Throwing out the rattle

Researchers at Purdue University have created a model that simulates vibrations in a car headrest which could one day help to eliminate annoying squeaks and rattles in vehicles.

According to Douglas Adams, an associate professor of mechanical engineering, the headrest and its seemingly simple adjusting mechanism have proved surprisingly complex. He and doctoral student Janette Jaques have applied mathematical models that simulate rattling headrests for analyses aimed at reducing vibration and enabling designs that eliminate the annoyance.

The same modelling and experimental techniques developed for the research could be used to reduce squeaking and rattling in other components, such as instrument panels, seats, transmission gears, suspension components and seatbelt mechanisms.

A new research paper describes rattling in the headrest, which is held in place by a small pin that fits into slots in one of the two posts connecting the headrest to the seat.

‘It's a surprisingly complicated little system,’ Adams said. ‘The mechanism has to be rigid enough to keep the headrest from falling down, but not so rigid that you can't easily adjust it. In other words, you have to put some mechanical free play into it, but you can't put too much because then it rattles.’

The model contains four equations corresponding to four key structural elements in the headrest system, making it possible to simulate headrest vibration. The engineers have tested their model by comparing its simulations with data recorded by shaking a car seat with hydraulic equipment. As the seat is shaken, sensors attached to various points on the headrest record vibration data.

The researchers used the model to identify how susceptible a certain design would be to rattle. The hydraulic shaker recreates the precise frequencies at which the suspension and seats vibrate, making it possible to analyse how the vibration causes the headrest to rattle.

Determining precisely which vibration frequencies are causing the headrest to rattle could enable car makers to better ‘tune’ suspension systems, altering the stiffness of shock absorbers and coils to reduce the rattling.