Solving Balancing Problems

Solving Balancing Problems
Uneven section weight

By Tim Kolhoff

We have all been challenged at one time or another with trying to balance a door that has uneven section weight. We usually experience doors that have equal section weight and they balance just fine, without requiring any fine-tuning.

Let’s think back to the primary goal in balancing a door. We need the door’s weight to be counterbalanced or properly supported by the horizontal track assembly throughout the cycle. When the door is fully closed, the springs should counterbalance the complete weight of the door. That weight must also be counter-balanced as the door moves through its cycle. As soon as the door begins to move, its springs begin to unwind and weaken. At this point, the springs are losing torque, although the design of the drum compensates for the change in weight. An operator is designed primarily to rotate the shaft. Counter-balance is the function of the springs.

Declining spiral portions of drums have been designed to compensate for this loss of torque. Either the door begins to lose weight almost immediately (standard lift); it continues to travel vertically, then transfers weight to horizontal tracks (high lift); or it never transfers weight (vertical lift). It’s the design of the drum that manipulates the ever-changing power of the springs. All remaining weight must be balanced, especially when the door is in the open position.

Therefore, what happens when you have uneven section weight? Two good examples of this are: 1) doors with glass windows, and 2) wide doors with strut bars located sporadically throughout its sections. As the door leaves the floor, the total weight is still in balance. Keep in mind that the springs are unwinding, becoming weaker. When the top roller is about two-thirds into the radius, the cable has peeled from a helical spiral on the drum, including standard-lift cable drum. Drums are designed to allow the weight to transfer to the horizontals at the same rate as the springs are weakening. This occurs as the cable peels from the “flat” portion of the cable drum. Although, when there is unequal section weight, there is unequal weight transfer. This is evidenced when a window section transfers its weight onto the horizontals. The remaining weight is less now, but the springs continue “pulling” at an even pace. This is why the door “shoots up.” It’s technically possible to design a drum that will balance a door through out its cycle, but such a drum would be costly to produce and market, hence there are none available.

Basically, springs are calculated from a multiplier to balance a door when it’s open or closed. “From a multiplier” means the springs are losing torque at an even rate throughout the door’s cycle, although the door’s weight these springs are counter-balancing is not evenly transferred to the horizontals. Computer programs are also “based” on this, although the ATL-OMI calculation program allows you to compensate via the program’s flexibility.

The easiest way to spring a door that has uneven section weight is to calculate springs to balance the door when it’s open or closed. On existing doors, the technician can equal the weight, or add weight to the door. The most common and inconspicuous method of doing this is to add strut bars. Additional face hardware—handles or footplates— can also be added. Although this adds to your cost, it may pay off in customer satisfaction.

An average 16-foot strut bar weighs about 15 pounds. You may also decrease section weight by doing the opposite. Operators are not intended to lift weight, but can “mask” some imbalance without overstressing the machine. Others have corrected this situation to their satisfaction by tilting the horizontal tracks or adjusting the friction on the weather strip or door stops. This situation is one reason you see some garage door windows made from lighter weight materials, so as to nearly equal the cut-out void.

If you have a proven method to correct improperly balanced doors due to uneven section weight, please share it with us. We’ll list ideas in a future issue.