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April, 1925 Ibrging - Stamping - Heat Tieating 119
F r o m T u b e to H O L B O L F r o m Strip t o H O L B O L
^m P""" PaMM US-A..»d Fcr.^n Couwrie. >. , _ p,MM, pa,mted USA. and Feign Countries
Tub* STcSt Poin,ed °va,ed Rounded c'°sed FHo?bofd
Blanked Drawn Re-drawn
and Pointed
Trimmed
Rounded Closed
FIG. 2—The tube is cut to length and the ends tapered. Succeeding operations are similar to those for making the balls
from strips. FIG. 3—Successive stage in the production of hollow balls from strip metal.
to put the balls through a further grinding operation, that a hollow ball having a wall thickness of from 8
which produces the requisite accuracy. Three grades per cent to 10 per cent of the ball diameter is equiva­
of hollow balls are being manufactured — Precision, lent in load carrying capacity to a solid ball of equal
Standard and Hardware — according to the service for diameter, provided both are made of the same metal
which they are intended. However, it is not necessary and subjected to the same heat treatment.
to dwell on these finishing operations as they do not The reduction in weight is of tremendous import­
differ essentially from the methods used in the manuance in practically^ all ball applications, but especially
facture of solid balls.
so in valve service. First of all the lighter ball im­
The weight of a hollow ball is a function of its wall proves the sensitivity of the valve mechanism because
thickness. This is illustrated in the diagram of Fig. 5. it responds more readily to changes in fluid pressure.
The two variables in this diagram are the wall factor. This is an essential improvement as solid ball check
which is the ratio of the wall thickness to the ball valves have been known to stick in the seat or at best
diameter and the weight factor, which is the ratio of respond sluggishly to the action of the fluid. Further­
the weight of a hollow ball to that of a solid ball of more, the lighter weight means a considerable reduc­
equal diameter. As an example consider the average tion in the shock effect, which increases the life of
hollow ball which has a wall factor of approximately both ball and seat. But what is most important is the
7 per cent. The weight factor in this case is 40 per tendency of the lighter ball to whirl under the in­
cent, which means that it is 60 per cent lighter than fluence of the fluid current. This causes a constant
a solid ball of equal diameter. When the wall factor change in seating position which eliminates localized
is reduced to 5 per cent, the saving in weight is over wear and "grooving".
two-thirds.
The necessity of having a lighter ball has long oc­
The process is adapted for making balls the wall curred to ball users and various attempts were made
factor of which ranges at the present time from 5 per to produce hollow balls. One of the best known meth­
cent to 10 per cent. Thus it is possible to produce by ods is to cast a ball out of bronze or iron, leaving a
this process aluminum balls with a weight factor so sand hole to remove the core and then plugging the
low that they can be used for float purposes. On the aperture. Balls of this dsecription are widely used
other hand for valve service, where the duty is likely in larger pumps and valves. A section of such a ball
to be severe, it is essential to get the wall as heavy as is shown in the center of Fig. 4. Another method was
is consistent with the desired reduction in weight. to fabricate a ball out of two hemispheres welded
along the equator. The objectionable feature
in such balls is their lack of balance and also
their weakness around the plug or the weld. The
lack of balance causes the ball to fall in its seat
constantly in the same definite position and a
groove is soon formed at the zone of contact.
The relative lightness of the cast ball is not
FIG. 4—Types of hollow balls. Left- Spun copper ball. Center— sufficient to compensate for this unbalanced con­
Cast bronze ball. Right- -Section of Holbol.
dition, the best reduction that has been achieved
This is achieved by maintaining the wall factor somewhat
around 8 per cent.
It is obvious that the core of a solid ball does not
contribute to its strength and if we imagine that this
is being gradually removed, we will find the following
situation. Up to a certain point the strength will not
be affected. When this "critical wall thickness", however,
is reached, the strength begins to decrease first
very slowly, then more rapidly. Theoretical calculations
show that this critical wall thickness is reached
somewhere between 8 per cent and 10 per cent according
to the metal and heat treatment. That is to say
in this way being about 50 per cent. Furthermore,
this method of construction is difficult to
apply to balls smaller than iy in., which accounts
for their restricted use.
One of the claims made for the balls is that their
perfect balance is not achieved by artificial means but
is inherent in the process itself. First of all the dies
are so designed as to give the blank an approximately
uniform wall. In the second place the remaining difference
of the wall thickness at various points is completely
obliterated during the rolling operation. Incidentally,
the process is in itself a test of the ultimate
have fitness of readily solid flaws of balls detected the than and ball. the such even bar The defects stock in raw a used material as superficial it in may the is manufacture
have less inspection.
liable can to be