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Working on a unit

11K views 56 replies 22 participants last post by  Derek Roger 
#1 ·
Can one of you engineery people tell me how you can work on one unit while the engine is still running? Can you disengage it from the crankshaft? If not how do you do it?
 
#4 · (Edited)
Beg to differ.
It depends on what the 'work' is. Should the con-rod still be connected to the crankshaft and the 'work' involve those moving parts, then I agree -NO!
However other 'work' is sometimes possible. There is even a video available of a tie bolt being renewed without stopping the engine on one of the very large Maersk container ships.
 
#5 ·
No you can not .
Stop the engine ; engage the turning gear ; make what turning is required to effect the repair . Then lock out the turning gear ( I used to remove the fuses and put them in my pocket ) so nobody can turn the engine with personal in the crankcase . Do your repairs and reactivate the system .
With a ship with two engines driving one shaft through a common gear box it is possible to shut down one engine for work . |However one has to physically block the clutch engagement of the engine being worked on and engaged the turning gear ; then proceed as above .
On a 2 Stoke on can hang up a fuel pump on a unit so there is no combustion in that unit ( allows a scavenge fire to burn itself out when running at reduced revs ) But you cannot work on the unit which is still turning .
On a 2 stroke it is also possible to stop the engine and "hang up a Piston at TDC " The piston rod has then to be disconnected . The connecting rod has then to be disconnected from the crankshaft and slung clear of the crankshaft throw and firmly secured . At that time some work could be done on a cylinder head etc. While the Engine is running on the remaining cylinders
Not to be recommended except in dire cir***stances .
As a Unit consists of all the running elements ; a Unit by definition cannot be worked with the engine running .
 
#11 ·
I've seen work done on a cylinder while the engine was run on the test bed in Harland and Wolff. The complete "engine unit" was stopped, lower con rod removed and cylinder isolated before engine re-started. In all my years there I only seen this happen once so can only assume it was some sort of test or demonstration. NOT a procedure I would like to replicate when at sea.
 
#13 ·
I can see this being done on an engine unit to allow some minor work to be carried out on the unit. With one cylinder isolated i.e. not functioning, the engine would be very much out of balance, though it could still run at a lower rated output.
I guess in general that the more cylinders an engine has, the better it will run with just one cylinder down. (Maybe a Vee 12 would run better than an In-line 7)?

Ron.
 
#14 ·
Run with a unit hung a couple of times (and with the fuel off a unit many times), in the manufacturers data there should be a table of barred speed ranges for various combinations of running gear removed, as Ron suggests dynamic balance is the problem. Also with 5 cylinder engines or less there is the potential problem of stopping in a "dead" position.
 
#16 ·
As Fred above said, depends on the type of work, and the type of engine installation.

Resetting a Woodward governor after repairs on a Fairbanks Morse (generator in this case) required the engine to be running. Oscillations can be frightening (saw that first on a tanker generator in drydock, syncroscope even wilder).

On steam turbine gen governors it was a fairly frequent job. But it tested the nerves.

i have heard of other jobs done with engine running, but I believe sanity (or lack of it) had something to do with it.
 
#19 ·
I have a 1920s everyman slim volume on Navigation. The preface says that a high up navy admin man went on board a ship, his first comment was "Goodness, it's hollow!"

Reading this thread, this starts to seem like a sensible comment.

I had no idea that you did this kind of thing in the engine room, and other bits you had sway over. I admire you for the skills you had.
 
#20 ·
spongebob - Great if you can do it, and I have, many times.

Ron Dean - The lower engine output with a unit 'out' slows the engine anyway and as Duncan112 states the manufacturer's information should be consulted for barred critical speeds. A further point is that there is a major difference in 'balance' with the con-rod disconnected and just the fuel shut off (fuel pump 'lifted'). SOP on the H&W double acting, opposed piston 2 strokes in the event of a scavenge fire.

Derek Roger - Well I guess I am not an engineer, because to me working 'on a unit' has a whole different meaning to 'doing a unit'.
 
#21 ·
Derek Roger - I hope you do not mind but I believe you missed two very important points in your description of 'hanging up a unit'.

1. It can also be done with 4 strokes.
2. The lubricating oil flow to the bottom end has to be 'secured' (plugged). I have personally seen one instance of where this was done according to the manufacturer's instructions and with the builders supplied 'gear'. One problem I do not believe it had ever been tested running because the 'gear' disintegrated with damage to the engine. The manufacturer shall remain nameless!
 
#22 ·
Bottom line working on a unit ( as understood by engineers with the engine running )is a crock of sh1t . There are minor adjustments that can be made ; that is it that's all .
With the mass of a unit ( Con rod ; piston rod ; piston all doing 2 revs per second ; what are you able to do . I have given examples of how work can be carried out after a shut down to disconnect various components . That is all that can be done .
I find this proposal to be the worst of all engineering posts .

Real engineers please comment . Derek
 
#24 ·
#14 and #15.

Never ran with unit actually hung up but once with fuel off. Burmpac Bahamas.

Telex below says most but not all.

The crack was not one detected by nuances of water in oil or pressure fluctuations across the engine but with an obvious split emitting a sheet of flame a metre or so in length across the tops with every revolution, each accompanied by a loud ripping sound.

On the following morning as I gazed at this triumph of Italian engineering - more in amazement than awe, a Dante-esque tableau with blue smoke gently ascending from many open and leaking parts of the exhaust system and blowby (the underpiston space was open to the engine room) - there was a loudish bang followed by a hiss. It would have sounded like a large compressor unloading were it not for the hiss continuing.

The cooling water had been isolated as well as the fuel and overnight had been enough to boil the remaining water (or perhaps to raise the rate) sufficient for it to blow out a gasket in the jacket water connection.

Only a day or so onboard but unforgetable.
 

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#28 ·
No, that happened before I was on her but I recall the story - like you say, character building. I was moved back to the chemical carriers after BB so missed the two Fin boats - if I recall correctly one of them suffered a very bad engine room fire?

Wasn't the Fiat main engine on BB prone to shooting the occasional piston bolt out the open scavenge and across the engine room?
 
#31 ·
To go back to square one . You cannot disengage it from the crankshaft while the engine is running .
A unit consists of a cylinder head with all its components ; a piston ; a liner and a piston rod and a crank driving the crankshaft .None of the above can be worked on while the engine is running .
I have explained what can be done after the engine has been stopped and the "bad " unit isolated .
While the engine is running the only work that can be effected would be to tighten cylinder head nuts or apply a coat of paint .
Which part of NO do you not understand .
Think I will go and poor a drink Derek
 
#36 ·
The question is similar to " can you replace a section of wire on a high tension grid wire of 400KV while it is still live "

Think about it . Derek
Weeeeellll you could but it or you wouldn't be live any more [=D]

Question... what's black and charred and hangs from the ceiling?


A Harland and Bluff spark repairing the lights (electrician for the uninitiated) (LOL) NOW see what you've started Derek...
 
#40 ·
Derek thks for clarifying what is meant by 'unit', although it was evident from the beginning what Endure was asking about. But to some of us that is a 'line'. A unit is the whole engine. A six cylinder in-line is a unit with six lines and the cylinder/piston/head assembly is one 'line'. (and if you ask me an 18cylinder vee is a curse, and a square four fullagar......but that is something else.)

Re the high tension wire thing, the birds knew that long before wiki. They also knew never to sit on adjacent phases and flap wings. But 'sparkys' are also known to do insane things in tight situations-like pulling out an exploded circuit brkr from a live 11kv busbar. You start breathing only when the hissing in the contact spouts stops.
 
#46 ·
A six cylinder engine is an engine ; not a Unit . It consists of 6 units .
When in port the Chief Engineer or 2nd Engineer says we are going to pull number 3 unit ; all understand that the complete set of running gear ( Number 3 ) will be pulled for inspection and probably survey . I t will consist of the Cyl Head removal ; grinding of the lip inside the liner if necessary ; removal of the piston ( normally to be replaced by a spare ) Cly liner calibrations to be measured ; checking of liner lubricating quills . Overhaul od piston rod gland and piston cooling water glands . Cleaning of the scavenge space .
In section and measurement of cross head bearing wear ; inspection of con rod bearing and also the checking od a least one main crankshaft bearing plus taking a set o of crankshaft deflections . In the case of a Sulzer engine with rotary exhaust valves these would also be inspected and corrected as necessary and the timing set .
It would also be an opportunity to check the fuel pump spill to ensure that that particular unit was performing to specifications .
A very busy day . Cannot be done with and engine running as any idiot could understand .

I don't know why some have so much difficulty in understanding what a unit is .

Changed multitudes of the beggers .

Derek
 
#45 ·
The fullagars i had to work with some years ago. Paired lines(units) have pistons cross connected by tie rods. Seen from inside crankcase when turned by air, it discouraged any work during operation. Still, since pistons were cooled by oil via telescopic pipes that were troublesome there was the occasional peek inside to see if something was wrong.

The Vee unit i had in mind is relatively modern. High power to weight ratio, it is highly stressed. Leads to many and frequent problems.

Not exactly what this thread asked, but then the type of engine was not indicated.
 
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