Re: Increasing the speed on Southbend 10-16” Models. Late 1930s-late 1940s.


I don't doubt the validity of the concerns expressed regarding the potential for damage to bearings and I appreciate being made aware. However, I was regularly running my OEM 1931 3/4hp GE 3 phase original motor on my 1931 13" SB lathe with a TECO VFD for 5 years without any known problems at all. Recently, I decided to switch to a 1.5hp modern 3 phase Baldor motor for more horse power, and the added advantage of lifetime lubricated bearings, and now that upgrade is running really nicely too. So, I suspect the bearing damage concerns may be more likely to show up in commercial type heavy use than the type of use we might see in hobby machining (about 15-20 hrs per week use in my case). I'm personally really happy with the flexibility that my VFD provides. I also added a rubber belt drive with the new motor upgrade because the leather belt couldn't transmit the upgraded power without slippage. Now, the lathe motor runs so quietly that I cannot even tell if the motor is on without looking at the switch position. And, I can take heavier cuts, even on larger work pieces. I had to machine out a new 2 groove pulley for the larger motor shaft diameter. Fortunately, my 9x20 handled that machining quite well while the SB was apart. In all, I ended up replicating the OEM rpm specs very closely - just more torque and hp.

Dave B.

On 7/10/2020 4:06 PM, glenn brooks wrote:
Thanks Milan,

Most of my motors are older builds. I�ve thought many times about adding VFD�s. After reading your comments, glad I stayed with my RPC.


On Jul 10, 2020, at 12:49 PM, Milan Trcka <milan.v.trcka@...> wrote:

My understanding of the problem lies in the damage to the motor bearings. The high frequency components of the drive currents in the stator generate currents in the rotor. Some of these RF currents flow through the balls of the bearings and erode them through discharge machining. This occurs as the balls roll on the surfaces and some spots generate small arc. As the spark damage progresses, more and more of the sparking is concentrated at the site of the original damage until the bearing fails. Newer motors have brushes that ground the rotor to eliminate this problem. I would expect this problem be less severe in low power motors. But the bearings would be smaller so perhaps not.

One of the articles:�


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