Finescale - of a sort?!

[Neil]

A suggestion, if you replace the pot fit it with a rubber tyred wheel rather than hand cut gear, likewise the quadrant. Looking at the gear teeth I can imagine all sorts of unhelpful forces happening when they enter and leave mesh. Friction drive will give a more consistent force at a consistent angle and be kinder to the pot. Also a lot easier to make.

 

[Osgood]

Time for a super bodge, Pete?

Not much room for this, but - unless you want to put a few more hours in on the file - I reckon the pinion needs to go a wee bit further out of mesh with the quadrant gear.

We had the the exact same problem with the ring gear and pinion on a rotary drying kiln when we replaced the old pinion.

No pic of the pinion, only this one showing the 1916 vintage cement clinker cooler in the process of being re-purposed in 1973.
The old pinion had been built up (more than a few times, I imagine) by welding and was not good.
It lasted 10 years but eventually a new gear (one of my first jobs was to make a template of the ring gear so David Brown Co could machine the pinion to match it) was the only solution yet it just wouldn't run right when set to the theoretically correct mesh.
The pinion would drive the ring gear ok, but on over-run it would jam up just like your gear set.
One of those times when theory goes out right out of the window and a practical solution has to be found.

 

[Osgood]

There is a kind of a railway connection here - just found this which shows the main kiln en route to Harbury in 1933:

Greaves Sidings: The complete rotary cement kiln transferred by rail from the APCM Cement Works at Ellesmere Port to Harbury Cement Works in May 1933

www.warwickshirerailways.com

This rotary kiln is, at 200ft long, considerably larger than the 65ft clinker cooler that we had (I think there were 3 kilns and 4 coolers in all).
I believe our cooler was installed secondhand at Harbury too, so may have been moved from somewhere else around the same time as this kiln.
The warwkickshirerailways website has some fascinating and varied stuff in it.

 

[Overseer]

Col, that "last resort" - sometimes referred to as a "1 number, tool, fine adjustment, for the use of" has come jolly close to being a preferred option in what is now turning out to be yet another sorry saga?!

I suppose the very notion of cutting gears by hand was probably bound to fail if the truth be known, and it should come as no surprise that it has indeed proved problematic, although not for any of the expected reasons !!

A great puzzle that has been vexing me is an extremely odd tendency of the gears to run sweetly on the bench, then completely jamb up after only a few test operations?! I could guess, but not prove that the gearbox was becoming twisted when attached (deep inside the boiler) to the bulkhead? Each time it happened, everything had to be dismantled for further investigation. Trimming various "suspect" teeth seemed to sort the issue, but each time only temporarily.

I felt that one possible answer was to fit an extra, fourth spacer in the small remaining corner of the box immediately adjacent to the potentiometer:

View attachment 156325

There was not really quite enough room for the bolt and collar, but after shaving a thin layer off the plastic knob there was an even line of daylight showing when it was all reassembled.

Worked fine on the bench again... but not for long...

Then a thought occurred:

All the teeth are profiled with a taper, so it is the basic law of physics that determines that when one set on the large lever pushes another on the small wheel the energy generated wants to travel in a line of least resistance - rather than turning at right angles to rotate the wheel !!
Also, being an old, once upon a time, serious cyclist, and therefore quite familiar with chain sprockets, should have known all along about the dynamics of engaging a small gear on the hub to a large bottom bracket - and having to stand on the pedals when forgetting to "shift" !!
Making the frame more rigid might divert some of the energy from trying to push the potentiometer outwards into the business of turning the wheel?

Some hope! That silly plastic knob must be the culprit then?

After the umpteenth dismantling, a hole was drilled in the box cover and another in the centre of the small wheel, then a tapered brass pin was pressed firmly in the gear to make a free running axle, thus ensuring that the distance between the axis remained constant.

View attachment 156328View attachment 156329

All set and powered up again, the new mods still looked reasonably pretty, and worked fine...

BANG !!

The energy couldn't go where it used to, so went somewhere else instead: It found the weakest link - in this case the aluminium shaft and bearing surface inside the potentiometer itself!

Lets face it, a little potentiometer like the one used here is usually only required to control volume and such like, by being twiddled (with even pressure) twixt someone's thumb and forefinger - and definitely NOT by a hard and unyielding set of metal cogs!

Replacing the potentiometer simply won't cure the fundamental design problem.

In utter desperation, and in order to avoid any temptation to apply that heavy hammer, I put the engine and stuff away on the shelf and slumped in front of the telly.
While watching (if that's the right word?) some completely meaningless nonsense, endlessly re-cycled plot, sci-fi film that was little more than a whole load of pretty, flashing lights, an idea suddenly sprang into my mind:

I am terribly sorry if I disappoint anybody, but it has absolutely nothing to do with zapping aliens!

It is nonetheless a cunning plan!!....

Pete.

I don't think there is anything wrong with the quadrant or pinion, just lateral force being applied to the pot which can't cope with it. I suggest making a pair of brass plate sides for the gear box, set closer together so just the thickness of the gears between the sides, and put the pinion on a steel shaft similar to the throttle lever shaft with a coupling to link to the potentiometer shaft. The potentiometer will need a bracket on the side of the gearbox to hold it in place. Possibly easier to make than describe, I could sketch what I am talking about if it doesn't make any sense.

 

[Peter Insole]

Wow, thanks guys, there is a lot to think about in your most welcomed replies!

Fitting some form of traction tyre arrangement was very tempting for a moment Neil, but then I realised again that the issue is twofold - namely that the tapered teeth push the wheels outwards, but the main issue is the dynamic one. Like the "derailleur" gears on my old bike, basically, the small drive ring will pull a large sprocket round with almost no physical effort - whereas the large chain ring will only move the small sprocket with a massive amount of effort - and that is only humanly possible once momentum is added to the equation! - Note; that sprinters with fixed top gears need to be held upright and given a push by an assistant at the start of each race!
I worry that the latter force could lead to potential slippage as a traction tyre becomes worn. The slightest error at either end of the lever stroke would put the "timing" out - with a risk of the regulator lever stopping in the fully "off" position, while the power remains "on" ?!

I guess it is those same dynamic reasons that cause the small drive pinions on the big drum to get chewed up on the "overrun" Tony, which I take to mean when the power is switched off and the momentum of the drum took over?!

You are quite right Overseer; a bit more of the belt and braces approach seems to be the answer? I have gathered some bits of brass sheet and steel rod already!!

Thanks again for the help,

Pete.

 

[michael080]

I don't think there is anything wrong with the quadrant or pinion, just lateral force being applied to the pot which can't cope with it. I suggest making a pair of brass plate sides for the gear box, set closer together so just the thickness of the gears between the sides, and put the pinion on a steel shaft similar to the throttle lever shaft with a coupling to link to the potentiometer shaft. The potentiometer will need a bracket on the side of the gearbox to hold it in place. Possibly easier to make than describe, I could sketch what I am talking about if it doesn't make any sense.

I had the same thought and I would like to add that the end position stops of these potentiometers are tiny and weak. Why not adding a motion limiter for the triangular gear wheel to limit its trave so that the pot wheel can't move more than 270° minus a safety margin.
Can also supply a sketch if not clear.

Michael

 

[Peter Insole]

Thanks Michael, you are quite right, but this is slightly embarrassing as I forgot to mention earlier that during all the fuss I have already got round to adding the prototypical style "limiter" stop bar on the regulator handle!

Here is the rather crudely repaired, original example on the poor old, somewhat bodged up remains of Wren at the NRM:



My own version is a separate, plain section of steel that is both flush rivetted (and soldered for good measure) to the back of the lever. While it lacks the subtly curved angles top and bottom, at least there is none of that lumpy weld all over it!



It is perhaps tad more beefy than the real thing, but this one has a job to do - particularly being handled by much bigger paws!

Anyway, thanks again Michael for the tip!

Talking of mitts, one thing that I had deliberately not made any further mention of was my "CUNNING PLAN!"... The reason being that I had ordered some parts and was unwilling to publicly commit myself to any new plan until I had the items in my hand. I felt unable to make any sort of reasoned judgement, simply based on website photographs?!

I was told not to expect a parcel until later in the week, but bing bong and hey ho, here it is...

Had a play, and wey hey! I fancy this might just be a goer...??



The best bit is that they are designed to be pushed and pulled from a knob near the top of the lever - let alone being less than two quid each !! I really felt I could splash out and buy an extra one - for just in case...!

It should only require a very simple pin and rod affair to drive it... with no nasty dynamic transmission forces to battle with anywhere along the way...?!

I can now start on a quick geometry drawing to work out where to drill some holes.

Pete.

 

[michael mott]

Linear stuff is often easier to sort that rotary.

Michael

 

[Osgood]

Except for clothes lines.

 

[Eastsidepilot]

 

[Peter Insole]

There are some well known words and phrases that are descriptive of certain types of attitude and behaviour: Many years ago I used to find myself pondering whether there was a specific moment when the term "Ladies man", "Jack the lad" or "one of the boys", turned into "dirty old man"?! Then one day I realised that it already happened, some time before, without me even noticing!!

Now I wonder if it is bravery, doggedness or tenacity that has become just plain "barmy" in somewhat similar fashion?

It must surely be so, considering how much time, effort and no little anguish that I have expended trying to get a model, mechanical regulator to work an electric motor in a convincingly prototypical, steam loco fashion?!

In a surprising moment of clarity, I thought it would be a good idea to wire up and check the new potentiometer before committing to any further action! I need not have been concerned, for despite it's very short, 20mm throw, the thing worked perfectly well. Thus satisfied, I felt confident enough to build a second acrylic gearbox...



This time, without the space required for the wide lever and round pot I could make the box shallower, while adding additional strength to the structure by mounting the bottom two screws slightly further inboard of the corners.

The next act required some very, very careful measuring indeed (for me anyway!) in order to ensure that the new pin was exactly in the right place. Get it wrong, and at worst the lever might overrun the delicate potentiometer stops at either - or both ends!



I was able to avoid mucking about with heat sinks by drilling the new hole slightly undersize, gently whizzing a slight taper at the end of the rod and with it still held in the drill chuck, pressing it fully home.

It was slightly saddening having to cut away all that previous work, but I doubt it would ever find further use in future anyway.



I was also lucky enough to have a short length of K&S tube, that being imperial was just sufficiently oversize on the metric pin to act as a free running roller.

A retaining cap was made using an offcut of 6mm rod, centre drilled the same size as the previous lever hole, then driven home and cut back afterwards.



The above arrangement should hopefully reduce friction and wear on the bearing surfaces.



At this point, everything seemed to be going swimmingly?!

I was still not quite clear in my mind precisely how to design and attach any kind of brass fitting onto the black plastic lever? I thought it best to quit while on top, and have a sleep on it.

No, it wasn't particularly comfortable!


Pete.

 

[Peter Insole]

Things had perhaps been going rather too well the day before, but in the cold light of a new morning; a horrible thought occurred...

When measuring and marking up for the lever modifications I had included an offset from the centre line in order to retain the relationship between the cotter pin holes and the set limits of travel of the permanently fitted operating rod. Prior to setting up the slider pot, the lever was free to flap around unrestricted, so I hadn't noticed a potential error - although it must have been rattling around somewhere in the deeper recesses as a nagging suspicion?! In a mild panic I checked it again...
Oh, what a turnip I am!! Somehow, I had successfully managed to cut the offset as a perfect mirror image of the one required. Too late, the "full on" position of the lever was now actually the "completely off"!

Oops!

Not all was lost though, as by great fortune I had previously decided to solder the pivot into the lever. It was therefore a relatively simple matter of scribing a new reference line at the correct angle, clamping the assembly in the bench vice, popping a length of piano wire through the cotter hole, sweating the joint with the little gas torch and jerking the pivot round to match the line when the solder "popped". Easy enough done, but I was very conscious all the time that it was extra work that should not really have been necessary!

Meanwhile, after considering every seemingly possible solution - almost to a standstill - on the subject of a slider knob alternative, I eventually seized upon the simplest arrangement-cum-bodge...



Although cut a bit roughly, and indeed wonky, it would hopefully do nicely for fitting in a dark place, and once the bearing proper was shaped and soldered on too?!



Pinched firmly in place, the little bearing "aint goin nowhere!" If the pot does fail at any point and has to be replaced, it wont matter at all if the plastic part is wrecked by the effort of prising the tongues back off for re-use?!



Although the arrangement does appear to be slightly odd, I felt that mounting the pot on one side would ensure that all the driving force of the lever would remain evenly delivered along the centre line of the pot - as it is designed for when fitted with a more conventional plastic knob.

The last act was to make a bracket. Only then would I know for sure whether I had got all the measurements right?!



After a test proved that good old MEK worked on both the plasticard and the gearbox base material, there seemed little point in making things more complicated than necessary?

It did turn out to be a bit fiddly trying to clamp the blighter in the correct position inside the closed up box - I couldn't do it any other way - before applying the solvent. I did at least have some brief wriggle time while it got to work!



Neither the pot or bracket were quite square when I had finished! A tweak on the bearing (perhaps a tiny tad overdone?) should keep it running freely?



Spot on!! Hurrah! At long, jolly long last!!

Well....

Not quite...

First time; way too much resistance... This time; not nearly enough!!

Control is utterly superb, precise and very smooth indeed. The problem is; that it is too much so - a mere breath is almost enough to throw the regulator over. Poor little monkeys with clumsy fingers could get scared off by an engine that can suddenly go tearing off like a scalded cat - or stop dead in it's tracks from a gallop like a wayward gee-gee?!

Groan!

I am now having to think along the lines of adding something pressing lightly on the axle in between the lever and the gearbox backplate to create a wee bit of friction and slow everything back down again ??

I shall be quite happy to leave that little task for "Springtime" though ?!

Pete.

 

[Osgood]

Why not just tighten the regulator stuffing box?

(You did incorporate a steam gland on the regulator shaft - didn’t you?).

 

[michael mott]

(You did incorporate a steam gland on the regulator shaft - didn’t you?).

Wicked!

Michael

 

[Tom Insole]

I will speak to one of my colleagues at work. they were building themselves a tripod head (from titanium of all things to make it lighter than it already was) He was playing with a rather sticky grease between two pieces of plastic that gave resistance.. with some of your fine precision a clamp around the shaft (with maybe an adjustable grub screw to loosen and tighten it) and then an application of some of that sticky grease then you can set the resistance on a the turn of a screw!


By the way, SOOOO glad it's worked!!

Tom

(Edit: quick look up the theory is based on a fluid head on a video tripod mount. the idea being the grease in the head gives resistance for smooth panning)

 

[simond]

Pete

I suspect it might be to do with angles and sines and cosines, and other arcane functions….

if you can arrange that the slider moves very little as the first movement of the regulator happens, and the movement increases as the handle gets to mid or full travel, it might help your case. This might need your “tuning fork” to be a bit longer, and the angle between the slider and the mid-point (or end-point) of the regulator travel to be altered.

if you can tell me how long the slider travel is, how long your little lever is, and the angle for the whole regulator travel, I can do you a sketch.

atb
Simon

 

[Peter Insole]

Thanks for the offer Simon. There is already an element of that speed differential by the simple principal of turning a rotary motion into a linear one. There is so little resistance anywhere within the gearbox that it is possible to "feel" a very slight tightening around the centre of travel. I imagine that it might be due to a tiny increase in drag as the slider starts slowly, speeds up and slows back down again at each end of the stroke. It is not very much, but there is only a 2mm vertical deflection in the 20mm stroke on the "yoke", but it seems to make a difference?! I allowed 5mm depth to the slotted hole forming the yoke just to accommodate any likely dimensional or constructional errors on my part!!

Trouble is, now the box has been screwed back down inside the boiler I can't get my vernier anywhere near it. The only clues I can provide is a photo of the crude and scribbly drawing I used to establish the basic principal.



Sorry, but I doubt there is enough of the information that you need in the above?!

The angles of the triangle (20mm x 22mm) represents the travel of regulator - although the pitch of same differs in that the handle is just to the right of vertical in the fully open position.

Sticky grease in a bearing sounds really interesting Tom, but I would be a bit concerned about it staying there! One tiny misplaced drop could spread very quickly and deep into a wood and fibre structure?!

Thanks too Tony, for as ever, your comment made me smile! Funnily enough though, thoughts around that packing gland have been focussed and shared by our own Rich as well - a very tempting idea!

Where's my leather and knotted string... ?!!

Pete.

 

[simond]

Hi Pete,

not sure if this is the right interpretation but here goes.



The travel on your slider is 20mm and the regulator lever arm moves through 54 degrees to do that, based on the 22mm radius of the arm.

If you arrange the slider as I understand you have, parallel top the 20mm dotted line, the first 5 degrees of the movement gives you 1.75 mm of slider movement.



Whereas if you arrange the slider parallel to the solid line (aiming for 4 o'clock) the slider movement is only 0.74mm for 5 degrees movement

Other angles of the slider axis will give different travel of the slider for a given angle of regulator movement.

This may not be possible, and you might run out of slider movement before you run out of regulator movement, then said little hands might be causing damage by accident,

another approach might be to fit a suitably inaccessible switch that connects an extra resistance in series with the slider resistance, although that will perhaps do more to limit top speed than to make the acceleration less fierce.

hth
Simon

 

[simond]

oh, and Pete, the versine movement "throw" is 2.4mm for the dimensions you've given - you've probably got enough clearance but it is important that the pin doesn't bottom in the tuning fork or your slider potentiometer's life will be short...

atb
Simon

 

[Peter Insole]

Thank you so much Simon for your kindness and for taking precious time to explain to me the principals with those drawings.

Carefully looking at your measurements, and taking heed of your warnings, I must admit that I appear to have got away with it yet again! The differences between what I had hoped for, and the errors I actually ended up with, seem to have more or less cancelled each other out ?!
Certainly, more by luck than judgement, there is actually no risk of an "overrun" on the slider pot, and unbelievably, there is even a very small gap between the pin and yoke at the bottom dead centre!

I do take particular note of that latter point however, and fully recognise just how close to the wind I'm sailing! I now think it would be very sensible to take the gearbox out one more time (hopefully for the last in a good while anyway?!) and make some essential adjustments. Doing so, at the very least would provide an assurance against problems arising from the inevitable heavy handling that the gear will have to endure, let alone regular wear and tear?!

Pete.