Introduction
When a mechanical clock runs slow, the cause is almost always related to pendulum length, friction, or power loss. Even small changes in lubrication, wear, or setup can slow the rate noticeably. This guide explains the most common reasons a clock runs slow, how to diagnose the issue, and how to correct it without damaging the movement, the paper dial, or the winding arbors when using the key.
Primary Causes of a Slow‑Running Clock
Pendulum too long
The most common cause of slow running is a pendulum that is set too low. Lowering the bob increases the effective pendulum length and slows the clock.
Friction in the train
Dried oil, dirty pivots, or worn bushings increase resistance and reduce power, causing the clock to lose time.
Weak mainspring or weight power
A tired mainspring or insufficient weight power reduces impulse to the escapement, slowing the rate.
Escapement out of adjustment
Shallow lock, deep lock, or uneven impulse can slow the clock by reducing pendulum amplitude.
Environmental factors
Cold temperatures, humidity changes, and case movement can all affect pendulum behavior.
Diagnosing the Problem
Check pendulum length first
Raise the pendulum bob slightly and test for 24 hours. If the clock speeds up, the issue is pendulum length.
Check beat and amplitude
A clock out of beat or with weak amplitude will run slow. Adjust beat and verify that the pendulum swings freely.
Inspect pivot condition
Look for blackened pivots, oval pivot holes, or dried oil. These are signs of friction slowing the train.
Check mainspring or weight power
If the clock runs slower as the week progresses, the mainspring may be set or the weight may be too light.
Protect the paper dial during inspection
When removing hands or accessing the movement, shield the paper dial to prevent scratches or tearing.
Correcting a Slow‑Running Clock
Step 1: Adjust the pendulum bob
Turn the rating nut upward to shorten the pendulum and increase the rate.
Step 2: Clean and oil the movement
Old, sticky oil slows the train. Clean the movement and apply fresh, high‑quality clock oil sparingly to pivots only.
Step 3: Correct worn bushings
Oval pivot holes cause power loss. Install bushings where needed to restore proper depthing.
Step 4: Adjust the escapement
Ensure proper lock and drop. A poorly adjusted escapement reduces pendulum impulse and slows the clock.
Step 5: Wind fully with the key
Under‑winding reduces power. Use the key to wind the clock fully until resistance increases naturally.
Additional Factors That Slow a Clock
Hands rubbing the dial
If the minute or hour hand rubs the paper dial or bezel, friction slows the movement.
Case not level
An unlevel clock reduces pendulum swing and slows the rate.
Pendulum interference
Ensure the pendulum does not touch the case, chime rods, or movement parts.
Incorrect suspension spring
A longer or more flexible suspension spring slows the clock.
Temperature changes
Cold temperatures lengthen the pendulum rod slightly, slowing the clock.
Troubleshooting Flowcharts
If the clock runs slow all week
Raise pendulum bob → Check beat → Check friction → Clean and oil movement
If the clock runs slow only at the end of the week
Wind fully → Check mainspring set → Check weight power
If the clock runs slow after moving it
Level case → Check pendulum clearance → Check hand interference
Common Mistakes to Avoid
Over‑oiling the movement
Too much oil attracts dust and increases friction.
Forcing the key
Forcing the key past natural resistance can damage the mainspring or arbor.
Adjusting the hands without protecting the dial
The paper dial tears easily and must be shielded during hand removal.
Ignoring worn bushings
Worn pivot holes are a major cause of slow running.
Trying to regulate before fixing power loss
Regulation only works when the movement has full, clean power.
Checklist for Final Verification
• Pendulum length correct
• Beat even and amplitude strong
• Movement clean and properly oiled
• No hand or dial interference
• Key winds smoothly to full power
• Paper dial protected during adjustments
FAQs
Why does my clock run slow after oiling?
Too much oil increases drag. Only pivots should be oiled lightly.
Why does the clock run slow after moving it?
It may be out of beat or no longer level.
Can a worn mainspring make a clock run slow?
Yes. A set mainspring delivers weak power and slows the rate.
Why protect the paper dial?
Paper dials tear easily when removing hands or adjusting the motion works.
Does winding with the key affect the rate?
Yes. Under‑winding reduces power and can make the clock run slow.
0 comments