Force multiplication of PS 1

[Warpspeed]

Will the force multiplication of hydraulic power steering (feel) be directly influenced by hydraulic line pressure ?

Or is it a design function of the relative areas in spool valve and rack piston ?

I realise that there needs to be sufficient pump pressure to generate enough basic rack force, but above that, does increased line pressure effect overall force multiplication ?

 

[Tmoose]

I think that GM used to tune the feel of power steering (Trans AM vs passenger car) by varying the diameter of a small shaft or stem in the steering column.

 

[shanba]

I think the spool valve and shaft tune the on-center feel and gain of the system. The line pressure will affect the effort of the whole system, and the spool valve and shaft would be re-tuned to get on-center feel back.

 

[GregLocock]

Sorry, I haven't got a diagram handy. Between the steering column and the rack there is a torsion bar, limited to say 4 degrees of twist. When it is fully open, you get full pressure from the pump on the rack. The max operating pressure of the pump is fixed by a blow-off valve.

At less than 4 degrees then you get a reduced level of assistance from the pump. The shape of the curve of boost vs bar angle is critical, as is the stiffness of the bar itself, and the maximum bar angle.

The relationship between those and other parameters is extremely complex, you may find some useful stuff if you look around for literature by Bishop.

Further complexity is added when you have variable ratio and speed dependent systems.

If you increase the blow-off valve setting then you will get a reduction in steering wheel efforts, and a subjective mismatch between the torsion bar's stiffness and that of the hydraulic circuit. You'd get a similar effect by fitting a larger rack (ie piston).

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Warpspeed]

Thank you for the very detailed description, it is very much appreciated. Hydraulic servo systems can be tricky things and I did not realise there was a torsion bar involved, that explains rather a lot.