Cotton Weaving and Designing / 6th Edition

by John T. Taylor

Draw the curved line touching the extremities of the treadle bowl. This gives the outline of the tappet.

As previously stated, the movement of the heald must be quickest when the shed is nearly closed, and must gradually decrease in speed as the shed opens. The unequal spaces into which the lift of the tappet was divided give this eccentric movement to the heald. The curve of the tappet will approach nearer to a radial line as the shed closes, and the heald approaches the centre of its stroke. Referring to [Fig. 40], it will be seen that the treadle bowl is at rest from F to G and from H to K, or one-third of a pick at both the top and bottom of the stroke. Therefore the time allowed for change, or for moving the heald from top to bottom, or vice versâ, is equal to two-thirds of a pick. If a dwell equal to half a pick is required, it can be obtained by dividing the pick into four equal parts and taking the middle two parts for dwell. If two-thirds dwell is required, divide the pick into six parts and take four parts for dwell.

It is usual to give the tappet which operates the back heald a slightly larger lift than the tappet which operates the front heald. The difference required can be easily calculated. In looms with the fulcrum of the treadles at the front, and the healds connected to the treadles between the fulcrum and the treadle bowls, some of the required extra lift is obtained by connecting the back heald to the treadle at a point further from the fulcrum than the front heald is connected. In looms with the fulcrum of the treadles at the back of the loom, and the tappets acting between the heald and the fulcrum, there will be a greater difference between the size of tappets in proportion to the lift than in the former case.

Tappets for twills, and other simple weaves, having more than two picks to the round, are usually placed upon a counter-shaft, but outside tappets are usually worked loose upon the bottom shaft.

The following example will illustrate the principle of constructing twill tappets:—

Draw a tappet for a 3 up and 1 down twill. Distance from centre of shaft to nearest point of contact with treadle bowl 3 inches, lift 3 inches, bowl 2 inches diameter, dwell ½ pick.

FIG. 41.

At a radius of 3 inches describe the circle A ([Fig. 41]). At a radius of 4 inches describe the circle B (one inch added for treadle bowl). At a radius of 7 inches describe the circle C (3 inches added for lift). There being four picks in the pattern, divide the circles into four equal parts by the lines DE, FG. Then each quarter-circle represents one pick, and the tappets must be made to make one revolution for four revolutions of the crank shaft. As the dwell of the heald (when the shed is open) must be equal to half a pick, or half a revolution of the crank shaft, divide the first pick into four equal parts by the points O, L, M; make DP equal to DO, and FN equal to FM, and rule lines from P, O, M, N to the centre. The distance OM represents the half-pick dwell, and the distances OP and MN represent the half-pick which will be allowed for changing the heald from bottom to top of its stroke, and vice versâ. Divide OP and MN into six equal parts, and the lift of tappet, or the distance between the circles B and C, into six unequal parts, the largest in the middle and gradually decreasing towards the two circles. From the corners of the unequal spaces describe the small circles representing the treadle bowl at different parts of its stroke, and draw the outline of the tappet touching the extremities of these circles.

A tappet of this shape acting upon a treadle bowl two inches in diameter will take the heald down for one pick and allow it to go up for three picks. The heald will be held stationary for exactly half a pick when at the bottom of its stroke, and will begin to rise slowly, and gradually increase in speed as it approaches the centre of its stroke, and will gradually decrease in speed as it approaches the top of its stroke. The downward movement will be an exact counterpart of this. In this kind of tappet it will be noticed that the heald, when it gets to the top (if it is required up for more than one pick), remains stationary until it is required to come down. Thus the heald remains at the top while the circles revolve from N to P.