Idler/belt size and it's effect on platter speed.

[analogadikt]

Am writing this post so forum members can have a clarity on this, and are not mislead by mechanics with half baked knowledge.

Relationship between idler size and platter speed.
Let's consider the following scenario. The motor pulley of a turntable has a circumference of 20 millimeters. So, for every revolution of the motor, the idler edge shall be moved by 20 millimeters, and hence, the platter shall be pushed forward by 20 millimeters. If the idler has a circumference of 80 millimeters, it shall be completing one revolution for four revolutions of the spindle pulley. Now, what shall happen if the idler diameter is reduced, and the circumference is now 60 millimeters? Will the platter slow diwn? No! The platter shall still move 20 millimeters for every revolution of the spindle, because the idler edge is still moving forward by 20 millimeters for every revolution of the motor. BUT.....BUT.....BUT, the idler is now rotating at a faster speed, as it is completing one revolution for only three revolutions of the motor spindle pulley. Similarly, if the idler's diameter is incresed, it shall move at a slower speed without affecting the platter speed. The platter speed shall not be influenced as long as the motor rpm, spindle pulley diameter and platter inner edge diameter are not altered.

Regarding belts and speed, I shall be back in a day or two.

Regards,

 

[arj]

On Idlers, The motor speed, spindle circumference/collar, Idler circumference and the platter point in contact with the idler are all synchronised and adjusted to get the 33.3/45 etc speed. Changing any one of those needs to be adjusted across the above chain of contacts to keep the speed constant.

Most of the Idlers do have an option of fine tuning some of them in some way or the others while some like the Lenco are simple the others like the garrard are complex but refined .

 

[TD150]

Am writing this post so forum members can have a clarity on this, and are not mislead by mechanics with half baked knowledge.

Relationship between idler size and platter speed.
Let's consider the following scenario. The motor pulley of a turntable has a circumference of 20 millimeters. So, for every revolution of the motor, the idler edge shall be moved by 20 millimeters, and hence, the platter shall be pushed forward by 20 millimeters. If the idler has a circumference of 80 millimeters, it shall be completing one revolution for four revolutions of the spindle pulley. Now, what shall happen if the idler diameter is reduced, and the circumference is now 60 millimeters? Will the platter slow diwn? No! The platter shall still move 20 millimeters for every revolution of the spindle, because the idler edge is still moving forward by 20 millimeters for every revolution of the motor. BUT.....BUT.....BUT, the idler is now rotating at a faster speed, as it is completing one revolution for only three revolutions of the motor spindle pulley. Similarly, if the idler's diameter is incresed, it shall move at a slower speed without affecting the platter speed. The platter speed shall not be influenced as long as the motor rpm, spindle pulley diameter and platter inner edge diameter are not altered.

Regarding belts and speed, I shall be back in a day or two.

Regards,

Very good and useful information @analogadikt as there is less clarity and information on the subject. Many people get confused. This information will help them.

 

[reubensm]

Am writing this post so forum members can have a clarity on this, and are not mislead by mechanics with half baked knowledge.

Relationship between idler size and platter speed.
Let's consider the following scenario. The motor pulley of a turntable has a circumference of 20 millimeters. So, for every revolution of the motor, the idler edge shall be moved by 20 millimeters, and hence, the platter shall be pushed forward by 20 millimeters. If the idler has a circumference of 80 millimeters, it shall be completing one revolution for four revolutions of the spindle pulley. Now, what shall happen if the idler diameter is reduced, and the circumference is now 60 millimeters? Will the platter slow diwn? No! The platter shall still move 20 millimeters for every revolution of the spindle, because the idler edge is still moving forward by 20 millimeters for every revolution of the motor. BUT.....BUT.....BUT, the idler is now rotating at a faster speed, as it is completing one revolution for only three revolutions of the motor spindle pulley. Similarly, if the idler's diameter is incresed, it shall move at a slower speed without affecting the platter speed. The platter speed shall not be influenced as long as the motor rpm, spindle pulley diameter and platter inner edge diameter are not altered.

Regarding belts and speed, I shall be back in a day or two.

Regards,

This is very true and hence, when replacing idlers, always keep in mind the turntable model and part number. I have been a customer of Terry's Rubbers and figured this out from his advise. While he recommends increasing the size of a reel to reel pinch roller (he calls it extra-rubber) marginally, he is always exact when it comes to idler wheels. For motor pulleys, one has to keep an eye on the 50Hz vs 60 Hz angle as well.

From my experience with working on turntables and tape decks, the common scam with belts is to use tighter (smaller) belts as OEM replacement sizes may not be easily available. A tighter belt slows down the speed and mechs often scam customers by compensating using the motor speed control (belt driven TTs generally use DC motors). If the motor is an AC one without a speed control, a common trick used is to increase the size of the motor pulley (usually by wrapping it with adhesive tape). A tight belt puts undue stress on the mechs and will end up breaking soon too.

 

[Hiten]

Changing any one of those needs to be adjusted across the above chain of contacts to keep the speed constant.

The idler only serves as transfer of rotation. A slight variation in diameter will not affect speed. Only Motor pulley and platter rim need to be perfect. However any significant reduction idler wheel will have less torque resulting in speed changes.

 

[arj]

The idler only serves as transfer of rotation. A slight variation in diameter will not affect speed. Only Motor pulley and platter rim need to be perfect. However any significant reduction idler wheel will have less torque resulting in speed changes.

Thats true since in this case its the Angular velocity which is important but practically any reduction in idler size will cause a disconnect between the platter and the motor/pulley and make it irrelevant ,,