Vibration Isolation, Foundation for forging equipments

Introducion

The Foundation design for forging equipment is one of the crucial but grey areas in Indian forging industry. Foundations have to be designed for 2 kinds of loads : Static & Dynamic. Due to the nature of work, the dynamic load caused by these machines is quite high.

Presses also create vibrations which - although low in comparison to  hammers - cannot be neglected at least under certain site conditions. Hammer vibrations will on the other hand always be a problem if it occurs in proximity to residential/sensitive industrial neighborhoods, which is often the case in India. 

Problems in setting up a forging industry in India :

1. Arranging finance
2. Set up of the equipment
3. Marketing and Production
4. Side effects, for example vibrations in the neighborhood or disturbance to tool room or sensitive equipments.

You can handle 1 - 3 with your experience, but what would you do for the 4th ?!!

Spring – damper system

Cork pads below the foundation

Rubber Ring Sheet for Foundation and Floor

Friction/Belt drop hammers are still most common in India, however foundation of all types of hammers & presses are planned in some way or the other irrespective of soil properties or specific local  requirements.  The Indian govt. is still in the process of setting  up guidelines for acceptable limits of vibration levels. Non existence of such guidelines often creates additional problems and there are dozens of such plants that have hence been forced to close or leave the area.

Europe has some norms & the norms in Germany are like:

VDI Guide Line 4025 about the design of hammer foundations &

DIN 4150 about permissible vibration levels in buildings

Fig.1

Fig.2

Now, its not easy especially to meet DIN 4150 "part 2" (fig.2). Even if you have operated  your plant for last 50 years your neighbour may come, spend just 50 minutes & complaint regarding vibrations. You- the forging company will unfortunately need to solve the problem & satisfy. This problem is specific especially for small & medium size plants, that are in abundance in India. Irrespective of size, however the  disturbances in one's own machining centre, structure & human resource component is valid for all size of forging companies.

Vibrations are transmitted on each impact as a shock into surroundings. Excited frequencies vary from 5 Hz to 50 Hz. In normal soils they will be in the range of 15 – 25 Hz, but may be in soft soils as low as 5-10 Hz while anything over 25 Hz moves towards rocky.

Vibrations are typically measured in so called velocities and not accelerations or amplitudes, as human perception, as well as stresses in structures caused by vibration are proportional to those velocities.

Distance has obviously an influence on velocity levels, which decrease typically in a homogenous half space proportional to distance, so double the distance means half the vibration level. Only in some few cases when the vibration energy stays in a soil layer vibration levels decrease less, only with the square root of the distance which may become an additional problem if it happens.

Energy may, however, not only be distributed by distance, but may be consumed additionally by dampening in the soil where this is more common in soft soils than in rock.

The minimal mass of conventional hammer foundations, if properly designed according to the German VDI Guide Line 4025, should  be

W = 75 R (h/ho) = 75 R (v/vo)²

R = ram weight(t)  h = drop height(m)    v = impact velocity (m/s), with ho = 1.6m and vo = 5.6m/s  [Reference taken at the time VDI 4025 was being developed]

A rubber sheet or wood below the anvil (fig.3) will only protect the concrete surface below, but will not provide any measurable reduction of vibration levels in the surrounding as will not a special sand bed below the foundation.

The same is in general valid for presses where the design of the foundation may be a little bit more complicated because of press requirements, but as for the hammers, the size of the foundation itself will also have no influence on vibration levels.

Fig.3

The solution to solve the vibration, is by using a much more flexible support of the equipment or its foundation. The flexibility provided by those stone age technologies as mentioned above, rubber sheet or stack of wood below the anvil (fig.3) or sand or cork pads below the foundation, is – as mentioned above - extremely limited and may have even a very low life. After a few years or in some cases even few months, all these systems may become redundant & may not work at all anymore. The company doesn't even realize this, as earlier too it was really not working to a level that could be felt by humans!

Vibration control is directly related to the flexibility of the support system. Now how do you provide this flexibility & isolate the system from the rest of the world?

There are good, but impracticable ways, as for example, the best one - Put the hammer hanging on balloons!!

OR the practicable way - Use Springs

There are 4 types of elastic elements which are typically used for hammers and presses. One is Air mounts, while the other three are, DISC, LEAF or COIL springs made of steel.

With all of them you will get isolation, but which one is practical & popular?

Air mounts – the Cost & maintenance will become exorbitant, and load capacities are low. They are, therefore, not popular in this industry at all.

Leaf springs – They have good bearing capacities, but are flexible only in vertical direction, and can, therefore, only be used for hammers to isolate just the vertical vibrations and not for presses, as those excite vibrations  in all directions & quite substantial in horizontal. Leaf springs provide friction damping, but that means wear is very high, hence frequent maintenance & replacements. For poor performance & frequent maintenance, even truck manufacturers have started parting ways with them.

Disc springs – they have a quite high bearing capacity, but they too are flexible only in vertical direction. High local stresses lead to a very fast failure when undergoing high dynamic loads, especially when combined with horizontal ones. They can, therefore, not be used under hammers, and under presses only if their dynamic load is low and excitation only in vertical direction.

Visco-damper combination under an emergency diesel generator system

Coil Springs – They are flexible in all directions, so isolate horizontal & vertical vibrations effectively. But, they have no material damping, important to control machine motion, so they are always used with added heavy duty Viscodampers® -patented by GERB. These dampers provide necessary damping to ensure hammers come to stand still before next stroke. The life of this system is higher than that of any of the above ones & needs lesser maintenance.

In both cases, hammers and forging presses, the flexible support system was originally placed below their foundations (fig.4), where spring supported hammer foundation blocks may be smaller than conventional ones. Their size is governed not by mass, but by acceptable response amplitudes of the block with respect to permissible amplitudes for springs and dampers.

Fig.4

Fig.5

In the meantime spring systems are also used as a so called direct spring support (fig.5) where forging presses may need an additional steel frame as an interface between press and the spring – damper system (fig.6) to limit rocking motion after each stroke.

Fig.6

Hammer motion happens only in vertical direction and is limited only, as far as operation is concerned, when forging from a bar or with a pair of tongues, otherwise by the dynamic bearing capacity of springs and dampers.

Press motion happens mainly in horizontal direction caused by rocking of the press and should be limited, especially when forging with robots, on workpiece level to approximately +/- 2.5 mm. Attention has to be given here to the rocking natural frequency of the system which should be about 2.5 times higher than press speed to avoid resonance effects. This requirement governs the length of the above mentioned steel frame below the press.

Use of such highly flexible system leads to many advantages – major being vibration reduction & secondly foundation size reduction. In case of direct spring support, the foundation becomes a pit only (fig.7), the design and engineering of which is limited to a static approach only because of the high vibration isolation effect of the springs and dampers. Under normal circumstances, the thickness of their base mat gets limited to 500mm to 1m. Rubber sheets or a stack of wood below the anvil is no more required, while cork pads or a sand bed are efficiently replaced by the springs.

Fig.7

The vibration control effect is now dependent only on 2 things – 1) Soil frequency & 2) Natural frequency of the spring supported system. The first one can be measured, the second one calculated. So you come out from the world of unknown to known and in most cases vibrations are much below the acceptable range of levels, that of course depends on hammer size, local soils and neighborhood distance.

A graph below (fig.8) shows the typical vibration velocity reduction for forging hammers versus hammer capacity when using a standard spring system. With this graph it is possible to forecast the vibration levels at any place in the surrounding. If they are still too high an even more flexible spring system will do the job.

Fig.8

GERB is a 100 year old company that  put the first small hammer on direct spring support in the year 1921. Another early one is shown here on a drawing (Fig.11) with its pit, block and spring Viscodamper® system. This no more works professionally, but a small time museum exists here in a small shop in Berlin where some open die forging is shown still today (Fig.9,10,12).

Fig.9

Fig.10

Fig.11

Fig.12

GERB India was founded as a subsidiary of GERB Germany in year 1992 and provides all services and hardware indigenously while the support of GERB Germany is always available. More than 70 hammers and about 10 forging presses have already been installed by GERB India and if problems have to be solved - we are there.

Conclusion

Vibration control is one of the most important but neglected part of Indian forging industry. Latest technology of using high life, high efficiency flexible support system exists in India & as proven worldwide, the same is already being used by most Indian companies. Use of such system, not just reduces vibration transmission & protects humans, structures & sensitive machines, but also improves the machine life. Further, it's an insurance against foundation settlement / cracking, vibration pollution & your neighbour will be happy, who will no more force you to close or shift your factory.