How to Bend Hydraulic Pipe?
How to Bend Hydraulic Pipe?
How to Bend Hydraulic Pipe?
Description of pipe bending
Hydraulic Pipe bending is the process used to reshape pipes of a certain shape and design. Hydraulic pipes are frequently preferred as connecting elements in hydraulic systems. Hydraulic pipes have become preferred because they have the ability to expand under pressure and provide long durability. Their lifespan is quite longer compared to hydraulic hoses.
Various pipe bending methods are used depending on the material used and the degree of precision desired. The most common ones are; pull bending, wrap bending, compression bending and 3-roll bending
Pulling Bending
When bending by pulling is applied, the pipe is connected between the bending mold and the vise. Both parts rotate around the bending shaft and wrap the pipe on the bending mould. The function of the printing die (slide part) is to absorb the radial stress produced during the forming process and to support the outer end of the pipe to remain flat. If it is additionally applied in mandrel and spoon molds (mandrel bending), successful results can be obtained even in thin-walled pipes and narrow (tight) bending radii.
Wrapping Bending
Twisting by wrapping is similar to twisting by pulling. In this bending method, the pipe is clamped between the slippery slide and the fixed bending die. The slippery slide rotating around the fillet block bends the pipe as much as the radius of the bending die.
Press Bending (Press Bending)
When pressing bending is applied, the twisting device is pressed on two cross rollers manually or hydraulically. This movement causes the pipe to enter between the radius block and the cross rollers and causes bending around the radius. Since the pipe cannot be supported from the inside, this method is only useful for pipes with high wall thickness and large bending diameters.
Roller Bending
3-roll bending is also used to produce workpieces with high bending radii. This method is similar to press twisting except for the working cylinder and the rotation of two fixed cross rollers.
Twisting Devices
Twisting tools for pull bending were designed during this project (Figure 2). The bending die that directs the pipe to the rotation axle is the main device. The vise die used to grip the workpiece approaches the bending die and moves simultaneously (together with the bending die). Mandrel prevents the bending area from changing shape and collapsing.
The pressure die is designed to balance the bending forces and move linearly with the pipe, keeping the pipe in the correct position. The spoon die is placed between the groove of the bending die and the tangent line just behind the tube. With its wedge-shaped tip, the wiper prevents potholes and humps (3). Since there is no serious risk of potting in low radius pipes and sufficient wall thicknesses, it is not necessary to use a wipe mold for small values. Detailed explanations about these devices are given in the "Device selection" section.
Scope of application
Pipe bending has a wide usage rate. Bent pipes and profiles are used in business signs, construction industry, agricultural machinery used in agriculture, automotive manufacturing industry, various accessory equipment, industrial kitchen manufacturing, various work machines, heating and cooling industry, furniture industry, etc. It is widely used in sectors.
Pipe Bending Problems
Important Features
Appearance:
Aesthetic appearance is particularly important where the pipe is intended to be used visibly or will be part of an ornamental structure. In order to achieve a stylish appearance, flattening must be prevented.
Wall thickness of the pipe:
In some applications where pressurized liquid will pass through, wall thickness calculations are important to find the smallest force and thinnest values that the pipe can withstand without being damaged.
Geometry:
Different geometric values in pipes require different parameters. For example, it is necessary to take serious care when bending square profile pipes, as sharp edges are much more difficult in bending work.
Factors Creating Difficulties
Low wall thickness compared to pipe diameter:
If the ratio of wall thickness to pipe diameter is significantly small, cracks and wrinkles may appear on the pipe surface. There are specific rate limits for each pipe material.
Low bend radius:
Changes in the bending die diameter change the bending radius. Bending with low bend radii can be critical because this causes excessive stress on the pipe's spine and excessive stress on the inside. This may cause tearing or collapse in the back and inner bulge. Figure 8 shows a pipe bent with a small diameter bending die. If the diameter of the bending die was larger, the pipe could be bent without any flattening.
Stitch:
Care should be taken if the bent pipes have been previously stitched. Twisting should not be done in the stitched area as the stitched surface cannot withstand the tension forces and the stitch line may open.
Pipe Bending Problems
Wall thickness changes (section-inner-outer):
The tension force on the back of the pipe and the pressure force on the inside create a thinning on the back and a thickening on the inside of the opposite parts of the pipe in the bending area.
Hump on outside of pull off end:
When the pipe is connected for bending, there is a gap of 0.5-1 mm between the bending die and the vice die. This opening helps to better grasp the part to be bent and prevents the part from slipping. On the contrary, if the bending mold and the vice mold are completely stuck to each other, shifting will occur during bending and a hump will occur, which is not a desired thing.
Scratches in the vise area:
When the vise grips the part too tightly, scratches may occur in the vise area.
Ball bumps in the twisting area:
If the mandrel used has a ball, if the dimensions and material of the mandrel are not chosen appropriately, there may be ball traces visible from the outside in the bending area.
Scratches on the inside of the bend area:
The mandrel used may cause scratches on the inside of the bending area of the pipe. Therefore, in order to eliminate this problem, the appropriate mandrel radius and material must be selected.
Tool marks on the midline:
The pressure die (slide) serves to relieve radial stress and ensure that the back of the pipe remains flat. While doing this, the pipe slides into the mold. Therefore, tool marks may appear on the centerline if the printing plate material is not selected correctly. For example, this problem can be overcome with a material such as ductile cast iron, which has spherical grains in its microstructural area.
Recoil:
When the bending process is about to be completed, the remaining stresses in the body of the bent pipe cause a loosening of the part, which reduces the degree of bending. To solve this problem, the initial degree of twist can be increased to a degree determined through simulations. Another solution is to release the increase up to the elastic ratio unit (E) of the pipe material. Increasing E means making the part stiff enough to result in minimal loss of the part after the forces are removed.
Collapse: If the appropriate mandrel is not used in pipes where the wall thickness to diameter ratio is low, the outer part of the pipe may not be able to withstand the tension forces and may collapse. Appropriate mandrels with appropriate ball sizes should be selected. Additionally, the bending radius must be large enough to prevent collapse.
Tear:
Tearing may occur on parts with small bend radii. The bent part may not withstand the tension forces occurring on the back of the pipe and may tear. Choosing the appropriate material is the solution to this problem.
Pipe Design
Pipe Material Selection
Comparison between pipe material and geometry specific aluminum, copper-brass and stainless steel.
Features of Radiator Pipes
The material of the radiator pipe must have high thermal conductivity, high melting point, high tensile strength and high wear (or corrosion) resistance, as hot and high-pressure liquid will pass through it. The material must also have a low weight for lightweight structures and be able to hold solder in order to solder the fins to the radiator.
Material Selection
Mechanical and physical properties of Aluminum AA 3003, Brass UNS C26000 and Stainless Steel are shown in Table 1.
UNS stands for the unified numbering system of the United States. UNS C26000 is a 70% copper alloy machined cartridge brass and is commonly referred to as UNS C26000 copper-zinc brass. It contains 0.70% copper (Cu) and 0.30% zinc (Zn) in its composition (by weight percentages). [9]
AA stands for Aluminum Association. AA3003 is called wrought Alclad Aluminum alloy and contains 98.6% Aluminum (Al), 0.12% copper (Cu) and 1.2% Manganese (Mn). [9], [10].
Material
Intensity
(g/cm^3)
thermal conductivity
(W/m€°C)
Income
stretch
force
(MPa)
Flexibility
ratio
unit of
(GPa)
Heat
expansion
coefficient
(?m/m°C)
Melting
point
(°C )
Solder
temperature
(°C)
Aluminum
AA 3003 2.75 160.0 145 70 23.2 643-
655 600
Rice UNS
C26000
8.53 120.0 435 110 19.9 915-
955 600
Stainless
Steel 7.75 16.3 515-827 190-210 17.3 1371-
1454 620-1150
Table 1. Pipe materials comparison
Stainless steel could be used due to its high tensile strength and high wear resistance, but it is very expensive and its low thermal conductivity makes it not preferred in its field of use.
In addition to the fact that aluminum has lower density and is more conductive than brass, it is also important that the radiator pipe power is equal or greater. Undoubtedly, brass is much more durable than aluminum. Thanks to this total power, the difference between conductivity and weight in thin-walled pipes can be compensated.
Given the advantages of these properties—superior thermal conductivity, strength and corrosion resistance—manufacturers can go directly to thinner materials and therefore produce copper/brass radiators instead of aluminium, which have a lower total weight but the same (or better) heat dissipation capacity. [11th]
In Table 2 you can see the advantages of Copper/Brass material versus Aluminium:
Technical Commercial
High thermal conductivity Low production cost
High wear resistance Institutionalized recycling tradition (among structures in a place)
High tensile flexing strength High scrap value
High melting point Institutionalized aftermarket tradition (among structures somewhere)
Low coefficient of thermal expansion Known for high quality
High flexibility rate unit
Easy repairability
Table 2. Advantages of brass over aluminum
Plastic Deformation Analyzes in Pipe Bending
As stated by N. C. Tang, stresses in pipe bending have three distinct components; longitudinal σx, circumferential σc, and radial stresses σr. When the wall thickness of the pipe is much smaller than its radius, radial stress can be ignored. It also varies (stresses) depending on the inner and outer sections of the pipe. As can be seen in Figure 13, the outer semicircle α values of the pipe are between 0° and 90°, and the inner semicircle α values are between 90° and 180°.
Analytical calculations
Analytical calculations were performed by writing a Matlab code (matlab: is a high-performance technical programming language) and the results are shown in Table 4. To determine the values, the formulas given so far and the basic geometric and mechanical parameters in Table 3 were entered into the code and the results were calculated. These calculations are valid only for bending straight pipes. You can find the plastic deformation analyzes performed in Dynaform for the flanged pipe within the scope of this project in the following sections.
Description Value Unit
Outside diameter 15 mm
Wall thickness 1 mm
Bending radius 35 mm
Twisting angle 90 Deg
Flexural strength 435 MPa
New rate unit 110 GPa
Table 3. Main characteristics of the pipe
Description Value Unit
Thinning on the back 0.90 mm
Internal thickening 1.16 mm
Internal diameter shrinkage 4.85 %
Neutral axis deviation 1.18 mm
Twisting moment 99.13 Nm
No-load twist angle 87.38 Deg
Bending radius without load 36.05 mm
Post bend angle (over bend) 92.70 Deg
Radius after bend (over bend) 37.13 mm
Material length 56.51 mm
Critical flattening point 168.07 mm
Table 4. Matlab Analytical Calculation Results
The No-Load Bend Angle and No-Load Bend Radius in Table 4 show valid results for pipes bent at 90° (Table 3). So, if the pipe is bent 90°, the No-Load Bend Angle due to rebound is 87.38° and the No-Load Bend Radius is 36.05 mm. It is possible. To obtain the desired bend angle of 90°, Over Bend Angle is 92.7° and Over Bend Radius is 37.13 mm, as Table 4 shows. should be.
Tool Design
Tool Material Selection
Material selection of mold sets to be used during bending.
Cast iron
First of all, the printing die (slide) part material was discussed. The printing mold must be capable of allowing the material to flow without deforming, as it remains stable during the twisting operation. Cast iron allows the material to flow, and thanks to the spherical particles inside, unwanted scratches do not occur on the surface of the pipe. (It does not wrap around the material). It can easily be said that cast iron is the most suitable material for the printing mold part, as it has the necessary properties for the printing mold.
Some general properties of this material are in Table 5:
CAN BE MADE
IRON
tension force
(MPa)
Flexing Force
(MPa)
%Elongation
Hardened iron 414 276
18
Table 5. Properties of cast iron [16].
“Ampeo” Tin (tin) bronze
If the pipe were chrome or aluminum, Ampeo (amco) Bronze would have to be used as a stamping die. Steel or iron are not suitable for such materials due to their chemical affinity.
The information given so far about materials is about general material design principles of bending parts, especially the printing die. The project here requires the same materials to be used for each of the bending parts. Therefore, when choosing materials, it is necessary to consider the parts as a whole. In Table 7, all steels are compared in terms of the properties required for bending parts:
Material
hardness rockwell
(HRc)
standards
Flexural breaking strength
(Mpa)
Elastic rate unit
(GPa) Poisson's ratio
Intensity
(kg/dm3)
1050 Steel 52 (400°F) DIN 1.1210 (AISI 1050) 636 190-210 0.27-0.30 7.7-8.03
2080 Steel 66 DIN 1.2080 (AISI D3)
2379 Steel 62 DIN 1.2379 1532 209.9 0.27-0.30 7.67
(AISI D2)
Table 7. Part material comparison [9, 10, 17,18].
The materials commonly used in the manufacture of pipe bending mold tools are AISI 1050, AISI D3 and AISI D2 steels. As seen in Table 6, the qualities required for design, such as the elastic ratio unit, are almost the same in all three materials. The only difference between AISI 1050 and others is that its tensile strength and hardness are lower than others. This does not cause the part to fail because the strength of the pipe is less than that of the part, preventing serious stress from occurring. Additionally, the pipe is made of a material that is conducive to making it bend easily. Since the applied forces are extremely low, the part is not in any danger that may arise from high stresses during the bending process.
The only disadvantage of the selected steel compared to others may be related to its lower hardness. High hardness is generally sought in part design to avoid wear. Due to the low forces and chemical interaction, wear and tear in bending processes is not as significant as in other shaping processes. Based on this, it was concluded that AISI 1050 had a sufficient hardness degree and there was no need to choose another material.