Statistically, it is said that 50 percent of all the vibrations that are either assessed or addressed in the rotating mechanical equipment are due to unbalance in the rotating system. While these numbers sound overwhelming, in reality, these are at times even higher.

Let’s look into the composition of most of the industrial and general engineering equipment; almost invariably, most machinery will have at least one or more rotating equipment either as their prime mover or through their kinematics. These can be motors, gearwheels, pulleys and flywheels and the variants of these combinations in a typical equipment setup.

The term unbalance in any rotating element refers to the unequal distribution of mass over its rotational axis essentially meaning that the centre axis of rotation is being pulled with unequal forces resulting in varying vibration responses from the support location, which typically is the bearing housing.

Unbalance mass is a scalar quantity and its unequal distribution over the axis centre of a rotating body is influenced by the angular velocity (ω). The resulting force is thus defined as F = m r ω2, wherein m is the net unequally distributed mass, r is the effective radius where the unbalance mass is located and ω is the angular velocity of the rotating system

Basic equation of centrifugal force and unbalance

 

Bearings are the interface between the rotating and the machine's stationary body; the centrifugal forces induced by the unbalance, exert forces on to the bearing housing in tandem with the rotational speed of equipment. The higher the rotational speed of the equipment, the higher is the force exerted and the forcing function is a squared value of the angular velocity; based on the mass and stiffness of the machine body on which the rotating equipment is configured, proportional vibrations are exhibited and are quantified for resolving the problem.

What starts as a cyclic force due to unbalance, subsequently initiates higher stresses on the bearing and the connected equipment housing; the continued usage of the rotating equipment in the unbalanced condition deteriorates the bearing condition that further increases the vibrations to a state that the equipment becomes inoperable.

All rotating equipment possess critical speed/s (can be termed as its natural frequency) and when unbalance forces / frequencies at these operating speeds match up, a state of resonance is attained making the entire rotating and its connected housing system to vibrate with very high vibrations. Higher unbalance in variable speed rotating machinery has a severe consequence when operating or passing through critical speeds. AK1 Systems in this condition has high vulnerability to failure and catastrophic consequences.

While many quality processes, both by design and manufacturing exist to minimise the content of residual unbalance in any rotary equipment, depending on the end usage of the equipment and the overall assembly, installation and maintenance, some amount of unbalance is still carried into the final operational condition of the equipment.

In-situ balancing of large waste gas exhaust fan

 

Two approaches are adapted to address the unbalance issues; Firstly, all the sub-assemblies are individually balanced on specific dynamic balancing machines and their residual values are minimised to comply with applicable ISO 21940 standards. Sometimes, the entire assembly is also configured as single assembly for balancing trials on the dynamic balancing machine. This method and approach are the most ideal and accepted methodology to assure the overall residual unbalance before the rotating assembly is put to its equipment assembly and operation.

The alternate added approach is to conduct in-situ balancing of the rotating assembly in its operating setup and configuration. This methodology has its advantages that the entire rotating assembly with all its installed tolerances and deviations are considered while conducting the balancing activities. This approach uses vibration sensors to quantify machine housing vibrations at their bearing locations and uses a phase sensor to assess the phase of unbalance. Through few steps of trials, unbalance corrections are effected at specific locations on the rotating component either by addition or reduction of mass.

NV Dynamics has a vast experience of handling in-situ balancing activities of various sizes, speeds and in challenging installation conditions. From a 25-gram rotor to a 35-tonne synchronous condenser, from high-speed turbo-propeller to low head, low speed vertical hydro generator, our teams have both the engineering expertise, technical resources and the overall understanding of the holistic approach to conduct balancing exercises.