RDP Electronics provides all the sensor requirements and analysis
software for a non-destructive load testing rig, built in-house by
vehicle tail lift specialist, Ratcliff
Ratcliff has been building vehicle tail lifts for more than 40 years, and
while nowadays the company uses advanced computer techniques to simulate
the stresses in its designs, it is still bound by law to carry out
physical tests to verify statutory overload capacities. The traditional
method is to stack calibrated weights on the tail lift platform until the
yield point is reached - a time consuming and usually destructive
procedure. Ratcliff engineering director, Alan Barsby was far from
satisfied with this approach, not least because of the potential safety
risk with high payload lifts. Moreover, after the tests, there was no
clear indication of the primary failure mechanism, and comparative tests
of central, distributed and offset loading were impractical.
The company set about building a test rig, a rigid steel cage
incorporating a hydraulic ram capable of exerting forces of up to
125,000N at any point on a platform via a pressure pad matched to the
footprint of the intended load. It was to be capable of testing the full
range of products, from the smallest passenger lift to the biggest column
and cantilever models. Perhaps the most important elements of this test
rig, however, were the sensors - mainly displacement transducers and load
cells - so Mr Barsby prepared to find suitable suppliers. One phrase:
'displacement transducers' typed into an Internet search engine brought
him to the virtual front door of RDP Electronics.
I looked through the web site and found, from one company, everything I
needed. Displacement transducers, load cells, amplifiers and displays.
The most important requirement was the ability to plot a
load/displacement curve in real time. In that way we could view the
approaching yield point without destroying the test sample. So I made a
call. RDP's Fred Thorneycroft and Ken Irvine worked on the technical
requirements and Mr Irvine later visited Ratcliff to offer possible
solutions. In the end, it proved a relatively simple job, according to Mr
Barsby.
N ACT6000 300mm LVDT and E725 ac signal conditioner/indicator were chosen
to measure the deflection caused by the increasing load, and a
53-20,000lb load cell in an articulated knuckle joint with another E725
dc (calibrated to read in tonnes) was fitted to the base of the ram. The
two E725s were connected to a PC via the serial communications port to
provide a stress/strain plot, the software being prepared by Mr Irvine.
The rig works exceedingly well and we are delighted with the
performance, says Mr Barsby. We are able to explore the ultimate
strength limits of prototype lifts without causing damage. Currently, we
are using the equipment to establish the yield point of a heavy-duty
platform with a concentrated load of more than nine tonnes. This would
have been out of the question using dead-weight methods.