Measuring a clock movement shaft is an important part of installing or repairing a clock. The shaft size must be matched properly to the clock hands to ensure proper fitting and operation. There are a few simple steps involved in accurately measuring a clock movement shaft.
Why Measuring the Shaft is Important
When selecting new clock hands, it is essential to know the shaft size they will be attached to. The shaft diameter must match the hand hole size for a secure friction fit. Hands that are too loose on the shaft will slip and not keep proper time. If the hole is too small, the hands will be difficult or impossible to install.
Measuring the shaft also allows you to identify the clock movement model if unknown. There are common shaft sizes used by clock manufacturers that correspond to particular movement types.
If a clock is showing signs of timing issues or the hands are slipping, measuring the shaft can help diagnose issues caused by worn holes in the hands from many years of use. In some cases, bushings are available to adapt larger shaft sizes to fit worn hand holes.
Typical Clock Shaft Sizes
Some common clock shaft sizes include:
- 0.125 in – 1/8 in: Small mantle clocks
- 0.25 in – 1/4 in: Common size for many wall clocks and larger mantle clocks
- 0.375 in – 3/8 in: Grandfather and large wall clocks
- 0.5 in – 1/2 in: Tower clocks and large public clocks
Vintage clocks, especially antique styles, may also have metric shaft sizes like 8mm, 10mm or 12mm. Modern Asian movements often have 6mm shafts as well.
Tools Needed for Measurement
To accurately measure a clock shaft, you will need the following tools:
- Calipers – Dial, digital or Vernier calipers that can measure in inches and metric units
- Cloth or gloves – For handling polished or finished shafts
- Magnifying glass (optional) – For reading caliper measurements
Calipers provide the most consistent and precise measurements versus using a ruler. Digital calipers are easiest for getting quick and clear shaft measurements.
How to Measure the Shaft
Follow these steps to correctly measure the clock movement shaft:
- Remove the clock hands if they are still attached. Pull the minute hand directly outwards first, then the hour hand. The second hand usually pulls off easily last.
- Inspect the shaft. Make sure there are no residual burrs or uneven edges that could affect the measurement accuracy.
- Select an area of the shaft to measure that is smooth and away from the clock movement body. Approximately 1/4 to 1/2 inch (6 to 12 mm) from the tip is often ideal.
- Open the caliper jaws and position them at the measurement area, perpendicular to the shaft. Apply even, firm pressure so the caliper jaw faces are touching the sides of the shaft. Make sure the calipers remain at a 90 degree angle to the length of the shaft.
- Read the shaft measurement on the caliper display. Repeat 2-3 times rotating the shaft to find the most consistent measurement.
- For damaged or uneven shafts, take measurements at multiple places along the length and average the readings.
- Convert metric readings to inches if needed. 1mm = 0.03937 inches.
Tips for Accurate Measurements
Follow these tips when using calipers to measure a clock shaft:
- Use a soft cloth or wear gloves to avoid scratching polished metal shafts.
- Clean measuring surfaces of oil for slip-free gripping.
- Apply firm but not excessive pressure that could flex or indent the shaft.
- Take measurements at multiple points to account for taper or eccentricity.
- Use calipers with fine measurement gradations for greater precision.
- Use a magnifying glass to accurately read the caliper measurements.
Interpreting the Measurement
Once the shaft diameter is measured, you can:
- Match replacement clock hands to the shaft size.
- Look up clock movement models based on published shaft diameters.
- Determine if a bushing is needed for fitting new hands to a worn shaft.
- Identify metric movements like 8mm, 10mm or 12mm sizes.
For critical applications like tower clocks, taking multiple measurements along the shaft at different angles can indicate if replacement is required for excessive runout or wobble.
How to Measure Other Winding Shafts
Many vintage and antique clocks have a separate winding shaft to manually wind the mainspring. This winding arbor shaft has a square, hex or knurled end rather than a round press-fit diameter. To measure:
- Remove any winding keys, handles or pulleys from the winding arbor.
- Measure the diameter right above the square or hex shaped end, before any taper begins. This is normally the thickest portion.
- Compare this diameter to replacement winding keys to find the proper fit.
Knurled winding shafts are more difficult to measure precisely. You can often estimate based on the groove spacing or distance between ridges. Replacement knurled keys may list sizes like small, medium and large rather than exact diameters.
Measuring Clock Suspension Shafts
Pendulum clocks will also have a separate shaft that the pendulum suspension spring sits on. This shaft has threads for adjusting the bob up and down to regulate the timekeeping:
- Measure the diameter of the smooth lower portion of the threaded suspension shaft.
- This size must match replacement suspension springs.
- Measuring the thread diameter is also useful for identifying the thread pitch for bob adjustment.
Testing for Worn Shafts
You can check for a worn clock shaft by measuring at multiple points along its length. Significant diameter variations indicate a grooved or tapered shaft.
- Measure near the base, midpoint, and tip of the shaft.
- Compare the readings – major differences point to wear.
- Badly worn shafts may need bushing repairs or complete replacement.
Measuring the shaft hole diameter in used clock hands will also show wear. Enlarged hand holes are a sign the shaft is damaged. Bushings or hand replacement may be needed if the hole is oversized.
Conclusion
Knowing how to accurately measure a clock movement shaft is an important skill for repairing and maintaining clocks. With some basic tools like calipers and a magnifying glass, you can quickly determine the shaft size and identify any potential wear issues. This allows proper fitting of clock hands, suspension springs, and winding keys for smooth operation. Keeping this key part of the clock properly sized and in spec will ensure reliable timekeeping and prevent slipping and timing problems.