Travel and Travel Reserves

A spring support has a finite stroke. The software has to place the operating and installed positions inside it, with enough margin at both ends that the spring never reaches a stop. This article explains how that placement works and what the reserves are for.

Admissible travel and recommended travel

Every spring model has two travel ranges.

Admissible travel is the full physical stroke, from one end stop to the other. Working outside it is impossible.

Recommended travel is the inner band the manufacturer advises staying within — in practice, the middle 80 % of the stroke, leaving a tenth of the range clear at each end.

The reason for the inner band is tolerance. Real installations differ from drawings: elevations shift, the pipe settles, the movement calculated in analysis is not exactly the movement that occurs. A spring positioned close to its stop has no room to absorb that difference, and a spring that reaches its stop stops behaving like a spring — it becomes a rigid restraint, which is precisely what the support was installed to avoid.

How positions are determined

For a variable spring, the hot position follows from the load: the software finds where in the travel range the spring produces the required load. The cold position is then the hot position displaced by the vertical movement.

For a constant spring, the load does not vary with position, so there is nothing to solve. The software centres the required movement within the total travel: the operating and installed positions sit symmetrically about the mid-point.

Both positions have to fall inside the admissible range. The software checks both, not just one.

Travel reserves

Travel Reserve Up and Travel Reserve Down are optional fields on the support datasheet. They define how much of the stroke must remain unused at each end.

When you leave them blank, the software applies its own default margin. When you define them, yours replace the default — and they are taken in absolute value, so the sign does not matter.

You can define one and leave the other blank. The software then uses your value on that side and its default on the other.

When to define your own

Four situations justify overriding the default:

• The stress analysis produces special load cases with extended movements or with movements opposite of the normal operational movement. You may use the travel reserve to make sure that the spring will be within its load range in all load cases.
• The stress analysis is provisional. Extra reserve absorbs the movement changing before construction.
• A client specification requires a stated minimum reserve. Common in power and nuclear work.
• The installation is hard to reach. A support that cannot easily be re-adjusted deserves more margin.

Reserves are not free: a larger reserve means a longer travel is required, which usually means a larger and more expensive support.

The effect on constant supports

For constant springs the reserves do more than trigger a warning — they enter the sizing directly.

The total travel of a constant support is the required movement plus a margin at each end. Without reserves the software applies a default margin proportional to the movement, with a floor. With reserves defined, your values replace that margin, and the total travel changes accordingly.

The result is then raised to the minimum travel the selected size and product line require, and rounded up. So a constant support’s travel is rarely exactly the movement you entered — it is normally larger, and that is by design.

Reading the result

The datasheet shows the hot and cold positions together with the model’s travel range. Two things are worth a glance:

• Are both positions comfortably inside the range? A position within a few millimetres of an end is a warning sign even if no message appears, because the check may have passed on the admissible range rather than the recommended one.
• Is the total travel much larger than the movement? For a constant support this is normal. For a variable it may mean the selection is oversized.