Poka shells

Cardboard — (Panch cutting - Cardboard disca)—KDisc passing)--- puii ca 

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The construction of Poka is shown in principle as Fig.loS. A Poka shell consists of a fire chamber only, or a fire chamber and a fire prevention chamber. The fire chamber contains the parts which must be ignited and the fire prevention chamber contains the parts which must be protected from fire. 

The Poka shell sphere must be broken into two hemispheres along the joint when it explodes, otherwise all the contents will not be thrown out or they will be damaged by fire. This is quite a different requirement from Wariraono. The strength of the joint therefore must be a little less than that of the hemispheres. Fig.11 shows an accident which occured, when the joint was stronger than the hemisphere. 

The number of pasted layers of paper for an.ordinary Poka shell is calculated by the following formula ... 

The amount of the bursting charge for a Poka shell depends upon the strength of the joint, but generally it is as follows(Table 2 ). 

Table 2. The standard values for the bursting charge of a Poka shell (using black powder)...

The Warimono and Poka shells described above are the most representative in construction, but there are some other shells which lie between these two and some are quite different from them. 

Shells of this type are quite popular in Europe and the USA. The shells are cylindrical and resembles Japanese Poka shells in principle. The process of assembly may be understood in the following figures (Fig.116 and 117). 

Other important parts. In round Poka-shells a stopper is necessary. ]... 

Shells can be stuffed with some buffer like sawdust or chaff in the space to protect the content from the shock. (But chaff may be forbidden in exports due to quarantine regulations.) Shells must have an outside strength which stands the shock when fired. Sometimes we have an accident because a Poka-shell is broken at the mouth of the mortar when it is fired and the contents are projected at the same time being ignited by the flame of the propellant charge. In the case of Poka-shells or some signal shells... 

The diameter of the core composition of a main fuse is one of the problems. In Fig.142, (a) shows a large core as in European shells. This type of core is quite ignitable and does not cause misfire on firing, but when it is used for a Poka shell, it causes gas escape due to the big hole after the fuse is burnt. To avoid this, it is necessary to place a gas stopper at the end of the fuse. This is troublesome, and in Japan a (B) type fuse, which has a rather small core composition, is used. Some use an even smaller type,(C), but this often causes misfire on firing and sometimes a fire stop during burning. 

Early explosion. In this type of accident a shell explodes too early before the shell reaches the expected altitude in the sky. It often occurs, when the composition of the fuse is too soft and causes rapid fire, or when the length of the fuse is too short. When a Poka shell is too weak... 

Other important parts. In round Poka-shells a stopper is necessary to avoid a misfire due to the revolving of the contents. A safety device is important to avoid unexpected ignition, but in ordinary cases this is simply served by covering or packing each ignitable part with a piece of paper or paper tube which is difficult to burn. 

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