Fasteners for Thin Panels

In the current economic climate cost reduction is a relentless on-going activity. Therefore, in non-structural applications where low cost is the key driver, a sheet metal screw is often driven directly into plain pre-punched holes. In these applications the key challenge is usually getting the joint to survive the initial installation without thread strip.

The strip-to-drive ratio is most common test parameter used to quantify this characteristic. The greater the ratio of the torque required to strip the threads in the nut member to the thread-forming torque the greater the processing window during installation. For a given material and fastener size a key differentiating factor in this ratio is the under-head interface to the bearing material. For this reason tapping screws are available with recesses under the head that allow material to be drawn up into it during tightening, thus increasing resistance to strip-out. Figure 1 depicts two popular versions of these fasteners along side the standard AB tapping screw. Figure 2 shows the results of torque-to-failure testing of #10 (0.190″ dia) screws (the Fastite was actually an M5 though the 0.197″ nominal diameter is essentially the same) Manufacturer’s pilot holes diameter recommendations of 0.151″ for the AB and Crimptite and 0.160″ for the Fastite were used.

The material of both the clamp and nut member was 20 gage C1020 CR steel. As the results show the torque required to strip the new fastener’s, and in particular the Crimptite is quite a bit greater than the common tapping screw. An interesting aspect of the Crimptite which undoubtedly influences its good performance is that the ramps of the under-head serrations are pointed so that they dig into the surface when the fastener is tightened. It is customary, as with the Fastite, for under-head serrations to be oriented in opposite manner to maximize resistance to loosening. We performed a brief assessment whether the Crimptite was trading better installation performance for worst loosening performnce.

Measurement of removal torque seemed to indicate the two fasteners were about equal in that regard. However, what is more important is resistance to vibration loosening, which although related to off-torque, can not be predicted by it. The Fastite has a trilobular body, which generally will provide some loosening resistance benefit, though without more focused testing we really couldn’t predict the relative performance of these screws in this regard. Another aspect of tapping screw performance that wasn’t quantitatively tested, and where the Fastite will show superior performance is maintaining proper alignment during driving. The twin-lead thread form will resist the tendency of the standard tapping screw thread to align the helix of the thread with the surface of material thinner than the pitch of the thread. While we did not have this problem with any of the screws during this test, it is certainly an occasional headache with tapping screws in thin sheet.

As one might expect these advantages come at a price, a factor that will limit application of these new fasteners. Expect to pay at least twice as much for these improved fasteners as for commodity tapping screws, so a measurable payback will be required for justification. In most cases, being able to reduce sheet gage or eliminate separate nut members or hole extrusions will justify their use.

Figure 1. – Tapping screws tested in 20 ga. C1020 sheet

Torque

Figure 2. – Typical torque-angle plot of torque-to-strip testing. Numbers refer to the average strip-to-drive ratio calculated in testing
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