For Want of a Nail

Galibier Design - Calibrating a Drive Pulley
(click to enlarge)

As the classic poem begins:

For want of a nail, the shoe was lost
For want of a shoe, the horse was lost…

Chasing vanishingly small tolerances is the name of the game in turntable design, and the smaller the part, the more critical this becomes.

It’s the Little Things  …

The drive pulley is as critical to a turntable’s performance as the bearing. 

When multiple parts are combined into an assembly tolerance errors multiply.  Let’s consider a drive pulley and the motor shaft it mates onto.

  • Our motor manufacturer has excellent consistency, specifying a tolerance range for their shafts of .0004″ (four ten-thousandths of an inch). 
  • We manufacture our pulleys to the same specification.

You’d expect that the maximum combined error for these two parts would be .0008″ (a very good number).

In practice, the loosest fitting combination (the smallest motor shaft and largest pulley hole) will result in a larger error.

To achieve our runout (eccentricity) target, we place a completed motor pod onto our measuring stand and match pulleys to motor shafts.

Even the torque sequence of the mounting screws matters.

You can take a “near-perfect” combination (nothing is absolutely perfect) and add or subtract .0002″ to the eccentricity by merely by adjusting the procedure with which you torque the mounting screws.  So, we tighten/measure, loosen/re-tighten/re-measure until we find a satisfactory combination.

No Free Lunch

Our rigid drive belt architecture approaches the performance of an idler drive (no mean feat, folks!), and with it comes a requirement to manufacture them with increased precision.

Sometimes, we’ll get a great match (.0005″) and we’ll still hear a bit too much belt noise. We’ll match a different pulley to the motor (one that measures identically), and the noise is reduced.

If you want a quiet running belt, go with rubber, but if you want the best speed control, a rigid belt is for you.

Life is full of trade-offs.  Click here for more on belts and the sonic compromises of running a quiet, rubber belt.

In part-2 of this series, we discuss the challenges (and our success story) in developing the idler pulley for our Savoie (idler drive) turntable.

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Thom


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