Why Use Mechanically Attached Threaded Fasteners?
PennEngineering® offers a very broad range of mechanically attached threaded fasteners.
Limited or no access to one side of the joint
Potential for damage from loose hardware
Sheets too thin and/or weak for tapped holes
1. Limited or No Access
With loose fasteners access to both sides of the joint is required. First to place the fasteners and start the thread, and then to hold one member with a wrench while the other member is tightened with another wrench or power driver. In the case of no access to one side, such as a closed tank, aircraft wing, or midpoint of a long hollow tube, Atlas® Brand blind threaded inserts can even be installed with one side only access. Because these inserts provide both push-out and spin-out resistance, the mating threaded fastener (typically a screw) can be assembled and tightened with one side only access.
When there is access to both sides of the joint during manufacturing, other means of mechanical attachment can be used. Although self-clinching into ductile metal panels is the most common, broaching, flaring, welding, riveting, and reflow soldering (SMT) can also be used for mechanical attachment. The details of the application determine the most appropriate method of mechanical attachment. See our “Engineering Guide to Mechanical Attachment Methods” for details. All of these methods require access to both sides of the sheet for installation (attachment) of the fastener. However, all of these methods provide adequate levels of push-out and torque-out for engaging and tightening of the mating screw or nut without the need to hold the attached fastener with a wrench of any kind.
2. Potential for Damage from Loose Hardware
Loose fasteners will not cause damage in their assembled condition. It is only when they are present in the device in an unassembled condition that they have the potential to cause damage. So how can unassembled loose fasteners be present in a device?
There are three primary ways listed below, but there are many other ways that things can go wrong.
- Fastener was not assembled and could not be retrieved when the device was manufactured
- Fastener was inadvertently dropped and could not be retrieved when the device was serviced
- Fastener was properly assembled, but vibrated loose and fell apart in service
There is a wide range of severity of damage that unassembled loose fasteners can cause. Severe damage can be caused by an unassembled loose fastener that ends up in the intake manifold of an internal combustion engine. At the other end of the damage spectrum, an unassembled loose fastener that ends up in the bottom of a hollow automotive door panel may cause a rattle under certain driving conditions causing mild customer dissatisfaction. Because most threaded fasteners are metal and conductive, unassembled loose fasteners causing a short circuit is a concern in any device with exposed electrical conductors or PC boards.
Threaded connections always involve two components, one with external threads and one with internal threads. If only one of the two components is mechanically attached, there is still a chance that the other component may become disassembled in one of the three ways above.
In many applications it is acceptable for only one threaded component to be mechanically attached. For example, if mechanically attached fasteners are used inside a device and covers or other components are attached to the outside of the device with loose hardware, there may be minimal risk of damage. If the loose fasteners become unassembled, they will likely be on the exterior of the device with no potential for damage to the device. However, in other applications, any potential for unassembled loose hardware is unacceptable. In these cases, a captive panel fastener assembly allows both components to be mechanically attached. This allows virtually no chance of any unassembled loose hardware. These assemblies provide axial movement of the threaded member, which is typically externally threaded. When multiple fasteners are used to attach a rigid item, this axial float allows the fasteners to be disassembled and re-assembled one at a time.
3. Sheets Too Thin and/or Weak for Tapped Holes
In some cases a tapped hole is a viable alternative to a mechanically attached fastener with internal threads. For a tapped hole to be viable, the sheet involved must be thick enough and have high enough shear strength. Ideally threaded connections are designed so that the mode of failure is a tensile failure of the externally threaded member. This is the preferred failure mode for two reasons; it is easy to detect and easy to recover from.
If the externally threaded member is a loose screw, then a tensile failure of it can easily be recovered from by replacing the broken screw with a new screw. However, there are two other possible modes of failure which must be considered; external thread stripping and internal thread stripping.
The only way to prevent external thread stripping is to provide adequate length of thread engagement. It is an issue of geometry only and involves balancing the external thread shear area and the external thread tensile stress are area. See our Engineering Guide to Thread Strength for more details. As a rule of thumb this requires a length of engagement of about 0.8 d, where d is the nominal thread major diameter. For example, an M4 thread with d of 4 mm requires a length of engagement of 3.2 mm.
If the length of engagement is adequate, internal thread stripping must still be considered. For a given length of engagement, internal thread stripping is a function of the shear strength of the sheet material.
For most thread sizes, when the length of engagement is set at 0.8 d, the sheet material shear strength needs to be around 70 % of the screw material shear strength. If we assume the same shear to tensile ratio for the sheet material and the screw material, the 70 % can be applied to the more common tensile strength. For un-heat treated steel screws in the 500 MPa/74 ksi tensile range, many common sheet materials way be strong enough in the 350 MPa/52 ksi range. But for heat treat steel screws in the 800 MPa/120 ksi range, far fewer panel materials have the 560 MPa/84 ksi tensile strength needed for tapped hoes to be viable.
Historically the majority of mechanically attached fasteners are used in thin sheets. There is a recent trend in the transportation industry to use even thinner, higher strength sheets to save weight. But as explained above, even with very high strength that will avoid internal thread stripping, thickness in the range of 0.8 d is still required for a properly designed joint using a tapped hole.
It should also be noted that even if the sheet is thick enough and strong enough to accommodate a properly designed tapped hole, that may not be the most cost-effective solution. Tapping is typically an expensive operation and may require the part to be routed to a different piece of equipment. On the other hand, most mechanically attached internally threaded fasteners are automatically tapped at high production rates and can be automatically installed at high production rates. For these reasons, installed cost of a mechanically attached fastener in a plain punched hole may be less than the cost of a tapped hole.