What's new
What's new

Lathe troublems: tripping the thermal overload relay at high speed

rustola

Plastic
Joined
Dec 11, 2010
Location
San Francisco
I just bought a new (to me) lathe for my shop. It's an Enco 14 x 40 lathe which is about 15 years old, and had been sitting for the past year or so (owner passed away, purchased it from the daughter).

Seems to be working fairly well for the most part with one exception. When I try to run the lathe at 1800 RPM, it starts up slowly, runs for a few ( maybe 10) seconds, then trips the thermal overload relay. At the next lower speed (1170 RPM), the lathe will start slowly but eventually get up to speed in 5 seconds or so.

To my (possibly naive) view, it seems that the motor is "working hard" in both instances, which doesn't seem to be a good thing. To this end, if I remove the chuck, the lathe will get up to 1800 RPM and seem to run OK (not the most useful configuration though).

Any advice on what might be going on here?

Thanks much.

Rusty
 
Check to see the motor and controls are wired for 110V operation, if that's what you are running on. Also, no extension cords. Direct to an outlet, only.

You might change the oil and make sure the recommended weight is in it. Heavy oil will create drag at high speeds that can bog one down.

We have a similar one at work and it does not have enough power to pull a wet chicken turd off a doorknob at high speeds.
 
Some possibilities:
1) The motor is just too wimpy. What HP is it?
2) Your supply voltage is low.
3) The lubrication in the headstock is crummy (in which case bearings may already be damaged).
4) There's a pair of meshed gears in the headstock with insufficient clearance.
 
Thanks for the responses. Some more info:

The motor is 3HP, 3 phase. The lathe is powered by a rotary phase converter (powerwave xr5, 7.5 hp); about 15 ft of 10/4 SO cable to the machine.

I did open up the headstock gearbox to check things out ... seems like oil is getting everywhere. BUT, in the course of doing so I took it out of gear and tried turning the spindle by hand; there is a fair amount of resistance there - is this an indication of worn/bad bearings? How can I check for meshed gears?

Thanks all!
 
Has the motor been rewired to a different voltage?

The RPC appears to be adequate, 7.5 HP should start a 3HP motor OK.

"A fair amount of resistance"...... that isn't a *measurement* (You already knew that) so it's a little tough to decide if that is too much, normal, crazy stiff, or whatever.

Worn bearings normally are loosey-goosey, or, if they are re-adjusted for preload, are rough and not smooth.

It is perfectly possible for grease to settle in bearings that have sat, although SF isn't typically a hot enough place for that to be a real big issue..... But if it DOES settle, it may take being run a while to re-distribute the grease and get to normal resistance.

Then also, 15 years non-use can allow grease to separate and stiffen, leading to poor lube, heating, and stiffness. Also wear and damage if run too much like that.

Do the bearings get hot?

Do you know if they are greased or oiled? Even with an oiling system, that doesn't PROVE they are oiled.....
 
As far as I know, the motor has not been rewired ... was hooked up to 220V 3-phase.

My totally subjective assessment on the spindle is that it's pretty stiff - this is something I noticed previously when centering a piece on a 4-jaw chuck and turning the chuck by hand. The turning is not rough or loose, just stiff. Not sure if there's a way to measure this...

It would make sense to me if the lubrication in the bearings has settled, and I haven't run the lathe much yet (probably less than an hour total). The lathe was in Napa (gets a little hotter than SF), and sat for just a year, not 15 - it was used a week before the previous owner died last March. Would it be worth trying to let the lathe run at a lower speed for a while to hopefully redistribute lubrication?

Not sure how to tell if the bearings are greased or oiled - there is a little hole on the chuck side of the gearbox into which it would appear that splashed oil would enter and flow towards where I imagine the bearings are located (?).

Thanks!
 
Most lathes are *much* harder to start in high gear. You may need a larger RPC, or you may need to add some start capacitance. Very much doubt there's anything wrong with your lathe based on your symptoms.

metalmagpie
 
Forget about blaming the bearings. Blaming the bearing as the first instigator in an open problem out of the box is "blame the lame" conjecture. Look at simple things first. The 3 HP motor when running trom a phase converter may be too wimpy to start a massy 4 jaw chuck expecially especially with the jaws extended. Hence with a long start up til the OL pops. Another point is a 10" 4 jaw has considerable windage at 1800 RPM. The chuck itself has lots of intertia and since it's a 4 jaw it doesn't balance very well because there's no predicting the radial position at any given set-up.

Spindle preload certainly has an effect on the parasitic load of a running spindle but consider the whole problem. 3 HP at 1800 RPM about 9 ft lb so look into the problem in an orderly and rational fashion. The lathe transmission is not perfect. There is a belt drive and at least two pairs of gears in mesh for any spindle speed. Each of these results in a small loss (4% probably) which compounded is 1 / (1.04^4) = 86%. 86% x 3 = 2.6 HP so figure 7.4 lb ft available torque - if running from clean balanced three phase.

You have a problem and numbers can help you solve it. Without actual performance numbers, you're only pointing and guessing. Here's some simple tests: with the lathe off and the spindle in neutral, wrap a string around the chuck and connect a fish scale. Pull on the scale so the chuck rotates at a steady rate. Note the steady pull scale reading. Note the radius of the pull. Work the math to figure torque to rotate. Do the same tests with the spindle in gear for 1800 RPM so the string drives the whole train to the motor. Perform this test with the lathe cold and after it has a 20 minute warm-up at the third from the top speed and another 10 miilutes in second from the top speed - spindle running. Test again with the string and scale with spindle in neutral and with 1800 RPM engaged. Motor off of course. At the same time check the spindle bearing temp after the warrm-up. If should be no higher than 20 degrees F over ambient.

Round up a tong ammeter, record the motor amps on all three legs at different upper speed settings including starts noting espeically start Amps. Amp readings will make your information more complete.

Even the best rotary phase converter will de-rate the connected load motor to about 80 - 85% of nameplate ratings. A 3 HP three phase motor draws about 9 amps per leg on three phase. The weak generated phase results in less HP delivered by the shaft particularly on high inertia starts. It's not uncommon for a rotary phase converter to deliver about half the FLA to the generated leg on start and that somewhat out of perfect phase and power factor.

OK, now you have data. Work out the ft lb and make a chart.

Chances are the problem is primarily the weenie effect of the phase converter' generated leg and secondarily the inertia of the 4 jaw chuck. I bet the lathe will run as advertised on clean three phase. Have your consider $hit-canning the phase converter and using a VFD? Or maybe upgrading the motor to 5 HP (starter too of course)? No harm will come to your later by up-motoring the drive.

Not to say you can't run capacity 4 jaws safely at max spindle RPM but the machine will be happier starting a lower inertia load. Usually people run smaller chucks and collets at higher spindle RPM. I suggest you obtain a 6" 4 jaw if you need the set-up versatility and accuracy for smaller work.
 
Another thought is that the machine may just be cold. Are you starting this thing in top gear stone cold? My old L&S has plain bearings, but it has a hard time running at high speeds when cold. Not having an oil pump in the head, I always start off in backgear and let it run idle for a few minutes, then shift up to a low speed range, working my way up to full tilt over ten minutes or so if it is not summer time. I'd check that oil too. Highly likely somebody decided it needed rear end dope in it because of the gears.
 
OK, machine is 15 years OLD, and sat for a year. Got that wrong, I did.....

If all is well, the spindle might give resistance, but it will be "squeezy smooth", smooth but "damped" , like paddling a canoe in molasses.

bearings do not usually give much trouble, despite being suspect #1 for many folks. But stiff lube can cause trouble.

There is always a chance someone decided that oiled bearings needed grease, or greased bearings needed heavier grease. Bubba has a big family.

The machine should oughtta start a fairly substantial piece of work with 3 HP, so a lot of trouble with just a chuck seems wrong, unless it is one momma of a chuck.

I suppose the RPC might be at fault..... if it is not power factor corrected well ("balanced" many call it), it might fall down on starting a heavier load.

But at 1800 rpm, it's about 1:1 from motor to spindle, and rotational stiffness of the spindle may be an issue, especially when adding the drive train and mass of chuck etc.

But I'd still expect it should start up without popping the overloads, even if it isn't super happy about it. A torque measurement such as Forrest suggests is the way to get numbers on the stiffness.
 
Taking a bit to get wound up with a heavy 4 jaw chuck on 3hp isn't a shocker My hydra shift takes a few seconds to wind up to 1800 with a 8 inch 4 jaw or the 6 jaw for that mater. not that I ever run it that fast with the 4 jaw over 600 rpm and it starts to have vibration.
See how it behaves after it gets warmed up, high speed and large diameter don't really go together.
One other thought is it wired for 440 and not 220, wired for 440 will perform wimpy on 220 but it will run.
I have a similar power set up 7hp rotary, and it dims the work light when I start the motor not an issue on a lathe with a clutch, I suspect a VFD would fix the problem.
 
I had the same problem on my 14x40 - it was the rpc. The generated leg was sagging. Just put a voltmeter on T3-T2, and T3-T1 and watch it.

Take care with rpc "specifications" - is it 7.5hp "total connected load", or 7.5hp "starting", or do they not say? Upgrade the rpc, or replace with a decent vfd (rated for 3hp in CT mode).
 
Okay, some more info on this one. Thanks for everyone's contributions.

So I came in yesterday and again tried to fire up the lathe. No chuck at all, just spindle, and it still won't start up at 1800rpm. Figuring it might be just cold, I turned on the machine at ~500 rpm and let it run for 20-25 minutes. When I turned it off, the spindle was warm to the touch and there was noticeably more resistance when turning by hand. Machine still stalls out and trips the overlay relay at 1800, but now also stalls out at 1170 as well. Again, no chuck at all. So I let it cool down for an hour and no change, still doesn't start up at these speeds.

Kinda freaked me out, and I had some other work to do as well, so I put it to the side for the remainder of the day.

Today is a new day, and I came in with a fresh attitude. Tried the lathe and at least it's back to where it was before yesterday, starts up at 1170rpm but not 1800. Started thinking that maybe the bearings are not getting lubrication, so I spent some time trying to get oil to them by pouring oil into some little holes at the sides of the gearbox that are right next to the bearings (I'm fairly certain these are oil bearings, since I can see the rear bearing clearly when I open up the gearbox - it is openly exposed to the oil). That seems to help, with the spindle being (subjectively) maybe a little easier to turn. But still no dice on 1800 rpm.

I tried measuring the resistance in the spindle, using a technique similar to what Forest Addy suggested. I wrapped some webbing around a chuck, and used a bicycle scale (a hanging scale with a hook) to measure the resistance force. Multiplying the force by the radius of the chuck (4"), here's what I get:

Neutral: 56 in-lbs
1800 RPM: 60
1170 RPM: 64
90 RPM: 35

I neglected to mention previously that this lathe has a 2-speed motor, so the speeds listed above are those listed at the high speed. Shouldn't have any impact on the rotational resistance though. But this got me thinking about the motor itself, so I did a little checking.

Not my area of expertise, but here are the numbers from the nameplate:
The motor is type YD112M-8/4
1.5/2.4 kW (= 2.04/3.26 HP?)
230V , 60Hz
8.3/8.8 A
IP 44 , INS B, {delta}/YY CONN

So am I actually dealing with a 2HP motor here at high speed?

Any comments on the torque numbers and/or bearing lubrication?

I will get to work on taking some electrical measurements, but need an extra set of hands to help me before I do so.
 
Not going to be of much help to the OP, but work was a little slow this morning, so I pulled the peckerhead off the motor on the 1440. Just as I figured, the control transformer had been swapped to 120V, but the motor is still wired for 240. If we had a 240V circuit nearby, the simple fix would be to change the single wire on teh transformer and go back to 240V. Unfortunately, I have to put the motor back to 120V and there's no diagram for the controls... yet another reason to hate chinese lathes.
 
What info do you have on the RPC - is it a commercial unit?

Your motor is a 3.26hp in the high speed mode. I'd put money (yours ;) ) on your problem being the rpc output.
 
You didn't state the brand of the motor. My Nardini uses a Weg 3 phase 2 speed motor, 5/7.5 hp. I use a Rotary brand phase converter. They recommended a 25 HP unit because while Weg is a fine motor, it is 'THRISTY' (their word) on startup in high speed. Rather than the usual 2X factor for the converter, they recommended a 3X factor. It's worked fine for me for 3 years so far.

Like your lathe, mine has no clutch, which increases the load on startup.
 
Your torque readings seem odd. Generally, the lower the speed selected in the gear box will require greater torque applied to the chuck. For instance one turn of the chuck with the head stock set at 90 rpm means the motor must turn 20 times.
From my limited experience it sounds as if the motor has been rewired to 440V. In effect this puts each individual set of the three windings in series. For 220V operation of the three -2 piece coil sets must be in parallel. With the motor wired for high voltage you only get about one fourth of the power. Check the connections on the motor carefully. With a two speed motor it may be quite difficult. You may want to remove the motor and take it to a repair shop and have them cheeck it out.
Good luck!
 
Forgot about something that is worth looking at on this machine... The QC box on the machine at work was ruined when I got there a year ago. The right hand shift lever had galled up and broken, bending the shaft that drives the feed rod and leadscrew in the process. These lathes have no oil to the QC shifters and they are plain bearings, cast iron on steel, so unless you know to reach up under there and squirt oil directly on the gears, they can run dry and bind up. That would produce what would seem like a very heavy load on the spindle, but it would actually be coming from the geartrain. Worth checking all the bearings in the threading/feed geartrain to ensure they are free before blaming the spindle.

This machine was run dry until it wallowed the iron "bearing" so badly it wouldn't stay in gear. Somebody (names are changed to protect my job, as it was my know it all boss that aped the repair last time) bored it out and put an oil lite bearing in it, which promptly seized to the still galled shaft and shattered the shifter in to three pieces.

I managed to braze the shifter back together with a reinforced strap of banding material and drive pins around the outside, then bored it out for a bronze bushing to run on the shaft. It works, but it was run leaving a shoulder on the bushing to keep it from falling out. Sadly, the gears are so eaten up from the bad fits that it can't be trusted for threading work because it will fall out of gear. If anybody scraps one of these CT1440Gs (doesn't matter whose name is on it), I need the guts for the QC box.
 








 
Back
Top