An important measure of hose flexibility is the Minimum Bend Radius (MBR). While most everyone understands that this is the smallest radius to which a hose can be bent, the method of measuring this radius is often misunderstood. Additionally, while most hose constructions list a single MBR, corrugated metal hose lists two: a static and a dynamic MBR. Let’s take a minute to review the differences between static and dynamic MBR’s and when they should (or should not) be used.
Definitions
According to the NAHAD Hose Safety Institute Handbook for the Design and Specification of Hose Assemblies, here are some key definitions:
- Bend radius: the radius of a bent section of hose measured to the innermost surface (R1) of the curved portion. Some manufacturers may measure to the centerline (R2) of the curved portion.
- Bend radius, static: the smallest radius at which a hose can be used without kinking while bent or flexed into a fixed position.
- Bend radius, dynamic: the smallest radius at which a hose can be used without kinking while constant or continuous flexing occurs.
For corrugated metal hoses, the minimum bend radius is typically measured to the centerline of the hose, or R2 in the above diagram. However, which bend radius should be used in an installation: static or dynamic? It depends on if dynamics are present.
What are “Dynamics” as related to metal hose?
Most articles that discuss the flexibility of hoses refer to dynamics as the constant or intermittent flexing of the assembly. While this is true, there are other application variables that should also be classified as dynamics, as they can have a significant effect on the cycle life of the assembly.
Here are some examples of dynamics:
- Flexing/bending the assembly
- Vibration
- Large pressure swings
Also, if severe dynamic pressures are encountered (i.e. pressure pulsations or spikes), additional derating factors should be applied to protect the corrugations from these extreme dynamics.
When to use the Static vs. the Dynamic Minimum Bend Radius?
So, when is it okay to install a metal hose to its minimum static bend radius? Unfortunately, the answer is “not often”, because most hose applications include dynamics. Let’s face it: if there is no need for a flexible hose to accommodate movements or vibration, then the pipe fitter would hard-pipe the whole thing and be done with it! Where we do see static applications is typically in installations where some type of offset is present in the piping system that is easier to accommodate by using a flexible hose rather than a series of pipe fittings. Not only is the hose easier to install, but there are fewer connections points, each of which is vulnerable to leakage.
The best course of action is to install a corrugated metal hose in as gentle of a bend as possible, even in static applications. But when that is not possible or practical, understanding the allowable radius to which a hose can be installed is critical. Additionally, knowing the installed radius in an application allows the use of hose length calculation formulas to ensure there is enough hose in the assembly to accommodate offsets and movements. If you are not sure that an application fits neatly into the “dynamic” or “static” category, it is recommended to use the dynamic minimum bend radius for that installation. It is also good to remember that every application is unique, sometimes requiring engineering involvement to ensure proper assembly design, installation, and use. Our team of sales managers and factory representatives are happy to assist you with application expertise, on-site surveys (live or virtual), and product recommendations. So, don’t get bent out of shape! Contact our friends at to make sure every metal hose assembly provides maximum service life, safety, and value.
The original article was written by Frank Caprio and can be found on Hose Master's website here.
For more information about metal hose applications or to speak to an engineer, contact Gallagher Fluid Seals.