One of the most common issues related to poor performance of Rivnut® fasteners can be attributed to the improper method of installation. The Rivet nut industry had introduced many new power tools over the last 2 decades and offer more choices than ever before, making it difficult to determine how to select the proper installation tool.
The OEM installing the fastener may also be limited on choice based on their supply chain. Each manufacturer has their own recommendation based on their brand and type of Rivet buts offered. The intent of this article is to educate manufacturing engineers on the different types of tools available and provide an outline of various factors that should be considered when purchasing Rivetnut® installation tools.
I would also add, that it is always recommended to seek an experienced source of supply when selecting Rivet nut fasteners. Care should be taken to select a source of supply with access to a wide range of tools and fasteners, capable to provide experienced fastener assistance, and SUPPORT the total assembly process.
Considerations: Although cost is always a consideration, it should be used only in comparing "like type" tools. A properly installed Rivet nut provides the highest performance consistency. A fastener "under upset" has a greater potential to spin, while "over-pull" the fastener may damage the threads, and impede core screw installation. Both conditions increase the performance.
Type & Size of Fastener - Proper tool selection correlates to the type and size of fastener being installed. Each tool has limits to the force that is generated to properly upset (install) the fastener. The force required is impacted by:
Type: Round, Full Body Hex, Length, Thin Wall or Standard - The wall thickness, shape and length of part requires different amounts of force to move the rivet nut and properly set it in the parent material. The rivet nut provider should be able to provide the amount of force required to upset their brand of fastener. Plusnuts®, or slotted rivet nuts are longer and require more stroke.
Size: The larger the thread size, the more force required to upset the part. The most common tools available today generate up to 5,000lbs of force. Generally, enough to install most fastener types up to 3/8-16 thread size. Larger thread size rivet nuts and full body hex parts may require more force and needs to be considered during tool selection. Sherex offers a tool capable of pulling 18,000lbs to install their large thread size series.
Range: It is important to understand the range of parts that the tool can install. Care should be taken to select a tool that can install the widest range of parts that is intended to be used. Some manufacturers offer lighter weight tools, but may only generate enough force to install up to 1/4-20 thread size.
Method of Installation - Proper understanding of the different methods available will prevent potential installation and operator issues.
Pull to Stroke - Common among older models, and limited to installations with consistent material thickness. The tool is pulling to a distance that is specific to the grip range of the fastener, and the gauge of material being used. More specific installation instructions is required to assure the operator has the tool properly set. Damage to the tool or fastener may occur if the operator "double upsets" the part.
Pull to Force - Common among most newer tools and typically the most operator friendly tool. The tool is set to a specific force for the fastener being installed, and can be installed in various thickness (gauge) of material without re-adjustment. This method eliminates over/or under pulling the fastener, which is one of the primary causes of down time and fastener performance issues.
Commonly referred to as Spin-Spin: This method is commonly used, but is really limited to installing thin wall (L-Series, K-Series) type fasteners only, and requires the most care in mandrel maintenance. This type of tool is NOT pulling on thread, but applies a torque to threads that cause part to collapse. Tool torque generated is limited to specific thread size and multiple tools may be required to install various thread sizes, as tool repair and air supply need to be properly selected for each thread size.
Weight / Ergonomics - Operator comfort should be considered to precent fatigue, potential carpel tunnel issues and proper access to the application. The rotary tool is pistol shaped and offers the lightest weight. Most direct pull tools today are 4.5lbs or less. Newer direct pull tools offer balanced handle for ergonomic comfort.
Trigger Function - direct pull tools typically have either a single or double stage trigger. Most common tools today offer self-threading, single stage trigger that upsets part, and automatically reverses from part. Rotary tools have a toggle trigger that spins forward to install, and reverse to exit from the part.
Wear Items - Excess wear creates frequent down time, reducing productivity. Rotary tools require more frequent mandrel and bearing replacement than Direct Pull Tools. Direct Pull Tools are Hydro-Pneumatic and require proper oil maintenance. Clean air supply is recommended for both types.
Power Source - Hydro-Pneumatic and rotary tools require a minimum of 90psi air supply to install properly
Several manufacturers have introduced battery powered direct pull tools to the market. Bollhoff recently released the B2007 Electro-Hydraulic tool capable of 600-800 cycles with 1 battery. Industrial Rivet introduced a "Smart" process monitor batter tool capable of Wi-Fi data transmission, with data logging available.