How to Measure a Shaft and Stuffing Box Before Ordering Packing

Most packing that arrives wrong was measured wrong. Ordering by “the size we had last time” carries forward whatever error was already there, and a packing set that is a millimetre out will either refuse to seat or leak from the day it is fitted. The measurement takes five minutes and settles the order completely.

What you are actually measuring

Gland packing is specified by section size and length. Section size is the square cross-section of the packing, and it comes from the gap between the shaft and the stuffing box bore. Length is how much you need to make up the rings. Both are derived, not guessed.

Take three dimensions

• Shaft diameter (d) — measure the shaft or sleeve itself, at the point it passes through the box.
• Stuffing box bore (D) — the internal diameter of the housing the packing sits in.
• Stuffing box depth — how deep the cavity is, which tells you how many rings it takes.

Work out the section size

The packing fills the annular gap, so the section is half the difference between the bore and the shaft:

Section = (D − d) ÷ 2

A 50 mm shaft in a 70 mm bore gives (70 − 50) ÷ 2 = 10 mm section. Round to the nearest standard size rather than ordering a non-standard section — and if the result sits awkwardly between two sizes, tell us the raw figures and we will advise.

Measure the shaft properly

Measure the shaft where the packing runs, not at a convenient spot further along. A shaft that has been running with worn packing is often grooved or stepped exactly in that zone, and measuring the undamaged section gives you a gap that does not exist. Take two readings at ninety degrees to check for ovality.

Check the shaft condition while you are there

This matters as much as the dimension. A scored, pitted or worn shaft will destroy new packing quickly, and harder packings make it worse — aramid is abrasion resistant but hard, and on a soft or already-scored shaft it accelerates wear. If the shaft is grooved, the sleeve needs attention before new packing goes in, or you will be repacking again shortly.

Calculate the length you need

One ring takes roughly the mean circumference of the packing annulus:

Length per ring ≈ π × (d + section)

Multiply by the number of rings the box depth takes, then add an allowance for cutting. The gland packing calculator does this for you from the same three measurements.

Record it for next time

Write the shaft diameter, bore, depth, ring count and packing grade on the pump record. The reason the same pump gets measured wrong repeatedly is that nobody wrote down the right answer the first time.

What to send us

Shaft diameter, box bore, box depth, pump or valve type, the medium, and the operating temperature and pressure. Add whether the shaft is in good condition. With those we will confirm the section, the length and the right packing — see choosing the right gland packing for the grade decision, and how to install gland packing for fitting it.