A prototype Lavet stepper motor

The bent up arms are 8 * 3.4

Across the outside they are 33

The bobbin is 25 dia * 28

The gap from arms to rotor unenergised is <1

The offset of the rotor is <1

Any more just shout

Thanks Duncan - that's great!

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Sorry, Tony ... I should have explained:

Near the top right corner of the linked page is a vertical column of three dots

Click that to open a menu ... one option is Download PDF

MichaelG.

No problem Michael - it was much easier to find on the laptop & I now have a paper copy of the whole patent to peruse - thanks.

Makes an interesting read. It seems that the rotor can have any (even) number of arms on each side, so in principle you could make one with 30 arms top and bottom that would give 60 pulses per rev, which would be handy, although the machining for that would need to be very precise indeed (or built rather larger).

Of course, if you were to build two 60-step motors and one 12-step...there would be no need for gears...Hmmmm.

But that would mean that at 5 to 3 (for instance) the hour hand would be resolutely pointing to 2. Better to build 2 off 60 step motors and only pulse the hour one once every 5 minutes.

It might actually be easier to make a 2 phase stepper with 60 notches per rev, the only extra is another coil I think, and the rotor is simpler, just cylindrical rather than the snail shape

No necessity to use hands, of course, you could use a digital style HH MM SS display.

Edited By Tony Jeffree on 27/03/2021 16:59:58

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I’m having some difficulty interpreting the ‘perspective’ sketches in the patent ...

it is reasonably clear from this photo, and from Fig.1, that items 10 and 11 are cut from a closed tube shape ... but Fig.3 and Fig.4 [which purport to clarify things] appear to show them cut from flat plate.

Is this an over-simplified sketch, or an optical illusion ?

... what do they really look like ?

MichaelG.

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A sketch for the purposes of explanation, as implied I think in p2 line 85. I suspect that in manufacture the rotor polepieces were stamped from flat plate then bent in two places, then finally milled to form the tapered circular forms on the actual poles.

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Yes, I agree regarding the purpose, John ... but I’m trying to comprehend the three dimensional shape, which [regardless of the actual manufacturing method], appears to be rather like something you might cut from a ferrous equivalent of the plumber’s ‘stop-end’

**LINK**

https://www.jewson.co.uk/p/altech-end-feed-stop-end-22mm-copper-ALTEF068

MichaelG.

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Hopefully: one of the 3D CAD wizards might come to my rescue, or Duncan might be able to provide a close-up photo [unfortunately, the photo that you grabbed doesn’t quite do it].

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Edited By Michael Gilligan on 28/03/2021 11:30:36

Michael -

I found Figure 2 from the patent was the best one for visualising what is going on. You are right - the two pole pieces (10 and 11) are best visualised as being cut from a steel version of one of your plumbers' stop-ends. If you look at part 10 in fig 2 and attempt to screen out the "noise", the top surface of 10 looks very much like the profile shown in Fig 4, with the cylindrical portion of the part (which appears from the photo to be essentially rectangular if you were to cut it away and flatten it) has been thinned, so it is full thickness at the end nearest the electromagnet's pole pieces and tapers to almost nothing at the tips. The tips therefore clear the pole pieces of the electromagnet by the minimum clearance plus almost the thickness of the metal. The effect of this is that in its un-energised state (as shown in Fig 2), the permanent magnet will align the rotor so as to minimise the gap between the rotor's pole piece and the electromagnet's pole piece (which is the state shown in Fig 2). However, the thinner tail of the rotor's pole piece is near enough to the next electromagnet pole piece to ensure that when the electromagnet is next energised, with the reverse polarity, the rotor will rotate in the direction of the next pole.

Thanks, Tony

I actually started 'working on' Fig.2 last night ... That's when I realised that I seemed to be missing some of the devilish detail; so I paused for thought and advice, in preference to making gross assumptions.

MichaelG.

As I had to mess with it today to change the hours I've taken some more photos. The crescent shaped bits are tapered, see photos. Intuitively I can't see why, I would have guessed that having a simple stepped profile would work. If I start any more projects SWMBO has threatened to lock the workshop door, too many outdoor landscaping projects in the queue, so I'll have to rely on others for the time being. The brass disc is a loose fit, some kind of rattle damper? (see posts from Joe Noci on ELS)

Trawling the memory banks brought to mind something from the SMEE archive, so I've searched through it and found a photo of 'experimental silent slave' from a Ron Hough, so I've sent an email to SMEE to try and contact him. Hopefully I won't need a ouija board

Edited By duncan webster on 28/03/2021 13:50:18

Edited By duncan webster on 28/03/2021 13:55:29

The tapering is all about ensuring that the rotor's rest (un-energised) position is as shown in Fig 2. That is important so that when the next polarity reversal happens, the tapered horns are attracted to the next stator pole, and the rotor will rotate. It controls the direction of rotation of the rotor. If they weren't tapered (or bent inwards to create more clearance at the tips, which would have the same effect), the rest position would be somewhere more nearly half-way along the rotor poles, and it would be uncertain as to which direction the rotor would move on the polarity reversal.

You could achieve the same effect by keeping the thickness constant and the clearance between the rotor and stator poles constant, but making the "flying" part of the rotor poles taper to a point in the axial direction.

constant thickness but varying gap seems a lot easier to achieve, in mass production a press tool, or as a one off just machine a cup , fret out the arms and tweak the rim. Or as I said before, a stepped profile.

Some of the details which seem odd may have been added to differentiate this from other patents...

Pretty obvious, but I might as well post it for reference:

Fig.2 in the Gent’s patent is too sketchy to be be directly scaled.

I have tweaked the proportions a little, to make it a reasonable fit in a square, with the smaller circles fitting quite nicely ... But: the all-important ‘rotor arms’ are then distorted.

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Having ‘got the general idea’ ... I can now concentrate on the shape of the rotor arms.

MichaelG.

Update:

I may be wrong [ it has been known! ] ... but :

I think I now understand the elegant geometry of the tapered ‘horns’ on those rotor-arms

Although it looks like the production items might have been stamped from sheet; it should be a relatively simple matter to machine them from solid.

I will leave you guessing for a while

MichaelG.

Edited By Michael Gilligan on 29/03/2021 08:32:36

What a tease, Michael c'mon...spit it out!