Reliability often requires ballscrew assemblies with internal redundant load paths that can support design loads in the unlikely event of primary load path failure
Since ballscrews were first developed commercially in the 1940s, they have served in almost every use imaginable: aerospace, military, agricultural, industrial, automotive, marine, mining, medical, recreational, and nuclear, to name some. Uses such as military aircraft (see this month’s cover) demanded high reliability early in the ballscrew’s development.* Redundant load paths are one means of assuring high reliability. They have allowed ballscrew applications to expand to increasingly severe environments where other linear motion devices have proved unsatisfactory.
“Load path” refers to path by which load transfers from the ball nut to the ballscrew shaft. In normal operation, and by design, the primary load path in a ballscrew assembly is the circuit (or circuits) of bearing balls within the ball nut, Figure 1.
Without a redundant load path, the ball nut would be free to slide along the ballscrew shaft if all of the bearing balls were to be lost from the assembly. Although unlikely, bearing balls can be lost due to:
• “Misrigging” of the ballscrew assembly in its application, causing the ball nut to either contact some external structure and damage or separate the ball-return system components, or to run off of the ball thread portion of the screw shaft — for example, onto an adjacent smaller diameter of a stepped shaft.
• Extreme accumulation of debris in the ball-return system, causing the ball circuits to jam up, resulting in damage to or separation of the ball-return system.
•Misassembly of ball-return system components in the field.
• Extreme corrosion or wear in the ball grooves of either the ball nut or ballscrew shaft, or extreme corrosion of the bearing balls, or all of those.
Getting redundant load paths
To enhance reliability and safety of ballscrew assemblies, we have used the following redundant load path designs:
Multiple-start ball threads. Where the lead will allow, multiple-start ball threads, with ball circuits in each start as in Figure 2, provide redundancy. A comprehensive DFMEA analysis and design study can help predict the “cascading” margins of safety if one or more circuits are lost from a multiple-start assembly. For ultimate reliability, each individual circuit should be designed to react operational loads independently.
Multiple circuits. For the same reasons as multiple-start ball threads, multiple circuits as in Figure 1 increase reliability. You can use them with either single or multiple- start ball threads. One recent design consisted of a double-start ball thread with two circuits of bearing balls in each ball thread. For maximum reliability, the circuitry was designed such that with one of the four circuits lost, the assembly would actually survive more than twice the design intent of 40,000 cycles, Table 1.
Structural deflectors. When required, specially designed “yoke type” ball deflectors can be used for load-path redundancy, Figure 3. The ball groove geometry, deflector cross section, and material are all designed to maximize load capacity in the event that all bearing balls are lost and the deflectors engage the screw shaft in Acme fashion — thread to thread. In normal operation, the deflectors are in clearance with the screw shaft.
Structural scraper/wiper system. Scraper/wipers, assembled in each end of the ball nut, can also be designed to react structural loads and work as an Acme type male thread if engaged with the screw thread, Figure 4. Ordinarily in clearance with the screw thread, structural scraper/wipers carry load only if the system’s bearing balls are lost.
Integral inverted groove. For extremely high structural integrity, an inverted groove form can be machined integral to the ball nut, Figure 4. The geometry, localized heat treatment, and finish of this groove form can be designed for optimum strength and wear life. Should all bearing balls be lost, this inverted groove form engages the screw shaft. Because it is integral, there is no concern for reliability of fasteners, assembly techniques, and so forth.
Load path inserts. Depending on the design envelope available, several versions of inserts can be assembled to the ball nut to provide load path redundancy. One of the most economical and simple inserts resembles a spring with a pitch equal to the ballscrew lead and a cross section slightly smaller than the bearing ball diameter, Figure 5. Available in several materials, these inserts are allowed to float radially and axially during normal operation.
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Where extreme reliability is required, we have designed as many as five redundant load paths into one ballscrew assembly. When you take such action, you must consider several parameters:
• Relative “timing” for engagement of the redundancies. the geometry and tolerancing of each redundant feature must be designed to support the intended order of engagement, such as scrapers first, deflectors second, integral thread third, and so forth.
• Material, heat treatment, and plating selection of redundant features. Intended fatigue and wear life of the redundant load paths when actively engaged must be accounted for during original design. Consideration must be given to the preferential wear and anticipated wearin of materials in contact.
• Decrease in system efficiency with engagement of redundant load path or paths. Frictional characteristics of the potential redundant load-path-andballscrew sliding couple must be known. Ideally, design validation tests should be made to verify system efficiency, heat generation, and predicted life of the ballscrew assembly when it operates on redundant load paths. When specifying a drive system for a ballscrew assembly, you must consider the potential increased power requirements if redundant load paths are engaged.
• Structural integrity of redundant load path. Comprehensive static and fatigue stress analysis should be documented. Tests to support this analysis are recommended.
Table 2 offers some guidelines when considering what types of redundant load path to use in a given design.
More about load path integrity
Ballscrew assemblies have historically been used to primarily react pure axial loads. The componentry used in these standard ballscrew assemblies can be damaged, and life greatly reduced, if a radial load, overturning moment, or combination of both is applied. Our experience and testing show that equivalent radial loads exceeding 10% of the applied axial load can damage a conventional ballscrew assembly.
Where radial loads or overturning moments may be applied inadvertently to the ballscrew assembly, circuitry specifically for these loads can be designed. Here, one portion of the circuitry reacts radial loading and is not engaged axially, and the remaining circuitry reacts axial loads and is not engaged radially, thus reducing friction and wear under this combination loading. n
Redundant load paths in ballscrews from Thomson Saginaw Ball Screw Co.
*See also, PTD, “Down-to-Earth Linear Actuation From High in the Sky?” 2/93, p. 56.
David A. Lange is Manager, Product Engineering, Thomson Saginaw Ball Screw Co., Saginaw, Mich.
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