Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~ WO95/11120 21 74639 pcT~Kg4loo388
A mixer
The invention conc~rns a mixer for a material, such as
..~lete, and comprising a stirring unit as well as a
mixing vessel with a closable discharge op~ g.
A typical example of a mixer of this type is a concrete
mixer. When a charge is to be mixed in such a concrete
mixer, ayyleyates usually in the form of stone and sand
are first added, and then the cement is added under con-
n~ dry mixing. Finally, optional additives plus waterare added, and the mixing process now continues as a wet
mixing until the concrete mix has obtAineA the desired
state ready for use. Until then, frequently about 2.5 mi-
lS nutes will have elapsed from the moment when filling wasinitiated.
Then the discharge op~ing is opened to empty the vessel.
At this time the concrete mix is in an almost stiff and
very viscous state which impedes the discharge process and
has as a result that the time it takes to discharge is
relatively long. Thus, it is not unusual that the actual
A, i ~,h~rge alone takes about ~ minut, or a consumption of
time of the order of 30% of the actual charging and mixing
time.
To this should be added the problem of emptying the vessel
completely of concrete mix. Even with such a great con-
sumption of time it has been found in practice that empty-
ing of the known concrete mixers can very well leave resi-
dues of up to 2% for each emptying. This circumstance is
not only extremely unfortunate for economic re~so~, but
may mo eo~er have as a result that a disuniform and defi-
cient quality is imparted to the concrete, because freshly
added materials are uncontrollably mixed with residues
WO95/11120 PCT~K94100~8
2 ~ 74639
-- 2 --
from the preceAi~g mixing cycles.
It has been attempted to solve the above problems by means
of a mixer which is described and shown in the Swedish
patent 225 460. In this case, the vertical shaft of the
stirring unit pivotably mounts an ejector in the form of a
rectangular plate which extends vertically from the bottom
of the vessel up to the maximum height of the mixture. The
pivot shaft of the plate is located somewhere between the
center of the mixing vessel and its side wall, there being
provided a biassed spring which is so adapted as to try to
pivot the plate outwardly transversely to the direction of
rotation of the stirring unit. However, the strength of
the spring is not so great that it can overcome the resis-
tance which the plate meets when it is driven through the
material during the mixing process. During this process
the plate is therefore automatically aligned in the direc-
tion of rotation.
When the vessel is to be emptied, the ~i schArge op~1 ng in
the bottom is opened, and the material then begins rllnn1ng
out of the vessel in the normal manner. The level of the
material hereby gradually sinks below the upper edge of
the plate, so that the pressure of the material on the
plate sllccec~ively decreases, thereby causing it to begin
pivoting outwardly and to be transverse to the direction
of rotation in the final phase where the vessel is empty
or almost empty.
This structure can give a certain reduction in the dis-
charge time with respect to the previously known art, but
the time gained in this manner is relatively modest. The
reason is that the plate per se is only capable of contri-
buting to emptying the vessel by degrees in step with the
material sinking in the vessel. Only in the final phase
when the vessel is empty or almost empty is the plate
21 74639
~_ WO95/11120 PCT~K94/003
r~onAhly effective.
During filling and initially also during the mixing pro-
cess, the plate will almost be in the way and tend to in-
crease the overall time required by the mixing process.
Another drawback of this known structure is that there is
no or only limlted space for its ~is~-h~rge plate $n a con-
crete mixer which is based on a planet gear having mixing
stars whose blades pass the center of the mixing vessel
during the mixing process. Such a concrete mixer typically
operates according to a counterflow mixing principle,
where the flows of material take place in a manner quite
different to the one of the known structure which employs
an ordinary stirring unit rotatably mounted on a central
axis. In the counterflow mixing principle the ~i~ch~rge
plate will not be oriented in the manner which is required
in the patent specification, viz. in the direction of ro-
tation during the mixing process. On the contrary, the far
more complicated flow state which is present in counter-
flow mixing, will tend to make the discharge blade assume
other positions where it may impede the int~nA~ flow and
reduce the efficiency of the mixing process.
Accordingly, there is a need for a mixer of the type men-
tioned in the op~ing paragraph which has a device capable
of providing faster emptying with less residues of mate-
rial than known before both in an ordinary mixing process
and in a counterflow mixing process.
This problem is solved according to the invention by means
of a scraper mech~n~sm which is caused to describe an
orbital movement about a central axis in the vessel by the
stirring unit in operation and which is adapted to raise
at least one scraper blade above the material during mix-
ing and to lower it down into it during emptying. This
woss/11120 ? 1 7 4 6 3 9 PCT~K94/00388
structure ensures that the scraper blade is entirely free
of the material while the mixing vessel is filled and the
material is mixed, so that this part of the process is in
no way disturbed by the pr~sence of the scraper blade.
This property is of decisive lmportance in particular when
mixing takes place according to the counterflow principle.
During emptying the scraper blade is effective from the
beg~ nn~ ng. The ~schArge time is hereby minimized.
The mixing vessel typically has a round side wall and a
bottom which closes it downwardly. In this case it is
advantageous when the scraper blade in the lowered state
forms a negative acute angle with a radius through the
inner edge of the blade, and its outer edge is present at
or in the vicinity of the side wall.
For the bottom to be scraped completely clean of residues,
it is moreover eXp~ient to position the scraper blade
with its lower edge close to the bottom. Since the scraper
blade is substantially transverse to the direction of
rotation, it pushes the material along for each rotation.
Owing to the negative acute angle with the radius, this
material will simultA~eol~sly be pushed out toward the
periphery, and when the scraper blade meets the discharge
opening, which is usually positioned in an area at the
side wall, the material drops out through this opening.
In counterflow mixing the blades of the mixing stars pass
the center and thereby ensure that no residues are left in
this area, even though the scraper blade itself does not
extend beyond the center. The scraper blade, optionally in
cooperation with conventional side scrapers, hereby en-
sures rapid and effective emptying and cl~Ani~g of the
mixing vessel for each mixing cycle.
`~ WO95/11120 2 1 7 4 6 3 q rCI/DK94/00388
A very effective mixer is described in the applicant's
European patent application 89 908 400.8-2113 "A method
and an apparatus for mixing materials". In this mixer,
which operates according to the counterflow mixing prin-
ciple, the stirring unit is based on a planet gear havinga ring-shAp~A planet housing which is caused to rotate
about a central axis by means of one or more motors during
rotation. This planet housing rotatably mounts mixing
stars which are in gear wheel engagement with the statio-
nary sun wheel of the planet gear and therefore rotatewith respect to the planet housing when this rotates about
the central axis. The mixing stars carry blades which di-
rectly serve to mix the material.
In this structure, the scraper mechAn;sm may advantage-
ously be provided on the ring-shaped planet housing. Then
the scraper mechAn;sm and thereby the scraper blade will
follow the rotation of the planet housing and always be at
an llnchA~ging relative distance - seen in vertical pro~ec-
tion - from the rotating mixing stars and their blades.
Both the mixing stars and the scraper blade must have a
great radius of action in order to be able to touch all
the material during the mixing ~Locess and afterwards to
be able to ~chArge it rapidly and effectively. The mix-
ing blades and the sc a~er blade will therefore work closeto each other. The above arrangement effectively ensures
that the blades and the scraper mechAn~sm with its scraper
blade do not collide with each other in operation.
When the scraper mechAnism is provided on the planet hous-
ing, it may have a lifting rod in a preferred embodiment
which is pivotally ~ournalled on a pivot provided on the
planet housing. The other end of the lifting rod then car-
ries the scraper blade, and a drive me~-hAn;sm is operative
between the planet gear and the lifting rod to pivot the
lifting rod up and down between a lower discharge position
21 7463~
WO95/11120 PCT~K94/003~ _
and an upper free position. This structure is simple and
sturdy and i8 of a size permitting it to be easily incor-
porated in the restricted space between the mixing stars
and the side scrapers.
In a particularly simple embodiment the drive mer,h~nism
may be a pneumatic or hydraulic drive cy~ er. In another
and particularly eYpP~ent embodiment of the drive mecha-
nism, said mech~nism consists of a combined rod and gear
wheel connection which can be engaged and disengaged with
the main gear wheel of the planet housing by a dynamic
brake, thereby causing the scraper mechAnism to lower and
raise the scraper blade respectively. The only component
to be directly activated in this connection, is the brake
which is stationarily mounted on the mixing vessel. The
brake can therefore easily be connerted to a power supply,
while it is somewhat more difficult to run a power supply
to the above-mentioned pneumatic or hydraulic drive cylin-
der via the planet housing which is rotatably mounted in
the mixing vessel.
The brake may be a pneumatic or hydraulic drive cylinder
having a piston which, in the activated state, serves as a
brake disc against another brake disc which is con~er-ted
with the combined rod and gear wheel conn~ction of the
drive mech~nism. In the activated state, the two brake
discs transfer the neCps-cAry force by means of friction,
whereby they simultaneously slidingly rotate with respect
to each other. A brake lining may advantageously be inter-
posed between the brake discs.
Also a ~u-,~elltional hydraulic brake or a magnet brake may
be used as a brake.
The drive mech~nism may generally be characterized in that
it must be adapted to lower the scraper blade toward the
' WO ~/11120 21 7463~ pCT~K94/00388
bottom of the mixing vessel upon braking and to raise the
scraper blade to its upper position when braking ceAC~s.
The invention will be explained more fully by the follow-
ing description of emho~;ments, which just serve as ex-
amples, and with reference to the drawing, in which
fig. 1 is a top view, with the uppermost portion removed,
of a concrete mixer having a stirring unit which is built
on the basis of a planet gear and operates according to
the counterflow mixing principle,
fig. 2 is a lateral, partially sectional view and with a
first embodiment of a scraper mechAn;sm,
fig. 3 shows the same, but with another embodiment of a
scraper mechAn;sm, and with the mixing stars of figs. 1
and 2 omitted,
fig. 4 is a lateral, partically sectional view of the
drive mechAn;sm A-cQor-;ated with the scraper mec~An;sm of
fig. 3, and
fig. 5 is an enlarged peL~e~ive view of the drive mecha-
nism of fig. 4.
Figs. 1 and 2 show a concrete mixer which is generally
designated 1. This concrete mixer is of the type which is
described in the applicant's European patent application
89 908 400.8-2113 "A method and an apparatus for mixing
materials". The concrete mixer comprises a mixing vessel 2
having a side wall 3 and a bottom 4. The mixing vessel
serves to receive the material which is to be mixed, and
which reAche~C the level N when the vessel is filled. Up-
wardly, the vessel is covered by a solid superstructure 5,
which carries a stirring unit 6 and a central pipe 7
wogsllll20 2 1 7 4 6 3 9 P~/DKg4/00388
-- 8 --
through which the materials to be mixed are fed to the
mixing vessel. Further, the bottom 4 of the mixing vessel
is formed with a side opPn~ng 8 (fig. 1) for removal of
the mixed concrete.
The stirring unit is built on the basis of a planet gear 9
(fig. 2) having a ring-sharea planet housing 10 which is
rotatably mounted on the superstructure 5. A top plate 11,
secured upwardly on the superstructure 5, mounts two mo-
tors 12, each of which is connected with a gear 13 havingan output shaft 14 with a gear wheel drive 15 which en-
gages an outer toothed rim 16 on the planet housing 10. An
isolating screen 17 for dampPn~ng the noise from the gear
13 is arranged around the gear 13.
Two output shafts 18 are mounted downwardly on the planet
housing 10. A gear wheel 19, which engages the stationary
sun wheel 20 of the planet gear, is arranged upwardly on
each of the output shafts. The lower end of each output
shaft 18 carries a mixing star 21 having inclined down-
wardly directed arms 22 whose lower ends mount mixing
blades 13 close to the bottom 4.
Further, the planet housing 10 mounts side scrapers 24
which are pulled along the side wall 3 and serve to scrap
off material that might stick to the side wall.
When the motors 12 are co~ected, they cause the ring-
sh~peA planet housing 10 to rotate at a speed of rotation
which is typically 10 revolutions per minute in the above
European patent application. Simultaneously, the mixing
stars 21 are caused to rotate against the flow in the
direction shown by the arrows in fig. 1 because of the
gear whéel engagement with the stationary sun wheel 20.
The mixing stars have slightly different diameters and
corresponA;~gly rotate at slightly different speeds of
~ wog5llll20 2 1 7 4 6 3 ~ pCT~K94/00388
rotation with 43 and 48 revolutions per minute, respec-
tively. This results in the greatest poss~hle mixing ef-
- ficiency, since the mixing hl ~S pass beyond the center
of the mixing vessel and get in contact with all the mate-
rial present in the mixing vessel. Thus, the mixing stars
21 have a considerable radius of action, and in combina-
tion with the side ~cLa~ers 24 they therefore restrict the
space available in the vessel for il-~ol~oLation of other
compor e~lts .
The mixing vessel 2 moreover accommodates a scraper mecha-
nism 25 which is mounted on the planet housing 10 by means
of a bracket 26. The bracket has a pivot 27 about which a
lifting rod 28 can pivot up and down. A scraper blade 29,
whose function will be described more fully below, is pro-
vided at the opposite end of the pivot 27 or the free end
of the lifting rod 28. A pneumatic or hydraulic drive cy-
l~nAer 30 operates between the bracket 26 and the lifting
rod 28. This drive cyl~n~er is pivotally susp~n~ from
the bracket 26 by means of a pivot 31. The piston rod 32
of the cylinder is simultaneously connected with a point
on the lifting rod 28 by means of another pivot 33. As
shown, the pivot 31 of the drive cylinder is spaced above
the pivot 27 of the lifting rod, so that the drive cylin-
der when activated will affect the lifting rod 28 with a
torque around the pivot 27 and can thereby pivot the
scraper 29 from the bottom position shown in fig. 2 to a
position above the level of material N, and vice versa.
When a charge of materials is to be mixed to fin~he~
~on~lete, the mo~o~x 12 are connected first. The motors
hereby cause the planet housing 10 to rotate via the gear
wheel engagement between the gear wheel drives 15 and the
toothed rim 16 on the planet housing. The side scrapers 24
are pulled along the side wall 3 of the mixing vessel at
the same speed of rotation as the planet housing, and the
WOgS/11120 2 1 7 4 6 3 9 pCT~K94/00388
-- 10 --
mix$ng stars 21 with the mixing blades 23 are simultane-
ously caused to rotate with respect to the planet housing
h~ .ce of the gear wheel engagement between the gear
wheel 19 of the mixing stars and the stationary sun wheel
20. The scraper blade 29 has been lifted by the drive cy-
linder above the level N which the material re~ches when
the vessel is filled. In this position the ~ a~er blade
does not interfere with the mixing process, which can
therefore proceed optimally.
In the concrete mixer structure shown in fig. 2 the mate-
rials are fed through the central pipe 7. However, it is
no condition of the present invention that the materials
are fed in this manner. Ohter embodiments of the concrete
mixer structure may e.g. have side op~ings (not shown)
for the introduction of the materials.
The aggregates stone and sand are added first and are dry
mixed. Then cement is added with cont~ A dry mixing, and
finally optional additives as well as water is added, and
then the total amount of material is wet mixed until the
mix is ready for use. The overall period of time spent on
this part of the mixing cycle is typically about 2.5 mi-
nutes.
As soon as the actual mixing process has been completed,
the lateral opening or discharge ope~i~g 8 is o~ened, and
the drive cylinder 30 is activated. This initiates the
discharge, which takes place without stopping the motors
12. The side scrapers 24 and the mixing stars 21 therefore
continue to work in the same manner as during the actual
mixing process.
The drive cylinder 30 now activated pivots the lifting rod
28 downwardly so that the scraper blade 29 is forced down
toward the bottom 4 of the mixing vessel 2, as shown in
~ wos5lll120 2 1 7 4 6 3 q PCT~K94/003~
-- 11 --
fig. 2. The scraper blade 29 is now rotated in the con-
crete mix at the same speed as the planet housing 10. The
scraper blade acts as a kind of plough which pllsh~c the
concrete mix in front of it around, while the concrete mix
is moved out toward the lateral wall. The latter function
is due to the fact that, as shown in fig. 1, the scraper
blade 29 is positioned with an obliquely rearwardly di-
L~c~ed inclination with respect to the direction of rota-
tion, or at a negative angle ~ with respect to a radius
through the inner edge of the scraper blade.
The usually stiff concrete mix has a very viscous consis-
~ell~y and can therefore normally be discharged only slowly
from the mixing vessel. Now the emptying takes place as a
combined process, during which the scraper blade coope-
rates with the side scrapers and the blades of the mixing
stars. The side scrapers pull down the concrete mix from
the side wall 3 of the vessel, while the blades of the
rotating mixing stars bring along the concrete mix, pre-
sent above the central area of the bottom 4, in an outwarddirection, so that also this part of the concrete mix will
be within the reach of the scraper blade 29, which there-
fore does not have to extend beyond the center.
The overall effect of the above-mentioned arrangement is
very good. It has thus been found that a typical concrete
mixing vessel can be discharged completely after just 3-5
rotations of the planet housing 10. The residues left in
the mixing vessel are down to 0.1-0.2%, while residues of
up to 2% for each emptying have frequently been left in
corresponA~g concrete mixers without a scraper mechAn~sm
having a scraper blade. The reduction in the time spent on
the discharge is considerable and is of the order of 10-15
seconA~ or 30-40%. The time spent on a total mixing cycle
can hereby be reduced by about 10~, while the residues are
reduced by more than 90%.
WO95111120 PCT~K94/00388
21 74639 12 -
In the embodiment of the scraper mech~nism shown in figs.
l and 2 the compressed air or oil must be fed to the drive
cyl;n~?r via a rotatable pipe ~oint, since the entire
scraper mechAn~sm is mounted on the planet housing lO
which rotates with respect to the mixing vessel 2 in ope-
ration. It is relatively difficult to establish such a
rotatable pipe ~oint, in part~c~llAr when the central area
of the concrete mixer is occupied by the central pipe 7
for the charging of the materials. This drawback is re-
medied by the embodiment of the scraper mech~nism which isshown in figs. 3, 4 and 5.
In this case, too, the scraper blade 29 is arranged at the
end of a lifting rod 28 capable of pivoting up and down
about a pivot 27 on a bracket 26 which is firmly mounted
on the planet housing lO. This entire part of the scraper
mech~nism is therefore rotated in the mixing vessel at the
same speed of rotation as the planet housing.
Spaced from the pivot 27, the lifting rod 28 is pivotally
co~nected via a pivot 46 with a co~necting rod 34, which
is in turn pivotally co~nected with a moment arm 35 via
another pivot 47. The moment arm 35 itself is moment-con-
nected with a horizontal shaft 36 which is rotatably
mounted in the planet housing lO. A screw spring 37 is
wound around the shaft 36 and is biassed by a spring
force, which acts on the lifting rod 28 in the lifting
direction via the moment arm 35 and the ~o~- e.-ting rod 34
and has a sufficiently great strength to lift the scraper
blade 29 above the level of material N.
A first toothed segment 38 is fixed on the end of the
shaft 26 disposed opposite the moment arm 35 and engages a
s~conA toothed segment 39 which is fixed on the underside
of a toothed rim 40. This tooLhed rim 40 is in turn rotat-
ably mounted on the planet housing lO in an area below its
21 74639
~_ wossllll20 PcT~K94100388
- 13 -
toothed rim 16. The toothed rim is formed with a slot 44
which extends concentrically with the LooLhed rim 40 and
the planet housing 10. A pin 45, which, as shown best in
fig. 5, extends freely up into the slot 44, is moleGver
fixed on the planet ho-lQi ng 10 . The rotation of the
toothed rim 40 with respect to the planet housing 10 is
hereby restricted to an angle whose size is determined by
the relative displacement of the pin 45 in the slot 44
being xLu~ed by the slot ends 44a and 44b, respectively.
A vertical shaft 41 is rotatably mounted on a stationary
part of the mixing vessel. A gear wheel 42 engaging the
toothed rim 40 is secured on the lower end of this shaft.
The upper end of the shaft 41 is affected by a dynamic
lS brake 43.
As shown in fig. 4, this brake is mounted on the top plate
11 which forms a fixed component of the mixing vessel 2.
The brake 43 with A-~OCi ated parts therefore do not parti-
cipate in the rotation of the planet housing 10 in opera-
tion.
In the case shown, the brake consists of a pneumatic or
hydraulic drive cylinder 48 having a piston 49 which can
be moved axially up and down in the cylinder 48, but is
fixed against rotation with respect to it by means of a
key and keyway connection 53. When the drive cyli~r 48
is relieved of pressure, the piston 49 is pushed up into
the upper end of the cylin~r 48 by a compression spring
52. The piston serves as a first brake disc 49. The upper
end of the shaft 41 is shAp~ as a ~econ~ brake disc 50
having a brake l~ n~ ng 51. The drive cyli nA~r receives
compressed air or oil via a conduit 55 which is uo~ ected
with the drive cylinder by means of a union 54. Further, a
valve 56 is inserted into the co~ t 55.
Wos5/lll2o PCT~K~4/00~ _
2 1 7463q
- 14 -
The brake described above may also be arranged in the
manner that the piston or the first brake disc is mounted
axially slidably, but fixed against rotation on the upper
end of the shaft 36, while the ~conA brake disc is sta-
tionarily secured in the drive cyl i nA~r. The drive cylin-
der shown in fig. 4 is single-acting with a compression
spring 52 for the return movement. Instead, a double-
acting cylinder may be used for displ A~i ng the piston in
both directions.
In addition to the brake structure described above, also
other brake types may be used, e.g. hydraulic brakes or
magnet brakes. However, the brakes must be dynamic in all
cases. They must therefore e.g. have two brake discs, one
of which is to be mounted fixed against rotation in the
brake and the other fixed against rotation on the shaft
41. When the brake is activated, and the brake discs have
hereby been engaged mutually, they must be capable of
transferring a sufficiently great frictional moment to
drive the scraper blade 29 via the scraper mechAnism down
through a concrete mix in the vessel to its bottom 4
against the action of the screw spring 37. However, the
frictional moment must not be so great as to defeat this
purpose, since the rotation of the planet housing 10 would
then be imp~A~A when the pin 45 hits the end 44b of the
slot 44, and the planet housing 10 thereby brings along
the toothed rim 40 in the rotary movement. Owing to the
gear wheel engagement between the toothed rim 40 and the
gear wheel 42, the shaft 41 now also begins rotating, so
that the two brake discs will rotate slidingly with re-
spect to each other and overcome the frictionAl moment
which occurs because of the applied brake force.
It is noted that a brake moment similarly enabling a mu-
tual movement between two brake parts can also be obtAine~
in other ways, e.g. by means of a pump in a liquid circuit
21 7463~
woss/11120 PCT~K94/00388
which is throttled when the brake is to be activated.
When a charge of material has been mixed to ready con-
crete, the ~ ~chArge op~n~ ng 8 in the bottom 4 of the
vessel is opened, and also the valve 56 is opene~, so that
the piston or the first brake disc 49 is moved down to
frictlonAlly touch the brake 1 ~ n; ng 51 on the ~eço~ brake
disc 50. The two brake discs are now pressed against each
other by a pressure which is determined by the pressure
over the first brake disc 49 and its diameter. The braking
moment is transferred via the seco~A brake disc 50, the
shaft 41 and the gear wheel 42 to the toothed rim 40. This
stops the rotation of the toothed rim 40 with respect to
the planet housing 10, which cont~n~les to rotate.
Till then, the scraper blade has been present in its upper
position over the level of material N. This position is
always maint~ ne~ when the brake is not activated. The
biassed screw spring 37 will then have rotated the moment
arm 35 in a clockwise direction (fig. 5) and will thereby
have lifted the pivotable rod 28 and thus the scraper
blade 29 above this level via the ço~neçting rod 34. The
first ~oo~hed segment 38 has simultaneously been rotated
clockwise, so that the toothed rim 40, because of the
toothed engagement between the first and the -4~' :V~
~ooLhed segments 38, 39, has been rotated counterclockwise
with respect to the planet housing 10 until the pin 45
hits the end 44a of the slot 44.
When the toothed rim 40 is braked by the braking effect
which occurs as described above when the brake 43 is ac-
tivated, the toothed rim 40 now rotates clockwise with
respect to the planet housing 10 until the pin 45 hits the
other end 44b of the slot 44.
wogs~ 20 2 i 74639 PCT~K94/00388
- 16 -
During the travel performed by the pin 45 between the two
ends 44a, 44b of the slot 44, the first toothed segment
now rotates counterclockwise b~cA~ce of the toothed en-
gagement with the ~econ~ toothed segment 39 on the toothed
rim 40, so that the scraper blade 49 is lowered to the
bottom 4 of the vessel against the spring force of the
screw spring 37. The lifting height of the scraper blade
is determined by the length of the slot 44.
As will be seen, the scraper blade 29 is maintA~ ne~ in its
upper position by means of a passive force applied by the
biassed screw spring 37 when the brake is not activated.
This is the case when the concrete mixer stands still and
during the part of the working cycle where charging and
mixing takes place. The scraper mechAn~sm uses no energy
during this. On the other hand, when the brake is acti-
vated, the scraper blade is driven down to its lower posi-
tion by means of an active force which is transferred from
the planet gear and thereby from the motors 12. In this
connection the brake just acts as a servomechAn~sm.
A certain frictional loss between the brake discs occurs
during the ~chArge process, but this loss is rather mo-
dest since the brake, in its capacity of a servomechAn~sm,
does not need any great braking pressure, and since the
~chArge process only accounts for a minor part of the
overall mixing cycle time.
A particular advantage obtA i ne~ by means of this sPcond
embodiment of the scraper me~han~sm is that it is only
neceSSAry to feed power to the brake in order to activate
the scraper mechanism. Since the brake is stationarily
mounted with respect to the mixer, the cumbersome rotat-
able pipe joint required in order to feed power to the
first scraper mechanism embodiment shown in figs. 1 and 2
is avoided.
~ WO95/11120 21 7463~ PCT~K94/00388
As mentioned before, the scraper me-rhAnism of the inven-
tion provides a considerable reduction in the time it
takes to empty the mixing vessel completely. This time can
be redl~ce~ further by mounting two or more scraper mecha-
nisms in the mixing vessel instead of just one.
The invention has been exemplified above by means of a
concrete mixer. However, the invention may be applied in
~,o~ e~-tion with many other mixers and for many other mate-
rials where rapid and complete emptying of the mixingvessel is desired.
