Review of Lange Datograph

Part 3

By Peter Chong, November 1999

The column wheel

Depending on the state in which the chronograph mechanism is in at that moment, the column wheel will coordinate the

  1. start of the chronograph
  2. stop of the chronograph
  3. reset of the chronograph
  4. retour en voul, or flyback without stopping.

Each move of the column wheel causes the fingers of the chronograph to either fall within the space between two turrets or pushed out by the turret.
From the chronograph running position shown right, if the column wheel is moved one position (clockwise), the finger 1 will drop towards the center, whilst the the finger 2 will be lifted by and sits at the left edge of the turret 3. The action on 2 swings the secondary chrono wheel away from its contact with the main chrono wheel, stopping power from the movement to the chrono. This action on 1 causes the brake lever to clamp onto the chrono wheel, stopping it. Note that because of the relative positions of 1 and 2, the action on 2 happens sooner than the action on 1.

On the next move of the column wheel, 1 will be lifted by the next turret, and 2 move from the left edge of turret to the right edge, ready to drop for the next command. The action of 1 lifts the brake lever from the chrono wheel, freeing it, while the reset lever snaps over the heart cam, returning the chrono wheel to zero position. With the non-action of 1, the power from the train remains away from the chrono.

Hence, in a classical chronograph, the column wheel is essential to keep the coordination of all activities in order.

The chronograph works thus:

  1. when the start button is pressed, the column wheel rotates one position clockwise. As seen earlier, this causes the brake (the highly polished edge of the brake finger is seen labelled 4) moves towards the top of the picture and releases its grip on the main chrono wheel (labelled 3). Immediately upon release, the secondary seconds wheel (the wheel on the left marked 2), is moved clockwise and comes into contact with the the main chrono wheel (marked 3), transferring the power from the wheel train to the chrono hand which is mounted on the main chrono wheel. As the secondary chrono wheel (2) is in constant contact and turning in unison with the fourth wheel of the base train, it rotates at the rate of one revolution per minute. Hence, the main chrono wheel, which carries the chronograph seconds hand, moves at the rate of 1 revolution per minute.

    Note that the chrono wheel carries small teeth, and the secondary wheel carries much larger teeth. This is designed as such to improve the lag between contact and start of the hands, and also to reduce backlash.

    Tolerances for the amount of movement the secondary chrono wheel moves is critical, and is adjusted by a screw slot shown as 1.

  2. When the stop button is pressed, the column wheel coordinates the split seconds between each of the 2 steps required: 1. move the secondary chrono wheel counterclockwise out of the way, removing power to the chrono hand, 2. engaging the brake finger (labelled 4) to swing clockwise and coming into contact with the chrono wheel, braking it. This allows the chrono hands to be still, and the timing read.
  3. When the reset button is pressed, the column wheel coordinates the release of the brake, and the striking of the reset finger (shown partially hidden by the top edge of the 3, towards the top edge of the number). The flat edge of the reset finger comes into contact with the heart shaped cam (The bottom sharp part of the cam can be seen partially hidden under the chrono cock…direct at the point of the label 3’s arrow), forcing the cam to rotate until the top part of the heart comes to rest with the flat edge of the lever. This causes the seconds hand to reset to zero.

The flyback mechanism is shown right. For this function, the same column wheel is required to perform steps 2 and 3 combined, with just one push of the reset button, and restart the chronograph.

The diagram clearly illustrates that when you push the flyback button, the mechanism moves into the dotted line position. It also shows that as long as you keep holding the flyback button, the chrono will not restart, as the flat part of the reset finger rests firmly at the top edge of the cam’s heart.

Note also that the secondary chrono wheel (shown as the only wheel carried by in a chaton) moves out of the way during the reset, removing power from the chrono hand.

Go to Part 4.

Copyright © 1999 Peter Chong