Re: [Phys-l] stress in a wound (clock) spring

[Brian Whatcott <betwys1@sbcglobal.net>]

At 10:41 AM 6/28/2008, you wrote:
> Occasionally a clock smith will repair a broken spring using rivets
> (scab on).  I suspect the stored E is mainly shear (bending) instead
> of tension.  Enormous, comparatively, force is necessary to stretch
> w/ such a large cross section of spring steel.  Therefore, not much
> stress on the broken part .  The rivets will be under mostly
> tension?  Am I correct?   The reason I ask is a poster on a clocks
> group thinks "this is a recipe for disaster".  The energy stored is
> rather great, and when one goes, if not ina barrel, ruins the clock.
> Occasionally barrels are broken.
>
> bc
>
> p.s. We'll possibly know as a Polish list member has done the repair
> using screws in a (rare?) multiple face clock (master w/ slave faces)
> being unable to find a replacement.

It's interesting to recall that the force to bend a strip of mild steel
through a certain angle can be similar to that required to bend a strip
of alloy steel of similar cross section.      The mild steel yields to a
permanent deflection at a certain limiting angle, while the alloy
 material can return elastically from a greater deflection angle.

 The surface fibers take the most stress in bending in this way.
While main springs store energy in bending, coil (helical) springs
 see a mix of different stresses: torsion, bending, shear.

   Springs made of animal tendon are said to offer the highest
energy storage density, one reason why I suppose tendon damage
is difficult to make good.    It is not hard to see why clock
 repairers would prefer to replace a mainspring than attempt
 a repair.   Still, I see examples published of screw and rivet
 repairs.

There is a style of articulated garage door more popular in
 Europe than here in the US,  designed to carry the upper
 edge of the rigid door on horizontal side rails along the
interior ceiling, whose lower door edge describes a
 semicircle, controlled by side arms, pivoted both at the
 door's bottom edge, and at the garage door side frames.

The weight of a proportion of the door's weight is
balanced by two large helical springs, fixed both at the side
frame base, and at the side arms, on extensions set to
 aid the opening rotation.   All of this is to report that
these long tension springs sometimes break, and contrary
 to expectations, can be successfully rewelded.

With this in mind, I observe that the archetype of a welded
spring strip joint is to be found on metal-cutting band saw
blades, which typically do not fail at the welded joint
on industrial bandsaw blades.

 On a speculative note, I imagine that an alternative to a
bandsaw blade welder such as those found near larger
 metal bandsaws might be found in a portable resistance
 welder of the kind now to be found on the retail market.
These only provide spot-welds however, rather than the
desired continuous end weld but would seem to offer the
 possibility of a low profile joint.



Brian Whatcott    Altus OK    Eureka!