Hydraulic-locking in Bassett-Lowke ‘Enterprise’ and ‘Super Enterprise’ locos.
26th June 2017
The following notes are based on long-experience of these engines ( 40 years ). Much of the information is fact; some is opinion. I shall try to make it self-evident which is which. Clearly the ‘facts’ can’t be changed, but other folk are certainly free to hold differing opinions. There is a lot to read and to comprehend; so don’t necessarily expect to be able to do this with just a single, quick read-through !
Some facts which are pertinent to the phenomenon of hydraulic locking in Bassett-Lowke locos
1) When first steamed from a ‘cold start’ most ‘Enterprise’ locos ( hereafter denoted ‘E’ ) and ‘Super Enterprise’ locos ( hereafter denoted ‘S.E.’ ) do experience some measure of hydraulic-locking. Often this can be readily cleared, but occasionally it is so severe as to make a loco unrunnable. One contributory cause of the hydraulic-locking lies in the type of cylinder and valve which is used. Another contributory cause is the use of quite a lot of ‘advance’ in the setting of the valve-timing. A MAJOR cause is inadvertent over-filling of the lubricator tank. ( See later ).
2) The cylinders of these engines employ piston-valves of the ‘Greenly double-ported type’ ( see diagram which shows a vertical section through a typical B-L cylinder ). The ‘spools’ of the valves are of full, circular, round cross-section.
3) These piston-valves are normally a very close-fit in their bores and, unlike most ( usually flat-surfaced ) slide-valves, they cannot lift clear of the port face so as to release any entrapped water or oil.
4) In this Greenly set-up, the two longer, inner spools are the working valves, the two shorter outer spools are just moving seals to prevent leakage of steam to atmosphere out of the end of the valve-bores.
5) The working valve-spools each have ‘lap’ or ‘overlap’ at both their working edges ( ends ). Usually, with an ‘E’ or ‘S.E.’ the valve is given more lap at the steam-admission end of the valve than at the exhaust. But I have found that, with B-L. locos, little should ever be taken as certain! The diagram actually shows a valve having four equal amounts of lap. Such a valve is sometimes found on some B-L engines - the B-L Mogul loco for example
6) In addition to the rotational, angular-setting of the valve-drive eccentric ( a ‘slip-eccentric’ in the case of the ‘E’ and the ‘S.E.’), the presence of lap on the ends of the valve-spools influences just when a valve opens a port and when it closes it, and this, in turn, has a significant bearing on the severity of any hydraulic-locking.
7) With an O-gauge or other small-scale engine, it is not uncommon for a designer to arrange that steam-admission to the cylinder occurs at the very moment the main piston reaches either its forwardmost or rearward most position … sometimes termed ‘front dead centre’ and ‘rear dead centre’ respectively. A consequence of this is that, usually, the exhaust-valve closes the exhaust-port quite late during the exhaust stroke such that most of any excess water or oil that has collected or formed within the cylinder barrel is usually able to escape. Hydraulic-locking is thus usually prevented.
8) In contrast, for arguably goodish reason, the valve-timing on an ‘E’ or ‘S.E.’ is usually set so as to be somewhat ‘advanced’, with the result that not only does the steam-admission valve open ‘early’ but the exhaust port also CLOSES early ….. and this is the main reason that any entrapped water and oil can’t escape and the engine displays hydraulic- locking.
9) A less-common cause of hydraulic-locking is that a mis-set assembly can result in the main piston passing fully over ( and thus wholly closing off ) one or other of the steam-ports in the wall of the cylinder barrel.
10) The steam delivery-pipe passes through the oil tank on its way to the cylinders. Oil is drawn out of the lubricator tank through a very small-hole ( ‘pin-hole’ ) in the wall of the steam-delivery pipe. Unfortunately, B-L took too little care in the positioning of this pin-hole and its position varies from engine to engine. THIS MATTERS, because the oil tank should be only filled ( part-filled that is ) up to the level of the pin-hole. The volume required to do this has been found to vary by a factor of almost two (!!) from as little as 9 ml ( or c.c ) to 17 ml . There are several techniques available for determining just where the ( usually invisible ) pin-hole is on any particular engine. One of these techniques will be described separately. ( See later ). The removable screw in the centre of the front of the smoke-box should thus be treated only as a ‘drain-screw’ for the oil-tank, and not as a ‘setting the oil level’ screw.
How to seek to avoid severe hydraulic locking ( opinion ) :
a) Avoid over-filling either the boiler or the lubricator - particularly the lubricator. This really does matter !
b) Warm up the loco with it either supported level on blocks, or on a purpose-made stand, rather than on a track. In this way the driving wheels are free to turn. With an ‘E’ , once the wheels show a genuine desire to turn, assist them gently; do not unduly force them. With an ‘S.E’., some folk find it best not to open the regulator until ‘a good pressure of steam’ has been created in the boiler. When there clearly is a good steam pressure available, open the regulator slowly and perhaps only about half-way. Perversely and conversely, other folk find that, with their own engine, it is best to have the regulator fully open from the outset ! So, experiment, and get to know your own engine.
How best to clear severe hydraulic-locking ( opinion ) :
In my experience, when severe hydraulic occurs, it is best to abandon any attempt at having ‘ a run’ and, instead, remove the meths burner and, when the engine is cool enough to be comfortable to handle, fully drain both the boiler and the lubricator. What one does next depends on whether or not one is able to readily unscrew the ‘cross-head pins’ ….. these are the pins which attach the front end of the connecting-rod to the rear end of the piston-rod. They are very small screws with a slotted head for which one requires a suitable, very small screw-driver. ( For the record, the thread is 10 BA ). These pins were not secured in place with glue at the factory, but, if your engine was bought second-hand, it is possible that some previous owner might have bonded the threads with a modern glue ….. in which case you might need to apply heat in order to help release them. DON’T get over rough with them and thus risk chewing up the slotted-head ! Support the back of the cross-head whilst pressing on the head of the screw so as to minimize any risk of bending the piston rod.
If you are able to remove these cross-head pins, you will then be able to turn the driving-wheels freely so as to move the valves into any position you wish. When a valve is either fully forward or fully backwards, then one or other valve-port is fully open. At this stage one can then, by hand, move the piston wholly independently just by pushing and pulling on the slotted ‘knuckle’ fitting on the outer end of the piston-rod. [ I find it convenient to do this with the engine upside-down; one can then see the valves more easily …. but beware, any subsequent discharge of oily water via the exhaust pipe and chimney will then probably go all over your trousers ( or kilt or skirt ! ) or onto the carpet ! ]. In this way, one can drive the hitherto entrapped fluid out of the end of the cylinder. Keep this pumping action going until it is obvious that the piston is fully reaching both end of the cylinder-barrel and that all the excess oil and water has been driven out. One can then replace the cross-head pins ( do them up tightly ) and all should be well ….. until the next time !!
Alternatively, if the cross-head pins cannot be removed, one can turn the engine upside down and ‘play around’ with the slip-eccentric whilst turning the wheels back and forth and try to get a combination of valve-position and piston movement which frees the entrapped oil and water. This is a fiddly ‘hit and miss’ method, but, with patience, does usually clear the problem.
Locating the position of the oiler pin-hole within the oil-tank.
Fully drain both the boiler and the lubricator. Remove the safety-valve from the boiler. Remove the top screw from the oil tank ( this screw is just ahead of the chimney ). Leave the other screw in place ( the one in the centre of the smoke-box front ). OPEN THE REGULATOR FULLY - if the engine has one that is. Devise some means of applying air pressure to the boiler …. the quieter the method, the better. ( Even lung pressure may well serve; I have just tried it ). Put your ear close to the top ‘filler’ hole of the lubricator ( oil-tank ). Listen to hear a gentle flow of air out of the pin-hole in the steam-delivery pipe. This air is going into the empty oil-tank. Inject about 8 ml ( 8 c.c. ) of water into the oil-tank, re-apply air pressure to the boiler and listen again. Repeat this procedure adding further water to the oil-tank – add about 1 ml at a time. When the ( very ) quiet air flow turns to ‘gurgling’, you know the water has now reached the position of the pin-hole. Drain and collect the water from the lubricator tank and measure its volume. This volume is just a little more than the ‘safe’ maximum volume of oil that can be put into the oil tank without over-filling it. So, when preparing the loco for a run, start with an empty oil-tank and put in a volume of oil 1 ml less than the volume of water you have just measured. That way, the oil level will never be above the level of the pin-hole and one major cause of severe hydraulic locking will have been avoided.
And that’s pretty much it. Over the years, I have ‘sorted out’ a lot of other folk’s ‘Enterprise’ and ‘Super Enterprise’ engines. Only very few times have I had to resort to the fairly major measure of resetting the valve-timing. ( This involves stripping down the front-axle and its fittings. The valve-gear on one engine had been set up so ‘advanced’, that, when on blocks, if one attempted to run the wheels ‘forwards’ they always self-reversed ! ).
A final thought ….. pretty much any engine will run on air on the bench …. it is a track test in steam that is the true test.
[posted by Martin]