Myford ML7 motor

[robgreenwood2018]

Hello everyone,

Hope your all well,

I bought an old Myford ML7 lathe, the previous owner said it was running the day before I collected it.

When I try and run the motor on the bench it locks up, tight then the fuse trips.

Once it has been turned off if I spin it a bit by hand it frees off,

I’ve never seen anything like this before, can anyone help?

I will try and get a photo of the motor

Thanks

[robgreenwood2018]

Here is the image I have at the moment, will get a better one later.

ibb.co/Xx3z5VV

Thanks again

[ettingtonliam]

Tell the previous owner he lied, and you want your money back, or at least enough to buy a new motor!

[keith1500]

Just a thought....

What happens if you give it a good spin then turn it on?

[jackrae]

If it's a mechanical seizure then there are at least 4 possibilities
a) Armature has expanded (unlikely if it frees off)
b) Bearings are seized (possible but unlikely
c) Swarf has entered the motor (they are not sealed motors) and is jamming the armature against the stator
d) The centrifugal switch has disintegrated and is jamming the armature
A capacitor failure will not cause motor seizure

Do you hear any rubbing or grinding noises when you turn it by hand and once if frees off does it turn freely. Once free does it run

[Roger]

The test Kieth suggests is an excellent one, because it eliminates anything to do with starting windings, capacitors and the centrifugal switch.
If it still locks up, then it's almost certain that the armature is being pulled out of its central position and there's metal to metal contact with the stator. I've seen this many times on high speed spindles which spin freely until the power is applied. The problem exists in all types of motors because the clearances have to be very small to get the best efficiency.

Most motors have one fixed bearing and another in which the outer cage slides in a pocket with a wavy washer behind to create the necessary preload. It's possible that the bearing outer is stuck in the housing and there's no preload, although I'd be surprised if that was enough to allow sufficient movement for this problem to happen. Sliding the motor shaft axially while moving the lathe could cause this to happen.
Anyway, it looks like the motor needs to come off to inspect and grease the bearing housing and check it's all moving as it should.

[uuu]

I'm voting for the swarf hypothesis. There's no guard in your picture, and the lathe/motor has been moved, possibly dislodging a hidden strand. As noted, the internal clearances are tight. If you're lucky a blast with an airline might clear it, otherwise it's covers off.

Wilf

[silverfox]

Rob

Where do you live ( areawise) i might be able to recommend a good repairer

[robgreenwood2018]

Thank you once again for all your replies, 

I did as you all mentioned, it turned out it needed a good clean.

I stripped it down and cleaned it with the air hose, and it started straight away.

Does anyone know what type of motor i have here? I've nerer seen one like this inside

[uuu]

It looks like a normal capacitor-start induction motor. If it makes a click as it slows down that's the speed-sensitive switch that operates the starting windings re-engaging.

Wilf

[jackrae]

Are the shaft bearings oilite type sleeves or ball races.
If sleeve bearings and seriously worn, they may permit the rotor to touch the stator. Can I suggest you check each for wear and if badly worn, replace them. Also check the lubrication channels to ensure oil can reach the bearings. On one of your photos the drive-end lubrication port appears to have a slotted plug fitted

[twombo]

Ahh! sweet Success, Rob! now, off to the workshop with you, Lad! and make up a nice pile of scarf to have scones and Tea by! do You do have a pot handy for those wintry nights? I wish myfords were not so hard to find here in the “Colonies”. Post a picture or two of your “precious”! congratulations!

[timb]

You don't show the rotor but it looks more like a slip ring motor where the mains is connected to the rotor and the stator.
The usual alternative is a squirrel cage motor where the mains is connected to the stator only, inducing a current in the rotor.
High starting torque withthe slip ring but low efficiency.

[jackrae]

That's the centrifugal switch you see in the background. The two bright faces are the 'rubbing' face that contacts the rotating mass part during start-up only. The actual switch is at the bottom of the yolk and will be open with the rotor removed. Extremely unlikely the motor would have a slip-ring rotor. The stator laminations show show signs of contact by either the rotor or debris within the air-gap.

[chris vine]

I once had problems with an induction motor like this on a myford lathe.
The culprit was cast iron dust which flowed in with the cooling air.

Quick clean out, as described above and all was sweetness and light again.

Chris.

[robgreenwood2018]

You are all amazing! starting back out in this hobby was daunting but having you guys to help when needed is priceless.

I gave the motor a good clean, i am going to give it a lick of paint and clean out the oil passages.

Thanks once again for your help! off to the workshop i go.....

[uuu]

Talking of oil - these motors need very little. Where you'd oil the lathe frequently, you should do the motor sparingly and only occasionally.

Wilf