The (Load) Path to Ball Screw Reliability

Design considerations from aviation experience

—by David A. Lange, Director, AGD Product Marketing
Danaher Motion — Linear Motion Systems

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. In the high dynamics of aviation, actuation and operation of flight control surfaces have become more sophisticated and demanding as flight stresses have increased. The trend has been to supplement the pilot’s physical strength and coordination with electronics, hydraulics, mechanics and computers.

Aircraft designers must weigh a number of trade-offs and variables when choosing the methods and equipment used to changing the aircraft flight path. The advent of 8,000 psi hydraulic systems was thought to be the next step. It has been found, however, that these systems incur accelerated seal wear, require totally redesigned hydraulic fluids, increased control system weight, and are unable to maintain control position if hydraulic feed lines are severed.

Reliability and load path

Since the first ball screw linear actuators were designed and built by the Saginaw Division of General Motors for landing gear on the B-29 Superfortress, aerospace engineers have demanded high reliability in the ballscrew’s development. Redundant load paths 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. 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.

Load path design considerations

To further enhance reliability and safety of ballscrew assemblies, there are several load path designs available for consideration:

Multiple-start ball threads — Where the lead will allow, multiple-start ball threads, with ball circuits in each start, 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 increase reliability. They can be used 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.

Structural deflectors — When required, specially designed “yoke type” ball deflectors can be used for load-path redundancy. 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. 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. 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 & pitch equal to the ballscrew lead and a cross section slightly smaller than the bearing ball diameter. Available in several materials, these inserts are allowed to float radially and axially during normal operation.

Design considerations

Several parameters must be considered when selecting ballscrew assemblies with redundant load paths:

• 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 wear-in of materials in contact.

• Decrease in system efficiency with engagement of redundant load path or paths — The frictional characteristics of the potential redundant load-path-and-ballscrew 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, one 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.

More about load path integrity

Historically, ballscrew assemblies have been used primarily to react pure axial loads. The components used in these standard ballscrew assemblies can be damaged, and life greatly reduced, if an excessive radial load, overturning moment, or combination of both is applied. 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. In this circumstance, 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.