Articles
Split Lock Washers Proper Use
-
Abstract
Helical spring lock washers, aka split lock washers, can be used with low strength fasteners such as Machine Screws, ASTM A307 Grade A, SAE J429 Grade 2, and ISO 898-1 Class 5.8. However, split lock washers are not advised to be used with higher strength fasteners. Under high clamp loads required by heat treated fasteners (Grade 5, Class 8.8 and above) split lock washers will tend to open at the split. Despite their name, split lock washers do not prevent vibration loosening. -
Discussion
Split lock washers find their utility when used with non-critical, low strength, low carbon steel fasteners and with low carbon, low strength steel mating material (sheet/plate). When split lock washers are used in this type of application, the tangs are permitted to embed into the fastener and mating material to assist in locking the joint; that said, the tangs do not always embed. Though a split lock washer does have a helical nature, the spring effect induced on the joint is minimal (approximately 5 to 10% of the fasteners clamp load). This utility of split lock washers differs slightly from NASA’s Fastener Design Manual (March 1990) which states:
“The lockwasher serves as a spring while the bolt is being tightened. However, the washer is normally flat by the time the bolt is fully torqued. At this time it is equivalent to a solid flat washer, and its locking ability is nonexistent. In summary, a lockwasher of this type is useless for locking.”
Figure 1: Image taken from NASA Reference Publication 1228, Fastener Design Manual, 1990, page 10. The left washer shows tang ends while the right shows square ends.
DIN 127 titled Spring Lock Washers with Square Ends or Tang Ends states in the scope the washers are designed to be used with “fasteners of property class 5.8 or less…” and goes on to say “[Spring lock washers] do not effectively prevent loosening of the assembly under varying radial load and are designed for use with short bolts predominantly subject to thrust.” This statement was written into DIN 127 at some point before its withdrawal in 1987.
Compounding the doubt of using split lock washers with high strength fasteners is the possibility the recommended clamp load causing the washer to open at the split. While it is uncommon to observe this phenomenon in practice, it has occurred in such frequency that it is worth mentioning herein. When the washer opens like this the washer plastically deforms into various degrees of a C-shape. The fastener then experiences eccentric loads as there is no longer an even distribution of stress. The washer has lost all spring function and because of its new geometry, the joint is at greater risk of loosening versus a standard flat washer. -
Conclusions
In summary, split lock washers used in non-critical, low strength, placeholder applications are perfectly acceptable. For critical applications, or, applications where high strength fasteners are used, split lock washers are useless and/or can be detrimental to the functionality of the joint. -
Solution
A holistic approach should be taken to determine the appropriate locking product for an application. Applications could be, e.g. high heat, low heat, galvanic corrosion concerns, general corrosion concerns, shock loading, vibration loading, thermal loading (coefficient of thermal expansion variation in joint members). Most commonly vibration resistance is the primary concern; therefore the options include but are not limited to: (1) pre-applied or point-of-use thread lockers (e.g. Loctite, ND Industries), (2) two piece wedge lock washers (e.g. Nord-Lock, Heico-Lock, Sherex TEC Series™), (3) serrated disc spring washers (e.g. Schnorr® safety washer). Contact your fastener consultant for the appropriate locking products for your application needs.
Respectfully,
Tyler Olson
Fastener Enthusiast
Polycrew Contributor
Caution: All information in Polycrew Articles is advisory only and the use of this information is voluntary. Polycrew has made a determined effort to present the contents accurately.
