Member postings for Simon Williams 3

It wasn't today, it was yesterday, but this is an object lesson in leaving well alone.

I bought a bandsaw from my local secondhand shop, did the insulation test and earth continuity, plugged it in, happy days.

The main lead had pulled through its cable gland into the motor terminal box, so yesterday I thought I'd while away 15 minutes putting that right. Simple enough.

Until I took the cover off the terminal box, and realised that the black bits in the bottom were broken insulation off the motor leads. Oh dear.

Can't leave that alone, so here's what I found when I took the end cover off the motor:

Not the best photo ever, sorry about that. The green lead RHS of picture is one of the main winding leads, the two once-upon-a-time white ones are the starter winding on a 6 pole single phase capacitor run motor of about 3/4 HP. These wires are the original tails connected to the motor windings, and look as though they have been overheated though I can't explain why there is no evidence of damage to the windings themselves, and no "burnt" smell. The insulation is brittle and cooked, and the wire itself is tarnished and oxidised.

I cut the damage out, grafted some PTFE insulated wire in with heat shrink sleeving and threw it all back together. Not the best job ever, but better than it was.

I could have made a better job if I knew how to withdraw the stator lamination stack from the motor outer housing. Any clues anyone? The only thing I can find on the internet assumes the windings are already toast, and you can pull the whole thing to pieces by brutality. In my case there seemed to be nothing wrong with the windings and I'm reluctant to get it rewound when it still works.

So it runs, takes a sensible looking load current, insulation at 500V is 85 megohms. I've put it back together and it'll do.

Rgds to all

Simon

For the same price go with the extra protection and adaptability of the single phase push button starter with an overload device and the option of a remote E Stop. I take the point from an earlier contributor that it's a bit anachronistic, but not as much so as the yellow pushbutton thing. Either way, that's the price you pay for bringing modern day safety technology into a 60 year old environment.

Besides, if as you say the Kedu type switch would have a cable to a 5A plug, (which won't be locally fused?) where is your overcurrent protective device and what rating would it be? And what else would it have on the same circuit?

Rgds Simon

Er...

One of those yellow pushbutton kedu type switches is by definition a direct on line (DOL) connector (switch) though the convention is that the word "starter" indicates that an appropriately sized overload device is included. As several have said, yer gets yer no volt release (NVR) function either way.

The IET wiring regs (BS7671) determine that a overload protection device must be fitted to automatically disconnect the supply to a motor in the event of overload (usually overcurrent) if the motor is over a certain size. From memory that size is 0.375 Kw (1/2 HP). A motor thermal overload relay is a far more sensitive device than any of the usual types of MCB in that it is designed to imitate the thermal characteristic of a motor and monitor its recent service history as well as the presence of an instantaneous fault. An MCB - even of the various different types (A, B, C or D) - is a different beast entirely..

Small overloads do exist, but below 0.375 Kw it's a bit of a lottery, as the current/load characteristic of small induction motors is a black art, and needs to take into account the phase angle of the load current. So protecting a small (sub 1/2 HP) motor with a plug top fuse isn't ideal, but is probably as good as it gets without major complication.

Using DOL starters on single phase is easy enough, provided the coil voltage for the maintain/NVR function is 230 volts not 400 as it might well be on a starter designed for three phase use. Remember to double back the load current through the spare poles of the overload on a three phase starter used on single phase so all three poles of the overload see the same current, as most are sensitive to current imbalance and will trip if one pole is left unused.

HTH Simon

Edited By Simon Williams 3 on 26/04/2019 14:15:37

Hello Terry, good afternoon.

It's your lathe --- but.....

I'm in the no don't do it camp, though not because I wouldn't buy a lathe which had been modified so drastically. I might look at it and marvel at the bravery, but I suspect the resale value of said machine has just plummeted. I'm also not too bothered by the argument that you will have affected (detrimentally, but how much I know not) the stiffness of the machine.

I'm anti because I don't think you're going to be able to resurface a disc even after chopping a bit out of the lathe bed. Firstly I don't rate your chances of mounting the disc in a three jaw chuck (run out and grippability) but I don't think you'll get a surface finish worthy of the name. It's a big flat disc and the tool surface speed at the outer periphery is going to be off the scale and chattering (screaming) like fun. As I understand it a 290 is inverter drive, you need low speed torque for this job, not speed.

The finish on the disc would be much better ground, and frankly I think you'll get better results with a linishing disc on a hand held angle grinder!

And how certain are we that there is enough meat on a modern brake disc to allow for a re-surface anyway? What's the thickness lower limit?

There, that's put the kibosh on it! Sorry!

Best rgds Simon

SOD - Dave - thanks for the explanation, but it still doesn't wash. There is no torque applied to the strut, only a force acting to compress it. The strut of course provides a force reaction to this compression; the magnitude of this force can be measured in Newtons, lbf, Kg (wrongly, but at least it's understood), poundals (if we must) etc. etc. All are dimensions of force, not torque.

Maybe it's just a typo. (Occam's razor!)

Thanks as always

Simon

I don't understand the force indicated on what I take to be the plastic wrapping for the strut in Nm? Am I missing something?

Rgds Simon

To my shame I'd couldn't remember the gent's name, let alone conjure up the correct reference. So thank you Roderick for giving credit where it is properly due.

Rgds Simon

Hi David, good morning. Sent you a PM just.

Brian correct me if I'm wrong but I believe on a Mk2 S7, and with 33 or 34 Teeth your new drop arm isn't needed? If you want to go bigger you need it, my recollection is that a 34T mandrel gear just fits in place and the slot in the standard banjo is just long enough to drop the banjo and line up the transfer gears with a smidgeon of clearance.

I seem to remember you might need to tickle the slot in the standard banjo with a round file, but it should fit together. I've not tried this experiment - mine's the earlier MK1 S7 where it doesn't fit at all and anyway it's irrelevant.

Best rgds Simon

No doubt Brian will be along shortly, but here is my three penn'orth for the time being:

To deal with the questions you ask one by one:

Firstly you can set up the change wheels any way you like so long as the maths works out and the bits go together, so the set up as per the label you have included is perfectly valid, but does require (obviously) that you not only have those gears identified but also the banjo onto which they are mounted. This is NOT the same banjo as the standard one with fixed centre spacings which is included with the standard quick change gearbox. So your expense factor is obvious.

The gear pairs are splined together by the centre sleeve, which has a key way machined into it. So the 50 and the 45 tooth gears are fixed together and rotate as one assembly, hence giving the ratio change you need. The same is true of the 60 and 63 t gears, they are two separate gears fastened together and rotating as one complete assembly.

What we will call "Brian's" method, as described in detail in his book, has a simpler solution and works just as well as the approximation represented in the gear set Myford drew out in the label you have noticed. However it has several advantages over the "standard" metric kit as you only need a couple of extra gears to do most of the commonly used metric pitches, you don't need a new banjo nor do you need the centre keyway boss. Whole lot of financial happiness there, however the biggest advantage isn't about cost at all, it's that you don't forfeit the fine feed system of the reversible 19/57 gears cluster because you use it as part of the gear assembly. Brian's book has the details.

However you do need a 33 and/or a 34 tooth gear to fit on the first mandrel spindle (below the tumbler gears). Again, Brian;s book describes the details. Now the snag of this is that these two gears are not the easiest of things to find, indeed the 34 tooth one seems to be in national shortage mode at present. But then neither is the metric set, and the special banjo is shades of unicorn poo...

With due deference to the proprieties of advertising on the forum (which is out of order, and I'm not going to do it) I have been making a batch of 34t gears (proceeds to charity) in response to similar questions elsewhere, so if you want one send me a PM and I'll explain further.

And no, I'm not making a profit - I'm not even covering my expenses making them the old fashioned way but if it helps someone try out "Brian's" elegant and simple adaptation, and makes a contribution to charity no-one's the worse for it, so I hope that's Ok with the moderators.

Hope this helps

Simon

Karl (OP), Nigel(mgnbuk)

Gents I have sent you a PM.

HTH Simon

Bob (9fingers) - I have sent you a PM.

Rgds Simon

Ah Hah!

At last someone's invented a gismo for correcting the effect of chopping a bit off the end of the rule in the guillotine.

Rgds to all

Simon

Erm...

Not necessarily.

Depends on whether it's an imperial (e.g. 3/4 inch x 16 TPI) thread, a metric thread (e.g.M20 x 1.5 pitch) or a Pg thread which goes by the nominal bore size of the conduit.

If you can measure I.d and pitch of the thread in the hole we'd be in with a chance. The other clue is what make and nationality is the motor, and how old?

HTH Simon

Paul Kemp makes a perfectly valid point in that he is correct, the PED doesn't proscribe the inspection and test intervals for a pressure vessel, it is to be determined on the basis of a risk assessment done by a competent person. My point was that a regular disciplined regimen is written in law.

The fact that many of us don't comply doesn't make it right.

And no, a casual user cannot be the "Competent Person" (which phrase has a specific meaning) making the risk assessment.

HTH Simon

Uh OH!

PED (Pressure Equipment Directive) doesn't differentiate between home use and commercial use, on the assumption that if anything does go bang it's not going to be choosy about who it kills. So use of a LPG cylinder outside its design parameters (wrong gas for a start) is straightaway negligence for a starter.

The other thing I haven't noticed in the above discussion is any consideration of the statutory requirement for a pressure vessel (exceeding the exclusion clause minimum volume x pressure, which this does) to be tested annually. No test, no insurance.

Rgds Simon

Has anyone else noticed that about 20 minutes after making an outgoing phone call I get a nuisance call?

Has all sorts of implications for how the concept of subscriber privacy is perceived by the telephone provider.

Rgds Simon

Hello Raphael, good evening again.

As I said before I don't know this particular machine, but I'd be leery of using motor oil in any compressor because of the detergent content. Compressors suffer from condensation by virtue of the water in the compressed air coalescing on the internals; this water gets mixed into the oil and emulsifies if the oil has a detergent content. You actually want an oil which does the opposite and is hydrophobic, i.e. it rejects the water, not dissolves it. Of course - just like a car engine - if you get water mixed in the oil it not only goes a nasty grey sludgy treacle but it also ruins its lubricity properties.

I spent a lot of time in a previous life playing with Atlas Copco industrial compressors, mostly reciprocating, some screw compressors. The reciprocating compressors normally used an oil called POA which stands for poly alpha olefin, and which has the long life lubricating and water rejection characteristics. Others will hopefully tell us if this is a suitable lubricant for your application, if so it's available in the after market quite readily. It's not cheap but it certainly isn't as expensive as the prices you mentioned above.

We used to take this oil and discard it, which sounds a bit barmy but we used the compressors in the food industry, injecting compressed air directly into the food process. This meant that the compressor oil had to be food safe, so we actually used an oil made by Rocol called FoodLube. Again I don't feel confident to say definitively that this is suitable for your compressor, but I'd be happy to give it a go if it were mine.

Hope this helps, I'm a bit reluctant to recommend an oil, but I'm pretty certain straight motor oil isn't compatible with your compressor.

Best rgds Simon

Original post says he has three pinions, 3,3,1 ; can't see that's going to matter too much. It's not ideal, but it's a second order correction.

He also has three jaws which I would call a mismatched set in that he has got 1,2,3 but different serial numbers. How much error this will introduce is anyone's guess, but if they fit in the correct slots (and subject to the above comments about missing teeth or a damaged scroll) this should give a usable if not precision chuck. There's a lot of good work come out of a poorly chuck by dint of machining everything at one setting.

If the concentricity is close but not close enough then we might be into jaw grinding territory. I'd be chary of committing to this except as a last resort.

Some of the posts above seem to have got fixated on not having a 1, 2 3, set of JAWS, this isn't the case.

What would be interesting to know is how far out of true the chuck is with all three jaws fitted. If it's country mile out then there is something wrong with the teeth or the scroll. This might be helped by shuffling the jaws or putting them in with one tooth delayed, but it's a long shot and is going to be a PITA for all time, Interesting but not serviceable.

Given a lathe is only as good as the chuck, bite the bullet and buy a new (i.e. replacement) one. Nothing wrong with buying another second hand one with all the jaws but be choosy. Alternative is three (six including the outside jaws) new jaws - money well wasted since we don't know the scroll isn't damaged and we've still got the riddle of the mismatched pinions which indicates this chuck has some sort of bodge-it history going on.

HTH Simon

+1 from me, Bill got there first.

Just to be sure though, you have checked the compressor is running freely and is not trying to start against the stored back pressure from the air in the tank? Hopefully there is some sort of decompressor or venting system so that the motor only has to accelerate the compressor internals up to speed. After the motor has got going then the compressor starts pumping and generating back pressure.

If the stored back pressure is on the compressor piston when the motor is stopped then that can make it stall, though usually it will at least try to start.

I'm not familiar with the compressor you have, but most of them have some sort of decompressor controlled by the pressure switch. There is usually a non return valve located at the pressure tank, and some sort of bleed off valve which allows the air trapped between the compressor outlet and the tank non-return valve to bleed away when the pressure switch shuts off the motor. Often the bleed off valve is part of the pressure switch mechanism.

But a capacitor is first suspect and a cheap fix. It worked for my shower pump, which exhibited exactly the symptoms you describe.

Best rgds Simon

Hi Raphael - certainly those symptoms sound like it is the capacitor that is the problem - I assume the shut down after failing to start is the overload trip popping.

A replacement shouldn't be too expensive - here's one such , I just put "motor start capacitor" into the search string on ebay and this was the first entry. You need to scroll down the page and select the uF value to suit.

The voltage rating looks like it is way over the top, but this isn't so. In fact the 270 volts RMS rating is approximately equivalent to the 450 volts of the advert, as this is a peak to peak value.

Try this link as a starting point - you may wish to find an equivalent located closer to you to keep the carriage charges down.

Motor start capacitor - select 50uF

It doesn't generally matter too much to get the value spot on the same as the original - the actual uF value of the new one will be subject to a wide tolerance - typically +/- 20% at least.

Good luck, do let us know if it cures the problem.

Best rgds Simon