difference between internal and external bending spring
Why is the difference between inner and outer bending springs crucial for a successful installation?
In professional installation of heating systems, where we work with copper pipes or thermally resistant plastics (PEX-AL-PEX), we often encounter situations where the plumber has to bypass obstacles in tight spaces. It may seem like a simple task – bending the pipe. In practice, however, it is an operation that requires a precise understanding of the material mechanics. One of the most common technical decisions a plumber makes before starting the work is the selection of the correct type of bending spring. At first glance, all the springs in the box may look the same: they are long, flexible steel spirals. The reality, however, is much more complex, and an incorrect choice can lead to kinks in the pipe, collapse of the pipe wall, or even the impossibility of pulling the spring out after the job is done.
The difference between inner and outer bending springs is not only in where you insert them, but also in the force you apply to the pipe material and how its geometry changes during the process. These two types have different physical principles of operation, different applications, and different risks when used incorrectly. This article will explain in detail why there are two categories, how to use them correctly in real conditions, and how to avoid common mistakes we see on construction sites every day.
The physics of bending: What happens to the pipe wall?
When you bend a pipe, you must be aware of what is happening at a microscopic level. The outer side of the arc is subjected to tension, while the inner side is subjected to compression. If we had no reinforcement, the thin wall of the pipe would fold into wrinkles on the inner side and could stretch too much on the outer side, leading to cracking. This is where bending springs come into play.
An inner spring has the task of filling the entire cross-section of the pipe and providing internal support against collapse. It works on the principle of "filling." When you insert it inside, it prevents the pipe wall from wrinkling on the inner side of the bend. It is an essential tool for larger pipe diameters, where the wall is thick enough to maintain the shape, but still needs an internal support beam. Without it, you risk the pipe "flattening" at the bend and reducing the flow area, which leads to increased hydraulic resistance in the system.
An outer spring works on a completely different principle. It does not act against internal pressure, but rather against deformation of the outer circumference. It is mainly used where it is important to maintain the circular shape of the pipe on the outer side of the bend and where an inner spring could be impractical or unnecessary. It is often used with thin-walled pipes or specific types of connections where an inner spring could interfere with the subsequent connection. Its design is intended to curve along with the pipe, but support the outer surface, thus preventing the formation of a "wave" on the outer side of the bend.
When and why to use an inner bending spring?
Inner springs are the standard for most installations, especially when working with copper pipes and multi-layer composite pipes. Their main function is to prevent the internal collapse of the pipe wall. When bending a pipe, the inner side of the arc shortens. If the wall is thin and the spring is missing, the material simply "folds" into wrinkles, similar to when you compress an empty soda can. These wrinkles not only reduce the cross-section of the pipe, but also create turbulence in the fluid flow and can serve as a place where dirt accumulates.
Practical example from practice: Imagine the installation of floor heating, where you have to bypass a load-bearing beam. The pipe has a diameter of 16 mm and a wall thickness of 2 mm. If you use only a manual bender without a spring, at a radius smaller than five times the diameter, the pipe will immediately flatten. The correct choice is Bending spring - 16x2; outer (see note below, in this case it is a specific application, but for this diameter, inner springs are used more often). For a typical installation of water supply lines in a bathroom, where you need to make a sharp bend around a pipe, it is essential to use a spring that fits inside. Choosing the correct size is critical – the spring must be slightly thinner than the inner diameter of the pipe to slide easily, but also strong enough to maintain the shape.
When selecting an inner spring, you should also consider the length. If you plan to make a long, smooth arc, you need a longer spring to support the entire length of the bend. If you use a spring that is too short, only part of the bent section will be supported, and the rest of the pipe will be unsupported, leading to irregularities. More information on how to determine the length you need for your installation can be found in our article what length of spring do I need for pipe installation.
When and why to use an outer bending spring?
Outer springs are a specialized tool that is often confused with inner springs, although their use is different. They are intended for situations where the inner space of the pipe is occupied or where the pipe construction is such that an inner spring could cause problems. For example, with certain types of connections where it is necessary for the inner surface to remain perfectly smooth and clean, or when bending pipes with very thin walls, where an inner spring could cause localized overloading of the material.
The main advantage of an external spring is that it does not interfere with the pipe cross-section. This means that after completing the bend, you do not have to deal with extracting the spring, which is often the biggest problem with internal springs. If the spring gets stuck, it can cause permanent damage to the pipe or you may have to cut it open. An external spring simply slides back after bending and that's it. This is crucial in installations where access to the back side of the pipe is limited and you cannot open it for inspection.
Practical example: When installing heating units, where it is necessary to make a precise bend to connect to the boiler, Bending Spring - 18x2; external is often used. This spring is designed to wrap around the outer shell of the pipe and provide the necessary stiffness against deformation on the outside of the arc. It is ideal for pipes with a diameter of 18 mm and a wall thickness of 2 mm, where an internal spring might tend to bend at the joint due to higher friction.
An important aspect is also the diameter. External springs are often available in specific dimensions that fit the outer diameter of the pipe precisely. If you use a spring that is too large, it will not press effectively against the pipe and the bend will be irregular. If it is too small, it could break during the bending attempt. Therefore, it is critical to follow the manufacturer's specifications. For a detailed comparison of the quality of spring steel from different manufacturers, which influences durability and safety, we recommend reading the comparison of spring steel quality from different manufacturers.
Technical analysis of parameters: Diameter and wall thickness
The choice between internal and external springs primarily depends on two factors: the outer diameter of the pipe and the thickness of its wall. It is not possible to use one universal spring for everything. Here is a technical look at how these parameters influence the decision.
| Parameter | Internal spring | External spring |
|---|---|---|
| Application | Prevention of inner wall collapse, thick-walled pipes. | Support of outer circumference, thin-walled pipes, special joints. |
| Installation | Insertion inside, extraction after bending (risk of jamming). | Wrapping around the pipe, easy removal (no extraction needed). |
| Suitable diameter | More commonly used for diameters of 16 mm to 32 mm and above. | Specific for 16 mm, 18 mm, 20 mm (depending on the model). |
| Risk of error | Spring jamming, damage to the inner surface. | Incorrect bend radius, slippage during handling. |
In practice, we often encounter the fact that installers use Bending Spring - 32x3; internal for larger diameters, where the need to prevent internal collapse is maximum. At a diameter of 32 mm and a wall thickness of 3 mm, the force required for bending is high, and without internal reinforcement, the pipe would immediately lose its circular shape. Conversely, for smaller diameters, such as Bending Spring - 26x3; internal, this spring is used where higher stiffness is needed for precise bends in tight spaces.
It is important to realize that the spring diameter should always be 0.5 to 1 mm smaller than the inner diameter of the pipe (for internal springs). If the spring is too tight, a large friction force will be created, making it difficult to insert or extract. If it is too loose, it will no longer perform its protective function. For external springs, it is important that they fit tightly to the outer diameter to prevent shifting during bending.
Gradual bending and prevention of damage
One of the most common questions we hear in consultations is how to make a bend without damaging the walls. Gradual bending is a key concept. You should never try to make a 90-degree bend in one go, especially if you are using a spring. The process should be carried out in several stages: first, the pipe is slightly bent, the spring adapts, and then the bend is continued.
You have probably already encountered the topic of gradual bending of pipes without damaging the walls. The key is hand movement. With an internal spring, you need to regularly change the direction of pressure to evenly distribute the spring along the entire length of the bend. With an external spring, it is necessary to ensure that the spring does not shift from its position. If you use the wrong type of spring, you may encounter the situation where the pipe starts to twist instead of bending, which leads to so-called "twisting" (torsion), a condition that is unacceptable in the system.
Safe pressure during the bending of copper and plastic pipes is another important factor. Copper pipes are relatively soft and easy to bend, but they tend to wrinkle. Plastic pipes (PEX) are more flexible, but when overheated or under excessive pressure, they can lose their properties. Therefore, it is necessary to follow the recommended temperatures and forces. If you feel confident in these procedures, you can look at our common questions about the purchase and use of bending springs, where you will find answers to specific scenarios.
Common mistakes and how to avoid them
Incorrect spring selection is one of the most common mistakes. Installers often use the same spring for different diameters, which leads to inefficient work. Another common mistake is ignoring the length of the spring. If you need to make a long arc and use a short spring, the result will be an uneven bend and possibly damage to the pipe at the points where the spring ends.
Another problem is common mistakes during the installation of bending springs. For example, if you do not secure the end of the spring before it enters the pipe, it may happen that the spring "bites" into the pipe wall and causes a crack. This is why it is important to use high-quality springs with a smooth surface. It is also important to make sure that you do not use too fast a movement during bending, which could cause an impact load.
There is also a problem known as "jamming". If the spring gets stuck in the pipe, how do you remove it? This is a topic we address in the article why the spring gets stuck in the pipe and how to remove it. The main cause is usually an incorrect diameter ratio or the presence of dirt in the pipe. If the spring gets stuck, never pull it with force, as this can damage the pipe. Instead, try gently tapping the pipe or using special extraction tools.
Maintenance and storage for long life
Willingness to invest in quality tools is only half the success. The other half is proper care. Bending springs are made of spring steel, which is prone to corrosion if not stored properly. After each use, the spring should be cleaned of dirt, oil, and moisture. We recommend storing it in a dry place, ideally in the original packaging or in a box where it will not be damaged.
Regular elasticity checks are also important. If the spring loses its elasticity, it will no longer perform its function and may cause pipe damage. Maintenance and storage of bending tools for long life is a topic that deserves the attention of every professional. Do not forget that a spring used for bending hot water should be thoroughly dried after cooling to prevent internal corrosion.
Conclusion
The choice between internal and external bending springs is not just a matter of preference, but a technical decision that will affect the quality and safety of the entire installation. Internal springs are essential for protecting the inner wall of the pipe from collapse, while external springs offer an alternative for specific applications where it is important to preserve internal space or avoid extraction. The correct choice depends on the diameter, wall thickness and specific application. Remember that the quality of the spring and its proper use are key to a successful installation. If you have any doubts, always consult a specialist or study our educational materials.
Bending Geometry: The Effect of Radius on Cross-Section Deformation
Bending moment principle: Why the external spring works differently
Critical extraction point: Where the spring most often gets stuck
Frequently Asked Questions (FAQ)
Can I use an internal spring for external type bending?
No, it is not suitable. An internal spring is designed to act against internal pressure. If you use it for external support, it will likely not achieve the correct shape and may damage the pipe. Each type of spring has a specific design for its purpose.
What to do if the spring gets stuck in the pipe?
Never pull it suddenly. Try gently tapping the pipe to free the spring. If that doesn't help, try using a special extraction tool or consult a specialist. Strong pulling can damage the pipe and the spring.
How long should the spring be for a 90-degree bend?
The length of the spring should be at least equal to the length of the arc you want to make. For a 90-degree bend with a 5-times pipe diameter radius, the spring should be approximately 1.5 times this radius in length to ensure the entire bend is supported.
Why does my spring corrode quickly?
Corrosion is caused by moisture and dirt. Make sure to clean and dry the spring after each use. Store it in a dry place. If the spring is exposed to aggressive environments, consider using a spring with a chrome surface treatment.
Is there a price difference between internal and external springs?
Prices may vary depending on the manufacturer and material. In general, specialized external springs can be slightly more expensive due to their specific construction, but the differences are often minimal. It is more important to choose a quality product that meets your requirements.
How often should I replace the bending spring?
You should replace the spring if it loses its elasticity, has visible cracks, or is heavily corroded. In case of intensive use, it is recommended to inspect it after every ten uses. If the spring is damaged, it can cause damage to the pipe, so it is better to replace it in a timely manner.
Do you have a question on this topic?
Not sure or dealing with a specific situation in your home? Write to us – we are happy to help.