| 摘要 |
A torsion coil spring type counterbalance system for an upward acting door includes one or more sleeves disposed over or within the spring coils and engageable with a pre-determined number of spring coils during operation of the spring to modify the spring rate to more closely approximate the required counterbalance forces exertable on the door when the door moves between open and closed positions. The counterbalance system is particularly advantageous for upward acting sectional doors which have one or more sections which are heavier than the other sections, including an uppermost section which may be heavier due to the provision of windows or other structural features of the section.
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