A counterbalance spring is one of the simplest parts in a garage door assembly and one of the easiest to order wrongly. Most failed orders trace back to the same five or six values, so it is worth being precise about each.
1. Wire size
Wire size sets how much torque the spring delivers for a given number of turns. It is normally quoted either as a decimal inch (0.207", 0.218", 0.225", 0.234") or as the corresponding millimetre value. A difference of a few thousandths of an inch changes the number of turns a door needs, which is why the wire size should be specified rather than inferred from the old spring's outside diameter.
2. Inside diameter
Inside diameter — not outside — is the controlling dimension, because the spring sits on the shaft. Measure the inside of the coils rather than the outside, and give the nominal figure (1.750", 2.000", 2.625") rather than what a caliper happens to read on a worn spring.
3. Free length
Free length is the length of the unloaded spring, measured before it is wound onto the shaft. It is often confused with the installed length, which is shorter.
4. Wind direction
Counterbalance springs come in left-hand and right-hand wind, and most doors use one of each. Wind direction can be read from the direction the coils travel when you look down the spring from the winding cone end. Getting this wrong produces a spring that simply cannot be installed.
5. End type
Standard counterbalance springs are wound onto cone and plug ends. If your assembly uses a different fitting, the end detail has to be on the drawing.
6. Quantity and life cycle
Springs are consumables with a rated cycle life. If you are a distributor, tell your supplier the cycle rating your customers buy against, and the annual volume per size, because it determines how material is purchased.
What this means when you send an enquiry
If you send nothing else, send the six values above plus the quantity you buy per year. If you have the old spring in hand, a photograph next to a tape measure will usually let a manufacturer confirm the rest.