High quality spring guards for hose protection

Function and Application of Bend Restrictor Springs

Bend restrictor springs protect flexible hoses at fittings from damaging bends, kinks, and premature wear. They distribute bending stress over a defined radius of curvature, thereby extending the service life of hose assemblies in pneumatic, hydraulic, and media technology systems. Typical areas of application include mechanical engineering, conveying technology, assembly automation, and laboratory equipment, where moving or frequently redirected hose lines experience high cyclic loads.

Materials and Corrosion Protection

Bend restrictor springs are predominantly made from brass, nickel-plated brass, and stainless steel. Brass offers a good combination of formability, electrical conductivity, and corrosion resistance in dry, non-aggressive environments. Nickel-plated brass additionally increases surface hardness and improves corrosion protection as well as the visual appearance, which is relevant in industries with increased hygiene or cleaning requirements. For aggressive media, saltwater applications, or elevated temperature requirements, stainless steel (e.g., AISI 304/1.4301 or 316/1.4401) is the preferred choice due to its resistance to chemicals and temperature fluctuations. When selecting the material, environmental conditions, cleaning agents, EMC requirements, and mechanical stresses must be considered.

Designs and Dimensions

Bend restrictor springs differ in length, inner and outer diameter, wire thickness, and coil density. Spiral springs with constant coil density have proven effective for uniform bend limitation. Conical designs are used when a controlled transition from the rigid connection to the flexible hose is required. The internal clearance must match the hose diameter: too large an internal space reduces the protective effect, while too narrow an internal space can pinch the hose and affect media flow. Common sizes for pneumatic and hydraulic fittings cover nominal diameters from 4 mm to 25 mm; special designs are possible for larger cross-sections and customized applications.

Connection Types and Installation

Bend restrictor springs are mounted directly onto the hose fitting and, in many cases, securely fixed by clamping rings, crimps, or threaded rings. In push-in fitting systems, the spring is pushed over the fitting and held in place on the fitting by form-fitting clamping seats. For fittings with O-ring seals, care must be taken to ensure that the spring does not exert pressure on the seal to avoid leaks. When used with crimp or press-fit hose assemblies, it is recommended to use a spring that is adapted to the outer diameter of the crimp to ensure flush load distribution.

Seals, Tolerances, and System Compatibility

The integration of bend restrictor springs into fitting systems requires knowledge of tolerances and seal design. Seals such as O-rings, flat gaskets, or metallic seals must not be subjected to lateral stress by the spring. Tolerance play between the spring's inner dimension and the hose's outer dimension must be strictly maintained: typically 0.2–0.5 mm clearance, depending on temperature and material expansion. In applications with pressure-bearing hose lines, the static and dynamic load must be checked to avoid the transfer of bending moments to the sealing surfaces.

Temperature and Media Resistance

Material selection depends on the operating temperature and the media conveyed. Brass and nickel-plated variants are suitable for -20 °C to +120 °C, depending on the alloy composition and galvanic layer. Stainless steel variants often cover a larger temperature range (-50 °C to +300 °C) and maintain mechanical strength and corrosion resistance. For aggressive media, solvents, or high sterilization cycles, coated or specially alloyed springs are necessary. In high-temperature applications, changes in length and relaxation of the spring coils must be considered.

Selection Criteria: How to Choose the Right Bend Restrictor Spring

Select the spring based on the hose outer diameter, required radius of curvature, expected bending cycles, environmental influences, and required corrosion protection. As a rule of thumb, the protection radius should be less than or equal to the minimum permissible bending radius of the hose used, so that the spring bears the load. Check mechanical load changes, temperature cycles, and chemical influences before deciding between brass, nickel-plated, or stainless steel. Also, pay attention to compatible fastening types for the existing fitting and ease of installation for maintenance and replacement.

Practical Examples

Example 1 – Pneumatic Assembly Axis: On a pick-and-place unit, 6 mm PU hoses with quick-release fittings are used. The nickel-plated brass bend restrictor spring is pushed over the fitting and fixed against the fitting with a conical seat, the radius of curvature reduces bending stress due to frequent cycles and prevents premature bursting of the hose. Maintenance interval: visual inspection every 3 months, replacement after 1.5 million cycles.

Example 2 – Hydraulic Connection on an Industrial Robot: For high-pressure hydraulic lines with crimp connections, a stainless steel bend restrictor spring with a corresponding internal fit is pushed over the crimp. The spring is fixed to the crimp by an additional collar clamp, so that under dynamic load, the bending moment is not transferred to the sealing surface of the connection. Result: fewer micro-cracks in the hose jacket and extended system life.

Example 3 – Laboratory System with Chemical Conveyance: In a process line conveying aggressive solvents, a stainless steel spring in combination with a PTFE hose is used for chemical resistance. The spring protects the hose during redirections and facilitates visual inspection for leaks without compromising the chemical resistance of the line.

Installation Instructions and Care

Mount the spring so that the winding direction supports the natural bending movement of the hose. Avoid sharp edges on fittings and crimps that could damage the windings. Clean nickel-plated and stainless steel surfaces regularly, use suitable cleaning agents without chloride-containing ingredients to avoid galvanic corrosion. After installation, check the routing of the hose in the working area and reduce over-twisting by additional hose clamps or strain relief elements.

Standards and Testing Requirements

Bend restrictor springs are part of the overall system and must be tested in conjunction with hoses and fittings regarding pressure, temperature, and service life properties. Relevant standards may refer to the hoses used, connections, and pressure tests; an individual test of the spring usually includes corrosion tests, dimensional accuracy, and resilience after bending cycles. Document test and maintenance intervals in the machine file and conduct load tests when the spring and hose are combined in new applications.

Compatible Products and Further Information

For technical details on fittings, materials, and application examples, please refer to our overview pages: technical data on fittings and connections can be found at https://maku-industrie.de/technik. Practical descriptions and application examples of other components are documented at https://maku-industrie.de/anwendungsbeispiele. Use this information to select the spring, hose, and fitting in a system-compliant manner.

• Important selection parameters: hose outer diameter, radius of curvature, environmental conditions, fastening type, material (brass, nickel-plated brass, stainless steel), temperature and media resistance.

FAQs

1. Which bend restrictor spring is suitable for my hose?
Select the spring based on the hose outer diameter, minimum bending radius, and environmental influences. For dry, non-aggressive environments, brass or nickel-plated variants are suitable; for corrosive media and higher temperatures, stainless steel.

2. Does the bend restrictor spring affect the seal of the fitting?
When correctly dimensioned and installed, the spring relieves stress on the seal. Ensure a clearance of typically 0.2–0.5 mm between the spring's inner dimension and the hose's outer dimension and avoid jacket contact that could exert lateral pressure on seals.

3. How often must a bend restrictor spring be maintained or replaced?
Perform visual inspections regularly; in dynamic applications, checks every 3 months are recommended. Replacement intervals depend on load cycles, environment, and material; documented service life tests or manufacturer specifications should be consulted.