Selection and application of mandrel in bend

ABSTRACT: The deformation of pipe in cold bending is introduced, and the method of bending ideal small radius pipe fittings is achieved by choosing mandrel reasonably and mastering its correct use method.

Key words: stress; mandrel; relative bending radius; relative wall thickness

Introduction
Process Analysis
Choice of Bending Way
Selection of mandrels
Correct use of mandrels

1. Introduction

Bending pipe technology is widely used in boiler and pressure vessel industry, central air conditioning industry, automobile industry, aerospace industry, shipbuilding industry and other industries. The quality of bending pipe will directly affect the structural rationality, safety and reliability of products in these industries. Therefore, in order to bend high quality pipe fittings, it is necessary to master the processing skills of pipe fittings under different technological conditions. It is very necessary to select the formation of mandrel reasonably and to master the correct use method for cold bend.

2. Process Analysis

In the case of pure bending, when the tube with outer diameter D and wall thickness S is bent by external moment M, the outer wall of the neutral layer is thinned by tensile stress_1 and the inner wall is thickened by compressive stress_2 (see Fig. 1a). At the same time, resultant forces F1 and F2 deform the cross-section of the tube bending to be approximately elliptical (see Fig. 1b), and the inner tube wall may also appear unstable and wrinkle under the action of_2 (see Fig. 1c). In order to bend the ideal tube, corresponding measures should be taken to prevent the above defects, of which the core bending is one of the most commonly used effective methods.

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Figure 1 Stress and deformation of the pipe when it is bent
(a) Forced condition when the pipe is bent (b) Sectional deformation when the pipe is bent (c) Wrinkled inside of the pipe when the pipe is bent
The so-called cored bend is that when the bent pipe has a relatively small bending radius R/D or a relative wall thickness S/D, in order to obtain a high quality pipe, a pipe is inserted inside the pipe during the bending process. A suitable mandrel to prevent flattening and wrinkling at the arc when the tube is bent (see Figure 2).
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Figure 2 Schematic diagram of core bend
1. Bending die plate 2. Mandrel head 3. Anti-wrinkle block 4. Tube 5. Mandrel rod 6. Clamping block 7. Pressure guide die

3. Choice of Bending Way

Generally speaking, there are two ways of cold bending: one is coreless bending, the other is coreless bending. The relative bending radius R/D, relative wall thickness S/D and bending angle alpha value of the bending pipe need to be analyzed before deciding which conditions to use coreless bend, which circumstances to use coreless bend and which mandrel to choose when to use coreless bend. The relationship between R/D, S/D and alpha values and bend mode and mandrel shape is shown in Table 1. Satisfactory results can be achieved by referring to this table when bending.
From Table 1, it can be seen that for pipes with the same outer diameter D and wall thickness S, when bending different arc radius R, because of the different relative bending radius R/D, relative wall thickness S/D and bending angle alpha, the following methods can be selected respectively: 1. Coreless pipe bending, 2. Using hard mandrel pipe bending, 3. Using soft mandrel pipe bending, etc. When R/D is greater than 3 and S/D is greater than 0.05, coreless bend can be used; when R/D is less than 2.5, S/D is greater than 0.05 or R/D is greater than 3, S/D is greater than 0.025, the expected effect can be achieved by using hard mandrel; when R/D and S/D are both smaller and bending angle is larger, soft mandrel must be used in the process of pipe bending.
Table 1
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Note: 1. Anti-wrinkle block should be used when bending pipe under dotted line; 2. N – means that no mandrel can be used (i.e. no mandrel bend); 3. H – means that hard Mandrel Bend can be used; 4. F – means that soft Mandrel Bend Pipe needs to be used, the number of balls recommended is the following figure.

4. Selection of mandrels

The shape of mandrel is varied. Different mandrel shapes should be selected for different pipe fittings with different relative bending radius or wall thickness and different processing requirements. Generally speaking, mandrels can b e divided into two categories: hard mandrels (see figs. 3a, b, c) and soft mandrels (see figs. 3d, e, f).

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Figure 3 Basic shape of mandrel
(a) cylindrical mandrel (b) cylindrical ball head mandrel (c) claw mandrel (d) chain mandrel (e) flexible shaft mandrel (f) ball socket nodal mandrel
When choosing hard mandrel, cylindrical mandrel (or cylindrical ball-end mandrel) is simpler in shape, more convenient in manufacture and more common in use than claw mandrel; while when choosing soft mandrel, because the ball socket mandrel can be flexible in many ways, the spherical joints between the ball joints can adapt to various deformation, so it is often used in bending thin-walled or relatively small bending radius pipe fittings. At the same time, the number of spherical pits should be selected according to different relative bending radius, relative wall thickness and bending angle. The number of spherical pits can be selected according to Table 1. If the number of ball pitches is small, the expected effect will not be achieved; if the number of ball pitches is large, it will be difficult to create and not easy for the pipe to penetrate. For the values of R/D, S/D and alpha between the values listed in Table 1, we can select the mandrel and determine the bending mode by referring to the compromise values in this table.
After choosing the shape of mandrel, it can not guarantee the quality of pipe bend. The gap between mandrel and inner diameter of pipe is also an important factor affecting the quality of pipe bend. If the diameter of mandrel pitch is too small, the inner part of the arc may be corrugated when the tube is bent (see Fig. 4A), and it may not prevent the flattening of the outer part of the arc; if the diameter is too large or the outer diameter of the pitch is not smooth enough, the tube wall will be pulled, and the outer part of the arc of the tube may be bulged or even broken. Choosing reasonable diameter of mandrel and lubricating it adequately are indispensable factors for guaranteeing the quality of pipe bend.
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Fig. 4 The inner wrinkling and the outer flattening occur when the diameter of spherical nodes is small.
Core rod diameter dimension D can be selected by referring to the following empirical formulas:
D(0.94-0.98)D
D-mandrel diameter, mm
D – Nominal Dimension of Pipe Inner Diameter, mm

5. Correct use of mandrels

After the shape and size of the mandrel are determined, the position and size e of the insertion tube at the beginning of bending (see Fig. 5) is also an important factor affecting the quality of the bend. If extended forward (e value is too large), the outer wall of the steel pipe may be fractured; if inserted insufficiently, the corresponding anti-flattening effect can not be achieved, and the inner part of the arc may also wrinkle.
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Fig. 5 Position and size of mandrel inserted into pipe
The initial position size e of the mandrel insertion tube can be selected according to the following formula:
E <(1/4-1/2)D
The distance between e-mandrel insertion and the beginning of pipe bending, mm
D – Nominal Dimension of Pipe Inner Diameter, mm
In order to bend high-quality pipe fittings, after selecting e value according to the above formula, the bending test should be carried out first, the bending mode and the shape of mandrel should be adjusted according to the bending test situation (different number of ball pits should also be considered for mandrel pits), the mandrel should be adjusted to the appropriate position, the springback of the same batch of materials should be tested and the bending compensation angle should be adjusted, so that the bended pipe fittings have their arcs. The cross-sectional shape is basically not elliptical. If combined with anti-wrinkle block and booster device, the wavy wrinkle of inner wall of circular arc can be eliminated and the thinning of outer wall of circular arc can be reduced, so as to achieve the purpose of bending thin-walled or small radius pipe fittings.
In addition, when bending the opposite (moment) steel pipe, the selection, use and bending method of the mandrel are basically the same, except that the cross-section shape of the die and mandrel is different from that of the circular steel pipe.

Source: China Pipe Bend Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

If you want to have more information about the article or you want to share your opinion with us, contact us at sales@steeljrv.com

Please notice that you might be interested in the other technical articles we’ve published:

  • The production process of stainless steel pipe bend

  • How to Calculate a Pipe Bend

  • Difference Between A Pipe Elbow And A Pipe Bend

References:

  • https://www.yaang.com
Summary
selection and application of mandrel in bend - Selection and application of mandrel in bend
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Selection and application of mandrel in bend
Description
The deformation of pipe in cold bending is introduced, and the method of bending ideal small radius pipe fittings is achieved by choosing mandrel reasonably and mastering its correct use method.
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www.steeljrv.com
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