Lecture8_Machine Foundation08.pdf
- 1. Design of Machine Foundation
- 2. Vibration Isolation Vibrations may cause harmful effects on adjoining structures, machines and the persons working in the area around the machine. However, if the frequency ratio is kept outside the critical range of 0.4 and 1.50, and the amplitude is within the permissible limits, the harmful effects are considerably reduced, especially if the system is damped. Transmission of vibrations can be controlled and the damaging effects considerably reduced by isolating either the source (active isolation) or by protecting the receiver (passive isolation). The following measures are generally adopted.
- 3. Vibration Isolation The following measures are generally adopted. (i) The machine foundation should be located away from the adjoining structures known as geometric isolation. (ii) Additional masses known as dampers are attached to the foundations of high-frequency machines to make it multiple-degree freedom systems and to change the natural frequency. (In reciprocating machines, the vibrations are considerably reduced by counterbalancing the exciting forces by attaching counterweights to the sides of the crank.) (iii) Vibrations are considerably reduced by placing absorbers, such as rubber mountings, felts and corks between the machine and the base.
- 4. Vibration Isolation (iv) If an auxiliary mass with a spring is attached to the machine foundation, the system becomes a two-degree- freedom system. The method is especially effective when the system is in resonance. (v) If the strength of the soil is increased by chemical or cement stabilization, it increases the natural frequency of the system. The method is useful for machines of low operating frequency. (vi) The natural frequency of the system is modified by making structural changes in the foundation, such as connecting the adjoining foundations, changing the base area or mass of the foundation, or use of attached slabs. (vii) The propagation of waves can be reduced by providing sheet piles, screens or trenches.
- 5. Block foundation consists of a massive block of concrete resting directly on soil or supported on piles or a pedestal resting on a footing. It has a large mass and, therefore, a smaller natural frequency. Box or Caisson type foundation is provided to increase the natural frequency as the mass of the foundation is less.
- 6. Plate foundation consists of a continuous plate made of concrete resting directly on soil or supported on piles. The plate has high static stiffness and is easy to construct. Spring foundation consists of a concrete slab or a steel frame on which the machine rests. The slab/frame is supported on a set of springs. This type is used, if the subsoil is weak or we need to isolate the subsoil from the transmitted vibrations.
- 7. Information Needed for Design 1. Static weight of the machine and accessories. 2. Magnitude and characteristics of dynamic loads imposed by the machine operation and their point of application 3. The soil profile of the site and dynamic soil properties such as dynamic shear modulus and damping 4. Trial dimensions of the foundation. These are generally supplied by the manufacturer. This will give the total static weight. 5. An acceptable method of analysis i.e., a mathematical model to determine the response of the foundation-soil system 6. A criteria for adequate design.
- 8. Soil Profile and Dynamic Soil Properties Satisfactory design of a machine foundation needs information on soil profile, depth of different layers, physical properties of soil and ground water level. This information can be obtained by usual sub-surface exploration techniques. In addition, one must determine dynamic shear modulus, material damping, poisons ratio and mass density of soil for dynamic analysis of the machine foundation.
- 9. Soil Profile and Dynamic Soil Properties Dynamic shear modulus of a soil is generally determined from laboratory or field tests. Material damping can be determined from vibration tests on soil columns in the laboratory. The values of dynamic shear modulii and damping may be estimated from empirical estimations for preliminary design purposes. Geometrical damping is estimated from elastic half-space theory and appropriate analogs.
- 10. Design Criteria for Reciprocating Machine The machine foundation should be isolated at all levels from the adjoining foundations. The natural frequency of the foundation–soil system should be higher than the highest disturbing frequency and the frequency ratio should be less than 0.4, however if not possible then the frequency ratio should not be less than 1.50. The amplitude of vibration should be within the permissible limits or any specified permissible limits. For most soils, another criterion, amplitude for low-speed machines is usually taken as 0.2 mm. According to another criterion, the amplitude in mm is limited to 4/f frequencies less than 30 hertz and 125/f2. For higher frequencies, where f is frequencies in hertz (cycles/sec).
- 11. Design Criteria for Reciprocating Machine Concrete block foundations should be used. However, when the soil is not suitable to support block foundation, cellular foundation may be used. The size of the block in plan should be larger than the bed plate of the machine. There should be a minimum all round clearance of 15 mm. The total width of the foundation measured at right angles to the shaft should be at least equal to the distance between the center of the shaft and the bottom of the foundation. The eccentricity of the foundation system along X-X and Y-Y axes should not exceed not 5% of the length of the corresponding side of the contact area.
- 12. Design Criteria for Reciprocating Machine The combined center of gravity of machine and foundation should be much below the top of foundation. The depth of foundation should be sufficient to provide the required bearing capacity and to ensure stability against rotation in the vertical plane. The stresses in the soil below the foundation should not excess 80% of the allowable stresses under static loads. The base pressure is limited to half the normal allowable pressure qna in extreme cases. Where it is not practicable to design a foundation to give a satisfactory dynamic response, the transmitted vibrations may be reduced by providing anti–vibration mounting either between the machine and foundation or between the foundation and the supporting system.
- 13. Design Criteria for Reciprocating Machine The machine should be anchored to the foundation block using a base plate and anchor blots. Bolt holes should be backfilled with concrete and the space below the plate should be filled with 1: 2 cement mortars. A number of similar machines can be erected on individual pedestals on a common raft. The analysis for such machines can be made assuming that each foundation acts independently up the raft into sections corresponding to separate machines.
- 15. Design Procedure for Impact Machines The hammer system usually consists of a frame that guides a falling, weight, known as the tup, that strikes down on the anvil that is supported on the foundation block. Typical tup weights range between 2.5kN and 100kN and the drop height is in the range of 0.3 m to 2 m or more. The anvil gets repeated blows from the tup so that the piece of metal held by the anvil gets forged to the desired shape or is broken. High impact energy is transmitted to the anvil from the falling tup. A part of the energy is used in forging and the rest gets transmitted to the soil below.
- 16. Design Procedure for Impact Machines To avoid breakage of the concrete foundation due to impact stresses, an elastic pad made of felt, cork or rubber is placed between anvil and foundation block. To ensure that the compressive stresses induced in the pad do not exceed the specified value. This is an additional design criterion for block foundations for a hammer. In the design of these foundations, the damping in the soil is neglected since the period of impact is very small compared to the period of natural frequency of vibration of machine-foundation-soil system. The rest of the design procedure is the same as that of block foundation for reciprocating machines.