Metric Compression Fittings (DIN 2353): The Global Standard for Hydraulic Connections

Technical Breakdown: How DIN 2353 Fittings Work

Metric compression fittings, standardized under DIN 2353 and ISO 8434-1, are a type of flareless tube fitting designed to connect metric-sized rigid tubing. They consist of three main parts: the body, a ferrule (also called a cutting ring), and a nut. When assembled, the ferrule "bites" into the surface of the tube, creating a strong mechanical grip and a reliable metal-to-metal seal. The inside of the fitting body features a 24° cone seat, and the ferrule is forced against both the tube and this cone as the nut is tightened. No O-ring or sealant is required, as the seal is achieved by the ferrule deforming and cutting into the tube.

Pressure Series: LL, L, and S

DIN 2353 fittings come in three series to accommodate different pressure requirements:

• LL (Very Light): For very low-pressure applications.
• L (Light): For low to medium-pressure systems, with some sizes rated up to ~4500 PSI (315 bar).
• S (Heavy): For high-pressure applications, with a more robust design capable of handling pressures up to ~7200 PSI (500 bar) or more in certain sizes.

The threads on these fittings are metric, which corresponds to the tube OD and series (e.g., an M18x1.5 thread might be used for a 10mm L-series or 8mm S-series fitting).

Why Are DIN Fittings So Widely Used?

Metric compression fittings are extremely popular because they provide a reliable, reusable connection for rigid tubing without needing to flare the tube end, which is a significant advantage over JIC systems. They are essentially the metric world's standard for high-performance tube fittings.

Common Applications

• European and Asian Machinery: Equipment from Germany, Italy, Japan, and other regions heavily utilizes DIN fittings. A CNC machine, a high-end press, or an excavator from these regions will likely feature DIN 24° cone fittings on its rigid hydraulic lines.
• Factory Automation: The clean, robust plumbing provided by DIN fittings is ideal for automated manufacturing lines.
• Mobile Hydraulics: Many pieces of modern construction and agricultural equipment use metric tube fittings for control, brake, and steering lines.
• Instrumentation: Bite-type fittings are also a standard in testing equipment and instrumentation panels where precise, leak-free connections are required.

Common Issues and Best Practices

Proper Assembly is Key

The reliability of a DIN 2353 fitting hinges on the correct "pre-setting" of the ferrule. This process involves using a pre-assembly tool (or the fitting body itself in a vice) to tighten the nut a specified number of turns (e.g., 1.25 turns past snug). This initial tightening drives the ferrule into the tube, creating the bite and forming the seal. Once pre-set, the fitting can be installed in its final location and tightened to its final torque value.

Troubleshooting Leaks

• Under-tightening: If the ferrule did not bite sufficiently into the tube during the pre-set, the connection will leak.
• Over-tightening: Excessive torque can crush the ferrule or crack the nut, compromising the seal.
• Tube Quality: The tubing must be of appropriate hardness. Tubing that is too soft may not allow the ferrule to bite correctly, while tubing that is too hard may prevent it from biting at all.
• Debris: Always ensure the tube end is cut square, deburred, and perfectly clean before assembly. A small metal chip can interfere with the seal.

Reusability

A major advantage of DIN fittings is their reusability. Once a ferrule is set on a tube, that connection can be disassembled and reassembled multiple times without issue. However, the ferrule itself should not be reused on a new piece of tubing; a new ferrule should always be used for a new connection.

Conclusion

Metric DIN 2353 compression fittings are reliable, high-performance connectors that are the global standard for rigid hydraulic tubing in many industries. They allow for clean, robust plumbing without the need for flaring, and once understood, they are straightforward to work with. For any business dealing with international equipment, getting comfortable with DIN 2353 fittings is essential for effective service and support.