ModelWorks Pride of Penrhyn Overtype Steam Lorry

Burrell Builders Home

Problem Solutions & Enhancements
Along the way I have encountered a number of problems most of which I have chosen to solve myself.  Unfortunately I did not keep notes so this is the best I can remember. 

For constructors not wanting to "go it alone" ModelWorks offer free technical support plus a guarantee of all  parts for 12 months following delivery of the last kit.  For addition support I would recommend joining a local model engineering society.  

The early kits were relatively straight forward. At times the instructions were misleading but nothing too difficult to fathom out. The first real challenge was kit 7 which included the crank , motion and valve gear plus the boiler lagging.  

The pivot for the reverser link arm would not fit, so I made a replacement on my lathe. I  think this must have been a general problem since Modelworks included a replacement in kit 11.  Since my version was working fine I have not even looked at the replacement.  Quite a bit of "fitting" was required to get the reverser working correctly - the correct travel and freedom from fowls.  The crankshaft had an ugly step in the middle which turned off in my lathe.  Fortunately the ends of the shaft we centred and this was a simple between centres turning job.

Keen to test the engine I made a brass button to place over the passage from boiler to steam chest, a plug for the safety valve hole and supplied air via the oil connection. Having carefully set up the valve gear I turned on the air in anticipation of motion.  The pistons moved fractionally then stopped.  Turning over the engine by hand I could feel it trying to work but not succeeding.  In an attempt to isolate the problem I disabled one cylinder (by removing the eccentrics) and immediately the engine sprang into life.  Then it struck me one cylinder was driving forwards while the others backwards! A check of the instructions confirmed I had assembled the valve gear as directed - the instructions were wrong!. My solution was to swap one pair of eccentrics. I chose the arrangement that allowed forward motion with the reverser lever closest to the boiler.  (5/6/05)   I have been studying the valve gear and I believe the way I have assembled the it may have reversed the angle of advance.   I now have it assembled exactly as shown in the ISO. see expanded write up

The next challenge was the boiler lagging.  This is a welcome addition over the previous Winson POP but the instructions were of little help and the supplied pictures too fuzzy to be of any use.  Eventually I got the lagging to fit and am pleased with the results, I seem to remember I had to do a few tweaks but can't remember the details.

Kit 8 included the pinion gears and drive chains.  As designed the fly wheel and pinion gears are loctited onto the crank shaft making it impossible to remove the crank from the assembled model.  I adapted the design to incorporate keyways on all gears so that the crank can be removed in the future.

As noted in the instructions the chain drive fouled the boiler lagging.  This necessitated open heart surgery on the completed lagged and painted boiler.  Had I known I would have delayed painting and fitting the boiler until after I had checked the clearance around the drive chains.

I do not recall any significant problems with Kits 9 & 10. (Smokebox and water tanks)

Kit 11 included most of the pipe work and fittings.  It was a significant challenge and without the use of my well equipped workshop I would have had to return many of the components for rework.  My Model Engineer set of tap and dies was much in demand (Tracy Tools).  The instructions gave no guidance (other than what goes to what) as to the route to be followed by the pipe runs and it took a long time to plan and bend the pipes to suit.  I cheated on two of the runs and shorted the pipes as this enabled a neater finish.  Anyone experiencing difficulty with this kit would be well advised to get help from an experienced model engineer.

Kit 12 was the final kit and included the bodywork and lubricator and final fittings. On assembly of the lubricator I noticed that the spring in its check valve was too strong.  I replaced the spring with a weaker spring from my junk draw. Due to a supply problem Modelworks have yet to supply the pressure gauge. (Arrived Jan 2004)

A tricky problem to solve was that of shedding tires. I have taken to moving the POP around the workshop wheelbarrow fashion and unfortunately the loads imposed on the front tyres caused them to separate from the rims.  The rubber tyres are glued to the alloy rims using loctite 401.  This glue is brittle and the distortion caused by the rolling load causing it to crack.  Not having a suitable flexible glue to hand I decided to bolt the tyres to the rim.  Having fixed the front tyres I have now experienced the same problem on the rear tires.  Since these have only been subjected to normal forces it is indicative of a design problem. 

This page contains a pictorial account of how I bolted the tyres to the rims.

Differential
Following steaming I found problems with the differential.  When I swept up I found a nut.  I hunted high an low and could not find where it had come from.  I was about to put POP away when I experienced what felt like a wheel spin.  I tracked this down to the differential slipping.

After I removed the back board I could see that the differential was in a sorry state.  The nuts on all three spiders were missing.  Luckily I found two on the workshop floor and together with the one from the sweeping I had all three. I replaced these and gave them a good tightening. 

This was not the cause of the slip.  I found that the drive axle was slipping in the crown wheel.  I had previously had trouble in this area in which both crown wheels were push out.  To prevent this happening again I installed brass bushes on the shaft so that there was nowhere for the crown wheels to go. I also cut small flats on the shafts to give the grub screws a better chance.  

I realigned the grub screw above the flat an gave it a really good tightening.  I shall watch this area as it looks like a weak point in the POPs design.  If I get any more trouble I will have to devise a better solution. Incidentally, the differential runs very smoothly. 

(27/3/04) When I steamed POP yesterday I tried it out on my very steep drive.  At 1 in 5 it put such an extreme load on the diff that it began to slip again but only after many successful runs.  I have now strip and Loctited the gears in place.  Originally I was disliked using Loctite but with so many parts coming lose I am gradually using it more an more.

Lubricator
Other than the problem of the strong check valve spring noted above I was concerned that the lubricator did not appear to work at low speed in the early cutoff valve setting.  I observed that the knurled  wheel wobbled but do not go round.  Despite this oil placed in the reservoir was used relatively quickly with no sign of it escaping where it shouldn't.  Having seen a picture of a POP with link arm moved up the actuating arm I decided to try this but I wanted to be able to easily revert to the original position. 

I came up with this arrangement pictured below.  I drilled three 7/64 holes with the lowest hole at the site of the original attachment.  I milled a 110 thou slot between these holes and made a spring loaded plunger to operate in this slot.  By depressing the plunger I can select one of three displacements.

To my surprise the improvement was not as much as I would have expected.  At the original position it was worse.  On investigation I discovered that backlash in the linkage was the cause of the problem and so I fitted a bush for the exocentric end of the linkage.  I also remade my plunger to eliminate backlash at this point.  With the backlash gone the lubricator works well.  I am not convinced that as drastic a solution is warranted and I would recommend first trying reducing the backlash in the linkage. (27/3/04) I now believe backlash in the Stevenson valve gear was the main source of the problem. When I originally fitted the vale gear I was too enthusiastic cleaning up the reverser slot and shoe.

Steering
The was a lot of slop in the steering, a definite MOT failure! It was impossible to steer a straight course and so I was determined to fix it.

I identified two problem areas in the steering box.  Red Arrow. The worm gear moved up and down since there was excessive clearance against the brass bushes. Green Arrow. Where the gear engaged on the worm there was play because it was not engaged to sufficient depth.  I presume that ModelWorks make it this way so that everybody's kits fits together.

The clearance against the brass bushes was relatively easy to solve. I knocked out the end bush with some stout blows and remanufactured an oversize bush.  I made it so that the gap for the worm was about 5 thou too small.  Holding the worm gear using a collet in the lathe I took a series of 1 thou cuts of the end, removing and checking for fit after each cut.  I was satisfied when I achieved smooth rotation of the worm gear with no up/down movement.

Solving the gear engagement depth problem was a little more tricky. I needed to move its centre towards the worm in small increments. It stuck me that I could use a similar technique to that employed on my rotary table.  If the bearing hole though the center of the bush was off centre, I could vary the engagement by rotating the bush.

I remanufactured the gear's bush with the central hole 25 thou off centre.  I tried 50 thou but the gear fouled the housing! Assembly was by trial an error to find the correct position.  I marked this and loctited the bush in position. A similarly located bush was manufactured for the cover plate

On assembly I found no discernable wobble at the drive arm.  About 45 degree movement of the steering wheel is needed to overcome the dead space in the steering; this is a tremendous improvement.  I have yet to try steering a course outside and don't kwon how long it will be before the backlash returns, finger crossed.

Ground Clearance

On firm ground the clearance under the fire box is just 0.82 inches
.

On softer ground the wheels are bound to sink in slightly reducing the effective clearance.

Solutions that come to mind:-

• Raise the boiler

• Altering the ash pan

• Fit larger diameter wheels

• Fit raising blocks above the axles

Raising or redesigning the  boiler would be and attractive option if I was building another POP from scratch.  With only 0.82 inches clearance raising the ashpan by an 1/8" would be worthwhile and I hope this would not aversely affect the draft.

I have looked around for suitable alternative wheels but have not come across any that are suitable. So I shall be investigating putting raising blocks between the axles and the leaf springs. 

As an experiment I have added 3/8 packing pieces above the axles. After clamping the leaf spring together with tool makers clamps I slacken the bolts and slipped in the packing pieces. This has increased the ground clearance to 1.2 inches as shown in the picture below.

With additional packing under the front axle the front is higher than the back.   With me stilting on the back the front is 1/8 inch (3mm) higher.   It is worth noting that the front was 1/4 inch lower than the back before adding the packing. Since I prefer having the boiler slightly up hill I shall leave the back axle where it is.

Regulator Improvement 1Sept 2004 I have found it difficult to start the engine smoothly.  Achieving a slow tick over is even more difficult since the tiniest movement of the regulator either causes the engine to stop, or gain a lot of speed.  I have decided to file a notch on the valve face in a similar manner to the Burrell.
I removed the regulator cover and housing to reveal the valve face and masked off the remainder of the engine (picture).  Using a small triangular needle file I filled a small nick in the vale port (insert).  I then refaced the vale face using wet and dry paper supported by a flat angle plate.  I took the precaution of temporally blocking both admission and exhaust ports to prevent emery getting into the works.  Since the original gaskets designated I was tempted to try reassembly without gaskets (as per Burrell) but I chickened out at the last moment and made and fitted new gaskets.
Testing the modification on compressed air proved successful and I shall take POP to the park this weekend for a steam test. 3rd Oct: While thing were better, I was not satisfied so I decided to investigate further.

Overall
I am very happy with my POP though the final proof will be in the steaming.  Unlike my previous projects progress has be much quicker and very satisfying. Having the odd problem to solve has added to my enjoyment and makes me feel that it is more than a kit I have just bolted together.  I have a well equipped workshop and several models built from stock materials behind me.  Perhaps one of the most useful skills I have employed is the ability to use a file.  Many of the parts, require cleaning up to attain an high standard of finish.  This is particularly true of the lazar cut parts which are supplied in the raw state. Some brass brazed parts appeared to be in an appalling state but with a little file and emery work came up to a high standard. A few parts needed machining to get them to fit! I recall doing the valve clevises.   

I cannot say whether a constructor with limited model engineering experience and only the kitchen table would be happy with their POP.  I suspect that some of the later kits would be challenging.   Such a constructor described their experiences with kits 1 through 7 in Old Glory.  It has now been several months since the last installment and I wonder how Paul is getting on?

I have just received the steaming instruction and it is clear that a novice constructor is going to need experienced help to setup and operate the engine for the first time.

Burrell Builders Home