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Welcome to our latest Engineers Guide To Spring Technology. In this issue we take a look at compression spring end types, explain some key terms and introduce our new feature, Under The Microscope which looks at a scenario that recently involved randomly breaking steel circlips.

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COMPRESSION SPRING END TYPES
 
There are a few options when it comes to end type selection for a compression spring. The two most common are 'closed and ground', and 'closed not ground'. Generally speaking, end grinding is an expensive process, and is therefore to be avoided wherever possible. The 'closed not ground' option can often be selected where spring end squareness is not critical. As the spring index (mean diameter/wire diameter) ratio increases, the need to grind reduces, as large index springs tend to stand fairly square without grinding. Grinding can be difficult on large index springs anyway, as the spring yields too easily under the pressure applied by the grinding wheel.

For small wire diameters, grinding is also impractical, so unground ends are normally specified for wire under 0.5mm, although it is possible to grind smaller wires, with care, if absolutely necessary.

In many applications however, there is no question that the springs will need to be ground. For instance, valve springs will need to be very square to ensure that the valve seat seals properly. End grinding can also help with spring stability; an unground spring is more susceptible to buckling under load than one that is ground.

Other end type possibilities exist, such as 'open', 'open and ground', and 'half-closed' (ground or unground). These are used comparatively rarely however, as spring stability is poor. Multiple inactive coils are sometimes used at the ends of compression springs. Generally this is done to help prevent tangling. When compression springs are packed in bulk, springs will often tangle (dependent upon the design). Using perhaps three or four dead coils at each end means that these close-coiled sections are longer than the gap between the active coils in neighbouring springs, and are therefore less likely to lock together.
 
The Mystery Of The Chain Mail Glove
 
Some time ago, we came across a baffling situation involving random breakage of stainless steel circlips. The circlips we supplied were for use as links in chain mail gloves, used, not in suits of armour, but in the butchery trade, to protect workers' hands from knife cuts.

Our customer had developed an accelerated test protocol, designed to replicate the cleaning processes that the gloves would be subjected to over many years of use. The problem was that he was finding that the circlips were breaking for no apparent reason. The circlips were not being subjected to any significant stress, and neither were any aggressive chemicals being used in the cleaning process.

Having witnessed the cleaning procedure and examined some broken parts under a microscope, we were at a loss to conclude the failure mechanism. Further discussion with our industry research organisation, eventually highlighted what was going on.

The cleaning process, although only using water, involved blasting the gloves under very high pressure.
 


Whilst it is often assumed that stainless steel is an inherently corrosion resistant material, this is not actually the case; it contains something like 70% iron and it derives its corrosion resistance from a passive surface film which forms naturally over time. Without this layer, it will readily corrode.

It transpired that the high pressure water jet was removing the passive film, rendering the circlips liable to corrosion. Spots of rust would form before the passive film was recreated. During the next cleaning session, the passive film was again blasted away, together with the rust, leaving corrosion pits. These pits naturally held moisture which again prompted further growths of corrosion, only for this to be removed during the next cleaning cycle. Eventually, a sufficient proportion on the wire section was eaten away to prompt the wire to snap under the pressure of the jet.

The mystery was therefore solved. The fact that failures didn't occur during actual use confirmed that it was the accelerated nature of the test procedure that was causing the problem; if the circlips were allowed time to regrow their passive surfaces, there was no problem.
GLOSSARY
 
Angular relationship of ends
The relative position of the hooks or loops of tension springs, or the legs of a torsion spring, to each other.

Closed and Ground Ends (also known as Squared and Ground)
The ends of a compression spring where the pitch of the end coils is reduced to zero and the ends are ground square with the spring axis.

Compression Spring
A spring whose dimension, in the direction of the applied force, reduces under the action of that force.

Torque (M) also known as Moment
The product of the distance from the spring axis to the point of load application, and the force component normal to the distance line. Usually expressed in N.mm
 


Helix Angle
The angle of the helix of a helical coil spring.

Hooke's Law
is a principle of physics that states that the force needed to extend or compress a spring by some distance is proportional to that distance.

Deflection(s)
Relative displacement of the ends of a spring under the application of a force.

Dead Coils The coils of a spring that do not affect the spring rate.

Stress Correction Factor (k)
Factor that is introduced to make allowance for the fact that the distribution of shear stress across the wire diameter is not symmetrical.