tough specifications
demanded of modern drive components, today's units providing the
technical performance and cost effectiveness much sought after by
designers. We take a look some examples that make installation less of a
headache
The appeal of conventional electromechanical drives for linear movement
continues, despite more recent developments such as linear direct drives.
This is due to a number of factors: locating/locating bearing type screw
drive spindles and driven screw drive nuts are able to generate higher
feed power than the linear motor, offering advantages in terms of greater
moved masses and higher process forces. They also come with a
significantly lower price tag than more innovative solutions.
Continuous development and new ideas associated with the concept of
electromechanical drives means that screw drive bearings have undergone
extensive improvement to cater for new demands on drive components. A
modern screw drive bearing provides both improved technical performance
and cost savings, offering the designer many advantages.
In addition to the developments in bearing technology, cost-effectiveness
is becoming increasingly important. Simplified installation and lower
processing costs for the surrounding structure give a much better total
cost saving potential than simply specifying a less expensive bearing.
Screw drive bearings, pre-drilled for flange mounting, are ready to
install units. They provide state-of-the-art bearing arrangements for
screw drives and are installed simply by bolting down, with no additional
radial location being necessary. There are several configurations:
Thrust angular-contact ball bearings are ready-to-fit, self-retaining
components. Being directly mounted on the machine structure, the bearings
positively affect the rigidity and accuracy of the entire screw drive
system. For the majority of applications, they are delivered lubricated
for life, with the seal integrated into the bearing, making additional
parts in the surrounding structure unnecessary.
Some bearings have been specially developed for locating/locating bearing
type drive spindles, where the spindles are axially preloaded to
compensate for heat expansion during operation (tensioned spindles). This
design principle means the bearings are permanently more heavily loaded
in one direction due to the stretching force inside the spindle. To
accommodate this a third row of balls provides a one-sided support for
higher axial loads, a feature which also makes the bearing suitable for
the vertical axis of machine tools where the carriage mass constantly
subjects one of the bearings to an axial load.
Radial positioning of the screw drive spindle bearing to a reference (a
problem often encountered at the design stage) is easily overcome by
using 'bolt down' bearings. In such cases the position of the screw drive
is the reference, with the moveable screw drive nut serving as a
functional element for positioning the flange mountable screw drive
bearing.
An important consideration when using flat bolt-mounted bearings is that
of safeguarding against bearing slippage caused by radial forces such as
a drive belt. Tests undertaken by bearing specialist, INA show that
slippage does not occur until after the radial load-carrying capacity of
the bearing is reached or exceeded if class 8.8 screws are used for
mounting. If higher strength class (12.9) screws are selected along with
higher tightening torques, forces exceeding the bearing limit load in the
radial direction can be transferred. Furthermore, if the outer ring is
glued (for example with Loctite 638) the maximum transferable radial load
can be significantly increased.
All bearing types mentioned in this article are available from INA
Bearing Company
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