Balancing of Reciprocating Masses
Balancing of reciprocating masses refers to the process of minimizing the vibrations caused by the movement of pistons or other reciprocating components in machinery. This is achieved by counterbalancing the mass and inertia forces to reduce the overall vibration and improve the performance and longevity of the machinery. Balancing can be achieved through various methods such as using counterweights or dynamically balancing the components.
5 Key excerpts on "Balancing of Reciprocating Masses"
eBook - ePub- Clifford Matthews(Author)
- 2011(Publication Date)
- Wiley-Blackwell(Publisher)
...The objective is to maintain the operating vibration of the machine within manageable limits. Dynamic balancing normally involves two measurement/correction planes and involves the calculation of vector quantities. The component is mounted in a balancing rig which rotates it at or near its operating speed, and both senses and records out-of-balance forces and phase angle in two planes. Balance weights are then added (or removed) to bring the imbalance forces to an acceptable level. Figure 5.3 5.6.1 Balancing Standard: ISO 1940/1: 2003 The standard ISO 1940/1: 2003 (identical to BS 6861: Part 1: 1987): Balance quality requirements of rigid rotors is widely used. It sets acceptable imbalance limits for various types of rotating equipment. It specifies various (G) grades. A similar approach is used by the standard ISO 10816-1. Finer balance grades are used for precision assemblies such as instruments and gyroscopes. The principles are the same. 5.7 Vibration Vibration is a subset of the subject of dynamics. It has particular relevance to both structures and machinery in the way that they respond to applied disturbances. 5.7.1 General Model The most common model of vibration is a concentrated spring-mounted mass which is subject to a disturbing force and retarding force. Figure 5.4 The motion is represented graphically as shown by the projection of the rotating vector x. Relevant quantities are The ideal case represents simple harmonic motion with the waveform being sinusoidal. Hence the motion follows the general pattern: Vibration displacement (amplitude) = s Vibration velocity = v = d s /d t Vibration acceleration = a = d v /d t 5.8 Machine Vibration There are two types of vibration relevant to rotating machines Bearing housing vibration. This is assumed to be sinusoidal. It normally uses the velocity (V rms) parameter. Shaft vibration. This is generally not sinusoidal...
eBook - ePub- Dick Jeffrey(Author)
- 2013(Publication Date)
- Routledge(Publisher)
...Chapter 4 B ALANCING Principles of Balancing If the centre of gravity of a rotating machine element does not coincide with its centre of rotation, then the machine is said to be unbalanced. When the machine is stationary, the off-centre mass causes the machine element to settle in a fixed position. (Fig. 4-1) Fig. 4-1 Stationary machine with an off-centre mass. As the machine element rotates, a centrifugal force associated with the off-centre mass develops and imposes a fluctuating load on the shaft support bearings as shown in Fig. 4-2. Fig. 4-2 Rotating machine with an off-centre mass. The size of this force depends not only on the mass of the rotating element, but also on the extent to which the mass is off-centre and the speed of rotation. The resulting load imposed on the bearings cycles continuously through 360° with every rotation of the shaft. In addition to imposing high loads and fatigue stresses on the bearings, a fluctuating load of this type will set up vibrations that will be transmitted through the machine and the surrounding structure. Balancing is the term used to refer to the process of improving the mass distribution of a rotating machine element, so that it rotates in its bearings without giving rise to unbalanced centrifugal forces. A machine element in which the centre of gravity and the centre of rotation coincide will settle in any position when stationary, and when rotating will not impose any additional loads on the bearings due to unbalanced centrifugal forces. The only (radial) load imposed on the bearings should be due to the weight of the rotating element. In practice, it is impossible to achieve perfect balance and, even after sophisticated balancing techniques have been used, a rotating element will always possess some residual imbalance...
eBook - ePubCompressors
Selection and Sizing
- Royce N. Brown(Author)
- 2011(Publication Date)
- Gulf Professional Publishing(Publisher)
...If this is not possible, selective tuning and proper application of damping must be used. If the vibrations are left unchecked, damage to the compressor will occur, such as premature failure of bearings and seals or packing rubs, and, in extreme cases, major component fatigue and failure. Secondary effects may result from the shaking forces being transmitted through the frame to the foundation. Damage may result to the foundation, particularly with reciprocating compressors. Interaction with other equipment in the area is also a concern. If there are people working in the area of a shaking compressor, there may be motion sickness or other physiological effects. Because of the potential for damage or problems, a basic understanding of the nature of and possible remedies for the more common vibrations is necessary. Balance Basics Before the subject of balancing can be properly discussed, imbalance should be described. An imbalance exists when the mass center is displaced from the rotating center. Figure 9-1A shows a mass-less disk with a finite mass, m, located at radius, r, from the center of rotation. If the disk rotates at an angular velocity ofω, the force, F, exerted by the finite mass, m, is Figure 9-1 A one-mass (A) and a two-mass (B) model of an unbalance. (9.1) where ω= 2 πN (shaft speed), rad/unit time Equation 9.1 is the basic equation for imbalance. For such a simple arrangement, balancing referred to as static balance could be done by placing the shaft on knife edges. Initially, the location of the mass would rotate the disk gravitationally until the mass was on the bottom. If weights were added opposite the location of m until the mass of the weights equaled the mass m and were located at the same radius r, the disk would be motionless, regardless of the position of the angular placement of the disk on the edges. In Figure 9-1B, two masses are located exactly 180 degrees apart, but on two different weightless disks...
eBook - ePub- J. P. Den Hartog(Author)
- 2013(Publication Date)
- Dover Publications(Publisher)
...CHAPTER 5 MULTICYLINDER ENGINES 5.1. Troubles Peculiar to Reciprocating Engines. There are two groups of vibration phenomena of practical importance in reciprocating machines, namely: 1. Vibrations transmitted to the foundation by the engine as a whole. 2. Torsional oscillations in the crank shaft and in the shafting of the driven machinery. Each one of these two effects is caused by a combination of the periodic accelerations of the moving parts (pistons, rods, and cranks) and the periodic variations in cylinder steam or gas pressure. Consider a vertical single-cylinder engine. The piston executes an alternating motion, i.e., it experiences alternating vertical accelerations. While the piston is accelerated downward there must be a downward force acting on it, and this force must have a reaction pushing upward against the stationary parts of the engine. Thus an alternating acceleration of the piston is coupled with an alternating force on the cylinder frame, which makes itself felt as a vibration in the engine and in its supports. In the lateral direction, i.e., perpendicular to both the crank shaft and the piston rod, moving parts are also being accelerated, namely the crank pin and part of the connecting rod. The forces that cause these accelerations must have equal and opposite reactions on the frame of the engine. This last effect is known as ″horizontal unbalance. In the longitudinal direction, i.e., in the crank-shaft direction, no inertia forces appear, since all moving parts remain in planes perpendicular to the crank shaft. The mathematical relation describing these effects is Newton′s law, stating that in a mechanical system the rate of change of momentum equals the resultant of all external forces: This is a vector equation and is equivalent to three ordinary equations...
- Anthony Sofronas(Author)
- 2012(Publication Date)
- Wiley(Publisher)
...Chapter 6 Causes of Vibrations and Solutions In Machinery 6.1 Rotating Imbalance Just like an unbalanced tire on an automobile due to a heavy spot or lost balance weight, unbalance occurs on rotating machinery such as motors, centrifugal compressors, turbines, pumps, and other such equipment. This can be due to manufactured tolerances, fouling as in the case of steam turbines, missing parts, or a shaft bow. Shaft bow can be due to thermal problems because the turbine was not slow-rolled to equalize the temperature, gravity sag due to poor storage techniques, or lack of periodic barring over during downtime. The dynamic loading force or the force due to an imbalance is If acceleration is now cyclic, based on the displacement curve, By knowing the displacement X p and the frequency along with the weight of the part in vibratory motion, the force to cause this motion can be determined. This can be written as or, simplifying, where W = lb rotating and ε is the eccentricity (in.). 6.1.1 Case History: Motor Imbalance An interesting use was on a motor that was moving 0.0005 in. radially at each bearing journal. Was this excessive? One rule of thumb for balancing machines is that the force on each journal due to unbalance should be less than 10% of the rotor weight, divided by 2. This is the allowable unbalance force. Thus, if the rotor weighed 1000 lb, the unbalance should not exceed lb at the journal. Solving for ε on this 3600-rpm machine, with W equal to 500 lb., ϵ = 0.00027 in. This allowable is less than 0.0005 in., so the vibration is unacceptable using this simple criterion. The unbalance at each journal is 92 lb. In this example, the rotor was taken simply as a mass in space being moved by an unbalanced force. Do not confuse this with a single-degree spring–mass system. Figure 6.1 represents an industry standard used for new design specifications...
