Splitting Seconds:
Delving into a Rattrapante Pocket Timer

Part III:  Splitting the Second


A split-seconds mechanism really has two separate challenges:  the first is how to control the engagement and disengagement of the split hand, and the second is to have the split hand “catch-up” to the regular second hand.

Controlling the Second Hand

The most traditional way of constructing a rattrapante is to use a second column wheel, combined with two jaws, which alternately open and release a finely toothed wheel which carries the split second hand. The column wheel is rotated according to the same priniciple used in the chronograph.

However, this column wheel can be replaced by an oscillating pinion, much in the same way that these pinions have supplanted the column wheel in modern chronograph movements. This is the case here:

In this photo, the rattrapante jaws and second hand assembly has been removed, to clarify how the oscillating pinion works. There is a “U”-shaped pusher lever (A), which is retained by a screw with a flange at the base (B). This pusher lever has a fair amount of freedom of motion; the flanged screw just manages to keep the lever from falling out. The pusher acts on the oscillating control pinion (C), which has two pins that actually move the rattrapante jaws in and out (see below for how this works). The control pinion is held in one of two possible orientations by the detent spring (D). Also visible in the photo are the pusher return spring (E) and the mounting base for the rattrapante jaws (F).

The above photo shows how the oscillating pinion works.  (Note:  in the following explanation, the terms “lower” and “upper” refer to the relative positions visible in the photos.) When the rattrapante button is pressed, the pusher lever is moved toward the control pinion (step 1, red arrow).  The “lower” tine of the pusher engages in the control pinion’s slot (2, orange), which causes the entire pinion to rotate clockwise. This rotation moves the jaw pins (yellow arrows) to move in opposite directions, and simultaneously causes the horn on the oscillating pinion to move downwards (3). When the pinion horn comes in contact with the pusher lever (4), the pusher lever is rotated counterclockwise (green arrows), so as to cause the “upper” tine of the pusher to fall into position on the upper side of the pinion. The detent spring ensures that the pinion maintains a consistent position once rotated.

The process for moving the pinion back is similar:  when the rattrapante button is pressed, the pusher lever’s “upper” tine (1, red arrow) engages the corresponding slot on the control pinion. The control pinion rotates counterclockwise (2, orange), which causes the jaw pins (yellow arrows) to rotate back into the original position, and the extended pinion horn to move upwards (3).  When the horn contacts the pusher (4), the entire pusher lever is rotated clockwise (green arrows), which repositions the “lower” tine to fall into the correct position against the control pinion.

Thus, as the pusher is activated multiple times, the oscillating pinion moves back and forth between the two positions determined by the detent spring. While this action seems pretty difficult to design, it seems fairly reliable, as the parts self-align each time to ensure that there can be no mis-activation of the function.

The fact that these kinds of pinions have taken over from the column wheel in most modern low-end chronos, without significant reliability issues, shows that this kind of actuation is as reliable as the column wheel, if not quite as elegant in appearance.

The scan to the right shows the rattrapante jaws, which grip the split-hand wheel. Both the jaws themselves (inset) and the split-hand wheel are finely toothed, so as to ensure a firm grip with a minimum of motion when the jaws engage.

Recall from the above discussion that the jaw pins move back and forth with the motion of the control pinion: the pins end up where the colored circles have been added to the photo – the blue circles show the jaws in the open position, while the red circles show the jaws in the closed position. As is apparent, metal from the jaws has been removed around the red circles to allow the jaws to relax out of contact with the pins. Putting all of the above motions together yields the animation to the left. (Note that differences in the frame unrelated to the rattrapante motion have been greyed out in the frame.)

Catching Up

The second task of the rattrapante function is to allow the split hand to reliably catch up to the regular second hand. This has been done through an ingenious combination of parts located on the second hand assembly itself:

The above two photos show the side and top view respectively of the center second hand assembly. Parts have been labeled consistently between the two photos. The extended second hand pinion (A) rides inside the hollow fourth wheel arbor, and the motion is kept synchronized via a friction fit. The finely toothed split hand wheel (B), along with the split hand pipe (E) are loosely fit on the second hand pinion, and rotate as a unit freely.  The split hand rides the split hand pipe at the position indicated by the red arrow above (E), while the regular seconds hand is fit to the second hand pinion directly at the point indicated at (F).

There are two stacked heart shaped cams rigidly attached to the second hand pinion: (C) is the rattrapante cam, while (D) is the reset cam. Recall that when the chronograph is reset, the reset lever contacts the reset cam (D). This forces the second hand pinion to rotate independently of the fourth wheel arbor to the zero position.

The rattrapante cam is used by a cleverly-designed “finger” which is attached to the split hand wheel (right photo): the finger (H) is riveted at one end (G) and is free to move back and forth as indicated by the green arrows. The other end of the finger rides the rattrapante cam (C). (A model of this can be constructed by holding your left hand in front of your face, palm down, and wiggling your index finger up and down. The riveted end of the finger (G) corresponds to your knuckle, and the free end, which rides on the cam, is your fingertip.) The spring at (J) ensures that the free end of the finger is always pulled into contact with the rattrapante cam.

Since the whole split hand wheel assembly is free to rotate around the second hand pinion, when the jaws are engaged, they prevent the split hand wheel from turning, without stopping the second hand pinion. When the jaws release this wheel, the finger, which is pulled by the spring (J), causes the split hand wheel to rotate until the finger is on the “low” point of the rattrapante cam. Through careful finishing of the cams, this action is very accurate, and causes the split hand to catch up reliably with the regular second hand every time.

However, since there is a small amount of spring tension (J) which keeps the finger on the split hand wheel in contact with the rattrapante cam, there is a bit more torque required which the movement must overcome, which leads to a slightly lower amplitude when the split hand is engaged. Thus, the recommendation that the split hand function only be engaged for the minimum amount of time to take a reading.

Despite this warning, there is no possibility of damaging the mechanism if the split hand is engaged for multiple revolutions of the regular second hand – the finger on the split hand wheel simply rides up and down the rattrapante cam profile.

Concluding Remarks

As it turns out, the rattrapante mechanism on this pocket timer utilizes the exact same principles as the current IWC rattrapante – the IWC version, of course, being suitably refined with a jeweled control pinion and pins.  However, the design, from the two-detent oscillating pinion, to the split hand finger/cam system, uses the same principles as shown here. There truly is nothing new under the sun.

The rattrapante is one of the more involved devices to design and implement, because of the precise alignment and adjustment of forces which are required to make it work without stopping the movement. While this does not lend itself to too much difficulty in a pocket timer, where there is no time-keeping to interfere with, and the space available is much enlarged, the degree of difficulty is increased dramatically when shrunk down to wristwatch size. Combined with the true lack of practical need for such a mechanical device (quartz timing systems being the standard for split time measurement today at a fraction of the cost), we see the premium for this mechanism in today’s wristwatches of several thousand dollars over the price of a regular chronograph.

This is the end of Part III.

Click Here to return to Part II.

Copyright (c) 2000 Edward Hahn