Does Changing the Idler Diameter Affect Record Player Speed?

[tewitt1949]

I'm new to this site so excuse me if I'm posting in the wrong area.
We are having a discussion about motor, idler pully (yes wheel) ratios and we can't agree on this problem.
What we have is basically an old record player, The motor shaft is turning a single idler, and this idler is also turning the platten (the part the record sits on). The disagreement is. Will an idler of a different diameter change the platten speed? Some say yes, some say no. We need an expert that can explain the answer so us average minds can understand. Thanks Terry

 

[xxChrisxx]

I'm new to this site so excuse me if I'm posting in the wrong area.
We are having a discussion about motor, idler pully ratios and we can't agree on this problem.
What we have is basically an old record player, The motor shaft is turning a single idler, and this idler is also turning the platten (the part the record sits on). The disagreement is. Will an idler of a different diameter change the platten speed? Some say yes, some say no. We need an expert that can explain the answer so us average minds can understand. Thanks Terry

If a pulley is driving something, it's not an idler, it's an output :)

Pullys work in exactly the same way as gears.


So in a 2 pully system:
Input (driving) OD = 1
Output OD = 2

The gearing ratio is 1:2
The output gear will rotate at half the speed of the driving pulley.


In a 2 Pulley system:
Input OD = 1
Output OD = 4

Ratio = 1:4
Output turns half the speed of the case above, and quater of the input speed.


In a three pulley system, with an idler (ie a pully that just spins, and doesn't drive aanything). The idle pulley size is irrelevant to the input and output speeds.

 

[entropivore]

I suspect you really meant "idler wheel", not "idler pulley". This is a common setup on low cost phonographs. Often the motor shaft is machined to three different diameters along the exposed length, corresponding to 33-1/3, 45, & 78 rpm. An idler wheel runs against the motor shaft and against the inner diameter of the platter (not platten) rim. By moving the idler wheel up and down (that is, along an axis parallel to the motor and platter spindle axes), the platter speed can be changed.

You weren't asking about the different motor shaft diameters, though, so ignore that part for the moment. Think about surface speeds. Assuming nothing is slipping, and ignoring such nuances as compression of the rubber of the idler wheel, then the surface speed of the idler will be exactly the same as the surface speed of the motor shaft (at the contact point). Likewise, the surface speed of the platter rim at the contact point with the idler will be exactly the same as the surface speed of the idler (which is also the surface speed of the motor shaft).

A smaller idler wheel will turn faster (in terms of revolutions per minute) because the path around its perimeter is shorter but its surface speed is constrained to be the same by the lack of slippage. Similarly, a larger idler wheel will experience fewer revolutions per minute because the path around its perimeter is correspondingly longer, but the surface speed is again the same as that of the motor shaft and the platter rim.

If you do want to change the platter speed without changing the motor speed, you need to introduce a new surface speed into the system. One way to do that is by changing the motor shaft diameter as discussed above. Another way is to couple two surfaces together that have different circumferences. A common way to do this is to have two sections of different diameters on the same piece of material, drive one section and take your output from the other section. Since the two sections have the same rotational speed by definition, and since their circumferences are different, their surface speeds must be different.

You don't need an expert, you just need to sit back and try thinking about it in different ways.

 

[tewitt1949]

Thanks guys, that's exactually what I wanted to know. You also did an excellent job explaining why it is what it is.
Here is the link of our discussion. You will see there are a lot of ideas. Thanks again, Terry

http://www.audiokarma.org/forums/showthread.php?t=410357

 

[PJC01]

I would approach this question by first understanding the basic principles of rotational motion and how it relates to the motor, idler, and platten in this record player system. Then, I would use mathematical equations and experimental data to support my explanation.

To begin, let's define the motor, idler, and platten in this system. The motor is the source of rotational motion, while the idler acts as a middleman between the motor and the platten. The platten is the part that rotates and holds the record in place.

Now, let's consider the rotational speed of the platten. This is determined by the angular velocity, or how fast the platten is rotating, which is measured in radians per second (rad/s). The angular velocity is related to the linear speed of the platten, or how fast a point on the platten is moving, by the following formula:

ω = v/r

where ω is the angular velocity, v is the linear speed, and r is the radius of the platten. This means that the larger the radius of the platten, the slower the angular velocity will be for a given linear speed.

Next, let's consider the effect of the idler on the platten speed. The idler acts as a gear, transferring rotational motion from the motor to the platten. The ratio of the motor's rotational speed to the platten's rotational speed is determined by the ratio of the diameters of the motor and idler pulleys. This is known as the gear ratio and is given by the following formula:

Gear ratio = (Diameter of motor pulley)/(Diameter of idler pulley)

If we change the diameter of the idler pulley, we are essentially changing the gear ratio of the system. This means that for a given motor speed, the platten speed will change. If the idler pulley is larger, the platten speed will be slower, and if the idler pulley is smaller, the platten speed will be faster.

In summary, changing the diameter of the idler pulley in this record player system will indeed change the platten speed. This is due to the relationship between the angular velocity, linear speed, and gear ratio. I hope this explanation helps to settle the disagreement and provides a better understanding of how motor, idler, and pulley ratios affect rotational motion in this system.