Working

When the bottom shaft rotates in the direction shown, the disc also rotates in the same direction. When the bowl comes against the shoe it strikes the shoe. This causes the side shaft lever to depress and pull the lug strap through the connecting lever. So the picking stick is pulled inward with sufficient velocity to drive the picker. As the shuttle is in contact with the picker, it is pushed. The shuttle thus reaches the opposite box. The picking stick and side shaft lever are returned to their original positions by return springs. For the next pick, the disc on the other side of the loom will work and the shuttle will come out of the opposite box.

Thus, for one rotation of the bottom shaft, two picks are inserted.

The timing of picking can be altered by changing the position of the bowl in the disc slot.

The strength of pick can be altered by raising or lowering the lug strap. By lowering the lug strap the strength of the pick is increased and vice-versa.

Strength and Timing of Pick in Underpick Mechanism

Strength of pick or force of pick

The strength of pick in side-lever underpick mechanism may be altered in the following two ways :

By raising the fulcrum of the side lever, the strength of pick is increased and vice-versa. See Figure 3.20

By moving the shoe closer to the bowl, the strength of pick is increased.

However, these methods of altering the strength of pick have only a limited range of success.

Figure 3.20 Strength of Pick

The timing of the underpick mechanism is altered by moving the bowl A in the slot B of the circular disc C. If the bowl hits the at 180°, the picking is called normal picking. If the bowl hits the shoe 5° before or after bottom centre, then the picking is called early and late picking respectively (Figure 3.21).

Figure 3.21 Timing

Comparison between Overpick and Underpick Mechanisms

The position of the fulcrum of the picking stick is right below the level of the shuttle box in an underpick loom, whereas in an overpick loom the fulcrum is above the level of the shuttle box.

Overpick mechanisms are used extensively in narrow looms. The underpick mechanism is used in medium and broad looms.

The overpick mechanism consumes less power than the underpick mechanism.

The overpick mechanism is extensively used in weaving light cotton fabric. Under pick mechanisms are used for all types of fabric, especially for silk and man-made fibre fabrics.

The spindle on which the picker is mounted in the overpick mechanism requires to be oiled frequently. So there is a chance of oil falling in the shuttle box. An under pick loom is thus much cleaner than an over pick loom.

Movement of parts in the underpick mechanism is more direct, making it more suitable for high-speed looms running light shuttles.

In underpick looms, additional mechanisms may be added above the loom.