Vibration Effects



The Human's Tolerance of Vibration
Vibration can be harmful. If you expose yourself to enough of it you will feel tired more quickly. If you expose yourself to even higher levels you face the possibility of spine damage in addition to stomach and liver disorders.

In an attempt to improve the working environment, the International Organisation for Standardisation has defined the effects of various levels of vibration in its Standard ISO 2631. Vibrations in various directions are considered, but let us first focus on vertical vibration. The Standard defines "Fatigue Limits", corresponding to a particular amount and type of vibration. (quoted in "hours of exposure"). It is apparent from Fig. 1 that the human is least tolerant of vertical vibration in the frequency range between 4 and 8 cycles per second (indicated by the Fatigue Limit Line being at its lowest level between these two frequencies). The Standard also goes a stage further and defines two other exposure limits for guidance (see Fig. 2). Exposure beyond the "Safe Exposure Limit" involves definite risk to health, whilst exposure to vibration levels at or below the "Reduced Comfort Boundary" enables travel occupations such as eating, reading and writing to be undertaken without disturbance.
Figure 1	Figure 2
Consider now the fore and aft and lateral directions of vibration, the information presented by the Standard is equally concise. However, summarising the information from Fig. 3, it is apparent that the major difference is that the human is most sensitive to horizontal vibrations between 1 and 2 cycles a second. This then is the human response to vibration and it is important to consider it when evaluating seating for use in vehicles.	Figure 3

The Vibration Environment in a Work Vehicle
Different work vehicles expose the driver to different types and different levels of vibration. In addition, any vehicle's vibration pattern is affected by the terrain over which it passes. The tendency of each of the many spring/mass systems within the vehicle is to vibrate at, or near to, their particular resonant frequency, the level of this vibration being dependent upon the input to the vehicle at the frequency in question. The result? The driver's seat is subjected to a mixture of resonant vibration superimposed on top of a random input vibration background. In the case of an unladen truck, the vertical vibration pattern on the cab floor has maximum values which correspond to the resonance of the vehicle on its main ride springs, the cab system on its mounting system, the engine on it's rubber mounts, the unsprung mass (wheels, brakes, axles etc) on the tyres, and so on. Each resonance will be present to a greater or lesser extent dependent upon the nature of the road input and the interaction between one resonant system and another. A typical result is shown in Fig. 4 below. The same vehicle in laden form (Fig. 5) gives a much better ride because the main vehicle springing is operating nearer to its design load, and also the forces generated by the "secondary" resonant vibrations have less effect because they are acting on a much larger mass.
Figure 4	Figure 5
Considering now the fore and aft mode, the input to the seat mounting in an articulated truck can be equally severe, and cannot be ignored. Fig. 6 shows a typical fore and aft vibration spectrum for an unladen truck. In many respects the vertical pattern of an agricultural tractor is more difficult to generalise, despite the simpler layout of the basic vehicle. The principal spring medium is the tyre system and this results in a resonant frequency centred around 3 cycles per second (see Fig. 7). Secondary resonances are generally fewer and normally occur at frequencies above the range under consideration. However, the addition of a trailer, or a plough can dramatically alter the vibration pattern. A further variation results from speed changes - the faster the tractor goes, the more severe the resultant vibration input to the seat.
Figure 6	Figure 7
Other work vehicles also have their own vibration signatures, but this basically hostile vibration environment is always present to a greater or lesser extent.

The Solution for Vertical Vibration
Put a human into a typical work vehicle vibration environment and you have a recipe for the tiredness and physical problems already mentioned. A traditional static seat will begin to isolate you from harmful vertical vibration at a frequency of 5 cycles a second if the input is sufficiently high, but more normally you feel at least the full effect of the cab floor vibration up to 8 or 9 cycles a second. The reason for this is the need to achieve a compromise between acceptable comfort without excessive cushion deflection. The resonant frequency of the static seat is, therefore, inevitably 3 to 4 cycles a second (see Fig. 8 below). But this is with a fairly high vibration input. Subject the static seat and occupant to a lower level of input and the resonant frequency will be higher - 5 or 6 cycles a second. All this means that in the frequency range where the human is least tolerant of vibration in the vertical direction, the static seat occupant is being exposed to vibration levels that are higher than those actually being fed into the seat from the vehicle structure.
Figure 8
How does a suspension seat differ? It is built on completely different principles. The cushion is designed to resonate less than a static seat so that whilst it doesn't provide the occupant with much isolation from vibration, neither does it amplify vibration at its resonant frequency. The comfort comes from an entirely separate suspension system beneath the seat. Just like your car suspension, it has low rate springs and a damper that give ride qualities you would expect from a comfortable car. The occupant is isolated from vibration at a lower frequency than with a static seat - typically from the frequencies above 2 cycles a second. The resonant frequency of the driver on the seat is as low as 1 cycle a second and because of carefully chosen level of damping, there is only a small amount of amplification even at resonance (Fig. 8). The result? If your work vehicle has a primary resonant frequency of 3 cycles a second, you would feel nearly 250% of the level of vibration of the floor of the vehicle whilst sitting on a static seat, but only 60% if you sat on a suspension seat - that means removing 75% of harmful vibration! You can spend at least double the amount of time at the controls of the vehicle before you reach the same level of fatigue. The benefits are obvious.
Visit the KAB Institute for a program on Whole Body Vibration.