The cable didn’t snap. It just walked off the drum, and now the door hangs crooked, one corner on the floor and the other six inches in the air. You can see the cable lying in a loose coil. It looks like a five-minute job, wind it back on, done. It is not. The tension that put it there is still in the spring system, and that tension does not care that you have a pair of vice-grips and a ladder.
A cable comes off the drum because something else failed first. The drum itself cracked. The bearing seized. The track bent. The spring lost a coil and threw the balance off. Until you find and fix that root cause, the cable will walk off again, usually the first time you hit the opener button.
What the drum actually does
The cable drum is not a spool. It is a calibrated lever. On a standard-lift residential door, the drum groove is machined so the cable wraps at a changing radius. That changing radius compensates for the geometry of the door: the cable pays out faster when the door is low, slower when it is high, keeping tension even through the travel. A high-lift drum has a different profile. A vertical-lift drum is different again. Swap the wrong drum onto a spring system and the door will never balance. The cable will stack unevenly, jump the flange, and you are back where you started.
Most residential drums are die-cast aluminum. They crack at the keyway. They strip where the set screws bite. The cable groove wears a burr that catches the strand. You cannot see the crack until the drum is off the shaft. You cannot feel the burr with the cable under tension.
Why the cable walked off
Loose set screws. Vibration works them out. The drum rotates on the torsion shaft independently of the spring. The cable goes slack on one side, tight on the other. The door twists in the tracks. Rollers pop. The cable jumps the flange.
Broken spring. One spring of a pair snaps. The surviving spring winds the drum on its side. The other drum back-spins. Cable pays out into a bird’s nest. The door comes down hard on one corner.
Bent track or seized roller. The door binds. The cables pay out unevenly. One drum winds loose. The cable drops into the gap between drum and bearing plate.
Wrong drum installed. Previous repair used a standard-lift drum on a high-lift track, or mixed left and right drums. The cable stacks against the flange every cycle. Eventually it climbs over.
Cable too short or too long. Short cable pulls the bottom bracket off the door. Long cable stacks past the flange. Both end the same way.
What you are looking at when the door is crooked
The bottom brackets are still attached. The cables are intact. One drum has cable on it. The other drum is bare, the cable lying in the track or coiled on the floor. The torsion shaft may have slid sideways in the bearings. The spring cone may have slipped. The center bearing plate may be flexed.
Do not reach for the winding bars yet.
If you wind the spring to take tension off the cable, you are adding turns to a system that is already unbalanced. The shaft will rotate. The drum will spin. The cable will snag on a track bolt, a roller stem, the edge of the bearing plate. When it snags, the stored energy releases into your hands, your face, the ladder.
I have seen a winding bar punch through a windshield. I have seen a technician lose two fingertips because the cable caught a burr on the drum flange and the bar kicked back. The spring does not know you are careful. It only knows torque.
The correct sequence
1. Lock the door down. Vise-grips on the track under the bottom roller, both sides. The door cannot move. 2. Mark the drums. Paint pen or scribe a line across the drum face and the shaft. You need to know the original clocking. 3. Unwind the springs. Fully. Both springs. Count the turns coming off. Write the number down. This is the only way to reset the system to a known state. 4. Slide the shaft. Pull the shaft out of the end bearing. Inspect the keyway. Inspect the drum groove. Inspect the cable for broken strands, especially at the bottom bracket swage and where it wraps the drum. 5. Replace what is worn. Cracked drum? Replace both. Frayed cable? Replace both. Seized bearing? Replace. Bent track? Straighten or replace. Do not reuse a cable that has been kinked. The strand damage is internal. 6. Reload cables. Left cable on left drum (red paint mark), right on right (black). Seat the cable end in the drum anchor. Wind the cable onto the drum by hand, keeping tension with your thumb. No slack loops. The cable must sit in the groove, not on the flange. 7. Reinstall shaft. Engage the keyways. Center the shaft. Tighten set screws on the drums, hard. Tap the drum with a hammer to seat the key, retighten. 8. Wind springs. Use the turn count you recorded. Equal turns both springs. 9. Test balance. Remove vise-grips. Lift door manually. It should hold at waist height. If it flies up or crashes down, the turn count is wrong. Adjust in quarter-turn increments. 10. Cycle the opener. One full cycle. Watch the cables track. Listen for rubbing.
What goes wrong when you skip steps
You wind the spring with the cable loose on the drum. The cable crosses over itself. The next cycle it binds, the drum bends, the cable snaps under load.
You don’t mark the drums. You reinstall them 180 degrees out. The cable anchors pull the bottom brackets sideways. The door twists every cycle.
You reuse a cable with one broken strand. That strand was carrying load. The remaining strands take the extra. The cable fails at the bottom bracket, usually while the door is moving.
You tighten set screws on a cracked keyway. The drum spins on the shaft. The cable walks off in three cycles.
When to call
If you have never wound a torsion spring, this is not the job to learn on. The tools are specific: 18-inch winding bars, not screwdrivers. Not rebar. Not pipe. The technique is specific: body positioned to the side, bars fully seated, one hand on the bar at all times. The consequences are not theoretical.
If the drum is cracked, the bearing seized, the track bent, or the spring cone slipped, you need parts you do not have on a Sunday afternoon. Trucks carry standard residential drums, cables, bearings, and springs. The repair takes ninety minutes. The door works. The balance holds.
You want the door to open tomorrow. You want it to open the same way next year. That comes from finding the root cause, not reloading the symptom.
Call 778-873-7282, Metro Vancouver.