Portuguese seconds at six -- some facts

[wtchgk]

I have posted this previously in another forum. It is well known that there are reliability problems with the seconds at six movement. Because the 3714 is very desirable, members are constantly asking for information to help them decide whether to buy or not to buy. Responses range from ‘don’t buy, this is a ticking time bomb’ to ‘the movement is fine when properly serviced’. It is the latter response that concerns me. If someone spends $200 to $250 on a service then there should be a reasonably clear metric like x thousands of hours of operation between services or some other proven data that permits a valid projection. Unfortunately, most of the optimistic responses have been either extremely vague or, worse, demonstrated downright ignorance of the facts about the movement.

Below I shall provide details of how the design of parts of the seconds at six movement is substantially different from the design that is predominant, well understood, and proven in traditional watches, and, I’ll show the problems it introduces.

Hopefully, this can serve as a basis for obtaining real data on how the problems can be mitigated.

The seconds at six movement is roughly speaking a normal A7750 (or ETA7750) where the calendaring parts have been left off and replaced with parts that perform the transfer of the seconds from 9 to 6. The acceptable reliability of the A7750 portion of the movement has been pretty well established, exceptions not withstanding.

Going forward I’ll just discuss how the 9 to 6 gear design is different from that in normal watches and point out the resulting problems. I'll use very rough figures that I believe are sufficient to demonstrate my points. Please refer to the pics below for clarification.

A normal watch gear that makes one revolution per minute uses pivots of about .2 mm diameter that ride at the top and bottom in plates with jewels that have cups to hold lubricants by capillary action.

Of the five gears in the transfer portion three gears revolve around a fixed axis. For one of those gears the axis is about 2 mm in diameter and it is about 1 mm for the other two. The height of the jewels is at least double that of a normal watch jewel. In addition, two of those gears at any one time ride on a ring on the plates that is about 4 mm in diameter and .1 mm wide. This is metal to metal friction. The only exception where this metal to metal friction is not occurring or minimal is the 90 degree position of the watch.

Some rough calculations show that the surface area of the friction and therefore the force needed to move the wheels is about 30 times higher then the force that would be needed if traditional gear designs were used. This number accounts just for the contribution of the surface area to the friction. It does not account for the lack of the kind of lubrication found in a normal jewel bearing. The 7750 movement has sufficient force to move those wheels when the friction surfaces are clean and polished even without lubricant. Because the wheels have virtually no capacity to hold traditional watch lubricants, the friction surfaces deteriorate quickly thereby increasing the friction to a point where the watch stops. The one piece of data I have shows that this occurs after about 2000 hours or three months of continuous operation. I have no data on how much this varies.

If no modifications are made to the movement, various treatments of the friction surfaces have been proposed. In this case the question becomes, can these treatments hold for at least 18000 hours in order to achieve the often claimed two years of operation between services?

On to the pics.

This pic shows the configuration of the transfer gears. Gears 1 and 2 are below the plate. Gear 1 is the original output at 9. Gear 2 uses the same design as gear 4. Gear 3 revolves about the hour wheel axis which is about 2 mm in diameter. The teethed edge of the gear touches the top plate which has been removed. Gear 5 is the new output at six. It rides in a normal jewel in the top plate and a metal bearing with an oil cup in the bottom plate.

This pic shows the ring on the plate that gear 3 rides on.

This is a pic of gear 4. Gear 3 is the same. Both gears revolve around a fixed axis that is about 1mm in diameter and both ride on one side on a ring on the plate that is similar to that for gear 3. Unfortunately this pic is overexposed. But it shows the ground up metal area that goes all around.

In this pic the transfer plate has been removed and it shows the original output at 9 with its gear.

[bklynboy]

Thanks for posting, actually read this where you originally posted and gave me enough confidence to pull the trigger on a M2M sale here this week

Will hack it when its in the box but enjoying it to much to take off just yet...

[Renegade01]

Thanks for the post, I just purchased the IWC 3714 from another member.  Can you recommend a servicer should the friction issue stop the watch?  I haven't found much else written about this issue, or the specific actions people have taken to deal with the friction issue.  I'm wondering if anyone did a surface polish and has seen good life as a result.

[FxrAndy]

Nice post, do you have any experience with watches that have gone down the DLC route or the powdered graphite route to reduce the surface friction?

[martijnp]

Thank you for posting this mate I bought the IWC Portuguese via M2M sale last week, so i'm waiting for the delivery guy