Hi,
I have a QRK 12C turntable in great condition, but the idler wheel as a little indent probably due to the idler resting on the motor spindle.
The rubber is in OK condition and I found information on how to restore the rubber using different methods, including rubber rejuvenator products.
What are my DIY options? I found some replacement alternatives but they will cost me between $150 to $200 Canadian dollars.
Francesco
I have a QRK 12C turntable in great condition, but the idler wheel as a little indent probably due to the idler resting on the motor spindle.
The rubber is in OK condition and I found information on how to restore the rubber using different methods, including rubber rejuvenator products.
What are my DIY options? I found some replacement alternatives but they will cost me between $150 to $200 Canadian dollars.
- Can I sand the wheel? If yes, before rejuvenating or after?
- Can I machine the rubber to remove a layer and then put a rectangular o-ring around? Do I freeze the rubber before attempting this?
- Other options?
Francesco
For idler-drive turntables, the idler diameter does NOT influence turntable speed.
Only motor speed does.
The only way an idler can cause a speed issue is if it's slipping, miss-shaped or of a bi-level design, which this one is not.
Thanks for your input. I know there were many discussions on the web about the idler size related to the platter speed. I did not take the time to read the explanations yet... but I can't wrap my head around it yet. The motor always spins at the same speed. The platter has a set size. So if the idler is smaller or larger, it could make minute differences, similar to changing the tire size on a car (see tire comparator).
Regardless if it will make a difference or not, I will still have to machine the rubber wheel perimeter. After its machined, I will have 3 options:
I'm a bit of a slow learner... so it will take me a bit of time to understand the dynamics at play. I know there were many debates on many posts and forums, and I don't want to start another one. My goal is simply learning how to fix the little indent on the idler wheel. My last option would be to buy a replacement idler wheel.
Regardless if it will make a difference or not, I will still have to machine the rubber wheel perimeter. After its machined, I will have 3 options:
- Leave it the way it is with a very slight size difference and no impact to the speed
- Or add a layer of rubber to get back to spec and maintain the right speed
- Change the motor speed by machining the motor shaft spindle (not sure it's a good idea to even think about it)
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The proper way to machine rubber is with a course grinding wheel. You need to mount the idler wheel on a mandrel in a lathe making sure that the mandrel is running perfectly true. Secure the wheel to the mandrel and use a tool post grinder. Take light cuts and use air gun to blow away the dust and keep the wheel cool. Take light cuts until the wheel is running perfectly true.
Or............contact Terrys Rubber Rollers, he will rebuild your idler to like new condition with new rubber at a very reasonable cost.
On Edit: As far as the reduction in idler size making a difference in speed, it wont. The ratio between the motor shaft diameter and the inside diameter of the platter doesn't change when you reduce the idler diameter. The idler is just transferring the motion between the two contact surfaces.
BillWojo
Or............contact Terrys Rubber Rollers, he will rebuild your idler to like new condition with new rubber at a very reasonable cost.
On Edit: As far as the reduction in idler size making a difference in speed, it wont. The ratio between the motor shaft diameter and the inside diameter of the platter doesn't change when you reduce the idler diameter. The idler is just transferring the motion between the two contact surfaces.
BillWojo
I'll try to educate you about this... call it a lesson in physics.
I've been an electronics service technician for over 40 years, and serviced thousands of turntables over the decades...
The motor shaft diameter/surface runs at a certain fixed speed, which determines the platter speed - that is a "fixed" amount.
The driven part of the platter (inside) is also a fixed diameter.
Now...... the idler simply transmits the motor revolutions/speed to the platter.... and as long at the idler is not slipping, there is no change in what the motor is transmitting to the platter, regardless of the diameter of the idler.
Think: surface-to-surface transmission of motion - motor shaft rotational surface speed-to-idler surface speed-to platter rim surface.
It has nothing to do with how big or small the idler is - it's all about the surface of it.
It is nothing like a car tire diameter - let's concentrate on a tire not driven by the engine, just a tire traveling down the road.
See, your tire outer rim/surface is not driven by a motor at a constant speed, it's only driven from the road's movement beneath it, so naturally a larger tire will revolve slower at a given MPH, and vice-versa.
Thanks for the advice BillWojo. Much appreciated. Would you recommend working on the old rubber or after rejuvenation? Room temperature or very cold?
Thanks Wiseoldtech. My own idler is spinning
Would I be correct to say that 1 revolution of the motor shaft will generate 1 revolution of the idler wheel which in turn will make the platter do 1 revolution? And since the speed is set by the motor revolutions per second, that's the speed the platter will turn. 
The next question relates to the real motor speed. Does it turn at the right speed even if the AC can range between 120 to 125 volts? And what was the normal AC voltage 40 years ago? 115? Maybe I have it all wrong again
115V or 125V, it shouldn't matter, particularly if the motor is of the synchronous type.Thanks Wiseoldtech. My own idler is spinning
The next question relates to the real motor speed. Does it turn at the right speed even if the AC can range between 120 to 125 volts? And what was the normal AC voltage 40 years ago? 115? Maybe I have it all wrong again
And yes, if the motor, idler, and platter were all the same diameter, the speed would be the same for all.
BUT.... if you had a say, 1 inch motor pulley//shaft, and a 1 inch platter, then ANY size of idler would result in the same platter speed as the motor.
It doesn't matter - the idler ONLY transmits what the motor is capable of to the platter.
Hardened, glazed, shiny idlers will of course introduce slippage into the mix.
And no, it's not cool to shorten the tension spring for less slippage - that's dumb and not a proper (and reliable) fix, and I don't know why people do that, perhaps because they're too lazy to get a new idler.
I think what you are trying to say is there's a second order effect due to compression of the rubber around the motor shaft that is different from the compression at the idler/platter contact point. Yes, no doubt, but its 2nd-order w.r.t. idler diameter so can be ignored for the sort of modest resizing involved.
I would revitalize the rubber first, allowing any stresses to be relieved, before any sanding or machining takes place. The indent may be reduced by revitalizing. Old hard rubber is that way because the plasticizers have leaked out or its partially vulcanized. If the former revitalizing will have some useful effect I'd hope.
There's a trick that might be simple to try: find a rubber belt that's the right size to stretch over the idler - this provides a brand new surface and could be super-glued on and trimmed for a neat job. Then the condition of the old rubber is irrelevant (unless its gone liquid!).
I would revitalize the rubber first, allowing any stresses to be relieved, before any sanding or machining takes place. The indent may be reduced by revitalizing. Old hard rubber is that way because the plasticizers have leaked out or its partially vulcanized. If the former revitalizing will have some useful effect I'd hope.
There's a trick that might be simple to try: find a rubber belt that's the right size to stretch over the idler - this provides a brand new surface and could be super-glued on and trimmed for a neat job. Then the condition of the old rubber is irrelevant (unless its gone liquid!).
Sorry, you're wrong there.
Because the idler shaft is spring-loaded and moveable to compensate and keep pressure against both the motor shaft and platter rim.
Back decades ago, in the shop I've tried that "cheap fix" and the results were not very pleasing.
Better to have the idler rebuilt or just get a new one.
No, I am not wrong. A dent in the idler causes wow. No spring can compensate a deformed idler.
Did some quick reading on synchronous motors and I saw it's based on Ac frequency, which is very stable at 60Hz.
My original thinking about the platter speed dependencies was absolutely wrong. Thanks for clarifying and providing your perspective.
I was wondering if there is a way to simulate this in software with some type of gear simulation software. Found something on geargenerator dot com. I will study the contact points and see if I can simulate the system... and then play with the values to see the results.
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It's really quite simple!
One rotation of the motor pulley will impart a movement equal to its circumference to the idler wheel - say 10 mm.
At the same time, the idler wheel will impart a movement of 10 mm to the platter. This will be true regardless of the diameter of the idler wheel.
One rotation of the motor pulley will impart a movement equal to its circumference to the idler wheel - say 10 mm.
At the same time, the idler wheel will impart a movement of 10 mm to the platter. This will be true regardless of the diameter of the idler wheel.
That is correct, it can introduce a momentary "blip" in the rotation, but has nothing to do with the OP's questions regarding the "overall size" of the idler, which I'm discussing with him.
That is exactly right Galu.
To Tubestyle: as long as there is no slippage between any of the mating surfaces of the drive train (motor-idler-platter) the whole affair is dependent on the motor speed, period.
Thanks everyone for the information, comments and recommendations. I've learned more about the idler drive.
I will try to rejuvenate the rubber first, and see if it will help. Last resort, I will send my idler wheel to V-M audio Enthusiasts. Shipping to Canada is $32
but I could not find options on this side of the border.
If Gary Stork at VM (great guy to deal with) can't supply a replacement, perhaps Terry can.Thanks everyone for the information, comments and recommendations. I've learned more about the idler drive.
I will try to rejuvenate the rubber first, and see if it will help. Last resort, I will send my idler wheel to V-M audio Enthusiasts. Shipping to Canada is $32
I believe he's in Michigan.
http://www.terrysrubberrollers.com/Contact him for more info.