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02-23-2007, 06:18 PM #1
I'm thinking of converting my 11” Rockwell to direct drive and ditching the Reeves drive. It needs new belts and the sheeves are worn, though not junk. It has the big belt variable drive on the extreme left side of the cabinet, with a jack shaft converting to dual v-belts that drive the spindle at about the center of the cabinet. I won't be starting any time soon as I need 3hp (prefer 5hp) VFD which is not in the budget at the moment, but wanted to start the research process.
Anyway, I found a nice Baldor 3hp but it’s a bit long for my cabinet, in fact it is only a few inches shorter than the max width. But, it will put the pulley right below where the top Reeves drive belt is/was (ie. about 2” from the left side of the cabinet), and that is with the motor almost touching the cabinet on the right side! This seems to be a very nice “Inverter Duty” (so says the sticker) motor and should be ideal I think, given that the OEM motor was 1hp and swing is only 11”.
So I'll still be using the original jack shaft to get the v-belts lined up with the spindle pulleys. I already rebuilt it with new shaft and bushings, and the brackets/alignment/adjustment is already built in. I’ll simply replace the Reeves Drive wide pulley with another belt system going straight up from the motor. That allows me to choose any pulleys at my convenience and avoid the issue of building a bunch of stuff and dealing with the offset spindle pulleys.
For main drive pulleys, I’m thinking I may just use a single automotive serpentine in 1:1 ratio on moderately large pulleys (6” dia?). That gives me relatively cheap and smooth running belts that are very strong and long lasting, along with common and easy to find pulleys, albeit needing adapting to fit the shafts.
That would let my 1750 rpm motor drive the spindle at 1750 rpm at 60hz. The motor is rated “inverter duty”, so should withstand maybe 15-20 hz on the low end for 525 low rpm in direct drive, and 250 rpm over the stock maximum (1500 rpm) at 60hz. Then again, a slight under drive would let me hit that ~500 rpm (or a bit lower, I may shoot for 400? on the low end?) without going so low on freq, and most folks agree that 3ph motors can run fine for moderate duty cycles at up to 200% (i.e. 3000+rpm) so I would still have more rpms in the motor than the spindle could ever stand. For the low freq, low rpm stuff, it will need an aux cooling fan if you want to go below 30 hz, but that’s not a problem I think, particularly if I gear it lower than 1:1 so I don’t underdrive the motor as much or as frequently...
So, how does this sound? Have I understood all the aspects correctly? Any suggestions on VFDs? I don’t need much except variable freq and 1ph 220V input, so I think most any cheap VFD should work. And it seems I can use a 3 hp 1ph rated, or a 5 hp 3ph rated VFD only wiring L1 and L2 on the inputs. So that’s what I should be looking for on EBay etc. Anything else? Comments?
02-24-2007, 01:13 AM #2
I would not use a serpentine belt. A cogged timing belt can be run with less tension than a serpentine belt. Set the speed at 60 hz a little lower (1200/1300)to retain torque and use the vfd to give you the high end rpm.
02-24-2007, 04:12 AM #3
Sounds good. Gives me a viable rpm range of about 600 to 2000 without going too far in stressing the motor or losing torque. And could be stretched into ranges from 400 to 2400 if required. I know that seems pretty obvious to most of you, just checking my understanding...
Anyone think it is worth it to go sensorless vector for this application?
BTW, this may get accelerated onto the fast track now. I was turning some brass valve stem material this afternoon (damn tough too, more white than most) when I noticed a sort of “thugh-thugh-thugh...” coming from the drive cabinet at around 1000 rpm. Looked inside and the floating Reeves Drive arm was sorta oscillating up and down. Looks like one of the drive belts is separating. Called local industrial belt supply, $54 + tax EACH. Not much point in spending $110+ on patching up something I’m thinking of replacing anyway, so I guess I’ll add another $150+ to it ASAP to get a VFD. <sigh> And I really don’t have the budget right now, but I’ll very quickly be short a lathe if I don’t caugh it up...
02-24-2007, 11:15 AM #4
I used a 1140 baldor on my repower. It was a 1/2 horse but may want to look to see if they have larger in that rpm range ($50 on ebay, was new). I used a web program to set the pulley ratio to get the same jackshaft speed as the oem setup so that I could get equivalent speed at 60hz. That way I can still use the 4 pulley belt system and the backgear at OEM specs, but use the variable for high end (atlas/craftsman). Used those heavy cast iron pulleys from Grizzly.
Only think I think I need now is a tach. I mainly set it by what feels good for aluminum and drill rod but a tach would be nice for turning new materials.
02-26-2007, 01:35 AM #5
Is there any particular benefit to a 1140 rpm over a 1750? Seems like not much difference, not like it would be with a 3500 rpm where you would need serious underdrive and/or excessive low freq on the VFD. One thing I do like about the 1750 is that the moderate underdrive (roughly under drive 2/3 to ¾) gearing on the 1750 would keep it self cooling better while increasing power at the typical rpm ranges, resulting in better power at low freq as well (where it is needed most for large diameter low rpm stuff).
02-26-2007, 08:26 AM #6
Do you realize that when ditching the Reeves drive, you will be sacrificing torque at low speeds? Any gear / pulley ratio speed changer increases torque as speed decreases. With a VFD, torque stays the same as speed decreases, so by comparison you will have less torque at lower speeds than you used to. If you are OK with that then it will work fine.
If you are planning on running at lower speeds most of the time, use the slower base speed motor because it will have more torque to start off with. But if you use an 1140RPM motor and want to run it over speed to 1750 some times, you will now be LOSING torque once you get over base motor speed. Again, if that's OK for the work you want to do, then go for it.
02-26-2007, 01:23 PM #7
WHa Bad Dog says. You can't get away from the physics of power transmission. You need some form of reduction or your available power will decrease in proportion to your speed.
An alternative would be to install a 10 HP motor drive ensuring you'd get 1 1/2 HP at was once the Reeves drive min RPM. Don't get nervous about installing a monster motor. Your spindle was designed for whatever the Reeves drive put out in terms of torque which would be about 6 times the full speed torque.
Your power meter won't care either. You can use the same breaker. The VFD accel if set up for 3 seconds will not generate as much of a starting surge as the old 1 1/2 HP motor across the line. Thanks to the PWM design and its variable duty cycle the full 10 HP torque (30 ft lb) is available at min RPM but the power meter sees only about 1/6 of the motor's 48 single phase translated full load amps.
The alternatives are a step pulley or a lame drive having little power at min RPM.
02-26-2007, 08:47 PM #8
Don't know what you are turning nor what tooling you are using but if you are using high speed steel and turning mild steel then you need to run much slower than 600 rpm. in fact, you need to run 300 rpm for a one inch diameter work piece and slower as they get bigger. this is tough stuff for a VFD. You would need to do as Forrest Addy suggests and use a much oversize motor. A sensorless vector drive will give better low end torque but you are faced with another problem--motor cooling. If the motor will not cool itself below 30% speed then you have no advantage to using a sensorless vector drive. Just more to think about.
02-26-2007, 10:30 PM #9
And threading....personally I do not thread at 600 or 300 rpms.....
02-26-2007, 10:42 PM #10
I still have back gear guys. Those were direct drive rpm numbers. I don't recall the back gear ratio, but it's got to be at least 10:1 based on what I recall (I'm in Atlanta, so can't check). So wound down to 500 rpm at the motor would give me around 50 at the spindle in back gear.
And I thought a 3 hp motor replacing a 1 hp motor would be enough upgrade. I thought that's about what people were running, but I guess not. That 3hp motor will BARELY fit in the cabinet, no way I can fit a larger one without completely butchering it unless it is the same length but larger diameter.
So, what is the formula for torque loss? The Reeves drive achieves 1:1 with a 1 hp Dayton 3 phase motor at about 1000 rpms. High of 1500 is max OD and low of about 600 is max UD. So, that's roughly 2:1 Under Drive, or double the rated torque. If I geared the 3hp motor to 1000 rpm at the spindle, that would be almost 2:1 torque advantage there (being geared from 1750). Then OD at 200%/120hz gets me 2000 rpm top speed at lower torque due to over freq, no big deal since that will be spinning 1/2 or smaller stuff. And 30hz gets me 500 rpm direct (~50 back gear?) with whatever torqe a 3hp motor has multiplied by the belt underdrive, or roughly double rated. Am I missing something?
I don't see how a 3 hp motor at around 30 hz would possibly be much less than that? And I certainly don't see how it would take a 10hp motor at 30hz to equal a 1 hp at 60hz.
02-27-2007, 12:15 AM #11
Sounds like you have it covered then.
02-27-2007, 01:23 AM #12
Yeah, I thought so, but not sure, some of the guys have now got me worried...
02-27-2007, 12:09 PM #13
A little bit different application but, I put a 3 hp motor-(1725rpm Marathon) on an old Enco step pully mill with a vfd. The guy regularly uses it to tap 5/8- nc in small weldments. Doesn't use the back gear at all. We sized the timing pulleys to give 2500 rpm with the motor fed 120 hz. I think a 3 hp (if it fits) would be a good motor on a Rockwell 11. Again size the pulleys to give you a reduction from the motor speed to spindle and use the vfd to overspeed it.
02-27-2007, 12:17 PM #14
Aw, c'mon Russ - don't worry, be happy.
I'm afraid I started all this.
Bad Dog and I both have Rockwell 11s, and for lack of having any actual work for the lathe, I started toying with the idea of eliminating the Reeves drive given how a vfd seemed to eliminate the need for adjusting it.
A step pulley was my first idea, but that's pretty involved given the cramped working quarters in the cabinet.
BTW, the Reeves drive provided a 7:1 range and backgear gives a 6:1 reduction per the manual.
I found a 2hp motor and a 3hp vfd at a local factory closure and figured I'd start there. I got the motor in there more or less ok and got it so the belts don't rub against the hole through the chip pan.
During all the testing, the vfd (a Warner/Seco Nextdrive) worked fine, but of course once I set out to make chips in anger, it started faulting out all over the place so I've yet to make any useful observations.
02-27-2007, 09:50 PM #15
JH: Thanks for the info, that does make it sound reasonable.
Richard: Thanks, particularly for the ratio info. Sitting out here in Atlanta, I had no way to check.
And yeah, it's your fault! I had kicked around the idea a few times, but my Reeves was working well enough, so it was never a real goal. Then I saw your post about the same time I found the 3 hp Baldor for $20. Final straw was a few days later when one of my belts started to separate and I figured it was time to really start thinking this through, so here we are...
02-27-2007, 10:07 PM #16
I have extra belts if you want them. You know the price is right!
They have quite a set to them, but one time I let the lathe idle while holding a heat gun to them and I think that smoothed them out some.
02-27-2007, 10:27 PM #17
I might just take you up on that. Kind of a shame to ditch the reeves after I made new bushings and shafts for it a while back...
More thoughts on the applicability of a 3hp motor.
So, if a 3hp 1750 has roughly 3 times the torque of a 1 hp 1750, and since I don’t know what those numbers are (nor care to calculate) lets use unit values of “1” for the 1 hp and “3” for the 3 hp.
Based on Richards 7:1 max reduction in the Reeves drive, we have a low rpm limit of 250 rpm in direct drive. Can that be right? I didn’t think it went that low? So that would be 7 torque units for the 1hp motor. Assuming 2:1 gearing on the 3 hp motor to achieve roughly 900 rpms at 60hz, and constant torque down to 20hz, that gives 300 rpms at 6 torque units. A bit less torque with a slightly higher low speed, but enough to matter? And at 6:1 on the back gear, my min rpm will be 50 rpms with 36 torque units vs. an OEM 42 torque units.
But that also gives me a “safe” max rpm of 1800 rather than the OEM 1500 rpm max. If I limit the new drive to 1600, then 800 rpms is available at 60 hz and 250 rpms at 20hz. That gives about 6.5 torque units in direct at 250 and over 39 torque units at 42 rpms. Seems pretty darn close.
02-28-2007, 03:12 PM #18
To be precise: the manual gives a speed range of 220 to 1550 in direct drive, 45 to 250 in back gear. A little inconsistent, but the numbers hang together well enough.
While I've persuaded J Hendricksen to opine that the spindle bearings would withstand 2k rpm, and maybe even 3k rpm, I just received Enco's March promo email featuring their 12x36 bench top lathe which sports very similar specs to our Rockwells, right down to the tapered headstock roller bearings. And whaddya know? The speed tops at the same 1550 rpm. Why that is?
02-28-2007, 03:49 PM #19
"The speed tops at the same 1550 rpm. Why that is?"
Class 3 bearings instead of Class 5.
02-28-2007, 09:44 PM #20
No idea on the Enco stuff...
But does anyone have any comment on why/if the 3 hp is really underpowered for a motor swap? Is my estimation and "rough figuring" way off the mark? If this really needs a 10 hp motor to repower an 11" 1hp lathe, then I'm clearly missing something in a VERY big way...