A 500 mm Stroke, So Why Order a Longer Range: Working Out Range from Cylinder Stroke

When a magnetostrictive displacement sensor is specified, the usual approach is to measure the cylinder stroke, find 500 mm, and tick the 500 mm range on the specification sheet. On commissioning, a short stretch at each end of the travel reads nothing, either alarming or holding the last value.

The reason is straightforward: nominal range is not usable length. There is a structural dead zone at each end of the rod where the return signal cannot be resolved or is absent altogether. That length has to be deducted from the range, and what remains is the usable measuring span.

Range and stroke are two different numbers

Stroke is the distance the machine can actually travel, set by the cylinder or by a mechanical stop. It is a physical fact.

Usable range is the length over which the unit produces normal readings. It equals the nominal range minus the dead zones at both ends.

What gets ordered is the nominal range. Of the three, the process cares only about stroke, purchasing orders nominal range, and usable range is the bridge between them. Written out:

nominal range >= stroke + margin at both ends + dead zone at both ends

Ordering range straight from stroke sets both margin and dead zone to zero, and the ends are certain to lose readings.

Three steps from stroke to ordered range

• Measure the real stroke. Not the nominal figure on the drawing, and not an estimate from cylinder length. Measure from mechanical stop to mechanical stop. On older equipment the real stroke and the design value frequently disagree, and it can vary between batches of the same model.
• Leave margin at both ends. Margin exists to absorb assembly tolerance and mechanical deflection — the position the piston actually reaches usually differs from the theoretical limit by a few millimetres. How much depends on machine precision and assembly method: short-stroke, rigid machines need less, long-stroke and high-tonnage machines need more.
• Add the dead zone at both ends. The dead zone is a fixed length set by the structure. The figures differ between series and have to be looked up for the series actually being selected; one experience-based number cannot be applied across a range.

The result of the three steps is a lower bound. Step up to the nearest size in the specification sheet. That is why a 500 mm cylinder frequently ends up with a 560 or 600 mm unit.

Dead zones cannot be estimated

Dead zones exist because of the structure at each end of the rod: one end houses the excitation and pick-up, the other has an end reflection. The length these occupy cannot be recovered, only deducted from the range.

The important point is that the figure differs between series, so estimating from a rough number causes trouble:

Series type	Dead zone magnitude	Note
Standard built-in series	Tens of millimetres at each end	The zero zone and the dead zone are separate figures, not one number
Series with a different rod end	Over sixty millimetres at one end	Applying a standard figure underestimates it
Flexible long-stroke series	Grows with stroke length	The longer the stroke, the larger the front dead zone; no fixed value applies

For accurate figures for a specific series, consult that series documentation. Measured dead zone values are listed in dead zones at each end of the usable range — match the series, rather than applying one general number across the range.

Four places it goes wrong

Treating range as the length that can be measured. Nominal range is measured from the structural end face, and the usable span sits inside it, inset at both ends. Read the specification note to see which convention is used.

Leaving margin at one end only. Many machines have a hard stop at one end and a hydraulic limit at the other, so the margins needed differ between the two. Work them out separately rather than splitting evenly.

Ignoring travel outside production. During maintenance, die changes and setup, the machine may travel further than it does in production. Confirm these conditions separately, or the reading will be adequate in production and alarm during maintenance.

Assuming all cylinders on a machine have the same stroke. On a multi-cylinder machine the strokes can differ. Size to the longest, and for how ranges are aligned across axes see aligning range and synchronisation.

Three checks before the order goes out

Check the mounting space. Stepping range up makes the rod longer. Bore depth inside the cylinder and available space outside both have to be re-verified against the final range; for built-in types see three things not to get wrong before a built-in hole is drilled.

Check the accuracy class. Once range is increased, resolution and absolute accuracy — both tied to full scale — need re-checking against the process requirement. This runs together with stroke, mounting, environment and output in four steps.

Check the replacement case. On an in-service machine, if the new range differs from the old one, the range scaling in the control program has to change with it. It is not a fit-and-run situation; see three-layer check when replacing a part.