Violent Vibrating of Screw Elevator due to ‘Epicyclic Rolling’ of the
A screw elevator can, in relatively rare cases, vibrate so violently that it is completely impractical to continue working. This phenomena can develop with the machine runs empty of product and/ or when conveying material. The stiffness of an elevator screw is relatively low by virtue of their essential slender construction to move bulk material to a height many times the diameter of their centre shaft, so the auger tube can be deflected relatively easily. The screw fabrication also entails significant welding of the flights to the centre tube such that weld distortion and flight stresses tend to build-in out of balance stresses. Whilst good practice dictates that screws are straightened after manufacturer, they are rarely stress relieved or dynamically balanced.
This form of vibration is initiated if the screw shaft is deformed such that the tips of the screw flights contacts the casing and develops sufficient frictional grip to commence the screw ‘whirling’, by rolling around the inside of the casing like a coin running round inside a bowl. The effect is similar to that of an epicyclic gear, with the shaft displacement acting as a virtual ‘arm’ equal in length to the normal working clearance of the flights in the casing, carrying a planet gear. This ‘mechanism’ induces a multiple factor to the rolling frequency of the flight diameter divided by the flight tip clearance times the speed of rotation of the screw shaft. As the dimensions of the flight diameter to flight tip clearance ratio is typically of the order of 20:1 and screw rotation speeds upwards of 60 rpm, vibration frequencies can easily exceed 20 cycles per second.
The weight of the deflected screw rotating at such frequencies produces significant centrifugal forces due to the power factor influence on screw speed and, whilst the frequency of vibration normally well exceeds that of the first mode of natural frequency of the screw shaft, it can be sufficiently close to that of the third mode or that of the much stiffer casing as to exacerbate the situation. Synchronous casing vibration increases the screw deflection, to proportional increase the disturbing force.
The problem can be overcome by splitting the elevating length between two machines, but this is expensive. An expedient step is to fit a bearing sleeve in the casing to reduce the tip clearance and reduce the flight tip friction such that the conditions for rolling contact at the higher frequency cannot develop. A corollary of this method is to fit ‘bearing bands’ under the flight tips to reduce the local contact stress. Other techniques may be adopted, but are the domain of specialists.
Violent Vibrating of Screw Elevator due to ‘Epicyclic Rolling’ of the Screw.
