Valve springs success- ful in AUDI diesel engine - Scherdel GmbH

RESEARCH
Potential of modern spring materials for spring components
subjected to dyna mic stress in the valve train
New engine designs require springs with improved characteristics and enhanced load-bearing capacity
- Choice of material is crucial - paper presented at the conference of the Association of German Engineers (part 1)
TECHNOLOGY & PRODUCT DEVELOPMENT - PROCESS OPTIMIZATION - NEWS AND INFORMATION
Authors:
Dr.-Ing. Hilmar Brödler,
Dipl.-Ing. Wolfgang Stark,
Jürgen Hartmann
SCHERDEL GmbH,
Marktredwitz
Paper presented by Dr.
Hilmar Brödler at the 2nd
conference of the Association
of German Engineers,
"Valve train and cylinder
head", on 25./26.10.2006
in Leonberg
Diagram 1: Summary of valvespring
materials.
M
odern valve-train designs and
increasingly variable factors in
the cylinder head increase the working
load on the various spring elements,
which have to be adapted to meet future
requirements – among other things
and in particular by the use of higherstrength
materials. New materials,
processes and designs are available
and are being used to an increasing extent.
Introduction
The increasingly stringent requirements
relating to the emission and fuel
consumption values of internal combustion
engines, which are associated with
a large number of individual measures,
in particular minimization of mass and
the reduction of power loss due to friction,
as well as the greater demands
made on the dynamics of vehicle movement,
mean an increase in the requirements
relating to the different spring elements
in the valve train. The new
down-sizing concepts – often with supercharged
engines and, in some cases,
combined with high-speed concepts
– call for a valve spring with good dynamic
characteristics and, as a rule, of
the same dimensions or, as the case
may be, even smaller. Rising engine
temperatures due to direct injection
systems, increased metal-cladding of
the engine and various other influential
factors additionally increase the temperatures
which the components have
to withstand. Exhaust-gas soot filters in
diesel engines or additional filters in the
exhaust tract of petrol engines raise the
exhaust counterpressure at the exhaust
valve and necessitate a greater closing
pressure while the dimensions remain
unchanged.
These examples demonstrate how the
various types of spring in the valve train
– in particular the valve springs, springs
for switchable bucket tappets, compression
springs for hydraulic valve-play
compensation, torsion springs and spiral
springs for variable valve-train elements,
as well as springs for camshaft
adjusters and others – are subjected to
ever-increasing stress, while the performance
of the components must be continuously
improved and, in particular,
their weight and dimensions steadily reduced.
In addition to achieving an optimum
component design using modern design
tools, for years now particular attention
has been paid to the materials
used and the ways in which they are
processed, in order to meet the demands
made on the spring elements.
The following is intended to provide a
general idea of the materials currently
in use or in the process of development,
as well as their manufacturers
and manufacturing processes. Reference
is also made to the design requirements
and to the handling of these
materials, as well as to the techniques
and processes used.
Diagram 2a: Relative increases in the price of
valve-spring wire.
Materials
rod-wire manufacturers
Taking the valve-spring as an example,
the following describes the most
commonly used materials. Diagram 1
provides some reference values for
the three material qualities VD SiCr,
VD SiCrV and VD SiCrNiV, given that
the mechanical-technological parameters
refer to wire with a diameter of
3.0 mm.
The three materials SiCr, SiCrV and
SiCrNiV are the valve-spring materials
most frequently used today. These materials,
which are readily available from
various manufacturers, are characterized
by the fact that they are produced
using the Superclean process, i.e. deoxidizing
with silicon, and subjected to
elaborate testing procedures during billet
treatment, rolling and the various
wire-drawing stages. Particularly for
spring elements subjected to a high level
of dynamic stress, these three materials
provide an excellent surface quality
and a very low inclusion rate. The
Superclean process ensures that the
composition of inclusions in the threematerial
system SiO2-CaO-Al2O3 is
regulated in such a way that they are
ductile and therefore produce only a
very slight notch effect in the metal
lattice. In addition, there are specifications
relating to surface-crack detection
to a permissible depth of 40 µm and
relating to the defined depth of partial
decarburization.
The three materials mentioned have
the following characteristics:
SiCr - High-strength standard material
with good ductility and very good crackarresting
ability
SiCrV - Higher-strength material with a
finer structure
SiCrNiV - Super high-strength material
with very good temperature resistance,
particularly suitable for nitration.
In addition to the technical characteristics
of these materials, an important
factor is their cost. The following summary
provides a comparison of the cost
of the individual qualities of material.
The estimated proportion of the use of
these materials is also indicated.
(To be continued) (Dr. Hilmar Brödler)
wire-drawers
Diagram 2b: Estimate of the proportion
of the use of the different
valve-spring qualities in relation to
the total.
Supplier market
Today there is only a limited number
of suppliers who manufacture
these sophisticated materials. Here
we must distinguish between the
rod-wire manufacturers and those
involved in the next stage, the
wire-drawers. Diagram 3 illustrates
the current global situation in relation
to the supply chains – rod-wire
manufacturers / wire-drawers – for
valve-spring wires.
(as of: 10/2006)
TECHNOLOGY & PRODUCT DEVELOPMENT - PROCESS OPTIMIZATION - NEWS AND INFORMATION
6
1)
in the case of nitrated springs a
reduction in setting loss is possible.
7