Tappet Shedding Mechanism: Positive & Negative | Scope & Advantages

Last Updated on 04/04/2021

Experiment name:
Study on positive and negative tappet shedding mechanism.


  1. To know about the different parts formed this shedding.
  2. To learn the mechanism of tappet shedding.

Scope of tappet shedding:
A tappet is a cam having some dwell period which controls the up/down movement of heald shafts and the time of opening of the shed. Tappets are generally used for heald shedding. Tappet, cam and wipers are names given indiscriminately to those irregular pieces of mechanism to which a rotary motion is given for the purpose of producing, by sliding contact, reciprocating motion in rods and levers. When the rod is to receive a series of lifts, with intervals of rest, the piece is generally called tappet.

The tappet shedding is the simplest and least versatile shedding motion. The design is restricted to plain weave, simple twill and simple sateen or satin weave, simple honey comb and huck-a-back weaves. Any design more than 8 shafts requires dobby loom.

Types of tappet: 
Various kinds of shedding tappets are used in industries. They are of two main types:

  1. Negative shedding tappet
  2. Positive shedding tappet

In negative tappet, the treadle levers are pressed by the tappet, but do not guide it back to move up whereas in the positive tappet, the bowl moves inside the groves of the tappet and is positively guided.

Main parts of tappet shedding mechanism:

  1. Motor
  2. Motor pulley
  3. M/C pulley
  4. Crank shaft
  5. Crank shaft gear
  6. Bottom shaft gear
  7. Bottom shaft
  8. Tappet
  9. Connecting rod
  10. Heald shaft
  11. Top arm

Position of fixation of shedding tappet:
The shedding tappets are may be fixed in different parts of a loom. Namely-

  1. Under and over the centre of heald shaft.
  2. Under and over one end of heald shaft.
  3. Out side the loom framing.

Negative tappet shedding mechanism:
A pair of tappets A and B is fixed to the bottom shaft C at 180 degrees to each other (Figure 1). Two treadle levers D and E are connected to the loom back-rail by a bracket F. The bracket acts as a fulcrum for the levers. The two treadles have teeth to carry the lamb rods G and H, respectively. Two heald shafts J and K are connected to the lamb rods. A top reversing roller shaft Q carries two rollers of different diameters. The roller of small diameter N is connected to leather strap L to which the front heald shaft J is connected. The roller P of large diameter is connected to leather strap M to which the back heald shaft K is connected. The tappets A and B touch the anti-friction bowls or followers R and S, respectively, which are fixed to the treadle levers.

Negative tappet shedding mechanism
Figure 1: Negative tappet shedding mechanism

The heald shafts have heald eyes T and U through which the warp threads pass. X is the warp sheet and Y is the cloth. The odd ends are passed through one heald shaft while the even ends are passed through the other heald shaft. As the tappets revolve, they press the treadles that in turn make the heald frames move up and down, thus forming the shed to facilitate insertion of weft.

Positive tappet shedding mechanism:
In this type of shedding, the heald shaft is raised and lowered by the tappet (Figure 2). The tappet shaft A carries tappet B that has a groove C or track in which a bowl D is placed. The bowl is connected in turn to a tappet lever E, link rods G, links J and a heald shaft K. Each tappet is separately connected to a heald shaft through link rods and tappet lever. F and H are fulcrums for tappet lever and links G, respectively.

Positive tappet shedding mechanism
Figure 2: Positive tappet shedding mechanism

When the tappet is rotated, the bowl is also rotated. According to the shape of the groove, the bowl is moved up or down or remains still. If the bowl is moved up, the tappet lever moves to the right through the links G and J and the heald shaft is lowered. If the bowl is moved down, the tappet lever moves to the left and the heald shaft is raised. Since the heald shaft is raised and lowered by means of the mechanism, this tappet shedding is known as positive tappet shedding mechanism. When the bowl stands still, the heald shaft is in the ‘dwell’ stage.

Driving of shedding tappet:
In our loom tappet gets motion in the following way. At first machine pulley gets drive by belt from motor pulley. Then machine pulley, which is directly joined with the crank shaft, gives motion to bottom shaft. This crank shaft gives motion to bottom shaft via gearing. Two tappets are joined with the bottom shaft. So when bottom shaft moves then the tappets also rotate. When it gets contact with tradle bowl it lifts the tradle levers and the heald shafts are lifted by tradle levers via links. As this is a negative shedding tappet the heald shafts are lowered by their own weight.

Advantages of tappet shedding:
The main advantages of tappet shedding is that It is robust, simple and cheap, It is capable of lifting a heavy weight with less wear and tear than other shedding mechanisms. It can move heald shafts at great speeds, puts less strain upon the warp, consumes less power, gives greater output and requires less maintenance.

Disadvantages of tappet shedding:
It has some disadvantages also. If the weave is changed, it will be necessary to change the tappet and the change gear wheel in the counter shaft arrangement. So work involved in changing the weave is more. The capacity of a tappet to produce a pattern/weave is very much limited. A maximum of 8 or 10 tappets only can be used.

Tappet shedding mechanism is important for driving loom. It is also important for fabric design. By this experiment I learn this mechanism. This would be very helpful in my future life.

You may also like:

  1. Differences between Tappet Shedding, Dobby Shedding and Jacquard Shedding Mechanism
  2. Dobby Shedding Mechanism | Scope of a Dobby
  3. Construction of Shedding Tappet for Plain Weave
  4. Jacquard Shedding | Jacquard Shedding Mechanism

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