Assessment of Stone Crushing Characteristics and Optimum Dynamical and Structural Design Parameters of a Stone Crusher for Small and Medium Scale Entrepreneurs

Abstract

In Kenya and other developing countries, crushing of stones into aggregates using manual means is common. The activity is carried out by men, women and children along riverbeds, roadsides, homesteads and near towns. Manual stone breakers do heavy work, expose themselves to health problems like body, ear, skin and eye injuries. The aggregates produced is of low quality and the business is hardly profitable. Manual stone crushing has been necessitated by poverty and unemployment. Stones are predominantly crushed by compressive forces and the equipment used in the crushing process are Gyratory Crushers, Jaw Crushers, Cone Crushers and Vertical Impact Crushers. Jaw Crushers are the most appropriate for Small and Medium Scale Entrepreneurs because of their simplicity in its structure and mechanism, reliable performance, ease of manufacture and maintenance and affordable. This study therefore assesses the stone crushing characteristics of stones and present design parameters for the design of a more efficient and user-friendly mechanized stone crusher for Small and Medium Scale Entrepreneurs. This thesis reports the compressive strength of rocks used in construction industry. Laboratory tests were carried out on Schist, Gneiss, Tuff, Quartzite, Granite, Phonolite, Granodiorite and Grey Wecke taken from quarry sites in Bungoma, Uasin Gishu, Siaya, Nairobi, Meru, Thika, Nyeri and Machakos Counties. The results confirm that strength of rocks differ quite a lot, ranging from as low as 23MN/m2 for Tuff to as high as 127MN/m2 for Gneiss. Using the Vector Loop Closure method, the study presents equations from first principles of displacement, velocity and acceleration. Further analysis is carried out for both static force and mechanical advantage of the crusher mechanisms. The study shows that in the Single Toggle Jaw Crusher mechanism, the minimum angle of inclination of the swing jaw to the vertical is 159.7º while the maximum value is 161.6º. Thus, the range of variation of the inclination of the coupler (swing jaw) to the vertical for one complete cycle of rotation of the crank is less than 2º, hence the angular orientation of the swing jaw, during the cycle motion is insignificant. In the same cycle of rotation of the crank, the minimum value of the velocity of the swing jaw is found to be-0.476 radians per second while the maximum value is found to be 0.461 radians per second, showing clearly that the angular velocity of the swing jaw is generally small. For one complete cycle of motion of the swing jaw, the minimum value of its angular acceleration which occurs at a crank angle of 123.9° is found to be -13.208 radians per square second whereas the maximum value of angular acceleration which occurs at a crank angle of 291.8° is found to be 13.873 radians per square second. At the crank angle of 26.32º and 207.92º to the vertical, the angular acceleration of the swing jaw, for both vertical and horizontal components become zero. At these instances, the acceleration of the swing jaw becomes purely translational. In this thesis, the maximum value of the force transmission ratio is found to be about 3268 at the active crushing stroke, the minimum value is found to be 0. 61 and the mean value of 10.6. The force transmission ratio is very high at the beginning of the active crushing stroke, which is of advantage in communition process as it enables the crushing of brittle materials which fracture without undergoing appreciable deformation. In the Double Toggle Jaw Crusher Analysis, the result shows that the greatest amplitude of the crushing force occurs at the toggle action, which corresponds to the angle of 180º in the first phase and at 360º in the second phase. The Mechanical Advantage suddenly becomes high at 0º to the vertical in the first phase and at 180º in the second phase. This is evidence of the toggle phase which coincides with the commencement of the active crushing stroke, hence advantageous in the stone crushing action. The equations derived in this thesis can be used to investigate the effects of any alterations in the design of the Crusher Mechanisms, upon its kinematics and the

characteristic Mechanical Advantage can be used as a criterion for selecting such mechanisms. The recommendations presented at the end of this thesis are for the development of a more efficient Jaw Crusher which optimizes the Design Parameters. The Vector Loop Closure method used in this thesis is not itself new, but has not been applied before in the study of Jaw crusher Mechanisms.