Machine Foundation
What Is Machine Foundation?
Foundation provided below the superstructure of vibrating and rotating machines for installation, is known as machine foundation. It essentially consists of a mass of concrete. Design of machine foundation involves consideration of static load and kinetic forces.
The load of the machine is the static load which is of minor importance in the design of a machine foundation. The moving parts of the machine develop inertia forces which are the major forces to be considered in its design.
The kinetic or inertia forces are periodic and vibratory in nature, the magnitude of which depends on the type of machine
Types Of Machine Foundation:
1. Block Type Machine Foundation:
2. Box Or Caisson Type Machine Foundation:
3. Wall Type:
4. Framed Type:
Design Principles:
The general principles of machine foundation design are as follows:
1. It should be isolated from the adjoining parts of the building by leaving a gap around it to avoid the transmission of vibrations. The gap is filled with suitable insulators or dampers.
2. The foundation should be stiff and rigid to avoid possibilities of tilt in it.
3. In static state, the resultant of forces acting on the machine foundation should pass through the c.g of the contact area of the base.
4. The weight of the foundation block should be adequate. It should be able to absorb vibrations and resist resonance between adjoining soil and the machine. The weight of the foundation block may be assumed roughly 2.5 times the weight of the machine.
5. A vibration absorbing medium is introduced between the bottom of the foundation block and the floor on which it is resting. This medium may be in form of a rubber or leather gasket, layer of sawdust, etc.
Sometimes, springs are used below the machine itself to prevent the development of the vibrations. It is advised that relevant data of machine should be obtained from the manufacturer before any design of foundation is undertaken.
Vibration analysis
Machine Foundation is subjected to dynamic loads. These loads develop the vibratory motions which will transmit into the soil below the foundation. The effect on soil caused by these vibrations is analyzed using principles of soil dynamics and theory of vibrations.
Free Vibration in Machine Foundation
Free vibrations occur without any external force and they occur under the influence of forces in the system itself. But to start free vibrations, initially an external force or natural disturbance is required. Free vibrations may be of two types as follows
- Damped vibrations
- Undamped vibrations
Forced Vibration in Machine Foundation
Forced vibrations occur with continuous external forces on machine foundation. Let us say a damped system is subjected to exciting force F(t).
Vibration Analysis of Machine Foundation
To analyze the vibration theory of machine foundation we need to assume that the machine foundation has single degree of freedom. Normally machine foundation has 6 degree of freedom.
Mass (m)
Whenever the machine vibrates, soil below the machine foundation also vibrates. The mass of soil which vibrates due to machine vibration is termed as in-phase soil mass. Therefore, total mass (m) is equal to
m = mf + ms
Where,
mf = mass of the foundation
ms = in-phase soil mass = it varies from 0 to mf
Total mass (m) varies from mf to 2mf.
Stiffness (k)
The stiffness is dependent of type of soil below the foundation, embedment of foundation block and contact pressure distribution between soil and foundation. Stiffness is derived from the following methods.
Laboratory Method of Vibration Analysis
In the laboratory, a tri axial test with vertical vibrations is performed and modulus of rigidity is obtained. From this young’s modulus is determined with the help of Poisson’s ratio.
Young’s modulus E = 2G(1+u)
Stiffness k = AE/L
Where E = young’s modulus
G = rigidity modulus
U = positions ratio
Barkan’s Method
The stiffness can also be derived from the formula proposed by Barkan which is given below.
Where A = area of contact
Plate Load Test
A plate load test is conducted in the field and determine the stiffness of soil as slope of the load-deformation curve.
For cohesive soils, Stiffness K
For cohesionless soils, Stiffness k
Where,
B = width of foundation
Bp = diameter of plate
Resonance Test
By knowing the resonance frequency (fn), we can calculate the stiffness value. fn can be determined by placing vibrator of mass m on a steel plate supported on ground.
Therefore, stiffness 
Damping Constant (c)
When the vibration energy dissipates from the soil, damping occurs. The main reasons to develop damping is internal friction loss due to viscous effects and hysteresis, radiational losses due to propagation of waves through soil.
The damping constant c is obtained from the area of hysteresis loop of load deformation curve as follows
Where,
= work lost in hysteresis
W = total work done.
The damping constant varies from 0.01 to 0.1
FOUNDATION ISOLATION
Isolated foundation for active vibration damping for power presses, forging hammers, engine test rigs, and passive vibration isolation of machining centres, grinding machines, measuring & testing equipments, laser cutters and microscopes.
The ISOLATED FOUNDATION is required to reduce both active and passive vibrations. Vibration isolation mountings are required to reduce the transmission of vibration and shocks.
A foundation block or Vibration isolation mountings for high dynamic machines like power press, forging hammers, compressors, engine test rigs etc. is required in order to reduce the transmission of vibration and shock to nearby precision machines / building structures. To control the source of vibration disturbance through the use of resilient insulating materials is known as ACTIVE VIBRATION ISOLATION.
When it is not possible to prevent or sufficiently lower the transmission of shock and vibration from the source, a resiliently supported vibration insulating foundation block can be used for the PASSIVE VIBRATION ISOLATION of sensitive equipments like Machining Centres, Measuring and Testing Machines / Equipments etc.
ISOLATED FOUNDATION lowers the centre of gravity of the machine foundation system and adds to the stability of the machine. Machine remains aligned during dynamic load changes and rapid movements within the machines.
CONSTRUCTION OF THE ISOLATED FOUNDATION
Construction of the Foundation Tub should be carried out under the supervision of a Civil Engineer/ Contractor. Foundation must be 100% water-proof. The pit should be made to a depth as per the ground condition. The inner surface of the pit should be smooth. As per the layout supplied by DYNEMECH, place the vibration damping plates sets and gap filler in the pit. Paste them to the pit surface (i.e the base and the side walls) with the adhesive. Seal the joints with the adhesive tape supplied by DYNEMECH. Place a layer of PVC Sheet over the entire pit surface on the base and side walls. The PVC films should overlap. Seal the joints. Put a wooden ledge on the top of the foundation upto the ground level, which will be latter removed for creating gap for Polymeric Sealant, for preventing the water from entering the pit from ground. Place a layer of 5mm thick fibre board, which should be overlapped and joints sealed with the adhesive tape. A layer of wooden ply should be placed on the entire pit surface overlapped and the joints should be sealed with adhesive tape. Place another layer of PVC sheet over the entire pit surface on the base and side walls. The PVC films should overlap. Seal the joints. Pour concrete upto the depth of 15mm amd allow it to dry, which acts as a compression plates.Afetr drying of the compression plates, erect reinforcement and pour concrete in the rest of the pit area.For concrete layer deeper than 700mm, concrete should be poured in the layers of 300mm. The foundation block and the tub should not be in contact with each other. Now, remove the wooden ledge and fill the gap with polymeric sealent. Place the machine over the concrete block and levelling should be done with the help of edge mounts or screw support mounts.
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