Fastener Technology

Rivet nuts were discussed in a previous blog, but what tools are required to install this type of fastener?

Hand Tools

Smaller rivet nuts can easily be set by hand using simple tools. The nuts are threaded onto a mandrel by hand, inserted into the hole and then using a mechanical lever action, the mandrel is pulled into the tool causing the nuts to collapse on the back side of the hole. The tool must then be manually rotated in reverse to disengage from the nut.

Spin/Spin Tools

These tools are typically pneumatic and have three stages. The first spin assembles the nut onto the threaded mandrel. The nut is then placed into the prepared hole, and the second stage spin collapses the nut on the backside of the hole creating a bulge. This bulge squeezes the material, and secures it into place. The third stage reverses the rotation and backs the mandrel out of the nut. These tools are the lowest cost power tools, but can be a bit fussy about hole size. Too large of a hole can allow the rivet nut to spin and not collapse. The pressure that the nut applies once the bulge is formed is controlled by the amount of torque that the gun creates, so consistency can be an issue if air pressure and/or friction in the threads varies.

Spin/Pull Tools

These tools also have two stages and are typically pneumatic over hydraulic. The first stage spins the nut onto the mandrel while the second stage does not rotate, but instead pulls the mandrel into the tool collapsing the rivet by force. This tool comes in two versions:

• Pull to pressure – the amount of setting force is set by hydraulic pressure, which can be adjusted on the tool. This style works best if the material thickness is not consistent.
• Pull to stroke – the amount of setting force is set by the distance of the stroke, which can also be adjusted on the tool. This type of tool works well for materials that are very consistent in thickness.

Some high-end tools incorporate both pull to pressure and pull to force.

For questions about rivet nuts and or tooling, check out www.bossard.com or contact Bossard directly at ProvenProductivity@Bossard.com.

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Doug Jones
Applications Engineer
djones@bossard.com

A rivet nut is a special type of nut that is installed into a prepared hole in a piece of sheet metal and secured prior to assembly of a bolt. A special installation tool is required to ‘set’ the nuts into the prepared hole. The nuts are placed into the hole and ‘upset’ on the backside (much like a blind rivet) which secures them, eliminating the need for a wrench.

Standard rivet nuts have been around for years, and work well as attachment points for low strength joints that do not see much stress. These standard nuts are not high strength and generally cannot support enough clamp load for structural joints.

Uses

Standard rivet nuts must be used with through holes in harder materials such as steel or aluminum and do not work as well in softer materials like wood or plastic. The bulge formed on the backside must have unrestricted space to form and will apply pressure to the plate which could overly stress plastic or wood.

Specialty type rivet nuts exist, such as the BCT®* which employ bulge control technology to allow them to work in blind holes and/or soft materials such as plastic. There is also a high strength version of BCT® which may be used in structural joints and can create high clamp loads for use with high strength bolts.

If you are currently using weld nuts, or have areas with limited access to one side, consider looking at rivet nuts. Contact us at ProvenProductivity@Bossard.com for more information. Look for a future blog discussing tools for the installation of rivet nuts.

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*BCT® is a registered trademark owned by BBA srl Italy

How many threads should protrude through a nut in an optimal joint?

The generally accepted answer is 1 to 3 threads. Most externally threaded fastener blanks are manufactured with a header point prior to thread rolling, which leaves the first 1 to 2 threads undersized for ease of assembly. To ensure full load carrying capability for a nut and bolt combination, this rule makes sure we have fully formed threads throughout the entire thickness of the nut.

One notable exception to this is for nuts with a locking feature at the top – or toplock nuts. This includes all nylon insert nuts. For these, it is best to have a minimum of 3 threads protruding through the nut to ensure that the locking feature is engaged on a fully formed external thread. Anything less could compromise the locking affect.

Is there a rule for the maximum number of threads protruding through a nut?

Too many threads is a waste of material, adds unnecessary weight and can be a hazard or cause interference with other components. However, functionally there is no downside to having too much thread protrusion.

When selecting fastener lengths, be conscious of the standard length increments. Metric fasteners are generally available in 5mm length increments up to 70mm and 10mm increments beyond this. Inch fasteners have similar standards. When choosing your fastener lengths, it is best to select the shortest fastener that will consistently give you 1-3 threads protruding through the nut.

For questions, please contact our Bossard engineering team at ProvenProductivity@Bossard.com.

Doug Jones
Applications Engineer
djones@bossard.com

Thread forming screws per DIN 7500 produce a chip-free, gage correct metric internal thread into ductile metals up to 160HV hardness. Thread forming screws may be reused up to 20 times, but care must be taken when re-starting screws to avoid cross threading. They are not intended for use in brittle metals such as gray cast iron; however, they can be used in non-ferrous metals and light metals, as well as steel.

Screw Hole Preparation

Hole preparation and hole size are key to trouble free assemblies with thread forming screws. In general, the material thickness should be 1xd minimum (for example, a 6mm plate for a M6 screw). Punching, drilling or laser cutting are common methods for creating through holes for thread forming screws. Different hole sizes may be required for each method based on the break-out of a punched hole, or the size of the heat affected zone of a laser cut hole, which makes the surface harder and more difficult to thread. Different material composition and thickness will also require different hole sizes.

Good design practice requires a countersunk hole as thread forming will slightly raise the surface of the mating part without the chamfer. This can cause mating parts to not sit flush against the tapped component creating a gap.

Assembly Torque

Recommended assembly torque is 80% of the breaking torque of the screw, and half way between the driving torque and the breaking torque. Drive torque and strip/break torque is recommended to select the optimum hole size for your application.

Check out www.bossard.com for more recommendations for thread forming screws, or contact us directly at ProvenProductivity@Bossard.com to perform a drive/strip torque test for your application.

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Doug Jones
Applications Engineer
djones@bossard.com

Are you thinking about using stainless steel fasteners in your next application but aren’t sure if it’s the right fit? Read on to see if it’s the right fit for your product.

Austenitic Stainless Steel

Austenitic stainless steel is by far the most common material used to manufacture stainless steel fasteners. It has a chromium content between 15 and 20 percent and a nickel content between 5 and 19 percent. It offers the highest corrosion resistance than martensitic stainless steel and ferritic stainless steel. It is also considered to be non-magnetic. The tensile strength may vary between 72,000 psi and 115,000 psi. Austenitic stainless steel is not heat treatable however the strength of this material may be improved by cold working or strain hardening.

18-8

The material that is commonly referred to as 18-8, is a type of austenitic stainless steel that contains approximately 18% chromium and 8% nickel. Austenitic stainless steel includes AISI grades 302, 303, 304, 304L, 316, 32, 347, & 348.

For more information about stainless steel fasteners and how you can use them in your applications, contact us at ProvenProductivity@Bossard.com.

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Do you have a fastened joint that keeps coming loose? Before proposing a solution, we first need to understand why it’s coming loose.

Reasons for a Fastened Joint Being Loose

1. Is there visible damage to the surfaces under the head of the bolt or nut? If so, you may have an embedment issue where the surface of the clamped material is not hard enough to support the load of the joint. The best way to address this is to increase the surface area of the fastener by using hardened washers or flanged hardware.
2. If embedment is not your issue, but you have loose hardware, then rotational loosening is occurring. Some things to check for:

• What is the clamping length of your joint? 5 times the diameter of the bolt is recommended to ensure proper bolt stretch.
• Is the proper clamp load being applied? See your hardware supplier to help with bolted joint calculations or a joint study to determine the proper torque to achieve your desired clamp load.

Fixing the Loose Fastened Joint

 Sometimes it is not possible to employ the 5 times the diameter rule, and we still have loosening. Several strategies may be used to help combat this issue.

1. Thread locking
   • Thread forming screws
   • Pre-applied locking adhesive
   • Liquid thread lockers applied during assembly
   • Lock nuts
2. Locking at the bearing surface
   • Serrated hardware – make sure to use serrated nuts AND bolts
   • Locking washers
     • Rip-Lock™ – good for up to and including 8.8 property class
     • Ribbed lock washers – good for 10.9
     • Nord-Lock® – good for up to and including 12.9 property class

For more information on keeping joints tight, check out www.bossard.com or contact our engineering department at ProvenProductivity@bossard.com.

For more shopping options click here.

Doug Jones
Applications Engineer
Email: djones@bossard.com

Many companies that have been designing with inch (imperial) fasteners for years are very hesitant to switch over to metric. Why would anyone ever use metric hardware? Where we have seen the most interest is from companies who are expanding into global markets. Most countries other than the US are using metric fasteners and for them, imperial fasteners are a mystery.

Metric is simple once you get the hang of it, and some things even make more sense, such as the marking of property classes (grades) on nuts and bolts. The metric system uses numbers stamped into the head or face of the nut rather than symbols that we use for inch hardware. For bolts, the first number indicates the strength in MPa while the second number tells us the percentage of yield strength.

For example:

Property class 10.9 – 10 indicates a tensile strength of 1000MPa, 9 indicates that the yield strength is 90% of tensile.

Property class 8.8 – 8 indicates a tensile strength of 800MPa, 8 indicates that the yield strength is 80% of tensile.

Metric Fasteners Chart

A quick comparison of grades and property classes is shown below:

If you are considering a conversion to metric hardware, contact Bossard at ProvenProductivity@Bossard.com, or check our technical resources at www.bossard.com including an inch to metric calculator that is also available as an iPhone app.

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Doug Jones
Applications Engineer
Email: djones@bossard.com