Piston seals serve as a pressure barrier and prevent fluid from passing the piston, which is important for controlling the cylinder motion or maintaining the position when at rest. Piston seals are typically classified into single-acting (pressure acting on one side only) and double-acting (pressure acting on both sides) seals.
Double-acting piston seals have a symmetrical profile and identical sealing functions in both directions. Typically, double-acting piston seals consist of a slide ring and an energizer. Because double-acting cylinders contain fluid on both sides of the piston, a relatively thick lubrication film can be permitted between the piston seal and the cylinder bore to minimize friction and wear.
A single-acting piston seal is designed for cylinders where pressure is applied from one side only. The piston in single-acting cylinders may have oil on the pressure side only, while the opposite side is open to atmosphere. Therefore, the piston seal must leave minimal oil film when passing along the cylinder bore since the transportation of oil otherwise would result in a leakage to the exterior.
In single-acting cylinders, the open end may push air out and draw air in as the piston reciprocates. This air may carry moisture and contaminants into the cylinder, which can lead to seal damage. Vent filters can be fitted to the open side of the cylinder to reduce contaminants entering the inside of the cylinder. The cylinder bore may also be hard chromium plated to prevent corrosion.
Wiper Seals
Hydraulic cylinders operate in a variety of applications and environmental conditions, including exposure to dust, debris, or outside weather conditions. To prevent these contaminants from entering the cylinder assembly and hydraulic system, wiper seals (also known as scrapers, excluders, or dust seals) are fitted on the external side of the cylinder head.
Wiper seals maintain sealing contact to the piston rod when the equipment is stationary (static, no reciprocating motion of rod) and in use (dynamic, reciprocating rod). Without a wiper seal, the retracting piston rod could transport contaminants into the cylinder.
Rod guides are typically placed inward of both the rod and buffer seal and should be lubricated on assembly with the same medium as used in the system. The guide must receive ample lubrication at all times and should not be outside the rod seal. However, in certain conditions, guides with polytetrafluoroethylene (PTFE) added may be used outside the rod seal due to their self-lubricating properties.
Guide rings provide effective guidance of components that are in relative motion to each other and accommodate radial loads acting on the cylinder assembly. The selection of the right seal and guide for a given application requires consideration of many factors. Rod and piston guide rings prevent metal-to-metal contact between components, react to the radial load caused by side loads on the cylinder assembly, and keep the piston rod and piston radially centered in the cylinder assembly within acceptable limits for the seals. These functions are important for performance of the rod sealing system and piston sealing system.
Industrial seals are exposed to a wide range of challenging operating conditions such as high temperature, speed, pressure, and aggressive chemicals. To handle these and other harsh conditions, it is essential to select the most suitable sealing materials. Several factors impact material selection, including exposure to media, pressure, temperature, and potentially stringent regulatory requirements common in food and beverage or oil and gas applications. Types of sealing materials include:
Rubbers—NBR, FKM, and HNBR are commonly used rubber materials in hydraulic applications. They are extremely flexible and can be stretched and deflected by exerting relatively little force. Many of them deliver excellent resistance to mineral oils, greases, or other media.
Thermoplastic elastomers—These offers advantages typical of both rubber and plastic materials. SKF’s high-performance thermoplastic polyurethanes (TPUs) combine excellent abrasion and wear resistance, low compression set, and high tear strength and outstanding pressure resistance.
PTFE—Engineered to handle extreme conditions, PTFE and its compounds can withstand aggressive chemicals plus high temperatures and pressures. Due to their extremely low coefficients of friction, they can also tolerate dry running conditions.
Plastics—Plastic materials can meet higher temperature, chemical, and mechanical property requirements and can range from engineering plastics to high-performance plastics. Backup rings are typically made of plastics and used to enhance the pressure carrying capability of a rod or piston seal.
Designing sealing and guide systems in hydraulic cylinders requires careful attention to the interaction between all cylinder components and the operating conditions as well as the application requirements. The selection of the right seal profile and material for a given application requires consideration of many factors. For any application factors outside of the ordinary or to specify sealing systems in new hydraulic cylinder designs, a certain amount of expertise may be required.
Before seals can be selected, certain applications, parameters, and information should be collected. The following most common application considerations are almost always required when selecting hydraulic seals:
Fluid pressure range—the range of operating fluid system pressure, as well as frequency and severity of pressure peaks
Temperature range—the range of the fluid and cylinder assembly, both when operating and at rest
Speed—the stroking speed of the reciprocating piston rod
Fluid media—the type and viscosity of fluid used in the system
Hardware dimensions—the rod and bore diameters, seal groove dimensions and gaps (if already specified), cylinder overall length and stroke length, and surface finish specifications (if already specified)
Application of the cylinder—the type of equipment the cylinder will be used on and how the cylinder will operate in the equipment as well as installation, duty cycles, and environmental factors (external temperature or contaminants).
Performance issues in specific applications are not always solved with a standard or catalog range of products. For difficult and constantly evolving fluid sealing applications, seal engineers can develop a customized sealing solution. Development of this custom solution should include failure analysis and system operating conditions investigations, testing according to customer specifications and performance standards as well as technical training.
Hydraulic seals have a crucial impact on system performance in many applications. Factors such as temperatures, speeds, pressures, lubricants, and other application operating conditions can greatly impact seal life. Specifying the right seal helps boost machine performance, optimize operations,, and lower a machine’s total cost of ownership.
This material was contributed by Tadd McBride, Customized Molded Seals engineering manager at SKF. Click here to visit SKF’s website on hydraulic seals.
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