What are the basic failure modes of hydraulic fittings and why do they occur?

Hydraulic systems are one of those amazing pieces of engineering structures that allow huge forces to be transmitted through fluids under pressure. The strength of this transmission is dependent upon the weakest component of the system; in many cases, that weak point may be a simple hydraulic fitting. These small, yet very important pieces are what connect the hoses, tubes, and other pieces of the hydraulic system together and supply an airtight and robust pathway for hydraulic fluid to flow through.

Failure of a hydraulic fitting, when it occurs (in some cases), can cause catastrophic failures resulting in fluid leaks, downtime, safety liabilities, and expensive repairs. To understand the modes of failure of hydraulic fittings and the reasons for their failure is extremely important for anyone who designs, maintains, or operates hydraulic machinery. 

Let’s take a look at some of the most common ways that hydraulic fittings fail.

1. Hydraulic fitting failure due to leaking is the most frequently occurring cause.

Leaks are generally considered a minor failure, but can have serious consequences. A long-term leaking hydraulic fitting can cause contamination of the hydraulic fluid, decreased hydraulic system efficiency, environmental concerns, and ultimately lead to the failure of the hydraulic equipment.

How Leaks Develop:

• Improper Torque is the number one cause of leakage.
• If the hydraulic fitting was under-torqued, it would not have provided enough clamping force to provide a positive seal, which would allow fluid to leak out.
• If the hydraulic fitting was over-torqued, it may have deformed (or cracked) the fitting material and damaged the sealing surface (flare, O-ring) as well as stripped the threads. All of these conditions would provide a path for the leakage to occur.
• Contamination/Damage to the Sealing Surfaces will also allow the hydraulic fluid to escape.
• If there is dirt or debris between two sealing surfaces, it will not allow for a complete seal to be made.
• If the sealing surface has scratches, nicks, or corrosion, it will provide a path for the fluid to escape.
• Improper Type/Material for Fitting: Using a different type (e.g., NPT instead of BSP) or an incompatible seal type, e.g., thread or O-ring, may cause issues with the fitting. This also applies if you choose a fitting made of a material that cannot handle the fluid, temperature, or pressure.
• Constant vibration can eventually loosen fittings (especially if the hose has insufficient slack).
• Seals on hoses will lose their integrity because of repeated heating and cooling.
• Elastomeric seals (like O-rings) will lose their elasticity, may harden, or crack because of exposure to heat, chemicals, and UV light.

2. Cracks and Fractures

While leaks are serious, cracks or fractures in fittings are considered a significantly more severe type of problem. A crack or fracture in a fitting may indicate that the fitting's structural integrity has been completely destroyed. While the structural integrity of a fitting can be compromised by several different factors, the most common causes of cracks and fractures in fittings include those caused by:

• Torque: When a fitting has been over-tightened (i.e., more than the manufacturer's recommended torque), excessive amounts of torque may exceed the yield strength of the fitting material. This can cause a crack in the fitting, particularly near the collar of a flare fitting and near the body of a straight fitting.
• Fatigue: When a fitting experiences repeated stress cycles (i.e., pressure pulsations, vibration), microscopic cracks will develop in the fitting material over time until a catastrophic failure occurs. Areas of a fitting that are subject to continuous stress cycles are subject to fatigue.
• Manufacturing Defect: While this is rare, sometimes there are defects in the manufacturing of a fitting that create weak points (e.g., inclusions or voids in the metal) that will eventually crack during normal operations.
• External Forces: Physical impact (i.e., dropped tools, collision, improper handling) and/or thermal shock can cause immediate or latent cracks in a fitting. Thermal shock is defined as an abrupt and/or extreme temperature change that creates thermal stress in the fitting material.

3. Thread Strippers.

This type of failure occurs when the threads on the male or female component are pulled out so that the fitting cannot be compressed and will not hold pressure. While it is easy to see that this is a failure that can prevent you from being able to tighten or maintain lubrication in your fittings, there are several reasons why this can happen.

• Improper Torque: Stripping can happen when you apply too much torque to a fitting. A fitting with a soft material compared to the material that it is threaded into (or if it has fine threads) can result in a sheared thread.
• Cross-Threading: When you attempt to insert a fitting into a port without proper alignment, the threads may become damaged, and the fitting will no longer be able to be engaged correctly, resulting in stripping when additional pressure is exerted on the fitting.
• Incorrect Thread Type: Inserting a fitting with the wrong thread pitch/angle will result in the improper engagement of the fitting.
• Poor Quality Threads: If the manufacturer does not produce a thread profile that meets acceptable tolerances, especially with respect to thread depth, taper angle, and surface finish, the threads may be susceptible to stripping under pressure.

4. Corrosion

Corrosion is a slow breakdown or wearing down of metal due to chemical interactions with the surrounding environment. In hydraulic systems, corrosion will affect the strength of the fitting and how well the fitting seals.

Reasons for Corrosion

• Chemical Incompatibilities: The hydraulic fluid contains a chemical that has been found to react with the fitting material.
• Environmental Hazards: When a fitting is exposed to certain elements outside of the hydraulic system (e.g., Salt Water, Acids, High Humidity), it may begin to corrode externally.
• Galvanic Corrosion: The process whereby an electrical connection exists between two dissimilar materials (metals) when exposed to a conductive substance, such as an electrolyte, which could be water or an unknown contaminating substance within the system.
• Fluid Contaminants: Water or gases and/or acidic contaminants finding their way into the hydraulic fluid will accelerate the process of internal corrosion of hydraulic fittings.

5. Component Wear and Erosion

Internal wear or erosion is not as frequent as leaks or cracks, but still occurs in some pumping applications with high flow rates and/or high-pressure on occasion.

Causes of Erosion:

• Abrasive Contaminants - Solid particle contamination entering the oil may act as a sanding agent on the internal surfaces of the coupling, leading to the gradual removal of material, especially on bends or at the point of expansion and constriction where the oil shifts direction or accelerates.
• Cavitation - If the pressure of the oil is reduced below its vapour pressure due to an external effect, the vapour bubbles form and erupt with significant force back into the body of oil, sending micro-jets outward and eroding the steel surfaces.
• High Velocity Flow - Fluid flowing at high velocities can cause erosive wear, especially in sharp turns or narrow spaces.

How to Avoid Hydraulic Fitting Failures

Fortunately, most hydraulic fitting failures occur due to errors in the selection, installation, or maintenance of the fittings, so these problems can be prevented. The following steps will help you avoid this type of failure:

• Selection: Properly select the correct fitting type, size, material, and sealing method for your application based on the required pressure, temperature, and type of fluid.
• Installation: Be careful to follow the manufacturer's torque values exactly, use proper tools, and install the fittings without cross-threading.
• Cleanliness: Be sure that all parts and surfaces of the fitting and ports are clean before assembling them.
• System Design: Minimize vibration on the system, provide enough hose slack, and route hoses correctly.
• Routine Maintenance: Inspect your fittings for leaks, corrosion, and damage during routine inspections. Replace old or damaged seals before they fail.
• Fluid Management: Keep the hydraulic fluid clean and free from contaminants, and ensure it is at the correct operating temperature.

If we follow these guidelines, we can significantly increase the longevity of hydraulic fittings and therefore increase the reliability and safety of hydraulic systems as a whole.