Well, we have been working on the list we made in the last post, and have some positive news, and some better understanding of the problems…
I spent a couple of days with the engine on the bench and a dial indicator attempting to find and measure the wear or absence of same…. We have a page or two of measurements…
The good news is that the piston rod bearings have no measurable wear, and inspection of the bearing pins confirms that wear is at most a couple of tenths of a thou. So we have shelved the plan to replace the bearings with needle rollers, and to rework the oil feeds, as they seem to be working… phew….
However, the crosshead bearings showed a lot of “slop” in the LP crosshead. This measured in excess of 15 thou movement of the piston rod at TDC, and is clearly a problem and maybe the source of some of the racket the engine makes. What was more interesting was that machining 10thou off the crosshead cover plates to take up this slack did not improve situation, nor did a 2nd 10thou cut!
What became clear was that it was not the crosshead that was locating the piston rod, but the steam gland/lower cylinder cover – far from ideal…
But this might explain why previous attempts to fix the knock had such limited success.
Once this fact was realised, we continued shaving clearance from the cross-head cover plates until we had the required 3thou clearance. This also required a little more hand scraping of the crosshead bearing on the engine standard to remove a tight spot, but all-in-all I think we have made progress, but only time will tell if this has “fixed” the ongoing knock.
The rest of the measurements seemed to indicate that the crank bearings were still all good (phew), and the most noticeable problem areas are around the weighshaft bearings, some waggle in the weighshaft arms and wear in the ahead eccentric-rod/link pins (not so surprising).
It may be that there is some wear in the eccentric strap, but this will be checked again when the gear is more fully stripped.
The pump scotch crank drive
Having deal with this initial work, I turned my attention to the scotch crank for the pump drives, which has been a continuing source of failures and problems.
The picture shows the scarring and multitude of parts in my original design, and the new crank at the rear (yoke to follow).
The problems with the old crank were:
- The key locking the webs to the axle allowed the webs to rotate slightly, so that all the thrust was taken by one or other crank pin bearing.
- The key locking the webs (and axle into the gearbox) used to “walk” out of the shaft, (probably because of 1 above) and then foul/scar the yoke bars….
- The yoke bars had on one occasion come loose, and resulted in the failure of the rods transferring the drive to the cross-head at the top of the pump rams (large bang, enforced abandonment of day trip, and emergency return to workshop for more parts).
- The problem listed in 2, meant that the ends of the key got misshapen and it became near impossible to disassemble the drive – this being aggravated by the lack of access space. (much cursing).
So, in summary, this was quite a major failure of my design skills. The replacement is simpler in design, if more complicated to make. The key is now only used to transfer the drive from the gear (worm) box to the axle, Squares are machined on the end of the axle, and “V”s formed into the ends of the two parts of a split web, which are secured with more of my almost bottomless tub of 1/4BSF Allen bolts I bought at a Guildford Model Engineering bring-and-buy meeting in the mid 80’s!!!
The replacement yoke is to be machined from some 1.25″ x 0.5″ mild steel bar, and will have a slot machined to locate the needle-roller bearings on the crank pins. These bearings and pins have performed well in the previous construction, withstanding a fair few hours of running, and several spontaneous disassembly events – so should be OK in the new assembly, We will arrange a minimal clearance in the bearing slots (2-3 thou) to try to create a solution with minimum clatter! As the yoke will now be a single part, it will not require the carful “fitting” needed with the current “bars, rods & nuts” arrangement, and so should not be able to fall apart either!
Here, you can see the new crank in position in the worm box (before crank pins were fitted). I plan to refit this higher ratio (7.5: as opposed to 5:1) worm box to slow the pumps again (as speeding them up did not help with the miss-calibration of the vacuum gauge (see last post) duh!!!) However, I also want to increase the size of the rams in the boiler feed pumps, and discover what happened to the o-rings that were once fitted to said rams, but are missing now!
So, it’s the yokes next, the pump rams, then the weighshaft bearing blocks, arms and valve gear pins, and then we can start on trying to decide where/how to mount the new condenser! …and what on earth is going on with the LP valve timing, as exposed by the Digital Indicator analysis….