ModelWorks Foden Builders Diary - Kit 18 - Crankshaft, Con Rods & Motion

Thursday 22nd May: City Link delivered kit 18 this afternoon just as I was about to go shopping. On opening the box I was faced with a bit of a bran tub. The contents were packed in very finely shredded paper which wanted to go ever where. Stopping this going all round the house was made more difficult as ModelWorks have gone back to packing small items directly in the packing. This meant that I had to sift through the shredded paper rather than putting it straight into the recycling sack.

I was expecting it to contain the long awaited crankshaft but I am certain it was not hiding in the packaging. I don't know if this is an error in my kit or a planned omission. What makes me suspicious that it was deliberate is that the crank and flywheel keys were also omitted. More bazarly the bag supposedly contained the regulator screw, F42147, was completely empty.

Friday: I have exchanged emails with Steve and he confirms the cranks were left out and are to follow. Apparently there should have been a docs envelope on the outside of the box with a note explaining the shortage. My box had the plastic docs envelope but it had been ripped open and was empty. I can't see me starting this kit until the second half on next week. as I am busy at the moment preparing for the Sellendge rally and judging by the forecast (3" rain Sun/Mon) I may need a dingy.

Tuesday 19 August: Today I moved the Burrell into it's winter quarters and lifted it's transport trailer to the ceiling of my garage. Now the garage floor is available I will be able to assemble my Foden but it will have to be out by March 2009 so I can prepare the Burrell for the 2009 season. Since starting the Foden it has become clear that I do not have room for the Burrell and Foden and I plan to sell my Foden to the highest bid received by the end of March.

Thursday 21 August: I am hopeful that I will shortly receive my crankshaft from Steam Traction World. This will be am important milestone following the demise of ModelWorks. In anticipation of this I have been preparing the remainder of kit 18 for fitting.

The connection rods have been made as sets and the respective parts have to be kept together in the same positions. I followed my usual practice an stamped them either 1 or 2 to identify their mates. These stampings are on the underside of the bearings so will be hidden once the rods have been fitted.

An oilway needs to be cut to allow oil to flow from the lubricator around the bearing fixing stud to the big end. At first thought this was going to be difficult and I started to think of elaborate ways of cutting a grove in the lathe. However I decided to adopt the suggestion in the instructions and use a burr in my mini-drill.

I often find that on initial contact the bur jumps and wanders off course. To prevent this and give me something to guide the burr I planed to block the hole just below route of the oilway. Whilst I was thinking how to lock a 1/4 rod in position the setup shown in the following pictures came to me.

A 1/4" rod is held in the vice with just enough protruding to bring the burr in line with the existing oil hole. The burr spun by the mini-drill sits in a dimple at the top of the 1/4" rod. The oilway is they cut by rotating the bearing around the 1/4" pin.

Friday 22nd August: Today I received my crankshaft from Steam Traction World. I suspect I might have received my crank a little ahead of schedule as Steve told me the majority are away with the crank grinder for finishing. This, with a little ingenuity, should let me get the engine running on air. Steve also told me that contrary to previous advice it should be possible to fit the gears and remaining parts with the boiler mounted in the chassis. Consequently once I have the engine running on air I shall press on and fit the boiler to the chassis along with all the other bits I have waiting.

The final piece of good news; STW's provisional production schedule supports getting the Foden in steam by March 2009.

I used a wire brush to remove the residue left over from heating the crank received during manufacture. While this could have been from welding I suspect the cranks have been heat treated.

The eccentrics incorporate sealed ball races ball races and will not need any additional lubrication in use. I found it difficult to slide the ball race into the housing since despite my best efforts they kept twisting and jamming. The frustrating thing was that once on they easily slid off! On final assembly I shall of course be using loctite.

When It came to fit the eccentric key I was faced with the choice of two slots. The right one and the wrong one. Reviewing what I have done I see I have picked the wrong one! I guess during the machining of my crankshaft the keyway was originally machined in the wrong position and subsequently fixed.

When it came to fit the connecting rods I discovered that I could not get access to the little end pin without partially removing the truck guide. I mention this because some of the nuts are awkward to get to and perhaps I should have delayed fitting all the truck guide bolts until I had the connecting rods.

The crankshaft fitted no problems and turned freely. When I came to attach the big ends I found it tricky to align the big end bearings. At first, I though there was a problem but with a little patience everything lined up OK.

Turning the crankshaft was now stiff and reasonably smooth except at one point where it required significant force. In an attempt to identify which cylinder was stiff I slackened the big end bearing nuts a few turns. By rocking the crank about the tight spot I could sense which cylinder was the source of the problem. To my surprise it turned out to be the cylinder whose crank was at 90 degrees. With a little further playing around I sussed that something was preventing the connecting rod swinging up far enough. Looking into the cross head at the little end I could see that the corners of the little end were digging into the cross head. Once removed it did not take long to round the edges of the little end so that it no longer fouled the cross head.

It is important that the crank turns over smoothly without any extra tight spots. As I guide I can turn mine relatively easily by gripping the shaft where the splines go. There is resistance and the crank stops the moment I stop turning.

Sunday: I have spent most of today cleaning up the valve gear components. It is surprising how long it takes to get everything polished and looking nice. I noticed that the flats on the upper lifting arm pin did not extend inwards far enough to engage on the grub screws. This was simply fixed using a file with the existing flats acting as a guide.

Monday: It did not take long to fit the remaining vale gear components. Durring assembly I made use of a 5mm reamer to clean up the bearing holes for the pins. Using a 1/2" reamer I reamed the wayshaft bearings in situ. They was not much metal removed but it did ensure that once fitted the wayshaft turned freely.

With the valve gear fitted it was now possible to set the initial length of the valve rods. I find it easiest to rotate the crank and watch at which points the valve port begins to open. This happens at about top dead centre and bottom dead centre. I keep adjusting the length of the valve until the forward and reverse stroke start are the same point relative to their dead centre.

At this point I became uneasy that that there was something not quite right with the valve gear but did not identify the problem. I pressed on and fitted the valve covers and adjourned to manufacture a blanking plate for the regulator cavity. I made brass mushroom which I dropped into the passages from the boiler thereby preventing air escaping from the regulator cavity into the boiler.

I took the valve chest cover off so that I could watch the valve events. Effectively the Foden is a two shaft engine as the chain drive to the back axle does not change the direction of rotation of the 2nd shaft and the back axle. The point is that crank turns in the opposite direction to the road wheels. Armed with this knowledge I determined that the right hand cylinder was timed correctly and the problem was on the left hand side.

Knowing where the problem was I reviewed the instructions carefully but all this showed was mine was assemble as per instructions. To fix the problem I removed the eccentric arms from the left hand side and reassembled them the other way round such that the forward eccentric was on the outside. ie the left hand outside eccentric connects to the top of the expansion link. Putting this another way the eccentric pairs on their respective cylinders are not mirror images of each other as shown in the instructions. Looking from the back crankshaft end of the engine, from right to left, the eccentrics run Reverse, Forward, Reverse and Forward.

Listening to the engine there appears to be an uneven beat and I plan to recheck the valve positions to see if I can get 4 even beats.

Sunday 7 September: I assembled the remainder of kit 18 today. more to write .....

Monday: Keen to continue checking out the engine I jury rigged a reach road in order to connect the reverser to the valve gear. When I reconnected my compressor I was disappointed to find that the engine would only run in full gear. When the engine was notched up it would not run at all. Clearly something was wrong so I removed both valve chest covers.

Rotating the engine while observing the valve ports I could see that the valve was not moving much in relation to the steam admission ports. When fully notched up they did not open at all (both fwd and rev) and in the second forward notch they only briefly opened a tiny amount.

In an attempt to shed some light on the problem I decided to generate valve timing diagrams as I have previously done for my POP and Burrell. I attached a dial gauge to the side of the valve chest such that it indicated the displacement of the valve. To measure the angle of the crankshaft I made a cardboard protractor and stuck this to the back of my chuck which I then clamped to the end of the crankshaft.

The plots show the displacement of the valve gear in thou against the percentage of stroke. 0% has the piston at the front and 100% the back. There is a misshaped ellipse plotted for each notch. For each notches the sequence of events is clockwise around the respective ellipse. This includes the reverse notches as well!

To aid interpretation I have added straight lines at +-140 thou, the admission lap of the valve. While the valve is between -140 and +140 no steam can be admitted to the cylinder.

The dark blue ellipse is full forward gear. This shows that admission starts at 3% and finishes at 84% of stroke with similar timings in the reverse direction. This and full reverse is fine but examination of the other traces shows all is not well. When fully notched up in either forward or reverse no steam can be expected to be admitted. It looks like the middle forward notch might be made to work but admission is a bit late with 78% cutoff does not offer much economy.

At first sight it looks like the Foden has too much admission lap but having seen plots from other engines the most striking omission is the lack of "twist" to the ellipses. I would have expected the mid gear notch to have shown more movement.

Fortunately I have access the J Rex Foden drawings on which the ModelWorks Foden is based. The lap and other valve port dimensions appeared to agree so I looked at the drawing of the eccentric. It took me a while to understand the drawing as I was expecting to see an angle of advance marked. When I finally got my head around what it was telling me directly what I wanted to know.

As shown in my sketch the big end is at dead centre on the left of the main shaft (horizontally in line with the shaft). Now imagine the crank rotated through 180 degrees such that the crank moves to the other dead centre. Thinking about it carefully the back edge of the eccentrics would have moved back 280 thou. Thus ignoring backlash I would have expected to have seen 280 thou valve movement with mid gear selected.

In order to directly measure this distance I removed the crank together with the eccentrics from the engine and set them up between centres on my lathe. My measurements made the distance a little under 150 thou. Clearly I was on to something so I painstakingly measured the eccentric and found it was made to the J Rex drawing except that the 140 dimension was close to 70 thou. I believe this is the route cause of the problem. Unfortunately the solution requires a new keyway to be cut.

New eccentrics will be available from Steam Traction World however ....
I cut new key-ways using my lathe as a slotter

Location of existing and new key-way
Sorry this diagram is a bit busy

The new key-way prior to final clean up

Tuesday: I spent the day exchanging e-mails with Steve and we have tracked the route cause of the problem. The effect of the error has been to reduce the angle of advance from 25 to 13 degrees. The fix for the problem requires the key-way to be cut in a slightly different place and Steam Traction World will be manufacturing replacement eccentrics. If you have not done so already do not loctite the bearings into the eccentric housings as the eccentric centres need to be replaced.

Keen to press on I decided not to wait for replacements from Steam Traction World and attempt to move the key-way in the existing eccentric. This presented me with three challenges: -

I decided to silver solder a small block of steel into the original key-way. I was nervous as to the damage the heat might due but the block would need to be firmly fixed to withstand the pounding it was about to get when cutting the key-way.

I had a vague recollection of cutting a small key-way using a modified parting blade. The chuck was locked up so it could not rotate and the tool racked back and fro using the carriage as the tool was advanced into the work. It would be a lot easier if I could cut the 3/16 slot in one go but was unsure if my setup was up to it. From a stick of HSS I spent over an hour griding up a 3/16 wide form tool and tested it on a scrap steel ring. The rigidity of my lathe was challenged and I had to optimise the cutting angles but in the end I managed to cut an impressive key-way.

This left me with the final challenge that of indexing the new key-way. Here I got sneaky. Having worked with Steve I could calculate the distance between the existing and new key-way to be 134 thou. I adjusted my form tool to be on centre and then raised it by 134 thou. I then locked up the chuck so that the old key-way was in line with the form tool finally returning the form tool to centre height.

I could only advance the tool 2 or 3 thou at a time so it took a long time. To date I have cut new key-ways in two eccentrics and plan to check these out before tackling the other side.

Wednesday: The remains the minor issue of the ordering of the eccentrics on the crank shaft. The instructions show the forward eccentrics towards the inside with the reverse eccentrics at the outside. With the key-ways as originally cut and now re-cut this configuration will not work. Steve mentioned the possibility of producing handed eccentrics so that the original assembly sequence worked. I think the objective was to avoid the need to fill in the existing key-way as the other hand eccentric would have its key-way in a far removed location.

I have decided to follow this idea as it will save cleaning up two of the eccentrics after silver soldering. I though it best to check a real Foden first and found these pictures in my library. They show the forward eccentric on the outside on both sides. The upshot of this is that the special eccentrics need to be fitted on the right.

Friday: I fitted the modified eccentrics yesterday and made initial measurements of valve deflection. In mid gear the maximum valve deflection was +- 0.067" . This puzzled me as I was expecting 0.140 since this is the horizontal offset of the eccentric at dead centres. A first I put this down to backlash but when I tried to push the valve gear it was surprising rigid exhibiting no significant backlash.

Having measured and checked everything in sight I was confident that my Foden was now made to drawing so I turned my attention to analysis of the valve gear design. It took me a little while to work out how to enter the dimension in to WB Hall's excellent Stephenson Link simulation program. My difficulty was persuading the program to cross the connecting rods as the engine runs backwards with the reverser forward. Once mastered the simulation results were remarkably similar to the valve displacements I had measured. The explanation for my puzzlement is that between bottom dead centers and top dead centres the connecting rods go from un-crossed to crossed. This has the effect of effectively shortening the rods subtracting from effect of the 0.140" eccentric offset.

I have the eccentrics rods in what is commonly referred to as cross rod configuration. This is necessary to make the engine run in the direction to achieve forward road travel with the reverser forward.

I hope the figure on the right makes this clearer. In all instances the red eccentric produced clockwise rotation and the blue anti clock (crank on left, cylinder on right). In the open configuration the clockwise eccentric connects to the top of the expansion link where as in the cross configuration the anti clock is connected to the top. Perhaps a less ambitious way of stating this is the configuration is determined by the state of the rods (open/crossed) when the piston is at dead centres closest to the crank.

As I have discovered a consequence of the crossed rod configuration is an decrease in valve travel in mid gear. With open rods the normal crossing of the rods as the crank rotates has the effect of increasing the valve travel, while the crossed rods reduces the valve travel. This can be clearly seen on my diagram.

To investigate the magnitude of this effect I reran the simulation with the rods in the open configuration.

Discussion
The measured results (crossed eccentrics) are a great improvement but I have a number of concerns.

The first and third issues can also be found in the simulation and are therefore related to the design. The second issue could be cause by unequal eccentric rod lengths so when I get a chance plan to remove and check their length. It should be possible to live with the first and third issue however my simulation results indicate that swapping to the open rod configuration would completely resolve them.

Studying the Rex 4 1/2" Foden drawings seem to imply an open rod configuration. Their diagram that shows the eccentric keying labels the top eccentric as Forward. This corresponds to the red eccentric in my diagrams. Connecting the red eccentric to the top of the expansion link result in the open configuration but an engine that runs clockwise with the reverser forward. My understanding of the Foden's gearing is that this results in a lorry that goes backwards.

I am now fairly sure that the crossed eccentric configuration should be used. Looking at the pictures of the fullsize engine above the configuration of the eccentrics can be determined. The piston on the right had cylinder it at the crank end. The inside eccentric is up and connected to the bottom of the expansion link. The outside eccentric is down and connected to the top of the expansion link. Thus the eccentric rods are crossed when the piston is at the crank end. Further evidence can be gleaned by examining the left hand cylinder whose piston is in mid stroke. By carefully looking at the valve gear we can ascertain that steam is being admitted to the front of the cylinder thus the top of the flywheel would go backwards. Looking at the expansion link we can see it is down and also see the reach rod from the reverser is forward. Fullsize Foden TW4702 has crossed eccentrics.

Monday: I have been giving the valve gear a lot of though. The issue is not whether it will work but to optimize it to work well. If have tried simulating a number of variations and discounted most of them. The most promising refinement I have identified is a reduction of valve lap.

First a few notes to explain how to interpret the table.
The table shows a summary of my simulated and measure valve performance. At the head of each double column is a description of the configuration analyzed. Against each configuration there are two valve for each performance indicator, one for each stroke direction. Numbers are either in inches or percentage of stroke.

Lead: Ideally the lead should be positive as we want the valve to be open at the beginning of the stroke. A negative valve indicates that the valve indicate that steam is not admitted until the stroke is underway. The most direct way of increasing the lead is to reduce the lap. It is for this reason I have included configuration will the predicted performance using a lap of 0.116". For the simulated valve with everything else unchanged this gives a lead of 0.012" in full forward gear, close to the idea value.

Cut-off: This is the percentage of piston travel completed when steam is no long admitted to the cylinder and expansive working begins. In full gear we want a big number whilst when notched up smaller is better. If I could achieve 30-40% cut-off when notched up I would be very happy but this seem unlikely.

Valve Opening: This is the maximum opening of the admission steam port during the stroke. Values over 0.100" are great. Small opening limit the flow of steam and also causes issues as the engine heats up. I am particularly concerned at the values less than 0.020".

Compression: This is gives the point at which the exhaust valve closes. This give the percentage of stoke leading up to dead centre that steam from the previous cycle is compressed. I believe a small positive valve is desirable. Not sure whether it would been better to take figures away from 100%.

Discussion: The to drawing configuration appears workable but there is scope to optimise the gear to provide positive lead in full gear. To this end I simulated two changes, In the first I reduced the lap to 0.116" and in the second I changed the angle of advance to achieve positive lead without changing the lap. I had high hopes for the configuration with increased angle of advance but the results show it is a not starter.

My thinking at present is to make the minimum of changes to the established design and therefor am only considering reducing lap. Since I can predict the benefit of reducing the lap I shall delay cutting any metal for a while, perhaps until after my first steaming.

Checking the length of the eccentric arms
All the same

The first alert to the problem
The flats on the outer eccentrics were of differing length

Using a digital caliper to confirm the error in flat position on the outer eccentric

Tuesday: While it may be a bit of an exaggeration I feel I have just made a major break through but first I should mention the conversation I had last night with Lou Rex the original designer of the 4 1/2" Foden on which the ModelWorks variant is based. As well as picking up some useful pointers he stated that right from the start the design was for crossed rods. However the figure labeling the top eccentric as forward is in error and will be updated. With my confidence in the design restored I decided to press on and cut new key ways in my other set of eccentrics to bring everything "to drawing". Lou also confirmed that the eccentrics pairs are handed and so I will definitely be cutting the new keyways on the other side.

As I need to my first set of modified eccentrics as templates to set up the second set it was time to dismantle the crankshaft from the engine. This gave me a chance to carefully measure the eccentric lengths as previously I had only checked the pair not fitted to the engine.

I was immediately alerted to a problem when I examined the eccentrics removed from the engine. When placed side by side it was clear that the flats on the outer eccentrics were of different lengths. While not in itself important it signified that the depth of the flats was different between the eccentrics. I should mention that the other set of eccentrics I had previously checked were perfect and I had mistakenly assumed that those fitted to the engine were also.

The flat should be machined 1.582" from the centre line of the eccentric. This is difficult for me to measure so I used my calipers to measure the thickness between the flat and the inner diameter of the eccentric at it narrowest point. I took several readings and took the lowest value. The correct dimension derived from the drawings is 0.160" and the set not fitted to the engine were exactly this. However the set I had just removed from the engine were 0.155" and 0.125". The 30 thou difference would cause an effective difference in the eccentric rods of 30 thou and explain why my reverse not valve events were lopsided.

As I had the opportunity I checked that the eccentric arms were the same length by pinning them together and checking that there was no step in the bolting flat onto the eccentric. I also checked the inner eccentrics using a similar method to the outer and identified no further problems.

As each outer eccentric is different I have numbered them and will make shims to go between the eccentric and arm to bring the total rod length to the drawing dimension.

I have just made shims to fit over the undersize flats in an attempt to bring them the the same length. To check that when assembled both eccentric are identical I keyed them onto the crank shaft in the same orientation and pinned the two ends together. If the eccentrics were not the same length I would have not been able to get the pit in. Further if the keying (angle of advance) had been different as I rotated the pair about the shaft it would have got tight and jammed. As the pair rotated freely I am not confident I have a near identical pair and will press on an update the other set.

I have just cut the keyways in the second eccentric set and refitted everything back on the engine. So far I have only quickly set things up but early indications are that the engine should be capable of running in all notches. The first eccentric set, which is now fitted to the left hand cylinder,does not appear as good as the second set since its maximum valve opening when fully notched up is less. I am surprised the second set is better as I used the first set as a template to cut the keyways in the second.

Wednesday: I did not have much time to spend on the Foden but I did get a chance to measure the valve displacements for the right hand cylinder. The results show the benefit in getting the rods the same length as the traces for reverse gear are now closer to the forward gear counterparts.

While I am certain the engine would run with the original lap I still believe I would get better performance if I reduced the lap to 110 thou. With the existing lap of 140 thou my results show that when notched up admission will not start until 35 degrees past dead centre. If I reduce the lap to 110 I can get admission to start at 10 degrees past dead centre. I would also have the benefit of a small positive lead in full gear.

Saturday: I have finally found time to measure the valve displacements for the left hand cylinder and produced a combined left/right plot. The right hand valve displacements are shown dashed while the left hand displacements are shown solid in the same colour. See the legend on the plot to identify which notch each colour refers.

Contrary to my initial thoughts the performance of the left hand side is as good as the right. Looking at the plot it can be seen that the dashed traces for the R/H side take a similar course to the solid traces for the L/H side. Looking at the enlarged version of the plot (click) it can be seen that the Forward 1 and Reverse 1 (full notched up) traces also take similar courses. This confirms that I have got the Forward and reverse eccentric rods the same length.

I have now completed my measurements and am confident I have the gear put together correctly.

Monday 13 October: I have decided to pin the lifting arms onto the weighshaft. I found that the valve gear linkages on my Burrell received an unbelievable pounding. In the end I was forced to weld the joint in the reach rod to stop the screws slipping. I am not sure pinning is necessary but it was easy to accomplish at this stage and is show on the Rex drawings.

I used the existing grub screw to hold each lifting arm in place after setting its position on the engine. I then drilled a 3.4mm hole which I then finished with a taper pin reamer. When I refit the weighshaft I shall use both the taperpins and grub screws to hold the lifting arms in place. They better not move!

	To drawing		To drawing reduced lap		increased angle of advance		To Drawing		As measured		As measured reduced lap
	crossed rods		crossed rods		crossed rods		open rods		crossed rods		crossed rods
Lap assumed	0.140"		0.116"		0.140		0.140		0.140		0.116
Full Forward
Lead	-.012	-.012	+.012	+.012	0.014	0.014	+.012	+.012	-.022	-.022	+.002	+.002
Cut-off	76%	79%	81%	83%	71%	76%	70%	81%	83%	82%	85%	84%
Max valve opening	.124	.176	.148	.200	.131	.175	.154	.171	.117	.170	.141	.194
Compression	5%	5%	5%	5%	8%	8%	9%	6%	4%	4%	4%	4%
Notched up Forward
Lead	-.043	-.039	-.019	-.015	-.016	-.012	+.042	+.040	-.047	-.064	-.023	-.040
Cut-off	27%	41%	41%	53%	31%	41%	40%	48%	48%	53%	61%	63%
Max valve opening	.011	.025	.035	.049	.029	.040	.082	.082	.008	.018	.032	.042
Compression	12%	11%	12%	11%	18%	16%	27%	20%	8%	6%	8%	6%
Conclusion	Should work		Better		No merit		Backwards!		Does work		Work better?



Pictures of valve gear on full size Foden TW 4207 Note: Forward eccentrics on outside (both sides)

ModelWorks Foden Builder's Diary

Kit 18 - Crankshaft, Con Rods & Motion

ModelWorks Foden
Builder's Diary