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Posted on January 24, 2022

Safeguarding Demanding Applications with Hydraulic Flange Connections

John Joyce
Written by

John Joyce

Posted in
Hydraulic Safety

Hydraulic flange connections are used in demanding applications for their ability to withstand high pressure, vibration, and shock.  They also make an excellent connection between hoses, hose to pipe or to system components . . . when properly assembled. 

Pipe and hose connections that require fittings of greater than one inch can have problems with effective installation and stability. However, the primary advantage in large diameter applications (and often smaller diameters) of hydraulic flanges over threaded fittings is lower required torque assembly and ease of installation in tight spaces. In addition, a hydraulic flange distributes the hydraulic load to four bolts, instead of one fitting. The required torque for each bolt is much less, and the wrench and the wrench gap are smaller providing an advantage in tight spaces.

Most pipe flanges are forged for high strength and durability and meet the requirements of SAE J518, and are interchangeable with ISO 6141, DIN 20066 and JIS B 8363, except for the bolt sizes. And split flanges allow for ease of assembly, especially in close gaps by simply loosening all four bolts and removing only one clamp half. Split flanges are also more easily disassembled in hose assemblies. The ability of properly installed flange connections to withstand demanding applications is due in large part to most sizes withstanding high pressures, with standard Code-62 flanges able to handle pressure levels of up to 6,000 PSI (413 bar).

Improper Flange Installation and Dangerous Modifications 

Although many SAE 4-bolt hydraulic flanges might look alike or that different flanges might appear to be compatible, they really aren’t.  For instance, SAE Code 61, SAE Code 62 and Caterpillar 4-bolt flanges have dimensional variations and slight differences in configuration, plus the differences in allowable working pressure, do not allow them to be used together or interchanged.  Flanges should never be reworked or otherwise adjusted beyond spec to force fit an application.  As an example, referenced in an earlier blog, as described on The Fluid Power Safety Institute™ website: 

A diesel mechanic was installing a new hydraulic motor onto a machine. While reconnecting the hydraulic hose he inadvertently used a 4-bolt split flange that did not line up with the holes on the motor housing. Suspecting the holes in the motor were not tapped in the proper place he filed the holes in an attempt to have them line up with the flange. He essentially modified a 6,000 PSI SAE Code 62 4-bolt flange to mate with a 3,000 PSI SAE Code 61 motor connection.  

This was an unfortunate mistake and one that could have been catastrophic to the system if it had been the other way around, modifying a 3,000 PSI bolt flange with a 6,000 PSI motor. An obvious system failure would have occurred, or worse a system failure that blew the assembly off causing severe damage or physical harm. This is a prime example of the importance of using only matched and properly rated components. 

Avoid Uneven Bolt Stress 

An incorrect bolt-up procedure can cause some bolts to be loose and others overtightened, which can leak through the gaps and crush the gasket or O-Ring.  This will usually cause leaks, especially at high temperatures when the heavily loaded bolts relax, allowing for gaps to form between the flanges. 

Four-bolt flanges that conform to SAE J518 and ISO 6162-1 and -2, are leak-free components for assembly in tight quarters and for higher pressure applications when installed properly.  They provide excellent resistance to vibration and have outstanding reusability.  An O-ring is inserted into a ring groove in the flange head and seals on a smooth face female port.  It is held in place by two clamp halves (or a one piece clamp) which are held together by four bolts located in a rectangular pattern.  Connecting the flange assembly should be done in the same way of tightening the lug nuts when mounting a tire on a car which has 5 posts (bolts), where the nuts are tightened in a star pattern to ensure an even distribution of torque.  Similarly, the four bolts on a flange should follow proper tightening sequence.  First, they should be threaded on by hand, then tightened with a wrench in a cross pattern in the order of bolt #1, then #3, then #2 and finally #4.  This not only helps ensure proper seating of the O-ring but also ensures an evenly secured connection.  One of the most frequent causes of the stress on a flange assembly which leads to leakage is incorrect torquing of bolts, such as uneven torquing, inadequate torquing, incorrect sequence of bolting and over compression. 

Human factors are the primary cause of flange seal leakage in challenging applications.  The five primary underlying causes of failure are: 

  • Damaged flange  (25%) 
  • Faulty gaskets (22%) 
  • Loose bolts (15%) 
  • Misalignment of the flange (12%) 
  • Improper installation (26%) 

Studies show that seal failures caused by incorrect joint assembly, damaged parts, and misaligned equipment far exceed the number of failures caused by the gasket itself. 

There is no doubt that four-bolt hydraulic flanges are a trusted connection for high-pressure and demanding applications in mobile and industrial equipment . . . as long as they are properly assembled. 

Learn more about hydraulic maintenance by downloading "4 Considerations for Safe Hydraulic System Maintenance." 

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