Loud click when motor runs down, or, why you need 3 lathes

Today my 6" Chinese lathe started making a sharp clicking noise, or perhaps more like a hand slap, when the motor ran down to a stop. It is an induction 2 hp motor. I couldn't use the Hercus, as I have the cross slide off to measure and compare with one I am milling from a casting. I wanted to turn a bit of 11/16 bar as a test piece for this, so I used lathe #3, the ML7. Having three is more circumstance than plan, but it worked out well today. Any thoughts on the motor? I know nothing about electrics but have heard of a centrifugal clutch on the start winding and that was my first thought. The motor is mounted behind the lathe, right out of the way of swarf or oil. In 10 years I have replaced the capacitor once. I don't want to use it until I know what is going on.

I'd try running the motor up with the drive belt removed first....to determine if it is the motor

then remove/strip motor...as necessary...no point in guessing what could be wrong?

I agree with John... If it's not something caught in the belt or the belt perished and its coming from the motor it could be some sort of centrifugal switch mechanism trying to re-engage on run down. Just a thought 🤔

Edited By Johnboy25 on 17/08/2016 09:28:42

Simon,

Others will have a better idea perhaps, but the usual noise made by the centrifugal switch on running down a capacitor start induction motor is more of a click, following by a kind of gentle brushing noise until the rotor comes to rest. The normal sound is not as hard or sharp as a hand slap, and it is not activated until the rotor has slowed appreciably from normal running speed,

​What has changed in your case I don't know as this appears to be new behaviour, but does the rotor still turn freely by finger tip spinning? The only other components inside that are affected by rotary action are the bearings on the rotor, but I would expect those to be making sounds of distress when the motor is at speed rather than as you describe.

I suspect you will have to split the motor to establish what has gone wrong. It is not difficult, but make scratch mark the bell housing joint faces with the body of the stator to restore alignment on reassembly before unbolting the four tie rods through the motor holding the bell ends together. Open with care, especially at the cable entry end, the bells may well be a tight fit and tapping with a nylon faced hammer or similar is recommended. Exploit any joint gap opening with balanced gentle leverage on the opposite side.

The centrifugal switch may have balance weights that are flung out sideways against a retaining spring, clean all parts with switch cleaner and test the action. There might be two springs opposite each other, one may have broken in which case find and remove the debris, and replace both springs with a similar pair.Also clean the switch contacts with a strip of fine emery folded in half slipped between them and then remove abrasive residues.

Bearings, if simply needing lubrication, will feel tight until the oil has done its job. If worn they will be sloppy, noisy and feel gritty and need changing.

I don't think they are your problem, but look anyway. Rebuild the motor and reconnect it electrically just as it was when you started. Use the sketch you made of those connections before you started.

Now test it. If it is not back on the machine, then wedge it securely on both sides to stop the starting torque reaction trying to roll it over. Don't try holding it by hand, good solid blocks either side are what is needed.

Regards Brian

Thanks all. I tried to remove the belts today to check just the motor in isolation, but I will have to loosen the motor on its mounting, and to get at it, I first have to move a heap of stuff out from behind the lathe: sheets of copper and brass and steel, an old door etc.. I will ask one of the clever electrical types at my club about it, and hopefully, to fix it. I have never opened a motor and would be pretty nervous about doing so.

You need three lathes ?

Damn, I'll have to sell 7 or so

Hi Simon

If the click happens with the belts disconnected it is almost certainly the centrifugal start switch re engaging as the motor slows down, it will be switched off again when the motor restarts. Not necessarily a big problem I have had several motors over the years that clicked the same way. If the motor runs cool, smoothly and without abnormal vibration compared to its former self I would not stress about it, in fact I would just use it as is.

Currently I have a drill press (Chinese motor) that clicks as it slows also a 2.5 HP compressor (Australian built motor) that does it. both have clicked for years.

If you cant resist opening the motor case be very careful and gently ease the end caps off while looking inside, the switches and the Bakelite mounts they are fitted to can be easily broken. Spares can be hard to find.

Regards
John

Some motors(my Taiwanese lathe) have the switch gear under a cover held on with 3 or 4 screws on the outside of the motor at the opposite end from the pulley. My one had poor insulation, and sometimes arced to the frame on start up, a friends drill press did the same, easy cure with a bit of tape. Advantage, you don't have to open the motor to service the switch gear.

Ian S C

John, that is the sort of advice I like to get, thanks. I certainly can resist opening the motor. The click is variable too, sometimes really sharp and loud, and sometimes much less alarming. I'll check it more today and perhaps try to record it. The trouble is, even getting the belts (double belt drive) off is very difficult, especially with my bad back. The service bloke showed me how to move the belts from high speed to low speed pulleys by getting a cloth under the belts and pulling, working them off the larger set, but when I tried that yesterday, all I did was hurt my back.

Ian, if it arced to the frame, presumably you would see a spark? It is pretty dark down there and I saw none, but I'll check again.

I just checked and there is a spark making the noise! The spark is under the cover of the bottom capacitor. Just replace it?

The capacitor housing was full of oil! So were the spade connectors. I can see oil leaking out of the lathes’ head/gearbox, some of which is dripping on to the top of the motor. I don’t know how it is getting from there to the cap, unless it is going inside the motor housing at the top, and running from there down inside the motor to the lead exit, tracking along the lead onto the spade connectors and beyond. The part of the motor around the exit point of the leads is not oily, that is why I think it might be coming from inside. I have a new cap in stock so I’ll fit that. The old one also has some sort of deposit, like a leaky battery, around the front seam.

I think the "oil" is in coming form the run capacitor, it's can is leaking. Maybe its time to replace both capacitors, especially if they are awkward to get at. I think the leaky one will be the run one which is in use all the time the motor is running. The other is only in use as the motor accelerates away, it will be high value special AC electrolytic type. The capacitance value will be on the can/container, both are available from RS or Farnell and RS and I'm told RS don't charge for postage, but do check.John

it

I wondered if the stuff (what is it?) might be coming from the capacitor. I replaced the bottom one and no more sparks, but there is a delay in the motor starting up. I'll replace the top one too.