Skip navigation

Web Exclusive: The four culprits — where to look to reduce hydraulic system leakage

Controlling and reducing hydraulic system leakage with dry technology is easy when you know what are the most common causes of leakage.

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.

System design

System design is equally critical to ensuring leak-free operation and can be cited 20% of the time when identifying the cause of fluid loss. Proper system design first involves selecting the right components to fit the application. The system designer must thoroughly examine all major considerations and how they will affect the choice of components. Once a designer looks at the application and determines which components can meet the requirements, other more subjective factors can be considered.

Correctly routing, bending and clamping tube lines is critical to system development. Failing to adhere to tube line fabrication best practices can result in a system that is inefficient and costly. By following recommended procedures, designers can help ensure a system that is leak-free.

Routing — Proper routing involves making a connection line from one point to another through the most efficient means. Routing, among all system design considerations, is perhaps the most significant and difficult.

Figure 1 shows several common routing configurations and their preferable alternative. In all instances, it is important to leave fittings as accessible as possible. Hard-to-reach joints are problematic to assemble and tighten properly as well as time consuming to service.

The ideal path should also avoid excessive strain on joints (a strained joint will eventually leak) and allow for expansion and contraction. Lines should be run straight and parallel when possible.

Figure 1

Figure 1. Proper routing of tube lines.
Select figure to enlarge.

Bending — The two most important rules to follow when fabricating a tube line are measure exactly and bend accurately (Figure 2a). A single measuring or bending error will result in a tube line that does not fit (Figure 2b) — a waste of both time and money.

Figure 2a

Figure 2a. Accurate measurements coupled with exact angles will result in a tube line that fits at all points (A-D).
Select figure to enlarge.

Figure 2b

Figure 2b. A measuring error on the second leg (B-C) resulted in a tube line that cannot fit at point D.
Select figure to enlarge.

Using proper tools is the best way to achieve good results. There are a number of bender types to choose from (Figure 3) for smooth, wrinkle-free bending without excessive tube flattening:

  • Hand-held lever (individually sized for tube sizes ranging from 1/8 to 1 in. and 5 to 14 mm)

  • Manual crank, table mount or vise mount

  • Hydraulically powered

  • CNC

Figure 3

Figure 3. Common types of tube benders.
Select figure to enlarge.


There are several steps to follow to ensure a good bend each time:

  1. Measure and mark exactly

  2. Always bend in the same direction

  3. Clamp tubing securely in bender

  4. Make certain the length mark is tangent to the desired angle on the radius block or in line with the desired degree on the link member

  5. Bend accurately to the desired angle plus spring-back allowance

  6. Remove tube and double-check bend angle

  7. Check measurement length with tape or a rule

Clamping —

With correct routing and bending accomplished, proper clamping must be considered to avoid premature tube line failure. When a line is left unsupported, mechanical vibration can shake the tube, causing fittings to loosen and leak (or the line to fail, in some instances).

Tube can be clamped individually or in sets, and can also be stacked. The most critical factor to any clamping system is having the correct number of clamps to achieve the proper result — that being a well-supported, vibration- and noise-free system.

Table 3 shows recommended spacing between clamps; Figure 4 shows an example. Clamp as close to each bend of the tube as possible (on both sides) to eliminate thrust in all directions.

Table 3

Table 3. Recommended tube clamp spacing.
Select figure to enlarge.

Figure 4

Figure 4. Optimal clamping configuration for tube support and vibration dampening.
Select figure to enlarge.

Quality of components
Approximately 15% of hydraulic system leakage can be traced back to poor component quality. It is essential to work with a reputable manufacturer that consistently provides high quality tube fittings that are made to SAE standards and that are dimensionally exact.

Taking time to identify quality components is the best way to avoid using inferior products. Generic “knock-off” fittings have become prevalent in recent years. If you suspect a manufacturer does not adhere to basic quality control measures, such as making fittings in only company owned and operated plants regardless of geographic location; or that parts are not machined to the highest standards; or that insufficient plating may lead to premature corrosion in the field; in these instances, do not risk the long-term reliability and overall safety of your hydraulic system to save a few dollars up front.

Furthermore, never use a fitting for which the type, size, working pressure and manufacturer is unknown. Many (though not all) fittings are stamped with identification marks. Establishing a supply of high quality fittings from a known reputable manufacturer is the best way to avoid this problem.

System abuse
System abuse, the final and most often overlooked factor (5% of the time) to hydraulic system leakage, can also be controlled. While this is typically assumed to be the responsibility of the end user, through proper system development the potential for abuse can be reduced. Besides adhering to the bending, routing and clamping best practices previously discussed, other considerations include:
1. Providing enough space (wrench clearance) to maintain equipment properly
2. Providing specialty tools to which the user normally would not have access (such as captive O-ring insertion tools)
3. Maintenance manuals outlining not only OEM and manufacturers’ part numbers but proper assembly techniques for servicing as well and,
4. Carefully considered routing that reduces the probability of users standing or climbing on tube lines.

The high cost of leakage

While the causes of leakage are now identifiable, the results can vary widely. One sure consequence is that leakage will cost money. The obvious cost associated with leakage is the loss of system fluid. A small leak can seem inconsequential until the long-term impact is carefully considered. For example:

1 drop of oil = .002 cubic inches
3 drops of oil per minute = .006 cubic inches
180 drops of oil per hour = .36 cubic inches = .001558 gallons
8-hour shift x 3 = .037392 gallons of oil per working day
250 working days = 9.348 gallons of oil per year
$18 average cost of gallon of oil = $168 of oil per year

Remember this is for one leak point. Multiple leak points can drive the loss and cost of hydraulic oil up significantly over the course of one year. You can calculate the approximate cost of lost fluid using Table 4.

Table 4

Table 4. Basic oil loss calculator.
Select figure to enlarge.

In addition to system fluid loss, there are many costs associated with leakage that are often overlooked. These include energy loss, downtime, lost revenue, rising insurance premiums, maintenance costs, warranty issues, safety hazards, and environmental responsibilities. Escalating EPA and OSHA scrutiny in recent years makes avoiding the consequences (fines; litigation) of hydraulic system leakage ever more important. In the event of an oil leak or worse, it is important to do the following:

  • Stop the leak/release

  • Contain the leak/release

  • Clean up the leak/release (necessary training/certification of clean up personnel will vary depending on the type/amount of oil and the media impacted; i.e. soil, groundwater, wildlife habitat, etc.)

  • Dispose of or remediate impacted media

  • Dispose of contaminated devices

  • Replace contaminated devices

  • Report leak/release (if required) to applicable government agencies (i.e. EPA, Coast Guard); follow up with regulators

  • Implement a corrective action to prevent a future, similar leak/release

  • Modify leak/release control plans, if required

There is no substitute for proper knowledge when working to ensure a leak-free system. Consider enrolling your employees in a training course to learn/refresh hydraulic connection and safety best practices. Parker’s Dry Technology Guide (Bulletin 4300-DT) is also a valuable resource for system designers, assemblers and technicians. The full-color, 100-page guide is a complete reference manual that allows the user to build a base of knowledge for creating and maintaining leak-free systems. To obtain a copy, call Parker Catalog Services at (800) C-Parker or contact your local Parker distributor.

Jeremy Haller, CFPS, is Product Sales Manager, at Parker Hannifin’s Tube Fittings Division
And Gary Kleiner, is Value Capture Manager for Parker’s Fluid Connectors and Hydraulics Groups. Contact them at [email protected], (614) 324-8209 or [email protected], (330) 298-4043 or visit