Designing Flexibility Into a Winding System
Part 2- Meeting Flexibility Criteria & Controlling Operating/Maintenance Costs
by Mark Wilson, Applications Engineer
Part 1 of this article examined considerations for designing a flexible winding system. In it, a flexible system was described as one capable of winding different materials, each having different diameters or thicknesses. A flexible system also accommodates spools of varying sizes and diameters.
In any winding system, the linear drive moving the material guide back and forth must travel a specific linear distance per one shaft revolution. This is the system's pitch and its proper adjustment ensures material is laid onto the spool in evenly spaced rows. In a flexible winding system, each different material being wound has different pitch requirements. Further, as more material is spooled onto the reel, the take-up motor reduces in speed, but the pitch must remain unchanged. That is, for a specific material, the linear drive must continue to traverse the same linear distance with each shaft revolution regardless of the take-up motor speed.
So a winding system designed to handle a variety of materials and spool sizes must provide a method of synchronizing traversing unit pitch with the rotational speed of the take-up reel. An automatically reversing linear drive with variable pitch control enables this process via a simple, mechanical pulley system. Other systems must use costly external controls including electronic control systems, variable speed motors, controllers, valves and solenoids, gear head assemblies, sensors, clutches, encoders, etc. Cost of training staff to operate and maintain these systems also adds to project budgets.
To support successful and profitable production objectives, it is necessary to select a linear motion method which minimizes setup, operating and maintenance requirements without sacrificing required functionality or efficiency. Optimizing the linear motion system to meet winding application requirements while imposing the fewest operating and maintenance challenges will contribute to a more productive operation.
Mechanical Solutions Minimize Operating & Maintenance Requirements
"Rolling ring" linear drives have long been used for spooling and winding because the rolling ring operating principle enables efficient, automatically reversing, reciprocating motion without using clutches, cams, gears or other external controls. This minimizes system design costs and simplifies production requirements and maintenance
(See Figure 1).
The most common rolling ring linear drive systems are used for reciprocating linear motion with automatic reversal. Reversal with a rolling ring linear drive is practically instantaneous (2 to 70 milliseconds depending on drive size) and is achieved via purely mechanical means without resorting to complex, electronic controls. This usually holds true for rolling ring linear drives, even with special winding requirements.
For example, a winding operation may require ramping down the linear drive speed at the reversal points. This is generally to lessen the effects of "jarring" or "jerking" on the payload attached to the linear drive. For example, the need for a more gentle reversal arises when spooling delicate materials that could break or distort if the reversal is too sudden.
Meeting winding application requirements for ramping up or down during the reversal process normally involves designing-in clutches, cams and often complex control systems. However, the performance characteristics of rolling ring bearings make rolling ring linear drives uniquely adaptable to relatively inexpensive, mechanical modifications to the auto-reverse mechanism in order to lessen the intensity of reversal.
The rolling ring linear actuator manufacturer or value-added distributor is best equipped to engineer such modifications to the reversal mechanism. But a basic understanding of rolling ring bearing design and performance characteristics is helpful when considering this technology in setting up a system for smooth, automatic reversal in spooling applications.
A Machined Inner Race is the Key
A standard ball bearing and a rolling ring bearing look the same. Examining the inner race of the rolling ring bearing, however, shows that the surface has been machined (Figure 2).
Machining a standard bearing to make a rolling ring bearing is a precise, proprietary process. Once machined, the bearing has a contoured, central "ridge" running around the entire inner race surface, while a ball bearing has a smooth, flat inner race surface. It is the presence of this central ridge that gives rolling ring bearings their performance characteristics.
Mounted on a shaft, a standard ball bearing's purpose is typically to serve as a friction-reducing device in a rotating assembly, like a wheel. Shaft-to-ring contact is across the full surface of the standard bearing's inner race. As the shaft turns, the inner rotating core of the bearing absorbs friction as it turns on the balls in the raceway. But a rolling ring bearing mounted on a shaft contacts the shaft only on the apex of the central ridge on the bearing's inner race. There is clearance between the shaft and bearing on either side of the ridge.
The shaft clearance permits the rolling ring bearing to be "pivoted" left or right on the shaft, and still maintain point contact with the shaft. If the inner race was flat, as in a standard ball bearing, it would be virtually impossible to pivot and angle the bearing because there would be no clearance between the shaft and the inner race surface (see Figure 3).
Pivoting a rolling ring bearing on a rotating shaft so it is at an angle relative to the shaft, generates force against the bearing's central ridge. This causes the bearing to "roll" along the length of the shaft. The rotary input provided by the motor-driven shaft is thereby converted to linear output.
The housing or nut enclosing the rolling ring bearings moves with the rings and bears the payload. The linear direction in which the drive moves is determined by the adjustable angle at which the bearings contact the shaft.
In a rolling ring linear drive, like the one seen in Figure 4, an assembly of three or four rolling ring bearings is positioned within the drive housing. The ring assembly is set at an angle relative to the shaft to determine the traversing direction.
To reverse the traversing direction of the rolling ring drive, the entire rolling ring bearing assembly must be pivoted to its opposite or "mirror" position on the shaft. The bearings' central ridge provides the pivotal point on which the rolling ring bearing assembly may be pivoted.
The process of pivoting the ring assembly is purely mechanical and automatically controlled by a spring-actuated reversal mechanism on the bottom of the linear drive. The mechanism is attached to the rolling ring bearing assembly.
When the linear drive reaches the end of its stroke, end stops are used to trigger the reversal mechanism. End stops can be screws, bolts or air cylinders. They may be positioned anywhere along the shaft to cause reversal at any point desired. When reversal is triggered by contacting a stop, the angle of the rolling ring bearing assembly on the shaft changes. If the rolling ring bearing assembly is pivoted so the rings assume the exact opposite angle on the shaft, the result is immediate and automatic reversal (see Figure 5).
Controlling Linear Speed Independent of the Drive Motor
Besides controlling the traversing direction of the linear drive, pivoting the rolling ring bearing assembly to different angles relative to the shaft also determines the drive's pitch (linear distance traveled per one shaft revolution). Adjusting the pitch setting affects the linear travel speed of the drive relative to each revolution of the linear drive shaft, even if the drive motor speed remains unchanged.
For example, increasing the angle of the rolling ring bearing assembly on the shaft increases the rolling ring drive's linear speed per shaft revolution. The greater the angle, the faster the linear drive will move on the shaft relative to each shaft revolution. Increasing the angle (pitch setting) therefore causes the linear drive to cover a longer linear distance per shaft revolution. Conversely, decreasing the angle of the ring assembly decreases the pitch and the drive moves slower, and covering a shorter linear distance per shaft revolution.
It is important to note that these changes in linear speed and linear distance traveled by the nut take place without any adjustments to the motor speed or shaft rotation direction. This means a variable pitch system may be free of clutches, cams, gears and so forth.
In a rolling ring linear drive, the pitch or linear speed per shaft revolution, may be changed while the drive is running. Mechanical stops and levers are used to pivot the rolling ring assembly on the drive shaft which changes the drive's pitch. If the ring assembly is pivoted to be perfectly perpendicular to the shaft, pitch is essentially zero. Then, although shaft-ring contact is still perfectly intact, the rolling ring bearings turn on the shaft without causing linear movement (see Figure 6).
Mechanical Methods for Controlling Linear Speed & Reversal
The most common rolling ring linear drive setup is for automatic, instantaneous reversal (see Figure 7). In such a system, the rolling ring linear drive traverses in one direction until it contacts an end stop. The end stop flips the reversal mechanism, which pivots the ring assembly inside the housing to its mirror position causing reversal of the traverse.
While this reversal method is typical for winding applications, correctly exploiting the rolling ring bearing performance characteristics permits the use of a rolling ring linear drive to meet a variety of other linear speed requirements in reciprocating linear motion applications. Rolling ring linear drives readily enable these processes through purely mechanical means.
A rolling ring linear drive is typically supplied within a production framework (see Figure 8). The assembly is "dropped" into the manufacturing line. Adjustable stroke stops come installed on the assembly to control stroke length. Various hardware fixtures may be attached to the reversal mechanism to meet requirements for ramping up or down. The most typical application requirements are:
- Ramping down before reversal.
- Ramping up after reversal.
- Ramping down before reversal; ramping up after reversal.
A simple option for ramping down a rolling ring linear drive before reversal is called the K-Stop. This is an adjustable stop which partially rotates the reversal mechanism just before it hits the final end stop. As seen in Figure 9, the K-Iever contacts the stop and partially rotates the rolling ring assembly. This reduces the drive's linear speed (pitch) before the reversal mechanism is triggered.
If desired, the K-Stop may be configured to rotate the reversal mechanism so that the rolling ring bearing assembly is perfectly perpendicular to the shaft. This gives the linear drive a pitch of zero. The drive dwells on the rotating shaft with no linear movement until the ring assembly is again angled on the shaft. This is usually accomplished via an air cylinder or other device which is used to actuate the reversal mechanism.
Slow-Down & Ramp-Up
An application may call for deceleration (ramp-down) prior to reversal and then acceleration (ramp-up) after reversal. To do this, rolling ring linear drives use an H-stop to control manipulation of the drive unit's pitch (see Figure 10).
In this set-up, the H-Iever contacts the stop and begins to pivot the rolling ring assembly. The linear drive slows down. After reversal has been tripped, the other end of the H-Iever catches a second stop that prevents the rolling ring assembly from completely assuming its mirror position on the drive shaft.
In this case, reversal has been completed, but the rings are still being held at an acute angle and have not pivoted all the way on the shaft. Therefore, as the drive traverses back in the opposite direction, it does so at a reduced speed. As the linear drive moves, the H-Iever is gradually pulled away from the stop and the ring assembly is permitted to assume its fun pitch position. The linear drive ramps up to full linear speed.
For Best Results, Ask the Experts
In some cases, even rolling ring drive systems require extra controls to meet specific winding application requirements. An example is in situations requiring two-way shaft rotation.
Rolling ring drive systems may also be PLC-controlled if the take-up pattern required is unusually intricate. A PLC-controlled rolling ring drive setup is illustrated in Figure 11.
Special applications like these require the attention of rolling ring engineering experts. If you use a rolling ring linear drive or are considering doing so, consulting with a rolling ring linear motion engineering firm can help you make sure your production process receives the full benefits this technology has to offer. An experienced rolling ring applications engineer can guide you in designing, fabricating and installing the appropriate system for your application requirements. This is your best bet for ensuring that the rolling ring system you design enhances your production process and reduces maintenance and operating costs.