Antique and Vintage Mechanical Clocks

There are four general categories of clocks: quartz, electro-mechanical, electric, and mechanical. Mechanical clocks—whether antique or vintage—are the focus of this discussion on why a clock might run slowly.

We’ve become accustomed to the accuracy of quartz clocks, which lose or gain only milliseconds per week. Contrast that with an era when people were content to accept that their mechanical clock might be a minute fast or slow over the course of a week. It was common practice to make small adjustments throughout the run cycle of a clock.

In fact, a typical American spring-driven clock in properly serviced condition may gain or lose a couple of minutes per week as a norm. Weight-driven mechanical clocks that gain or lose only a few seconds per week are considered to be much more accurate.

That said, how many mechanical devices do you know that still run (relatively speaking) perfectly after 120 years?

A clock can run slowly for a variety of reasons.

• Environmental
• Pendulum too low or too high
• Pendulum of incorrect weight
• Suspension spring length is incorrect
• Suspension spring is not attached correctly
• Lack of lubrication
• Gummed-up lubrication (over-oiled)
• Balance wheel needs adjustment
• Weak mainspring
• Changes or alterations during servicing
• Clock cycle time variance
• Slipping or binding
• Bent gear teeth or arbors

Let’s explore each one of these factors:

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Environmental Factors

Mechanical clocks are subject to environmental conditions that may cause them to gain or lose time over the year. These include heat, cold, and humidity. Warmer temperatures can slow down a clock due to the expansion and lengthening of the pendulum, unless it has a compensating pendulum with mercury or dissimilar metal rods. Denser air can also cause the pendulum to swing more slowly.

Even a change in elevation, such as moving a clock from sea level to a higher altitude, can affect the speed of the clock.

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Pendulum Too Low or Too High

The lower the pendulum, the slower the clock will run. Many pendulum clocks have an adjustment screw at the bottom or on the bob itself. If not, there is often a regulator on the clock face. You can use the small end of a double-sided key to insert into the dial and adjust the speed.

Shortening the pendulum speeds up the clock. Anything that increases the pendulum’s effective length will slow it down.

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Incorrect Pendulum Weight

A pendulum that is too heavy lowers the center of gravity, which causes the clock to run slowly. Using the correct weight for your clock ensures proper and reliable operation.

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Incorrect Suspension Spring Length

When someone unfamiliar with the mechanics of a clock replaces a suspension spring with one of the wrong length or thickness, the result is a clock that may run too fast or too slow. Always ensure the spring matches your clock’s specifications.

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Suspension Spring Not Attached Correctly or Kinked

The suspension spring connects the top post to the pendulum leader and allows the pendulum to swing. If it’s not installed securely, the pendulum may not swing properly or may wobble, reducing efficiency and affecting the clock’s accuracy.

A kinked or damaged suspension spring will impede the smooth action of the pendulum rod.

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Lack of Lubrication

Dry pivot holes mean there is no lubricating barrier between the pivots and the bearing holes—even if the movement appears clean. Apply a small drop of clock oil to each dry pivot hole to ensure smooth running. Without oil, steel pivots will wear the brass holes, eventually causing gear misalignment and stopping the clock.

Note: Only a small drop of oil per bushing hole is needed—no more.

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Gummed-Up Lubrication

When a clock runs slowly, the instinct may be to add more oil. But if there’s already old, dirty oil—often blackened or greenish—it will mix with new oil, forming an abrasive paste. Though this may offer a temporary improvement, the clock will soon begin running slowly again.

The only solution is proper servicing: disassembly, thorough cleaning, addressing wear, reassembly, and testing.

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Balance Wheel Needs Adjustment

For those clocks that have a balance wheel instead of a conventional escapement arrangement.

The escapement is regulated by sliding the two small weights on the balance wheel. Slide them inward to speed up the clock, outward to slow it down. Use the adjustment “finger”—moving it to the right increases speed, to the left decreases it. One dot of adjustment usually changes the time by about 10 seconds per day.

Look for markings near the balance wheel: “S” for slow, and “F” for fast.

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Weak Mainspring

Many antique clocks still have their original mainsprings. These springs were often made from high-quality steel, though they weaken over time—a condition known as becoming “set.” A set mainspring won’t run a full cycle (8 days for eight-day clocks or 30 hours for one-day clocks).

While repair shops often replace mainsprings as standard practice, most properly serviced original mainsprings still perform reliably. If replacement is necessary, use a correct-size, high-quality American or German mainspring for dependable performance. Avoid springs made in India at all costs!

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Changes or Alterations During Servicing

Altering a mechanism—such as replacing a gear with one that has the wrong tooth count—can affect timekeeping. Even if parts look identical, manufacturers often made slight variations over the years. Using incorrect parts may lead to a slow or fast clock.

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Clock Cycle Time Variance

American spring-driven eight-day clocks typically run a little faster at the beginning of their cycle (when the mainspring is fully wound) and slower as the power diminishes. This is considered normal and usually does not require adjustment.

Weight-driven clocks provide constant power, so any time variance from the beginning to the end of a cycle is more likely due to wear or other issues.

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Slipping or Binding

If your clock is losing hours per day, something is slipping or binding inside the movement. If it’s losing minutes per day after all adjustments have been made, worn bushings or components may be the cause.

Clockmakers check for end shake—the slight lateral movement of gears between the movement plates. Without sufficient end shake, gears may bind, slowing the clock. Ensuring proper end shake is a standard part of any professional servicing.

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Bent Gear Teeth or Arbours

Bent or slightly out-of-true arbors or gear teeth can cause intermittent resistance, slowing or halting the clock temporarily.

Final Thoughts

Your situation may be unique, and if your clock issue isn’t covered in this article, I recommend consulting a professional clock repairer. If you have limited experience, attempting your own repairs may lead to irreversible damage.

Working with mechanical clocks also involves risk. Mainsprings store a significant amount of energy and can cause serious injury if mishandled.

Understanding why your clock runs slowly is the first step. Addressing the problem is the next. Beyond that, periodic maintenance and the use of quality parts are key to a long and reliable life for your clock.