Rolling ring linear motion assemblies may be used in a wide range
of finishing machines, performing applications such as painting,
coating, positioning, cutting, spooling/winding, spraying, slitting
and packaging. Rolling ring linear motion benefits finishers in
three ways:
- Elimination of complex, electronic controls reduces overall
machine cost;
- Simplified operation and maintenance reduces investment in specialized
skills and training;
- Less downtime for repairs and control adjustments results in
longer periods of uninterrupted production time.
These types of machines incorporate a linear drive assembly to
move the tool mounting head (drive head) back and forth (Fig. 1).
A primary design goal is smooth, accurate linear motion, possibly
with requirements for automatic reversal and the ability to change
linear speed.
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Figure
1
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Traversing or reciprocating systems offering automatic reversal
of the tool mounting head and the ability to adjust pitch (linear
speed) almost invariably involve a variety of components, such as
sensors, clutches, encoders and gear boxes. Depending on the linear
motion requirements dictated by a specific finishing process, however,
it is frequently possible to simplify system design, operation and
maintenance by using mechanical methods for motion control demands.
Simple
operating principle
Standard ball bearings typically serve to reduce friction
in rotating assemblies, much like the bearings in the hub of a wheel.
Rolling ring bearings, however, are designed to create linear output
from rotary motion input.
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Figure
3
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Figure
4
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A rolling ring bearing begins as a standard ball bearing. A special
machining process creates a sort of "ridge" that runs around the
center of the bearing's inner race surface. When mounted on a shaft,
a rolling ring bearing contacts the shaft only on the apex of this
ridge (Fig. 3).
There is clearance between the shaft and bearing on either side
of the ridge. This clearance permits the bearing to be pivoted,
angled left or right on the shaft and still maintain point contact
with the shaft.
A three or four rolling ring bearing assembly is fixed within the
housing. Each bearing is held at a specific angle relative to the
shaft (Fig. 4). When the shaft rotates, the rolling ring bearings
generate axial force on the central ridge. This causes the bearing
assembly to roll along the length of the shaft. The rotary input
provided by the motor-driven shaft is thereby converted to linear
output. As the bearing assembly moves, it carries the tool mounting
head with it.
A simple spray painting process, for example, probably does not
need a complex electronic control system to move the spray head
back and forth. Rolling ring linear drive assemblies are commonly
used in these situations, because in most instances rolling ring
engineering eliminates the need for these controls as well as the
need for clutches, cams, gear and variable speed reversible motors
(Fig. 2).
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Figure
2
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Instant
reversal without special controls
The angle at which the rolling ring bearing assembly
contacts the shaft is adjustable. The travel direction of the tool
mounting head is determined by this angle. Changing the angle of
the bearing assembly is done mechanically making the reversal process
totally independent of the drive motor or other controls.
Reversal occurs when contacting a hardware fixture called an "end
stop" triggers the spring-actuated reversal mechanism (Fig. 5).
When the reversal mechanism is triggered, the entire rolling ring
bearing assembly is flipped on the shaft to its opposite to mirror
position and reversal is instantaneous (Fig. 6). The end stops are
user-positioned to determine the system's stroke length.
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Figure
5
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Figure
6
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At no time does the rolling ring bearing lose contact with the
drive shaft. This is how rolling ring linear drive assemblies prevent
backlash, because there is no play between shaft and bearing. Furthermore,
the shaft on which a rolling ring bearing operates is not threaded,
which means dirt and debris cannot be trapped and cause clogging
or jamming. Mechanical control over linear speed.
Manipulating the angle of the rolling ring bearing assembly on
the shaft also controls the pitch or distance traveled per shaft
revolution. This is useful when a process calls for a gradual decrease/increase
in speed before or after reversal. This "ramping" down or up is
usually used to lessen the effects of jarring or jerking on the
payload attached to the linear drive.
For example, suppose a finishing operation involved the attachment
of a buffer on the end of an extension arm. The extension arm, mounted
to the tool mounting head, travels back and forth when the system
is in operation. If reversal is not gentle, the inertia and whip-like
action of the arm could wrench or torque the system, possibly damaging
system components. Meeting application requirements for ramping
up or down during the reversal process can mean designing in clutches,
gearboxes and control systems. With a rolling ring system, however,
controlled changes to linear speed are achieved with relatively
inexpensive mechanical modifications to the auto-reverse mechanism.
The linear speed of the tool mounting stage may be changed while
the drive is operating simply by adjusting the pitch control (Fig.
7). Adjusting the pitch control changes the angle of the rolling
ring bearing assembly on the drive shaft. This increases or decreases
the drive head's travel distance relative to each revolution of
the shaft. This translates into an increase or decrease of linear
speed, even if the drive motor speed and rotational direction remain
unchanged. Thus, a variable speed system may be driven by a relatively
inexpensive, single-speed, uni-directional motor.
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Figure
7
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Figure
8
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The reversal mechanism may also be configured so that it rotates
the rolling ring assembly to be perfectly perpendicular to the shaft.
This causes the drive head to dwell. The pitch is zero and there
is no linear movement, even though the shaft-ring contact is still
intact (Fig. 8). When the ring assembly is again angled on the shaft,
usually accomplished with an air cylinder or other actuating device,
linear motion resumes.
A rolling ring linear drive assembly is typically supplied within
a production framework (Fig. 9). The end stops are preconfigured
to provide the desired linear motion.
Technical
considerations
In some cases, rolling ring drive systems do require
additional controls to meet application requirements. An example
is a spot grinding operation where the tool head must be accurately
positioned forward and backward on the shaft.
This requires two-way shaft rotation and the rolling ring system
might employ a PLC controller as illustrated in Fig. 10. Generally,
a rolling ring system readily lends itself to various types of mechanical
manipulation that exploit the unique performance of the rolling
ring bearing to permit control over the reversal and pitch of the
traversing housing.
Accuracy with rolling ring systems is typically to within + 0.005
inch, at speeds up to 13 fps over distances of 16 ft maximum. Some
rolling ring linear motion systems offer increased accuracy to within
+0.0004 inch. Rolling ring systems generally are not used if incremental
linear movement in the <0.0004 inch range is required.
Rolling ring assemblies are designed to perform finishing processes
requiring reciprocating and positioning linear motion. For processes
involving repetitive, reciprocating motion such as slitting or spraying,
rolling ring assemblies pose a practical alternative to avoiding
the operating and maintenance costs associated with traditionally
developed linear motion systems.
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Figure
9
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Figure
10
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Low
operating/maintenance costs & less downtime
Since, reversal of direction and changes to linear speed
and travel direction are possible without slowing or stopping the
drive motor, production machinery may be operated for longer periods
without spotting for adjustments. Freedom from a complex and costly
control system results in reduced set-up costs, low operator training
costs and lower maintenance expenditures. With simpler controls
there are fewer electrical and mechanical problems, and production
machinery downtime is reduced. Periodic lubrication of the drive
shaft is the only maintenance required.
Built-in
overload protection
Because there are no threads on the rolling ring drive
shaft, a rolling ring system requires no bellows assembly to protect
the shaft from debris and dirt. The shaft remains clog-free even
in dirty working environments. No shaft cleaning means less downtime
and better production rates. Additionally, the smooth shaft will
"slip," not jam, in the event of overload, whereas a threaded shaft
may continue to "churn," causing damage to expensive system components.
Tool
head accepts load directly
There is no need to purchase linear bearings, slide mechanisms
or other load carrying devices. The tool may be attached directly
to the rolling ring tool head stage or "nut."
Free
movement on shaft
Rolling ring systems generally feature a free movement
lever. This permits manual or pneumatically actuated positioning
of the tool head while the system is stopped. Screw-based systems
require the shaft to turn in order to move the nut. The free movement
lever eliminates the need to start-and-stop, or "jog" the system
to position the tool head. This saves time and expedites production.
Backlash
free
The design of a rolling ring bearing assures continuous
point contact with the drive shaft that eliminates backlash. To
prevent backlash, other devices can require the purchase of expensive
preloaded nuts or other device to load the gear train. Rolling ring
linear motion machines can make finishing operations simpler by
reducing the need for a number of complex controls, specialized
training and an uninterrupted project line due to repairs and control
adjustment.
