I’ve seen a fair amount written about the problems with Apple Powerbook power supply cables wearing at the end near the power supply. I’ve seen various clip-ons or do-it-yourself gummy solutions that claim to reduce this issue.
Everyone’s assumption seems to be that there is not sufficient cable stress relief, meaning the cable bends tightly at one spot, rather than making a smoother bend over a longer distance, thus relieving the stress at the power brick.
Apple’s stress relief is, indeed, minimal. And yes, the methods suggested do extend that cable relief. But that only solves the problem if the assumed cause of cable failure is correct.
Observing what is actually happening with these cables, they are very supple; Apple would not want to ship a stiff cable, which cannot be wound-up conveniently. And this suppleness means the cable does not fatigue much from being tightly bent; this would be one reason why Apple does not supply an extended stress reliever.
Instead, the motion that I see causing wear in the cable has to do with the sheath not being bonded to the interior components, and a rotation occurring within the cable sheath. This twisting motion eventually stresses the cable sheathing, the internal wires, or both, causing cable failure.
Most stress relief designs do not address this issue. Anyone who has spent time on sailboats knows about the problems that side-winding a rope produces; wind it to the wrong side, and not only will you kink the rope, you may actually open up the strands, significantly weakening it. The ideal solution to avoid kinking, is to straight-wind a line or cable from the end, not the side, both winding and unwinding.
Applying this to power supply cables, the cause of twisting at the end, causing eventual failure of the cable is the method we use to wind and unwind the cable. If we wind it up overhand, but then, instead of reversing that process to unwind it, pull the cable over the end of the power supply, we are building a series of kinks into the cable, and increasing this twisting force every time we store, then release the cable.
Apple has designed their cable storage solution with end plates that force the user to straight-wind the cable both on and off the arms. However many of us do not open up these clips and store the cable on them, but instead wind the cable around the body of the power supply, or simply wind it up on it’s own.
This has two problematic results. First, it straight-winds the cable onto the power supply, or our fingers, but in most cases we pull the cable off the end of the supply, or sideways out of its self-coil to release, it, thus side-winding kinks into the cable in the process. The second problem is that the Apple winding arms are positioned to contain the short stress reliever, and eliminate force against it, or repeated motion of the cable at that point during storage and travel. Any other cable storage method will leave the cable exposed to force and repeated bending at this point, shortening its overall life expectancy.
So while a conveniently supply cable is prone to wear, the best solution to Apple power supply cable wear appears to be storing the cable in the manner Apple planned, plus being aware of the dangers of twisting the cord, and straightening out any kinks that may form, to avoid ongoing twisting of the cable at the point of attachment.
C. David Tobie