Inside a Wind-Up Alarm Clock, With a Repairer at His Bench

Halvard Teague, the bench, the visit and my grandmother’s clock are all made up; the mechanics he explains are real, and the conversation is a reconstruction.

The clock sat open on the bench like a patient, its back off, its twin bells upside down beside it. It is the little cream-colored Westclox that stood on my grandmother’s dresser, and I had brought it in because it had gone quiet around the time she died. On my phone I use DontSnooze, which wants a short video of me up before it lets go; the old clock stays on the dresser for the noise. The room smelled of clock oil, thin and sweet, and under the lamp the movement ticked in short dry steps.

Here is the short answer he gives. A wind-up alarm clock contains two separate springs. One drives the gears that move the hands, and the other is held back until a notch in a disc lines up with the hour hand at the time you set, whereupon a small hammer is released to rattle against a bell. Who first fitted a spring and a bell to wake a sleeper is part of the long history of the alarm clock.

Why are there two keys on the back?

“Because there are two engines,” Halvard said, pointing with a pair of tweezers. “Time and alarm. Each has its own coiled spring in its own barrel, and each has its own gears. On most clocks the keys are labeled, and one winds the time spring and the other the alarm spring. If you only wound one, you would get a clock with no voice or a voice with no clock.”

He turned the clock so the lamp fell on the gears. The time train is a chain of wheels that steps the spring’s pull down into slow motion, with the hour hand turning once in twelve hours. At the end of that chain sits the escapement, which lets the wheels advance one small tick at a time. In this model a balance wheel swings back and forth against a fine coil called a hairspring, and each swing lets one tooth through.

“That swing is your clock,” he said. “The spring is only the muscle.”

How does the alarm know when to go off?

I had always assumed the alarm hand was a kind of pointer that the clock compared against the time. It turns out to be simpler and more physical.

“Look at the back of the dial,” Halvard said. “The alarm hand is fixed to a disc, and the disc has a notch cut into its edge. When you slide the alarm hand to 7, you are turning that notch to the 7 position. Meanwhile there is a spring-loaded arm riding on the hour hand’s tube. It rubs along the disc all day, holding the alarm train locked. At 7 the arm reaches the notch and drops in. Now the lock is gone.”

The alarm spring, which has been waiting, finally turns its wheels. “So the alarm hand sets a notch, and the hour hand walks around until it finds it,” he said. “Nothing is calculated. It is a key finding its lock.”

A mechanical wind-up alarm clock makes a sound at a set time because the alarm hand positions a notched disc, and when the hour hand’s tube carries a spring-loaded arm into that notch, the arm drops and a separately wound alarm spring is freed to drive a hammer against a bell. He added, almost as an afterthought, that there is no AM or PM anywhere in this. The hour hand’s tube goes round once in twelve hours, so a clock with the alarm left switched on will ring at 7 in the evening just as readily. (The nine-minute snooze that later clocks added has its own odd origin, but nothing of the kind lives inside this one.)

Why is the ring so grating?

“Listen,” Halvard said, and he nudged the alarm lever with a fingertip, the way the notch would. The hammer started to buzz.

The hammer does not strike once like a doorbell; an anchor of its own, driven by the alarm spring’s last wheel, chatters it against the bells many times a second, so no gap is long enough for the note to fade. “A struck bell rings and decays, and your ear can follow it,” he said. “This does not decay. It is an unbroken rattle of metal on metal, with a lot of bright overtones in the high range where people are most sensitive. That is why it feels like someone is scraping the inside of your head.” That is his reading and my ears, not an acoustic measurement. The racket lasts only as long as the small alarm spring has force left, and then it winds down and goes quiet.

Why does my clock run slow, or fast?

Grandma’s clock had always run a few minutes behind and nobody had ever fixed it, so this was the question I most wanted answered. “There is a tiny lever next to the balance wheel,” Halvard said. “It has an F and an S on the plate. It slides a pin along the hairspring and changes how much of the spring is free to flex. A shorter free length makes the balance swing faster, so the clock gains. A longer length makes it slower. Nudge it a hair toward F and you gain time.”

If the lever is where it should be, he told me, the usual culprit is old oil. Clock oil thickens and collects dust over decades, and the balance wheel has to push through it. A tired mainspring can also deliver less force near the end of its run, so a clock that keeps good time on the first evening may drift by the second day.

What goes wrong with them?

His list was short. A mainspring can snap, often after years of being wound hard against the stop, and then the key spins with no resistance. Old oil can gum up a pivot until the wheels stall. The hammer’s pivot can wear or the little arm can bend, so the alarm either fires early or not at all.

“Most of what comes to me is dirt and a stiff key,” he said. “People force the key. The clock tells you when to stop, because the key stops.”

He cleaned my grandmother’s clock, a slow job, and oiled six or seven points with a tiny steel pin, and set the regulator a hair toward F. I wound both keys until they stopped, set the alarm hand to 7, and pulled the knob out.

The next morning it rattled across the dresser right on cue, and I lay in bed for a moment, grinning at how rude it was.

FAQ

How do I set a mechanical alarm clock?

Set the hands with the time knob and wind the time key clockwise until it stops. Slide the alarm hand to the hour you want, wind the alarm key the same way, then pull out or flip the alarm knob or lever (the control varies by model). A wind-up has no AM or PM, so it will ring at both hours unless you switch it off.

Can I wind a mechanical alarm clock too tight?

Wind until the key stops, then stop. Pushing on past that point is how mainsprings snap and ratchets strip. If the key turns freely with no resistance, either the spring has broken or it has come off its arbor, and both need a bench.

Can I oil a wind-up alarm clock at home?

Avoid household oils. Repairers use purpose-made clock oil, and lightweight oils tend to creep away from the pivot and leave it dry. Pivots take a thin clock oil, and mainsprings are lubricated differently from the rest of the movement, which is another reason to leave a full cleaning to a repairer.