When the need to use a screw and a nut presents itself, engineers must evaluate several considerations when deciding which screws and nuts best fit mechanical and metallurgical applications. The most commonly used screws and nuts are the ACME brand, a forerunner to their Craftsman brand, which was introduced the late 1800s by Sears and Roebuck Co. The brand now comes in three versions: general purpose ACME, centralising ACME, and stub ACME.
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Basic Thread Specifications
What makes a stub ACME thread different from a general purpose ACME thread or a centralising ACME is simply the thread height. The stub ACME thread height is specified to equal 0.3 times the pitch, while the thread height for the other ACME screws is equal to 0.5 times the pitch.
Also, of the five available ACME screw classes, stub ACME screws and nuts are only covered in the 2G class.
These stub ACME thread specifications are standardised jointly by the American National Standards Institute (ANSI) and the American Society of Mechanical Engineers (ASME) with their national standard ANSI/ASME B1.8 (Stub ACME Screw Threads). The federal government has adapted this national standard within its FED-STD H28/13 (Screw Thread Standards for Federal Services, Section 13).
Stub ACME Variations
Two variations to the standardised stub ACME are modified forms, and are known as M1 stub ACME and M2 stub ACME. The thread height specifications of the M1 stub ACME are smaller than that of standard stub ACME. The thread height specifications of the M2 stub ACME are larger than that of the standard stub ACME.
While national standard ANSI/ASME B1.8 (Stub ACME Screw Threads) allows for the usage of M1 and M2, both ANSI and ASME suggest that their usage is avoided whenever possible.
Method for Choosing Correct Stub ACME
Mechanical engineers consider four different application factors before deciding which stub ACME screw and nut to use. The mechanical engineer needs to know the load used to install the screw measured in pounds or newtons, the speed used to install the screw measured in inches or millimetres per minute, the length between bearings measured in inches or millimetres, and whether the screw will be fixed by zero, one or two nuts.
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