Why does my clock run slow?

There are four general categories of clocks; quartz, electro-mechanical, electric and mechanical. Mechanical clocks be they antique or vintage types are the focus of this article.

Topic areas covered are:

  • Pendulum too low
  • Pendulum is the incorrect weight
  • Suspension spring length
  • Suspension spring not attached correctly
  • Lack of lubrication
  • Gummed up lubrication (over-oiled)
  • Balance wheel needs adjusting
  • A weak mainspring
  • Changes or alterations
  • A typical clock cycle explained
  • Slipping or binding

We are quite accustomed to the accuracy of quartz clocks which lose or gain mere seconds per week. Contrast that with an era when folks were content to accept that their mechanical clock would be a minute fast or slow through the week. 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. My weight driven clocks that gain or lose several seconds per week are considered accurate by me.

Periodic maintenance and fully servicing of mechanical movements is the key to long life
one-weight Vienna wall clock
Miniature one-weight Vienna wall clock with large pendulum bob and rate adjustment on the bottom
Ogee clock showing replacement pendulum bob
Ogee clock showing replacement 2.2 oz rate adjusting pendulum bob

Mechanical clocks are subject to a number of environmental factors which may cause them to gain or lose speed. These factors include heat, cold and humidity. Increasing the ambient temperature of a clock will slow it down from the expansion and lengthening of the pendulum unless the pendulum is a compensating type. Denser air causes the pendulum to move more slowly. Moving a clock to a higher elevation will slow it down as the air is thinner.

Numbers on bob correspond with the movement
Pendulum bob with rate adjustment – French time and strike

The following are reasons why a clock might run slowly.

A typical antique or vintage mantel clock with a pendulum will be used as an example.

Pendulum too low: The lower the pendulum the slower the clock will run. Many pendulum clocks can be adjusted either by a set screw at the bottom of the pendulum or or by the small end of a double sided key inserted into the dial face of the clock. Shortening the pendulum will speed up the clock. Anything that increases the length of the pendulum will reduce the rate of the pendulum and result in a clock that is running too slow.

Parts of the clock related to the pendulum
Parts of the clock related to the pendulum

Pendulum is the incorrect weight: If the pendulum is too heavy it causes the centre of gravity to be too low consequently the clock will run slower. Get the right weight pendulum for your clock. No, any old pendulum will not do.

Suspension spring length: Often, when a clock is repaired by a person who has limited knowledge of the effect a replacement spring will have on the running of a clock they will frequently install an incorrect spring.

Rate adjustment on clock face. Use small end of double-sided key to make the adjustment

Suspension spring not attached correctly: This is located at the top of the pendulum rod and is the flexible part that allows the pendulum to swing. If it is not installed securely the pendulum may not swing correctly.

Let’s assume your mantel clock has either a pendulum or balance wheel escapement.

Lack of lubrication: The other day I inspected a simple time-only kitchen clock movement and discovered that the pivot holes had dried up. There was no trace of lubricating oil in the pivot holes. The movement was otherwise very clean. Small drops of Keystone clock oil were applied to the pivot holes resulting in a dramatic effect on the running of the movement.

Gummed up lubrication (over-oiled): Unfortunately when clocks run slowly the first thought is to re-oil it even though it may be obvious that there is a build up of old blackened oil in the pivot holes. There will be an immediate improvement but not a lasting one. In as little as a few days or weeks perhaps, the clock will begin to run slowly again as the new oil mixes with the dirt and grime of the old oil. When this occurs the oil becomes an abrasive paste. The only solution in this case is a thorough servicing which includes disassembly and cleaning of the movement.

Balance wheel needs adjusting: Regulating for fast and slow consists of sliding the two-small weights attached to the center of the balance wheel. Inwards for fast and outwards for slow. Hold the wheel and push the small adjustment “finger”. Moving the finger toward your right will be faster and visa versa. Moving the finger one dot represents a change of 10 seconds per day.

Balance wheel

A weak mainspring: Quite often the mainspring you will find in your antique clock is the original one(s). The steel used an the old clock was generally of excellent quality. Mainsprings become weak over time. Weak mainspring are called “set” mainsprings. If “set” you will find your clock does not run a full cycle, 8 days for eight day clocks, a full 30 hours for one day clocks or whatever the cycle is designed to be. If at all possible I avoid replacing mainsprings.

A mainspring bought today may well be of inferior quality compared to the quality steel used in antique clocks. Most clock-makers stay away from mainsprings made in India. If you replace the mainspring with a correct size quality American or German made mainspring you should not go wrong.

Changes or alterations: Changing or altering the mechanism such as replacing the escape wheel or a gear with incorrect number of teeth will slow down a clock assuming that the parts are not an exact fit for that movement.

A typical clock cycle explained: American spring driven eight-day clocks will run slightly faster at the beginning of their cycle when completely wound and run more slowly through the week as the power of the mainspring unloads. A clock that is one or two minutes fast at the beginning of the week might be a minute or two slower at the end of the week. This is normal and in that case no adjustment is necessary.

Slipping or binding: If your clock is losing hours per day something in the mechanism is slipping or binding. If your clock is losing minutes per day after all adjustments are made, it is likely bushing wear or no end shake. You only want enough end shake to allow freedom of lateral movement when the plates are tight.

Knowing why your clock runs slowly is the first step in diagnosing the problem. Beyond that, periodic maintenance and fully servicing with quality parts is the key to long life for your clock.

2 thoughts on “Why does my clock run slow?

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