By Tim Kolhoff

At times we are asked questions regarding the use of ATL/OMI (or similar) cable drums. A few of these questions and the appropriate answers are listed below. The most common question involves using a drum of a different design in place of another.

No, not without also changing springs. Changing drums and not changing springs will affect the amount of torque transferred from the springs. Say you are using 1/8” cable on a 4” drum; therefore, the high-moment arm will be 2.0625”. If the installation has a pair of 2” x .225” x 23” springs with 7-1/2 turns, the springs would lift a 255# door. (35.1 IPPT x 7 1/2 turns = 263.25 MIP. 263.25 MIP / 2.0625” = 127.64#. 127.64 x 2 = 255.28#). By changing to a 5” drum, only 205# would be lifted. (263.25 MIP / 2.5625” high-moment arm). The opposite would be true if you switch a 5” drum to a 4” drum.

Yes, but more spring wire will be needed and the cable capacity of the high-lift drum’s spiral portion must not be exceeded. The flat-moment arm of the high-lift drum becomes the low-moment arm for calculating purposes. When you divide the high-moment arm by the smaller rate-of-rise of the high-lift drum you will find you need more turns; thus more spring wire. Therefore, even though “yes” is the answer, this is not a cost-efficient, long-term solution.

I would further suggest reviewing an ATL/OMI long-form worksheets to calculate the necessary spring rates, and then compare the results of both the vertical-lift and high-lift drum.

This portion of the drum compensates for the vertical travel of the door as it opens from the floor before it transfers any weight into the horizontal. The springs begin unwinding immediately and lose torque/counterbalancing power. This torque loss needs to be accounted for. That is done by the reduction of the moment arm (cable’s peel-off point) throughout that short vertical travel before any weight is transferred.

As can be expected, it’s true to say these are the diameters that have been used in our industry for years. An important consideration in the beginning was drum width, insuring that cable would properly spool onto drums when peeling on and off. The smaller the drum diameter, the less the spring requirement. Theoretically, there could be no drums and you’d save spring wire, but you’d have to thread the shaft (very expensive and highly impractical), and spooling 8’ of cable onto a shaft would also result in a great angle from the peel-off point to the bottom bracket, when the door is open.

This situation would obviously cause the cable to want to wrap over itself and then mess up the balancing. A large diameter would require more headroom and costly bearing plates. These are just some of the considerations when determining cable drum diameters.

To better understand these solutions, we recommend you perform a few ATL/OMI worksheet calculations at your convenience. We are sure that doing this will shed further light on the subject.

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