Instructions for Assembling Driving Wheels, Axles and Crankpins

Transcription

1 Instructions for Assembling Driving Wheels, Axles and Crankpins (Version 1; October 2008) Introduction These instructions explain how to assemble Exactoscale 4mm scale driving wheels, axles and crankpins to produce a wheelset that is has the desired back to back setting, has wheels that run true and crankpins with a consistent throw. The design features that help to achieve this result will only work properly if the correct assembly procedure is followed. It is therefore important to read these instructions carefully and understand the reasons for the sequence that is described. Successful assembly of the wheels and axles (and subsequent removal of wheels from their axle) depends on the use of a small press specifically designed for the purpose. Attempting to assemble or dismantle wheelsets without using the press is likely to result in damage. The drawings in these instructions show a 6 8 driving wheel but the methods used (and the press tool) will be applicable for any future wheels introduced that use this design of axle. Concept The design features a taper joint between the wheel and axle. Tapers of a shallow enough angle, when pressed firmly together, form a secure locked joint which provides the kind of accurate location needed for a true-running wheelset. No adhesive is needed in the taper joint and this means that the taper can be released and the wheels removed by pressing the axle out of the centre of the wheel... and then reassembled by pressing the wheel back on to the tapered axle. The difficulty with shallow-angle tapers is that the position at which they lock is very sensitive to small changes in the dimensions of either of the components involved - a manufacturing nightmare if the aim is to make an axle with a taper on both ends which will ensure both wheels are locked at the required back to back dimension. Our solution is a three-part axle which is assembled by the modeller - this forming the first part of the assembly sequence. We strongly advise that, when wheelsets are dismantled, care is taken to keep each axle with the wheels it was initially assembled with. Wheel rim Plastic stub axle Axle sleeve Moulded wheel centre Crankpin bearing (4CR 603) Centre boss with bearing surface Axle cap (4DW 103) M1 screw (4CR 601) Aluminium insert (4CR 602) This shows the cross-section of an assembled wheelset - the three-part axle and taper joints can be seen together with the wheels with their bearing surfaces integral with the wheel. The grooves in the bearing surfaces are to assist oil retention.

2 The press tool shown in cross-section below consists of a frame and screw with two small components - one to fit in the frame ( A ) and one to fit on the screw ( B ). These enable the tool to be used in several different ways. Removable anvil A Removable sleeve B Operating screw Frame Assembling an Axle An axle consists of a central metal sleeve with two plastic stub axles. This section describes how to assemble these to produce an axle of the required length so that the taper joints will lock at the required back to back dimension. First, prepare the stub axles. These are made of a special hard plastic (Torlon) and need care when they are being detached from their sprue to avoid lifting the surface skin. Score the mould feed with a scalpel where shown in the cross-section view below and break the stub axle from its sprue by bending in the direction shown. Use a scalpel to clean off any remaining roughness that protrudes above the round surface of the stub axle. Score here Mould feed Round section Taper section Stub axle Before going further, examine the wheels carefully to make sure that there is no mould sprue material sticking out beyond the back face of the wheel rim. The mould is filled at a point on the back face of the wheel in line with the crankpin boss, and it is here that you may find a small raised mould pip (see below). Trim it flush with the back of the wheel with a scalpel. Take a wheel and stub axle, check that the taper of the stub axle and the bore of the wheel are clean, and push the taper section of the stub axle into the wheel so that it is finger-tight - to give the result shown on the left. Even at this stage, you should be able to feel some locking effect between the stub axle and the wheel.

3 Now put the other end of the stub axle in the removable anvil (component A ) of the press tool and put component A with the stub axle and wheel in position in the press tool frame as shown in the smaller picture below. Put the removable sleeve B in position on the end of the operating screw and screw the screw in until the end of B engages in the centre of the wheel, pressing on the metal centre boss, as shown below. Tighten the screw, making sure the end of B is pressing on the metal boss and not the plastic moulding, and then tighten the screw very firmly - but using just fingers on the screw knob - do not use other tools to get it even tighter. Repeat all this for the other wheel and stub axle so that you have two wheels firmly locked on their stub axles. Now to assemble the axle - and please note the following: i) A dry run axle assembly is worth doing - the work needs to be completed reasonably quickly, before the Loctite sets; ii) When using Loctite, avoid contact with the liquid and observe the safety instructions provided; iii) Do not pay any attention at this stage to wheel quartering - that comes later. Apply a small drop of Loctite 603 (or 638) to the surface near the inner end of one stub axle and spread it round the circumference. Park the wheel while you put Loctite in one end of the axle sleeve (a more generous drop is OK here). Now push the stub axle into the sleeve, twisting the sleeve to spread the Loctite evenly. Apply Loctite in the same way to the other stub axle and into the other end of the sleeve and push the second stub axle into the sleeve, again twisting the sleeve. Wipe surplus Loctite off with a tissue and place the wheelset in the back to back gauge.

4 With finger and thumb, hold the wheels against the gauge, pressing gently along the line of the axle. Using a small screwdriver, adjust the position of the sleeve so that the gaps at each end between it and the inner ends of the wheel bearing surfaces are the same on both sides. Continue holding the wheels in place for a few minutes and then leave them standing for an hour or so - by which time the Loctite should have gone off sufficiently for the wheelset to be removed from the gauge. Best to leave the wheelset overnight, though, before doing any more work on it. You now have a wheelset that can be dismantled and reassembled, each time with the tapers locking at the correct back to back dimension and with the wheels running true. Working on the Wheelset A range of further steps will be necessary before the wheelset will be installed in a working locomotive - including, for example, fitting bearings, adjusting sideplay with washers, fitting crankpins, quartering (or whatever angle is appropriate between the crankpins on opposite sides of your loco), fitting return cranks, fitting balance weights and so on. Many of these require dismantling of the wheelset and for others, while dismantling may not be essential, it can make the job easier. This section therefore covers dismantling and reassembling the wheelset. First, a comment on loco design. To use the press tool to dismantle a wheelset, it is necessary for the wheelset first to be removed from the loco chassis. Some kits and approaches to chassis design do not allow for this and it is suggested that modifications are made so that removal is possible. Trying to dismantle a wheelset while it is still installed in a chassis is likely to result in damage to the wheelset, chassis, or both. To dismantle a wheelset, remove components A and B from the press tool and place the wheelset in the press tool as shown on the left below. Screw the operating screw in, making sure the narrow tip of the screw bears on the end of the plastic stub axle and does not catch on the metal boss of the wheel - see the right-hand picture above. Tighten the screw until, quite suddenly, the taper is released and the axle drops out of the wheel boss. This should not need anything more than firm finger pressure on the screw knob of the operating screw. The procedure can be repeated for the other wheel, though for most work on the wheels, removing one wheel is likely to be sufficient. A dismantled wheelset is shown on the right, with the three-part axle in its final, assembled, form.

5 These instructions will not (for now, at least) cover most of the other work that might be done on the wheels - though notes on crankpin assembly and quartering are included later. But once any work has been completed, the wheelset will need to be reassembled, as follows. Push each wheel boss onto its end of the axle, then gently squeeze the wheelset between thumb and forefinger (at this point, the tapers will be binding sufficiently for the quartering to be tested - see next section). Place components A and B in position in the press tool, though this time with component A reversed, with the solid end (which fits inside the wheel moulding and engages with metal wheel boss) facing the operating screw. Place the wheelset in the press tool, fitting the front face of one wheel on component A and screwing the screw in until component B sits in the recess in the front face of the other wheel, as shown on the left. Do not tighten the scew further yet. Now place a back to back gauge between the wheels, as shown on the right. The back to back gauge will rest conveniently on the press tool. Tighten the screw so that the wheels are pressed more tightly onto the tapered ends of the axle. As you do this repeatedly check how easily the back to back gauge moves up and down between the wheels. At some point, the play between the wheels and the gauge will be eliminated and the wheels will just begin to bind on the gauge - this is when the wheels are correctly gauged. Undo the operating screw and remove the gauge and wheelset. Check that the gauge still can slide betwen the wheels without any play and then roll the wheelset on a flat surface to confirm that the wheels run true. When the wheelset has been assembled for the final time, axle caps (4DW 103) can be glued into the wheel centres to give the proper axle end appearance. We suggest an adhesive that does not set too hard (possibly even Blutack) is used to minimise the risk of damage if removal is eventually necessary. Quartering No special provision has been made for quartering and it is therefore necessary to do it by eye or make or obtain some appropriate fixture. Remember, though, that it is not necessary to achieve an exact angle - just to get the same angle on all the wheelsets. The ability of the locking tapers between the wheels and axles to be lightly locked with finger pressure, tested for free running and then adjusted if necessary by simple twisting of the wheels may help. Final setting to the correct back to back dimension (as above) would then only be done when satisfactory quartering has been achieved.

6 Crankpins The Exactoscale crankpin range is based on M1 countersunk stainless steel screws, located in an aluminium insert that fit in a recess moulded in the back of the driving wheel. A shouldered ( top hat ) threaded phosphor bronze bearing is screwed onto the M1 screw, clamping the moulded wheel centre between the aluminium insert and the top hat bearing. The cross-section drawing below shows these components. Moulded wheel centre Threaded crankpin bearing Aluminium insert M1 countersunk screw To assemble a crankpin, first check the recess in the back of the wheel. It is important that there is nothing preventing the aluminimum insert fitting correctly in the recess as otherwise it (and the screw head) will not be flush with the back face of the wheel. It is also important not to deepen the recess as that could prevent the insert and the crankpin bearing clamping the wheel moulding properly. Then push an aluminium insert into the recess, checking that it is flush with the back of the wheel (see first drawing below). Take an M1 countersunk screw and check that there are no burrs at the end of the slot in its head that could prevent it sitting correctly in the countersink. Use a needle file to remove them if there are any. Push the M1 screw into the aluminium insert (see second drawing below). Take a threaded crankpin bearing (reduce its length if it is too long for the thickness of the coupling rods being used) and, holding the bearing surface in a pin chuck, screw it onto the M1 screw. If you are confident that the crankpin will not need to be removed (or have its position adjusted to get a return crank correctly angled), put a drop of Threadlocker (Loctite 243) on the screw before finally tightening it to clamp onto the moulded wheel centre. The third drawing below shows the crankpin in position. The Exactoscale range of crankpin components includes a threaded bearing extension, spacer washers and two sorts of crankpin nut (round and hexagonal). It may be possible to use the round crankpin nut reversed for the recessed nut that is sometimes needed on the leading coupled wheels of an outside cylinder loco, though considerable thinning of the top hat crankpin bearing will probably be needed to give an acceptable overall crankpin length. It may also be possible to use the threaded bearing extension to create a robust return crank. Thread a bearing extension onto an M1 screw (right up to the head). Drill a 1.0mmØ hole in a thick piece of aluminium (or MDF might do) and place the M1 screw in it - so keeping the bearing extension square to the surface of the aluminium/mdf. Open out the return crank to 1.5mmØ and place it front face downwards over the bearing extension, then run a bead of solder round the join between the return crank and bearing extension. Unscrew the M1 screw and screw the return crank assembly tightly onto the crankpin, check its position and work out the amount of any angular adjustment needed. Remove the return crank from the crankpin, loosen and retighten the cranpin having made the necessary adjustment in the position of the screw slot and replace and retighten the return crank. It may take several attempts but it should be possible using this method to get the return crank tight in the correct position. As will be obvious, this section does not yet reflect much actual experience of using these components. It will be extended and updated in due course. Feedback from modellers to help with this will be welcomed.