Pneumatic actuators are key elements of material-handling and automation systems used for moving, clamping and positioning goods, boxes, flaps, gates, etc. A typical pneumatic cylinder (Figure 1) contains several static and dynamic seals. The dynamic seals are:

Double-acting piston seal (often two U-cup seals) Rod seal and dirt wiper (often combined in one element) Two cushioning seals for the adjustable pneumatic end cushioning.  The dynamic seals have to work well with: Oil-free compressed air using only the minimal grease lubrication that was applied during cylinder assembly Operating pressures to 100 psi with excursions to 360 psi during cushioning Creep velocity of 3+ ft./sec. A temperature range of -4°F to +176°F With a typical cumulative lifetime travel of at least 4,000 miles, the area of dynamic contact must maintain grease lubrication. A flexible sealing lip geometry provides low radial forces, and a rounded contact area allows the lip to float on a grease film.

If hydraulic seals were used in pneumatic cylinders, the high radial force and sharp sealing edges would scrape the grease away from the contact area. The grease would collect at the ends and the ensuing poor lubrication would result in high friction, heavy wear and shortened useful life.

Seal design refinedThe seal design follows the principles described above. Additional features ensure proper function under any operating condition.

Piston seal: U-cup-type piston seals have been used in pneumatic cylinders for many years. The design in Figure 2 has several special features. It has a thin connection between sealing lip and body to achieve low radial force. Toward the contact area, the lip is larger to give stability under pressure.

Notches at both lips and on the outer diameter of the back side avoid malfunctions. The notches on the side of the outer sealing lip ensure proper activation when the pressure increases. Notches at the outer diameter of the back and on the inner lip act as pressure-release channels in case pressure is trapped between the two U-cups in a double-acting piston. Without these notches, a pressure trap, together with friction, can tilt the seal in the groove, which results in an inoperable cylinder. The rounded contact area of the sealing lip was optimized by finite element analysis to ensure good function over the whole pressure range.

A polyurethane material was developed for the specific demands of pneumatic applications. The 83 Shore A hardness is relatively low to minimize the radial force. With its good compression set, low friction and extraordinary wear resistance, it provides the preconditions for good functional properties and long service life.

Rod seal: The rod seal is designed to suit grooves for pneumatic cylinders to ISO 15552. The seal has a sealing lip and a dirt wiper lip, and is fixed by a retainer «nose» at the outer diameter. The seal can be mounted into an unsplit gland by pushing it into the bore until the retainer nose snaps into the mating groove of the housing bore. The outer, static sealing lip is chamfered for ease of installation.

As the seal is fixed only by its retainer nose, the seal material must be stiff enough to withstand the axial forces of pressure and friction. Therefore, a harder material (94 Shore A) is used to maintain the proper seal orientation.

Cushioning seals provide smooth piston movement at the dead-end positions. The cushioning seal (Figure 5) works better than O-rings. The specific design features are:

Dynamic sealing lip gives good tightness and low friction Axial sealing edge with check valve functionality Combined notches at the lip side and on the outer diameter to allow air flow

The cushioning seal is active only at the end of the stroke. A pressure chamber, which is formed between cushioning seal and piston seal, works like an air pillow to absorb the kinetic energy smoothly. To ensure that the cushioning seal doesn’t affect movement when the cylinder reverses direction, the seal has an integrated check-valve function that forces the pressure to act on the full cylinder area. If O-rings are used as cushioning seals, the movement is delayed because the air pressure can’t act on the full cylinder area until the cushioning rod has moved out of the O-ring. Tests validated performanceA test program performed with commercially available pneumatic cylinders confirmed the material and design-development efforts. The cylinders were equipped with a rod seal-scraper, two piston seals and two cushioning seals. The test program included:

High-pressure test Bursting pressure test Low-temperature test Temperature cycling test Minimum speed test Endurance test The test procedures reflected harsh, but realistic, operating conditions using oil-free compressed air with an oil content < 0.01 mg/m3 (class 1 to ISO 8573-1). Testing monitored two indicators of seal functionality: the leak rate at two pressure values (29 psi and 145 psi) and the break-off pressure in both directions. The break-off pressure is the minimum pressure needed to move the cylinder. It’s different for outstroke and return stroke because the pressurized area of the cylinder is smaller on the rod side.

High-pressure test: This test reflects the situation at peak pressure during the cushioning cycle. To avoid having to produce and dissipate excessive energy, the test equipment included a pressure intensifier to generate an operating pressure of 360 psi. The cylinders have a short stroke of 0.59 in. to simulate only the cushioning cycles. Special high-pressure lines and high-pressure valves are used.

After 100,000 cycles, the sealing function wasn’t compromised by physical damage, seal wear or leakage. Break-off pressure was the same as when new. The sealing system proved its robustness and reliability even under extreme pressure peaks during the pneumatic end-cushioning cycle.

Bursting pressure test: This test confirms seal robustness and safety in case of abnormally high pressure. For the rod seal, this test shows the safe fixation by the retainer nose. The cylinders functioned at the target pressure of 725 psi without problem. The first leakage occurred at 942 psi, when the rod seal was partly extruded out of the housing, but no seals were damaged. After re-installing the rod seal, the cylinder was fully functional with a leak rate and break-off pressure the same as new.

Low-temperature test: This test shows the minimum temperature under which the cylinder functions properly. The cylinders were installed in a temperature chamber and connected to an external measuring station to enable measuring of leak rates and break-off pressure. An additional air dryer helped avoid water accumulation in the system. After cooling down the system, the measuring was done quickly to avoid frictional heating that would affect the result.

The low temperature limits of the piston seals depend on their location on the piston, although the two piston seals are identical. The piston seal at the bottom of the cylinder is the most critical position because it is measured with the rod extended, when the guidance clearance causes a misalignment between the piston and the cylinder tube. The piston seal at the rod side is measured with the rod retracted, when piston and cylinder tube are in good alignment. Under such good conditions, the piston seal works well at lower temperatures. So, the degree of misalignment leads to a low-temperature limit between -4°F (piston seal bottom side, rod extended) and -22°F (piston seal rod side, rod retracted). The rod seal is tight down to -40°F because the volume shrinkage from temperature reduction helps to keep the lip in sealing contact.

To summarize, testing showed that the pneumatic cylinder remains fully functional down to -4°F under worst-case conditions, and down to -13°F under normal conditions.

Temperature cycling test: This test cycles between minimum and maximum operating temperature to show the seal material’s ultimate behavior. The test equipment was the same as for low-temperature testing. Temperature cycling revealed no leakage and no change in break-off pressure, which proves that frequent changes of operating temperatures don’t affect the service life.

Minimum speed test: This test explores a cylinder’s operation at creep velocity. The cylinder, equipped with a side-load weight, is pressurized on one side while a throttle valve on the other side is closed until the cylinder begins to exhibit stick-slip mode.

The test results demonstrate how a sealing system can be optimized with material technology and design knowledge. The thorough consideration of functional aspects and use of latest development tools produce better performance. The material development, focused on the specific demands of pneumatic applications, was the second component of a high-performance polyurethane sealing system. Pneumatic equipment manufacturers and end users can be sure of the best functionality under extreme conditions, absolute reliability and outstanding service life. Thus, the costs for replacement and consequential costs from production shutdowns can be reduced.

screamnews.com

The pump ideally suits portable, battery-operated equipment used in a wide range of OEM applications, including gas sampling, explosives detection devices, anesthesia systems, and medical analyzers, among others.

Among features, the pump’s elastomeric diaphragm promotes high efficiency and delivers oil-free operation without any risk of contaminating the sampled gas. Pumps can be specified either with an ironless core DC motor or with a brushless DC motor offering longer service life. PEEK housings and pump heads contribute high stiffness and temperature and chemical resistance.

Depending on model, these pumps can achieve flow rates to 2.1 l/m, vacuum to 650 mbar (11 in. Hg) absolute, and continuous pressure to 0.6 bar g (9 psig). All are engineered to perform maintenance-free with low power requirements and are supplied ready for installation to operate in any position.

Chemically resistant versions are available and pumps can otherwise be customized to meet particular application demands.

KNF Neuberger, Inc.
www.knfOEM.com

Designed for use across a range of applications such as hydraulic and pneumatic systems, pumps and compressors and in power generation, the switches provide a stable switching point setting.

The 400 series switch converts pneumatic and hydraulic pressure into switching functions and is designed for applications that require accuracy under high loads.

The switching point is fully adjustable through an adjustment knob with the adjustment range, and includes a locking mechanism.

AMS Calibration’s switch is SPDT, and can be normally open (N.O.) or normally closed (N.C.) depending on the wiring.

The 400 Series Switch is fitted with DIN EN 175301-803 Form A connectors for fast and easy installation.

It is also available with an LED indicator on the plug for visual monitoring of the switch status, or an M12 x 1 (4-pin) electrical connection.

The switch features a repeatability of ±2% of full scale, 360° rotatable male connections, high loadability (shock 30g, vibration 10 g) and Hirschmann (DIN EN 175301-803 Form A) with optional 36″ cable.

www.ams-ic.com.au

Surgeons report that medical devices such as endoscopes with the F-treated seals respond better to their touch than non-treated elastomer seals. Medical product designers claim custom F-treated seals improve nasal and liposuction canula.

Laboratory studies of a broad range of elastomer materials show the reduction of coefficient of friction of F-treated molded components. Since the modification occurs as a reaction with the polymer, the most dramatic improvement is seen with low hardness, polymer-rich elastomers such as a 55 Shore A compound.

Minnesota Rubber and Plastics
www.mnrubber.com

::Design World::

The BalContact™, an electrically conductive component that employs precision-engineered canted-coil™ spring technology, is being showcased at Elektrotechniek 2009 by U.S-based manufacturer Bal Seal Engineering, Inc. And according to the company, interest in the simple, versatile Bal Contact is expected to be high among engineers and manufacturers who are seeking a simple, flexible solution to high, mid and low current conducting challenges.

“When we demonstrate how the Bal Contact performs, people immediately recognize it as a solution they can apply quickly and easily,” said Josien Suntjens, a Bal Seal Territory Manager working at Elektrotechniek this week. “They’re consistently impressed with its elegance and simplicity, and they like the fact that it can help them lower manufacturing costs and simplify maintenance.”

According to Suntjens, the Bal Contact is available in a wide variety of wire sizes, coil heights and materials. Its spring is designed to maintain a nearly constant force over a
broad range of working deflections, compensating for large mating tolerances and
temperature changes without significant deviation from its initial force. Each coil in the Bal
Contact works independently, maintaining contact with the mating surface and ensuring
maximum conductivity, Suntjens said.

According to Bal Seal, typical applications for the Bal Contact include busbar connections, gas-insulated switchgear contacts, circuit breakers and industrial fuse boxes. In some environments, the Bal Contact can also be used for EMI shielding/grounding purposes.

Elektrotechniek attendees can learn more about the Bal Contact and other conducting, connecting, sealing and shielding solutions by visiting Bal Seal Engineering’s exhibit in Hall 09BV, Stand B060. A photorealistic animation of the Bal Contact at work in three different gas-insulated switchgear applications can be viewed at http://mediaroom.balseal.com/vod2

Bal Seal Engineering, Inc.

www.balseal.com

Elektrotechniek

www.elektrotechniek-online.nl

::Design World::

The materials engineered by Trelleborg Sealing Solutions for its new range of static seals for hygienic-design couplings have undergone extensive testing in commonly used Cleaning-In-Process (CIP) media. The tests prove that the materials are well-suited to these applications. In particular, Ethylene Propylene Diene Monomer (EPDM) E7502 and Fluorocarbon (FKM) V8T41 gave best-in-class results for their material types, with V8T41 demonstrating exceptional performance for a fluorocarbon sealing material.

“These results enable Trelleborg Sealing Solutions to give qualified advice and to recommend the optimum material for specific customer applications,” says Steven Farnsworth, Food, Beverage and Pharmaceutical Segment Manager for Trelleborg Sealing Solutions Americas. “Based on our tests we are confident that our new range of static seals for hygienic-design couplings in recommended materials will give long-term sealing integrity. This will contribute to a lower meantime between planned maintenance and overall reduced production costs for users.”

The three materials recommended for hygienic-design couplings, EPDM E7502 and FKMs V8605 and V8T41, underwent various tests, one of which was immersion in 21 commonly used CIP fluids. Along with four other characteristics, the sealing materials were measured for volume swell, a critical determinant of suitability for use in hygienic-design grooves. In all cases but one the volume change of the materials was well below ten percent, the percentage of volume swell that is deemed allowable in such applications. In fact, in most CIP media the change was less than five percent, the percentage of volume swell that is deemed perfectly acceptable in such applications.

The FKM V8T41 gave the best overall performance and one which is exceptional for a Fluorocarbon sealing material. In the demanding conditions created by a combination of a two percent solution of acidic acid, citric acid and formic acid at +140°F/+60°C volume swell of V8T41 was well below five percent. In a one percent concentration of acidic CIP media with oxygen at +104°F /+40°C, virtually no swell was recorded. In addition, the material is also suitable for hot water and steam applications up to +338°F/+170°C.

Trelleborg

www.trelleborg.com

::Design World::

Conductors are silver plated copper on the airside in order to reduce contact resistance and range in diameters of .142 inches to .375 inches. Pin densities include single to 4 pin configurations.

Airside plugs are included with all assemblies and include crimp style contacts for wire/ cable installation. Standard assembly installations include weldable, ISO quick flanges or conflate flanges.

Solid Sealing Technology

www.solidsealing.com

::Design World::

Interstate’s Specialty O-Rings can be made from perfluoroelastomer (PTFE), Viton®, neoprene, nitrile, EPDM, solid and cellular urethane or silicone, and santoprene. Designed to match specific application requirements, they can be spliced, vulcanized, and supplied in sizes from 1/32” to 48” I.D. with 1/16” to 1/4” cross sections, depending upon material and construction.

Material selection for Interstate’s Specialty O-Rings depends upon customer requirements with regard to fluid compatibility, temperature and different- tial pressure-, abrasion- and tear resistance, configuration, service conditions, installation considerations, and other factors. They are available in hardness from supersoft to 90 Durometer.

Interstate Specialty Products, Inc.

www.interstatesp.com

::Design World::

The XploR™ range of advanced elastomers includes compounds in HNBR, FKM and Isolast® Perfluoroelastomer (FFKM), each of which demonstrates best-in-class Explosive Decompression Resistance (EDR) for its material type. In independent tests, materials within the XploR™ range were able to satisfy both the NORSOK M-710 sour service and Rapid Gas Decompression (RGD) test conditions. The XploR™ range is available in all standard international O-Ring sizes and cross sections, along with custom-engineered versions and specially designed seal profiles.

Trelleborg Sealing Solutions
www.trelleborg.com

::Design World::

Baldor’s MotiFlex servo drive can be networked using TCP/IP or the deterministic Ethernet-compatible Powerlink protocol.

Developed by a leading builder of structural and custom test equipment, Dynamic Testing & Equipment (DTE), the new machine fully automates the accelerated life testing of the flexible boot seals used to protect ball joints in automobile steering assemblies. The use of programmable AC servos – instead of conventional hydraulic actuators – provides an unprecedented level of control flexibility, enabling test parameters such as joint articulation angles to be varied on the fly.

Capable of testing up to six boot seals simultaneously, testing regimes can include continuous articulation of the ball joints, hot and cold brine sprays, elevated humidity levels and air temperature cycling from -25 to +80 degrees Celsius.

DTE’s boot seal testing machine uses two Baldor Ethernet TCP/IP controlled servos

DTE’s boot seal testing machine employs two motorized movement axes – one vertical and one horizontal – each driven by a Baldor MotiFlex e100 3-phase servo drive and servomotor fitted with a multi-turn absolute encoder. The precise feedback signals enable both axes to be programmed to absolute zero, facilitating optimal positioning of the machine’s tooling for parts loading and unloading, and ensuring ease of start-up.

The servo drives are connected via industry-standard Ethernet to the test machine’s host computer, which runs National Instruments’ LabVIEW software, and are controlled via TCP/IP using the built-in ActiveX commands in Baldor’s Mint programming language.

Baldor was able to supply servomotor 3D CAD files to DTE, for use with SolidWorks’ 3D CAD software.

According to DTE machine designer Glenn Siddens, “We were looking for servo manufacturers who could support ActiveX. Baldor was also able to supply 3D CAD files for the motors. As we use SolidWorks’ 3D CAD software for much of our design work, the availability of these files was a major time-saving advantage. What’s more, Baldor has proved to be extremely supportive. As well as helping us with ActiveX/LabVIEW integration, applications specialists from the company visited our facility on several occasions to help set up and fine-tune the servo systems.”

Designed primarily for automotive component manufacturers, DTE’s boot seal testing machine can be used to influence component design, or to demonstrate conformance with end customers’ performance standards.

Dynamic Testing & Equipment (DTE)

www.dynamic-testing.com

Baldor Electric Co.

www.baldormotion.com

::Design World::