Pendulum bob weight – should it make a difference?

Welch movement on test stand
E. N. Welch movement on test stand

I was working on a E. N. Welch (ca. 1875) mantel clock movement in the late fall of 2019, giving it a thorough cleaning during which I installed 6 bushings.

When I purchased the clock at the NAWCC clock and watch mart in June of 2019 in Springfield Ma. it had, what I thought, was a heavier than necessary pendulum bob at 3.7oz. Was the heavier bob put in the clock to compensate for some sort of power issue with the movement? I began to wonder.

Does pendulum weight make a measurable difference?

NAWCC clock and watch mart

According to my research, most experts would agree that pendulum weight is not a critical factor.

The pendulum is passive. The weight of the pendulum is not linked to the clockworks; wheels, pinions, pivots, bushings, etc. Its entire weight is suspended from a suspension spring, and at worst if it is too heavy the clock will stop.

Pallets on American time and strike movement

As for the pallets, any given pendulum will produce pallet wear but a heavier one might accelerate wear. The difference in wear rates among two, four, and 8 oz pendulums would be difficult to measure or assess, five years running. It is possible that a heavier bob accelerates wear but it may not be that significant.

On the test stand, I observed one curious anomaly with the heavy pendulum bob. While not quite a wobble I observed some stutter, otherwise, the movement ran well. I checked the suspension spring and found it straight, without creases or bends and the crutch loop adjusted correctly to give sufficient impulse but when I substituted the 3.7oz bob (104 grams) for a much lighter 1.8 oz (51 grams) the stutter disappeared and the movement ran well. In this particular case, it made a small difference but I will leave that one for the experts to explain.

Quite often clocks do not come with their original pendulum bobs but if missing they can be sourced through an aftermarket supplier. If it is missing on your clock, source a pendulum that is more or less the same weight as the original pendulum but a difference of a couple of ounces either way is not critical.

So, if your newly acquired American time and strike clock came without a pendulum bob it is unlikely that a slightly heavier or lighter one would make much of a difference.