How to Choose the Right Brake
There are many types of brakes from which to choose. Different sizes, models and configurations exist. As well as a long list of potential suppliers. Choosing the right brake for your unwind application can be difficult. My attempt with this tech tip is to make your decision a little easier.
There are many types of brakes including pneumatic, magnetic particle, hysteresis and mechanical. Three of the most common types are:
Pneumatically operated brakes utilize air pressure to increase or decrease torque output. The more air pressure you supply to the friction pad, the more torque is supplied through the friction disk to your unwind shaft. Pneumatic brakes for converting are generally designed for medium to heavy duty applications. Advantages to using this type of brake is that they can dissipate a lot of horse power and supply large amounts of torque. Their disadvantage is that their torque output is not linear relative to the pressure controlling them. I make these statements as very general rule of thumb. Different types of pneumatic brakes will perform differently in terms of heat dissipation, torque output capability, accuracy and linearity.
Magnetic particle brakes use electric current to energize a magnetic field. There is a rotating rotor mounted in bearings internal to the brake assembly. Ferrous particles reside in a narrow gap between the magnetic field generator and the rotor. By energizing the magnetic field, the ferrous particles stick together. Varying the energy supply to the brake changes the intensity of the magnetic field and adjusts to what level the ferrous particle cling to each other and the rotor. These brakes are used in the light to medium duty range. They can be very accurately controlled and their torque output relative to control input is almost a straight line adding to accuracy. They have limited heat dissipation, so are not well suited for high speed applications.
Mechanical brakes come in many designs, including leather strap brakes and mechanically adjusted friction pads. The main concern here is the initial cost. Mechanical brakes are typically used in applications where low initial cost is the deciding factor. These brakes are mechanically adjusted and cannot be controlled with a typical tension control. Adjustments can only be made manually.
OK, now that you have an idea in which type of brake you may be interested in the question is how do you select a model and size? There are three main considerations here:
Horsepower (heat dissipation) requirement. This is a measurement of how much heat your application will generate. The brake you select must dissipate your maximum horsepower generation. To calculate horsepower for your application, first multiply your maximum total web tension in pounds X your maximum web speed in feet per minute. Then divide that number by 33,000. So, HP = (tension (pounds) X speed (FPM)) / 33000.
Maximum torque requirement. This is the measurement of how much power your brake must bring to your unwind application in order to unwind your web at the tension level you require. To calculate your maximum torque (in pound inches) requirement, multiply your maximum roll diameter in inches X your maximum total web tension in pounds. Then divide that number by two. So, Tq (pound inches) = (diameter (inches) X tension (pounds) /2.
Minimum torque requirement. This is the lowest amount of torque your application requires. This is a very important consideration because brakes have a minimum torque they can supply and nothing controls to zero torque. To calculate minimum torque (in pound inches) requirement, multiply your core outside diameter in inches X your minimum total tension (considering your minimum web width) in pounds. Then divide that number by two. So, minimum Tq (pound inches) = (core O.D. (inches) X minimum tension (pounds)) / 2.
Once you know your horsepower requirement and your minimum to maximum torque requirement, selecting a brake becomes a process where you compare your requirements to horsepower and torque graphs of available brake models and sizes. Make sure your operating range for minimum to maximum torque requirement falls within the torque range of the desired brake.
As you can see, in order to calculate the size of any brake you MUST KNOW YOUR TENSION. Statements like "Light", "heavy" or "whatever we need to stop the roll from freewheeling" do not help when calculating brake size. A number (in pounds) must be plugged into the equations described above. A guess at that number is still just a guess and whether the brake is properly specified and sized will be a guess as well. That been said, tension is often the hardest number to understand and actually know for your process because it is a transient force that is not readily measureable without a load-cell display. To understand more about tension and ways to measure tension, you may want to visit:
http://www.converteraccessory.com/papers and click on "The Mechanics of Tension Control White Paper".
One last bit of information. Brakes (and clutches) are used in all types of industries. Many have none of the concerns that converters and their operations have regarding tension control. Before considering which type of brake you require and then running through the aforementioned calculations, make sure you consider a brake that is specifically designed for working in converting applications.
201 Alpha Road, Wind Gap
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