By Tim Kolhoff

In Part I (PDD April 2006), the definition of “zero balance point” was discussed. In this part, we will discuss additional considerations and calculations. I do want to emphasize here that this “zero balance point” concept was considered in the design of all current drums, so you do not need to address this concern but be aware of its existence.

We will then continue by revisiting four important considerations when selecting a high-lift cable drum for an installation:

• Weight capacity—never to be exceeded.
• High-lift capacity requirements —never to be exceeded.
• Flat portion capacity—remember it must be more than the door’s height, less than high-lift amount.
• Cable diameter requirements, based on weight limitations—also never to be exceeded.

Once these four points are verified, it is a good practice to check the variance of our selected drum to its zero balance point. This will insure acceptable manual operation of the door.

At this time, I’d like to mention that the most optimum door height and high-lift combination—the zero balance point—for any given high-lift drum is the point where the published multiplier is equal to the rate of rise of the spiral portion. When the multiplier is less than the rate of rise, the door will be “heavy” until its top roller enters into the horizontal tracks. The more the multiplier varies from the rate of rise, the more the imbalance in vertical travel (an electric operator only masks this situation).

Multipliers may be obtained from your drum supplier. The combination of the door height and the high-lift measurement is used to calculate multipliers. The rate-of-rise is the measurement of changing radius throughout the spiral portion of the drum. The rate-of-rise is normally listed on most high-lift drum specification sheets.

Always choose a high-lift drum with a small overall diameter and the least amount of excess spiral capacity.

One common outcome is that of cable riding over the face of a door. This most often occurs on high doors that have small amounts of high-lift. In these situations there is a lengthy horizontal travel of the cable to the drum. This should be avoided if possible. There have been cases where cable drums have been shaved to lessen this horizontal travel, although this process will weaken the drum and is never recommended. If the amount of high-lift is similar to panel height, it may be best to add a “dummy” section to the door, making it a standard lift application, thereby increasing the balance weight while now requiring a standard drum.

Again, the zero balance point concept was considered in the design of all current drums, so you do not need to address this concern, but simply be aware of its existence.

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