Note: Descriptions are shown in the official language in which they were submitted.
Case 4018
1038358
BACKGROUND OF THE INVENTION
The invention pertains to a method and apparatus for
protecting a mill consisting of a horizontally revolving pul-
verizing bowl and stationary crushing rollers that are posi-
tioned for rolling upon a race on the bowl, in which the crush-
ing rollers are attached each to a shaft through an oil lub-
ricated bearing, said rollers thus transferring the crushing
pressure to the pulverizing race.
In this type of mill, sometimes a single lump of es-
pecially hard consistency or of foreign material is not rolledover and crushed to pieces, but acts like a drag shoe to block
the crushing roller. The blocked roller, instead of grinding,
pushes the softer grinding material to the sides in a snow-plow
fashion and slides over the bare race. There is no noticeable
change in power consumption. This may last until the lump
wears off by attrition, or until the foreign material falls
;~ aside. During that time a flat spot is worn on the circumfer-
ence of the roller due to sliding, which further favors the
blocking of the roller at the same position. This leads to
enlargement of the flat spot. The larger the flat spot the
greater is the danger that the crushing roller will block at
the same position due to minor causes, such as a change in
hardness of the feed material, reduction in grinding pressure
or damage to the bearing. This has the disadvantage that a
continuation of the operation reduces the grinding yield and
leads, through the rapid wear, to the failure of the mill which
results in costly outages and expensive repair work.
It is customary to monitor bearings during operation
in order to detect early any irregularities in the pressure of
the lubricating oil, or in the oil level and oil temperature
in immersion lubrication. A drop in oil level means a loss of
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oil and an excessive rise in the oil temperature is an indica-
tion of lubrication deficiency. An increase in the temperature
of the bearing is a warning of an eminent bearing failure.
Methods for remote monitoring and control of the tem-
perature and of the liquid level of liquids are well known.
However, the continuous surveillance of the oil temperature
and/or oil level during operation in the bearings of crushing
rollers as in the previously described mill has not been known.
SUMMARY OF THE INVENTION
The purpose of this invention is to substantially elimi-
nate the generation of flat spots on the circumference of the
crushing rollers. A solution to this problem is, according to
this invention, that during the normal operation of the mill
the oil temperature and oil level in the immersion lubricated
bearings of the crushing rollers are monitored by remote in-
dicators. The mill is stopped and re-started when, during the
mill operation, the oil temperature in one of the bearings
drops a certain amount and/or the oil level increases by a cer-
tain amount.
The normal start-up and shut-down switch is used to
accomplish the proposed protection method. In addition, a
number of electro-sensors are used that are shrouded in metal
sheaths. Also a number of electronic transmitters and remote
indicators are used, which react to temperature differences of
the sensors or to change of electrical capacitance in them.
These devices can eventually be connected to alarm devices.
According to a feature of the invention, apertures are
formed or drilled in each of the roller shafts, in which are
located temperature sensors connected to a remote indicator,
so that the sensors reac~ into the oil sump of the bearing.
The metal sheath of the temperature sensor is electrically
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lnsulated from the walls of the aperture and together with them
provides two opposing surfaces of a condenser with an annular
space between them, into which the oil from the sump enters r
from the bottom up to the oil level. The metal sheath is con-
nected to a remote indicator which is sensitive to changes in
electrical capacitance.
The temperature sensor responds, through heat transfer,
to the temperature of the oil and through an electronic remote
transmission may be made visible on an electronic instrument.
The capacitance of the condenser formed by the metal sheath
and the surface of the aperture of opening changes depending
on the oil level in the annulus of the condenser, because of
the effect of the dielectric coefficient of the oil itself.
The dielectric coefficient of the lubricating oil nearly doubles
the capacitance of such a condenser. The change in oil level
thus causes a change in capacitance. The level of the oil is
then made visible on an electronic instrument through the re-
mote transmission of the change in the capacitance.
Through the unique arrangement of the sensors in the
openings or apertures of the roller sha~ts, a way is provided
in the mill to remotely monitor simultaneously both the tem-
perature and the oil level in the sump of the immersion bear-
ing of each crushing roller.
In operating immersion bearings, in which there is an
oil film between the gliding and rolling surfaces, the oil
temperature always ad~usts itself to a point above the tem-
perature of the surroundings, because of fluid friction. When
a bearing stops operating due to a blocked roller, the oil
friction also stops, and the oil temperature in the sump of
the splash bearing drops in a short time due to heat exchange.
The temperature drop indicates that the roller is blocked.
During the start-up of the mill a part of the oil is
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1038358
removed from the sump to lubricate the bearing~ so the oil
level drops. When a roller is blocked the entire oil collects
in the sump and the oil level rises.
If during the normal operation of the mill there is a
temperature drop and/or an oil level increase in the sump of
a bearing of a crushing roller, this provides a warning for
the operator that the crushing roll is blocked and that the
mill should be stopped and re-started before a larger flat
spot will be ground on the circumference of the tire.
At the instant of stopping, the tire rolls back a bit
on the race from the especially hard lump or from the foreign
material. This results in the grinding bowl being slightly
turned back, so that the lump becomes free and during the sub-
sequent start-up is shoved away or is popped off the raceway
by the roller. As a precaution to insure that a previously
blocked roller actually became free, or if not, to be ready to
shut-down the mill again and re-start, the operator should
preferably observe the behaviour of the oil levels, because
they react during start-up much faster than the oil temperatures.
When it ls observed that the oil level in one of the oil sumps
does not drop~ this means that the corresponding crushing rol-
ler is blocked and that the mill should be stopped again and
re-started to remove the danger.
In order to recognize more easily a temperature drop
of the oil in any of the crushing rollers it is advantageous
to compare continuously the oil temperatures o~ the three
bearings. The measured oil temperature o~ a blocked roller
indicates its actual oil temperature and the other oil tem-
peratures indicate how high the oil temperature of the blocked
roller would be if it would operate properly. The comparison
permits an easy recognition of the temperature drop.
The advantages achieved by the invention are that
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through continuous monitoring of the oil levels and of the oil
temperatures, an early warning of imminent damage to bearings is
obtained. Further, it permits a considerable reduction in the
grinding of flat spots on the circumference of the tires, through
which an improvement in mill availability is achieved, because of
avoidance of excessive wear, costly shut-downs and maintenance work.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 shows a cross section of a part of a crushing
roller mill;
Fig. 2 shows a diagram for monitoring the oil levels and
the oil temperatures of the crushing roller bearings; and
Figs. 3 and 4 show plan and elevation views respectively
of a pulverizing mill in which the invention is applied.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Of the three non-orbiting crushing rollers Al, A2, A3 of
a pulverizer shown in Figs. 3 and 4 only roller A2 is shown in Fig. 1.
The crushing rollers are stationary relative to the pulverizer
housing and roll on a ring shaped grinding race or strip of a hori-
zontally rotating bowl or plate 2 which is driven by a motor 3
(shown schematically only) through a gear 4. The switch 5 represents
the normal device used for start-up and shut-down of the mill.
The crushing rollers Al and A3 are attached in a similar
manner as is crushing roller A2 to a shaft 7 through a combination of
roller bearings 6 as shown in Fig. 1. The hub 21 of the roller,
together with a bearing cover 22, enclose the roller bearing 6 and
the shaft 7 to form a tight enclosure that is filled with a mineral
or other lubricating oil to a level B2. The oil collects in the sump
D2 of a splash lubrication system in which the bearing rollers
repeatedly immerse during the rotation of the crushing
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rollers. This way a part of the oil is carried out of the
sump by the bearing rollers and the oil level drops to a
level B2, but rises again to the original level B2 when
the rotation stops because no oil is carried out and all
the oil collects again in the sump D2.
Furthermore, during the rotation of the crushing
roller? heat that is generated by the fluid friction of the
lubricating oil is added to the oil fill. T~s heat is trans-
ferred to the shaft and to the housing through a temperature
gradient. Therefore, the oil temperature may rise above the
temperatures of the shaft 7 and housing 23, but drops again
; by a certain amount when the rotation is stopped because the
heat generation ceases and the heat transfer to the shaft
and housing continues until the temperature of the oil
equals that of the shaft end housing 23
When a crushing roller blocks, the rotation of the
roller stops and the temperature of the oil drops by a cer-
tain amount and also the oil level rises from B~ to the
original level B2.
When the crushing roller rotates, but an irregular-
ity develops in the bearing, then the oil temperature rises
too high and if the lubrication fails or an oil loss occurs,
the oil level will fall below B2.
~ A temperature sensor 8 is provided to measure the
oil temperature. The sensor is shrouded with a metal sheath
9 and reaches into the sump D2. me temperature sensor 8
is connected through electrical leads with a measuring de-
vice 10 and an indicating instrument E2 to indicate the oil
temperature. The metal sheath 9 serves to measure the oil
level. It is connected by electrical leads with a trans-
mitter ll and an indicator F2.
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~ Case 4018
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To perform the remote transmission of the oil level
indications B2 or B2 the metal sheath 9 is so arranged in
the aperture or opening 12 of the shaft 7, that it is elec-
trically insulated and forms, together with the surface of
the aperture or drilling, two oppositely located condenser
surfaces An annular space 13 is defined between these
surfaces, in which the oil contained in the sump D2 pene-
trates from the bottom to a height of the oil level B2.
Above the oil level B2 the aperture or drilling 12 is pro-
vided with a perpendicularly drilled hole 12' for pressureequalization within the roller bearing space. Oil is con-
tained in the section 14 of the space of the condenser an-
nulus 13, so that the capacitance of that part of the con-
denser is about doubled due to the dielectric coefficient of
the mineral oil. When the oil level drops to the B2 height
some of the oil flows out of the annulus 13 and the capaci-
tance of the condenser changes. This change of the capaci-
tance is utilized for the remote transmission of the oil
levels B2 or B2. A transmitter ll sensitive to changes of
capacitance is used, therefore, for remote indication of the
oil level.
Each crushing roller Al, A2 and A3 has a temperature
sensor 8 with a metal sheath 9 arranged the same way in rela-
~ tion to the apertures or drillings 12 and 12', as shownschematically on Fig. 2. Reslstance thermometers are provided
for the measurement of the oil temperature, with the tempera-
ture sensor 8 used as part of a Wheatstone bridge 10 and the
instruments El~ E2 or E3 measuring the corresponding current
in the bridge for each of the rollers Al, A2 and A3; respec-
tively.
m e transmitter ll which reacts to changes of capaci-
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Case 4018
1038358
tance consists of a high frequency generator 15 with an oscil-
lating circuit composed of an inductance 26 and a capacitance
27, which is excited by a transistor 28 connected across the
capacitance. The condenser formed by the metal sheath 9 and
the drilling 12 is part of the oscillating circuit. mus the
frequency of oscillation is changed wnen the capacitance
changes due to variation of the oil level from B2 to B2, or
the reverse. m e frequency of the high frequency generator
15 is tuned by a Wien-Robinson bridge circuit 16, consisting
of resistors and capacitances which are tuned to the high
frequency generator 15, so that with a change in the frequency
of oscillation in the bridge a change occurs in the electrical
potential. This electrical potential, upon amplification by
a transistor 31, controls a direct current circuit wnich de-
livers the measurement signal to the indicating instrument F2
The indicating instrument F2 s'nows for example that the oil
level has risen from B2 to B2
The continuous supervision of the operation of the
bearings is achieved through observation of the indicating
instruments El, E2 and E3, which can also be coupled with
alarms or other annunciating devices to make sure that the
oil temperatures in the sumps Dl, D2 and D3 are not, compara-
tively, too hot or too cold Furthermore, attention must be
given to the indicating instruments Fl, F2 and F3 to make
sure that the oil levels Bl, B2 and B3 do not drop too much
due to loss of oil
If an observation is made on one of the instruments
El, E2 or E3/ for example on the instrument E2, that the oil
temperature in sump D2 dropped in comparison to the indica-
tions on the instruments El and E3, this does not mean thatthe running conditions of the bearing have impro~ed, but that
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1038358
the crushing roller A2 is blocked. In that case the indication
of the instrument F2 does not mean that the oil in the sump D2
was replenished, but that the rise of the oil level from B2 to
B2 indicates also that the crushing roller A2 is blocked.
me roller pushes the unground material from the race 1 (see
Fig. 1) and glides on a bare race, so that the race wears off
and a flat spot G2 (Fig. 1) starts to appear on the circum-
ference of the crushing roller A2. Such occurrence is detri-
mental and an increase in the size or num~er of flat spots G2
can be avoided by stopping the mill and starting again.
When, during normal operation of the mill one of the
indicating instruments, for example E2, indicates directly or
through a warning signal that the oil temperature is dropping
and/or if it is determined through a direct reading of the in-
strument F2 or through a warning signal that the oil level
rises from B~ to B2, then for the safety of the mill it is
best to stop the mill by actuating the switch 5 (Fig. 1) and
to re-start it a~ain using the switch 5. m rough this action,
the especially hard lump or the foreign material that had
blocked the crushing roller is shoved aside or pops off the
grinding race.
During the re-start care must be taken to ascertain
that the indicating instrument F2 shows a drop of the oil
-level from B2 to B2 in order to be sure that bearing damage
does not prevent the rotation of the crushing roller. The
same safety procedure is performed when any of the other
crushing rollers Al or A3 are blocked.
While there is illustrated and described herein a
specific embodiment of the invention, those skilled in the art
will understand that changes may be made in the form of the
invention covered by the claims, and that certain features of
Case 4018
' ~0383S~
the invention may sometimes be used to advantage without a
corresponding use of the other features.
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