High level roller and ground ball screws seem to work almost identically in
a wide range of mechanical applications. However, the surface of the raceway,
which is very similar to the traditional machine screw threads found in common
screw / nut systems, differs by the manufacturing process used to create these
shapes. In addition to the reinforced ball bearings, which allow rotation
movement, the ball screw system consists of two basic components: a lead screw
with external threads and a ball nut with internal threads. Identifying two
unique manufacturing processes used to manufacture these components is
important to understand both the strengths and limitations of roller balls and
ground ball screws.
Rolling and Grinding:-
The threads of the rolling
ball screws are produced by pushing each blank uncut piece of bar through a
rotating tool die in one operation. The driving principle of this process is
the cold work deformation process, which literally molds the blank material with
a rolling tool array designed to form the inner and outer threads of the feed
screw and ball nut. Lamination dies are available in various shapes and sizes
to laminate different thread sizes, but the deformation principle is the same
for internal and external threads. The lamination process is very efficient
because it does not physically remove the material from the cutting parts, as
in the cutting or grinding process. This process is widely used throughout the
industry because it can be easily automated to achieve high automation rates.
In contrast, ground screw threads are generally manufactured through abrasion processes that occur in a horizontal environment, in which the work-piece is rotated along a long axis between two machining centers, and an abrasive cutting thread. Very hard cuts raw pieces designed to cut several threads at once. Similar processes are used for cutting internal and external threads of lead screws and spherical nuts. This progressive polishing process requires that the blank material be cut to its final length before grinding. Various sizes and styles of cutters are used to rectify internal and external threads, but the conceptual process is the same.
Strengths and
Limitations
The rolled threads are generally of greater resistance than the
rectified threads due to the significant reinforcement of the material achieved
as a result of the large plastic deformation induced in the material to produce
threads during the rolling process. Rolled thread is a more cost effective
option because it can be produced quickly in an automatic laminating machine
configuration. The lamination process is considered a "one-time"
process, so manufacturing errors in the laminating threads are difficult to
correct. Rolled screws also tend to have a thick surface roughness than
rectified screws, which can be particularly problematic because thick surface
screws can reduce mechanical efficiency due to the high rotational friction
that occurs in ball bearings. The roughness of the rough surface can contribute
to the overall reduction of the wear resistance of ball screw systems.
Finally, ball screw
systems are particularly susceptible to dimensional deformation as a result of
the wear of the die tools. The cause of these errors can lead to ball / nut
systems with excessive clearance due to the mismatch between the diameter of
the bearing and the threads of the screw shaft, and the positional accuracy
that can be achieved is drastically reduced.
In contrast, the thread grinding process is inherently
slower than the rolling process, in part due to the special grinding tools
required for cutting operations. This means that the price of "off
shelf" for ball screws is much higher than ball screws. However, because
each wire is grounded one at a time, there is a significant advantage for
higher cost processes, since the ground wire can be subjected to stricter
tolerances with respect to the material's center line in white. For this
reason, ground ball screws are used almost exclusively in applications where
high precision and repeat-ability are required.
Finally, there is one last note to mention regarding heat
treatment and / or housing hardening methods that are typically used for two
types of ball screws. In ground wires, the blank of the material usually
hardens before the wires are grounded. The threads of the rolling screws go
through a similar process of hardening of the carcass only after the thread has
been wound due to the fact that the hardened surface of the carcass is too
difficult to perform the rolling work. The main disadvantage of this type of
post-rolling heat treatment is that distortions can occur in the threads of the
roller due to the inherently unpredictable rate of expansion of the material in
the thread geometry.
Conclusion:-
Roller ball screw systems are generally much less expensive
than ground ball screws and should be more than available for most
applications. Ball and ball screws are available in a variety of rod lengths,
diameters and capacities, which makes them suitable for a wide range of
applications. However, if a very high level of positional accuracy is required,
the choice of a ground ball screw system with high repeat-ability may be worth
more than the cost. Engineers must carefully consider these differences when
choosing between the two for a given mechanical application.
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