Single Roller safety/ Double roller

[davidd20042001]

Hi all,
been trying to understand the difference between a Single Roller Safety Action and a Double Roller Safety.. Still not quite clear on this matter.. and which is the more common one in wristwatches?.. Appreciate any help

 

[psfred]

A single roller watch will have one plate holding the roller jewel. There is a notch in the plate matching the position of the jewel, and a vertical pin on the pallet fork that prevents the pallet fork moving across to the opposite banking unless the jewel is in the fork.

A double roller watch has two circular plates, one holding the roller jewel and the other, smaller plate having the notch. The safety pin (sometimes called a safety dart) is horizontal and projects underneath the fork. It also prevents the fork from crossing to the opposite banking unless the jewel is in the fork.

A double roller is more "positive" in the engineering sense in that the operating distances are smaller and the parts are stiffer, which should result in less occasions where the watch goes out of action from the fork being on the wrong side of the banking when the roller jewel comes around.

I have never seen a single roller wrist watch excpet possibly an old pin lever one I have floating around that I cannot take apart. Might be some out there, but I don't know of any examples. I would assume any wristwatch with a jeweled train would have a double roller, and even a 7 jewel Caravelle I cleaned recently does.

Peter

 

[Smudgy]

The biggest difference in action of the two is that the double roller has less of an effect on the balance when the safety dart strikes the roller (from being bumped or jarred in normal usage). The reason for the diminished effect is that the double roller is of smaller diameter than the single roller, so the dart bumping against the roller table imparts less of a torque to the balance. The net effect is that the double roller escapement is more accurate.

I think most, if not all, watches (with the possible exception of some very low quality movements) had changed over to the double escapement prior to the wrist watch becoming commercially viable (which was early in the century, becoming very popular during the first world war). I think most manufacturers had changed over to the double roller by late in the previous century.