Lord of the O-rings
Installing an external O-ring by hand is a four-step process. The operator picks up a lubricated O-ring, slips it onto a tapered sleeve, places the sleeve over the part, and slides the O-ring down the sleeve until it snaps into a groove on the part. Depending on the ring and the part, a skilled operator might be able to install some 600 O-rings per hour.
With a semiautomatic machine, however, the process becomes a lot simpler. The operator picks up a part and presents it to the machine. The O-ring is installed automatically, and the operator doesn't have to fumble with small, slippery seals. "With a standard machine, you can triple the manual rate," says Bob Leo, president of The Amara Co. (1992) Ltd. (Cobourg, ON, Canada). "A dedicated operator who's fairly dexterous can easily install 2,400 pieces per hour. And, you get more consistent results."
O-rings are found in many products, including valves, faucets, refrigerators, tractors and medical devices. They are used for both static and dynamic seals.
These doughnut-shaped components are available in myriad sizes and materials. Nitrile rubber is the most common material for O-rings. The material resists a wide variety of chemicals, including oils, fuels, chlorinated solvents, and alkaline and salt solutions. Depending on the formulation, nitrile rubber can handle temperatures ranging from -65 to 300 F.
Fluorocarbon elastomers, such as Viton, withstand very high temperatures. Some formulations can be exposed to a temperature of 600 F for 16 hours without significantly affecting their physical characteristics. The material resists aromatic solvents, aliphatic and halogenated hydrocarbons, diester oils, silicate ester oils, and mineral acids, but it's attacked by highly polar fluids, such as ketones, hydrazine, anhydrous ammonia and certain hydraulic fluids.
Ethylene-propylene rubber resists water, acids, alkalis, salt solutions, ketones, alcohols, glycols and phosphate esters. It has excellent resistance to ozone and steam, but it's incompatible with petroleum-based oils and greases.
Silicone is commonly used in medical devices and high-temperature applications. However, its mechanical properties are generally poor, so it should not be used for dynamic seals.
Other common O-ring materials are styrene butadiene rubber (SBR), neoprene, fluorosilicone, polyacrylate and polyurethane.
Standard automated equipment can install O-rings ranging in size from 0.15 to 2 inches OD, says Thomas J. Barney, marketing director at Automated Industrial Systems (Erie, PA). Machines can be designed for semiautomatic operation or for integration into fully automated multistation assembly systems. Machines are available to install internal and external O-rings, and parts can be presented to the installation head vertically or horizontally. Besides O-rings, automated equipment can be used to install quad rings, backup rings, lip seals, cup seals, flat seals, square seals, split seals and Teflon seals.
Besides speed, a key benefit of automatic installation is that the process does not twist, roll or damage the O-ring. "If the O-ring gets twisted, it won't seal correctly," Leo warns.
O-rings are usually fed from small vibratory bowls. From the bowl, the O-rings slide, by gravity, into a short, angled track. If the O-rings are small and not too floppy, it's preferable to feed them standing on end.
With feeding, the biggest concern is warped O-rings. "O-ring suppliers are notorious for putting freshly molded parts into bags while they're still hot," says Leo. "Inside the bag, they get folded and bent until they look like potato chips. If an O-ring is warped by more than 25 percent of its cross section, it will climb over other O-rings and jam the feed track.
"There's only so much clearance that you can build into the feed tracks. And if you let a ring inside the track, you have to make sure it goes all the way."
A typical semiautomatic machine for installing external O-rings works like this: From the track, individual O-rings are positioned in front of a tapered mandrel in the installation head, explains Barney. As the machine cycles, the mandrel is pulled through the O-ring, expanding it and positioning it for installation.
"To install an O-ring, an operator manually places a part into the external end of the mandrel and gives it a light push," says Barney. "A sensor activates a pneumatic cylinder that pulls the mandrel back through a set of jaws that strip the O-ring onto the part. Simultaneously, another O-ring is positioned on the mandrel for the next cycle."
A light lubricant is used on the mandrel so the O-ring can slide easily off of the mandrel and onto the part. Different O-ring sizes can be accommodated by changing the installation head.
The process for installing internal O-rings is similar, except that tooling compresses the rings instead of expanding them.
As an alternative to semiautomatic, benchtop machines, Techno-Sommer Automatic (New Hyde Park, NY) has developed a six-finger pneumatic gripper for installing external O-rings. The gripper can be mounted to a pick-and-place device or a robot for fully automatic assembly.
The gripper has two pistons, which are driven independently by a three-way, two-position valve. When compressed air is applied to port A, the first piston moves the six fingers to pick up and expand the O-ring. Once the O-ring is expanded, compressed air is applied to another port, activating the second piston. This second piston forces the ejector jaws to push the O-ring forward, off the pickup fingers, and onto the part.
The six fingers stretch the O-ring evenly, minimizing the amount that the ring must be stretched to slip over the part. This puts considerably less stress on the O-ring than a three-jaw gripper. The stroke of the expansion jaws can be adjusted to accommodate a range of O-ring sizes. The jaws travel within T-slots, giving them good resistance to side loads.
Two inductive proximity switches can be added to the sides of the gripper. The sensors can signal a controller that an O-ring has been picked up, that a part is present, or that the O-ring has been installed.
Model GS65 has a horizontal stroke of 6 millimeters and can handle O-rings ranging from 4 to 20 millimeters in diameter. Model GS1015 has a horizontal stroke of 12 millimeters.
The choice of O-ring material depends more on the product requirements than on the installation method. Most O-ring materials are compatible with automated installation equipment, though some are easier to feed and manipulate than others. Ideally, the O-ring should be stiff enough to stand on end, but flexible enough to stretch over the part. The maximum amount that an O-ring can stretch depends on its size and composition, but it's usually less than 100 percent of its diameter.
"Any material with good elasticity, such as nitrile rubber or neoprene, works well for automatic installation," says Mike Hunter, design engineer for Kinotec Inc. (Nashua, NH). "Teflon-coated O-rings don't work extremely well. If the coating is too thin, it flakes off. If the coating is too thick, the O-ring is too stiff and doesn't stretch."
Another material that can sometimes cause problems with automated equipment is polyurethane, says Hunter. "Some urethane O-rings are pretty soft and very sticky," he explains. "They'll stick to any feeding surface. We have to put a special coating on the feeder bowl to keep them from sticking. One of our customers puts talcum powder in the bowl to keep the rings from sticking together."
Static electricity can be a concern when feeding silicone O-rings. "Static electricity will make the O-rings stick to the sides of the feeder, so they don't feed very well," explains Leo. Blowing ionized air into the bowl often solves the problem.
The groove, or gland, on the part should have tapered edges, to avoid damaging the O-ring. Ideally, the groove should be located as close to the end of the part as possible.