What You Need To Know About Mounting Screws
Posted by NavePoint on May 3rd 2021
Rack screws are used to attach equipment to the vertical uprights of a 19” rack. Rack screws can be part of a cage nut set or a standalone screw mounting directly onto threaded mounting rails. There is no universal standard thread type for server racks, however, the four thread types that are most used include 12-24, 10-32, M5, and M6. Sometimes, one type of thread will work better than another, and the following explains why.
ANSI vs ISO
First, we need to address the differences due to origin between a 10-32 vs an M5 or a 12-24 vs an M6 screw. 10-32 and 12-24 threads originate from Unified and American Screw Threads for Bolts, Nuts, and Machine Screws standards published in 1974 by ANSI B1.1. Being an American screw standard, it follows the English unit of measurements like inches. M5 and M6 threads originate from the International Organization for Standardization published under the standard called ISO 68-1 in 1998. The main reason why we see both English and Metric screws in the market is that a significant portion of the world's inventory and parts are designed and produced in metric from countries like China and Germany. However, 10-32 and 12-24 threads exist when a part or design originates from the US which is predominately English-driven like inches and feet. Due to differences in rounding, English and Metric screws can be very similar but they will not and should not be used interchangeably.
In metric threads, the "M" stands for the metric unit, and the number behind is the nominal major diameter of the screw. Meaning, when you measure the diameter of an M5 screw, you should measure a value close to 5mm.
For English screws, the number before the dash is used as a size designator that corresponds to a published size chart as shown below:
From the chart above you can see that 10-32 thread corresponds to screw size #10, which should measure to be 3/16” or 4.76mm. 12-24 thread corresponds to screw size #12, which should measure to be 7/32” or 5.556mm.
It is very difficult to tell the diameter difference between a #10 and an M5 screw. The same goes with trying the tell the difference between a #12 and an M6 screw. Screw thread diameters have tolerances that blur the diameter differences between these English and metric screws further, which is why it is a better practice to determine which screw you have by looking at the thread pitches instead of measuring the screw diameter.
Thread pitches determine the frequency of each thread rotation within a defined distance. Thread pitches can be categorized into a different level of "fine". For example, both 10-32 and 12-24 are under the United fine-pitch threads (aka. UNF) thread pitch standard as shown below:
Unified form thread designations are represented by abbreviations such as UNC (unified coarse), UNF (unified fine), UNEF (unified extra fine), UNS (unified special), and UN (unified constant pitch), with the thread major diameter preceding the designation. A thread is more “fine” when there are more thread revolutions within the same distance.
Figure 1 Credit: Atlantic Fasteners
Taking the 10-32 thread as an example, the number after the dash, or the “32”, refers to the 32-thread pitch in threads per inch. Meaning, the 10-32 screw should have 32 full-thread revolutions for every inch in length.
In the world of metric threads, coarse pitch threads are the default unless mentioned otherwise. Meaning, the technical representation for the M5 screw used in mounting rack equipment is M5 x 0.8. The technical representation for the M6 screw is M6 x 1.0. Unlike English threads, the 0.8 in an M5 thread stands for the distance between each revolution:
Figure 2: Credit: Atlantic Fasteners
For a coarse M5 thread, you would measure a new peak every 0.8mm, and you would measure a new peak every 1.0mm for a coarse M6 thread.
For example, an M5 x 0.8 screw will have 20.32 revolutions per inch, which is roughly 12 revolutions less than a 10-32 thread, but 4 more revolutions than a UNC 10-24 thread.
Try over stacking two screws over its threads. The threads will fit together nicely if the pitch is the same and they will not fit if the pitches are different. Inspection professionals often use a pitch gauge to measure the pitch of the thread that is being inspected.
Beware, it is possible to force a metric threaded screw into an English threaded hole with the force of electric drills or vice versa. Doing so would undermine the thread’s structural integrity, so please test the screws by hand if you are not sure what screws are mixed in the bag or bin.
There are two types of screw strengths. Tensile strength, or the ability to withstand pulling force, and shear strength, or the ability to withstand shearing forces. Screw/thread design can be very complicated and engineering intensive for different applications and industries. What we need to know in the world of mounting equipment onto our racks or cabinets is that the screws we use are most likely not the weakest point. When mounting equipment with cage nuts, any of the four threads discussed should be sufficient in nearly all standard applications. However, you should be aware of the different threaded mounting rails in the market and how you should interact with them.
Most mounting rails are manufactured with steel metal gauges between 18 and 14, or 1.5mm to 2mm in thickness. There are racks and cabinets design in the market with a gauge 18 sheet metal with 12-24 threaded holes. The problem with this design is 24 revolutions per inch equates to 1.06mm per revolution. Meaning, you are only getting 1.4 revolutions of thread engagement, making the design prone to thread stripping. If the rack was designed with a gauge 18 sheet metal with 10-32 threaded holes, you will get 1.9 revolutions of thread engagement, which might not look like much, but it is nearly 35% more.
The proper way to design threaded mounting rails is to pair a 12-24 thread with a sheet metal gauge between 14 and 11 (lower gauge number = thicker sheet). A gauge 11 sheet metal is 3mm thick and will provide you with a 2.8 revolution of thread engagement. Gauge 11 sheet metal is not popular in the market because thicker sheet metal leads to both higher material cost, higher manufacturing difficulties and will be harder to bend.
Threaded mounting holes can be a huge time saver when installing multiple pieces of equipment. This blog provides you with more insights about threads and know the right questions to ask. NavePoint is a trustworthy brand that tries its best to think through the details of each rack and cabinet design. I look forward to sharing more about rack and cabinet designs.
Contact NavePoint’s sales or customer service team at 888-505-1363 to learn more about how we may be able to help with your specific need!
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