Simple one. Some extension springs are wound with an initial tension which acts to pull the coils together, even when not under external loading.
When calculating the preload in the spring, should the initial tension be added to the spring force (e.g. initial length x spring rate)? I have seen a few examples and some people include it, others don't. Which is correct?
After being pointed in the correct direction by an acquaintance I think I have the answer now:
It appears that initial tension should be included when calculating load in an extension spring, as shown in the formulae above. However, in some applications the initial tension may be insignificant compared with the load at a given extension. People either ignore it due to it being negligible, or in error.