Cutting rings: why a pre-assembly machine is needed and how the LT-KT42 works
The reliability of a DIN 2353 joint is determined by one operation — the ring edge biting into the tube. We explain why it is done on a machine, how pre-assembly on the LT-KT42 works and how to check the result.
The cutting-ring joint to DIN 2353 remains the most common way to connect a rigid tube to a hydraulic system. It is simple in composition — a fitting body with a 24° cone, a union nut and the ring itself — but its reliability is entirely determined by one operation: the ring edge biting into the tube. That is why serious workshops perform this operation not with a spanner on site but on a pre-assembly machine. Below is how it works and what it gives.
How a cutting ring holds
When the nut is tightened, the body cone compresses the ring, and its hardened front edge cuts into the outer surface of the tube. Ahead of the edge a collar of displaced tube material forms — it is what keeps the tube from being pulled out, while the tight fit of the ring to the cone provides the seal. The joint works only at the correct bite depth: too shallow, and the tube slips out under pressure; too deep, and the tube wall deforms while the ring loses its resilience.
The drawbacks of this design and the alternative — tube end forming — are covered in the article on Walform technology. Here we deal with how to make the classic joint correctly.
Why pre-assembly is done on a machine
Biting the ring is the most critical and most laborious part of installation. It can be done on site, in the fitting body, but this approach has four problems:
- Force. On 25–42 mm tubes the bite requires a tightening torque that a fitter with a spanner applies with difficulty, and in a confined space with the risk of under-tightening.
- Fitting wear. When the bite is made in the body, the 24° cone acts as a tool: after several assemblies it wears and the seal deteriorates.
- No control. While the ring is in the body, the result cannot be seen. The collar can only be checked by taking the joint apart.
- Scatter. "One and a half turns after contact" is done differently by everyone; on a batch of a hundred tubes that means a hundred different joints.
A pre-assembly machine moves the operation to the preparation area. The bite is made in a hardened 24° sleeve rather than in the fitting body, with a set force and stroke. The tube arrives at installation with the ring already seated, and the fitter only has to insert it into the body and tighten the nut with a small final turn as instructed by the fitting manufacturer.
How the LT-KT42 works
LT-KT42 is an electrically driven machine for the pre-assembly of cutting rings on tubes from 6 to 42 mm. The operator places the nut and ring on the tube, inserts the tube into the pre-assembly sleeve to the stop and starts the cycle: the drive moves the assembly through a set stroke, and the ring edge bites into the tube to the design depth. The cycle takes seconds, and the force is the same for every tube in the batch.
- Tooling for L and S series. Pre-assembly sleeves for the light and heavy DIN 2353 series from D6 to D42 — the full size range of the standard.
- Steel and stainless steel. Materials handled — ST37.4 and AISI 316Ti; the corresponding tooling is provided for stainless tubes.
- 37° flaring. An optional unit turns the machine into a JIC (SAE J514) flaring machine: the same drive forms the 37° cone for American-standard connections.
- 220 V supply. Single-phase mains, 245 W motor — the machine sits on a bench in any workshop.
| Parameter | Value |
|---|---|
| Pre-assembly range | D6×1 – D42×4 mm |
| Materials | ST37.4, AISI 316Ti |
| Supply / power | 220 V, single phase · 245 W |
| Tooling | 24° pre-assembly sleeves (L/S), 37° flaring unit — options |
| Dimensions / weight | 650×600×260 mm · 71 kg |
Inspection after pre-assembly
The main advantage of the machine is that the result is visible before installation. After the cycle the tube is removed and inspected:
- Ahead of the ring edge, all around the circumference, a continuous collar of tube material must be visible, filling the space in front of the edge.
- The ring must not move along the tube; slight rotation is permissible.
- The tube must not be deformed: a narrowed wall or a dent is a sign of excessive stroke.
- The tube end must be at the stop: a tube not fully inserted into the sleeve gives a bite in the wrong place.
Final assembly on site is then reduced to seating the tube in the body and tightening the nut a fraction of a turn — as instructed by the fitting manufacturer.