This is an important factor to be considered; however, there are other capacities that need to be considered as well. Weight capacity is a very important safety consideration in the decision. Another is cable diameter requirements.

The required “flat capacity” or the cable capacity of the constant radius portion of a high-lift drum must also be considered, although this has been considered in the design of most current drum. Flat portion requirements increase as high-lift decreases. This fact tells us that the higher the door’s height, it requires higher high-lifts.

The following example may clarify this situation. Using a door that is 12 feet high (144 inches) that has 24 inches of high-lift, we need to be sure the drum we select has 120 inches of cable capacity on its flat (144” - 24” = 120”). A note on this is printed on all of ATL and OMI’s high-lift drum specification sheets.

An additional consideration is the “zero balance point” of the drum. This term refers to the optimum door height that will be properly balanced throughout the doors vertical travel with a specific drum. The drum’s rate of rise is used in this calculation. (Rate of rise refers to the constantly increasing radius of the spiral portion.) There have been drums on the market, which have an erratic “zero balance point.” This means that the door will be greatly out of balance during its operating cycle. A door will still be balanced on the floor and in its open position while it is unbalanced as it travels vertically. This poor operation is masked when a jackshaft operator is used.

These situations make it very difficult for most installers to determine why the door operates erratically. Initial reactions may be inaccurate springs or incorrect cable length when actually the design of the drum may be at fault.

We suggest trying to stay with +/- 2 feet of a drum’s balance point to assure a door’s best performance. 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 situation is that of cable riding over the face of a door. This situation 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 situation should try to 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, increasing the balance weight while now requiring a standard drum.

I again want to emphasize that the “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.

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