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Specifying a Drive Belt

Galibier Design - Kapton BeltFrom time to time, we like to revisit our design assumptions  – to see whether they still have validity.

Several months ago, we demonstrated various drive belts for a customer (link here).

In this post, we’re going to cover some design considerations for selecting a drive belt for our turntables.

Conflicting Goals

Throughout the vinyl playback chain we’re fighting two opposing attributes:  isolation and coupling.

Our experience has shown us that isolation compromises speed stability, while coupling adds noise, and therein lies the challenge – finding the balance that works for your musical values.

Musical Values

We prioritize coupling, and I need to state that my musical values place an emphasis on rhythmic integrity, conveying a sense of propulsion in the music, and being able to unravel complex, multi-voiced melodic lines.  Harmonic integrity (“tone colors”) is important as well.  This latter attribute is not as inconsistent with the other characteristics as you might think.

Putting this into context, my torture test records are bluegrass, Carribean jazz, 20th Century string quartets, along with a bit of 18th & 19th Century romantic music.

Individuals (in general) who are aligned with my priorities tend to favor idler and direct drive turntables over belt drives.

But … Belt Drives Can Deliver the Goods

Early on (ca. 1999), we realized that reducing the compliance (springy-ness) of a belt goes a long way toward resolving many of what I would consider belt driven turntables’ shortcomings (springs are another problem).

I’ll return to this concept in a future post, but suffice it to say for now, that the problems we see in a rubber belt relate to mechanical resonance in the drive system.  A belt is a mechanical filter, which (just like an electrical filter) resonates.

As the stylus tracing the record groove modulates the platter speed (yes, it does!), a rubber belt contracts and stretches.  It’s in a constant state of reaction – never truly locking on speed.

Comparing Belts

Galibier Design - Belt ExperimentSo, in the link at the top of this post, we described the listening group’s observations of various belts, and as you’ll read there, nothing has changed.  The rubber belt was the big loser, and by no small amount.

Stiffness Counts, but Mass Matters

We’ve been through a variety of materials and configurations, and learned that a key element involves keeping the mass of the belt as low as possible, while not compromising its rigidity.  Lower mass transmits less noise into the platter.

Kapton has proven itself over the years – for both rigidity, low mass, and durability.  Kapton belts were specified in airplane flight recorders (black boxes) – a “no fail” application if there ever was one.

The Challenge

Our suppliers are no longer willing to produce Kapton belts in the .003″ thickness we most recently specified.

The Future

We recently specified a prototype run of .005″ thick Kapton.  To compensate for the added mass, and we’ve reduced the width from .375″ to .200″.

Sometimes, you can outsmart yourself, and the proof is in the auditioning.  Our sense is that belt grip on the pulley will be unaffected, as the wear pattern on our .003″ belts shows only about 40% of the belt is actually contacting the pulley.

Will the increased thickness reduce the contact surface on the pulley further, and will this compromise speed stability?  That’s why you prototype.

There may be a hidden benefit to these narrower belts:  reduced airborne belt noise.  With the implementation of our faster turning motors (about 600 rpm for 33-1/3), we’ve observed a bit of “belt release flutter” – the slightest bit of noise emanating from the belt as it passes over the pulley.

We’ve isolated it to two things:  the unsupported section of the belt (that doesn’t contact the pulley), and drive system runout (eccentricity).

With respect to runout, precise machining of our pulleys (to .0005″ – five ten-thousandths of an inch) is key, and those are the limits of manufacturing.  When you consider that motor shafts have a similar variance, there’s an additive effect, and you can only control one of them (pulley).  We minimize the additive effects of these two parts by individually matching pulleys to motors.

Stay tuned …