Centec 2B - New arrival and Q&A

Without reading back up to 5 pages, what is the current motor shaft diameter, how much could you remove from the pulley for a sleeve? Would boring the pulley remove the current keyway? Is it a one horse power motor?

I found plenty of larger diameter twin sheave pulleys available but when I looked they were all of larger diameter. Not quite so important with variable speed motor control, I suppose (I overlooked that at the time).

If it is the motor/pulley at fault, the motor mounts may be ‘adequate’. Out-of-balance issues can exacerbate/exaggerate mounting shortcomings.

There are normally 4 long bolts holding both ends of the motor casing to the stator part of the case. Sometimes long bolts, sometimes studding with nuts at each end. Bearings are usually fairly easily removed from the end caps or warming (gently) assists, particularly if the end caps are aluminium. Generally need a bearing puller to remove the bearings from the motor shaft. Mark where everything fits to make rebuilding easier.

It is not difficult to fabricate a bearing puller for these types of motor - a piece of plate to slide over the shaft behind the bearing, a couple of suitable length bolts and another plate with two holes for the bolts and a central threaded hole for a further bolt to draw off the bearing.

The non-drive-end bearing is almost always the better of the two (if money saving). If bearings are dry, lubing may possibly work (but bearings are relatively common and cheap). Out of balance running could have destroyed the bearing. If changing bearing(s) don’t buy the cheapest. I nearly always go to my local stockist (ABC) for belts, bearings, etc.

For this application, fully sealed bearings would suffice, doing away with the need for further lubrication, but bearings with one side sealed would enable future lube, generally by grease gun. Sleeve bearings are unlikely, but would be lubed either by gun or a felt wick.

Edited By not done it yet on 09/08/2021 09:14:49

The motor shaft diameter is 3/4" and the keyway is approx 0.080"/2mm deep and 0.200"/5mm wide. I assume with a <0.020" oscillation that I technically only need to remove about 50 thou - which would leave the keyway in situ but would require me to use sheet material bent into a sleeve rather than machining from solid, right?

What's the stator part of the case? (shame!) The motor is 1hp.

Re: balance issues - the motor was canted about 15 degrees out of parallel with the mill spindle - would that also be a factor?

Edited By William Ayerst on 09/08/2021 11:26:42

What's the stator part of the case?

There are only 3 parts.🙂 The end caps hold the rotor clear of the stator.

Much more than that and the belt may have jumped off - except it looked so tight (too tight).  That most certainly would not help matters - particularly if there is a hardened section(s) where the belt has stood idle over the pulley(s).

Over-tight belts ruin bearings.  It is one reason why there are two belts to transmit the power, particularly with the small contact area on the small pulley.

I’m no expert regarding the sleeve, but at 80 thou deep for the keyway I would be making a sleeve as close to that as I could.  I would mill a keyway for a key of less than the width of the pulley/sleeve and shrink or loctite the sleeve in position.  I would, if possible make the keyway deeper and make a new key to suit. So much for the belt part (of belt and braces) I would then likely install a couple extra grub screws, as well, to make sure the sleeve would never move.

Edited By not done it yet on 09/08/2021 12:49:56

A sleeve with 25thou wall thickness should be quite possible to turn though I expect less will be needed and a thinner sleeve is quite possible or easier still just loctite in a solid plug and then bore that out to size as I said earlier

May be possible to cut the keyway in the pulley a little deeper and cut a slot in the sleeve to clear it so a "C" shaped sleeve. Keyway is more likely to be 3/16" on a 3/4" shaft 0.1875" so a piece of 1/4 x 3/16 keysteel could be used to make a new key as that is a stock size. However if the pully has been wobbling about then the keyway in the shaft may be worn. That is why I asked earlier what the condition of the motor shaft is like as it will be hard to fit a new or modiied pulley to a shaft that is bu**ered

Yes any out of parallel between motor and mill's axis will add to your problems

I'll get a picture of the motor shaft shortly - hopefully when I get it out of the case and have a look at the bearings.

From memory it looked a little rough but the key isn't a rattle fit by any stretch.

I don't want to question the knowledge here, but is it really possible that a 20 thou oscillation is causing all of this?

Edited By William Ayerst on 10/08/2021 12:33:00

Just bringing this to the current page - any thoughts on the mount? I'm wondering if two wooden legs underneath the outside of the existing angle might give me be a short term solution while I'm using the mill to fix itself with proper mounting brackets, etc.

…. I don't want to question the knowledge here, but is it really possible that a 20 thou oscillation is causing all of this?

You just did! The following have been mentioned and one, all or any permutation of them could be implicated.

Belt (unlikely), pulley, shaft, bearing(s), motor mounting. Motor alignment has been raised by yourself, too.

It is always difficult to diagnose exactly from a distance. I am expecting it to be the off-axis pulley and the motor mounting which are, together, setting a resonant oscillation which is making things worse. Likely to cause an early failure but equally might be blamed for poor finishes or short-lived cutters when operational difficulties arise.

Check out the Tacoma Narrows bridge failure back in 1940, on you tube, The Romans knew that marching soldiers should ‘break step’ while crossing bridges to avoid the risk of bridge collapse. Single phase motors can cause far too much noise if used in that base with that machine without extra bracing. All these are examples of unwanted frequency vibrations causing nuisance, damage or even destruction.

Edited By not done it yet on 10/08/2021 17:06:20

Seems that the bearing cover on the motor comes off separately - it's grease filled from a screw-lid cup. The only noise at all is definitely coming from this bearing. However, there's no play at all in the motor spindle and though the surface is rough I can detect less than a thou overall runaround:

I removed the bolts on this side - they don't go all the way through. On the other side, the bearing cap is closed. Any ideas on how to proceed? Still worth replacing? I spoke to ABC and they suggested getting the exact spec of the bearing (I can just about read it under the grease). Like the pulleys, is it best to replace both? The other SOUNDS fine.

Edited By William Ayerst on 11/08/2021 12:00:39

Edited By JasonB on 11/08/2021 12:06:46

Sounds awful, to me. Needs investigating to see what exactly is causing that apparent clicking sound.

Not sure why you might be needing to change both pulleys.

They will be ‘C’ spec bearings. I expect the bearing will be 20mm ID, or the shaft may well need cleaning up with abrasive (preferably before dismantling).

Edited By not done it yet on 11/08/2021 16:58:13

I meant to ask whether I should change both bearings, not pulleys, sorry

The bearing seems to be 3/4" ID, 2" OD, 11/16" wide. It is marked as 'Hoffman England Imperial NM M8 v2' so I assume it doesn't have a 20mm bore? With that in mind this looks like a drop-in replacement: https://simplybearings.co.uk/shop/p510033/Major-Brand-MJ3/4-Imperial-Open-Deep-Groove-Ball-Bearing-3/4x2x11/16-inch/product_info.html - but am I better off getting a sealed bearing like this? https://simplybearings.co.uk/shop/p42035/MJ3/42RS-Imperial-Rubber-Sealed-Deep-Groove-Ball-Bearing-3/4x2x11/16-inch/product_info.html or ebay: 253824082621 ?

SImply Bearings offer a C3 variant which seems to be over-size I/D clearance if I understand correctly? I guess I need to pull it before I order a replacement, eh?

I don't suppose you have any tips on which of the 7000 types of grease I should be buying as well? Learning so much today!!

On those lines I've got the spindle out of the motor and it looks in good condition. No obvious damage. The bottom of the inside of the motor housing (not sure of the terminology) seems to be smoother than the top? Not sure what that means but there's no damage per se to any of it.

Edited By William Ayerst on 11/08/2021 17:47:43

Edited By William Ayerst on 11/08/2021 19:18:36

C3 variant

This is the clearance within the bearing, not the inner race ID nor the outer race OD. Allowance needs to be sufficient for changes in temperature while operating.

Seems

Going by the number on the bearing outer race is best. No mistakes that way. Many ‘Imperial’ bearings are actually metric. Sensible fitment is with the numbered side visible when fitted.

per se

There should be clearance between the rotor and stator. No foreign bodies rolling about, either. Loose steel and magnetism is not a good combination in an electric motor.

grease

​​​​​​​I would likely fit a shielded bearing and forget about further lubrication. They are not that much more expensive - and should last a long time if good quality named brands are purchased. I simply use a good quality Lithium based grease for nearly everything.

The application lists are invariably found on the internet. I recently found more than enough of an equivalent lubricant for my jack hammer for less than a fiver at my local ABC. Sellers advertising the same grade were asking £15 or more for the small amount required. They hyped it up as a special grease specifically for that purpose, of course.

If not sure, either seek local advice or just replace the bearing(s). Over £25 seems, to me, expensive. But I don’t often buy larger bearings myself and haven’t bought anything bigger than 16mm OD recently.

Thanks NDIY - ordered a pair of KSM (japanese?) sealed imperial bearings - figured I may as well replace both while I'm here - and components for a brace across the inside-rear face of the stand.

Ordered a pair of AX71 belts and an A35.5 for the VH just to be on the safe side, too. Based on your earlier advice I guess I'll knock up the support by 1/4" or so to reduce the tension a little?

I've got a line into a metal fabricator who I'm friends with who is prepared to make me up whatever I need for the motor supports, but it's likely to be a month or two - so I'll knock up some temporary legs for the back of the motor plate down onto the plinth to stop the wild oscillation for now, and get him to build me what I need - when I figure what that is!

Edited By William Ayerst on 11/08/2021 19:48:24

I bored out the pulley from 0.750 to 0.780 and the eccentricity is now removed. I found a sheet of 0.015" brass so used that as a sleeve, cut to π(0.750)-0.188 with a hole for the existing grub screw to fix onto the shaft and I'm pretty pleased:

It is a solid fit on the motor shaft with no play whatsoever.

I have noticed that the end caps for the motor housing seem to have integral lips to fit against the groove in open bearings, and I have ordered sealed bearings. Hopefully no dramas, but if not I'm not sure whether to send back the bearings for open ones, or shave off the lips on the inside of the caps...

Anyway, my slab cutter also arrived

Edited By William Ayerst on 12/08/2021 21:56:42

Do you mean sealed or shielded? For a motor where what you are trying to do is keep grease in rather than muck out I'd use shielded, and if you have grease nipples prise one side shield out to let the new grease in. Having said that, for hobby use a pre-greased shielded bearing will last a very long time without new grease

Choices, choices!. One, or both, seals can be removed from the bearings , if necessarry. Alternativelly a suitable shim gasket could be employed to attain the correct clearance, if important.

Removing the outer seal would still leave the assembly sealed, anyway? What you don’t want is too much grease in the bearing such that it gets in the motor, so just needs care when assembling.

For steel, you will need to use only the lowest gear and plenty of cutting fluid with your new cutter, a suitable feed rate to make each tooth cut (not rub). Depth of cut will be limited by the motor power available (or the tightness of those spacers on the arbor).

That looks quite an ambitious cutter for a Centec! Don't go trying to climb mill with it, it will definitely be exciting!

Paul.

Looks like either someone has tried to enlarge the keyway or it has become seriously worn where the pulley has been moving about on the shaft. Worth sorting out so it does not allow the pully to start moving again.

Worth making a new one I think. Considerable damage can be done once worn components start flopping about. And not just to the worn part itself - shock forces hammer through the drive train, taking years off the life of bearings etc and causing poor cutting finish.

Decent challenge for a keen beginner too!

Dave

William,

Stop faffing about and do the job properly! Bits of brass shim is a 'bodgers' answer to the pulley problem. Bore it out with a slight recess and make a top hat bush which is a good press fit into the pulley, remount and finish bore to the 3/4" bore.

Then put a keyway into the bush to suit - use the toolpost to mount a keywaying tool then by taking 0.005" cuts shave the keyway to size (obviously you don't run the spindle for this and its best to lock the spindle in position ) by hand feeding the tool through the bore.

That gets the pulley sorted!

Re-con the motor with new bearings and refit it. Whilst awaiting the motor mount brackets use a car jack to support the outer overhung end of the motor.

You will need a good vice for using that 3" slab mill! Not my choice for a small hobby machine as they are more suited to high horsepower industrial machines where time is money. I have a wide selection of slab mills that I don't think I've ever used.

Martin

I’m not keen on youtube amateurs and just realised we are being used as ‘sounding boards’ for some questionable output - a bit like a learner-driver advising on the Advanced Motorist Driving Test.

Bye.