Screw threads applications

Three basic applications for screw threads: 1) to hold parts together, 2) to adjust parts with reference to each other, 3) to transmit power

Definitions: Mr. G, pg. 383

Thread Forms: the cross section of thread cut by a plane containing the axis of the screw

• 60° Sharp-V thread (originally called the United States Standard thread or the Sellers thread); only used occasionally; not standardized
• American Nation: replaced the Sharp-V
• Unified National : standard agreed upon by U.S., Canada, and Great Britain; the American Nation standard form was modified slightly to adapt to the Unified National standard; designed to correct production difficulties in the former standard and for standardization and interchangeability
• Metric Thread: standard agreed upon for international screw thread fasteners
• Square thread: theoretically ideal for power transmission because its face is nearly at right angles to the axis;
• Acme:  replaced the square thread
• Knuckle thread: usually rolled from sheet metal, but sometimes cast
• Buttress thread: designed to transmit power in only one direction: used in large guns, jacks, and other mechanisms

Pitch: distance between corresponding points on adjacent threads, parallel to the axis
Pitch = 1/(# threads per inch)

Right handed V.S. left-handed threads

• A thread is considered to be right-handed unless otherwise specified
• A left-handed thread is labeled LH on a drawing

Multiple threads are used wherever quick motion, but not great power is desired (ex: valve stems)

Thread symbols: schematic, simplified, and detailed

• Threads of small diameter (1” on drawing) are represented with schematic or simplified

Thread Designation for inch threads:

• nominal diameter in decimal form
• # threads per inch
• thread form and series
• class of fit

Thread notes:

• A complete thread note for a blind tapped hole provides the tap drill and depth, although in practice they are often omitted and left to the shop
• Thread notes for holes are preferably attached to the circular views of the holes
• Thread notes for external threads are preferably given in the longitudinal view of the threaded shaft
• For a left-hand thread, LH is added to the thread not (otherwise it is assumed to be a right-hand thread)
• If the thread is a multiple thread, the word DOUBLE, TRIPLE, or QUADRUPLE should precede the thread depth

Classes of fit:

• Class 1A (external) and 1B (internal) fit: used where clearance between mating parts is essential for rapid assembly and where shake or play is not objectionable, this class produces the loosest fit
• Class 2A (external) and 2B (internal)  fit: recommended for the bulk of interchangeable screw thread work, used in normal production of screws, bolts, and nuts
• Class 3A (external) and 3B (internal)  fit: recommended only in cases where a particularly close or snug fit is essential and the high cost of precision tools and machines are warranted

Thread Designation for metric threads:

• M- for metric profile
• Nominal size (in mm)
• Pitch (in mm)

Example: M16 X 1.5; nominal diameter =16, pitch= 1.5

 You will sometimes see metric thread notes like the following: M16 X 1.5 - 6H/5g6g

The information following the pitch is the tolerance grade (numbers) and positional tolerance (letters)

• Tolerance grade
• Grade 6 tolerances are generally used for medium-quality and medium length of engagement (6 is similar to class 2 in inches)
• Grades below 6 are intended for fine quality and/or short lengths of engagement
• Grades above 6 are intended for coarse quality and/or long lengths of engagement
• Positional tolerances
• External threads (lowercase letters)
• e (large allowance)
• g (small allowance)
• h (no allowance); Note: 6h is similar to class 2A in inches
• Internal threads (uppercase letters)
• G (small allowance)
• H (no allowance)
• If there are two number/letter combinations as in the example above, the first is with respect to the pitch diameter and the second is respect to the crest diameter.  If there is only one set, the tolerance grade and tolerance position is the same for the pitch diameter and the crest diameter

Thread Designation for pipe threads

• nominal diameter in inches (inch does not need to be specified unless being used on a metric drawing)
• number of threads per inch (sometimes omitted)
• thread form and type (NPT or NPS- NP stands for American National Standard, or National for short and S= straight pipe thread; T= tapered pipe thread

When pipe is ordered the nominal diameter and wall thickness are given.

Taper pipe threads are recommend for general use.  Standard taper is 1 in 16 or .75" per foot.

Straight pipe threads are used for pressure-tight joints for pipe couplings, fuel and oil line fittings, drain plugs, free-fitting mechanical joints for fixtures, loose-fitting mechanical joints for locknuts, and loose-fitting mechanical joints for hose couplings

Specs for Bolts and Nuts

• Nominal size of bolt body
• Thread spec or note
• Length of bolt
• Finish of bolt
• Style of head
• Name

Thread lengths:
For bolts or screws up to 6” in length: Thread length = 2D + ¼”
For bolts or screws OVER 6” in length: Thread length = 2D = ½”

• Thread length: length of full or perfect threads
  
  

Tapped Holes

• The bottom of a drilled hole is conical in shape, as formed by the point of a twist drill
• When an ordinary drill is used in connection with tapping, it is referred to as a tap drill
• Blind bottoming holes should be avoided wherever possible.  A better procedure is to cut a relief with its diameter slightly greater that the major diameter of the thread
• Never use a blind hole when a through hole of not much greater length could be used
• Good practice to give the tap drill size in the thread note
• Thread length in a taped hole depends on the major diameter and the material being tapped
• Both parts steel: minimum engagement length X = D (major diameter of the thread)
• Steel screw into cast iron part: X = 1.5D
• Steel screw into aluminum, zinc, or plastic: X=2D

Clearance for bolts or screws

• Drilled 0.8 mm or 1/32” larger than the screw up to 10 mm or 3/8” diameter
• Drilled 1.5 mm or 1/16” larger than the screw for larger diameters

• For precision work: divide the numbers above in half

• It is not necessary to show the clearance spaces on each side of a screw or bolt passing through a hole in an assembly