Motor Swap RPM Impact

[MarvinsP32]

I bought a Craftsman 101.21400 about 4 months ago with the plan to leap right into a restoration. Unfortunately, life got in the way of hobbies and I had to shelve it for a while. I'm just getting back to it and have a question I thought the group might be helpful with from a newbie.

The original motor is with the lathe (1/2 HP, 1725 RPM 110V) and I was considering upgrading it with a brand new 3/4 HP, 3450 RPM, 110V motor I have from a Grizzly shaper that I never use. I've read about people upgrading to a 3/4 HP motor in other forums/videos, but I've never heard them say anything about how they handle the 2X increase in RPMS.

I'm guessing it's by using a different pulley on the motor shaft, but I'm not entirely sure. Any thoughts?

 

[jbaccell]

Without changing the pulley's, a 3450 RPM motor would be much too fast for the lathe. The least complicated way is to replace the original motor with another 1725/1750 motor.

Thant's my $.02.

Good luck.

 

[markba633csi]

It may be difficult to adjust to the higher rpm- for operations like threading you need slooooowww
Have you considered cannibalizing a treadmill for the dc motor? Variable speed is great on small lathes

 

[Eddyde]

Don't use the 3450 rpm motor unless you reduce it to near the 1750 rpm the machine is designed for. IMHO it's an unnecessary "upgrade". A ½ hp should be more than sufficient for a small lathe like that. I have a South Bend heavy 10 that has a 3 phase ¾ hp motor. For years, I ran it off a static phase converter, meaning it only produced ½ hp, yet I never felt the need for more power.

 

[higgite]

I've read about people upgrading to a 3/4 HP motor in other forums/videos, but I've never heard them say anything about how they handle the 2X increase in RPMS.

What makes you think they increased the RPMs? Motor HP and RPMs don't go hand in hand. They come in all different combinations of HP and RPMs.

Tom

 

[Beckerkumm]

In addition to screwing up the rpm, you are actually downgrading the motor. A 3/4 hp 3450 rpm motor creates less torque than a 1/2 hp-1750 rpm motor. A 1 hp 3450 motor is basically the same but more poles are preferred for a lathe. Makes for a smoother motor. Dave

 

[RJSakowski]

I made that exact same swap on my Atlas/Craftsman 6x18 maybe forty years ago. I replaced the OEM pulleys on the motor and countershaft with 1.3" and 6" single sheave pulleys resulting in a 4.5:1 primary reduction in speed. This resulted in an approximate 10% increase in speed at the spindle.

 

[RJSakowski]

Replacing a 1/2 hp 1725 rpm motor with a 3/4 hp 3450 rpm motor and changing the pulleys to reduce the countershaft speed to match the OEM speed will result in a 50% increase in torque. Torque (lb-ft.)= hp x 5252/rpm

 

[MarvinsP32]

Replacing a 1/2 hp 1725 rpm motor with a 3/4 hp 3450 rpm motor and changing the pulleys to reduce the countershaft speed to match the OEM speed will result in a 50% increase in torque. Torque (lb-ft.)= hp x 5252/rpm

I figured that was the case but wasn't sure. I need to figure out how much to increase/decrease the size of the pulleys on the new motor's shaft to get the countershaft speed back to the OEM speeds.

I also thought Eddye's comment about just refurbing the original 1/2 hp and using that is well worth considering.

 

[matthewsx]

I'd say live with what you have until you can articulate what the need for an upgrade is. I've upgraded my lathes with 3 phase motors and VFD's so I have variable speed, that's far more useful than slightly more speed or slightly more HP IMHO.

John