Abstract
This work presents an analytical model for studying the stick-slip phenomenon. The vibratory
system is schematised, and the friction force in the contact between the piston-rod and the
seal, and the force measured on a test bench, are examined. This enables the time response
equations during the consecutive slip-stick cycles to be solved.
The model explains the response in force obtained: base curve and superimposed oscillatory
force. Moreover, the model enables stick-slip situations in shock-absorbers to be predicted,
calculating the expected frequency and amplitude.
In order to apply the model, some simple tests are needed to determine the type of oil and
seal. Starting from this point, the behaviour of any combination of oil and shock-absorber seal
can be deduced when faced with stick-slip, in any kind of working conditions whatsoever.
On the one hand, a testing programme was conducted to define the type of seal (stiffness,
damping, modal mass and natural frequency) and the oil (friction force compared to sliding
speed), the most suitable method of definition being deduced on a test bench.
On the other hand, these tests permit the model to be validated with combinations of oil and
seal that produce noise and with others that do not, when they are tested with the same shock-
absorber over a wide range of conditions. These tests on a test bench include noise spectra
measurements of the seal-lip.
The proposed model is coherent with the data obtained, which endows it with reliability for
predicting the effects of using different seals, lubricants or additives that modify the friction
curve.
Keywords - Stick-slip, Friction, Shock-absorber, Seal, Piston-rod