Method for bending a metal pipe

A method and hot bending apparatus for metal pipes in which the temperature of the pipe is kept constant during "gradation bending". The temperature may be adjusted by adjusting the power applied to the heater, or alternatively by adjusting the relative movement of the pipe with respect to the heater. While a feedback system may be utilized, a predetermined program is preferred under control of a microprocessor.

BACKGROUND OF THE INVENTION 
This invention relates to a method and apparatus for hot bending a metal 
pipe, and especially to a method of keeping the heating temperature 
substantially constant while gradation bending, in which the bending 
radius is changed gradually at the start and the end of the bending 
process to produce smooth bends while avoiding abrupt changes in pipe wall 
thickness. 
The prior art includes a method for hot bending a metal pipe, wherein the 
pipe is heated locally with a circular heater such as induction heater or 
the like, and where the heated zone is moved relatively to the pipe by 
means of moving the pipe and/or the heater while bending moment is applied 
to said heated zone to cause bending, and after which the pipe is cooled 
in the vicinity of the bend. 
But is it not well known how to prevent abrupt changes in pipe wall 
thickness due to abrupt change of radius of curvature at the start and the 
end of the bending process when the relative bending radius (i.e., the 
ratio of bending radius to pipe diameter R/D) is very small. However, it 
is very important to prevent such abrupt changes in pipe wall thickness 
because of problems that make the bending itself very difficult, e.g., 
swelling or wrinkling at the start of the bending process, and severe 
concentration of bending stress. 
In relation to the method to make said change of pipe wall thickness gentle 
and smooth, a Japanese Patent Application No. 51-150809 has been laid open 
in which the bending radius is changed gradually at the start and the end 
of the bending process and in which the mean radius of bending is made 
equal to the desired radius. This process is call "gradation bending". 
Gradation bending is based on basic principle of hot bending and covers 
many cases wherein a pipe to be bent is heated locally with a circular 
heater such as induction heater or the like and the heated zone is moved 
relatively to the longitudinal direction of the pipe by means of moving 
the pipe to be bent and/or the heater while a bending moment is applied to 
the heated zone to cause bending. After bending, the pipe is cooled in the 
vicinity of the bend. Further, bending may be started at a larger radius 
than the desired radius and reduced gradually until it becomes slightly 
smaller than the specified radius within a certain predetermined small 
range of bending angle. 
In a case in which heating temperature changes significantly as a function 
of the relative speed of the heated zone to the pipe to be bend. Such 
change can happen in the case of typical prior art induction bender shown 
in FIG. 1 where the pipe 1 is fed at a constant speed and heater H is 
displaced gradually for "gradation bending". 
In FIG. 1, 1 is a pipe to be bent, 2 is a bent portion of the pipe, 3 is 
the center of heated zone where deformation of bending arises, H is a 
heating means (such as induction heater) equipped with cooling means in 
one body, 4 is a bending arm which clamps pipe 1 at the top of it and can 
rotate freely around a center 0, 5 and 6 are guide rollers to guide and 
support the pipe 1 against the bending forces, P is the thrust to feed 
pipe 1 and exert bending moment at the heated zone 3, W is the speed of 
pipe 1 to the right, h is the speed of heater H to the left, and A is a 
point which is an intersection of the axis of pipe 1 and a plane which is 
vertical to pipe 1 and includes the point 0. 
In normal bending, heater H is located at point A or in the vicinity of it 
and then radius of bending is kept substantially equal to the effective 
length Ro of bending arm 4. 
In the case of gradation bending, heater H is first located at point 3 of 
FIG. 1 spaced from A by certain proper distance towards bending arm 4 and 
is displaced gradually to point A in order to change the radius of bending 
from large to small gradually. 
With reference to FIG. 1, the change of bending radius R is accomplished as 
follows: 
Within a minute interval of time .DELTA.t, the pipe 1 is fed to the right 
by a minute length dS.sub.1 at a constant speed W, while heater H is moved 
to the left by a minute length dS.sub.2 and the pipe is bent by a minute 
angle d.theta. where the length of pipe before and after bending is 
assumed unchanged. 
##EQU1## 
where dS=dS.sub.1 +dS.sub.2 
If heater H is not moved and fixed, then: 
EQU R=dS.sub.1 /d.theta.=Ro (2) 
Formula (2) means that radius of bending R is substantially equal to the 
effective length of bending arm Ro when the position of the heater is 
fixed. 
From formulas (1) and (2) above: 
##EQU2## 
Since as dS.sub.1 /dt=W, and dS.sub.2 /dt=h: 
##EQU3## 
The relative speed V of the heated zone to the pipe is: 
EQU V=W+h (5) 
If for instance, bending is started at a radius R twice as larger as Ro, 
then from formula (4), 
##EQU4## 
When heater H is moved at a high speed, heating temperature becomes very 
low if heating power is kept constant. If doubled effective heating power 
would be supplied, then the heating temperature should be kept 
substantially constant. 
It is normal to control heating temperature by means of controlling heating 
power corresponding to a deviation of heating temperature measured with an 
instrument, but such feedback method is not effective when the change of h 
(or V) is very large. 
The present invention is directed to a program for keeping heating 
temperature substantially constant by controlling the heating power supply 
or alternatively keeping the relative speed V constant from the start to 
the end of bending by means of controlling W and h separately.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
This invention relates to use of above two methods of keeping heating 
temperature constant while gradation bending is performed. A very large 
heating capacity is required in order to cover large changes in heating 
power in case-1, so that case-2 where heating power is kept constant is 
much more preferable. But case-b 1 may be useful when the capacity of the 
heating power is large enough because of the simpleness of the control 
mechanism that is only changing effective power supply corresponding to 
the change of relative speed of heated zone to the pipe to be bent. 
In case-1, the change of radius of bending is achieved as follows: 
For example, let Rs be the specified or desired radius of bending, D be 
pipe diameter to be bent and let Rs/d=1.5. At the start of bending, the 
speed h of the heater H to the left (FIG. 1) is taken equal to the speed W 
of the pipe to the right (constant during bending) and is thereafter 
changed to zero gradually within a certain small range of bending angle 
.theta.. Thus, changing the speed V from 2W to W, the radius of curvature 
is changed from 2Ro to Ro gradually. 
It is true theoretically that Ro should be a little bit smaller than 
specified radius Rs in order to make mean radius of the bend equal to Rs, 
but the difference between Rs and Ro is so small as to be within the 
normal allowable deflection of a bending machine. 
At the end of bending, radius R is again changed gradually from Ro to 
normally 2Ro in above case by means of changing speed h from zero to W 
gradually and changing speed V from W to 2W. 
In case-2, it is important to make the program to change W and h separately 
so as to keep V constant and to change radius of bending according to the 
predetermined program. 
The principle would be explained with a simple example in which radius of 
bending R is changed hyperbolically corresponding to bending angle .theta. 
as shown in FIG. 2. 
In case-2 and with reference to FIGS. 1-3: 
EQU V=W+h=constant (7) 
From formula (4): 
EQU R/Ro=V/W 
Let Rm be the largest radius of bending at the start, Ro be effective 
length of bending arm, a be start point at the horizontal coordinate, 0 be 
range of gradation and .phi. be an angle within .theta., then, 
##EQU5## 
Value a has been introduced in order to prevent starting with an 
infinitive radius of bending, and to start bending at a proper radius (for 
instance 2Ro) so that if a=2, then a=.theta.. 
Bending angle .phi. must be counted zero at point a' in programming W and h 
in relation to bending angle .phi. at the start of bending, and gradation 
bending is operated from .phi.=zero to .phi.=.theta. (normally less than 8 
degrees) and finished at point 0.sub.1. 
At the end of bending, it is convenient to take another symmetrical 
coordinate as shown in FIG. 2 wherein original point of horizontal 
coordinate is 0', where bending is finished at the point a', and .theta. 
is range of gradation (less than 8 degree). 
In programming, gradation starts at point .theta..sub.2 and programmed 
angle .phi. must be counted from .theta..sub.2, being zero at 
.theta..sub.2 and .theta. at a' where bending is completed. 
At this stage, the program should be naturally be: 
##EQU6## 
As the result of gradation bending according to program (11) and (13), 
speed V which is equal to (W+h) is kept constant and then heating 
temperature is kept constant only by keeping heating power constant, while 
W and H is changed as shown in FIG. 3 and therefore the radius of bending 
is changed as shown in FIG. 2. 
It must be noted that gradation range .theta. should be not larger than the 
required minimum value and preferably should be less than 8 degrees, 
because a large gradation range should be compensated with a small radius 
of bending between the start and the end gradation in order to achieve a 
mean radius of bending equal to the specified radius Rs. More preferably, 
5 to 6 degrees of gradation range is adopted, because in such small 
gradation the deviation of bending radius can be made negligibly small. If 
a very large range of gradation should be adopted, it would cause 
difficult mechanical problems and would cause impreciseness of the bending 
radius. 
The above program control may be accomplished with a microcomputer, 
electric instruments using electric motors or hydraulic equipment. 
On the other hand there is a simple mechanical method to keep V constant. 
With reference to FIG. 4, elements which are common with FIG. 1 are 
nominated with the same numeral. Further, a thrusting means 7 is used to 
clamp the tail end of pipe 1 to feed pipe 1 with thrusting force P, a 
driving means 8 drives thrusting means 7, a screw 9 is installed between 
the thrusting means 7 and the heater H to give constant relative speed V, 
a nut 10 is provided to move the screw 9 while supported with a bracket 11 
and rotated at a proper constant speed with a geared variable speed motor 
12. Bracket 11 is fixed on the thrusting means 7 and the heater H is 
displaceable on a rail parallel to the pipe 1. 
As is clear from FIG. 4, the relative speed V (i.e., the speed of heated 
zone relative to the pipe 1) is kept constant as long as rotating speed of 
nut 10 is kept constant, and the value of V is taken equal to normal 
proper bending speed. To provide gradation at the start of bending, speed 
W of pipe 1 is changed slowly from small (normally V/2) to large (V). At 
first, when W is smaller than V, heater H moves to the left and when W 
becomes equal to V heater H is stopped at point 0. Thereafter, bending is 
performed at a constant radius Ro for a while and at the end of bending 
the speed W is made smaller than V gradually until it equals the starting 
speed (normally V/2) at which point bending is completed. 
In FIG. 4, the location of heater H shows the point when bending is 
completed. 
Further in FIG. 4, roller 5' is installed at the opposite side of roller 5 
near point 0. Roller 5' is used for controlling excess enlargement of 
bending radius R caused by misoperation or some other effects, but roller 
5' may be omitted if some other control mechanism to regulate R is 
equipped. 
The reason why gradation range .theta. is taken smaller than 8 degrees and 
preferably should be 5 to 6 degrees is to avoid excess deviation of radius 
R from Ro and to minimize excess reaction force at the pivot 0 and other 
parts of the bending machine while at the same time performing precise 
bending. In this case, a method would be adopted in which an auxiliary 
feedback temperature control system including means to measure heating 
temperature may be used to get the heating temperature more precisely to a 
constant, but it is effective only when speed V is very small. 
Further, FIG. 5 shows another program which is a little bit improved than 
the case based on the hyperbola illustrated in FIG. 2. At the early stage 
of gradation, the R-.phi. curve may be taken much more steep than the 
hyperbola and at the end of gradation the curve should be more gentle than 
the hyperbola. Such improved curve is more natural in regard to connection 
with constant radius curve III and makes the start of bending easier 
especially when Rs/D is very small. 
According to methods mentioned above, very smooth, small Rs/D bends can be 
produced and bending temperature is kept adequate and constant, and 
consequently this invention is useful to supply ideal bends mechanically 
and metallurgically.