Geometry
A tube can be bent in multiple directions and angles. Common simple bends consist of forming elbows, which are bends, and U-bends, which are 180° bends. More complex geometries include multiple two-dimensional (2D) bends and three-dimensional (3D) bends. A 2D tube has the openings on the same plane; a 3D has openings on different planes. A two plane bend or compound bend is defined as a compound bend that has a bend in the plan view and a bend in the elevation. When calculating a two plane bend, one must know the bend angle and rotation (dihedral angle). One side effect of bending the workpiece is the wall thickness changes; the wall along the inner radius of the tube becomes thicker and the outer wall becomes thinner. To reduce this the tube may be supported internally and or externally to preserve theProcesses
Tube bending as a process starts with loading a tube into a tube or pipe bender and clamping it into place between two dies, the clamping block and the forming die. The tube is also loosely held by two other dies, the wiper die and the pressure die. The process of tube bending involves using mechanical force to push stock material pipe or tubing against a die, forcing the pipe or tube to conform to the shape of the die. Often, stock tubing is held firmly in place while the end is rotated and rolled around the die. Other forms of processing including pushing stock through rollers that bend it into a simple curve. For some tube bending processing, aPress bending
Press bending is probably the first bending process used on cold pipes and tubing. In this process a die in the shape of the bend is pressed against the pipe forcing the pipe to fit the shape of the bend. Because the pipe is not supported internally there is some deformation of the shape of the pipe, resulting in an oval cross section. This process is used where a consistent cross section of the pipe is not required. Although a single die can produce various shapes, it only works for one size tube and radius.Rotary draw bending
Rotary draw bending (RDB) is a precise technology, since it bends using tooling or "die sets" which have a constant center line radius (CLR), alternatively indicated as mean bending radius (Rm). Rotary draw benders can be programmable to store multiple bend jobs with varying degrees of bending. Often a positioning index table (IDX) is attached to the bender allowing the operator to reproduce complex bends which can have multiple bends and differing planes. Rotary draw benders are the most popular machines for use in bending tube, pipe and solids for applications like:Roll bending
During the roll bending process the pipe, extrusion, or solid is passed through a series of rollers (typically three) that apply pressure to the pipe gradually changing the bend radius in the pipe. The pyramid style roll benders have one moving roll, usually the top roll. Double pinch type roll benders have two adjustable rolls, usually the bottom rolls, and a fixed top roll. This method of bending causes very little deformation in the cross section of the pipe. This process is suited to producing coils of pipe as well as long gentle bends like those used in truss systems.Three-roll push bending
Three-roll push bending (TRPB) is the most commonly used freeform-bending process to manufacture bending geometries consisting of several plane bending curves. Nevertheless, 3D-shaping is possible. The profile is guided between bending-roll and supporting-roll(s), while being pushed through the tools. The position of the forming-roll defines the bending radius. The bending point is the tangent-point between tube and bending-roll. To change the bending plane, the pusher rotates the tube around its longitudinal axis. Generally, a TRPB tool kit can be applied on a conventional rotary draw bending machine. The process is very flexible since with a unique tool set, several bending radii values Rm can be obtained, although the geometrical precision of the process is not comparable to rotary draw bending. Bending contours defined as spline- or polynomial-functions can be manufactured.Simple three-roll bending
Three roll bending of tubes and open profiles can also be performed with simpler machines, often semi-automatic and non CNC controlled, able to feed the tube into the bending zone by friction. These machines have often a vertical layout, i.e. the three rolls lie on a vertical plane.Induction bending
An induction coil is placed around a small section of the pipe at the bend point. It is then induction heated to between 800 and 2,200 degrees Fahrenheit (430 and 1,200 C). While the pipe is hot, pressure is placed on the pipe to bend it. The pipe can then be quenched with either air or water spray or be cooled against ambient air. Induction bending is used to produce bends for a wide range of applications, such as (thin walled) pipe lines for both the upstream and down stream and on- and off shore segments of the petrochemical industry, large radius structural parts for the construction industry, thick walled, short radius bends for the power generating industry and city heating systems. Big advantages of induction bending are: * no need for mandrels * bend radii and angles (1°-180°) can be freely selected * highly accurate bend radii and angles * accurate pipe spools can easily be produced * significant savings can be obtained on field welds * wide range of pipe sizes can be accommodated in one machine (1” OD thru 80”OD) * excellent wall thinning and ovality valuesPacking
Ice packing
The pipe is filled with a water solution, frozen, and bent while cold. The solute (soap can be used) makes the ice flexible. This technique is used to make trombones.Pitch packing
A similar techniques using pitch was formerly used, but discontinued because the pitch was hard to clean out without excessive heat.Sand-packing/hot-slab forming
In the sand packing process the pipe is filled with fine sand and the ends are capped. The filled pipe is heated in a furnace to or higher. Then it is placed on a slab with pins set in it, and bent around the pins using a winch, crane, or some other mechanical force. The sand in the pipe minimizes distortion in the pipe cross section.s
A mandrel is a steel rod or linked ball inserted into the tube while it is being bent to give the tube extra support to reduce wrinkling and breaking the tube during this process. The different types of mandrels are as follows. * Plug mandrel: a solid rod used on normal bends * Form mandrel: a solid rod with curved end used on bend when more support is needed * Ball mandrel without cable: unlinked steel ball bearings inserted into tube, used on critical and precise bends * Ball mandrel with cable: linked ball bearings inserted into tube, used on critical bend and precise bends * Sand: sand packed into tube In production of a product where the bend is not critical a plug mandrel can be used. A form type tapers the end of the mandrel to provide more support in the bend of the tube. When precise bending is needed a ball mandrel (or ball mandrel with steel cable) should be used. The conjoined ball-like disks are inserted into the tubing to allow for bending while maintaining the same diameter throughout. Other styles include using sand, cerrobend, or frozen water. These allow for a somewhat constant diameter while providing an inexpensive alternative to the aforementioned styles. Performance automotive or motorcycleBending springs
These are strong but flexible springs inserted into a pipe to support the pipe walls during manual bending. They have diameters only slightly less than the internal diameter of the pipe to be bent. They are only suitable for bending soft copper pipe (typically used in household plumbing) or PVC pipe. The spring is pushed into the pipe until its center is roughly where the bend is to be. A length of flexible wire can be attached to the end of the spring to facilitate its removal. The pipe is generally held against the flexed knee, and the ends of the pipe are pulled up to create the bend. To make it easier to retrieve the spring from the pipe, it is a good idea to bend the pipe slightly more than required, and then slacken it off a little. Springs are less cumbersome than rotary benders, but are not suitable for bending short lengths of piping when it is difficult to get the required leverage on the pipe ends. Bending springs for smaller diameter pipes (10 mm copper pipe) slide over the pipe instead of inside.See also
*References
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