Rotary draw bending apparatus and method

A rotary die pipe bending apparatus comprising a bend die mounted for rotation about a bending axis and a clamp die for clamping a pipe to the bend die. The clamp die holds the pipe to be bent as the bend die and the clamp die rotate about the bending axis. A pressure die directed against an outer surface of the pipe for restraining a portion of the pipe spaced away from the bend to be formed in the pipe. A reverse die engaged with an inner surface of the pipe at a location spaced away from the clamp and bend dies and near the pressure die for resisting the bowing of the pipe in back of the bend die and for avoiding the formation of an indentation at the end of the bend.

FIELD AND BACKGROUND OF THE INVENTION 
The present invention relates in general to pipe bending and, in 
particular, to a new and useful rotary draw bending apparatus and method 
which avoids the formation of indentations or humps in the pipe at the end 
of the bend and resists the bowing of the pipe in back of the bend die, 
resulting from the application of excessive force on the pressure die. 
The technique of rotary draw bending derives its name from the concept of a 
pipe or tube being clamped, held and rotated around a bend die. During the 
process, the pipe is pulled or drawn forward. This is different from the 
so-called compression bending where the pipe is stationary and is wrapped 
around a form. The minimum tools or dies required for rotary draw bending 
are a clamp die, a bend die, and a pressure die. The clamp die holds the 
pipe or tube against the bend die and rotates with the bend die. The 
pressure die holds back reaction forces to create the bend. All bending 
occurs within a narrow zone just prior to and slightly following the point 
where the pressure die is tangent to the bend die. It is within this range 
that the yield point of the pipe or tube material is exceeded thereby 
forming indentations or humps. 
The machine operators are known to apply excessive force on the pressure 
die in an attempt to improve the ovality of the tube or pipe within the 
bend. The application of excessive force may cause the formation of 
indentations or humps in the pipe at the end of the bend and may cause the 
pressure die to over travel and actually bow the pipe in back of the bend 
die. The present invention avoids the formation of indentations or humps 
in the pipe at the end of the bend and resists the bowing of the pipe in 
back of the bend die, resulting from the application of excessive force on 
the pressure die. 
SUMMARY OF THE INVENTION 
The present invention relates in general to an apparatus and method which 
avoids the formation of indentations or humps in the pipe at the end of 
the bend and resists the bowing of the pipe in back of the bend die, 
resulting from the application of excessive force on the pressure die 
during operation of the rotary draw bending apparatus. 
Accordingly, it is an object of the present invention to provide a rotary 
draw pipe bending apparatus comprising a bend die mounted for rotation 
about a bending axis; a clamp die directed toward the bend die for 
clamping a pipe to the bend die, the pipe having an inner surface engaged 
by the bend die and an outer surface engaged by the clamp die, the clamp 
die holding the pipe to be bent as the bend die and clamp die rotate about 
the bending axis; a pressure die engaged with an outer surface of the pipe 
for restraining a portion of the pipe spaced away from a bend to be formed 
in the pipe; and a reverse die engaged with an inner surface of the pipe 
at a location spaced from the clamp and bend dies and near the pressure 
die for resisting the bowing of the pipe in back of the bend die and for 
avoiding the formation of an indentation at the end of a bend to be formed 
in the pipe. 
A further object of the invention is to provide a method of bending the 
pipe using rotary draw bending, which includes counteracting the force of 
the pressure die using a reverse die to resist the bowing of the pipe in 
back of the bend die and to avoid the formation of indentations in the 
pipe. 
The various features of novelty which characterize the invention are 
pointed out with particularity in the claims annexed to and forming a part 
of this disclosure. For a better understanding of the invention, its 
operating advantages and specific objects attained by its uses, reference 
is made to the accompanying drawings and descriptive matter in which a 
preferred embodiment of the invention is illustrated.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
As shown in FIG. 1, the known rotary draw bending process involves clamping 
the pipe 20 between the clamp die 22 and the bend die 12 and engaging the 
outer surface 18 of pipe 20 next to the bend die 12 by a pressure die 24. 
As the clamp die 22 and the bend die 12 rotate in the direction of arrow 
A, around the bending axis 14, the pipe 20 moves in the direction of arrow 
B to accommodate the newly formed bend. 
FIG. 2 shows a known compression bending arrangement where the clamp die 
22' holds the pipe 20 against a fixed bend die 12'. A movably mounted 
pressure die 24' moves in the direction of arrow C to form the bend in the 
pipe 20. 
FIG. 3 shows the undesirable effect, encountered with the known 
apparatuses, whereby an indentation 40 is formed at one end of the bend 
whenever excessive force is applied on the pressure die 24 causing the 
latter to engage the pipe 20 too tightly. 
FIG. 4 shows another undesirable effect, encountered with the known 
apparatuses, wherein the portion of pipe 20 positioned in back of the bend 
die 12 is deformed downwardly, as indicated by the phantom line 20a, by 
the downward pressure resulting from the application of excessive force on 
the pressure die 24. 
FIG. 5 illustrates the invention and comprises a rotary draw bending 
apparatus 10 which includes a bend die 12 mounted for rotation about a 
bending axis 14. The bend die 12 has an outer surface engaging the inner 
surface 16 of a pipe 20 which is to be bent. The outer surface 18 of the 
pipe 20, near one end of the bend, is engaged by a clamp die 22 which 
holds the pipe 20 firmly to the outer surf ace of the bend die 12 and 
rotates therewith. A pressure die 24 is engaged with the outer surface 18 
of the pipe 20 behind the bend area and is fixed with respect to the bend 
die 12 and clamp die 22. 
According to the invention, a reverse pressure die 26 is engaged with the 
inner surface 16 of pipe 20, behind the bend area, and at least partly 
overlaps the pressure die 24 for counteracting the application of 
excessive force on the pressure die 24, thereby avoiding the undesirable 
indentations or humps in the pipe 20 at the end of the bend and resisting 
the bowing of pipe 20 in back of the bend die 12. The reverse pressure die 
26 comprises a frame 28 which carries a plurality, in this case three, 
rollers 30 that engage the inner surface 16 of pipe 20. The reverse 
pressure die 26 can be repositioned to any desired location to counteract 
the force applied to the pressure die 24 thereby avoiding the indentation 
or hump 40 in the pipe 20 at the end of the bend, as shown in FIG. 3, and 
resisting the bowing of pipe 20 in back of the bend die 12, as shown in 
FIG. 4. 
The use of a reverse pressure die in the rotary draw bending process, 
according to the present invention, provides the advantages of eliminating 
the indentation in the pipe at the end of the bend and resisting the 
bowing of the pipe in back of the bend die, improving the ovality of the 
pipe within the bend, allowing pipes having thinner walls to be bent, 
adding rigidity to the bending process, eliminating excessive wear of the 
bend die post and key ways and extending the life of other tools used in 
the bending process. 
The apparatus of the present invention is particularly effective when used 
in the rotary draw bending machines utilized by the boiler tube industry. 
It should be recognized that the present invention is also readily 
adaptable in the manufacture of other products requiring the use of rotary 
draw bending machines. 
While a specific embodiment of the invention has been shown and described 
in detail to illustrate the application of the principles of the 
invention, it will be understood that the invention may be embodied 
otherwise without departing from such principles.