A Garage Door That Slams Shut Is a Safety Failure

What causes it

These are the causes we find most often, in roughly the order we check them.

1. 1

   Broken or fatigued torsion spring

   The spring is what holds the weight. Once it snaps, or loses a serious share of its tension, nothing is counterbalancing the door and gravity takes over the moment it starts down. A 90kg sectional door falling a metre does real damage to whatever is underneath it.

   What you can check

   Look for a visible gap in the coil on the shaft above the opening, and think back to whether a bang preceded the problem. From ground level you can also see whether the two springs look different from each other in coil spacing.

   What we do

   We replace the springs as a matched pair sized to the measured weight of the door, then re-balance so it holds still at waist height and lowers at a walking pace. We also inspect the cables and drums, because a spring failure often takes them with it.

2. 2

   Springs wound with too few turns

   Torsion springs are set by turns, not by feel. If a previous repair left them a turn or two short, the door will still lift with the opener helping, but it has nowhere near enough tension to control its own descent. It behaves normally going up and slams coming down.

   What you can check

   Pull the release and raise the door to head height, then let go with your hands ready and your feet clear. If it drops rather than holding position, the tension is short. Do this only if you can step back immediately and never with anyone else in the opening.

   What we do

   We reset the winding to the correct number of turns for the door's height and weight, then check the balance at three points through the travel. A properly balanced door holds at the top, at waist height and near the floor without any help from the opener.

3. 3

   Cable slipping on a loose drum

   The drums are locked to the torsion shaft by set screws seated on a flat. If those screws loosen, the drum turns on the shaft under load and the cable pays out. The door loses support on that side and can drop suddenly, usually with a bang and a sideways jolt.

   What you can check

   Watch the cables and drums as the door lowers. A cable that goes slack partway, jumps a groove, or wraps unevenly on the drum is the tell. Any sudden lurch on one side during travel is reason enough to stop using the door immediately.

   What we do

   We reset both drums on the shaft flats, torque the set screws correctly, re-tension the cables to matched wraps and re-check the balance. Shafts with chewed flats and drums with rounded set-screw seats are replaced, because they will simply work loose again.

4. 4

   Failed soft-stop or brake on a DC opener

   Modern DC openers slow the door over the last 200mm and hold it electronically. If the board's soft-stop fails, or the encoder loses position, the door runs that last stretch at full speed and hits the slab hard. It sounds like a spring problem from inside the house but is not.

   What you can check

   Watch the last half-metre of travel. A door that comes down smoothly then accelerates only at the very end points at the opener rather than the springs. A door that feels heavy by hand right through its travel points at the springs.

   What we do

   We test the encoder, the board and the motor brake, replace the failed component and re-learn the travel profile. Where the board is obsolete we tell you honestly whether a replacement opener is the more sensible spend rather than chasing a part that no longer exists.

5. 5

   Stripped opener gear releasing mid-travel

   A nylon drive gear with worn teeth can hold for most of the cycle and then let go under the highest load, which is near the floor. The door is released with no counterbalance behind it, and the final section of travel becomes a free fall onto the slab.

   What you can check

   Look for white plastic swarf under the opener head, and listen for a grinding or crunching sound just before the door drops. If the door slams only sometimes rather than every cycle, an intermittent gear failure is a strong candidate.

   What we do

   We replace the gear and sprocket, re-grease and reset the limits, then correct the door balance that wore the gear out in the first place. A gear that failed on a heavy door will fail again within a year if the springs are left alone.

6. 6

   Tilt door with a failed spring and no containment

   Older tilt-a-doors use extension springs down each side of the frame. They are simple and they work, but when one lets go there is nothing holding the panel and it swings shut fast. Doors installed before containment cables became standard drop with genuine force.

   What you can check

   Look at the springs on both sides of the frame. One hanging slack, one visibly longer than the other, or a spring lying on the floor tells you straight away. Check whether a thin cable runs through the centre of each spring.

   What we do

   We replace the springs in matched pairs, thread proper containment cables through them, and inspect the pivot arms and frame mounts that take the shock. On tilt doors we also check the anti-drop catches, since those are the last line of defence.