John Palmer wrote:
JFS wrote:... however, the lock does not hold the lever until it has moved from the middle to the "3/4" position.
Aye, there's the rub, since a very common arrangement with push-pulls is for them to operate signals controlling movements in opposite directions over the same route, eg a crossover, and ideally for this you need a nib that holds the push-pull in its normal mid-quadrant position until released, for example by reversal of the points over which the push-pull pair of signals read.
It hadn't occurred to me that full travel tappets are more difficult to lock - the release locking I've mentioned for a push-pull must be an exception to any such general principle, albeit one of fairly minor significance.
Keith's suggestion for the dets is the kind of thing I had in mind. An excursion to Youtube quickly brought up a source for a suitable soundtrack, with a bit of editing: https://www.youtube.com/watch?v=4x-FKMYJYP8
Not sure if we misunderstand each other here John (and I do know how they work for I have worked one in a real box!). The lever would be retained by its catch handle in the mid position, but - if it were locked - for example by the non reversal of the points) then the lever could be moved SLIGHTLY before encountering the lock but not enough to "work" the signal. The movement from the mid to the 3/4 position being quite only 1/4 of the lever stroke. Fair enough if you find this an unacceptable compromise weighed against the need to make all the locking by hand!!
Full travel tappets are a pain because of the need to avoid "false locks" where ports pass through intermediate tray slots ("troughs" in Stevens-speak)- any bridles in these trays (which are obviously intended to engage with other ports in the same tappet) must be made a different size (larger) so that they will not engage should two levers be moving at once. Not sure how many of these locking trays you have looked in, but the range of shapes and sizes is wonderful to behold, but a real pain to make! Such potential conflicts need much more careful consideration and avoidance. I agree that Push Pulls are but a minor compensation for this!!
Stevens, having invented tappet locking, were rather saddled with quite old fashioned locking arrangements consisting of a series of two-trough trays each with three bridles, spaced along the height of the lever. Thus each tray had a different locking pitch - and "mass producing" an etch is impossible. Worse, the tappets must be curved to the radius of the lever at the point of attachment - again different for each tray. In bigger boxes these were supplemented by a much larger tray at 45 degrees driven by a bracket off the lever - for the modeller, these have the disadvantage that the leverage from the signalman's "pull" is very great, necessitating some pretty hefty locking if it is not to break under an over-scale pull. Perhaps such challenges were the reason why they went for economising devices in quite a big way - such as "Route" levers and "Russel" levers, which would certainly be an interesting challenge!
To look at the locking in a largish Steven box (one or two of which still exist / existed until recently in Scotland) and compare it with that in a Westinghouse box (some excellent examples of which still exist in Sri Lanka) is to see what a difference reduced travel locking makes.
I did quite a bit of work to investigate etched catch handle locking - including all four of the most common patterns on the Real Thing - but all foundered on the rock of creating a strong enough catch handle, not to mention the extra precision which is implied by some types.
But of course, none of that applies in the case of a small box - which are "easy" enough to do by hand-methods, and small enough to adopt the beautiful but expensive SMS frame. But if you had more than say, 25 levers, it might focus the mind
Just in passing, it is interesting to note that lever frame types which do not exist in museums here can be seen in museums overseas - for example, there is a Saxby Dog and Rocker in Lousado (Portugal), a Saxby Duplex in Madrid, and M & H Cam and Soldier in Perth, W. Australia.
There - now I have revealed just to what extent I need to get a (modelling) life...
Interesting ideas about the dets - making a big bang might be reserved for the co-acting dets at the Homes!! If trains get to the locations of the worked dets, they need stopping!