Note: Descriptions are shown in the official language in which they were submitted.
78~
The present invention relates to trucks for rail
vehicles and in particular to apparatus for improving the
stability of such trucks.
It is of course well known to provide a rail vehicle
with a pair of trucks located at opposite ends of the vehic]e to
support the body of the vehicle. Conventionally these trucks
are provided with a pair of axles and are pivoted to the body of
the vehicle to permit the trucks to negotiate a curve. A
commonly used truck assembl~ includes a pair of longitudinal
side frames with a pair of axle assemblies extending between the
side frames at opposite ends. The axle assemblies are
supported so they may rotate about a horizontal axis to allow
the truck to roll along rails. The side frames are
interconnected by a bolster that is mounted to the side frame
through a set of springs to accommodate vertical and to a
certain extent lateral loads. The bolster is pivotally
connected to the vehicle to provide the connection between the
vehicle and the truck. The bolster may be displaced vertically
relative to the frames in slides so that there is freedom to
move vertically but not longitudinally. This permits the
transmission of longitudinal forces between the bolster and the
side frames. The connection of the bolster to the side frames
permits each side frame to pivot relative to the boLster about a
horizontal axis and as such allows the wheels to move vertically
with respect to one another. This allows the truck to travel
1 l 9~B~7
over track which is uneven and maintains a good load
dist~ibution between the four wheeLs of the t~uck~
This arsangement of truck provides a very stiff constraint
against any out of phase yaw displacemerlts of the wheelsets
(that is, it maintains the wheelsets parallel to one another).
However, the arrangement offers very little restraint to inphase
yaw displacement in which the wheel sets remain parallel to one
another but not perpendicular to the side frames. This inphase
yaw displacement is commonly known as lozenging and results in
two undesirable characteristics. Firstly, an unstable condition
known as hunting can occur in which the yaw displacements occur
on a continuous occillatory manner excited by the action of the
wheels agai~st the rails. Such a motion promotes high wheel and
rail wear, causes high shock levels to be transmitted to rails
and the vehicle body and can, in extreme cases, lead to
derailment of the train.
The second action occurs on curvesO ~hen the vehicle
travels on curves of sufficiently small radius to cause the
leading wheelset to come into flange contact with the outer rail
the wheelset experiences a yaw torque which turns it toward
the outer rail. This creates a very high angle of attack of the
leading axle with the ~ail and it is well known that such high
angles of attack result in high levels of wear and noise as well
as creating high force levels and the possibility of derailment.
One solution to such lozenging has been the proposal to
1~9~7~3~
use trucks having a rigid H frame. In this type of construction
the bolster and side frames are integrally formed so that
relative lonyitudinal displacement between the side frames
cannot occur. Such frames tend to be extremely rigid so that
their ability to accommodate vertical movement between the axles
is not very good, and surprisingly have a relatively low
critical velocity that is the velocity,at which instability
occurs.
It has also been suggested to use braces extending
diagonally between the side frame and rivoted to the side frames
at spaced locations. This construction is also inherently rigid
and therefore has the disadvantages of low stability associated
with the H f,rame truck.
Proposals have also been made to use diagonal braces
between the journal boxes of the wheel sets. However, this
arrangement becomes complicated by the movement of the journal
boxes relative to the side frames and increases the unsprung
weight of the vehicle. Further, the arrangement can only
conveniently be used on trucks in which the frames are located
in-board of the wheels as the diagonal struts tend to interfere
with the wheels when the side frames are in the conventional
outboard position. This complicates the structure used to brace
the axles and further increases the unsprung weight.
It is therefore an object of the present invention to
provide a truck in which the above disadvantages are obviated or
'7~'7
mitigated.
According therefore to the present invention there is
provided a truck comprising a truck frame including a pair of
laterally spaced side frames, a pair of wheelsets extendirlg
between said side frames at opposite ends thereof and supported
for rotation about a horizontal transverse axis, first resilient
means interposed between said wheelsets and respective ones of
said side frames to provide flexibility between said frame and
said wheelset to permit controlled movement of said wheelsets
from a mutually parallel position and thereby determine the yaw
stiffness of said truck, and bracing means extending between
said side frames to oppose relative longitudinal movement
therebetween, said bracing means comprising a pair of struts
inclined to the longitudinal axis of said truck and second
resilient means associated with said struts and deformable upon .
relative longitudinal movement between said frames, said first
resilient means and bracing means providing an increased lateral
stiffness to provide a controlled flexibilty in shear to said
truck to improve the stability thereof.
According also to the present invention there is
provi~ed a truck comprising a frame having a pair of laterally
spaced side frames, a pair of wheelsets extending between said
side frames at opposite ends thereof to support said side frames
and having ~irst resilient means interposed between said
wheelset and each side frame to determine the yaw stiffness of
said truck, bracing means extending between said side frames and
second resilient means connected between said side frames and
said bracing means, said first and second resilient means
providing a lateral stiffness for said truck, said stiffnesses
4 -
8'7
being selected such that an increase in at least one of said
stiffnesses results in a lower critical velocity of said truck.
According to a further aspect of the present invention
there is provided for use in converting a truck having a pair of
side frames with transversely extending axle assemblies
supported at either end, a stabiliser kit comprising first
resilient means to be interposed between said side frames and
said wheelsets to provide increased flexibility between said
frame and said wheelset in a direction to permit controlled
movement of said wheelsets from a mutually parallel position and
thereby decrease the yaw stiffness of said truck, bracing means
to extend between said side frames and to be resiliently secured
thereto, said bracing means including a pair of struts and
second resilient means to be interposed between opposite ends of
said struts and said side frames, said struts and second
resilien~ means being selected to provide an increase in lateral
stiffness, said changes in stiffness being selected to increase
the critical velocity of said truck.
According to a yet further aspect of the present
lnvention there is provided a method of increasing the stability
of a truck having a pair of laterally spaced side frames with a
pair of axle assemblies at opposite ends thereof, said method
comprising the steps of selecting values of yaw stiffness and
lateral stiffness lying on a characteristic curve of maximum
critical velocity on a plot of yaw stiffness versus lateral
stiffness locating first resilient means between said axle
assemblies and said side frames to the yaw stiffness of said
~ - ~a -
;
7~
truck and connecting brace means including second resilient
means between said side frames to provide said selected value of
lateral stiffness for said truck.
According to a yet further aspect of the present
invention there is provided a method a method of increasing the
stability of an existing truck having a pair of laterally spaced
side frames with a pair of axle assemblies at opposite ends
thereof, said method comprising the steps of determining the
existing values of yaw stiffness and lateral stiffness,
selecting an increased value of at least the lateral stiffness
of said truck, locating elastomeric members between said axle
assemblies and said side frames to determine the yaw stiffness
of said truck, and connecting brace means including resilient
means between said side frames to provide said increased value
of lateral stiffness.
According to a yet further aspect of the present
invention there is provided a method a truck comprising a pair
of longitudinally spaced transversely extending wheelsets each
having a pair of wheels with an axle extending ~herebetween, a
truck frame including a pair of laterall~ spaced side frames
each of which has an axle receiving member at opposite ends
thereof, each of said receiving members being located on a
respective one of said axles to maintain said wheelsets in
spaced parallel relationship, first elastomeric means interposed
between each receiving member and said axle to permit controlled
movement of said axles from a mutually parallel position and
thereby determine the yaw stiffness of said truck, brace means
,~ 4b -
787
extending between said side frames and resilient coupling means
connecting said brace means to said side frames, said
elastomeric means and said coupling means providing an increased
lateral stiffness for said truck to provide a controlled
flexibility in shear and increase the stability thereof.
According to a still further aspect of the present
invention there is provided a method of a method of improving
the stability of a truck having a frame including a pair of
longitudinal side frames supported at opposite ends on a pair of
transverse wheelsets comprising the steps of inserting resilient
means between said wheelsets and respective ones of side frames
to reduce the resistance to out of phase yaw displacement
between said wheelsets and locating brace means between said
side frames to increase the resistance to a force tending to
displace laterally one of the wheelsets relative to the other.
An embodiment of the invention will now be described by
way of example only with reference to the accompanying drawings
in which,
Figure 1 is a side view of a truck,
Figure 2 is an underview of the truck shown in Figure 1,
Figure 3 is a detail of a portion of the truck shown in
Figure 2,
Figure 4 is a series of curves showing the relationship
between yaw stiffness and lateral stiffness for a given truck.
Referring now to the drawings, a truck 20 includes a
pair of longitudinal side frames 22 - 24 supporting a pair of
wheelsets 26 - 28. Each wheelset includes a pair of flanged
wheels 30 secured to an axle 32, the ends of which are supported
- 4c -
7~
in journal boxes 34. The journal boxes 34 are secured in yokes
36 formed at each end of the side frames 22 - 24 so that the
axles may rotate about a generally horizontal axis relative to
the side frames 22 - 24. An elastomeric pad 37 is positioned
between each journal box 3~ and yoke 36 to provide a primary
suspension for the axle assembly and to permit limited
controlled movement of the wheelsets out of parallel.
A bolster 38 extends between the side frames 22 - 24
and passes theough an aperture 40 formed in the central portion
of each side frame. The ends of the bolster 38 are supported on
a spring assembly 42 to permit vertical movement between the
bolster 38 and the side frames 22 - 24 and bears against slides
44 connected to the vertical edges of the aperture 40 so that
the bolster may move vertically but not longitudinally relative
to the side frame.
A pair of webs 48 are welded to the side frames 22 - 24
between the yokes 36. The webs 48 are e~ually spaced from the
centre line of the truck and are inclined relative to the
longitudinal axis of the truck. A pair of struts 50 extend
between diagonally opposite webs 48 so that the struts intersect
on the centre line of the truck. The struts are received in the
~ebs 48 in the manner best shown in figure 3. Each strut 50 has
a reduced portion 52 at each end that terminates in a thread
54. A hole 56 is formed in the flange 48 o~ a diameter greater
than the diameter of the reduced portion 52. A pair of
27~7
elastomeric inserts 58 is located between the reduced portion 52
and the wall of the hole 56 ~nd each has a radially directed
shoulder 60 that bears against the face of the web ~B. Washers
62 are mounted on the reduced portion 52 to engage the outer
faces oE the elastomeric inserts 58 and a nut 64 attached to the
thread 54 to compress the shoul.ders 60 of the inserts between
the washers ~2 and the outer faces of the web 48. The inserts
58 there~ore provide an elastic connection between the struts S0
and the web 48 and provide a controlled flexibility bet~een the
side frames of the truck . The struts 50 are attached to one
another at their point of intersection by means of a shackle 66
to inhibit vertical vibration o the struts S0.
In,operation, the struts 50 are effective to oppose
lozenging of the side frames, that is relative longitudinal
movement between the side frames but by virtue of the elastic
connection and their flexibility in bending do not introduce
undue rigidity which would inhibit verkical displacement between
the wheelsets 26. The elastomeric inserts 58 are chosen to
provide the desired degree of shear flexibility in the truck for
dynamically stable configuration
he elastomeric pads 37 between
~e axle and side frames assist in providin~ ~e optinum value of such fle~ibility
hut the prim~ contribution is from the inserts 58.
Figure 4 shows a typical family o~ curves for a given
truck that illustrates the relationship between the truck
stiffness and the critical velocity which is the velocity at
-- 6 --
~ ~J~7 ~ ~
which a tcuck experiences instability. The para~eter KB is
defined as the resistance to out of phase yaw displacement
between the axles, that is the resistance offered to a couple
tending to move the wheelsets out of a parallel condition. The
stifness Ks is the stiffness offered to a force tending to
laterally displace one of the wheelsets 26 relative to the
other. The relationship between KB and Ks is determined by
the dimensions of the truck, the disposition of the struts 50
and ~he rP~ nce of the elas~ric inserts 58 and pads 37. Ihe curves
indicated Vl, V2, V3 etc., represent characteristic critical
velocities at which a truck having a yaw stiffness KBn and a
lateral stifness KSn that lie on the characteristic curve
Vn will have a critical veLocity Vn. Beyond that velocity
the truck becomes inherently unstable. It will be observed that
the~e is a curve Vc MAX that interconnects the saddle points
of the characteristic curves and represents the maximurn velocity
of the truck before instability occurs. Therefore, by selecting
values of Ks and ~ that lies on the curve Vc MAX~ the
critical velocity of the truck may be ~ maximum. The curves are
subtended by a line ha~ing a value Ks = l/a2 RB where "a"
is haLf the distance between the axles 26, 28. For all
practical considerations it can be shown that Ks ~ l/a2 KB
for a truck in which the wheelsets are not directly coupled. The
configuration of thê struts 50 and the stiffness of the inserts
58 and pads 37 may ~fore be ch~ bo ensure that the
-- 7
7~'~
maximum critical velocity is obtained.
The characteristic curves beyond the curve Vc MAX
each have a critical velocity lower than Vc MAX~ Therefore a
rigid truck such as that utilisi,ng an H frame or employing
rivetted cr,oss,,braces would,haye a low critic~l velocit,~ which
explains the surprîsing instability of such trucks. Similarly,
a truck having a high degree o flexibility, such as a
conventional 3 piece truck assembly, would also have a low
critical velocity. However~ by introducing the elastic bushes
to provide controlled flexibility in shear~ a truck with the
desired critical velocity may be obtained.
In tests performed by the applicant a Barber Type S2
truck was mQdified by utilising the structure shown in the
drawings. The struts 50 were inclined at an angle of 66 to
the longitudinal axis of the ~n~ and the inserts 58 ~ ~ku~d from
an elastomeric material having a hardness of 70 durometer. The
inserts had an outside diameter of 3 inches so that an annulus
of material of area 6 s~uare inches and l inch thick was
positioned between the washers 62 and the flange 4~. The length
of the struts between the washers 62 was 82.5 inches and the
struts were made from low carbon steel with an outside diameter
2 inches. ~ith this arrangement the stiffness Ks
increased from a value of 2xlO N/M to 2.10
N/~. The critical velocity of the truck was calculated to be
increased from 31 m.p.h. to 74 m.p.h. (neglecting the efects of
-- 8 --
friction damping)
It will be apparent that the structure disclosed is
particulartly adaptable to retrofitting to existing trucks in
order to increase their critical velocity. Such a retrofit rnay
be accomplished with the simple addition of the webs 48 to the
truck frame or the utilisation of existiny holes in the ~ide
frames of the truck i-f such are available~ The stiffness
imparted to the truck may be varied by the selection of the
elastomeric material and by the dimensions of the inserts 58.
The stiffness KB is not greatly affected by the retro~itting
of struts 50 and the existing value of KB may therefore limit
the increase in critical velocity that can be achieved by a
simple retrofit to below the value Vc MAX
It will be seen therefore that the disadvantages
associated with the prior art are obviated or mitigated in a
simple, convenient manner. Although in the embodiment described
the struts 50 pass beneath the bolster 38~ they may iE practical
pass through apertures in the bolsterO This allows the struts
to be positioned close to the rotational axis of the wheelset
and minimise the tendency to twist the side frames about their
longitudinal axes~ The provision of the elastomeric blocks 37
also contributes to the improved performance of the truck. The
effect of these blocks is to lower KB and it can be seen from
figure 4 that a reduction in this value leads to a reduction of
critical velocity~ However, a similar plot showing the
7~
characteristic curves of angle of attack would show that a
L eduction inKB reduces the angle of attack and so improves
the curving characteristics of the truck. The pro~ision of the
strut 50 then increases the value of KS without unduly
affecting KB so that an increased critical velocity is
obtained whilst retaining the improved curving characteristics.
-- 10 --
