US3664151A - Spindle for centrifuges - Google Patents

Abstract

A spindle which forms part of a suspended centrifugal and is mounted in the machine frame at its upper end. The bowl or basket is attached to the lower end of the spindle and the latter tapers from the lower toward the upper end so that its flexural strength is at least substantially constant all the way from the hub of the bowl or basket to the bearing in the frame.

Description

United States Patent Pause [451 May 23, 1972 SPINDLE FOR CENTRIFUGES References Cited [72] Inventor: Kurt Pause, Grevenbroich, Germany UNITED STATES PATENTS [73] A i M hi f bfik B k W lf m 2,213,004 8/ 1940 Hickman ..267/57 gesellschafl Grevenbroich Gennany 96,186 10/1 B8110] 1 0/366 [22] Filed: 1969 Primary Examiner-Benjamin W. Wyche [2]] Appl. N 866,882 Assistant ExaminerRanda.ll -lea.ld

At'torneyMichael S. Striker [30] Foreign Application Priority Data [57] ABSTRACT Oct. 16, 1968 Gennany ..P 18 032 49.0 A Spindle which forms part of a Suspended centrifugal and is mounted in the machine frame at its upper end. The bowl or basketisanached tothelowerend ofthe spindle and thelauer tapers from the lower toward the upper end so that its flexural [58] Freld of Search ..64/1, 210/366, 267/57 Strength is at least substantially constant a the way from the PEDTL BERANL'E OF Q\ F A hub of the bowl or basket to the bearing in the frame.

2 Claims, 1 Drawing Figure SPINDLE FOR CENTRIFUGES BACKGROUND OF THE INVENTION The present invention relates to centrifuges in general, and more particularly to improvements in spindles for centrifuges of the type wherein the spindle is designed to support the basket or bowl in such a way that the basket or bowl is suspended on the spindle and can perform swinging or pendulum movements. Suspended centrifugals are typical of such classes of centrifuges.

Swingably mounted baskets or bowls are preferred in centrifuges wherein the load is likely to produce unbalance. This is the case in centrifuges which are used in sugar refineries and which are operated periodically, i.e., whose rotational speed varies so that the components must stand repeated acceleration and deceleration, for example, between a minimum and a maximum rotational speed or even between zero speed and a maximum speed or vice versa.

The rotary parts of a centrifuge with a swingable bowl or basket constitute a so-called symmetrical heavy gyroscope or top. Thus, the center of gravity of the gyrating system is located below the point of suspension. When in rotary motion, the system constantly tends to maintain the moment of inertia of its mass in a position which is symmetrical with reference to the axis of rotation. Based on the physical law of impulse preservation, the gyrating system compensates for each interference due to an unbalance by producing an oppositely directed gyroscopic couple of equal magnitude.

If a sudden or gradual unbalance develops in the plane of the center of gravity of the gyrating system, the basket tends to move in parallelism with the axis of the revolving structure. The newly formed axis of rotation is then parallel to the original axis. If the unbalance develops outside of the center of gravity plane, the basket tends to establish a new axis of rotation which is inclined with reference to the original axis.

Of course, the spindle of the centrifuge opposes such shifting of the axis of rotation because one of its ends is joumalled in the frame of the machine. Consequently, the basket must flex the spindle in order to establish a new axis of rotation. The flexing is terminated when the bending moment (which equals the gyroscopic couple) is balanced by the moment produced by the bending stress upon the spindle. The force which counteracts the bending moment due to correction for unbalance is transmitted to and is taken up by the frame or foundation of the machine. Such counteracting force is transmitted through the bearing means for the spindle, i.e., through the center of the gyrating system, and it travels in the frame at the rotational speed of the basket to excite" the frame to the extent determined by the characteristic deformability or plasticity of the frame. Such excitation of the frame can produce resonance, especially since the rotational speed of the gyrating system normally covers the full range from zero speed to maximum speed. If the frame is caused to perform resonant vibratory movements, the center of the gyrating system begins to move in space which, in turn, initiates swiveling movements of the system. Thus, the axis of the gyrating system travels along a conical path at a frequency which is a function of the mass and of the distance between the center of gravity and the center of the gyratory system.

Depending on the circumstances at which the resonance develops, the swiveling movements of the gyrating system are either damped or amplified. Such swiveling movements are invariably assisted by friction which develops at the swingable bearing cup; at the very least, such friction delays the damping of vibratory movements and the return of the frame to quiescent condition.

The just described swiveling movements of frames in centrifuges of the suspended basket type are undesirable because the dimensioning of centrifuges limits the extent of such movements to a rather narrow range. Thus, the revolving parts are likely to rub against stationary parts whereby the gyrating system reacts by jumping or knocking as soon as the rotational speed decreases in response to frictional engagement between stationary and rotary components. The thus released energy causes changes in shape, flexing of the spindle, damage to the protective shell and/or to the basket, or complete destruction of the machine.

Based on the above considerations and experiences, the art has developed certain types of spindles for use in suspended centrifugals or like machines. Such spindles were designed to exhibit sufficient flexibility in view of anticipated stresses due to unbalances and also to offer to bending stresses a resistance which is less than that at which the forces travelling in the support for the spindle would attain an undesirable magnitude. As a rule, presently known spindles are of cylindrical shape and their diameter is constant all the way from the point of suspension in the frame to the point of fixedly mounting the spindle in the hub of the basket. The bending moment is zero at the point of suspension and is greatest at the point where the spindle is mounted in the hub.

Since the trend in the refining of sugar and in analogous industries is toward largest possible sizes of baskets which can take up ever increasing quantities of material and are driven at very high speeds, the bending moments acting on the spindles of such machines are extremely high even if the basket is dynamically balanced. The only presently known solution is to use a stronger spindle, i.e., to increase the diameter of the spindle. Of course, this enhances the resistance which the spindle offers to bending stresses and causes transmission of greater forces to the frame or foundation of the centrifugal machine. Consequently, the recently developed large centrifugal machines with sturdy spindles are less stable than earlier machines employing weaker spindles and smaller baskets.

The designers of centrifuges are thus faced with two conflicting problems. On the one hand, the moment of resistance of the spindle is to be increased to avoid excessive bending and eventual destruction or danger of accidents with oversized centrifuges. On the other hand, a stronger spindle offers a greater resistance to bending with the result that the frame is subjected to excessive stresses; this again increases the likelihood of damage, accident or destruction.

SUMMARY OF THE INVENTION An object of the present invention is to provide a novel and improved spindle for use in suspended centrifugals and analogous centrifuges and to design the spindle in such a way that it can take up and/or resist substantial bending stresses without, however, permitting excessive stressing of its bearings or other stationary parts of the machine.

Another object of the invention is to provide a spindle which is designed and constructed by full consideration of static and dynamic factors which arise when the spindle is used to support a load-accommodating container in a centrifugal machine.

The improved spindle is preferably of the type which is mounted in upright position and is joumalled in the frame at its upper end. The lower end of the spindle is connected with and supports the hub of a bowl or basket. In accordance with a feature of the invention, the spindle tapers from its lower end toward its upper end in such a way that its flexural strength (resistance to bending) is at least substantially uniform all the way between the two ends.

BRIEF DESCRIPTION OF THE DRAWING The single FIGURE is a fragmentary axial sectional view of a centrifuge including a spindle which embodies the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The spindle of the present invention is configurated, constructed and machined in such a way that it constitutes a body having a uniform resistance to bending all the way from the point of attachment of the hub of a bowl, basket or a like loadaccommodating container to the point where the spindle is mounted in the bearing structure of the machine frame. Otherwise stated, the bending resistance or flexural stiffness of the entire exposed part of the spindle is at least nearly constant. This is achieved by imparting to the spindle a conical shape, i.e., the spindle tapers from the hub of the basket toward the point of suspension of the spindle.

The just described novel spindle was designed on the basis of the following considerations: The flexing of a spindle for the container of a centrifugal machine, especially a suspended centrifugal, is analogous to the stressing of a cantilever beam one end of which is mounted in a support and the other or free end of which takes the load. If the dimensioning, and hence the moment of resistance of the beam is uniform along its full length, the maximum flexing stresses. develop at the point of the maximum bending moment, i.e., at the point where the beam is fixedly mounted in the hub.

Otherwise stated, the value of the tangent of the bending angle is greatest at the point of mounting of the beam, and such value decreases in a direction toward and is zero at the point where the load acts on the free end of the beam. The maximum permissible tangent of the bending angle is a function of the material of the beam. If the deformation of the beam is to remain within the elastic limits of its material (i.e., if a permanent deformation is to be avoided), the maximum value of the tangent cannot be exceeded. The characteristic curve of bending of the beam is a parabola of higher order. The maximum deformation (i.e., the extent of movement of the free end of the beam from its position in unstressed condition of the beam) can be determined in advance.

With the above purely statical considerations in mind, the beam can be designed in such a way that its moment of resistance remains the same at each point of its length, i.e., the moment of resistance can be constant and just large enough to insure that the flexural strength of the beam is uniform from end to end. Thus, the value of the tangent of the bending angle at each point of the beam is the same. The characteristic bending curve of the thus designed beam is a portion ofa circle. If the moment of resistance of the beam at the point of its mounting is the same as in the previously considered case (beam of constant diameter from end to end), the total deformation of the modified beam at the same length and assuming that the load thereon is the same will exceed the total deformation of the previously discussed beam. The results of such considerations open new perspectives when applied to spindles of centrifugal machines.

As a rule, one attempts to minimize the deformation of parts which develops in actual use of a machine. It was now discovered that, contrary to such belief, substantial flexibility of a spindle for suspended centrifugals is desirable and advantageous, as long as the flexibility does not exceed the elastic limits. Contrary to the situation when a cantilever beam is subjected to a bending stress of constant magnitude (static conditions), the bending moment upon a spindle in a revolving suspended centrifugal decreases in response to corrective movement of the container toward its ideal position, i.e., toward the position in which the moment of inertia of the moving masses is symmetric with reference to the axis of rotation. This automatically reduces the forces acting on the bearing for the spindle.

A spindle which is constructed in accordance with my invention exhibits the additional advantage that the moment of resistance at the point where the spindle is mounted in the hub can be greater than in known spindles to thus further reduce the likelihood of permanent deformation at the point which is subjected to maximum stresses despite the fact that the spindle, as a whole, is more readily deformable and insures better control of forces which act on the frame of the centrifugal machine.

The drawing shows a portion of a centrifuge having a housing or frame 5 for a swivel bearing 6 which surrounds a sleeve 7 at the upper end 4 ofa tapering spindle 3. The lower end 4a of the spindle 3 is fixedly connected with the hub 2 of a container or basket 8. The character 9 denotes a mass of sugar in the interior of the container 8. The mass 9 has an inwardly extending protuberance 10 which causes unbalance of the revolving parts. The numeral 11 denotes the point where the spindle 10 is subjected to the action ofa bending force P The force which causes unbalance is shown at P The bending moment M is b.P B, and the moment of unbalance M is a.P An equilibrium is established when M equals M Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features which fairly constitute essential characteristics of the generic and specific aspects of my contribution to the art.

What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims:

lclaim:

1. As a novel article of manufacture, a spindle for use in centrifugal machines of the type wherein the spindle is swingably and rotatably mounted in the region ofits upper end and fixedly mounted in the region of its lower end in the hub of a load accommodating container carried by the spindle, said spindle tapering continuously from the region of said lower to said region of its upper end.

2. A spindle as defined in claim 1, wherein the taper is such that the flexural strength of the spindle is at least substantially uniform between said ends thereof so that if an imbalance occurs in the container during rotation of the spindle and the container fixed to the lower end thereof, said spindle will bend along a curve substantially forming a portion of a circle, whereby permanent deformation of the spindle in the region of the lower end thereof is prevented.

Claims (2)

1. As a novel article of manufacture, a spindle for use in centrifugal machines of the type wherein the spindle is swingably and rotatably mounted in the region of its upper end and fixedly mounted in the region of its lower end in the hub of a load accommodating container carried by the spindle, said spindle tapering continuously from the region of said lower to said region of its upper end.

2. A spindle as defined in claim 1, wherein the taper is such that the flexural strength of the spindle is at least substantially uniform between said ends thereof so that if an imbalance occurs in the container during rotation of the spindle and the container fixed to the lower end thereof, said spindle will bend along a curve substantially forming a portion of a circle, whereby permanent deformation of the spindle in the region of the lower end thereof is prevented.

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