The outside dimensions of seamless rolled rings can range from a few inches to 30 feet, height can reach from less than an inch to higher than 9 feet, and weight can be as much as 350,000 pounds. The shape of seamless rolled rings can be configured to be flatter, like a washer, or to have high walls like a hollow cylinder.
Unlike other types of ring forming, this forging process creates a seamless ring with great integral strength, capable of resisting high degrees of internal or external pressure. This is why wheel hubs, bearing housings, pressure vessels, gears, jet engine bearings, engine turbines and wind turbines – all applications which apply significant force – are usually made from seamless forged rings.
Seamless rolled rings are also utilized in nuclear reactors, off-road equipment, couplings, rotor spacers, sealed discs and cases, flanges, valve bodies and many other high stress applications.
A seamless rolled ring begins as a solid, circular block of metal that is heated to forging temperatures and then punched through the center to form a “donut” shape. This preform is then placed on a rolling rig with a roller touching the inside of the donut and a roller touching the outside of the donut. The two rollers begin to rotate in opposite directions, turning the ring, and at the same time the rollers slowly squeeze inwards, thinning the ring wall and causing the diameter of the ring to increase.
Some ring-rolling machines also have a roller that presses down on top of the ring as it turns, governing the height of the ring wall. The process continues until the ring has been thinned, flattened or otherwise shaped to its desired dimensions, at which point it may receive secondary processing such as size fine-tuning or heat treatment. Rings can be shaped at high forging temperatures for faster forging or, for rings that require the highest pressure resistance, such as aluminum car wheels, the rolling can be done at lower temperatures over a longer period.
Lower forging temperatures and longer forging times result in greater tensile strength. The ring-rolling process can be modified in any number of ways to produce a wide range of precise contours both on the inner dimension and outer dimension. Seamless rolled rings are unequaled in tangential strength, ductility, and adaptability.