A push-in fitting seals on the outside of a plastic tube with an O-ring and holds it with a toothed gripping ring. A cutting-ring fitting bites into a metal tube and forms a mechanical joint that cannot be pulled apart. One is assembled by hand in two seconds and reused freely; the other takes a spanner, a torque figure and a tube that has been cut and deburred properly. They are not alternatives for the same job.
This guide covers where each belongs, what pressure each really holds, how to assemble both without creating the leak you will spend a shift chasing, and what happens when someone mixes tube materials.
The two joining principles
Push-in (plug-in) fittings. The tube is pushed through a gripping ring into the body. An O-ring seals against the tube's outside diameter. The gripping ring's teeth angle inward, so pulling on the tube tightens the grip; pressing the release collar retracts the teeth and the tube comes out.
Everything about the joint depends on the tube's outside diameter being round, clean and within tolerance. That is why push-in fittings are specified for a nominal tube size, not a thread, and why they belong on plastic tubing where that tolerance is controlled.
Cutting-ring (compression) fittings to DIN 2353. A nut drives a hardened ring along a 24° cone in the fitting body. The ring's leading edge cuts into the tube wall and turns up a collar of material ahead of it. That collar is the mechanical lock; the seal is the metal-to-metal contact between ring, cone and tube.
The joint is made once, properly, with a defined number of turns past finger-tight. It can be disassembled and remade, but the ring stays with its tube and the assembly is never quite as good as the first time.
Side by side
| Push-in fitting | Cutting-ring fitting (DIN 2353) | |
|---|---|---|
| Tube material | Plastic: PA, PU, PE | Metal: steel, stainless, soft copper |
| Typical working pressure | 10 to 16 bar | 100 to 800 bar depending on series |
| Assembly | By hand, seconds | Spanner, defined turns, often a pre-assembly tool |
| Disassembly | Unlimited, press the collar | Possible, ring stays on the tube |
| Vibration tolerance | Good, the tube flexes | Excellent, if correctly pre-assembled |
| Temperature limit | Set by the tube, typically 60 to 80 °C | Set by the seal, up to 200 °C or more |
| Where it fails | O-ring on a damaged tube surface | Under-tightened ring, or an over-tightened one that cuts through |
The pressure difference is the headline. Push-in fittings live in a 10 bar world. Cutting-ring fittings in the L (light) series handle 100 to 315 bar and the S (heavy) series goes well beyond. If the line is hydraulic, the question does not arise.
Series, not just size: L and S
DIN 2353 fittings come in series, and the series is part of the specification, not an afterthought.
- LL (extra light) for instrumentation and very low pressure.
- L (light) for general hydraulics up to roughly 315 bar depending on tube diameter.
- S (heavy) for high pressure, with thicker tube walls and a more substantial body.
L and S fittings of the same nominal tube size are not interchangeable. The cone angle is the same 24°, but thread sizes and ring dimensions differ. An L nut will start on an S body and then jam, which is how cross-threaded fittings and stripped bodies happen in a hurry.
Tube wall thickness matters as much as diameter. A 12 mm tube with a 1.5 mm wall and a 12 mm tube with a 2 mm wall need different rings and reach different pressures. Specify outside diameter and wall.
Assembling a cutting-ring joint that does not leak
- Cut square. A tube cutter, not a hacksaw. An angled cut means the ring bites unevenly and the joint weeps on one side.
- Deburr inside and out. The internal burr restricts flow and eventually breaks off into the system; the external burr stops the ring seating.
- Pre-assemble in a pre-assembly tool or a hardened pre-assembly body where the fitting series calls for it. Pre-assembling directly in the fitting body wears the cone on the part that has to seal for the rest of its life.
- Tighten to the defined turns past the point of resistance, following the manufacturer's figure, not feel. Under-tightening leaves the ring short of cutting; over-tightening cuts through the tube wall and the tube fails at the ring under vibration.
- Check the collar. After pre-assembly, a visible collar of material should stand proud ahead of the ring around the full circumference. No collar means no joint.
- Final assembly into the fitting body, tightened to the specified torque.
The most common failure found in the field is a joint that was made by feel, held on the test and failed three months later under vibration. The turns figure exists because feel is not repeatable between fitters.
Assembling a push-in joint that does not leak
Far simpler, and still worth getting right.
- Cut square with a tube cutter. Side cutters crush the end oval and the O-ring cannot seal on an oval tube.
- Check the tube's outside diameter is undamaged for at least the insertion depth. A scratch running along the tube is a leak path straight past the O-ring.
- Push fully home, past the gripping ring, until the tube meets the internal shoulder. A tube that stops at the ring holds at rest and releases under pressure.
- Pull back gently to confirm the teeth have engaged.
- On tubing that will be pulled or that runs vertically, use fittings with a locking clip if the range offers one.
Mixing materials: what goes wrong
Plastic tube in a metal cutting-ring fitting. The ring will cut through the tube wall. Some ranges offer a support sleeve that lets a plastic tube be used in a compression fitting; without it, the joint fails, usually not immediately.
Metal tube in a push-in fitting. The gripping ring cannot bite a hardened surface and the O-ring may seal against a tube that is out of tolerance for it. Some manufacturers list push-in fittings rated for soft copper or thin-wall metal tube; if the datasheet does not say so, do not.
Brass fittings with stainless tube in a corrosive environment. Dissimilar metals in the presence of an electrolyte produce galvanic corrosion, and the fitting is normally the part that goes. Match materials, or accept and plan for the consequence.
Where each one belongs
Compressed air on a machine, 6 to 10 bar, plastic tubing, frequent reconfiguration. Push-in. Assembly speed and free disassembly are the whole point.
Hydraulic line, any pressure above about 20 bar, metal tube. Cutting-ring to DIN 2353, in the series the pressure calls for.
Instrumentation, small bore, moderate pressure, metal tube. Cutting-ring in the LL or L series, or a dedicated instrumentation fitting where the application needs the extra precision.
Line that will be broken and remade often, at pressure. Neither. Use a quick coupling designed for repeated connection.
A specification checklist
- Tube material and its outside diameter, with wall thickness for metal tube.
- Working pressure, and the series it puts you in for DIN 2353.
- Temperature at the fitting, which sets the seal material.
- Medium, for compatibility with both the seal and the body material.
- Body material, matched to the tube material to avoid galvanic pairs.
- Thread form on the port side (BSP, NPT, metric), and how it seals: O-ring, elastomer washer or taper.
- Assembly tooling available on site, if the series calls for pre-assembly.