Axle pump problem

[jj]

I've been having a problem with something squeaking and binding at one point of the wheel revolution on my Modelworks Britannia. It happens after a few laps as the engine heats up, and causes the motion to be quite jerky. It rolls perfectly freely when cold. I thought at first it was lack of cylinder lubrication causing the piston rods to bind in their PTFE glands, but the problem persisted after overhauling the lubricators and fitting new O-rings. Last time I ran, after a few laps of squeaking and binding, I disconnected the connecting rods and the piston rods would slide quite freely. We then sloshed oil over the axle pump eccentric and ram and it was very much improved, so the problem seems to be there. Yesterday I dismantled the pump and found that the ram (which is brass in a brass body, with an O-ring) slid quite freely, but the front half of the eccentric strap was very badly worn - 2mm longer on the front-to-back diameter than top-to-bottom - and the brass at the front side had been deformed to form a lip as shown in this photograph:




The strap has obviously been subjected to extreme pressure, not just wear through lack of lubrication.

I've left the pump off and I can happily run with just my two very reliable Chiverton injectors, but I'm puzzled as to what is causing the binding. The problem started after I dismantled the axle pump to fit a new O-ring, and I wasn't then able to replace the graphited string gland packing because of difficulty of access, but I didn't think that this served any real purpose. Any ideas?

Regards, John

[jj]

I think I've worked the problem out for myself - it often helps to write things down!

The steel eccentric has quite low shoulders on either side of the bearing surface as shown in this photo:




and I think that the strap, after some initial wear has jumped up onto one of the shoulders and jammed on it. The photo also shows from the unpainted strip on the axle that the eccentric has moved sideways, presumably wrenched round after the initial jamming. This explains the distortion on the edge of the strap. So the problem is not caused by heat but just random movement. Perhaps dismantling the pump to fit the new O-ring somehow changed the alignment and caused the problem. If I get a new eccentric made I'll take more care to lubricate it every time!

Regards, John

[springcrocus]

I think you are probably correct, John. Here is a picture of my eccentric from the Spink drawings. You can see that the shoulders are quite a bit higher.



Steve

[Alan]

what a horrid eccentric with minimum shoulders. Pump should just go back and forwards and shouldn't need shoulders, but it help keeps things lined up

[jj]

Other Modelworks owners may like to know that I was able to buy a spare pump eccentric strap and pump O-ring from Engineers Emporium (aka LA Services) who took over the Modelworks stock of 5" gauge spares. Surprisingly they make no mention of this on their website.

Regards, John

[Jim]

That's very good news for you John. Glad to hear you have been able to sort out the eccentric which really was a shocker. Hope Oliver Cromwell is soon back in business.

Jim

[jj]

Thanks Jim. The remaining problem is that the eccentric has moved slightly sideways on the axle, as shown in the photo above with the strip of unpainted axle showing, and even after removing both grub screws and using a Stillson wrench on it I cannot move the eccentric - it's jammed absolutely solid. I don't want to use heat for fear of weakening the Loctite on the driving wheels. Any ideas for softening the paint on the axle, which is presumably what's causing the jam?

Regards, John

[Jim]

I hate to say it John but as both Steve and Alan have said the eccentric's shoulders are too small to properly contain the eccentric strap. 

Jim

[Roger]

It might be best to very carefully saw the eccentric from the axle, a tedious and fiddly job, and make some split new ones with bigger flanges. If the eccentric strap is that badly worn, the eccentric isn't going to be round any more and I agree that those shoulders are too small.
As an aside, I've used one of my fixing holes under the strap as an oil reservoir, with a piece of felt in it. I wonder if anyone else has drilled a hole in an eccentric like that for this purpose? Obviously you would avoid putting a hole on the side where the greatest load is.

[jj]

I agree that the shoulders are less than ideal, but really the problem is entirely my fault for not lubricating the eccentric diligently enough over the five years that I've been running the engine. There is an oil reservoir which can be seen towards the top right of the first photo above, but it's difficult to get at and frankly I took the view that a bit of wear on the eccentric wouldn't matter that much, so didn't bother. The front half of the strap must have worn by about 2mm through lack of lubrication before it jumped onto the shoulder. If I can re-align the eccentric on the axle and then fit the new strap and lubricate it every time, it should last a long time.

I might drop the driving axle assembly from the frames in order to get a better grip on the eccentric to realign it.

Regards, John

[gingerneer]

John something that might help ensure you get oil to the eccentric and up ram, is to have a bit of copper pipe (about 1/8") bent up leading from the running board to the above the the said parts. The pipe needs to drop down to ensure the oil flows down it, a squirt of oil every couple of laps should keep the pump oiled.

Will

[jj]

Thanks Will, that's a very good idea.

Regards, John

[GWR 101]

John, hope you don't mind an intrusion but I was wondering about lubrication for the axle pump on my Juliet and have been thinking along the lines mentioned by Will. Because of height constraints (top of eccentric to bottom of ash pan) it would appear very difficult to get oil into the eccentric strap. I was wondering if a small pipe in my case screwed into the strap and as Will suggested brought to near the foot plate would be a viable idea. Because of the movement it could only be secured at the one end, but I wondered if due to its short length and light weight it would be workable and not cause an out of balance problem with the eccentric. This may be a completely illogical idea and a recipe for disaster or some other solution may have already been tried and tested. Regards Paul

[jj]

Hi Paul

I was thinking along the same lines myself. On my Britannia there appears to be plenty of space behind the driving axle to take a short pipe up from the rear of the eccentric strap and ending in a small cup that would oscillate around in the space behind the rear end of the horizontal stretcher. This could be reached with an oilcan poked under the running board and through the dip in the main frames between the wheels. I can't imagine that the small extra weight oscillating at most a few times per second would cause any vibration.

Regards, John

[4fbuilder]

Good evening John,

From the photograph I suspect the original strap was from brass, ideally gunmetal, but better still cast iron for the strap on a steel eccentric; I'd follow the suggestion of dropping the axle and sawing and machining away the original eccentric and making up a split replacement, all time consuming but on completion will last the life of the locomotive. You say you have two perfectly good injectors on the locomotive, why not just stick with them and abandon the axle pump altogether along with it's band brake. I ran 5" gauge locomotives every weekend for years in public passenger hauling with only two injectors and not even hand pumps. Just a suggestion.

Regards,

Bob

[Jim]

Hi John,
I must say Bob's suggestion make sense to me. I haven't fitted the axle pump to Boadicea for exactly the reason Bob gives. If it were me, I'd save myself a lot of work and leave as is and continue running with your two injectors which from what you say, is the way you operate anyway. 

Jim

[fostergp6nhp]

Any pipe sticking up from the eccentric strap will very likely fatigue and snap from the osilating motion and the momentum of the pipe.

[Roger]

I don't suppose there's any reason why you can't connect a very fine hose to the oiling point on the eccentric strap and bring the other end to an accessible point. I've seen some ultra flexible pipes that would cope with the motion.

[GWR 101]

Hi John, this is my proposed oil feed as you will see it is probably only a couple of inches in length and bringing it from the front and bending it around a curve my help reduce any possible inertia stress as mentioned by fostergp6nph. I would be interested in using a flexible pipe if that was suitable, not sure if Roger has some details ?. I believe this has the added advantage of a much greater oil carrying capacity than the original design and the extra work involved is minimal obviously the original oil hole would need to be capped. Apologies for the quality of the sketch (not quite back of fag packet) but hopefully its better than trying to describe it. Regards Paul

  

[jj]

Hi Paul

Yes that's exactly what I had in mind, except coming from the rear of the eccentric in my case. Bear in mind that the end of the pipe will rise and fall by 1/2" as well as going backwards and forwards, so you may want it to be in line with the top of the frame at its highest point, and roll the engine to that position to fill it.

I'd be very surprised if the inertial forces (1/2" displacement at a frequency of 10 cycles per second at the very most) cause any problem, but I suppose a thin reinforcing strut from the end of the pipe to another point on the eccentric could be considered.

Regards, John