View Full Version : motor for south bend 9A

07-09-2007, 09:59 PM
my SB9A motor is a very old craftsman unit with 1/2" shafts out both ends. the shafts are quite tore up and motor is starting to vibrate some. my question is that atwoods farm supply sells a 5/8" shaft 1/2 HP fulley inclosed motor that can be wired to turn clockwise or counter clock wise. will this work for my lathe or should i look for something else?

07-09-2007, 10:46 PM
IT should be fine. make sure it is 1725 RPM. My south bend has a 1/2 HP Leeson motor . Factory unit.

07-09-2007, 10:49 PM
You can get up to a 1-1/2hp motor on the same 56 frame base.If it were me I would bump up the hp to 3/4hp.If you can get an open drip proof motor for the same or less money I would also do that,since the TEFC motor will have a fan shroud and conduit box that may get in the way of things.Might be a good idea to set the old motor next to the new prospect to see if everything will fit before buying.

07-09-2007, 10:55 PM
i'll get some more info, i think it was in the 1700 rpm range. and is a capasitor start motor.

Mike Burdick
07-10-2007, 01:41 AM
If your lathe has flat belts I don't think additional hp would help you any - the 1/2 hp motor you suggested should be okay.
One other option you might consider...
Since you are going to get a new motor you might look into a 3-phase motor and VFD combination.

Your Old Dog
07-10-2007, 07:04 AM
The 1/2hp on my 9a has enough steam to completely mangle up projects if something goes west. Mine uses a flat belt but I up-converted to a high-tech man made material that apparently has zip stretch in it.

07-10-2007, 07:04 AM
please excuse the nievness, but what are the advantages of 3 phase.

07-10-2007, 07:48 AM
I ask why not pop the ends and press in new bearings or bushings? Cheap and the motor is probably better quality than alot of new ones. As for the chewed up shafts dress down the burrs, they dont have any effect on the function unless you are constantly taking the pulleys on and off.

07-10-2007, 08:40 AM
Depending on the old motor it may have a feature that the new motor doesn't. The original SB motors are 117vac "instant reversing" type. This means that they can be "plug" reversed even when operating at full rpm. This is usually only available with three phase motors but is a feature of the SB motor. In detail, it means that you can throw the switch to reverse at any time and the motor will reverse almost instantly, within less than one second.

If you are going to replace the motor with another 117 vac single phase motor you should ask about a "plug reversing" instant reverse motor. Here is an example:


07-10-2007, 08:46 AM
please excuse the nievness, but what are the advantages of 3 phase.

If you hook the 3 phase motor up to a VFD - (you can get a vfd to run off 115
or 220 single phase ) you now have a (with in sensible reason ) variable speed motor, that you can reverse at the touch of a button,go further and use the aux strip provided with the vfd and you can add controls to the machine like start stop reverse buttons, limit switches, ectect whatever

the vfd can be programmed to run in a number of differant ways , I have abig shaper , i run the motor up to speed graduly in case i have left the clutch out , or you can brake the motor quickly especially if you put in a braking resistor.

Lots written on 3 phase and vfds so look through the archives of here and PM

One of these days I am going change my SB9 over to 3 p and a vfd

07-10-2007, 09:18 AM
Hey, guys Watch those instant reverse motors on any lathe with screw on chuck. Could be a severe problem. JIM

07-10-2007, 09:27 AM
I have never had a problem with the chucks on my lathe and I don't hammer them on either. The reversing action isn't as sudden as it is with a three phase motor. It's a fast deceleration/acceleration without the jolt that three phase has (without VFD). Another benefit of the instant reverse type is that they are rated for a much higher proportion of start/stop cycles than a regular motor.

07-10-2007, 10:26 AM
Heh...someone beat me to it:)

I was going to say "it means that you can throw the switch to reverse at any time and the motor will reverse almost instantly, within less than one second."....and spin the chuck and the work off on the floor.

I would think that running a threaded chuck in reverse....much less instantly reversing...when you are applying force in the "remove the chuck" direction is pretty iffy.

No one has said *why* you would plug reverse a motor on a small lathe? I am guessing threading operations in which you don't want to pull the cutter from the thread for the next pass?? I suppose in that case, there is limited force on the work other than the initial resistance to the rotational force from the weight of the chuck.


07-10-2007, 10:35 AM
I've wondered about reversing the motor with a threaded spindle. I've not tried it myself since I haven't had the need. I suppose it would be useful when threading to a shoulder and not opening the half nut.

The reversing action isn't as sudden as it is with a three phase motor.

Evan (or anyone), in what circumstance do you use this 'not-so-instant' reverse. I also have a SB9. Mine is the A model. Early '50's vintage. While I have run the motor in reverse (just to verify, yep it runs backwards alright :) ) I've never done so while turning something.

I wouldn't have a problem reversing If I were holding the work in a collet with a drawbar for example. Nothing to unscrew there. While I don't have any 3c collets I did make up a drawbar to use with MT2 collets (from my mill) and the reducing sleeve for small work. Just trying to incerase my knowledge on the subject.

07-10-2007, 10:37 AM
Instant reverse is very handy as a way to stop the lathe. You just "blip" the switch to reverse and no coast. It doesn't spin the chuck off. I have never had it happen, threaded chucks don't come off that easily (at least they shouldn't. Mine don't). The only time I have had a chuck unwind unintentionally is when backing out a stuck drill bit (large sizes). And yes, it is a very nice feature when threading.

If it were a major problem I don't think South Bend would have offered it. I also don't have problems threading in reverse. I also use reverse for polishing as it is safer and never have any problems. I also use reverse all the time when doing ordinary work as it can save tool changes on face end work.

07-10-2007, 10:38 AM
No one has said *why* you would plug reverse a motor on a small lathe?

Probably for operator stupidity, so you don't destroy the motor by flipping the reverse switch while the lathe is running.

A large portion of the SouthBend 9's market was schools and hobbyists.

07-10-2007, 10:43 AM
It won't destroy the motor if it doesn't have instant reverse. It just won't reverse is all. It keeps going whatever direction it was running.

07-10-2007, 10:50 AM
Ok. Sounds like a useful trick to know. I may try it sometime with nothing on but the chuck first though. I'd like to change the knob on my drum switch to something more like a handle first. I'd like something I can grip a little better with oily hands. Most pictures of SB's show a shaft with a ball on the end for the drum switch. Mine has round knob with finger holds molded in. Its action is a little stiff and the knob is smooth. Hard to grip when my fingers are oily. Also a longer lever would make it easier to 'blip the switch'.

I suppose the reverse isn't too hard on the gears if they are adjusted properly.

07-10-2007, 11:15 AM
please excuse the nievness, but what are the advantages of 3 phase.

Primarily, that 3-phase motors have a much smoother torque profile than single phase motors.

Most commerical machinery is built with 3-phase motors for this reason, and many of us here (and on PracticalMachinist, and on the woodworking forums) have replaced single-phase motors with 3-phase motors, and noticed a huge improvement in surface finish.

On single-phase motor, the voltage to the stator cross zero voltage twice per phase (120 times/sec for 60 hz), so the torque drops to zero, and pure inertia carries the rotor pass the stator.

On three phase motor, the 3 phases also cross zero 120 times a second, but the phases are shifted so there's always the same average voltage applied to the rotor, and the output torque of the motor is very smooth.

One of the guys on the Yahoo SouthBendLathe group made an excellent spreadsheet calculating the torque at any point in the 60Hz cycle for a single-phase and three phase motor:

1 phase vs 3 phase torque.xls (http://f1.grp.yahoofs.com/v1/8J6TRto9NyVOrORLaDaV96lWIcTa66XDwq2huYgxPJI4ggEEU0 aNQCEi1JHPpKhFBMA9Z8xyUYSX_e4ASqhajo_5G7msg3VwmW9X SA/1%20phase%20vs%203%20phase%20torque.xls)

This is a screenshot from his spreadsheet comparing the torque provided by a 3-phase motor (the red line), and the torque from a single-phase motor in (teal):


This shows the 3 overlapping phases of a 3-phase motor. You can see that anytime one of the phases is crossing zero volts, and not contributing to the motor torque, the other two phases are near their maximum torque, compensating for the "weak" phase:


07-10-2007, 11:16 AM
The instant reverse isn't a sudden jolt. I have a three phase drill press and when you plug it it sounds like it should hurt something so I turn off the phase converter first. With instant reverse it is a very fast but smooth slow down to a halt and then normal restart in the other direction. I would advise getting or making a switch handle of the lever type. I don't think it would be nearly as useful with a knob, especially for threading stops and similar.

07-10-2007, 09:24 PM
i have used the revers when tapping, i would not want to try and leave the cutter in the thread ( when single point threading ) and revers, with the play in the gear train i'm sure it would lose time with the thread and cause damage to the work peace.
i went ahead and purchesed the 1/2 HP motor from atwoods. i thought about the 3/4 HP motor but don't think it will help at all, can already slip the belts off with the old motor. as for refurbishing the old motor would have to weld up the shafts and turn back down. they are just to chewed up. i'll save the 3 phase and vfd for when i get a bigger lathe.