Energy loss due to unbalance in rotor–shaft system

Abstract

Purpose

One of the major problems faced by industry is vibrations in rotating parts. Vibration is a to-and-fro movement of rotating mechanical parts and has many detrimental effects on machinery. It is obvious that no movement can be achieved without consumption of energy. All the energy consumed in vibration of mechanical parts is useless. Unbalance is one of the most common reasons for vibrations. This paper aims to experimentally evaluate the effect of unbalance in a shaft–rotor system on power consumption. An experimental setup consisting of a shaft and a rotor mounted on antifriction bearing was built-up. The shaft was driven through a flexible coupling, by a variable speed DC motor. The shaft–rotor system was rotated at different speeds and electrical power consumed by the system was measured at specific speeds varying from 1,200 to 2400 rpm. The rotor was balanced to grade G6.3 at 1,200 rpm. The power consumption by shaft in balanced condition was taken as baseline data for the further work. The rotor was then made unbalanced by adding different masses at 60 mm radius, and power consumption was recorded again at the same speeds. It was observed that average power loss due to unbalance is of 0.11watt/gm.mm unbalance. This can amount to 2.75 kw if there is unbalance of 50 gm at a radius of 500 mm. This work is meant to emphasis on the fact that the power consumption can be reduced if the vibrations can be kept under control.

Design/methodology/approach

The experimental setup consisting of a rotor–shaft system was fabricated. The shaft was supported on two anti-friction bearings. The shaft is driven by a 0.25 HP DC motor. The speed of the motor can be varied by a speed controlling device. A digital ammeter and voltmeter are connected to measure the input current and voltage to the system. The rotor was rotated at different speeds after two-plane balancing and the parameters like voltage, current drawn, rms velocity (average of drive and non-drive side bearing) and displacement at 1× frequency were recorded. The base line data for the balanced shaft–rotor system were recorded for further use.

Findings

Power consumption increases with increase in unbalance at each of the speeds. Total power consumed at resonant frequency is high. The average power consumed “W/gm.mm” increases at higher speed due to increased damping force of lubricant in bearings combined with the effect of resonance. Average power consumed due to unbalance is about 0.11 W/gm.mm unbalance. It is important to reduce the vibration to save power which can be effectively achieved by balancing the rotating parts in the machinery.

Research limitations/implications

The experimentation is done on a small rotor. When the same work is done on real situations where the rotors are heavy, we may expect some differences in the actual effect of unbalance on the power consumption.

Practical implications

The experimental work have a huge application in industry in condition monitoring. The power may tend to increase not only because of the unbalance but also due to other reasons of vibrations like misalignment, loose foundation, poor bearing conditions, etc. The power loss may increase due to any other reasons mentioned above. The degree of power saving due to steps taken for reducing vibration will depend on the existing vibration levels.

Social implications

The work highlights the effect of power loss due to vibrations. Even (1 per cent) small amount of power saved can save millions of dollars in industry, as there are many rotating parts which run 24 × 7. The emphasis is on condition-based monitoring which will help in power saving beyond the conventional advantages of condition monitoring.

Originality/value

The experimentation clearly quantifies power loss in absolute form that is the power loss is expressed per gm.mm of unbalance and not as the percentage of electrical or mechanical power, input or output. The percentage values may be misleading some times, as SMALL percentage of large values is also LARGE and hence should be taken into consideration.

Citation

Bulsara, M.A., Hingu, A.D. and Vaghasiya, P.S. (2016), "Energy loss due to unbalance in rotor–shaft system", Journal of Engineering, Design and Technology, Vol. 14 No. 2, pp. 277-285. https://doi.org/10.1108/JEDT-09-2013-0064