User manual

Appendix F: Mechanical Dynamic Loading

Approximate Solution for Elastomer Spring Selection

8580/8590 Vehicle-Mount Computer User Manual F-5

To simplify matters, of the 6 possible degrees of freedom, we will only consider those with

the greatest deflection in the case of the 8580. In other words: We observe the display as it

oscillates towards or away from us (a combination of rotational and longitudinal

oscillation).

Comparative measurements for the precise arrangement displayed in Table-top attachment

with elastomer springs diagram on the previous page:

Attachment (construction of the mounting bracket, quantity and position of the elastomer

springs) show that the individual spring must be stiffer by a factor of 25 for the mathemati-

cal model stated above to be applied.

As a result, this model gives a value of 26 N/mm x 22.5 = 585 N/mm for the required single

spring constant.

The next step is to look through the manufacturer’s data sheets (such as those from gmt-

gmbh.de or simrit.de) to find the right types of elastomer springs and rubber buffers.

Here we have decided to use springs with an M8 thread and cylindrical body made of

natural rubber (NR). Based on the data sheet for a diameter of 30 mm and a height of 20

mm, for example, we arrived at the pressure load:

Compressive force 539 N / Displacement 1 mm = 539 N/mm for a Shore hardness A 70.

This value lies below the default value. What is the natural frequency?

The following formula can be used to calculate the natural frequency:

Important: Factors for other mountings with elastomer springs must be calculated

through testing!

Where:

= natural frequency in Hz

c = total spring constant = 539 N/mm (calculated from datasheet

values)

* 3 (springs) / 22.5 (factor) = 71.9 N/mm

m = oscillatory mass = 5 kg

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