Threads and Threading - Sportpilot.info

Machinery's Handbook 27th Edition
BUTTRESS THREADS 1849
BUTTRESS THREADS
Threads of Buttress Form
The buttress form of thread has certain advantages in applications involving exceptionally
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 perpendicular
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
45˚
45˚
d
5˚
50˚
50˚
d d1 d
g
e
3˚
33˚
33˚
SCREW
f
f
Fig. 1a. Fig. 1b. 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 .
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 1850)
except: 1) A basic depth of thread of 0.4p is used instead of 0.6p; 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.
Lowenherz or Löwenherz Thread.—The Lowenherz thread is intended for the fine
screws of instruments and is based on the metric system. The Löwenherz thread has flats at
the top and bottom the same as the U.S. standard buttress form, but the angle is 53 degrees
8 minutes. The depth equals 0.75 × the pitch, and the width of the flats at the top and bottom
is equal to 0.125 × the pitch. This screw thread used for measuring instruments, optical
apparatus, etc., especially in Germany.
Copyright 2004, Industrial Press, Inc., New York, NY
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