Method and apparatus for monitoring mechanical power transmission systems

A method for monitoring mechanical power transmission systems is disclosed, wherein the power transmission system includes at least one power transmitting element, and which is characterized for a reliable and simple measurement of elongations on the power transmitting element by the following steps: First two substantially parallel planes are established on the unstressed power transmitting element, each plane being defined by at least three space coordinates or respectively measuring points on the power transmitting element. Then, the relative change in position of the two planes to one another is determined on the stressed power transmitting element. Finally, the determined relative change in position of the planes is correlated with the local elongation or compression on the power transmitting element. An apparatus for carrying out the method is likewise described.

BRIEF SUMMARY OF THE INVENTION 
The invention relates to a method of monitoring mechanical power 
transmission systems, wherein the power transmission system comprises at 
least one power transmitting element. Furthermore, the invention relates 
to an apparatus for monitoring mechanical power transmission systems, in 
particular for carrying out the method of the present invention. 
Power transmission systems in the meaning of the present invention are 
systems, wherein forces are transferred to a tool or the like, such as, 
for example presses, punches, extrusion press installations, forging 
hammers, injection molding machines, chill casting installations, etc. The 
power transmission systems utilized in the foregoing apparatus all 
comprise at least one power transmitting element which exerts a 
predeterminable force on the tool. Within the scope of this power 
transmission, the power transmitting element is subjected to great 
stresses and thus accordingly to great elongations and/or compressions. 
Bending stresses and/or bending moments which occur simultaneously, are 
normally the cause for the failure of the power transmitting element, when 
the fatigue strength of the material is exceeded. Consequently, it is 
almost absolutely necessary for the purpose of avoiding damage to the 
power transmitting element, to monitor the power transmission system or 
respectively element as regards elongations or compression and stress 
respectively occurring in the course of power transmission, i.e., also as 
regards the occurrence of maximum bending moments. In particular, this 
monitoring is necessary, inasmuch as frequently the tools connected with 
the power transmitting elements are incorrectly adjusted or exhibit 
unintentionally any kind of foreign bodies in their operating range. These 
facts lead, when tool parts impact on one another, to at least a slight 
tilting, which in turn causes a bending of the power transmitting element 
operatively connected with the tool. This bending may easily lead to a 
stress above the prolonged alternating stress strength. 
Known from practice as of now are methods and apparatus for monitoring the 
mechanical power transmission systems under discussion. This will be 
explained below with reference to the example of a pressure die casting 
machine: 
According to DIN (German Industrial Standards) 24480, sheet 2, a pressure 
die casting machine is divided into four subassemblies, namely: a die 
closing unit, a casting unit, an ejecting unit and a core withdrawing 
unit. The die closing unit serves to shift the movable die halves, thereby 
closing the die, keeping it closed, and opening it again. The die closing 
unit which must, among other things, apply the required holding force, 
includes a stationary clamping plate, a movable clamping plate, and 
columns which on the one hand serve to guide the movable clamping plate, 
and on the other hand to absorb the holding force generated by the die 
closing unit. These machine parts which are named columns in pressure die 
casting machines, are power transmitting elements in the meaning of the 
present invention. 
To avoid an overload of the column, which ultimately may lead, according to 
the foregoing description to a breakage of the column or a breakage of the 
power transmitting element, it has in the past been possible to monitor 
the columns in different ways. Thus, for example, the holding force of the 
die closing unit is measured, i.e., the force at which the closing unit 
with a certain tool counteracts the occurrence of a die bursting force. To 
this end, a so-called "cushion" is mounted in known manner between the 
stationary and the movable clamping plate, by which a force is applied 
hydraulically. The force applied via the "cushion" corresponds to the 
holding force of the pressure die casting machine, when it barely holds 
closed. 
Furthermore, it is possible to determine tensile and compressive forces via 
the elongations in the columns, inasmuch as in accordance with the Hooke 
Law, a linear relationship exists in the elastic range between the stress 
occurring in the column and the elongation. To measure this elongation 
under stress, various methods are in turn available in practice. On the 
one hand, the length variation of a column under stress is measured on the 
front end of the column in the so-called neutral axis. Bending stresses in 
the column are not measured in this process. Furthermore, it is possible 
to measure the length variation under stress on a certain segment in the 
region of the die. To this end, it is possible to use dial gauges which 
indicate the movement of the column via blocks and a steel strip clamping. 
The blocks may also be mounted on the columns adhering thereto by means of 
a magnetic tape. 
Finally, a method is known, by which wire-strain gauges are applied to the 
columns. The measuring principle of a wire-strain gauge is based on the 
change in resistance of an electric conductor, when the latter is 
elongated or compressed by the action of force. However, the application 
of wire-strain gauges necessitates a substantial expenditure. When pulling 
the column, the measuring assembly is normally destroyed, which makes this 
method extremely expensive. The measuring results are always dependent on 
the position of the wire-strain gauge, so that for a complete monitoring 
of the power transmitting system or element under discussion, it would be 
necessary to provide same with wire-strain gauges practically over its 
entire surface. However, this is impossible for reasons of both 
practicability and costs. 
The above-described known methods of monitoring mechanical power 
transmission systems either can be carried out only within the scope of a 
"blank determination", or cause on the other hand a considerable 
expenditure of time and costs. 
It is therefore the object of the present invention to indicate a method of 
monitoring mechanical power transmission systems, which enables in a 
simple manner a reliable monitoring of the power transmission system 
during its operation. It is another object to describe a corresponding 
apparatus for applying the method of the present invention, it being 
possible to variably use the apparatus in different power transmission 
systems. 
In accordance with the invention, the method of monitoring mechanical power 
transmission systems solves the foregoing problem, and comprises the 
following steps: first, two planes arranged substantially parallel to one 
another are plotted on the unstressed power transmitting element, with 
each plane being defined by at least three space coordinates or measuring 
points on the power transmitting element. With respect to the latter, in 
the present invention these two planes are established directly on the 
power transmitting element. The next step of the method occurs on the 
power transmitting element under stress. In the stressed condition, the 
change in position of the two planes relative to one another is 
determined, which is caused by the stress or elongation occurring in the 
power transmitting element. Finally, the determined relative change in 
position of the planes is correlated with the local elongation or 
compression occurring on the power transmitting element, with the Hooke 
Law applicable to the elastic range forming the basis. 
At this point, it should be briefly pointed out that the change in position 
of the two concerned planes relative to one another as a result of an 
elongation or compression of the power transmitting elements occurs in 
direction of the power transmission. The term elongation is defined as a 
change in length with respect to the overall length, the latter being the 
distance between the planes in the unstressed condition of the power 
transmitting element. Accordingly, the elongation .epsilon. results from 
EQU .epsilon.=d1/1 
with 1 being the distance between the planes in the unstressed condition of 
the power transmitting element and d1 the change in position. 
The tensile stress .sigma.z results according to the Hooke Law from the 
elongation and the material-specific modulus of elasticity E as follows: 
EQU .sigma.z=E.times..epsilon.. 
The tensile forces are calculated from the cross sectional surface as well 
as the tensile stresses, as follows: 
EQU Fz=A.times..sigma.z, 
with A being the cross sectional surface of the power transmitting element. 
The bending stresses can be calculated in the same manner as the tensile 
stresses, likewise in accordance with the Hooke Law: 
EQU .sigma.b=E.times..epsilon.b, 
with .sigma.b representing the bending stress and .epsilon.b the 
elongation. 
The bending moments Mby and Mbz are then calculated, as follows: 
EQU Mb=Wb.times..sigma.b, 
with Wb being the moment of resistance. 
Furthermore the maximum bending moments are calculated from 
##EQU1## 
Finally, the lines of action of the bending moments, i.e, the directions of 
the bending moments are calculated from 
EQU tan.alpha.=Mbz/Mby 
To calculate the elongation within the power transmitting elements or the 
columns of the pressure die casting machine, it is necessary to establish 
a system of coordinates. To this end, reference is made to FIG. 2. As one 
can note from this Figure, the x-axis is placed along the longitudinal 
axis or neutral axis of the column or power transmitting element. The 
vertical thereto is defined as z-axis, and the horizontal accordingly as 
y-axis. To calculate the stresses and flexures in the power transmitting 
element, at least three measuring points are needed on the power 
transmitting element. The three measuring points define a plane in the 
system of coordinates. When now the power transmitting element is 
stressed, the position of the measuring points will change. When a change 
occurs only in direction of the x-axis (y, z=0) of the origin of 
coordinates (here, the first plane serving as reference plane is defined), 
a mere tensile stress is present. Under uneven stress, tensile, 
compressive, and bending stresses are present which are calculated with 
the plane equation. In the unstressed condition, the plane should extend 
perpendicularly to the power transmitting element. In the stressed 
condition of the power transmission system or element, the direction and 
position of the respective measuring points will change. 
As indicated above, the method of the present invention allows to calculate 
in a particularly advantageous manner the tensile stress according to the 
Hooke Law from the elongation and the material-specific modulus of 
elasticity of the power transmitting element. In a further advantageous 
manner, is it possible to calculate the tensile force from the cross 
sectional surface of the power transmitting element and the calculated 
tensile stress. Likewise, in accordance with the Hooke Law, it is possible 
to calculate the bending stress from the elongation and the modulus of 
elasticity of the power transmitting element. From the calculated bending 
stress again, it is possible to derive the maximum bending moment. 
Likewise, it is possible to determine the position of the maximum stress. 
As regards the two planes plotted on the power transmitting element, it is 
of special advantage, when in the unstressed condition of the power 
transmitting element, the planes extend substantially orthogonally to the 
direction of the power transmission. This arrangement has the great 
advantage that when the method of the present invention is applied to the 
monitoring of columns in pressure die casting machines, the planes extend 
in the unstressed condition of the columns substantially parallel to the 
clamping plates and orthogonally to the columns or power transmitting 
elements respectively. 
With respect to the determination or establishment of the space coordinates 
of the two planes, it is particularly advantageous, when this 
determination occurs by means of noncontacting displacement measuring 
sensors. Noncontacting displacement measuring sensors are especially 
advantageous, inasmuch as the "absence of contact" discontinues all 
further power transmission, which renders the monitoring difficult. These 
noncontacting displacement measuring sensors measure each against a 
measuring object. The displacement measuring sensors and the measuring 
objects associated thereto are stationarily arranged on the power 
transmitting elements in the region to be monitored, i.e., about firmly 
connected with the power transmitting element. The space coordinates of 
the one plane or the displacement measuring sensors respectively result at 
the location, which corresponds to the zero point of the measurement of 
the displacement measuring sensors. The space coordinates of the other 
plane can be derived from the measurement of the displacement measuring 
sensors against the measuring objects. Consequently, the displacement 
measuring sensors form with their location the one plane, and the objects 
to measure, against which the displacement measuring sensors measure 
noncontactingly, the other plane for monitoring the mechanical power 
transmission system. 
Furthermore, it is particularly advantageous, when the method of the 
present invention constitutes not only a kind of stationary monitoring, 
but that rather the correlated or calculated data of the power 
transmitting element under stress are fed back to the power transmission. 
Such a feedback can lead in an advantageous manner to a power control, a 
readjustment of the power transmitting element, or even to an emergency 
stop of the power transmission system, thereby effectively avoiding damage 
to the power transmission system. 
To carry out the above-described method, an apparatus for monitoring 
mechanical power transmission systems with at least one power transmitting 
element is provided, and which is characterized in that a clamping device 
is provided for engagement with the power transmitting element. The 
clamping device comprises two clamping pieces practically freely movable 
relative to one another and adapted for engagement with the power 
transmitting element in a defined position relative to one another. The 
first clamping piece in turn comprises at least three, preferably 
noncontacting displacement measuring sensors which measure against 
measuring objects associated to the second clamping piece. 
In accordance with the invention it has thus been recognized that the 
planes to be established in accordance with the method of the present 
invention can be defined by means of a suitable clamping device, the 
latter being secured in position on the power transmitting element. To 
enable now the two planes to move relative to one another in the stressed 
condition of the power transmitting element, the clamping device comprises 
two clamping pieces practically free movable toward each other, the 
clamping pieces or the clamping device being attached in the unstressed 
condition of the power transmitting element, to or on top of the power 
transmitting element at a defined distance from one another. Since now the 
plotting of one plane calls for at least three space coordinates, the 
first clamping piece is provided with at least three, preferably 
noncontacting displacement measuring sensors. These displacement measuring 
sensors form already the first plane with their local arrangement and the 
zero points respectively determined thereby. These displacement measuring 
sensors measure against objects which are associated with the second 
clamping piece, with the second plane resulting from this measurement, 
i.e., from the position of the measuring objects. Finally, in accordance 
with the method of the present invention, the change in position between 
the planes is determined by means of the displacement measuring sensors in 
the stressed condition of the power transmitting element. 
In a particularly advantageous manner, the displacement measuring sensors 
under discussion operate by the eddy-current principle. However, it is no 
problem to also use noncontacting displacement measuring sensors which 
operate by induction or capacitance, it being always necessary to observe 
that electric or electromagnetic fields do not influence the measurement 
of the displacement measuring sensors. 
As regards the constructional configuration of the clamping device or its 
elements, it is of special advantage that the clamping pieces are 
constructed as angle brackets, whose legs form an angle of preferably 
90.degree.. Naturally, these angle brackets may also form a different 
angle, it being necessary that on one hand the angle and other hand the 
size of the legs be always adapted to the power transmitting elements to 
be monitored. The angular clamping pieces are in particular suitable for 
monitoring cylindrical power transmitting elements. To be able to 
associate the clamping pieces with the power transmitting element, for 
example the column of a pressure die casting machine, in a clear and 
point-by-point accurate manner, the inside surfaces of the clamping pieces 
are provided with contact elements having preferably a circular cross 
section. Such a configuration of the contact elements effects that the 
clamping pieces, when being placed or tightened on the power transmitting 
element form with the latter only a point contact on each leg, which leads 
to an extremely precise definition of the respective plane. 
To be able to firmly attach the clamping pieces to the respective power 
transmitting element, the free ends of the clamping pieces are provided on 
the one hand with a clamping means preferably in the form of a chain and 
on the other hand with a tensioning device for tightening the clamping 
means jointed or attached to the other end. Once the clamping piece is 
brought in contact with the power transmitting element, the chain is 
guided around the portion of the power transmitting element which is not 
enclosed by the angular clamping piece, attached to the other free end of 
the clamping piece, and tightened by means of the tensioning element. 
Thus, a tight fit of the clamping piece on the power transmitting element 
is ensured. In this manner, both clamping pieces, i.e. the clamping piece 
carrying the displacement measuring sensors and the piece serving as 
reference object are secured in position. 
Furthermore, it is of special advantage, when the displacement measuring 
sensors measure about orthogonally toward the surface defined by the first 
clamping piece against the second clamping piece. This second clamping 
piece for itself is intended to form likewise a surface which is largely 
orthogonal to the direction of power transmission and thus arranged 
parallel to the first clamping piece or respectively the surface defined 
by the first clamping piece. 
In order that the displacement measuring sensors associated to the first 
clamping piece determine reliably the second plane or respectively the 
distance between the first and the second plane, at least three measuring 
objects corresponding to the number of the displacement measuring sensors 
and facing the first clamping piece, are associated to the second clamping 
piece. The arrangement of the measuring objects has the advantage that 
while they are physically independent of the second clamping piece, they 
all undergo as a result of their stationary jointing to the second 
clamping piece, the change in position of the latter which the 
displacement measuring sensors determine regardless of the distance 
between the displacement measuring sensors and the measuring objects. In 
so doing, it is particularly advantageous, when the measuring objects 
project from the second clamping piece in direction of the displacement 
measuring sensors or in direction of the first clamping piece, so that the 
distance between the displacement measuring sensors and the measuring 
objects is kept as small as possible, thus easily permitting a 
noncontacting displacement measurement based on the eddy-current principle 
or by induction or capacitance respectively. This distance of the 
measuring objects relative to the displacement measuring sensors can be 
adjusted, preferably by means of a micrometer screw or the like, so that 
in the unstressed condition of the power transmission system, all 
distances can be set to the same values, regardless of slight inclinations 
of the two planes toward each other. 
To protect the arrangement o displacement sensor and measuring object 
against mechanical influences, and to adequately shield the displacement 
measuring sensor, for example, with respect to external electromagnetic 
fields, the displacement measuring sensor is capsulated in a particular 
advantageous manner, namely: in a sleeve attached to the first clamping 
piece and extending in direction of the second clamping piece. The 
measuring object projecting from the second clamping piece respectively 
extends into this sleeve in direction of the displacement measuring 
sensor. As an alternative, the measuring object could be an integral part 
of a sleeve attached to the second clamping piece and extending in 
direction of the first clamping piece. In this instance, the displacement 
measuring sensor projecting from the first clamping piece would extend 
into the sleeve. Both aforesaid arrangements are basically possible. In 
the latter instance, the measuring object could advantageously be formed 
by the bottom of a bore provided in the sleeve, so that the sleeve itself 
would represent the measuring object. 
As regards the assembly of the clamping device or respectively the 
adjustment of the clamping pieces, it is particularly advantageous, when 
at least two spacers are provided for insertion between the clamping 
pieces and connection with the latter. These spacers remain installed 
between the clamping pieces until the clamping pieces are secured to the 
power transmitting element. Thus, the clamping pieces are arranged 
absolutely parallel to one another and adjusted accordingly. Only when the 
clamping device or clamping pieces are secured in position on the power 
transmitting element, are the spacers released, preferably via screws, and 
removed from the clamping pieces to allow a practically free mobility of 
the latter relative to one another. 
To be able to prepare or process the values measured by the displacement 
measuring sensors in the meaning of the method of the present invention, a 
computer with a corresponding evaluation program is provided for 
processing the measured values. If need be, a display and/or printer will 
be useful for a graphic representation of the evaluated data of the 
measurement. 
The use of the apparatus for monitoring the die closing unit of a pressure 
die casting machine is only by way of example. In such an instance, at 
least four power transmitting elements are provided, which are the columns 
of the die closing unit of the pressure die casting machine. Thus, it is 
possible to effortlessly monitor with the apparatus and by the method of 
the present invention the columns of the die closing unit, with the 
monitoring occurring during the normal pressure die casting process. 
The utility and advantages of the present invention are numerous and are 
not limited to the most preferred embodiment disclosed in detail below.

DETAILED DESCRIPTION 
In the following, the apparatus for monitoring mechanical power 
transmission systems in accordance with the invention will be described 
with reference to the example of a power transmission system used on a 
pressure die casting machine. It should be emphasized already at this 
point that while the following description serves only to explain the 
teaching of the present invention by way of example, it is by no means 
limited to pressure die casting machines. 
FIG. 1 schematically illustrates the problematic situation arising in 
principle in power transmission systems to the extent that power 
transmitting elements 1 constructed in the form of columns are bent to a 
greater or lesser extent during the closing operation. The schematically 
illustrated die closing unit of a pressure die casting machine comprises 
according to the illustration of FIG. 1 besides the power transmitting 
elements 1, two stationary clamping plates 2, 3 and one movable clamping 
plate 4. In the here-selected illustration, the die closing unit has just 
been closed and is held closed, so that a pressure casting die 5 is pushed 
into the clamping plates 3, 4 in such a manner that the latter 3, 4 
deform. 
To monitor the power transmission system of the pressure die casting 
machine under discussion, or respectively the power transmitting elements 
1 of the pressure die casting machine, a clamping device 6 as shown in 
FIGS. 3 and 4 is provided to be secured in position on the power 
transmitting element 1. This clamping device 6 comprises two clamping 
pieces 7, 8 which are practically freely movable toward each other and can 
be secured in a defined position relative to one another on the power 
transmitting element 1 as is depicted in the various Figures. The first 
clamping piece 7 comprises noncontacting displacement measuring sensors 9 
which measure against measuring objects 10 associated to the second 
clamping piece 8. The displacement measuring sensors 9 operate by the 
eddy-current principle. 
As can further be seen particularly clearly in FIGS. 3 and 4, the clamping 
pieces 7, 8 of the clamping device 6 are constructed as angle brackets 
whose legs 11, 12 form an angle of 90.degree.. The inside surfaces of the 
clamping parts 7, 8 are equipped with contact elements 13 which have a 
circular cross section. 
Arranged at the free ends of the clamping pieces 7, 8 are on the one hand a 
tensioning element 14 in the form of a chain, and on the other hand a 
tightener 15 for tightening the tensioning element 14 jointed to the other 
end. The chain, a cable or the like are particularly suitable for use as 
tensioning element 14, since they occupy only little space and, thus, 
contribute to a space-saving configuration of the clamping device. 
As is further shown in FIGS. 3 and 4, the noncontacting displacement 
measuring sensors 9 are attached to the first clamping piece 7. Two of the 
displacement measuring sensors 9 are associated to the outer leg regions 
and one displacement measuring sensor 9 to the angle region of the first 
clamping piece 7. As is best seen in FIG. 4, the displacement measuring 
sensors 9 measure about orthogonally to the surface defined by the first 
clamping piece 7 against the second clamping piece 8. Associated to the 
second clamping piece 8 are three measuring objects 10 corresponding to 
the number of displacement measuring sensors 9 and facing the first 
clamping piece 7. These measuring objects 10 extend from the second 
clamping piece 8 in direction of the displacement measuring sensors 9 or 
respectively in direction of the first clamping piece 7. The distance of 
the measuring objects 10 relative to the displacement measuring sensors 
can be adjusted by screws 16 preferably constructed as micrometer screws 
or the like. 
Essential now is that the measuring object 10 is an integral part of a 
sleeve 17 attached to the second clamping piece 8 and extending in 
direction of the first clamping piece 7. The displacement measuring sensor 
9 projecting from the first clamping piece 7 extends into sleeve 17, but 
is not firmly connected therewith. The measuring object 10 could now be 
formed on the one hand by the bottom of a bore provided in the sleeve 17, 
or on the other hand by the blunt end of a bore arranged in sleeve 17. 
To mount the clamping device or respectively adjust the clamping pieces 7, 
8, six spacers 18 are arranged according to the illustration of FIG. 4 
between the clamping pieces 7, 8, which are connected with the clamping 
pieces 7, 8. Once the clamping device 6 is secured in position to the 
power transmitting element 1 as shown in FIG. 3, the spacers 18 are 
removed so as to enable a relative movement between the clamping pieces 7, 
8. 
To carry out the method of monitoring mechanical power transmission systems 
in accordance with the invention, reference should be made to the general 
part of the description. Referring only to FIG. 2, it should however be 
emphasized that the planes to be established or plotted on the power 
transmitting element, are defined by three space coordinates or measuring 
points on the power transmitting element. To determine these planes, the 
system of coordinates illustrated in FIG. 2 will serve, wherein the x-axis 
is placed along the longitudinal axis or neutral axis of the power 
transmitting element. The vertical thereto is defined as z-axis, and the 
horizontal accordingly as y-axis. 
Finally, it should be remarked one more time that the foregoing description 
of only one embodiment of the present invention does not limit the 
teaching of the invention to the selected embodiment.