Myford Super 7 - Crompton Motor cuts out at high speeds

Hello, This is my first post....I recently aquired a second hand Super 7with gearbox. While trying it out, I noticed a rather odd problem. If I try and run it at 2150 RPM, the motor initially runs fine, but after 30 secs, it starts slowing down and speeding up again (as though a saftey switch is stopping overheating ?). On 1480 rpm, I can run as long as I wish.

Both are free running, i.e. no work in the chuck or cutting tools involved. The motor is a DC Crompton running off mains supply, running at a single speed. The only difference is one uses Headstock Drive 1 and the other Headstock Drive 2.

I am trying to machine some EN16 so would prefer to use the higher speed.

Any ideas or help most appreciated.

Nicholas Hill (Myford for vintage motorcycle restoration)

What size is your motor? The S7 needs a 3/4hp motor to run at full speed, rather than the 1/2hp which was adequate for the ML7.

Just a thought,

Rod

How quickly does the speed vary? It's possible the motor has internal thermal protection that may be misbehaving.

Another option is the spindle bearings may need adjusting. If there is too little clearance on the front bearing it will over heat and bind surprisingly quickly. Next time you get chance run the lathe at top speed for a minute or so then stop machine and turn off motor. Feel the front bearing, if it is bad enough to be causing the problem you will be able to feel the heat on the bearing and nose of the spindle.

Edited By Nick Taylor 2 on 02/05/2017 23:42:05

Disconnect the belts and run the motor and see if the problem is actually the motor. If no problem with belts disconnected the problem may be bearings tightening up in the countershaft or headstock.

My S7 runs very rough on the higher speed range but doesn't behave like that. But unless you are turning small diameters that sounds a high speed to want to use.

Hi, sorry for the delayed reply. I have only just had a day that I could play with my Myford.

I have just looked at the data plate on the motor, and it says for BHP 1/3KW.....so I think that relates to about 1/2 HP.

So, as there is little heat generation, I am assuming that, based on the above, i need a more powerful motor to run the highest speed.

I am only turning 1/2" to 1/8" at the moment, so I don't need the highest speeds immediately. I just assumed that if Myford supplied the motor, it would be sufficient for the job.

In the long term, any views of 3 phase? Is the extra speed control worth it?

Many thanks again, and sorry for the delay in replying,

Nicholas

I converted my S7 to 3 phase with a Transwave kit I didn't do so to get anymore speed I needed a new motor so decided to go 3 phase and inverter. I find it usefully to be able to vary the speed without changing the belt positions and also (with care) run in reverse to spin off a die.

H

1/3 KW is 333 watts, 1/2 HP is 373 watts, so yes it is slightly less than 1/2 HP.

I run 1/2hp on my super 7 and it copes on top speed easily, even with my biggest chucks. Unless the motor is faulty then it should be fine. You say there is little heat generated; are you referring to the front spindle bearing?

if the motor is fine on other speeds, even whilst taking a cut, but struggles to run the lathe on top speed without a cut on then I would be checking for spindle binding before I spent money on a new motor. Not to mention the damage that could be happening to the spindle/bearing.

I understand quite a number the early S7s were fitted with 1/2HP motors. David Standing 1's assertion that 333 watts is slightly less than 1/2HP is only partially true. Motors are inductive. As a result current and voltage are not in phase so some of those precious Watts are not turned into output power. It also makes no allowance for energy lost as heat because of the resistance of the winding amongst other things, I suspect that 1/3kW of this particular motor is actually producing something closer to 1/3 HP. IMO this is not really enough to do any useful work with on the higher speed ranges of a S7.

I suspect that 1/3kW of this particular motor is actually producing something closer to 1/3 HP.

That depends on how the motod was rated. Historically the motor gave the output stated, so the input was slightly more, to take care of inefficiencies. Some specs now quote input power, which will obviously be higher than the power output.

Like compressors. Decent manufacturers quote free air produced while the cheaper, inefficient end of the market quote displacement, which is not the same by a considerable margin!

Hi Nicholas,
It is very unusual to see a Myford fitted with a DC motor. First some questions. Have you checked the brushes ? Is the motor shunt wound or series wound (Also called universal.) ? What kind of rectifier is used, The old selenium type or a siicon diode bridge. ? Can you post a picture of the information plate on the motor ?

Les.

I have a 1956 Myford Catalogue sheet, Publication No. S713 in front of me that states:-

"A 1/2 h.p. resilient motor is recommended. To ensure satisfaction a suitable motor can be fitted at the factory.State whether A.C. or D.C., exact voltage and phase."

In 1956, some power stations were still pumping out DC, Brighton was one I believe.
BobH

Hi, thanks for all the info, I am now getting more confused / worried!

I have been reluctant to play with the spindle as i thought it was set in the factory, and problematic to realign? But I can't see any other option. The motor is a single speed, so it is only the drive belts that are changing. So the load on the motor should not be increasing with speed.

This leaves only the spindle speed that varies, and hence the heat expansion theory is looking sound.

I think i need to look into this first, otherwise, as mentioned, it may cause serious damage if the power of the motor is increased. I don't want the spindle suddenly seizing and potentially distorting.

Many thanks for the ideas and info, it has got my brain working again, and hopefully on the right track.

Nicholas

My early seventies instruction book for the S7 says that the recommended motor is 1/2 hp three phase or 3/4 hp single phase, resilient mounted. Whether that is relevant to the OP's problem I know not but I offer it to add to the mix.

Rod

.

Nicholas,

I think it might be helpful if you could post a photo of that data plate.

You mentioned, originally, that "The motor is a DC Crompton running off mains supply, running at a single speed." ... and, as noted by Les, this would itself be rather unusual.

I'm sure folks will be able to give best advice if they know exactly what they are contemplating.

MichaelG.

If you had headstock bearings binding, you would feel the heat by hand in no uncertain manner. Too hot to hold your hand on = problem. Ok to hold your hand on = OK.

You need to check too the countershaft bearings. They could be binding or overheating. Ditto the motor bearings themselves. Best way to do this is loosen off the belt from the headstock spindle to the countershaft pulley and run the motor and see if it slows down. Then remove the belt from motor to countershaft and check the motor running on its own.

Also check for slipping belts. Could just be the belt slipping on the stepped pulley on the highest speed. Might be just a matter of belt adjustment, or you might need a new belt if the old one has gone hard and lots its grip.

Hi again,

Sorry, I have only just been able to try some things out.

It appears to have fixed itself......Today, I tried the idea of running de-clutched, all ok.

Tried again at the highest lathe speed, and again, the same symptoms with the added note of the continuous clicking of the motor.

Checked all the bearings, no, heat at all.

As a thought, I tried running it in reverse, again at full speed, ran for 2 mins, no problems???

Based on this, I tired again in forward, and three times in a row, each of about 2 mins, no problems at all!

So i can only assume that something was catching / rubbing, and by reversing, it freed it...I am wondering if the belt was getting caught in the back gear cluster, as the pulley and the gear are close to each other, and a frayed bit of belt would cause a lot of resistance?

Thanks again,

Nicholas

I'd be suspicious of the clicking in the motor. I think you said its a DC motor of some sort, so maybe you have worn brushes and/or build up of carbon around the commutator area and running in reverse has cleared it up a bit, for now. Best thing if it is working OK is probably to carry on using it and see what happens.