Before the Rotor Went All the Way Around

The Ingenious Halfway World of the Bumper Automatic

Pick up a modern automatic watch and there is a good chance that somewhere beneath the caseback sits a weighted rotor capable of travelling through a complete circle.

Move your wrist and gravity does the rest.

It seems such an obvious solution today that it is easy to imagine automatic watches have always worked that way.

They haven't.

Before the familiar full rotor became the dominant solution, watchmakers explored several different ways of turning the movement of a wrist into energy stored inside a mainspring.

One of the most successful was beautifully mechanical.

The rotor didn't go all the way around.

It went backwards and forwards.

And today we call it the bumper automatic.

A Problem Worth Solving

The idea of a self-winding watch was considerably older than the wristwatch itself. Watchmakers had experimented with using movement to wind pocket watches during the eighteenth century.

But a wrist presented a rather different opportunity.

Unlike a pocket, which spends much of its time relatively still, an arm is constantly changing direction. Walking, reaching, working and simply going about the day creates movement that could potentially be harvested by the watch.

The question was how.

In the early 1920s, English watchmaker John Harwood began working on an automatic wristwatch.

His motivation wasn't simply convenience.

A watch mainspring does not deliver exactly the same torque throughout its entire reserve. As it unwinds, the power being delivered through the train changes.

Harwood recognised that continually replenishing the energy being used during the day could keep the mainspring operating within a narrower and more useful working range.

His original patent is remarkably clear about the advantage. Under ordinary conditions he expected the spring to run down by only around nine to twelve hours before the movement of the wearer wound it again, providing what he described as “a more even torque” to the train.

There were other advantages too. Removing the conventional winding stem allowed the case to be better protected against dust and moisture, while the mechanism incorporated protection against overwinding.

So the automatic wristwatch wasn't simply about saving its owner the trouble of winding it.

It was also about keeping the movement supplied with more consistent power.

Harwood received a British patent for his design in 1923 and a Swiss patent followed in 1924. His production watches reached the market later in the decade.

A Weight With Somewhere to Go

At the heart of Harwood's system was a weighted mass mounted inside the movement.

As the wearer's wrist moved, gravity caused the weight to swing.

But unlike the rotor in most modern automatic watches, it couldn't continue around a complete circle. Its journey was deliberately limited.

At either end were sprung buffers which stopped the weight and encouraged it back in the opposite direction. Through a ratchet and winding train, that movement was used to replenish the mainspring.

Backwards.

Forwards.

Backwards again.

The principle at its heart is one we still recognise today:

use the movement of the person wearing the watch to help keep the mainspring within a useful working range.

Harwood's system was sufficiently different from the watches we're accustomed to that his early examples didn't even have a conventional winding crown. The hands were set using the bezel instead.

Then the Rotor Went All the Way Around

It would be tempting to describe the bumper as something watchmakers used simply because nobody had yet worked out how to make a rotor rotate through 360 degrees.

But that's not quite what happened.

In 1931 Rolex introduced and patented its Perpetual rotor system, using a centrally mounted oscillating weight free to rotate completely around the movement. It established the basic architecture that would eventually become familiar throughout automatic watchmaking.

Yet the bumper didn't immediately disappear.

Different manufacturers continued developing automatic winding in different ways, and limited-arc bumper systems remained a perfectly viable engineering solution.

By the 1940s some of Switzerland's finest manufacturers were still building them.

Which brings us to a name particularly familiar on the Past2Present bench.

Omega Takes Up the Idea

Omega introduced its first series-produced automatic movement, the 28.10 RA PC, in 1943.

It was a bumper.

The oscillating weight travelled through a limited arc before meeting sprung buffers at either end. Later versions of the family would be given the more familiar three-digit Omega calibre numbers, and bumper movements would find their way into some of the company's most recognisable early automatic watches.

When the Seamaster appeared in 1948, bumper automatics formed an important part of the early range.

And Omega wasn't alone.

Jaeger-LeCoultre developed bumper automatics of its own, and versions of the principle appeared in watches from other respected manufacturers.

These weren't crude experiments waiting for somebody to invent a proper automatic watch.

They were sophisticated movements produced during a period when automatic watchmaking itself was still evolving.

An Automatic You Can See Working

Take the caseback from a bumper automatic and its ancestry becomes immediately obvious.

Instead of the familiar semicircular rotor sweeping freely around the movement, there is a weighted oscillating mass with a clearly defined range of travel.

Move the watch gently and you can watch it swing. At the end of its journey, the buffers do their job and the mass changes direction.

On some examples you can even feel that movement through the case when the watch is worn.

That's where the modern nickname becomes useful.

Bumper.

But the bump isn't really the interesting part.

What's interesting is being able to see an engineering solution that belongs to a particular moment in the development of the automatic wristwatch.

The problem is familiar.

The answer is different.

Keeping It Wound

There is a connection here with the automatic watches we work on today.

Automatic winding is particularly good at maintaining the energy in the mainspring while a watch is being worn.

A mechanical movement generally benefits from having a healthy amount of power available from its mainspring. As the reserve becomes depleted, the torque available to the movement reduces and its rate can become less stable.

That is why, on the bench, we don't regard waving a completely stopped automatic watch around as the best way to get it going.

Where the movement allows manual winding, give it some initial wind first. Then put it on your wrist and allow the automatic winding system to do what it was designed to do particularly well:

keep it wound.

The technology has changed enormously since Harwood's first watches, but that underlying piece of watchmaking makes the connection rather nicely.

Evolution Rather Than Obsolescence

Eventually the full-rotor automatic became dominant.

Automatic winding continued to develop, with more efficient winding systems, improved bearings and increasingly sophisticated ways of capturing the movement of the wrist.

Seen from today, it would be easy to regard the bumper as an evolutionary dead end.

We don't think that's quite fair.

Watchmaking history rarely progresses in a perfectly straight line. Different manufacturers solve the same problem in different ways. Some solutions survive and others are superseded.

Occasionally something that was once completely ordinary becomes fascinating precisely because we no longer make things that way.

The bumper automatic belongs in that category.

There is something particularly satisfying about servicing one because its architecture makes the development of automatic winding almost tangible.

The movement isn't merely old.

The idea inside it is old.

A modern automatic gives very little indication that there was ever another way of doing things.

A bumper movement does.

Eventually the rotor would go all the way around.

For a while, it didn't need to.

And more than seventy years later, many of those watches are still quietly winding themselves every time somebody puts one back on their wrist.