One of the most frustrating obstacles that someone new to Watch Repair, is fitting the train bridge onto the movement, aligning the pivots and into the pivot holes, and then being able to tighten down the train bridge without crushing or bending the pivots. This is the last system to cover with the ST36 watch movement and sets you up to now be able to start disassembling the watch and performing the needed inspections.

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The Power Train of a Watch

Understanding the Heartbeat and Muscles

Have you ever heard the saying that the balance wheel of a watch is its heartbeat, and the power train is like its muscles? Well, it’s true. The power train transfers all the energy coming out of the mainspring barrel to the escapement, and it’s crucial that it’s in good working order.

Taking Apart the Train Bridge

As you delve deeper into the world of watch service and repair, you’ll encounter bridges of various configurations. Sometimes the barrel bridge will simply hold the barrel, and the train bridge will hold the rest of the train wheels. In other cases, the escape wheel may have its own bridge. No matter the configuration, all the principles in this article apply to all the different situations you will encounter. So let’s start by taking apart the train bridge.

This train bridge is secured to the plate with two screws and some steady pins beneath the bridge. To remove it, simply unscrew the screws and use a screwdriver to loosen the bridge from the plate. Then, use your tweezers to lift the bridge away.

ST36 Train Bridge
ST36 Train Bridge

Here you have the third wheel, the fourth wheel, sometimes called the second’s wheel, and the fifth wheel which is better known as the escape wheel.

Wheels of the Train Bridge
Wheels of the Train Bridge

The Wheels of the Barrel Bridge

Now, let’s move on to the barrel bridge. The wheels of the barrel bridge are straightforward to identify. You have the first wheel, better known as the mainspring barrel, and the second wheel, which is often called the center wheel when it’s in the center of the movement, but in some movements, the second wheel can be placed elsewhere. 

Barrel Bridge Wheels
Barrel Bridge Wheels

The Motion Works and Gearing Ratios

As you may recall from our video on The Motion Works, the motion works operate on a reduction ratio. The cannon pinion turns the teeth on the minute wheel, and the minute wheel’s pinion turns the teeth on the hour wheel, slowing things down. However, with the power train, it’s the complete opposite. The goal is to speed things up. These wheels start slow and gradually get faster, all the way through the power train to the escapement.

The gearing ratios will vary depending on the beat of the watch. In this case, the beat is 18,000 beats per hour. The mainspring arbor of this watch makes one revolution every eight hours, the second wheel arbor makes one rotation every hour, the third wheel makes one rotation every seven and a half minutes, the fourth wheel pivot makes one rotation every 60 seconds, and the escape wheel pivot makes one revolution every six seconds.

Train Ratio of 18000 BPH
Train Ratio of 18000 BPH

Understanding the Workings of a Watch

So, you may be wondering, how does all this information help you with putting on a train bridge? To be honest, it doesn’t. But it’s still important to understand the workings of a watch, don’t you agree?

Teeth and Pinions of the Wheels

As you know, the teeth of the mainspring barrel turn the pinion leaves of the center or second wheel. The second wheel’s teeth, in turn, turn the pinion leaves of the third wheel. You may have noticed that the pinion on the third wheel on this movement has a cutout to make room for the fourth wheel. The third wheel’s teeth turn the pinion leaves of the fourth wheel, which carries the seconds hand, and the fourth wheel’s teeth turn the pinion of the escape wheel.

Reassembling the Wheels and Bridges

Step 1: Install the Mainspring Barrel and the Second Wheel Bridge

We’ll start by installing the mainspring barrel and then the second wheel. You can see the teeth of the mainspring barrel turn the pinion of the second wheel.

Installing Barrel Bridge Wheels
Installing Barrel Bridge Wheels

The barrel bridge is straightforward to line up since the mainspring barrel and the center wheel are in the center.

Installing the Barrel Bridge
Installing the Barrel Bridge

Step 2: Placing the Train Wheels onto the plate

The order in which the wheels are installed may or may not matter, depending on the movement. In this case, we’ll install the third wheel first, then the fourth wheel, and finally the escape wheel.  Install the wheels so that the pinions and teeth are straight up and down as if the bridge was in place. This will make it easier to line up the bridge jewels with the pivots of the wheels.  

Train Wheels in Position
Train Wheels in Position

When installing train bridges, it’s always a good idea to tighten the screws gradually and check that the wheels and pivots are still rotating freely. Always turn the screws to tighten them and stop once you met resistance. Then check to make sure that the wheels still turn freely and finish tightening the screws. 

If you are not familiar with how much to tighten a screw, review the video on The Keyless Works. 

Step 3: Installing the Train Bridge

When bringing in the train bridge, keep an eye on the orientation of the jewel holes, the orientation of the pivots, and the locations of the screw holes. Ideally, the pivots would go straight into the holes, but this is rarely the case. If the pivots don’t go into the holes, you’ll need to adjust the wheels.

Positioning the Train Bridge
Positioning the Train Bridge

Use a hold-down tool to steady the bridge and check the free running of the wheels by turning the center wheel. If all the pivots go into the holes, you’re good to go. If not, you’ll need to adjust the wheels and try again.

Step 4: Move the Wheels into their Jewel Holes

More than likely when you first start doing this, all your pivots will not fall into place right away. To move the wheels into their jewel holes, use a light touch, holding the bridge down and look for the pivots in the jewel holes.  Gently move the wheel pivots in the right direction so that they fall into the jewel hole.

Using Probe to move Wheel Pivot
Using Probe to move Wheel Pivot

To ensure the pivots are in the holes, turn the barrel or center wheel until you see the escape wheel spinning. The goal is to have the wheels and pinions spinning before adding in the screws to hold down the bridge.

Tips for Reassembling the Wheels and Bridges

When reassembling the wheels and bridges, it’s crucial to check the free running of the wheels and ensure that the pivots are engaging properly. As you tighten the screws, be sure to feel for resistance and stop turning the screw when you feel it.  Bending pivots is one of the main issues you will face if you do not follow the procedures I have laid out for you.

Struggling with Installing the Train Bridge

If you find yourself struggling to get the train bridge on, take a moment to relax, walk away and come back to it later. There is nothing wrong with you because almost nobody gets it on their first attempt.  It’s important to be patient and avoid applying too much pressure on the plate, as the pivots are very fragile and can easily bend.

Protip

One of the most valuable tools in watch repair is a probe, which can be easily made at home. All you need is a piece of peg wood with a hole drilled in it and a sewing needle inserted and polished. This simple yet effective tool will be used frequently in hairspring work and adjusting wheels.

Home-Made Probes
Home-Made Probes

In conclusion, taking the time to understand the inner workings of a watch and having the right tools will save you time and money in the long run. Remember to be patient and take breaks when needed.