High durability springs and their working stresses
This is calculated quite arbitrarily at 70% of the elastic limit of carbon steel and bronze, and 80% for stainless steel and music wire.
Please note that this can be altered to suit; a spring which has to do no more than sit there and exert a force can be worked higher as long as it never exceeds the elastic limit. One which carries a dead weight with some little movement needs 70%; on the other hand, one which may have a light load but is repeatedly stretched (a lubricator ratchet spring, perhaps) might be better designed at a lower figure.
There is no harm in using a low stress — if you don’t mind using heavier wire and more of it. The one exception to this is, perhaps, the case of engine valve springs.
This is a classic case of the “Severe Duty” spring requiring high durability springs. The severity is not so much due to fatigue as to the fact that the inertia of the valve spring itself may mean that the whole load is carried on just the top few coils at the beginning of each lift. Also, such springs may vibrate axially from coil to coil, again increasing the local load.
Deck hardware springs and spring design
We have to preserve some sense of proportion over this matter. Most springs whose in-situ load are important to having means of adjustment provided; with others, it does not matter a great deal if they are slightly larger than designed.
Though it may be important that a spring does not bind on, e.g., on a valve-stem, though here the spring-back will tend to ease it rather than the reverse. Less than one in a dozen of the springs need the meticulous attention described above; indeed, many springs just “adjust” lengths cut off from commercial “stock lengths”.
Initial tension in high tensile strength springs
Most tension springs, whether boating equipment springs or not, are wound with “Initial Tension”. That is, it requires a definite force to be applied before the spring begins to stretch at all. Contrary to common belief this feature is not achieved by winding the spring with the wire at an angle to the circumferential line of the previous coil. True, this does impart a small tendency for the coils to nestle close to each other, but it does not cause any measurable initial tension.
The spring derives its ability to exert a force from the torsional stress in the wire. To obtain an initial tension, we must wind the spring with a locked-up torsional stress in the wire. This is done by twisting the wire as it is laid onto the mandrel, the twist being in the direction which will tend to cause the coils to press one against the next.
This is easily done on proper spring winding machines, and both calculation and experience can be used to determine the amount of twist per revolution of the mandrel needed to provide the designed initial tension. The one point on which care is essential is that the speed at which the winder is rotated MUST be steady and uniform. If not, then the degree of initial tension will vary along the spring, and in service, one part will start to stretch before the rest, and may well be overstressed at full extension.
As with all other manual operations, the effective winding of UES Int marine hardware springs does require some practice.