Knowledge · Connection Technology
Assembling a cutting-ring fitting correctly
A cutting-ring fitting only seals when the ring cuts cleanly into the tube, and that comes from the right number of turns, not from feel. DIN 2353 requires a two-stage, turn-controlled assembly for it: first the pre-assembly, in which the ring raises the collar, then the final assembly in the fitting body. This guide states the turn counts per phase and manufacturer and the inspection criteria by which you recognize a good pre-assembly.
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Pre-assembly
The hard cutting edge has to bite in a defined way into the tube outer diameter (tube bite) and raise a visible collar of material ahead of the edge (Bundaufwurf) that work-hardens. Parker names three effects: tube bite, tube clamping and a spring effect that absorbs pressure surges and bending cycles. That is why the ring is pre-assembled separately first, before the fitting is finally closed.
Pre-assembly is done in a hardened pre-assembly stud or directly in the fitting body. The hardened stud protects the reusable body and delivers a reproducible bite; per manufacturer guidance it is replaced after about 50 pre-assemblies. The tube must be cut square, burr-free and to length; the series and their dimensions are explained in the article DIN 2353: series LL, L and S.
For small sizes and single assembly, the one-stage direct assembly in the fitting body is often enough, without a separate pre-assembly stud. EXMAR states 1 1/4 to 1 1/2 turns for the cutting ring, Landefeld about 1 turn for the DIN cutting ring and about 1 1/4 for the NC clamping ring. For series assembly, large sizes and stainless steel, pre-assembly in the hardened stud stays the safe choice, because it drives the ring evenly and reproducibly into the tube and protects the reusable body from wear.
- Cut the tube square and burr-free, deburr inside and out. A slanted or burred cut prevents the clean raised collar.
- Lightly oil the thread, cutting ring and tube, no grease and no dry running. Slide the ring on the right way round: the cutting edge points toward the tube end.
- Push the tube up to the felt stop into the pre-assembly stud or body and hold it there. The tube must not lift off the stop during pre-assembly.
- Turn the union nut on by hand until finger-tight. This point is the reference point for counting turns.
- From the finger-tight state, turn the nut on by the specified amount: about 1 1/4 turns (VOSS, HANSA-FLEX in the body) up to 1 1/2 turns (Parker optimized EO pre-assembly, HANSA-FLEX in the hardened stud). Eaton Walterscheid states 1 to 1 1/2 turns.
- Remove the tube with the ring and visually check the pre-assembly before final assembly.
Final assembly
In final assembly the pre-assembled tube is inserted into the final fitting body and the union nut is tightened until the ring reaches its end position and seals metallically. The reference point here is no longer finger-tight but the clearly noticeable rise in force.
The rise in force is clearly perceptible: the resistance at the nut increases sharply as soon as the pre-assembled ring seats on the 24-degree cone of the body. From this fixed point the small re-tightening angles count, not from finger-tight. Parker has optimized the earlier final assembly of 90 degrees, that is a quarter turn, down to 30 degrees to lower the assembly force; Eaton Walterscheid likewise controls final assembly over 30 degrees. Which value applies is stated by the instructions of the fitted brand.
- Insert the pre-assembled tube into the fitting body until the ring seats on the 24-degree cone.
- Turn the union nut on by hand and tighten until the rise in force is clearly noticeable.
- From this point tighten firmly: about 1/4 turn for steel (VOSS, HANSA-FLEX), about 1/2 turn for stainless steel and for repeat assembly (HANSA-FLEX).
- Optimized methods work angle-controlled: Parker EO at 30 degrees (about 1/12 turn), Eaton Walterscheid at 30 degrees controlled. These reduce the assembly force; follow the instructions of the fitted brand.
- For repeat assembly, tighten the nut only up to the rise in force and a short way beyond; the ring is already cut in.
| Manufacturer | Pre-assembly | Final assembly |
|---|---|---|
| VOSS (ES-4) | 1 1/4 turns (1 to 1 1/2) | 1/4 turn |
| HANSA-FLEX | 1 1/4 turns in the body, 1 1/2 in the pre-assembly stud | 1/4 (steel), 1/2 (stainless) |
| Parker EO (optimized) | 1 1/2 turns | 30 degrees (about 1/12 turn) |
| Eaton Walterscheid | 1 to 1 1/2 turns | 30 degrees controlled |
| EXMAR | about 1 turn (with pre-assembly stud) | 1/4 to 1/2 turn |
The most common assembly mistakes
Most leaking cutting-ring fittings trace back to an assembly mistake, not to a defective part. Almost every one arises in one of the two assembly steps and is avoidable with a clean pre-assembly and the visual check of the raised collar. How to narrow down a leaking fitting systematically is in its own article; here are the mistakes to avoid at assembly:
- Tube cut slanted or burred: the ring cuts unevenly, the joint leaks or loosens under pressure.
- Tube not pushed up to the stop: the raised collar stays too small, the ring does not hold the tube securely.
- Too few turns, rise in force ignored: the ring does not cut deep enough, the tube can slip out under pressure.
- Too many turns, nut overtightened: the ring is crushed and the tube constricted, cracks and fatigue failures follow.
- Worn pre-assembly stud (beyond about 50 pre-assemblies): the bite becomes uneven and insufficient.
- Visual check of the raised collar skipped: a faulty pre-assembly goes unnoticed and only shows in operation.
- Assembled dry or with the wrong lubricant: friction falsifies the turn assembly, the ring can gall.
- Parts of different series or systems mixed: geometry and angle do not match, the joint does not seal.
Matching fitting and tube from stock
Cutting ring, union nut, fitting body and the matching tube come from PTS in one place, in the L and S series and in steel or stainless. State series, size and material, and the technical team assembles the combination and the assembly tool.
Prices and stock are visible in the catalog without signing in.