New Range of Locomotive Wheels for P4
See the new range of locomotive wheels at Scalefour North on 17th/18th April 2010.
For some time we have been working towards introducing the planned range of 21 wheels (20 new + 1 existing) at Scalefour North and have been comfortably on course to achieve this. Recently, however, small errors in the tooling have been discovered - mostly affecting the crankpin recess - and these are currently being corrected.
To give more detail - initial production batches of all 20 wheels were produced some time ago and all looked good. It was only when fitting crankpins to the driving wheels that some small misalignments between the recess in the back of the wheel and the hole in the front of the wheel became apparent. On three wheels, this was bad enough to cause the crankpin, once fitted (with difficulty), to be visibly angled. Subsequent detailed investigation revealed that 10 of the 13 driving wheels had at least some misalignment and we decided that all these should be corrected.
As a result, although examples of all types in the range will be on our stand, it now looks as though few, if any, of the driving wheels will be available to purchase at the exhibition. Of the new wheels, only the LNER outside cylinder 5’ 8” wheel was right first time.
That’s the bad news. The good news is that our supplier of steel wheel bosses responded very well following our previous problem and has replaced all our stock with bosses with the correct taper. With these and good stocks of all sizes of rims, once all issues with the mould tooling have been resolved, producing top quality wheels to meet all requirements will be very straightforward. Production of all types will be underway immediately after Scalefour North and stock levels should soon build up so that all types will be available from stock.
Replacement of previously sold 6’ 8” LNER driving wheels with bosses with an incorrect taper will be possible towards the end of April - watch this website, S4Webforum or P4_talk.
Assembling the wheels has been very satisfying as they simply go together as intended, very quickly producing a true-running wheelset to the chosen back to back. Removing a wheel and adding crankpins (putting on one side the effect of the errors I found) has similarly been straightforward.
Some points worth mentioning have emerged:
For driving wheels -
Some wheels with rims at the minimum width tolerance need a scalpel run round behind the rim to ensure no plastic gets in the way of setting the back to back dimension (this is in addition to making sure no residue of the mould gate remains in the way, as mentioned in the instructions);
The plastic (Torlon) stub axles are quite difficult to clean up where attached to the sprue. Torlon is very hard and difficult to cut and attempts to clean up the attachment area with most tools can result in a burr which will then make easy insertion in the central sleeve difficult. Len’s experience is that a diamond surfaced file (available from Eileen’s Emporium) does the job easily;
Loctite 603 is effective for assembling the stub axles in the central sleeve. It is tempting to use one blob placed on a hard surface for the assembly of several wheelsets but experience indicates that it becomes less effective if left out for long. Best to use a fresh (small) blob for each wheelset;
The crankpin assembly uses an M1 countersunk screw. Occasionally, these have burrs at the end of their slot and these must be removed if the screw is to seat correctly in the aluminium bush that fits in the back of the driving wheel.
Check that neither the aluminium bush nor the M1 screw stick out beyond the plane of the back of the wheel rim. They shouldn’t (getting this right was an issue with two of the designs) and if they do it is likely to be because some debris is preventing the components going fully home.
For carrying wheels -
A range of axle lengths is available covering inside bearing wheelsets, outside bearings with plain bearings (25mm long) and outside pinpoint bearings (26mm long). Additionally, a 30mm long extended axle with plain ends is available;
As with some driving wheels, some excess of plastic may need to be removed from the back of each wheel with a scalpel before assembly (it is important the whole back of the metal tyre can make contact with the back to back gauge);
A central sleeve is available to make the axles look like solid 2mmØ axles. We are using a standard component and this is slightly over-length for this application. Approx. 0.2mm needs to be removed (hold the sleeve in a pin chuck and rub each end in turn on some fine emery paper placed on a flat surface). It is important to reduce the length of the sleeve enough so that the wheels can be pressed firmly against the back to back gauge (make sure they can ‘pinch’ the gauge with the sleeve in place before starting final assembly);
The axle end cap for inside bearing carrying wheels does not seem to sit flat with the wheel boss face - it goes into position but can wobble. This will be looked at further but for now it may be best to use an adhesive, holding the cap flat on the wheel face by positioning with sticky tape across the boss while the adhesive sets;
Don’t forget the carrying wheelsets are not designed to be dismantled, so any bearings, etc. that need to be on the axle must be threaded on before final assembly.
See:
- the Summary Table for dimensional and other details of the wheels;
- the Price List for prices and details of all the components involved;
- the Instructions for details of the driving wheel design and assembly method (note - this is a very large file).
Improved Check Chairs
It has long been an ambition to introduce check chairs which fit the rails better, so making them easier to use and giving more accurate control of the flangeway dimension. Progress has been extremely slow with each adjustment to the mould seeming to involve a cycle taking many months. We have, however, now moulded a set of chairs with the correct P4 flangeway of 0.68mm which meets all the objectives in terms of good rail fit, accurate flangeway and a flat chair base with rails installed. The only residual issue is that the recesses in the chair base tend to produce slightly undergauge track when positioned on NewTrack pips - the error being around 0.04mm. Given the timescales and cost involved in yet another development cycle and given that the chair/pip clearance should be sufficient to enable the correct gauge to be achieved, we have decided to regard this version as final.
The next steps are:
1. To produce end (flared) chairs for the 0.68mm flangeway;
2. By making a straightforward adjustment to the computer model used to produce the tooling, produce a chair set for check rails set at the 0.58mm (S4) flangeway;
3. By making a another, similar, adjustment, produce a chair set for check rails set at a flangeway of 0.95 - 1.00mm. This will be suitable for 00 and EM standards.