Hydraulic Seals and Preventing Seal FailureJuly 10, 2018
To function properly seals must withstand extreme temperatures, high pressures, chemicals and contaminants. Seals can be composed of a number of different materials, and choosing the right seal for the job is not a trivial task; it takes consideration and knowledge of not only the machine or equipment that the seal will reside on or in, but also the potential effects of the seal’s environment.
Preventing Seal Failure
Seal failure will lead to equipment problems and costly downtime. And while they are durable if used in proper applications, in the wrong setting they can cause serious problems. Even if a hydraulic system continues to operate safely when leaking, leaks will be costly. Six drops of oil a minute leaking from a connector in a hydraulic system — assuming the volume of each drop is 0.5 ml — is 47.5 gallons over the course of a year and more than a hundred dollars’ worth of replacement oil. A steady stream leak of 1/16 in. diameter adds up to 520 gallons of lost oil per year. Now multiply that by the number of connectors used and it adds up quickly.
Such issues can be prevented, however, with an understanding of the causes of seal failure and how to prevent them. So while all seals eventually wear out, the most common causes of premature failure include improper installation, contamination, excess temperature, pressure or speed and chemical erosion.
Let’s look at these concerns one at a time.
Improper installation. An improperly-installed seal can fail in a number of ways, due to improper handling, contamination or even being an incorrect fit for the application. Improper installation can result in tears or nicks to the seal, consequently leading to failure. A seal can even be installed backwards, resulting in immediate leaks.
The location where the seal will be housed must be measured correctly so that it is a precise fit. Without the correct size, the seal can either break (if too big) or not seal at all (if too small). For example, an O-ring seal size is determined by the seal bore (the diameter of the hole in the housing where the seal will be fitted), the seal outer diameter and the seal width (the total width of the seal). Seals are sized to fit a certain shaft or rod diameter and are identified as such. A seal should only be used in the dimensions that it is designed for.
Contamination. Metallic shavings, powder, dirt, mud, grit and other minute solid particles can be picked up during operation and damage the seal, possibly getting into the protected area. Causes can include internal cylinder contamination, cylinder wear over time, clogged filters, dirty oil and poor wiper performance. Any liquid used in these applications should be properly filtered of debris.
Excess temperature. Heat can cause hardening of the elastomer and can cause the seal to degrade, eventually becoming brittle and cracking apart. Material can break off, causing further contamination. There is generally a stated temperature range that is optimum for a particular seal’s use and should be adhered to. Increased temperature also accelerates aging; the temperature of the application and constant friction can decrease a hydraulic seal’s life expectancy significantly. For instance, if axial cracks at the sealing edge are visible when replacing a seal, the seal may have been exposed to excessively high temperatures. On the other hand when the operating environment is too cold a seal may become brittle.
Pressure. When a seal is stressed beyond its limits and fails, it could be the result of over-pressurization. Failure modes range from cracking to complete structural failure. Check for pressure spikes and make sure the seal design is rated for the pressure in the given application. Like temperature, recommended pressure for optimum performance is specified for each seal by the supplier (such as Trelleborg Sealing Solutions whose products are specifically intended to suit a variety of hydraulic applications, from light-duty linear hydraulic to heavy-duty rotary seals and static applications, and where pressure and temperature specs can be easily identified during product selection). When recommended pressure is exceeded a seal wears faster. Additionally, using the wrong lubricant or oil may result in the fluid density becoming too thick or slippery, causing incorrect hydraulic pressure to be applied to the seal.
High speed rotation. If used in an environment that requires a certain speed (like a rotating shaft), the seal may not withstand the rotation speed. Seals are designed to ride on lubrication between the seal lip and a moving surface. The thickness of the lubricating film establishes the level of friction. When velocity increases, friction decreases initially, but over time at an increased velocity the frictional forces will climb, causing seals to wear. Speeds under and over the recommended range cause friction values to rise sharply, impacting the seal material.
Chemical deterioration. A corrosive fluid or incompatible chemical composition, such as oil additives, can cause the breakdown of seal material. This can occur when the improper seal material is chosen for an application. Connecting a seal to a hydraulic system using chemical fluid may create material fractures or extrusions to the material. Chemical deterioration can manifest itself in the swelling or shrinking of the seal.
Trelleborg Sealing Solutions
Leakage prevention involves not only selecting and installing the right component for the specific application, but maintaining the conditions that allow the component to continue performing without leaking. For optimum component performance, designers must be aware of the products available that achieve zero leakage for each specific application and how to install them. Products exist to reduce or eliminate the problem of leakage, but the key is in selecting the right product — and installing it correctly — for the application at hand.
Trelleborg Sealing Solutions offers a wide range of products to fit applications ranging from light-duty linear hydraulics to heavy-duty seals. With materials such as high-performance thermoplastics, polymers and elastomers, all seals on the online platform are specifically developed to help a system reach its optimal performance and maximum durability with minimum maintenance. The company is based in Stuttgart, Germany, and serves many areas of industry, including service and maintenance, repair, manufacturing, wholesale and research and development.
O-rings, the most popular seals, are used mainly in static sealing applications (meaning there is no movement between the sealing surfaces or between the seal surface and its mating surface). Seals-Shop offers O-rings with cross sections in the range of 0.8 to 8.4 mm and inside diameters up to 810 mm. O-rings can be used to seal practically all liquid and gaseous media and they are available in elastomeric and other materials, including ethylene propylene diene monomer (EPDM) rubber, fluoroelastomers (FKM) and nitrile butadiene rubber (NBR).
Hydraulic rod seals prevent fluid leakage from within the cylinder to the outside. The online platform offers a wide range of rod seals for both single-acting and double-acting systems. Rod seals are available as Turcon® polytetrafluoroethylene (PTFE) products and Zurcon® polyurethane (PU) U-cups, where the focus of the design is on low friction, compact form and simple installation.
Piston seals are installed in the cylinder head and seal against the cylinder bore to ensure that pressurized fluid does not leak. This allows pressure build up on one side of the piston, making the cylinder extend or retract. Piston seals can be double-acting (pressure on both sides) or single-acting (pressure on one side) and are available from Trelleborg in a variety of materials.
Wear rings guide the piston and piston rod within the hydraulic cylinder, preventing metal-to-metal contact. The online platform offers Slydring® hydraulic wear rings in three different material types: Turcite® for low- to medium-duty applications where there are limited radial forces; HiMod® for medium- to heavy-duty applications; and Orkot® materials for heavy-duty applications where there are high radial forces.
For more technical information on the handling of seals as well as installation guides, visit Trelleborg Sealing Solutions.