Replace your idler wheel. I wouldn't say the warmth will cause any problems. Is the ROK in a massive plinth or a box?
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At 33 rpm, assuming no significant slippage of the idler wheel against the platter, and assuming there is some sort of mat between the platter and the LP that ought not to be a very efficient conductor of heat, unless it's made of copper, I cannot imagine that it could be "normal" for the LP to heat up as much as 30 degrees F during one 20-minute play. Something is amiss. I am thinking about the bearing assembly in particular.
I'm thinking the massive ply plinth does not allow enough ventilation. Seems like after 1 hour of use when I remove the LP, it's like it was in a warm car. I've had tons of turntables, even a good number of idler drives, but this is my first idler ROK. With the clunky motor, the heat generated isn't so much my concern, its record wear from a warmed, softened LP.
I'd defer to any real scientist, but warming vinyl (up to some point which I don't know) might make it less susceptible to damage, not more.
Cooling vinyl to below room temperature reduces its elasticity. Vinyl subjected to impact stresses in that state will more easily chip or crack.
Warming vinyl to (somewhat) above room temperature presumably increases its elasticity. Vinyl subjected to impact stresses in that state would flex, then return to its original shape.
If the above is correct, *slightly* warming an LP should reduce the likelihood of stylus-induced groove wear or damage.
Caveat #1: there is *some* temperature above which vinyl loses its elastic tendency to return to its original shape after being deformed. Hot vinyl behaves more like a liquid than an elastic solid. This explains why LPs are made by pressing hot vinyl into a mold, and why the vinyl must be allowed to cool before removing the mold.
Caveat #2: warmed vinyl might be less susceptible to stylus-induced wear or damage, but the sonics of a softer groovewall would certainly be softer too; less HF information, less low level detail, softened microdynamics.
It's a compromise, like eveything with this wacky, loveable medium.
This uses a General industries induction motor, no phasing cap is needed-- but I have not taken apart and cleaned the motor. Bearing looks smooth, no visible issues or wear. I've seen some big idler tables with fans underneath, but that would introduce more noise and buffeting. If I cant reduce the temp, I'll just use the ROK on cold winter nights.
Cocoa, Have you felt around among the various culprits we've mentioned? The thing that is hottest may likely be the source of the heat. If the LP is very warm due to the motor, the motor must be very very hot, for example. If you own or can borrow a high quality multimeter, many of them have an optional heat sensor which permits very fine estimation of the temperature of any object. But meantime, just feel around under the tt (with the plug removed from the wall socket) after a few hours of use. Check the platter, the bearing assembly, the motor. One or more of those is going to be very "warm".