Motion Scenarios: Tensioning in winding

Most wire manufacturers have applications for which magnetic particle clutches and brakes are suitable.

Consider how a plant might control the rewind tension when transferring wire from one large roll to smaller rolls: As wire is rewound onto the smaller rolls, a dancer arm measures the diameter of wire applied. This is fed back into the controller, which controls the voltage going into a clutch, keeping a constant tension on the wire as it is rewound. Here, magnetic particle clutches are often used. Why?

Magnetic particle clutches are suitable for drag and winding applications because their voltage-to-torque relationship is almost linear, so accurate control is easily maintained.

Elsewhere, magnetic particle brakes control wire-spool tension. Consider a scenario in which wire goes through a load cell that measures wire tension as it is pulled into a wire twister. The load cell feeds back tension information to the controller, which in turn feeds voltage back to the brake. Voltage is proportional to torque (assuming a constant current power supply) in magnetic particle brakes, so this provides relatively simple tension control.

Because a wide range of tension is required as a spool pays off, the magnetic particle brake is particularly suitable: Again, magnetic particle brake torque output relates directly to the voltage input, so this is easily controlled by a combination of a load cells giving accurate wire tension and a controller providing variable voltage to brakes. Brake output is geared into each of the wire spools, simplifying the attachment and removal of both brakes and spools. Magnetic particle clutches and brakes are also totally sealed useful where winders require is an oil mist, which can adversely affect friction brakes.

Elsewhere, permanent-magnet hysteresis brakes control wire tension for coil-winding machines. In one setup using these components, wire is set on a spool on the ground. (The normal tendency for wire coming off a spool is to uncoil, so there is a bit of slack in the line.) Wire goes up to a pulley mounted on a shaft going through the brake. In some setups, the wire is also wrapped around the pulley twice, for a tight connection. Then the wire goes across an idler pulley and down to the coil winder. As the wire is drawn from the brake to the coil winder, it is drawn at a constant tension, regardless of coil winder speed.

Permanent magnet hysteresis brakes are suitable here, because many small coils do not change that much in diameter, so a constant-torque brake allows for a slight tapper tension on the end of the coil wind. Certain units deliver constant torque regardless of the slip speed so wire tension is constant even at high coil-winder speed. Adjustable brake units also accommodate different wire sizes, and in some cases, marks on the unit are simply set to the wire tension required.

Other winding applications

Besides coil winding and unwinding, light film tensioning for adhesive, video, and audiotape applications, for example often benefits from magnetic particle clutches and brakes. Alternatively, magnetic film tensioning sometimes leverages hysteresis tensioners: Instead of film wrapping around a pulley, it is usually compressed between two pinch rollers that use the its friction to transfer rag. Likewise, in medical installations, small, thin-walled tubing is common so hysteresis brakes that minimize differences in stick-slip (stiction) are best. Other friction devices with substantially different static and dynamic coefficients cause inconsistencies and even material breakage.

For more information, call (732) 271-7371 or visit ogura-clutch.com.

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Motion begets motion: Cable making

When we talk about cables in the motion world, it's typically regarding how they're used to transmit power and signals. However, cables require motion, too — during their manufacture.

According to John Gavilanes, director of engineering at cable producer LAPP USA, Florham Park, N.J., dozens of steps impart certain cables with higher mechanical performance for use on multi-axis robots, rotating and tilting tables, and other extreme-bend and torsion applications. Central to the company's production of rugged FD cables, for example, are planetary stranding machines that remove backtwist.

“We use planetary stranders that leverage sun and planet gearing to simultaneously spin payoff drums, which in turn ride in rotating cradles that hold all axes in a fixed direction, so the drums orbit one another,” explains Gavilanes.

This manufacturing approach prevents two phenomena from developing when cable goes into use in applications: detrimental backtwist (also called “memory”) and corkscrewing.

Use of planetary stranding machines for cabling can be more time consuming, but also allows for highly engineered arrangements of conductor layers — one example being concentric contra-helical laid conductors. Here, each well-defined layer has a reversed lay direction — and a lay length that increases in each successive layer. This type of cabling technique is usually used on continuous flex designs.

For more information, call (800) 774-3539 or visit lappusa.com.

Brake and roll considerations

What are some best practices for winding and converting large spools of cloth, paper, foil, film, or plastics? Jeff Damour, engineering manager at Converter Accessory Corp., Wind Gap, Pa., provides helpful commentary on tension control brakes and roll selection.

Brakes for tensioning

Air brakes are beneficial in high-speed, high-torque applications because have they dissipate large amounts of heat. In contrast, magnetic particle brakes are well suited for low-speed, low-torque applications where very accurate tension control is required. Their heat dissipation is lower, so these are not well suited for particularly fast, high-torque applications — but their nearly straight torque curve is great for applications demanding extremely accurate tension control.

Another option is to drive an unwind stand. Drives can be designed to handle high speed and torque for stringent tension control requirements — though driving an unwinder may be the most expensive method of controlling tension.

Rolls for wrinkle removal

Web spreading and anti-wrinkle devices abound, and curved axis or bowed-roll systems are perhaps one of the most familiar. Though effective, these can distort some web types.

Another effective option is expanding surface rollers. These straight rollers have adjustable end assemblies that can be independently tilted — even while the web is in motion. Web is fed onto the roll at the narrowest line on the circumference, and exits at the widest — which is how the wrinkles are removed.

The expanding surface rollers are insensitive to wrinkles at all tensions, and have surfaces designed for high traction, so they're suitable for high-drag web spreading. (A stretchable rubber sleeve maintains high traction without surface glazing; static conductive roll faces are also available.) No special tools are needed to adjust degree of spreading action, and the rolls accept and deliver web at any angle. Drive sheaves are available for light webs.

For specific technical support, call (800) 433-2413 or visit converteraccessory.com.