Machinery's Handbook, 31st Edition
2078
BUTTRESS THREADS BUTTRESS THREADS Threads of Buttress Form
The buttress form of thread has certain advantages in applications involving exception ally high stresses along the thread axis in one direction only. The contacting flank of the thread, which takes the thrust, is referred to as the pressure flank and is so nearly perpen dicular to the thread axis that the radial component of the thrust is reduced to a minimum. Because of the small radial thrust, this form of thread is particularly applicable where tubular members are screwed together, as in the case of breech mechanisms of large guns and airplane propeller hubs. Fig. 1a shows a common form. The front or load-resisting face is perpendicular to the axis of the screw and the thread angle is 45 degrees. According to one rule, the pitch P = 2 × screw diameter ÷ 15. The thread depth d may equal 3 ∕ 4 × pitch, making the flat f = 1 ∕ 8 × pitch. Sometimes depth d is reduced to 2 ∕ 3 × pitch, making f = 1 ∕ 6 × pitch.
f
P
f
P
f
P
NUT
h
33˚ 3˚
45˚
50˚ 5˚
g
d
d d 1
d
33˚
50˚
45˚
e
SCREW
f
r
f
Fig. 1a. Fig. 1c. The load-resisting side or flank may be inclined an amount ( Fig. 1b) ranging usually from 1 to 5 degrees to avoid cutter interference in milling the thread. With an angle of 5 degrees and an included thread angle of 50 degrees, if the width of the flat f at both crest and root equals 1 ∕ 8 × pitch, then the thread depth equals 0.69 × pitch or 3 ∕ 4 d 1 . Fig. 1b. The saw-tooth form of thread illustrated by Fig. 1c is known in Germany as the “Sägengewinde” and in Italy as the “Fillettatura a dente di Sega.” Pitches are standardized from 2 millimeters up to 48 millimeters in the German and Italian specifications. The front face inclines 3 degrees from the perpendicular and the included angle is 33 degrees. The thread depth d for the screw = 0.86777 × pitch P . The thread depth g for the nut = 0.75 × pitch. Dimension h = 0.341 × P . The width f of flat at the crest of the thread on the screw = 0.26384 × pitch. Radius r at the root = 0.12427 × pitch. The clearance space e = 0.11777 × pitch. British Standard Buttress Threads BS 1657: 1950.— Specifications for buttress threads in this standard are similar to those in the American Standard (see page 2079) except: 1) A basic depth of thread of 0.4 p is used instead of 0.6 p ; 2) Sizes below 1 inch are not included; 3) Tolerances on major and minor diameters are the same as the pitch diameter tolerances, whereas in the American Standard separate tolerances are provided; however, provision is made for smaller major and minor diameter tolerances when crest surfaces of screws or nuts are used as datum surfaces, or when the resulting reduction in depth of engagement must be limited; and 4) Certain combinations of large diameters with fine pitches are provided that are not encouraged in the American Standard. American National Standard Buttress Inch Screw Threads The buttress form of thread has certain advantages in applications involving exception ally high stresses along the thread axis in one direction only. As the thrust side (load flank) of the standard buttress thread is made very nearly perpendicular to the thread axis, the radial component of the thrust is reduced to a minimum. On account of the small radial thrust, the buttress form of thread is particularly applicable when tubular members
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