Verge Escapement - Disadvantages

Disadvantages

As might be expected from its early invention, the verge is the most inaccurate of the widely-used escapements. It suffers from these problems:

• Verge watches and clocks are sensitive to changes in the drive force; they slow down as the mainspring unwinds. This is called lack of isochronism. It was much worse in verge and foliot clocks due to the lack of a balance spring, but is a problem in all verge movements. In fact, the standard method of adjusting the rate of early verge watches was to alter the force of the mainspring. The cause of this problem is that the crown wheel teeth are always pushing on the pallets, driving the pendulum (or balance wheel) throughout its cycle; it is never allowed to swing freely. All verge watches and spring driven clocks required fusees to equalize the force of the mainspring to achieve even minimal accuracy.
• The escapement has "recoil", meaning that the momentum of the foliot or pendulum pushes the crown wheel backward momentarily, causing the clock's wheel train to move backward, during part of its cycle. This increases friction and wear, resulting in inaccuracy. One way to tell whether an antique watch has a verge escapement is to observe the second hand closely; if it moves backward a little during each cycle, the watch is a verge. This is not necessarily the case in clocks, as there are some other pendulum escapements which exhibit recoil.
• In pendulum clocks, the wide pendulum swing angles of 80°-100° required by the verge cause an additional lack of isochronism due to circular error.
• The wide pendulum swings also cause a lot of air friction, reducing the accuracy of the pendulum, and requiring a lot of power to keep it going, increasing wear. So verge pendulum clocks had lighter bobs, which reduced accuracy.
• Verge timepieces tend to accelerate as the crown wheel and the pallets wear down. This is particularly evident in verge watches from the mid-18th century onwards. It is not in the least unusual for these watches, when run today, to gain many hours per day, or to simply spin as if there were no balance present. The reason for this is that as new escapements were invented, it became the fashion to have a thin watch. To achieve this in a verge watch requires the crown wheel to be made very small, magnifying the effects of wear.