It is obvious that any other resting place may be used instead of the floor for the tram; thus in a locomotive the wheel guard may be used, the tram t being used to mark lines on the upper part of the wheel rim, instead of opposite the crank. To set the valve, place the fly-wheel on its dead centre, moving the fly-wheel as directed until one of the points (e or l, say e) comes fair with the point of the tram; then move the eccentric on the shaft until the steam port is open to the required amount of lead, and fasten the eccentric to the main shaft. Next move the fly-wheel around until on the opposite dead centre, and if the lead is the same in amount for both ports the valve is set. Suppose, however, that in this last case the lead is too great; then it shows that the eccentric rod is too long, and it must be shortened to an amount equal to half the difference in the lead. Or suppose that the lead when the wheel was tried on the last dead centre l, was less than for the other port; then the eccentric rod must be lengthened to half the amount of the difference. Assuming that the rod was too long by 1⁄32 of an inch, then it may very often be shortened by simply heating about six inches of its length to a low red heat, and quenching it in water. If the rod has a foot which bolts on a corresponding foot on the eccentric, then to lengthen it a liner of the requisite thickness may be placed between the two feet.
Fig. 2553.
Suppose there is an equal amount of lead at each end but the amount is not sufficient or is too great: then the eccentric must be moved on the shaft until the proper amount of lead appears at the port. The lead must then be again tried at the other dead centre. In moving the eccentric, however, it must, under all conditions, be moved in the direction in which it will rotate, for reasons already given. The best method of measuring the lead where the lines on a rule cannot be seen is with a lead wedge p, as shown in [Fig. 2553]; this, if slightly forced in, will mark itself, showing how far it entered.
Fig. 2554.
In some practice the position of the valve is transferred to the valve stem outside of the stuffing box or gland, as shown in [Fig. 2554], sectional view. The valve stem being disconnected from the rod or arm that drives it, the valve is moved by hand to have the proper lead, as at a; a centre-punch mark is then made outside the stuffing box and a tram b rested thereon; with the other end of the tram a mark c is made on the valve stem. A similar mark is made on the stem when the crank is on the other dead centre, and the tram and marks, applied as shown, are employed instead of measuring the lead at the ports themselves. This involves extra work, but gives no more correct results. It involves marking lines on the valve stem, which is objectionable. If several trials have to be made there is a confusion of lines on the valve stem, and the wrong one is apt to be taken. On the other hand it affords a facility for setting the valve without having the steam chest open, which may in some cases be desirable. If this plan be adopted the lines on the valve rod should not be defined by centre-punch marks, for they will cut the packing in the stuffing box.
When the eccentrics are secured to the shaft by a set-screw only, and not by a feather, it is an excellent plan, after they are finally set, to mark their positions on the shaft, so that if they should move they may be set to these marks without moving the engine around.