Direct drive/rim drive/idler drive vs. belt drive?

Hiho

Consider the speed of three things: firstly the belt on the drive side of the motor pulley, this being the section which is pulling on the platter; secondly the belt on the non-drive side of the pulley, this being the section where the belt is feeding off the pulley onto the platter; and thirdly the surface speed of the pulley itself.

Belt slip is where the speed of both parts of the belt is slower than the surface speed of the pulley.

Belt creep is where the belt speed on the non-drive side of the pulley is slower than the drive side, so the belt creeps over the pulley to make up the speed difference. This is because the belt is stretched by the torque it is transmitting and that amount of stretch must relax over the pulley. If the belt were equally stretched on both sides there would be no tension difference and thus no torque transmitted.

Mark Kelly

"If the belt were equally stretched on both sides there would be no tension difference and thus no torque transmitted."

Thanks for the explanation. On my system, the active pulley is the same size as the passive platter so I would have to assume there would no or little belt creep, right? I use two identical turntables to drive each other via a VHS tape. So far this is the best sound I got from a belt-drive system and the only thing that can match it is a quality direct-drive turntable with a coreless motor. Since I am lazy, I like the convenience of direct-drive table with some automatic features. :-)

Hiho

The creep is proportional to the strain in the belt divided by the parallel sum of the wrap lengths of the pulley and platter.

The compliance of the mylar belt is quite low, so there's not much strain present. The wrap length of your "pulley" is about 500mm rather than the 10 - 50 mm of conventional drives.

Your system will give about the lowest creep available (this side of direct drive of course).

Mark Kelly

Lewm, I did not mean to imply DD to be better, but equal in sound Performance characteristics w/ CMB, the latter having less-er friction. To tell you the truth I have both mechanical and CMB version and prefer the original mechanical bearing the best. It is not, i think, the magnetic bearing feature (which is horizontal BTW, the ceramic being the main spindle) but the ceramic spindle that hurts the overall performance. They have not optimized the stiffness spindle, me thinks and hence less stiff than the mechanical bearing metal ( steel)spindle.

Hiho

When I said "about the lowest creep of any system outside DD" I was thinking that the drive I designed for Thom Mackris was probably slightly lower. I was wrong.

I've run the numbers, assuming you are running a 1/2" x 3mil Mylar belt around two 300mm platters placed 400mm apart, and you win. We have slightly lower compliance (0.063 mm/N vs 0.117 mm/N) but you have a longer effective pulley length (236 mm vs 68 mm) so your creep is lower: 0.003 vs our 0.005, referred to a radius of 150mm.

As a reference, a typical belt drive system might have a creep number around 5 - 10 and a good idler system (like Win's) will have a number around 0.05, over 100 times better than the belt drive.

Mark Kelly