Today, engineers are successfully attacking problems from vibrational loosening to joint fatigue with a self-locking fastener called Spiralock, whose effectiveness has been validated in published test studies at leading institutions including MIT, the Goddard Space Flight Center, and British Aerospace. It has been used in extreme fastening applications, from medical implants, to the main engines of NASA’s Space Shuttle, to the Saturn Cassini orbiter and Titan Huygens probe.
Resolving Design Limitations
In traditional fasteners, the radial clearances between traditional male and female 60° “vee” threads permit relative sideways or lateral movement when shock, vibration, or transverse loading occurs. Stress concentration and fatigue at the first few engaged threads is also a problem, along with an increased probability of shear.
As a result, locking devices from wires and washers to chemical and drypatch adhesives are commonly added to retain clamp load. While adding weight and complexity, however, these methods do not always hold up under extreme conditions.
Testing at a wide range of leading institutions has demonstrated the effectiveness of a locking fastener called Spiralock. The standard 60-degree thread form was replaced with a unique 30- degree wedge ramp at the root of the thread that mates with standard 60- degree male thread fasteners.
The wedge ramp allows the bolt to spin freely relative to female threads until clamp load is applied. The crests of the standard male thread form are then drawn tightly against the wedge ramp, eliminating radial clearances and creating a continuous spiral line contact along the entire length of the thread engagement. This continuous line contact spreads the clamp force more evenly over all engaged threads, improving resistance to vibrational loosening, axial-torsional loading, joint fatigue, and temperature extremes.
Resistance to Vibration
In dynamic and static testing by Goddard Space Flight Center, Spiralock nuts (stainless alloy A-286 and alloy steel) were tested under vibration and static load conditions. The most severe vibration tests (Sine: 24.7 Hz – 2G and Random 20-400 Hz – 2 G RMS) did not loosen the nuts when subjected to both high amplitude and sine random testing.
The Tinius Olsen tension machine was used to pull the bolts in tension and calibrate the strain in the bolt for a given load. Static tests were performed to measure gapping and to determine the friction constant for both the lubricated and unlubricated nuts.
Vibration testing of Spiralock wire thread inserts by British Aerospace: Naval Warfare Division confirmed the locking fastener’s resistance to vibration. Testing was done on an Un brako Fastener Vibra - tion Machine using M6 × 25mm grade 8 bolts with wire inserts in L168 aluminum, and 20 samples of both Spiralock wire inserts and standard (60 degree UN thread) wire inserts. These were tested at 13.6 Hz and tightened to 1800 pounds of preload.
Results showed that Spiralock wire inserts yielded consistent vibration resistant performance with an average preload loss of 15%, while the standard wire inserts yielded erratic results, losing from 22% to 95% preload given the same test parameters.
Resistance to Joint Fatigue
A report by the Massachusetts Institute of Technology for Chrysler Corporation similarly studies vibration resistance and stress distribution in threaded fasteners. It compares the Spiralock thread form (on the nut) and the standard 60-degree thread form, both in combination with a standard 60-degree thread form bolt, on two counts; the load and stress distribution on the threads, and the resistance to relative rotation between the nut and the bolt.
According to the report, calculations show: the total bolt load is more evenly distributed over the engaged threads for Spiralock than the 60-degree thread form; the maximum stresses at the root of the bolt thread are of the same order of magnitude in both cases; and the movement required for relative rotation is significantly higher for Spiralock.
The Align™ Radial Head System from Skeletal Dynamics (Miami, FL) is one example of a medical technology that utilizes this self-locking fastener. This artificial elbow joint is designed to restore the natural function of the native radial head. Previously, prosthetic radial head designs followed one of two approaches, with significant drawbacks. While a traditional fixed mono-block design offered stability, it could not be aligned to the patient’s anatomy, which tended to wear away natural tissue such as cartilage. A bipolar radial head was an attempt to align with the patient’s native anatomy, as it was able to rotate in a polyethylene sheath, but would not remain in the correct position because it would not lock.
Because repetitive loads, shock, and loosening must be decisively handled for implant use, traditional fasteners susceptible to self-loosening rotational movement, stripping, and shearing are not always appropriate. Testing, in fact, has found that the first two threads of traditional fasteners can carry as much as 80% of the load, permitting stripping or shearing, while subsequent male threads “float” within the female threads.
When Skeletal Dynamics’ Align™ Radial Head System is surgically installed in a patient, proprietary instrumentation allows alignment of the radial head as it would be in the patient’s native anatomy. Once the surgeon orients the device in this natural position, the surgeon tightens the set screw in a Spiralock milled interrupted thread made of cobalt chrome, against a long titanium stem designed for 3-point fixation, to lock the device in the correct position.
In the medical field, because of their vibration-resistant, reliable self-locking features, the fasteners hold various components together in implants, artificial limbs, heart pumps, and MRI machines, and are also being considered for cardiovascular devices such as pacemakers and implantable defibrillators, as well as for dental and orthopedic surgical instruments and CT scanning applications.