South Bend 9a (undermount) Belt walks off under load

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dansawyer

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The lathe is a circa late 1945 South Bend 9a with undermount drive. The table is a custom made stainless steel table. In forward the belt runs centered at about 1/8 inch toward the tail stock, in reverse the belt is centered.
If when running in forward and back gears engaged I firmly, but not straining, put a load on the spindle the belt walks off the counter shaft toward the tail stock. When in reverse the belt runs off the spindle pulley toward the left, away from the tail stock.
The belt is an accu link size B 5/8. When running without load it is relatively smooth and quiet. However due to the construction there is some ability for the belt to distort. It is not immediately obvious to me why under load there would be a preference to distort in one dimension over another. (Whatever belt that goes on this lathe has to be spliced. The pulleys are fully encircled by the pulley mount casting. There is no way to mount a continuous belt.)
The table is solid and firm. The motor mount is well secured and stable. I am skeptical of the mounting system distorting under load as the first suspect. However I have to say that is where I am inclined to start.
I would appreciate any input on this.
Thanks Dan.
 
I do not know why the link belt acts in this way, but before trying to solve a link belt issue, I would purchase a flat belt from Al Bino Machining. I purchased a hybrid flat belt for my Southbend Heavy 10. This has inner rubber and outer nylon. They scarf the ends and provide two adhesives in the kit, one for the rubber and different one for the nylon.

With this kit you can make a continuous flat belt drive. This may track better than the link belt since the cones were designed originally for flat belts.

Al Bino Belting
 
I agree with Dave. I like and use link belts on v pulleys but think flat pulleys are not a good fit. I used an automobile serpentine belt that I scarfed and sewed together myself on my 10K. It barely took any pressure on the pulley to hook up without slipping on the flats. It worked 100 times better than the leather belt that it was originally equipped.


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I too stitched together an automotive serpentine belt for my heavy 10L, it never slips and runs true and quiet.
 
dansawyer said:
The pulleys are fully encircled by the pulley mount casting. There is no way to mount a continuous belt.)
Click to expand...
There is always a way. How did the pulleys get in there? Many Moon ago the shop I worked in still had some line shaft machines and we used to make up new belts,(Ox hide strips boiled in Beeswax, Linseed oil and Neatsfoot oil with a bit of Rosin),and stitch the last joint in place. You can also use 'Alligator' strips but they are noisier than stitching. Flat belts need a bit of flexibility. Are the wheels aligned? Are the shafts aligned? Is the crowning good on the crowned wheel?
 
NortonDommi said:
There is always a way. How did the pulleys get in there? Many Moon ago the shop I worked in still had some line shaft machines and we used to make up new belts,(Ox hide strips boiled in Beeswax, Linseed oil and Neatsfoot oil with a bit of Rosin),and stitch the last joint in place. You can also use 'Alligator' strips but they are noisier than stitching. Flat belts need a bit of flexibility. Are the wheels aligned? Are the shafts aligned? Is the crowning good on the crowned wheel?
Click to expand...

Not on the SB underdrive. In the second pic, you can see the bottom of the headstock. The belt has to go around both sides of it. No way to do it, but to glue/sew,assemble the belt around the frame.

drive belts.PNGSAM_1225.JPG
 
Hello Superburban,
I looked at the pictures you have supplied and immediately thought remove the spindle. I had an old Thornton lathe years ago and options were remove spindle or sew a new belt in place. Not a big job to take things apart and if a belt needed replacement the bearings probably need re-metaling and everything given a good clean. I did a quick search on the YouTube and found a couple of videos. One by mrpete222 in his Southbend section shows a headstock and drawings over the underdrive and he states that to replace a belt you remove the spindle or sew one in place. At the factory the belt would have been fitted before the spindle was bolted down. The other video is of a guy replacing the belt in situ with a glue up.
 
Look at this screen grab from your second link. There is a slot formed by the outer wall, and the under frame of the head stock. That is all one casting. The belt has to be threaded thorough there. No choice, Maybe not the best design. Mr Petes 9" is different, it does not have the outer wall.

sbhead.PNG
 
Interesting, gotta file that in my cerebral storage unit
 
I thought the same way until I got my SB 16, I decided the belt is good enough until I need to replace it.
 
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