Model Boat Mayhem
The Shipyard ( Dry Dock ): Builds & Questions => Steam => Topic started by: derekwarner on January 03, 2022, 11:48:57 pm
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This superb model engine is from the Midlands Engineering Exhibition & shown in MBM today
I have worked on the hydraulic control systems of engine turning gear on 9 cylinder, [edit] 40,000 HP 2 stroke diesels, however have not seen an example of turning gear on a model steam engine
The worm appears a single start, so by best guess, it is ~~48:1 reduction?, however the interesting point is that the gear appears to be fixed and not able to be dis-engaged & swung-out when not used?
Derek
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Well spotted, It’s got me stumped too Derek. Perhaps a sliding dog clutch out of view??? Need more pics.
Jerry C.
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As the baring gear has a ratchet handle I wonder if the shaft gear is fitted with a roller clutch?
Regards Ian.
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Interesting - indeed I cannot really see the purpose of this arrangement as there does not seem to be much room for any kind of clutch and why bother when a simple pivot of the worm would be much more effective.
The only think I can think of is that it was a temporary fitting so they could rotate the crank to set up the valve timing?
Cheers
Geoff
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Perhaps the shaft is splined and sits in a blind hole in the bottom bracket. A matching spline in the bore of the worm would allow the shaft to be lifted out, the worm removed and the engine free to turn. Obviously after first reversing the ratchet to back off the worm to generate a clearance and allow free removal.
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As Ian has said..."baring gear" and I agree this is a more appropriate term ......simply as used with the Bar to create the rotational motion .... as opposed to turning gear which is a term [more generally used] for a motor-driven function [be it an electric or hydraulic motor]
So we see here are a couple of potential swivel points,
a. the first swivel point under the column, but the main yoke appears to be bolted to the engine base
b. the second appears logical, with the secondary yoke swivelling within the main yoke, however we see no evidence of the bolted swivel or attachment
:o I find it difficult to understand why a builder of such detail and quality would simply produce the engine with the baring gear permanently engaged?....
[unless intending at a later time, to remove the Bar, and add a small electric motor drive to show the engine in slow motion?]
Derek
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(https://www.modelboatmayhemimages.co.uk/images/2022/01/05/Image22.jpg) (https://www.modelboatmayhemimages.co.uk/image/fhcnh)
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Lots more pikies on t'internet, "Dodman Compound Marine Steam Engine" but no clarity of worm disengagement.
Regards Ian.
Seen Baring gear on other full size engines. Despite "Self starting" 90deg. cranks some needed to be turned over centre.
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Ian, I once sailed on a ship with a three cylinder, 120 deg crank, opposed piston, two stroke, medium speed Doxford. It had an 'automatic' baring gear, the idea being that, being symmetrical, if it should stop with any cylinder at TDC or BDC a sensor would identify this and the automatic baring gear would engage and turn the engine out of the dead spot ready to start. This was a complete Heath Robinson affair consisting of hydraulic arms that engaged with pawls on a separate flywheel. During the first trials the 'Praying Mantis', as we called it, decided to engage while the engine was running.
The Praying Mantis was ripped off the tank tops, flung across the engine room putting a nice dent in the ship's side and prayed no more!
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It would seem to exist out of engagement. But I notice the worm is not central within the holding fork, is their some eccentric cam internally that swings the worm into engagement on operation of the lever ?
(https://www.modelboatmayhemimages.co.uk/images/2022/01/05/890AEBEA-E6E2-4E27-8351-B0E4BEE8381A.jpg) (https://www.modelboatmayhemimages.co.uk/image/fhSnQ)
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If you think about it the casting holding the worm is a single piece. Therefore the only way in which this could be assembled is by sliding the shaft through the top bearing and through either splines or a key inside the worm, which is slid in from the side, and right through to sit in a blind bearing in the bottom of the casting. If that is how it has to have been assembled then this is also how it can be disengaged.
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Is it also called "Jacking Gear"?
(https://i.postimg.cc/kg3dtdfQ/Jacking-Gear-2a.jpg)
https://maritime.org/doc/merchant/engineering/part3.htm
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That would be my take on it :-))
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Seems a very cumbersome way of doing things Phil, is that because it would be very rarely used?
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It would be used prior to starting with the drain cocks open to ensure the cylinders are clear of water and it would be used for maintenance for positioning the crankshaft, pistons and running gear to allow various parts to be removed and refitted as well as setting valve timing.
The Americans call it "Jacking Gear"
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If you think about it the casting holding the worm is a single piece. Therefore the only way in which this could be assembled is by sliding the shaft through the top bearing and through either splines or a key inside the worm, which is slid in from the side, and right through to sit in a blind bearing in the bottom of the casting. If that is how it has to have been assembled then this is also how it can be disengaged.
Bunkerbarge
If the worm was bored off centre , say 1/16" and a keyway cut in it the shaft going down thro' could have a matching keyway cut and dropped thro' with the key in it to match the one in the bore and slide into the worm.
A key passage would have to be cut in the top bearing to allow the shaft thro' the worm and locked off at the bottom, this would make it easy to remove for maintenance.
As the ratchet is turning anticlockwise and can only turn in that direction it only then requires to turn to engage and release..
George.
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As BB says....."assembled is by sliding the shaft through the top bearing and through either splines or a key inside the worm"....
If 'the worm was bored off centre' as put forward by George, then splined......the engagement we see could be the result of just one spline out O0
All of this appears & sounds the more logical....... so the 'as shown' depth of engagement between the worm & wheel is I think a red herring..
[The engines Reversing Gear worm & wheel quadrant displays optimal engagement which supports an error in the assembly mis-matching of a spline in the worm to shaft for the Barring Gear]
That image of a Liberty Ship engine with the words Removable Worm seals the deal [these Folk could have copied the design from others]
Either way, a suburb model .....https://www.stationroadsteam.com/1-12-inch-scale-alfred-dodman-compound-marine-engine-stock-code-6954/ (https://www.stationroadsteam.com/1-12-inch-scale-alfred-dodman-compound-marine-engine-stock-code-6954/)
[thanks Ian]
Derek..
oh PS Martin, Americans have always tended to have their own alternate names for most things......I have also heard them called Jacks O0
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Bunkerbarge
If the worm was bored off centre , say 1/16" and a keyway cut in it the shaft going down thro' could have a matching keyway cut and dropped thro' with the key in it to match the one in the bore and slide into the worm.
A key passage would have to be cut in the top bearing to allow the shaft thro' the worm and locked off at the bottom, this would make it easy to remove for maintenance.
As the ratchet is turning anticlockwise and can only turn in that direction it only then requires to turn to engage and release..
George.
George you are assuming that the key has to be fixed in the shaft. Think about how simpler the arrangement would be if the key was inside the worm bore, either held by locking plate or interference fit, and the shaft simply had an open ended clearance fit keyway machined in it. It would not need to be off centre and it would go clear though the top bearing.
Another option, and the more I think of it the more likely it might be, could be a hex shaped bore in the worm with a hex shaped shaft, which was a smaller diameter across the points than the top bearing. The lower bearing would then have to be smaller than the hex across the flats.
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George you are assuming that the key has to be fixed in the shaft. Think about how simpler the arrangement would be if the key was inside the worm bore, either held by locking plate or interference fit, and the shaft simply had an open ended clearance fit keyway machined in it. It would not need to be off centre and it would go clear though the top bearing.
Another option, and the more I think of it the more likely it might be, could be a hex shaped bore in the worm with a hex shaped shaft, which was a smaller diameter across the points than the top bearing. The lower bearing would then have to be smaller than the hex across the flats.
Hi BB
Yes that would be a better solution having the key in the worm and a keyway machined full length in the shaft and there is no need to cut a keyway in the top bearing
I don't think splines are necessary and that would be a costly operation, I do think that the worm centre would need to be off set to allow disengagement if the ratchet lever was a permanent fixture.
If however the complete assembly was removed after doing what it is intended to no off set of the worms centre would be required, this would mean that all dismantled parts would need to be very closely stored to avoid not being there when needed.
It was Baldric's post that mentioned the off set of the worm which is clearly seen on Martin's larger pic in the disengaged position, also on one of the pics in the link Derek posted.
It's a pity that the maker of the engine couldn't be contacted to explain, must say that it's a very well made model.
George.
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The eccentric vertical worm shaft holds water to engage and rotate the worm for baring using the ratchet lever being keyed in one direction operation but how to disengage? DOH, take the lever off so it ratchets in the opposite direction rotating the worm shaft in the opposite direction.
Nice logical thought Baldrick and George.
Richard, Saw the pawl and holes in flywheel idea in one of the model comics to restart a steam engine in a boat.
Regards Ian
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As Ian says, all you need to do is to lift the ratchet handle off the square end of the shaft, invert it, replace it upside down and give it a touch of movement in the opposite direction. This will give you a clearance to remove the worm.
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Ian & B.B.( who I assume is Richard,)
I don't see any reason for removing the worm which is only another reason not to in case as some things do, disappear when needed.
If the worm is in fact showing off centre in the pics it leads to thinking that the worm is bored off centre,
when the worm is in the drive position it is assumed that the main usage is to retime the engine and one of the pistons can be brought up to T.D.C and as we all know only a crack in the slide valve is required to set the engine in the direction that the crack will show and it can be used to turn the main shaft if required, all that then is required to disengage the worm is to use the ratchet and it allows the engine to be steamed.
Yes it's easy to turn the ratchet over and turn the engine or change rotation but the point is it's always available and out off gear , not stuck in a cupboard some place.
Well that's my thoughts
George
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I'm afraid I don't see it as being off centre.
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Me neither, but I think we have the answer, its a jacking engine to turn by hand the crank over to both expel any condensate and to assist with valve timing. The other use given that its a worm is to lock the engine in place so in real life the bottom big end bearings can be changed with no chance of the engine rotating into the well and crushing the engineers.
The fact that the modeler made this is just another piece of excellent workmanship to show off. I suspect that the crank lever and the inner shaft is removed to facilitate removal of the worm gear which would just slide out permanently and be stored which would be a simple exercise. In my opinion anything which can be slid out of the way temporarily has the potential to slide back again with disastrous results if the engine is turning.
Its an excellent model and the builder is to be commended!
Cheers
Geoff
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Me neither, but I think we have the answer, its a jacking engine to turn by hand the crank over to both expel any condensate and to assist with valve timing. The other use given that its a worm is to lock the engine in place so in real life the bottom big end bearings can be changed with no chance of the engine rotating into the well and crushing the engineers.
The fact that the modeler made this is just another piece of excellent workmanship to show off. I suspect that the crank lever and the inner shaft is removed to facilitate removal of the worm gear which would just slide out permanently and be stored which would be a simple exercise. In my opinion anything which can be slid out of the way temporarily has the potential to slide back again with disastrous results if the engine is turning.
Geoff, good points you may well be correct.
George
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Having followed Derek's link to Station Road steam there is a page of a collage of pictures showing the model of the engine, if you look to the R/Hand column of pics # 6 down there is an end view which shows the worm and the wheel, which is keyed to the main shaft.
If you drag and drop to the desk top a thumbnail comes up, double click on that and a full size pic can be had and if you enlarge the pic it shows the Worm Wheel keyed to the main shaft and the worm clearly disengaged. ( I use Windows 11 ) note the wheel's teeth are radiused to the worms dia.
Thanks Geoff for the explanation of the possibility of a mishap if the gear was to move if left in the disengaged position while the engine is running causing damage hence it would be removed to avoid accidents, which was a thing that didn't come to my mind at the time.
As the worm has no other method of engaging the wheel if it is not bored off centre with a key fixed to the bore and the ratchet spindle passing thro' the bore as B.B. has indicated , how does it engage in the wheel as the cast bracket to the base plate isn't hinged, ??
George.
P.S. this is the first time posting on a new H/P computer as my Mac has died and I am not replacing it, please excuse any spelling mistakes as I am still finding my way with the new computer.
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I think in conclusion, when I first said....."the gear appears to be fixed and not able to be dis-engaged" ....was clearly 'incorrect'
Obviously the Designers and Builders some ~~120 years ago, realised when the engine Baring Gear was finished with, had two choices
1. the eccentrically bored Baring Worm + shaft + key + ratchet etc, were simply to be removed and packed away until required again
or
2. the Baring Worm rotated eccentrically + keyed out to provide a positive disengagement to the Worm Wheel, and the ratchet stored away
If the engine was used in a marine application [with the seawater condenser] the decision in the alternate storage of the Baring Gear would have been that of the Chief Engineer, who would I suggest inspect the Baring Gear orientation in his Check List, prior to starting of the engine
[There certainly could be arguments for either option]
I too initially, was looking for something that was not there. Would engine Turning Gear be designed on similar lines today?, probably not
Derek