US 3760455 A
A door check includes a cam coupled to the door and having opposite profiles against which a follower on one end of a lever is stressed by a spring, the other end of the lever actuating a damping cylinder. Particular shapes for the cam and for the orifice for escape from the cylinder are shown.
Claims available in
Description (OCR text may contain errors)
United States Patent 9] Berry et al.
[ Sept, 25, 1973 DOOR CHECK Societe dEtudes Verrieres Appliquees, Neuilly-sur-Seine, France Filed: June 3, 1970 Appl. No.: 43,613
 Foreign Application Priority Data June 4, 1969 France 6918375 52 us. cr 16/53, 16/55, 1-6/51  Int. Cl. E051 3/10  Field of Search 16/51-62; 251/208; 137/360; 188/310  References Cited UNITED STATES PATENTS 1,307,502 6/1919 Knudson 16/57 1,313,763 8/1919 Thomas.... 16/57 1,947,637 2/1934 Bolster 251/208 X 1,449,652 3/1923 Capra 16/58 679,905 8/1901 Ocumpaugh 16/62 1,327,850 1/1920 Ziemer 137/360 X 1,151,326 8/1915 Anderson. 188/310 2,752,627 7/1956 Carlson 16/55 2,688,150 9/1954 Roussel 16/52 1,833,121 11/1931 Norton 16/56 X 2,493,117 1/1950 Diebel 16/58 X 1,871,030 8/1932 Bommer 16/55 3,273,196 9/1966 Voster et a1. 16/55 X 1,520,765 12/1924 Norton 16/55 UX 2,911,210 11/1959 Ferguson 16/55 X FOREIGN PATENTS OR APPLICATIONS 342,973 2/1931 Great Britain 16/55 850,026 9/1960 Great Britain 16/55 538,942 8/1941 Great Britain 16/55 1,421,530 11/1964 France 16/55 OTHER PUBLICATIONS Merriman-Wiggin, Civil Engineers Handbook, Wiley & Sons, Inc., 1930, Pages 1315-1318.
Elementary Fluid Mechanics, John K. Vennard, 1947, 2nd Ed., Wiley & Sons N.Y., QA 901 V4.
Primary ExaminerFrancis K. Zugel Assistant ExaminerPeter A. Aschenbrenner Attorney-Fannie, Edmonds, Morton, Taylor & Adams [5 7] ABSTRACT A door check includes a cam coupled to the door and havingopposite profiles against which a follower on one end of a lever is stressed by a spring, the other end of the lever actuating a damping cylinder. Particular shapes for the cam and for the orifice for escape from the cylinder are shown.
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Pierre Rene Berry BY Romon Fernandez .7 MiG/4... ATTORNEYS DOOR CHECK The present invention pertains to door checks or door closers to close a hinged door. The check of the invention may be constructed to close a door swinging open in either direction from a closed position. it includes a hydraulic braking element which according to one feature of the invention is substantially independent of ambient temperature in its operation.
The door closer of the invention comprises a helicoid compression spring operating on a lever which carries a follower bearing against a cam which is fixed with respect to the door. A hydraulic brake for damping the motion of the door includes a piston which is coupled to the lever, the piston working in a cylinder having a thin wall with a throttling orifice therethrough for the escape of hydraulic fluid from the cylinder on closing motion of the door. The entire mechanism of the door check is enclosed'within a casing which may be integral in construction. I i
In door closing apparatus of the type to which the invention relates, the spring must be quite stiff so that its stress increases rapidly as the door is opened. The result is that if the door is coupled directly to the spring so that with increasing open angle of the door the deformation of the spring increases, the effort required will increase with departure of the door from closed position. In accordance with one aspect of the invention, the coupling between the door and the spring is made to possess a changing drive ratio so as to compensate or correct for this tendency.
,lf friction is neglected it is possible to write:
F/C dB/dl w/v a wherein:
C is the restoring couple or torque; d6 is a small variation in the angle of the door; to is the angular speed of opening or closing; F is the stress of the spring;
: d! is the variation in elongation of the spring corresponding to d0; l v is the linear speed of compression or expansion of the spring; and a is the drive ratio. 1 It will be seen that by choice of a suitable function for the relation of a to 0 it is possible to modify the value of C as a function of 0, i.e. to modify the effort required at various values of 0 from the person opening the door in order to open it, consistently with retention of a desired restoring force at the closed position of the door, i.e. of the starting torque required to displace the door from its closed position, so as to insure that the door will fully close itself when released. It is thus possible to obtain with cams having a simple shape a more agreeable variation in the effort required of persons passing through the door and indeed to provide that that effort will go down as opening of the door increases.
In actuality, by reason of friction the torque Cr required to be applied to move the door toward greater open angle, and which is opposed by the mechanism of the door check, is greater than the theoretical torque C indicated by the calculation. On the other hand, during closing motion of the door the torque Cm available from that mechanism is smaller than the calculated value. Cm corresponds to the difference between the to overcome friction. It is customary to describe the efficiency by the average value of the ratio Cm/Cr 17.
Thedriving torque Cm makes it possible to define a fictitious available driving force or stress Fm for the spring. When the user lets go of the door this force must be sufficient to overcome rapidly the inertia of the door so as to have the door closed within a few seconds. It is however necessary to limit the speed of the door in the vicinity of its closed position. It is hence necessary to add to the spring a brake. This takes the form of a hydraulic damper including a piston working in a cylinder. Once the door has been set into motion, the throttled passage of oil out of the cylinder through an orifice in the cylinder absorbs most of the energy of the psring so as to limit the speed of the door to a satisfactory value.
More exactly, it is possible to write as follows:
.S is the effective cross-section of the damping piston;
w is the speed thereof; Ap is the pressure drop through the orifice; and y is the drive ratio between the spring and the damper. The rate Q at which hydraulic fluid is expelled from the brake cylinder by the piston on door closing is directly proportional to the speed of the piston thus:
By reference to the preceding equation it is thus possible to write:
w 'YQ/ and secondly,
Ap yFm/S However, for a given orifice, the rate Q is directly dependent on the pressure drop Ap, so that y depends on Fm via the parameters a and y.
In the door checks of the prior art the orifice, which becomes effective at the end of the closing motion of the door, takes the shape of a thin annular slot between a hole and a conical adjusting stud adjacent thereto. The effective size or section of this opening is not negligible, so that turbulence in the flow therethrough will be low. Its length is large or at least of the same order of magnitude as this width or effective dimension of the slot.
In these prior art devics the pressure drop is thus due primarily to viscous friction of the fluid against the walls bounding the annular orifice. Given the high viscosity p. of oils, the flow is laminar whatever the rate of flow Q, and the loss of pressure Ap can be expressed by analogy to Poiseuilles law as:
Ap 6 .tl/1'rra wherein:
l is the axial length of the annular opening, i.e. in the direction of flow therethrough;
r is the mean radius thereof; and
a is the width of the slot, i.e. radially.
It is thus possible to write as follows:
That is to say that in these prior art devices, the closing speed of the door is directly related to the viscosity and hence to the temperature of the oil. This means that a compensating thermostat will be necessary. Moreover, this closing speed depends importantly on the width of the slot and on variations therein due for example to the deposit of foreign matter therein. Impurities in the oil will likewise have a substantial efi'ect on the door closing speed. Lastly, this closing speed depends on the available force Fm such that the closing time may be substantially dependent upon undesired friction.
It is however important not to impose the braking or damping too early nor too suddenly, since to do so would make it necessary to provide initially an auxiliary circuit or to use an opening of variable section so as to avoid the undesired effects of too high a final closing speed for the door or of too long a closing time.
The present invention is intended to surmount these difficulties in door checks.
According to one aspect thereof, the door check of the invention includes a particular shape for the orifice, the pressure drop across which is due to turbulent flow. This orifice is formed in a thin walled diaphragm, and it constitutes a throttle or constriction whose length is substantially less than its diameter, instead of taking the shape of a narrow channel of narrow width obstructed by an adjusting element.
By this means the invention makes negligible the loss in pressure created by friction with the walls and leaves simply the classical turbulent pressure loss:
Ap ku /2 wherein u represents the speed of the fluid. This can be rewritten as follows:
AP A (Q/ab) wherein a and b are the principal dimensions of the orifice. It will be observed under these conditions that:
Such a solution seems a priori less desirable. In fact however the speed depends less on the dimensions of the orifice, so that adjustment is facilitated. This speed is in fact proportional only to the square root of the available force so that the closing speed is less influenced by variations in friction which can arise. In particular, it is independent of oil viscosity and hence of temperature. The consequence is that it is possible to do without the thermostat, without being obliged to modify the door check for temperature changes or the season of the year and to use an ordinary oil of relatively low viscosity.
Moreover, at the start of the closing operation the rise in speed is greater, since the pressure drop Ap varies with the square and not with the first power of the rate of flow Q. Since the orifice is not annular, it is possible to obtain a sufficient pressure drop without unduly reducing the cross-sectional dimension a of the orifice. To increase the coefficient of pressure drop it is advantageous for the diaphragm to have sharp edges.
According to a further feature of the invention, the door check may include a brake whose piston is directly coupled to the spring restoring or actuating lever while the orifice is stationary, being formed in a wall of the cylinder, only one orifice being provided. These two characteristics seem in principle to be contradictory since as a result thereof during closing of the door the damping or braking of the spring is substantially constant.
In fact however, referring again to equation (3'), the coefiicient A takes on only a single value and the drive ratio y remains practically constant. However, the speed of closing to still varies as the product av Fm, such that a suitable choice of the spring and of the cam profile makes it possible to take advantage of the constancy in time of the turbulent flow in order to simplify the construction without reducing the quality of the result. The throttling orifice is of intermediate size, making easier the formation and measurement thereof.
According to a further feature of the invention, the spring and indeed the entire mechanism are enclosed in an integral housing where all of the elements are immersed in the fluid of the turbulent flow damper. The housing comprises on one side a plate which closes off the cylinder ahead of the piston, except for the orifice. The housing also includes the bearings for a pivot pin on which the door rests and by which its motion is coupled to the mechanism of the check. In addition use of a brake employing oil as a fluid makes it possible to provide permanent lubrication of the moving parts of the door check.
The torque Cr opposes opening of the door and must be overcome to open it. For this reason, and having regard to the necessity of providing a starting torque sufficient to hold the door closed against such forces as wind for example, it is desirable to give to the variation of Cr with door angle a shape declining as rapidly as possible. This means that a should increase rapidly as the door is opened. Moreover, it is desirable to reduce friction so as to diminish the average effort to be applied in opening the door, consistently with maintenance of a sufficient torque Cm, and to reduce the necessary energy, i.e. the size of an entire apparatus. The value of the driving torque Cm becomes less variable and unpredictable since it is less influenced by undesired factors such as friction at bearing surfaces, and the reversibility of the door is thereby increased. It is only a high efficiency which makes it possible to give to the function Cr versus 0 a declining characteristic and indeed to increase the starting torque by taking advantage of the fact that the supplementary effort thus required will be short. Under even the best circumstances the solutions of the prior art do not make it possible to exceed efficiencies of 50 percent nor hence to reduce the final torque, i.e. that required to complete opening of the door, to less than 60 percent of the starting torque.
A first embodiment of the invention employs a restoring torque which rapidly declines with door angle 0. In this embodiment the spring and the brake are mounted in the usual fashion and are disposed on opposite sides of the axis of the restoring lever. The articulation of the spring to this lever is provided by a knife edge and groove or socket combination, made of hardened steel, similar to those used in balances for weighing. This arrangement, differing from knuckle joints heretofore employed, makes it possible to reduce friction and also holds the force of reaction exerted by the lever on the spring in the axis of the spring in such fashion that there will be a reduced tendency for the spring to deflect laterally. I-Inece guiding friction on the spring is substantially reduced. Moreover, the principal pivots may be provided with needle bearings. This improves the reversibility of the door, i.e. its ability to close in the same position no matter from which side it had been opened.
The door check according to the invention thus possesses exceptional efficiency since the quantity '1 does not fall below 60 percent over the whole length of the door motion.
It is possible according to a still further feature of the invention to give to the torque door angle curve a rapidly declining shape, the theoretical torque C declining to below 80 percent of its initial value after a 20 motion of the door and to below SO percent after a 90 motion.
As hereinabove' indicated this decline, obtained by giving to the cam a very high and then declining rise adjacent the closed position for the door so as to make the coefficient a increase rapidly and uniformly, is highly advantageous. It makes it possible to improve both closing and ease of opening. Nevertheless, it is not al ways acceptable especially for outside doors which are not lockedand in which the starting torque Cr is employed to prevent opening of the door under wind action. In such case one may employ a cam with a relatively high rise and give to the spring a long motion in order to obtain a greater variation in the force F. In another embodiment of the invention this result is obtained by reversing the mounting of the spring and by shifting the pivot axis of the lever in such fashion that the points of attachment of the spring and the brake are on the same side of that pivot. The spring is then mounted in compression between a traction member and a stirrup which opposes it and which is pivoted in the usual manner.
The invention will now be further described in terms of a number of exemplary embodiments and with reference to .the accompanying drawings in which:
.FIG. I is a plan sectional view of a door check according to the invention; 7
FIG. 2 is a sectional view in elevation along the line IIII of FIG. 1;v v
FIG. 3 is an axial sectional view of the damping piston in the check of FIGS. 1 and 2;
FIG. 4 is a sectional view of the aperture taken along the line IV-IV of FIG. 1;
FIG. 5 is a set of curves useful in describing operation of the door check of FIGS. 1 and 2;
FIG. 6 is a plan sectional view similar to that of FIG. 1 but showing a modified construction of the door check of the invention;
FIG. 7 is a sectional view in elevation along the line II-II of FIG. 6; and
FIG. 8 is a set of curves illustrating the operating characteristics of the door check of FIGS. 6 and 7.
Referring to FIGS. 1 and 2, in one embodiment thereof the door check of the invention comprises an outer housing 1 having a cover 2 fastened to thehousing at the lugs 3. The housing with its cover is intended to be disposed beneath the floor at the location of the door, at the hinged sidethereof. The operating mechanism of the door closer is further disposed within a casing 4 closed with a cover plate 5 and with two additional closure members 6 and 7 which constitute bearings for the pivot pin 8 on which the door is hinged.
The casing can be adjusted in position with reference to the housing I by means of abutment screws 9 for adjustment in the horizontal plane and by three abutment screws 10 for adjustment in vertical planes. The casing can then be locked in position with respect to the housing by means of the bolts 12 and the right-angle member llll seen in FIG. 2.
The interior of the casing 4 provides two substantially parallel spaces of approximately cylindrical shape. The first of these contains a helicoidal compression spring 13 which may be made as shown of two parts having unlike spring constants. The spring is supported at the right end, as seen in FIG. 11, on an adjustable abutment 14 supported at the cover 5 to the casing 4. The other end of the spring 13 bears against and supports an abutment member 15 which bears against a lever 16 pivoted at a pin 17 fixed in the casing 4 for rotation of the lever about a vertical axis. One arm of the lever, namely that against which the spring 13 bears, carries a follower 18 which is stressed by action of the spring against a horizontal cam 19 fixed to the pivot pin 8 of the door. The other arm of the lever is pivotally connected to a connecting rod 16' which is connected in turn' to a damping piston 20 working in a cylindrical cavity formed in the casing 4. It will be seen that as the compression spring 13 expands to conditions of lower stress, the piston 20 will be driven to the right as seen in FIG. I and vice versa.
The door pin 8 has affixed thereto a second cam 21 cooperating with a follower 22 also disposed on the lever 16.
As indicated in the sectional view of FIG. 3, the piston 20 has two passages extending through it with valves in those passages. In particular, a ball 23 imprisoned behind a cage 24 permits the flow of hydraulic fluid through the piston from left to right as seen in FIG. ll. This permits the cavity 27 to the right of the piston in FIG. 1 to fill with oil as the piston moves to the left. A ball 25 held against its seat by an adjustable spring 26 constitutes a safety valve'which permits hydraulic fluid to flow through the piston in the opposite direction upon the build-up of excessive pressure on the right side of the piston.
A partition within the casing defined by the elements 4 and 5 closes off a space 27 at the right of the piston from the remainder of the interior of the casing 4. The chamber 27 is thus bounded in part by a wall 27. The wall 27 has an opening therein which is closed by a hollow cylindrical plug 28. The plug, shown in detail in FIG. 4, has apertures 30 and 31 through its opposite walls. The smaller, of these apertures 30 is formed at the bottom of a counterbore 32, so that the aperture 30 exists in a thin wall and has an axial length (perpendicularly to the axis of the plug), for example 0.3 mm., small by comparison with its diameter which may be of the order of 1 mm. A threaded closing member 29 is formed with a conical cavity at the lower end thereof and can be adjusted vertically within the plug 28. The lower edge of the member 29 thus constitutes a blade which according to the adjustment of the member 29 effects greater or less obturation of the aperture 30. Otherwise stated, the member 29 provides a thin, sharp-edged diaphragm which can be advanced adjustably across the aperture 30.
Referring again to FIG. 1, the member rests at a knife edge against a complementarily slotted abutment member 33 affixed to the lever 16. The knife edge and its socket on the member 33 may desirably be made of hardened steel.
By reason of this articulation of the spring 13 to lever 16, the friction between the two is reduced and the reaction of the lever is. effected substantially along the axis of the spring which diminishes the tendency of the spring to flex out of line and consequently reduces its friction against the walls of the casing 4.
The door pivot 8, the lever 16 and the roller 18 may be provided with ball or needle bearings as seen in FIGS. 1 and 2, to improve the operation of the apparatus and the uniformity of its operation in both directions of door opening.
The casing 4 is filled with fluid oil and is provided with seals or gaskets as indicated in the drawings.
FIG. 1 shows the door check in closed position. The door being openable in either direction, the cam 19 has two slopes of the same or similar profile so that'the operation of the door and also that of the lever 16 is the same in either direction.
The cam profile includes a concave portion 34 in which the follower 18 rests when the door is in closed position. To the two sides of this hollow portion there extend rapidly rising portions 35 which are substantially symmetric in the plane of the door.
If the door is opened clockwise for example, as seen in FIG. 1, the pivot 8 will entrain the cam 19 in the same sense. Follower 18 is repelled by the cam and rises along the convex cam surface 35. Lever 16 is thus forced to rotate counterclockwise about its axis 17 and to compress the spring 13 by operation of the coupling at 15, 33. More particularly, each of the cam surfaces 35 includes initially a neutral zone extending over a few degrees and having a substantially constant slope. This makes it possible to give a fixed, known value to the force holding the door in closed position. The cam then changes slope so that the torque-angle characteristic declines with further opening of the door and notwithstanding the increasing stressing of the spring.
After a rotation of the door through 20 the restoring torque is reduced to 50 percent of its initial value and after being rotated through 90 the torque is reduced to about 35 percent of its initial value.
While the spring 13 is being compressed the lever 16 carries the piston 20 to the left as indicated in FIG. 1. Under these conditions the valve 23 opens and permits oil to pass freely into the chamber 27 of the damper or brake.
The useful part of each cam surface is terminated with a notch such as 36, provided to cooperate with the follower. The notches 36 function as detents to hold the door in completely open position, for example after rotation of 105 from the closed position of the door. To reach this position it is sufficient to push the door far enough so that the follower l8 falls into the notch.
If the door is let go before it reaches this position or if it is pushed to get the follower out of the notch, the energy stored in the spring will effect rotation of the cam, pivot pin 8 and door toward the closed position. By virtue of the precautions taken to reduce friction the door will rapidly accelerate despite the relatively low value of the closing torque. The spring distends and the lever 16 rotates clockwise. The movement of the lever is transmitted by the rod 16' to the piston 20 of the damper. The valve 23 immediately closes and the piston forces the oil out of chamber 27 through the orifice 30. The energy absorbed by this flow rapidly increases and the loss of pressure resulting tends to op- Pose the force of the spring.
Given the shape of the orifice 30 and the short axial length of its walls, the flow is practically free of laminar flOw properties. As hereinabove described, the braking which results is very stable and does not depend on temperature.
As has already been shown, the torque available to close the door increases as the door returns toward fully closed position but the Speed of closing as limited by the damper tends on the contrary to diminish. The cam 21 and the follower 22 which is available to engage it impose any necessary retarding force on the door if the door tends to overrun the check, and to insure controlled, progressive motion of the door to closed position.
The torque-angle characteristic is sufficiently declining so that it is not necessary to provide a supplementary orifice for escape of oil from the chamber 27 to be operative at the start of the closing motion.
The door check operates in exactly the same way when the door is opened in the opposite sense, i.e. counterclockwise. The slight inequality of the two profiles of cam 19 visible in FIG. 1 is simply due to the need to compensate for the fact that the motion of the follower 18 is circular. By making the two profiles slightly different it is possible to obtain two identical closure, i.e. torque-angle, characteristics.
The valve 25 does not come into play except in the event of malfunction which makes it necessary to relieve excess pressure in the chamber 27.
FIG. 5 shows, in arbitrary units along the vertical scale, a graphical representation of the principal characteristics of the door check of FIGS. 1 and 2. The axis of abscissae represents compression of the spring but is also graduated in degrees of opening of the door.
The strength or the stress of the spring, which increases linearly with compression thereof, is indicated by the curve F. By virtue however of friction, the available closing force varies according to the curve Fm.
Curve C represents the torque exerted by the check on the door in the absence of friction in the check. The maximum value thereof is rapidly attained in the vicinity of the closed position of the door. For a door of normal dimensions this is adjusted, e.g. at the element 14 in FIG. 1, to a value of about 45 Nm such that the door will not oscillate about its closed position and resists well the effect of wind. The effort necessary to be applied to the door to open it is relatively high but it need be applied for only a short time, and the consequence is an agreeable sensation for the user since the force required of him rapidly and smoothly declines as his arm moves the door.
Again due to the existence of friction, the torque Cr required to be applied to open the door varies according to the curve Cr and the driving torque available from the check to close it varies according to the curve Cm.
It can be observed that the minimum value of the efficiency 1;, which is obtained at full opening of the door, is still above 60 percent. This remarkable result is due primarily to the adoption of a blade and groove articulation between the spring 13 and lever 16 and to the use ing lever, this speed is relatively high at the start of closing (i.e. at high values of and diminishes sufficiently with closure so that without increase of the closing time the door comes slowly to rest. Closure is thus assured in six seconds without high stress at the end thereof.
The embodiment of FIGS. 6 and 7 employs a construction similar to that described herein with reference to FIGS. 1 and 2, corresponding elements of structure having applied thereto the same reference characters. In the embodiment of FIGS. 6 and 7, the pivot 17 for the lever 16 is displaced to the side of the casing 4 so that the coupling at 37 of the link 16' and at 38, of the spring 13 are side by side. This permits a large change in compression of the spring, over the course of motion for the door, with an initial low compression of the spring when the door is closed. In order to preserve the integral construction of the casing and to keep the pressure chamber 27 and the throttling plug 28 at the end of the casing, the mounting of the spring has been inverted. The spring is compressed between a washer 14, at the end of the rod 39 adjustably screwed to the housing, and the washer 15 which is fixed to the stirrup 40. This stirrup in turn is pivoted at 38 to the lever 16. The rod 39 and the stirrup 40 are hence subjected to tension by the spring 13.
The second cam 21 of FIGS. 1 and 2 is absent, the portions of the single cam 19 remote from the parts thereof engaged by follower 18 being engageable with a follower 22 having the same function as the follower 22 of FIG. 1.
The lever is made of two plates 16a and 16b fastened together to provide the various pivots 17, 18, 37 and 38. The cam .19 and the stirrup 40 pass between the two plates of the lever.
The pivot 38 is located in position to remain out of the way of the cam 19 for all positions of the cam and lever. The knife edge and groove articulation 15 and 33 of FIG. 1 is not necessary with the large displacement of pivot 38 from the washer 15 of the spring, because Mil this large displacement reduces the tendency of the spring to flex laterally. The frictional losses are smaller than in the devices of the prior art and this insures a high efficiency.
FIG. 8 shows the characteristics obtained with this construction. It will be observed that the decline in the torque C with increasing 0 is smaller than that of the embodiment of FIGS. 1 and 2. Indeed this torque, and hence the effort required of the user to open the door, increases as the door approaches fully opened position. The closing speed to however has a characteristic similar to that of the embodiment first described.
1. A door check comprising a casing, a pivot pin for adoor joumaled in the casing, the pin and door being engageable together for the transmission of torque between the two, a cam affixed to the pin, a lever journaled in the casing, a follower arranged on the lever for engagement with the cam, resilient means engaged between the casing and lever stressing the follower against the cam, means defining an enclosure fixed with respect to the casing, said enclosure having a cylindrical portion, a piston slideable within said cylindrical portion, a link coupling the piston to the lever, a cavity communicating with one end of said cylindrical portion, and means defining an orifice through the wall of said cavity in said enclosure, said orifice having a crosssection whose maximum dimension is greater than the length of said orifice, said orifice-defining means comprising a hollow cylindrical plug disposed in the wall of said enclosure substantially parallel with said wall, said plug having aligned apertures through the walls thereof giving access between the interior and exterior of said enclosure, each of said apertures having a cross-section whose maximum dimension is greater than the thickness of the walls of said plug, said orifice-defining means further comprising a cylindrical closure member adjustable lengthwise within said plug for adjustable partial obturation of said apertures, said closure member having a concavity in the end thereof adjacent said apertures whereby the said end of said closure member constitutes a blade for partial obturation of said apertures.