The American Society of Mechanical Engineers

70 TRANSACTIONS OF T H E A.S.M.E. JANUARY, 1941
inner cylinder diameter. The “softness” or elasticity of the
flange resulting from its depth and high compressive loading
must make the two faces conform to each other much as two
pieces of rubber would a t pressures within our ordinary experience.
In the design of high-pressure turbine shells it is im portant
F io . 27
T a p e r e d T h r e a d s t o D i s t r i b u t e L o a d o n T h r e a d s o r
H e a v i l y L o a d e d S t u d s
Two further refinements are: (1) To plate the threads of the
nut w ith a thin copper coating; and (2) to make the clearance
between the nut and stud somewhat larger than normal, about
5 mils on 1-in-diameter and 10 mils on 3-in-diameter studs.
Both refinements seem conducive to ease in removing nuts from
studs after use.
All studs over 2 in. in diameter are made hollow to permit heating
for ease in setting up and loosening.
A refinement recently introduced is illustrated in Fig. 27.
This shows, first, a nut and stud combination in which the threads
are of the usual parallel construction, and second, a nut and stud
in which the threads on the nut are machined on a taper, shown
exaggerated in the figure, of course. In the nut with the parallel
threads the load m ust of necessity be concentrated on the first
few threads, if not even on the first, because of the tendency of
the remaining portion of the stud to stretch away from, and the
nut to compress away from, the load with resultant overstressing
of the portion of the stud adjacent to this first thread. The
tendency of the n u t to stretch circumferentially tends to reduce
this concentration,18 but the freedom to stretch circumferentially
is reduced in the nuts on studs which are set up by heating.
In studs screwed into castings rather than into nuts, such concentration
of loading is very pronounced. This concentration
of stress has caused some studs to crack at the bottom of the
nut or where entering the casting. If, however, threads on the
nut or casting are cut on a proper taper, as shown, the threads
a t the end of the stud are loaded first, and when tight the loading
is more uniformly distributed. The taper is sometimes put on the
threads of the stud. Such tapered threads also reduce the con-
I. STOP-VALVE BONNET
F i q . 2 8 C ir c u l a r F l a n g e J o in t s
3. FLANGE JO IN T ON PIPE
f o r H i g h P r e s s u r e s
T e m p e r a t u r e s
th at abrupt changes of diam eter be avoided as much as possible,
particularly to avoid a small “w aist” between two larger sections
because the normal perm anent distortions which accompany
heating and cooling are of such a nature as to cause the flange to
want to open up a t this small waist portion. Also, projecting
inner walls or rings attached to the shells should be avoided if
possible because they tend to heat up faster than the shell and
so by expansion, force the joint open.
A manufacturing refinement which has been introduced into
the manufacture of the bolts and studs is to mill these threads on
a special machine rather than to turn them as was formerly
done. Fig. 26 shows a magnified section of the threads on studs
on which the threads have been turned, and on which they have
been milled. The superior quality of the milled thread is apparent.
F i g . 29
T y p ic a l S t o p a n d C o n t r o l V a l v e s f o r T u r b i n e s f o b
H i g h P r e s s u r e s a n d T e m p e r a t u r e s
centration of stress if the studs are not perfectly aligned with the
threaded holes.
Fig. 28 shows three typical circular flange joints: First, th at
used on stop-valve bonnets; second, the type commonly used
on control-valve bonnets; and third, as used in pipe runs where
welding is undesirable because of dismantling requirements.
18 “The Distribution of Load on the Threads of Screws,” by
J. N. Goodier, Journal of Applied Mechanics, vol. 7, March, 1940, p.
A-10.