This independent study compares threaded and lockbolt fasteners for connecting permanent PV racking system joints. Once again, HuckBolts are proven to offer distinct advantages over traditional threaded fasteners.

A Tale of Two Solar Racking System Fasteners: Threaded vs. Lockbolt

Author: Jon Ness, Principal Engineer,
Matrix Engineering Consultants
April 2020 

Executive Summary

The important role of fasteners in ensuring safe and reliable solar racking and tracking structures is critical but often underappreciated. There are significant differences in strength and performance between the two main types of fasteners—threaded and lockbolt. This paper compares threaded and lockbolt fasteners and finds that lockbolts offer distinct advantages over traditional threaded fasteners for connecting permanent PV racking system joints.

Situational Analysis

Some of the most overlooked components within a photovoltaic (PV) racking and tracking system are the fasteners. Although relatively simple and inexpensive components, fasteners play a critical role in the structural integrity of PV systems. Each fastener connects two or more components to create a securely fastened joint which transmits both static loads (live, dead and snow) and dynamic loads (wind). In some systems, the joint also serves as a low-resistance electrical conduction path for grounding purposes. A typical PV system contains thousands of fasteners, underscoring the critical role they play in the structural reliability and safety of the system. A wide variety of fastener designs and materials are used in PV racking system joints and they can generally be categorized as either traditional threaded fasteners (nuts and bolts) or lockbolts.

A Tale of Two Solar Racking System Fasteners

Download the White Paper to read more.

 

Comparison of Torque-Tightened Threaded Fasteners and Lockbolts

 

Torque-Tightened Threaded Fastener

Lockbolt

Initial Preload Scatter

Preload scatter of a torque-tightened threaded fastener and range from ±20% on the low end to as much as ±60% on the high end. The preload scatter of lockbolts is generally considered to be approximately ±5%

Assembler Training Requirements

Assembler skill plays an important role in controlling preload scatter so ongoing training is generally required. Generally minimal.

Speed of Assembly

Highly dependent on the skill of the assembler and their tools. Lockbolts can often be assembled in less than 5 seconds.

Risk of Galling of Stainless-Steel Fasteners

High Minimal because of the direct tension method.

Resistance to Self-Loosening

Not resistant to self-loosening unless wedge lock-washers or pre-applied, high strength thread adhesives are used. Given the nature of their design, lockbolts are immune from the self-loosening since the internal and external threads are crimped(locked) together during the assembly process.

Residual Torque Auditing

Residual torque audits have been shown to be effective if conducted by a trained inspector. It is not possible to conduct residual torque audits if high-strength threadlockers and adhesives used. Witness marks on the crimped collar provide a visual indication that the minimum preload wasachieved during tightening, making auditing of lockbolts quick andeasy.

Ability to be Retightened

Yes, unless if high-strength threadlockers and adhesives used. No

Tightening Tool Cost

Relatively low. The expense of the special installation tools and equipment required for assembly could be uneconomic for small quantities but that is generally not an issue with PV racking systems given the number of fasteners.