Measuring Stripwound Hose: Relaxed vs. Extended State

By: Erik Kane | On: March 21, 2017

How can measuring a hose ever be an issue? The concept of measuring from point “A” to point “B” isn’t rocket science, and in the hose world, acronyms like OAL (over-all length) and ID (internal diameter) are common measurements that we see all the time. But stripwound hose is unique in the sense that unlike its various hose counterparts its ability to flex is mechanical; meaning that the hose’s corrugations operate by allowing sliding in and out of each other as individual parts. When a rubber or corrugated hose flex, the material is able to bend and stretch to accommodate the movement and then relax back to their neutral state. Because stripwound hose does not have this characteristic, it can make determining the true length of the hose difficult or seemingly inconsistent. Below we’ll take a look at the three different states that stripwound hose can be found in.

States of Stripwound Hose

Compressed Length & Extended Length

A stripwound hose is in its “compressed length” when all of the corrugations are completely closed. This is the shortest possible length the hose can assume, and also the least common. A hose is usually only fully compressed for some sort of special test or measurement, and typically won’t happen in service, but can give an accurate measurement. When the hose is fully compressed it means that the hose cannot get any shorter, and can only extend from that point. “Extended length” is simply the opposite of compressed length. The difference being that there are times when a stripwound hose will be measured in this length.

Relaxed Length

The “relaxed length” is when the hose is just midway between its extended and compressed lengths. This is the most typical state that you’ll find stripwound hose. As the intermediate state, if a hose is in service and is being flexed back and forth, and subsequently measured, it will be closest to its relaxed length. The best way to get an accurate measurement of the hose’s true relaxed length is to coil the hose up as tightly as possible (without damaging it), then roll it out like a tire until the hose is lying flat. This will balance the profile in the hose and give you a reliable length. Relaxed length is quite possibly the most important state for a stripwound hose, because this is the most common way for it to be measured.

When Will a Stripwound Hose be in a Given State?

A stripwound hose will occupy various degrees of the three states throughout its life in service to be sure, but the most critical time to know the state is before the hose is even manufactured. Whether or not the hose is going to be made into an assembly, or sold as a bulk length will determine how the hose is measured by the factory. If the hose is being sold as an assembly that will include fittings and other options so the hose is able to be put into service straight from the manufacturer, it will be measured and sold in its relaxed state. But if there are no options being added (liner, packings, fittings etc.) and the hose is being sold as bulk, it is measured and sold in its extended state. Adding to the confusion, long lengths of bulk hose are often too big to be shipped straight, and are rolled into coils for transport. When they are uncoiled at their destination, the hose will now be in its relaxed length! This difference in length between extended and relaxed is referred to by Hose Master as the “relaxation factor” and will often be about fifteen percent, which is a significant change in the length of the hose.

Understanding these industry practices can prevent unwanted problems when trying to specify a new hose or replace an existing one. Many end-users have a specific length of hose they need for their application, and the relaxation factor is much too large a variance to be off by when trying to specify a length. Asking the right questions before attempting to measure the hose can eliminate these issues. What state is the hose currently in? Was the length cut from a bulk reel? Has the hose been fully relaxed since it was in service? Are the corrugations fully open or completely closed along its length? If you need additional assistance with any issues with stripwound metal hose, call Hose Master inside sales at 800-221-2319 and we will be happy to help.


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