Common faults and solutions of external wire joints

External thread fittings (i.e. external thread fittings) are commonly used connectors in hydraulic, pneumatic, and industrial piping systems, and their failure may lead to leakage, pressure drop, and even system paralysis. The following is a detailed analysis and solution of common faults in external wire joints, covering the entire life cycle of design, installation, and maintenance.

1、 Common fault types and cause analysis

1. Leakage fault

External leakage (hydraulic oil/gas seeping out from the threads)

Reason:

The threads are not tightened or the torque is insufficient, resulting in insufficient pressure on the sealing surface.

Uneven application or incorrect selection of thread sealant (such as using ordinary sealant under high temperature conditions).

Seals (such as O-rings, combination gaskets) are damaged, aged, or installed in the wrong direction.

The low precision of thread machining (such as tooth profile angle deviation and excessive surface roughness) leads to seal failure.

Uneven cutting of pipelines, burrs scratching the sealing surface.

Internal leakage (medium leaks from the inside of the joint through the threaded gap)

Reason:

Defects in joint design (such as failure to use tapered pipe threads or unreasonable sealing structure).

Long term high-pressure impact causes wear or deformation of the sealing surface.

Excessive corrosiveness of the medium can damage threads or sealing materials.

2. Vibration and looseness

Reason:

The system vibration frequency resonates with the natural frequency of the joint, causing the thread to gradually loosen.

Insufficient thread pre tightening force, without considering the effects of thermal expansion, contraction, or pressure fluctuations.

The stiffness of the joint and pipeline materials does not match (such as metal joints connecting plastic pipelines), resulting in relative displacement.

3. Difficulty installing or getting stuck

Reason:

Thread corrosion or impurities getting stuck can increase the resistance to screwing in.

The installation angle of the sleeve type connector sleeve is incorrect, and it bites too deep or too shallow into the pipe wall.

Using non one-to-one tools such as adjustable wrenches can result in uneven stress on the threads and deformation.

4. Pipeline rupture or joint fracture

Reason:

The bending radius of the pipeline is too small, and forced twisting during installation leads to stress concentration.

The strength of the joint material is insufficient (such as the misuse of low-carbon steel instead of stainless steel), making it unable to withstand system pressure.

Excessive tightening leads to excessive stress at the root of the thread, resulting in cracks.

2、 Troubleshooting and preventive measures

1. Leakage fault handling

External leakage:

Re tighten: Use a torque wrench to re tighten according to the manufacturer's recommended value (such as 15-20N · m for carbon steel joints) to avoid thread damage caused by over tightening.

Replace seals: Check whether the O-ring or combination gasket is aging or scratched, replace it with a new seal compatible with the medium, and confirm that the installation direction is correct.

Apply sealant: After thoroughly cleaning the threads, apply one-on-one sealant (such as Loctite 542 for metal threads and Loctite 577 for plastic threads), evenly covering the male threads.

Attentive service sealing surface: If there are slight scratches on the threads or sealing surface, they can be polished flat with oilstone or sandpaper; Severe damage requires replacement of the connector.

Internal leakage:

Upgrade joint design: Use tapered pipe threads (such as NPT, BSPT) or add sealing structures (such as combination gaskets, metal retaining rings).

Replace corrosion-resistant (based on actual reports) materials: For strong acid and alkali media, use corrosion-resistant (based on actual reports) joints such as Hastelloy and titanium alloys.

Regularly replace seals: shorten the seal replacement cycle according to working conditions (such as replacing seals every 500 hours in high temperature and high pressure environments).

2. Vibration and loosening prevention

Add anti loosening measures:

Use double nut anti loosening (top nut) or spring washer.

Apply anti loosening adhesive (such as Loctite 243) to the thread and form an elastic locking layer after curing.

Use welding or clamps for secondary fixation of key joints.

Optimize system design:

Avoid resonance frequencies (such as adjusting pipeline length or adding support points).

Choose joints and pipelines that match the stiffness (such as metal joints connecting metal pipelines).

3. Difficulty in installation or stuck solution

Thread cleaning and lubrication:

Use a wire brush or ultrasonic cleaning machine to remove thread rust and impurities.

Apply hydraulic oil or one-on-one thread lubricant (such as Loctite 7649) to reduce friction.

Adjustment of sleeve type connector:

Reinstall the card sleeve, confirm that the opening direction of the front card sleeve is facing towards the end of the pipeline, and rotate the nut to the specified angle (such as 1-1/4 turn).

Use a card sleeve pre installation tool to protect installation consistency.

One on one tool usage:

It is necessary to use one-on-one tools such as torque wrenches and open-end wrenches to avoid thread deformation caused by adjustable wrenches.

4. Protection against pipeline rupture or joint breakage

Standardize pipeline installation:

Use a pipe bender to control the bending radius (≥ 3 times the pipe diameter) to avoid pipe collapse.

Reserve 1% -2% expansion and contraction during installation to prevent stress concentration caused by thermal expansion and contraction.

Material upgrade:

Select appropriate materials based on system pressure (such as stainless steel or alloy steel joints for high-pressure systems).

For corrosive media, inner lining anti-corrosion (based on actual reports) coating (such as PTFE spraying) or fully anti-corrosion (based on actual reports) joint shall be used.

Torque control:

Strictly tighten according to the manufacturer's recommended torque value to avoid excessive force causing thread breakage.