# Pulleys For Dc Motor Conversion?



## WesPete66 (Jun 3, 2015)

I am in process of switching over to a DC motor system for my QC54 lathe, and I discovered the old belt pulley has been damaged & should not be used.  What type and size of pulley is correct for use with the DC motor? Is there any reason to have a two groove pulley any longer (does the variable speed aspect negate the need for 2 grooves?)? What have others done/used?
Thanks,
Wes


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## Kernbigo (Jun 4, 2015)

i ran a thread mill motor on my atlas for 10 years tell i sold it worked great, you only need 1 pulley, can't r ember the size for sure, try 1 1/2" should work ok. I think i ran it in the spindle not largest but the next one down.


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## wa5cab (Jun 4, 2015)

The overall speed range in either direct drive or back gear (you will still have back gear) varies by a ratio of 12.36:1.  If you try to run the machine without the countershaft, you will have to vary the motor speed over that range.  I would expect you would have motor cooling problems in the lower 1/3 or 1/2 of the range.  You would be better off long term keeping the countershaft.  But you should be able to use a single step motor pulley and outer countershaft pulley.  What diameters you need will depend upon what the maximum CCS RPM rating of the motor is.  If it is around 2000 RPM, you could use the larger motor and smaller countershaft pulley diameters (to save doing a bunch of calculations on pulley diameter and belt length).  This would give you approximately the same upper and lower spindle speeds on each spindle pulley step with a motor speed variation of only about 3:1.  Should last longer and work better than trying to get it all out of the motor.


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## Silverbullet (Jun 4, 2015)

The main reason for the dual pulleys .is to transfer as much power to the machine without slippage. With one pulley if you get a tough job even in back gear it will slip possible burning of the belt is for sure.


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## wa5cab (Jun 4, 2015)

The only Atlas lathes with dual pulleys are the late model 12", and only on the spindle belt.  There is only one motor belt and one speed select belt.  And for reasons that I won't go into, I have been running my 3996 on only one spindle belt for nearly 30 years now. 

All the rest of the Atlas and Atlas built lathes except for the MK2 have 2-step and 4-step pulleys with each step having a different PD.  So they run one motor belt and one spindle belt.  All belt slippage on an Atlas is caused by one or more of four things:

worn out belt
contaminated belt
improper belt tension
link belt


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## Kernbigo (Jun 4, 2015)

i ran mine for 10 years direct drive,no counter shaft had no problems overheating or lack of power, 1 hp thread mill motor. If any one is interested the lathe is for sale now the guy i sold it to is selling his business and does not have room for it. O's small engine repair so.beloit il. 61080 $1500 atlas 10" x 54"


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## John Hasler (Jun 4, 2015)

Kernbigo said:


> i ran mine for 10 years direct drive,no counter shaft had no problems overheating or lack of power, 1 hp thread mill motor.


It might overheat if you ran it continuously at full power and low speed.  An external fan would probably prevent this.


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## Kernbigo (Jun 4, 2015)

my tread mill motor had a fan on it


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## WesPete66 (Jun 4, 2015)

Yes I am keeping the counter shaft and all, just replacing the motor with the DC unit.
I just checked and my DC motor is rated 3250 rpm. I will start out with a single groove pulley of a smaller relative diameter. That might keep the motor speed in the upper ranges & not lugging. Does that sound right?  I see now there will be several ways to set up to get any given spindle speed. Lots of flexibility!
I think I need one of those optical laser tachometers now! 
Speaking of motors, is there any interest out there in an original Atlas 1/3 hp motor? Do some prefer the original equipment?
Thanks again,
Wes


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## Ulma Doctor (Jun 5, 2015)

you can buy a commercially made motor pulley a nominal size 1 to 2" would most likely work for you.
as long as you keep the motor from getting too hot at low rpms you'll be just fine
keep in mind that the original motor was in the 1725- 1760 rpm range, that would be about 55% of the new top motor speed.(at 1:1 ratio)
lubrication of the headstock will become a concern,

you also have the real possibility of very high spindle speeds and the dangers associated with that too, as a heavy thought to consider.
i don't want to sound like a wet blanket, but i'd like to see you be as safe as possible
i wish you the best


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## wa5cab (Jun 5, 2015)

Kernbigo,

The fan performance decreases as the motor RPM decreases  To a first approximation, the motor RPM is directly proportional to the applied voltage.  The current draw, however, does not fall off proportional to the voltage or RPM.  So at low RPM, the fan may not adequately cool the motor.  You partially offset this problem by using a motor 33% larger than the maximum recommended HP.  But even so, if you had ever run it very slowly with a significant load, it would undoubtedly have overheated.

Wes,

If the motor that you are going to use is 3250 RPM max and is rated to run at that RPM continuously, then you should calculate the motor and countershaft pulley diameters required to get 2072 (or thereabouts) spindle RPM with the motor running at 3250 and using the same motor belt (so that the belt tensioner will still work to tighten both belts at the same time..  The required diameters will be somewhere between the existing two diameters of the two standard 2-step pulleys.  For comparison, If you were to use the existing low speed steps, the maximum spindle RPM would be approximately 1290 RPM at 3250 motor RPM.  On the high speed steps it would be 3904 RPM.  And probably quite dangerous.  But in any case, you don't want to use anything on the motor that is smaller than the small step as it is near the minimum recommended for a B-section belt.


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## rdhem2 (Jun 9, 2015)

The Ulma Doctor speaks wisely.  Heed his advice.  I also agree, once you drop below 30% of rated motor speed you have pretty much lost all cooling effect from the motors fan.  Is there another reason for the change other then the fact that you have the motor and can do it?  I have done fine on my little Logan with six speeds for over thirty years.  Change the geometry of your tooling and feed rate.  You will be surprised at what you can do without altering the speed!

Now lets get back to making *SWARF!*


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## Kernbigo (Jun 9, 2015)

I respectfully disagree i ran my Atlas for 10 years,never a overheating problem. Now i have a 9" south bend wide bed with a thread mill motor,not a problem, theory is great practice is better!


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## wa5cab (Jun 10, 2015)

Yes, but you had a motor on the 10" that was twice as large as recommended.  Fan efficiency versus rpm is practice, not theory.


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