Fluid leakage is a problem that virtually every hydraulic system user will experience sooner or later. What causes the leakage, however, might come as a surprise. The short-term consequences and long-term costs of leakage can be significant. By using a comprehensive and effective process for determining the best hydraulic connections, the four common causes of leakage can be systematically evaluated to determine the best solution and thereby control costs.

Improper installation
Improper installation is the leading cause of hydraulic system leakage, contributing to fluid loss in approximately 60% of all instances. Proper installation is specific to one of three main fitting connections that you are putting together:

37° flare fittings — As the most common fitting in the world, improper installation of 37° flare fittings can be generally attributed to insufficient tightening or over tightening. Many times these fittings are over torqued, which collapses the cone on the 37° flare and causes premature failure, resulting in leakage. And because they are a metal-to-metal seal, these fittings are prone to scratches on the sealing surface. A single scratch on the male cone or female flare is a potential leak path. These scratches can occur from both rough handling and the use of poorly welded tubing opposed to seamless tubing.

Parker ships its fittings with protective plastic end caps. If your fittings come with caps, leave them on until you are absolutely ready to assemble the connection. And when making the tube assembly, proper clamping and connection tightening is critical as severe system vibration — particularly in mobile applications — can cause the metal-to-metal seal to loosen up during machine operation.

Parker’s recommended assembly technique for 37° flare fittings is called Flats from Wrench Resistance (FFRW) or “Flats” method. A “flat” is referred to as one side of the hexagonal tube nut and equates to 1/6 of a turn. Table 1 shows how many flats to turn the nut past the point of initial wrench resistance.

Table 1

Table 1. Flats from Wrench Resistance (FFRW) values for Parker Triple-Lok 37° flare fittings.
Select figure to enlarge.


O-ring face seal fittings — The vast majority of O-ring face seal fitting problems are due to a problem with the O-ring itself. This can include a missing, pinched or partially extruded O-ring. Often times, the common misconception that O-ring face seal fittings can be finger tightened and will not leak can cause problems.

When making connections with O-ring face seal fittings, take a moment to double check to make sure the O-ring is present and properly seated in the groove. It is also important to lubricate the O-ring because friction created during installation — such as screwing down a hose — can cause the O-ring to pop out of the groove (hint: use two wrenches during assembly to prevent the hose from twisting). Proper assembly of O-ring face seal fittings also involves the use of a torque wrench and SAE recommended torque values (Table 2).

Table 2

Table 2. Assembly torque values for Parker Seal-Lok O-ring face seal fittings.
Select figure to enlarge.


Flareless bite type fittings — Improper installation of flareless bite type fittings, also known as 24° flareless fittings, typically involves improper presetting of the ferrule onto the tube. The initial mistake, however, is often an assumption that this type of fitting is a compression fitting, which it is not. Consequently, ferrule preset instructions are not always followed through to completion, and so the ferrule does not properly bite into the tube, which results in a poor quality seal.

Proper presetting can be accomplished manually using a hardened Ferulset tool or the fitting body, or hydraulically using a Hyferset tool or Hydra-Tool. Both methods are illustrated in detail in the “Ferulok Assembly” section of the Parker Tube Fittings Division catalog (Catalog 4300). Regardless of which method is used, it is important to complete each step in the assembly process leading up to installation: 1) cutting, deburring and cleaning of the tube; 2) ferrule preset; and 3) preset inspection.