Cutting ring fittings for connections in hydraulic lines

Hydraulic systems are characterized by a liquid medium and high internal pressures. They are also subject to large temperature differences.

These present major challenges for the assembly of the required lines, which can only be met with adapted construction methods. A typical form of connection for hydraulic lines is the cutting ring fitting. They can be used for both rigid and flexible lines.

Properties of the cutting ring fitting

The cutting ring fitting ensures a tight transition from one line section to the next. They can be produced in rigid lines made of plastic, steel, copper, or aluminum, as well as in rubber hoses.

For flexible lines, additional internal sleeves are required to absorb the compressive forces of the cutting ring during assembly. The production of a cutting ring fitting is a standardized process that is the same for all materials and diameters.

Elements of a cutting ring fitting

It consists of four elements:

• Line
• Cutting ring
• Union nut
• Connector fitting

A correctly assembled cutting ring fitting ensures consistent tightness without the use of an additional seal. Correct assembly is crucial for stability and freedom from leaks.

Assembling the cutting ring fitting

The cutting ring fitting is assembled as follows:

1. Prepare the line
2. Attach the union nut
3. Place the cutting ring
4. Insert the line into the connector fitting
5. Tighten the union nut hand-tight
6. Mark the union nut
7. Turn the union nut a further 1 1/4 turns

The line must be cut straight. Crooked cuts jeopardize the tightness of the connection. After the cut, the line is deburred. Internal chips must also be completely removed, as they would otherwise contaminate the hydraulic medium.

Errors with cutting ring fittings

A cutting ring fitting can be over-assembled or under-assembled. Over-assembly occurs when the cutting ring is pressed in too deeply. This often happens with smaller diameters. Technicians consider the connection not tight enough and tighten the union nut beyond the prescribed limit.

The opposite case often occurs with large diameters. Here, the technician’s strength is not sufficient to perform the required 1 1/4 turns. The connection remains under-assembled and can come loose. For large diameters, an extension for the open-ended wrench should therefore always be ready. This increases the leverage so that the assembly can be carried out correctly.

Avoiding galling

Identical materials have a tendency to gall. This means that they first jam against each other and then wear away internally, so that a firm connection is no longer possible. Therefore, the union nut and the connector fitting should consist of different materials. Common pairings are stainless steel to steel, aluminum to steel, or stainless steel.

If no different material pairing is possible, the threads must be pre-treated with a suitable lubricant. This sufficiently prevents galling so that the tightness of the connection remains guaranteed.