Time-In-Process and Belt Speed Control on a Conveyor Oven By Daniel Pierre III
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hen comparing ovens for a new investment, one factor that influences your decision is the range of time you can keep parts in the oven. To a lesser extent, another is whether you can control your speed digitally or through a variable speed potentiometer. If you have a specific application in mind, you may know what speed or Time-In-Process (TIP) you require. If you need an oven for general use, one tends to select ovens with the widest range of speeds to maximize your flexibility. Unfortunately, most specification sheets do not come with asterisks to further explain that you usually cannot get the entire range of speed indicated. To make things more complicated, there are additional speed ranges available for that oven. This lack of clarity illustrates that you need to utilize your relationship with your oven supplier to fully understand an oven’s capability. It is nearly impossible to compare “apples to apples” by just looking at the specification sheets. From an oven designer’s point of view, let me explain what goes on to develop the optimal drive system for an oven. First, ovens are designed from the inside out. You have known
wattages of heating elements and/or a known capacity of heating forced air. For electric ovens, there are limits to consider so you do not have nearby elements affecting another element’s performance. In the end, you have a heat chamber with the optimal heating power. Next, you determine how much mass can pass through the oven so it will not be overloaded, both physically and thermodynamically. These are the hard facts, and all sorts of formulas can assist in this design stage. Now comes the “art” of the design. An oven designer must guess the typical loads that will be used in these ovens. This is where relationships with our customers as well as relationships with the makers of spring coiling machines are beneficial. The more oven designers can understand the probable load scenarios, the better we can calculate the stress relief needs of the parts, i.e., TIP. You are left with a range of speeds that are likely to be needed for this oven. This is only half of the design, though. The harder calculations are understanding the torque
The more oven designers can understand the probable load scenarios, the better we can calculate the stress relief needs of the parts, i.e., TIP. needs based on the load range. The smoothness of the conveyor bed, the weight of the conveyor belt plus parts, the size of the drive drum and a customized torque wrench reveal the torque needs of the drive motor. Next comes the calculations based on the gear ratio (or choices of gear ratios) of various drive motors. All of this information will give you a specific range of speeds possible for a particular motor on an oven. Typically, for digital speed control, the range of speed is limited to about 90 percent of the motor’s range. In analog potentiometers, you can often see that the motor stops before you crank the dial all the way to zero. Digital controllers can cut that dead space out.
SPRINGS / Summer 2021 / 43
