Motion System Design
Tension brakes let it slide

Tension brakes let it slide

Brakes used in tension control for winding and unwinding material must accurately apply wide ranges of changing torque while handling the heat generated under continuous slip

Controlled-tension material transfer is essential in many industrial settings. Some are obvious, such as processing mills that feed raw material onto rolls for shipment and storage. Other common industries are metal stamping, textile, and printing, where raw material is pulled off of rolls for shaping, cutting and pressing. There is also a need for tensioning in marine equipment such as winches used to moor offshore floating platforms, and in the logging industry, where power yarders require controlled tension in cable riggings that haul logs to a gathering area.

Tensioning concepts

Tensioning involves winding or unwinding sheets or strands of material on or off of spools, drums, rolls, and so on. Ideally the material is kept at constant tension throughout the process; this is done with brakes. Tensioning brakes perform under near constant-duty, so heat and wear are affected by different dynamics than in shutdown and emergency- stop (E-stop) brakes.

Several basic arrangements are possible. Unwinding or rewinding from one coil or spool onto another is the most basic. Here the feed spool uses a brake and offers resistance against the motor-driven take-up spool. This causes a pull on the material, a force tangential to the point where it conforms to the spool. (Tension is conventionally measured as the pull per inch of material width – force divided by length.)

In the case where material is unwound from a spool but not rewound, (cutting bolts of fabric, for instance), it is simple enough to envision a feed spool with brakes applied as the material is caught and drawn out by means such as rollers. In a process where flat sheets or strands are wound onto a spool or “core” (turned by a motor), controlled tension is often imparted through some kind of braked traction device (again, like rollers).

There are implications to faulty tensioning. If the right tension is not maintained during a winding procedure, telescoping and improper roll density can occur. Telescoping happens when two brakes at each side of a feed spool are applied unequally; the material winds onto the take-up spool at a helical angle, wrapping into a cone or “telescope” shape rather than a straight cylinder.

Improper roll density, on the other hand, is a result of fluctuating tension throughout a winding process. The material may be wound tightly at the center of the core and loosely at the outer diameter, or vice versa. This can cause an uneven feed during reprocessing, leading to excess scrap.

During material unwinding, incorrect tensioning can lead to web distortion, which occurs in either of two ways: the material might not line up in the right tracks as it comes off the roll, or it could be stretched as it is laid out for reprocessing, causing variations in the thickness. Unnecessary scrap is again the outcome.

Tension can be sensed and regulated by several methods. A load cell mounted onto both ends of a roll might be used to weigh the pull on the material as well as the changing weight of the roll as it is processed. The resultant force (which varies not only with weight but with different angles of pull, or wrap angles) is analyzed, and the braking torque is moderated accordingly as the electrical control signal is converted into fluid pressure that actuates the brake.

A sonar head is another way to determine the tension. It uses ultrasound to measure the roll diameter. The required braking and acceleration torque is calculated and controlled as a function of the changing roll inertia, derived from the diameter.

The dancer system is a third technique. It uses position-based control involving a special idler roll. This “dancer roll” translates, moving against the web of unrolling material; the position of the roll is influenced by the tension. A sensor in the dancer roll sends a signal to the controller, which decides the required braking torque (based on the position of the dancer roll) and adjusts the braking pressure.

Heat generation

There are applications in which the tension is controlled yet varied, but constant- tension applications are far more common. Also typical, but to a lesser extent, is a constant payout, which is the linear material speed. The condition of both steady tension and payout offers a suitable base from which to establish thermal concerns in tensioning brakes.

Braking torque (a function of the frictional coefficient, the surface area of the friction material, and the actuating force) provides the tension. For this use, torque is defined as the required pull multiplied by the radius of the material roll – the tangential force times the moment arm.

When a spool of material is unwound under constant tension – and constant pull – the radius and hence the required torque goes down, but the speed of the roll goes up if the material payout is to be steady. Conversely, winding up (under constant tension and payout) requires a gradually increasing torque and decreasing speed.

Both speed and torque affect heat. But in fact, the increase (or decrease) in torque is proportionately offset by the decrease (or increase) in angular speed as the spool diameter changes, so in a constant- tension, constant-feed operation heat is always produced at a continuous rate.

To prove this, here’s a quick rundown of the governing equation. In a continuous-slip application like tensioning, the thermal horsepower quantifies the rate of heat generation. Horsepower is traditionally known as:

H = Tn/63,000

Where T is the torque (lb-in.) and n is the shaft speed (rpm). This can be revised to read:

H = Fv/33,000

Where F is the force (lb) and v is the linear velocity (ft/min). The force (the pull) is held constant, and if the material payout is steady, linear velocity is constant, and therefore the thermal horsepower doesn’t change.

Brakes are hot

The life of brake friction material is affected by the wearable lining volume and the applied thermal horsepower, but also by the ability of the brake to dissipate heat. The wear rate of a friction material is calculated in hp-hrs/in3, so the theoretical element life is a measure of the wear rate times the material volume divided by the horsepower. But heat transfer cannot be ignored: improper cooling will inevitably degrade the brake life and performance. Although heat generation (thermal horsepower) is continuous for an application with fixed tension and payout, heat can build to unacceptable levels within the friction components.

Heat dissipation from the brake has to be accelerated under higher levels of heat generation. Therefore tensioning applications are separated into light and heavy, according to the thermal horsepower requirements. Light tensioning usually refers to operations that are 30 hp or less, while those exceeding 30 hp are classified as heavy tensioning. As stated, the pull and linear speed of the material solely constitute thermal horsepower, so smaller operations with light-gauge materials or low speeds are likely to use brakes in the light tensioning category.

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Light tensioning is accomplished with air-cooled brakes. These brakes dissipate heat by conduction from the friction surfaces into the adjoining materials, by convection with the air, and by radiation into the surroundings. Convection is largely relied upon, hence the term “air cooled.” It is a fair means of heat transfer, and the air provides an infinite- capacity low-temperature reservoir. The surface areas of mating frictional surfaces play a large role in the convection process. Also of significance is the speed of the moving frictional surface, since higher windage increases convection heat transfer. Here the rotation of the material spool plays a part; the heat generation may remain the same, but as spool diameter increases and angular velocity decreases, convection is reduced and heat can build up more rapidly – and vice versa.

Forced convection is sometimes applied with an external cooling fan. This not only supplies good air movement even at low speed, it provides flow over all components, not just the one turning.

Radiation is influenced by surface area and by the enclosure. Conduction from the friction elements goes as far as the ability of neighboring materials to absorb and disperse the heat.

Typical air cooled brakes are caliper, air-cooled drum, and air-cooled disc. While caliper and disc brakes in automotive use are one and the same, here caliper and air-cooled disc brakes are distinguished by the construction; with disc brakes both friction surfaces are discs that mate all the way around, while calipers have pads mating with the rotary disc at limited points.

The choice between drum, disc, and caliper depends on the particular function. The pros and cons vary considerably, and these are a study in themselves. Overall size, frictional surface area, and speed and torque allowances are among a few of the basic parameters that are weighed.

Heavy tensioning requires greater cooling capabilities, and water-cooled brakes are generally used. Recirculated water greatly improves the transfer of heat from hot metal. Naturally, watercooled brakes are more complex than air-cooled.

Of course, water doesn’t make contact with the friction surfaces. It is circulated into cavities where it interfaces with the opposite surface or with highly conductive backings attached to the contacting plates. The material is kept cool as heat conducts away from the surfaces and discharges into the water by convection. A higher water circulation rate increases the heat capacity of the brake.

Cooling with water makes some brake sizes and materials possible that cannot be used in air-cooled units. Watercooled brakes typically come in single and dual piston disc as well as in drum configurations. Dual piston watercooled arrangements provide exceptional tension control across a wide range of conditions – two pistons can readily supply the full brake torque when the roll diameter is largest, and with lesser diameters, it can switch to a single piston.

Kristen Chilton is Product Manager with Eaton Corp., Airflex Div., Cleveland, Ohio.

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