: Hi mick,

: I think my use of the terms “softer” and “harder” caused some confusion. I
: used those terms in the metallurgical sense – softer means easier to
: plastically deform. The 300 series stainless steels can be strengthened by
: work hardening – that is, by deforming the material beyond the elastic
: limit. But they cannot be strengthened by tempering – which is a term
: loosely used to describe heat treatment. The key idea here is the
: distinction between elastic deformation and plastic deformation.

: Springs are designed to only experience elastic deformation. To describe a
: spring, “hardness” and “softness” are used in the sense of how much force
: is needed to extend or compress the spring – physically they describe the
: spring constant K in the equation F=Kx, where F is the force and x is the
: distance that the spring is elastically deformed. K is determined only by
: the spring geometry, the wire diameter, and the elastic modulus of the
: material. Elastic modulus is invariant – it does not change with work
: hardening or heat treatment.

: For a given geometry, wire diameter, and material (say 316 stainless steel) a
: spring made of a weaker wire will deform less before being permanently
: deformed than will the same spring made of stronger wire. However, up
: until the point where the weaker spring is permanently deformed, both will
: have the same K, and therefore both will have the same stiffness because
: the geomertry and the material are the same. The weaker wire spring will
: simply not act as a spring for as much stretch or compression (x) as the
: stronger wire spring. To reduce the effort needed to operate the foot
: pump, we need a spring with a lower stiffness K. The easiest way for us to
: achieve this is with smaller diameter stainless wire.

: The reason that heat treatable alloys, such as medium carbon steels, are used
: for many springs is that in the weak condition they can be deformed into
: the spring shape, then strengthened by heat treatment. In the strong
: condition, they deform elastically, so the spring returns to its original
: shape. Because 300 series stainless spring wire is supplied in a work
: hardened state, it is stronger than the same wire after heat treatment
: (tempering), but the elastic modulus is the same. So by tempering, we
: would only loose some of the compression that the spring could withstand,
: but the effort to compress it to that point would be the same as for the
: hardened wire.

: Sorry for the confusion, and I hope I haven’t made it worse!

: Sam

hey sam

thanks for the nice explanation.

-mick