Eccentric Rod: Part 14

  • BOOK, NEW, OLD?, ME? Colin Sims analysed that the length of the eccentric rods appears to be wrong as follows: “The eccentric rod issue I worked out in the following way: Link radius is 7.5″, with a ‘backset’ (not the right term, I know) to the pin holes of 3/4″, leaves a dim of 6 3/4″ from the pin to the crank centre. If I add the dim from the centre to top face of the eccentric strap to the foot to pin hole dim on the rod, I get 6 5/8″, i.e. 1/8″ short. This will not affect the valve travel, as this is determined by the eccentric throw, but it would affect the quasi-SHM of the valve movement subtly. Yes I know it is not true SHM, as the mechanism is effectively a slider-crank form.editor’s note: I have made the rods to the drawings, and they seem to fit, but I agree with Colin that the math sais they may the wrong length.
  • Having now assembled the valve gear (February 2014) I fear that this is proven – moving the gear from ahead to astern cannot be accomplished without the valves being deflected in mid gear, no matter where the crank is set. I think this is wrong, ah well, time will tell – I have managed to time the engine to run on air, but pretty roughly so far….

2 thoughts on “Eccentric Rod: Part 14

  1. sybefur Post author

    John, thanks for your comments, your explanation makes the whole thing very clear…. The fact that the engine can be made to turn so slowly (admittedly as a simple) suggests that things should be OK…. Hopefully we will be able to find out for sure in 2015.

  2. John

    I know this is a while back now, but maybe I should comment that with Stevensons motion the valve will have some movement with the reversing lever set in mid gear. Both eccentrics are normally set with a bit of advance, and in mid gear this gives the valve a small movement which should be such that the valve is just opening. This is a motion of the valve which is pretty much in phase with the crank movement. The motion due to the eccentric when it is in full gear is nearly 90 degrees out of phase with the crank. Because of the way this all works, the valve will always open just before top dead centre, and the point of closing will be earlier with short cutoff and later with long cutoff.
    It can get a bit more subtle than that, the opening point actually can vary too, Stevensons gear gives variable lead while radial gears like Walschaerts can give fixed lead.


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