Why does my clock run slow?

Your situation may be unique and if it is not a clock issue covered by this article I suggest consulting an expert in clock repair. There is also a certain element of risk working with mechanical clocks as the power contained in the mainsprings may cause serious injury if not handled correctly

There are four general categories of clocks; quartz, electro-mechanical, electric, and mechanical. Mechanical clocks, whether they are antique or vintage are the focus of our discussion on why a clock runs slow.

We are quite accustomed to the accuracy of quartz clocks which lose or gain mere milli-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 and it was a common practice to make small adjustments over 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 several seconds per week are considered to be very accurate. That said, how many mechanical devices do you know run perfectly after 120 years.

A clock runs slow for a variety of reasons.

Let’s look at some factors

  • Environmental
  • Pendulum too low or too high
  • The pendulum is the incorrect weight
  • Suspension spring length is incorrect
  • Suspension spring not attached correctly
  • Lack of lubrication
  • Gummed up lubrication (over-oiled)
  • The balance wheel needs adjusting
  • A weak mainspring
  • Changes or alterations when servicing
  • Clock cycle time variance
  • Slipping or binding
one-weight Vienna wall clock
One-weight wall clock with large pendulum bob and rate adjustment on the bottom
Ogee clock showing replacement pendulum bob
Ogee clock showing replacement 1 oz rate adjusting pendulum bob

Environmental 

Mechanical clocks are subject to a number of environmental factors which may cause them to gain or lose speed over the course of a year. 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 using Mercury or dissimilar metal rods. Denser air also causes the pendulum to move more slowly.

Moving a clock from sea level to a higher elevation will affect the speed of the clock.

Numbers on bob correspond with the movement
Pendulum bob with inset rate adjustment

Pendulum too low or too high: 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 by an inset screw on the pendulum. In the absence of an adjustment on the pendulum, there is a regulator on the clock face. Use the small end of a double side key and insert it into the dial face of the clock to speed or slow down 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 will run slower.

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

The 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. Having the correct weight pendulum for your clock ensures smooth running.

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 occasionally install an incorrect length or thickness of suspension spring. Choose the suspension spring that is correct for your clock.

Rate adjustment under the 12. Use the small end of the double-sided key to make the adjustment

Suspension spring not attached correctly: A suspension spring is located at the top of the pendulum rod and is the flexible part that allows the pendulum to swing. It is the connection between the top post and the pendulum leader. If it is not installed securely the pendulum may not swing at all or will wobble, robbing the movement of its energy.

Lack of lubrication: Pivot holes that have dried up means that there is no lubricating barrier between the pivots and their bearing holes although the movement may otherwise be very clean. Small drops of clock oil applied to the dry pivot holes will ensure the clock runs well and will have a long life. Without oil, the steel pivots will wear the brass pivot holes resulting in wheels that will not mesh properly eventually stopping the clock.

Gummed-up lubrication: When a clock runs slowly the first instinct is to apply more oil. Old blackened or greenish oil in the pivot holes is a sure sign the clock has been over-oiled. Although there is an almost immediate improvement in the running of the clock it will not be long-lasting. In no time at all the clock will begin to run slowly again as the new oil mixes with the dirt and grime in the old oil. When this occurs the oil becomes an abrasive paste resulting in exacerbated wear. The only solution is servicing which includes disassembly, cleaning of the movement, addressing wear issues, reassembly, and testing.

Balance wheel needs adjusting: Regulation of the escapement 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 vice versa. Moving the finger one dot represents a change of about 10 seconds per day. The movement will have a directional indicator with an”S” for slow and “F” for fast on the sides adjacent to the balance wheel.

Balance wheel escapement

A weak mainspring: Often the mainspring you will find in your antique clock is the original one(s). The steel used at the time the clock was made was generally of higher quality than the steel used today with some exceptions. By their very nature mainsprings become weak over time. Weak mainspring are called “set” mainsprings. If “set”, your clock will not run a full cycle, 8 days for eight-day clocks, a full 30 hours for one-day clocks, or the designed cycle. When a spring-driven clock is brought in for a professional repair the mainsprings are often replaced.

Most properly serviced clocks with their original mainsprings will run their full cycle. Should your clock require a mainspring replacement a correct size quality American or German-made mainspring should provide years of reliable service.

Changes or alterations: Changing or altering the mechanism such as replacing gear with an incorrect teeth count may speed up or slow down a clock. Although movement parts may appear to be similar, manufacturers often made small changes resulting in parts that may not be interchangeable with movements from other years.

Clock cycle time variance: American spring-driven eight-day clocks will run slightly faster at the beginning of their cycle by providing most of their power and run more slowly through the week as the power of the mainspring unloads. A spring-driven clock that is one or two minutes fast at the beginning of the week is often a minute or two slower at the end of its cycle. This is considered normal and no adjustment is necessary. The power on a weight-driven clock, on the other hand, is constant and the loss or gain in time at the beginning of the cycle will be the same at the end assuming no wear issues are slowing it down.

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 some other worn component is causing the problem. Clock repairers have a term called end shake. End shake allows freedom of lateral movement for each of the wheels between the movement plates. If the plates are tight and there is no end shake, too much resistance will slow a clock. It is why clock repairers always check for sufficient end-shake when servicing the wheels on a movement.

Final thoughts

Your situation may be unique and if it is not a clock issue covered by this article I suggest consulting an expert in clock repair. Be very cautious if you have little experience and choose to do your own work on an antique or vintage clock. The mistakes you make might be irreversible. There is also a certain element of risk working with mechanical clocks as the power contained in the mainsprings may cause serious injury if not handled properly.

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


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