Fig 61

the radius-rod BD; which latter is raised or lowered by means of the lifting link GH. Finally, the link BC is sustained or sus

pended by a link JK; K being a fixed centre of motion. Compared with the Stephenson gear, the present arrangement has more joints, and occupies a greater length; but these objections are

Gooch's Link Motion.

compensated in some degree by the improved valve movement, whereby the lead is constant for all positions of the radius-rod. That such is the case will be apparent from an inspection of the figure. The crank A L is supposed to be on the outer dead centre, and when in this position, the valve, if correctly set, gives the desired amount of lead. Now, since the link has a radius equal to the length of the radius-rod, and because the eccentric-rods being of equal length, the link is vertical, it follows that BD can be swept through the whole length of the link without altering the position of D; and since the valve spindle is stationary, the lead is unaltered.

It remains to find the characteristic line of the gear; and, having obtained this, to determine the action for full forward or full backward gear, or for any intermediate position.

Eccentrics Driving Obliquely. Before entering on this matter it is necessary to investigate the action of an eccentric

Fig 62.

D

driving a valve spindle in an oblique direction. Let A B (Fig. 62) be an eccentric, and B C an eccentric-rod driving a valve-rod CD in a horizontal direction. Draw the straight line A C connecting the centre line of the crank shaft and the end of the eccentric-rod. This line makes an angle CAF with the horizontal line A F. Draw the line AH, making angle B A H equal to angle CA F. At right angles to BA draw BJ, intersecting the line A H at K. Then A K is the throw and position of an eccentric, which, driving the valve spindle directly, would give approximately the same steam distribution as an eccentric A B driving the valve spindle obliquely. The foregoing construction, although not admitting of Fig 63.

D

exact mathematical proof, is so very near absolute truth as to be quite satisfactory for all practical purposes.

In Fig. 63 consider the eccentric A B driving the valve spindle in the manner shown. Join A C, and make angle BAH equal to angle CAF. Draw BJ perpendicular to BA, cutting the line AH at K. Then A K is the throw and position of an imaginary eccentric, which, driving the valve-rod directly, would give approximately the same action as A B driving it in an oblique

manner.

Valve Diagram for Gooch's Link Motion.-In Fig. 64 let A E represent the throw and position of one of the eccentrics in a stationary link motion, in relation to the real position of the crank А В. Make AC equal to the throw and position of the virtual

eccentric arm, obtained in the manner just explained. Draw the vertical CD, which is the characteristic line of the gear, and contains the extremities of the virtual eccentric arms for all positions. By dividing the line CD in the same proportion that the block divides the link, the equivalent eccentric for that position is obtained, and the valve action for that particular position can be shown by Zeuner circles. In the figure several positions have been taken, and the points of admission, cut-off, &c., clearly indicated.

Crank-Pin Diagram.—In figure 65 a crank-pin diagram has been constructed, for full and mid gear, showing, as the characteristic line indicates, a constant lead for both grades of cut-off.

Best Arrangement of Gear.-The stationary link is more sensitive to the point of suspension than the shifting link, and unless there are strong reasons for not doing so, the point of suspension should be placed in the arc of the centre line of the link, and midway between the eccentric pin holes. The correct horizontal position of K is somewhere on a vertical line which bisects the arc described by the central point of the link, J-an arc whose length is equal to twice the lap plus twice the lead. Finally, the link J K should be as long as convenient. The above conditions have been carried out in the design shown by Fig. 61. It is difficult to explain why these positions are the best; but plotting down the slip curves for various points of suspension will clearly show that they are.

As to the point G (Fig. 61), it may be anywhere on the radius rod, and is often placed half way between the extremities. The intermediate link-work between G and the reversing handle M N is necessarily one of convenience, the aim being to move the block of the radius-rod its full extent by a suitable movement of the handle on the reversing lever. To arrange for a movement of from 2 feet 3 inches to 2 feet 9 inches is a common allowance. When reversal is effected by a hand wheel and screw thread, as is common in locomotives and marine work, the mechanical advantage between the wheel and the lifting link is not restricted in any way. To balance the weight of the radius-rod and lifting link, the lever HM is made double ended, and carries a balance weight on the arm M O.

The proportions of the Stephenson link given on a previous page apply equally to the Gooch link gear, and can be relied upon giving good results.

case.

Allan's Straight Link Motion.-The Allan straight link motion is obviously a combination of the shifting and stationary link gears. In the present arrangement the link and radius-rod are moved in opposite directions. The eccentric-rods may be open or crossed. With open rods the lead of the valve increases as the link is brought into mid-gear; but with crossed rods the reverse is the It will be shown, however, that this variation is somewhat less than in the shifting link with the same length of rods; but as the Allan gear occupies more length than the former, the eccentricrods are generally shorter; so that finally, the lead variation is little less than with the common form. The two extra joints introduced are an objection; and altogether it is difficult to see that the straight link gear is an improvement on its predecessors. This much, however, may be said; the movement of the link and radius-rod being in opposite directions, the vibration of these parts between extreme positions is less; and, therefore, in confined situations its use may be desirable. Another point in its favour is the straight link, which is somewhat easier to construct than a curved one.

In the skeleton diagram, Fig. 66, A E and AF are the eccentrics relative to the real position of the crank A L. BC is the straight