Foundations for machines

UNIT - III

Chapter - 15 (B)

FOUNDATIONS FOR MACHINES

Dynamic Load and Vibrations in Machines

Machines produce dynamic forces in addition to the static forces. The design of foundations for machinery is therefore more complex than that for building structures.

Vibrations in the ground are caused by earthquakes or from construction activities such as pile driving, rock blasting, etc. On the other hand, vibrations also result from the dynamic loading of operating machinery. Such vibrations are transmitted to the soil through the foundation and produce several adverse effects.

TYPES OF MACHINES

1. Reciprocating Machines: Reciprocating machines produce unbalanced forces and work at low rotating speeds. The unbalanced force of the reciprocating machines is considered as sinusoidal, while analyzing the foundation.

Examples: Reciprocating Compressors, Reciprocating Engines, etc.

2. Impact Machines: Impact machines produce impact loads. In these machines, the dynamic load reaches a maximum value in a very short time and attenuates immediately.

Examples: Stamping Press, Forging Hammer, etc.

3. Rotary Machines: Rotary machines operate at frequencies ranging from very low to high. They are classified as low frequency up to 1500 rpm, medium frequency from 1500 to 3000 rpm and high frequency from 3000 to 6000 rpm. Examples: Electric Generators, Turbo Generators, Crushing Mills, etc.

1. REQUIREMENTS OF MACHINE FOUNDATIONS AND DESIGN CRITERIA

1. Static and Dynamic Loads: The foundation should be capable of carrying the static and dynamic loads acting on it, without undue settlement or shear failure.

2. Natural Frequency of Foundation System: When the operating frequency of the machine equals the natural frequency of the foundation, resonance occurs, causing large amplitude of vibration. To avoid unwanted resonance, the natural frequency of the foundation should be high for low speed machines and low for high speed machines.

3. Minimizing Unbalanced Forces: All reciprocating and rotating parts of a machine should be well-balanced in order to minimize the unbalanced forces and moments.

4. Combined Center of Gravity: The combined center of gravity of both the machine and the foundation should be on a vertical line passing through the centroid of the foundation base.

5. Amplitude of Vibration: The amplitude of vibration under service conditions should not exceed the safe and permissible limits, stipulated by the Code of Practice IS : 2974.

6. Limiting Vibration Propagation: Ground-water table should be as low as possible in order to limit the vibration propagation.

7. Separation of Machine Foundation: The machine foundation should be separated from adjacent parts of the building by air-gap on the sides and by a resilient mounting such as cork or timber sleepers.

8. Level of Machine Foundation: The machine foundation should go down to a level below the level of the foundations of adjacent buildings.

9. Heavy Concrete Block: The concrete block of the machine foundation should be heavy to provide damping. The concrete for the block should be of M20 grade and should be reinforced. The block should be cast in one continuous operation.