What Is A Load Spectrum?

When engineers talk about the ‘Load Spectrum’ what do they mean?

There is no simple answer, simple terms like load and spectrum can be used in different situations and therefore to mean different things. However the most common definition of load spectrum is as follows…

Step 1

Using a strain gauge instrument the power output side of the engine or generator.

Step 2

Using a strain gauge instrument the output side of a gearbox, power train or toothed wheel mesh.

Wireless telemetry will be required.

Step 3

Run the item under test to find the strain levels in the instrumented shafts.

Step 4

Convert the strain values to torque using the following in Prosig DATS,

Torque = \dfrac{Strain * \pi * ({D_o}^4 - {D_i}^4) * E}{16000 * D_o * (1 + v)}


D_o = shaft diameter outside

D_i = shaft diameter inside (adjust to 0 if not required)

E = Modulus of elasticity

v = Poisson ratio

Step 5

With the torque data over time, perform a peak and trough detection to find the turning points in the data. This is known as rainflow counting. The output of this calculation is called the torque count statistics.

Some engineers stop at this point and define the rainflow data as the load spectrum, however it is not.

Step 6

Using the rainflow data is it then possible to calculate the histogram. This histogram is the load spectrum.

This load spectrum is very import during the design phase or a refinement phase.

The information from the load spectrum can be used with test rigs or simulation software to reduce, but not remove, the need for field tests.

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James Wren

Application Engineer & Sales Manager at Prosig
James Wren is an Application Engineer and the Sales Manager for Prosig Limited. James graduated from Portsmouth University in 2001, with a Masters degree in Electronic Engineering. He is a Chartered Engineer and a registered Eur Ing. He has been involved with motorsport from a very early age with special interest in data acquisition. James is a founder member of the Dalmeny Racing team.

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