Hydraulic rod seals, also known as piston rod seals, U-cup seals, gland seals or simply rod seals, provide the seal between the piston rod and the cylinder head. They form the pressure barrier that keeps the oil inside the cylinder while the rod moves. This is the most critical seal in the entire assembly, because leakage here is immediately visible, causes oil loss and, in machines operating outdoors, also creates an environmental risk.
Hydraulic rod seals therefore largely determine whether a cylinder operates reliably. Techniparts supplies every brand and every size, metric and inch, for low and high pressure. If we do not have a size in stock, we will provide a technically equivalent alternative.
A hydraulic rod seal is installed in the cylinder head around the piston rod. The sealing lip makes contact with the surface of the moving rod and forms the pressure barrier between the hydraulic fluid inside the cylinder and the external environment.
As the cylinder extends, the rod moves outward past the seal. Normally, a very thin lubricating film remains on the rod. When the rod retracts, this oil film must be able to return to the cylinder.
Leakage at a rod seal is immediately visible on the outside of the cylinder. A worn seal can therefore lead to oil loss, contamination of the machine and loss of hydraulic fluid.
When hydraulic pressure builds up inside the cylinder, the sealing lip is pressed against the piston rod. This increases the sealing effect as the pressure rises.
At the same time, the rod must be able to move smoothly through the seal. The profile must therefore provide a good balance between sealing pressure, friction, lubrication film and wear.
At high pressure, soft sealing material can be forced into the clearance gap between the rod and cylinder head. This is known as extrusion. Depending on the profile, backup rings or anti-extrusion rings can therefore be used to support the seal.
When selecting a hydraulic rod seal, it must be determined from which direction the pressure acts on the sealing profile.
Many traditional rod seals are single-acting and are designed to seal pressure from the cylinder side. Examples include various U-cups and lip seals.
When pressure can occur from multiple directions, the complete sealing arrangement must be designed accordingly. Therefore, do not use dimensions alone as the selection criterion, but also check the pressure direction and the operating principle of the profile.
Hydraulic rod seals are available in different profiles and sealing arrangements. The correct version is determined by factors including pressure, speed, temperature, medium and available installation space.
The U-cup or U-ring is a commonly used rod seal. The open side of the profile is installed facing the pressure side, allowing hydraulic pressure to press the sealing lips more firmly against the sealing surfaces.
A PTFE rod seal can consist of a PTFE sealing element that is preloaded by an elastomer O-ring. This type of design offers low friction and can be advantageous in applications where stick-slip must be limited.
In certain T-profiles, the central sealing element is combined with backup rings that support the profile against extrusion.
For heavy-duty industrial applications, chevron packings, V-packings or stacked seal sets can be used. In these designs, multiple sealing elements are installed one behind another.
To select a hydraulic rod seal, at least the operating pressure, rod speed, temperature and hydraulic fluid being used are required.
At high pressure, the risk of extrusion and damage to the sealing edge increases. The clearance gap, material hardness and any backup ring must therefore be assessed together.
At low speeds and with frequent start-stop movements, stick-slip can occur. A low-friction sealing profile, such as a suitable PTFE combination, can offer advantages in these applications.
With biodegradable oil, water-glycol or other special hydraulic fluids, the chemical resistance of the sealing material must be checked separately.
In heavily loaded hydraulic cylinders, a buffer seal can be installed ahead of the primary rod seal. This helps absorb pressure peaks before they reach the main seal.
A backup ring or anti-extrusion ring supports the softer sealing material on the side of the clearance gap and helps prevent the material from being forced into this gap under high pressure.
Also view the available backup rings and anti-extrusion rings for applications where protection against extrusion is required.
The rod seal works together with the hydraulic wiper. Both components have a different function and must be properly matched to one another.
During extension, the rod seal leaves a controlled thin lubricating film on the piston rod. This film helps reduce friction and must be able to return to the cylinder when the rod retracts.
If a wiper completely removes this oil film from the rod or prevents it from flowing back, oil can accumulate between the seals. This can ultimately lead to unwanted pressure build-up and visible leakage.
Proper guidance of the piston rod is important for the service life of the rod seal. Under lateral loading or misalignment, the seal can be subjected to significantly greater loads on one side.
This can result in uneven wear, damage and premature leakage. Therefore, when sealing problems repeatedly occur, do not only inspect the seal itself.
Also inspect the hydraulic guide rings and hydraulic guide strips when overhauling the cylinder.
Because a hydraulic rod seal seals the transition between the cylinder and the external environment, leakage is usually relatively easy to recognise.
A clearly visible layer of hydraulic oil on the rod or oil running out along the cylinder head can indicate a worn or damaged rod seal.
In the event of leakage, also inspect the piston rod for scratches, corrosion, pitting and damage. A damaged rod surface can quickly cause a new seal to wear again.
Uneven wear can indicate clearance in the guiding system or an incorrectly aligned piston rod. Therefore, always inspect the complete cylinder assembly.
To select the correct hydraulic rod seal, three important dimensions are generally required: the rod diameter, groove diameter and groove width.
The inside diameter d corresponds to the diameter of the piston rod on which the sealing lip runs.
The outside diameter D is determined by the diameter of the seal groove in the cylinder head.
The groove width L is measured in the longitudinal direction of the cylinder. If a backup ring or another component is located in the same groove, the available width must be adjusted accordingly.
A used seal can become deformed or swollen due to pressure, temperature and hydraulic fluid. If in doubt, therefore measure the piston rod and groove directly.
A hydraulic rod seal and piston seal are both located inside the hydraulic cylinder, but perform different functions.
The rod seal is located in the cylinder head and helps prevent hydraulic fluid from leaking out along the moving piston rod.
A hydraulic piston seal is located on the piston and helps prevent hydraulic fluid from flowing internally between the different pressure chambers.
Hydraulic rod seals are used in hydraulic cylinders across virtually all mobile and industrial sectors.
In excavators, cranes, aerial work platforms and earthmoving machinery, rod seals are used in cylinders that often operate under high pressure and demanding environmental conditions.
Hydraulic rod seals are also widely used in tractors, agricultural machinery and telescopic cylinders.
Other applications include hydraulic presses, lifting equipment, marine applications, material handling and industrial production machinery.
Techniparts supplies hydraulic rod seals from a wide range of manufacturers. Existing seals can often be identified on the basis of type number, profile, material and dimensions.
This makes it possible to find an original type or a technically equivalent alternative for many existing cylinders, even when a particular profile or type number is no longer available as standard.
For older machines, inch sizes and non-standard grooves, hydraulic rod seals can be produced to size. Where possible, provide the rod diameter, groove diameter, groove width, operating pressure, temperature, medium and preferred material.
When overhauling a cylinder, it is advisable to inspect the other sealing and guiding elements in addition to the rod seal.
View our hydraulic piston seals for separating the pressure chambers inside the cylinder.
For protection against dirt and moisture, hydraulic wipers can be used.
For supporting the piston and rod, hydraulic guide rings and hydraulic guide strips are also available.
A hydraulic rod seal is installed around the moving piston rod and helps prevent hydraulic fluid from leaking out of the cylinder.
Possible causes include a worn rod seal, damaged piston rod, incorrect material selection, high pressure peaks or worn guiding components.
The rod seal prevents leakage along the piston rod to the outside. A piston seal prevents internal leakage between the pressure chambers of the cylinder.
An anti-extrusion ring or backup ring helps prevent soft sealing material from being forced into the clearance gap under high hydraulic pressure.
Common materials include PU, NBR, FKM, HNBR, PTFE and EPDM. The correct choice is determined by pressure, temperature, speed and hydraulic fluid.
Important dimensions are the inside diameter (d), outside diameter (D) and groove width (L). If the seal is deformed, preferably measure the rod and groove directly.
In addition to the new seal, also inspect the piston rod, wiper, guiding components, alignment and pressure peaks. A problem elsewhere in the sealing arrangement can damage a new rod seal again.
Yes. For older cylinders, inch sizes and non-standard grooves, custom-made hydraulic rod seals or technically suitable alternatives can be supplied.
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