Belt riding too high in Panasonic SG2080 Turntable

[Old Sparky]

After fitting a replacement motor and control PCB to the turntable from one of the above music centres, I find that the belt(recently replaced) rides up the motor pulley and jumps off. Given that the motor mounting bushes,  i.e. grommets, are now nearly 40 years old, my thoughts are that these should be replaced, although originally I had the idea to remount the motor's mounting bracket a few mm higher above the baseplate by fitting washers under the bracket's mounting screws, which, in theory, would raise the motor a few mm, and achieve the desired result.  Which, in the opinion of other members, would be the best way to go?

[Mark Hennessy]

I presume the motor has a crowned pulley?

There's a good explanation of how these work on this page: https://woodgears.ca/bandsaw/crowned_pulleys.html

(Be warned - there's a lot of good stuff on there. Prepare to lose an hour or two )

Anyway, if the pulley is crowned, you have wonder what is pulling it away from the centre of the pulley. An obvious thought is the motor mounting - is the spindle of the motor parallel to the rim of the turntable, or is it leaning in, as a result of worn grommets, etc?

[Mark Hennessy]

Just to add, the video you can see at the bottom of that page demonstrates this perfectly (don't recall watching this before). Towards the end, he runs up a bandsaw, which has a tilt adjustment on the top pulley, and shows how the belt can be moved from the centre of the crown.

[Old Sparky]

The motor does have a Crowned Pulley, Mark, but with a flange top & bottom. AFAIK the motor is correctly positioned and the shaft/pulley asembly is verticl, but maybe new grommets will solve the problem.

[Mark Hennessy]

Well, there's not much else to go wrong. The bandsaw video clip is a vivid demonstration of how a tiny, basically invisible misalignment causes the belt to wander off-centre, so clearly in your case the two can't be correctly aligned, no matter how it might initially appear. Raising or lowering the motor will have no effect, as the edge of the turntable will be flat. It's the angle of the motor relative to the turntable that matters.

[pwdrive]

Just to add, the video you can see at the bottom of that page demonstrates this perfectly (don't recall watching this before). Towards the end, he runs up a bandsaw, which has a tilt adjustment on the top pulley, and shows how the belt can be moved from the centre of the crown.

On large bandsaws the bandsaw blade doesn't always run dead center on the top crown wheel, it's offset to the left so that the bottom of the gullet overhangs the the top crown wheel by about 3 mm, if it doesn't the bands will start to crack at the gullet, once that starts and left running then the gullets either side of the first one will start to crack and on it goes, if left unchecked then eventually the original first crack in the first gullet will spread right across the bandsaw blade and will ultimately fail...catastrophic/deadly if any sawdust is packed tight between the bottom of the band on the bottom band wheel and the floor or base of the saw 'cos the band can shoot out like a snake.

With belt drive and crowned pulley, worn shaft bearings/shafts out of parallel can cause the belt to wander off.

Lawrence.

[tiv48]

I had this problem after changing the belt on a trio turntable .
I found the the new belt was too tight ,this was enough to pull the motor slightly off of true.
another belt from a different supplier cured the problem
Comparing the two the 1st one was 5mm shorter than the one that worked.
Rob T  

[Mark Hennessy]

On large bandsaws the bandsaw blade doesn't always run dead center on the top crown wheel, it's offset to the left so that the bottom of the gullet overhangs the the top crown wheel by about 3 mm, if it doesn't the bands will start to crack at the gullet, once that starts and left running then the gullets either side of the first one will start to crack and on it goes, if left unchecked then eventually the original first crack in the first gullet will spread right across the bandsaw blade and will ultimately fail...catastrophic/deadly if any sawdust is packed tight between the bottom of the band on the bottom band wheel and the floor or base of the saw 'cos the band can shoot out like a snake.

Out of interest, what does "gullet" mean in this context?

With belt drive and crowned pulley, worn shaft bearings/shafts out of parallel can cause the belt to wander off.

As I say, the shaft isn't parallel to the turntable - even if it isn't obvious to the naked eye. Whatever the cause - be it bearings (new motor?) or grommets - that's where the investigation starts. If replacing (or simply moving around) the grommets doesn't fix it, then judicious bending of the mounting plate might be needed.

[pwdrive]

Mark the gullet is what contains the sawdust as it's cutting through the timber, feed speed is related to band speed and gullet capacity, feed to fast and the blade will wander as the packed sawdust expands within the gullet.

The belt wandering is also similar to tape riding in reel to reel tape paths etc.

EDIT: Whoops....forgot the link, 'ere 'tis: http://www.carbideprocessors.com/pages/s...lator.html

Lawrence.

[Old Sparky]

I have found that by fitting a new belt of slightly smaller diameter, it now remains on the motor spindle. There are pre-et adjustments for playing speed which can be tweaked if neccesary, so hopefully it's a case of 'all's well that ends well. As the exsisting belt was riding too high and thus coming off the pulley, there was no way to compensate for that. New, thicker grommets would make the motor ride lower rather than higher, which is what is needed.