US3501910A

US3501910A - Tuning fork arrangement for electromechanical oscillators

Info

Publication number: US3501910A
Application number: US722319A
Authority: US
Inventors: Cecil Frank Clifford
Original assignee: Individual
Current assignee: Individual
Priority date: 1967-05-15
Filing date: 1968-04-18
Publication date: 1970-03-24
Anticipated expiration: 1987-03-24
Also published as: GB1195432A; DE1773340A1; FR1561892A

Description

March 1970, c. F. CLIFFORD 3,

TUNING FORK ARRANGEMENT FOR ELEGTROMECHANICAL OSCILLATORS Filed April 18, 1968 mlllll'l 1 Ceq/ Frank car wrewfw? United States Patent 3,501,910 TUNING FORK ARRANGEMENT FOR ELECTRO- MECHANICAL OSCILLATORS Cecil Frank Clifford, Newbridge Works, Bath, Somerset, England Filed Apr. 18, 1968, Ser. No. 722,319 Claims priority, application Great Britain, May 15, 1967, 22,329/67 Int. Cl. G04c 3/00 US. Cl. 58-23 ABSTRACT OF THE DISCLOSURE A mounting arrangement for a single reed or a tuning fork of an electromechanical oscillator in which an inertia weight is directly attached to one end of the reed or the nodal point of the tuning fork, and the inertia weight is mounted on a spring which is supported by a base, the spring preferably being arranged so that it will readily move in a direction in which out-of-balance forces of the oscillator will act, so that noise and other undesirable effects are reduced.

This invention relates generally to electromechanical oscillators, and more particularly to a mounting arrangement for the mechanically oscillating member of an electromechanical oscillator.

It is becoming usual practice to make the oscillating member of an electromechanical oscillator from a strip of metal which, if the oscillating member is a tuning form, is bent to the required U form and has a support attached to it at its nodal point, that is to say, at the center of the curved portion of the U. A fork of this kind is simple to construct and light in weight and both these features are important in a device which is required to be manufactured in quantities, in small sizes and at the lowest possible cost. A further advantage, which is gained both in the reed type oscillating member and the tuning fork, is that alloys having a substantially zero temperature coefficient of elasticity are available in suitable strip form and this makes it possible to provide a tuning fork which has a high degree of constancy in its frequency of oscillation despite variations in ambient conditions.

The reed or fork is, of course, provided with a pair of transducers, one of which is connected in the input circuit and the other in the output circuit of an amplifier by which the reed or work is maintained in continuous oscillation.

If an oscillating member of either kind is mounted directly on a base or on a clock movement which it is to control, any shocks to which the base or clock movement may be subjected are transmitted to the oscillating member and may momentarily disturb its steady oscillation. A more important circumstance is that the vibration of the oscillating member is transmitted to the base or movement and may set up a volume of noise which is quite unacceptable, more especially if the noise is transmitted to a case in which the clock movement is mounted and the case acts as a sounding board. There is another factor, of a technical nature, known as the support effect. Certain losses occur due to all conventional types of support and the effect of these losses is to reduce the Q of the fork, and usually, the frequency of oscillation. It has been found that when the oscillating member is mounted on a support of a conventional type the support has a natural frequency of its own. Hence, when the oscillating member oscillates, the support tends to vibrate at its own natural frequency and a certain amount of pulling results. This changes the oscillation frequency slightly. It has been established that the change in oscillation frequency may be such that it makes as much as 20 seconds per day difference in the speed of a clock controlled by the oscillating member.

5 Claims 1 3,501,910 Patented Mar. 24, 1970 The principal object of the invention is to provide a mounting arrangement for an oscillating member of the kind described which will prevent or substantially reduce the transmission of noise or shocks and substantially reduce the support effect.

The invention consists of a mounting arrangement for the mechanically oscillating member of an electromechanical oscillator comprising an inertia weight which is attached to the oscillating member, a spring attached at one 0 end to the inertia weight, and means by which the other end of the spring may be attached to a base on which the mounting arrangement is to be carried.

Where the oscillating member is a tuning fork the inertia weightis preferably attached to the fork substantially at the nodal point thereof, and where the oscillating member is a reed the spring may conveniently be constituted by an extension of the reed.

In order to facilitate the full understanding of the invention one embodiment thereof will now be described with reference to the accompanying drawings, in which:

FIGURE 1 is an elevation of one arrangement according to the invention;

FIGURE 2 is a side elevation of the arrangement of FIGURE 1;

FIGURE 3 is an inverted plan view of the same arrangement;

FIGURE 4 is an elevation of another arrangement according to the invention;

FIGURE 5 shows the application of the invention to a compound reed oscillator;

FIGURE 5a shows a strip of metal from which the compound reed oscillator of FIGURE 5 is made; and

FIGURE 6 shows the application of the invention to a simple reed oscillator.

Referring initially to FIGURES l to 3 of the drawings, a base on which the tuning fork arrangement is to be mounted is indicated at 11. This may in fact be one of the support plates for a clock movement which, is in turn, intended to be mounted in a clock case. A tuning fork 12 is made from astrip of material which is bent into a U form and is advantageously made from one of the nickel-iron alloys having a substantially zero temperature co-efficient of elasticity such,'for example, as the alloys known as Ni-Span C and Ni-Span D.

It will be understood that an amplifier is included and the tuning fork is provided with two transducers connected to the amplifier but these are not shown since they are not a part of the invention and constitute well-known art. The tuning fork 12 has attached to it, at or near its nodal point, an inertia weight 15 which, as shown in FIGURE 2, is suspended just clear of the base 11. A support block 16 is mounted on the base 11 by screws 17 and it has one end of a spring 18 attached to it by means of screws 19. The spring 18 contains a rightangled bend and its other end is attached to the inertia weight 15 by screws 20. As will be clear from FIGURE 3 the centre part of the end portion of the spring which is attached to the inertia weight 15 is cut away to permit the tine 13 of the fork to pass through it.

With this construction the major part of any vibration which is transmitted to the heavy inertia weight 15 is reduced by that weight and the resilient mounting by means of the spring 18 ensures that the remaining vibration is further reduced. The mounting block 16 is conveniently made of a synthetic plastics material such as nylon which helps to absorb the remaining small amount of vibration.

In operation, each tine of the fork 12 generates a small amount of centrifugal force and an amount of inertial force as it oscillates, and this tends to move the fork in the direction of the double headed arrow 21. This tendency is transmitted to the heavy inertia weight 15 which is attached substantially at the nodal point of the fork. If a comparatively rigid mounting were used then the vibration due to this combined centrifugal and inertial action would be transmitted directly to the 'base 11, resulting in the noise referred to previously.

By using the'heavy inertia weight 15 and arranging the spring 18 so that it will yield in the direction of the arrow 21 it is ensured that this vibration is not transmitted to the basell, or is only transmitted in heavily attenuated form. The support effect is also substantially reduced, if not eliminated. It was mentioned earlier that the support effect could alter'the rate of a tuning fork sufliciently to cause a time difference of as much as 20 seconds in one day. In such a case, mounting the fork in the manner described above can reduce this rate error to as little as 2 seconds per day.

The improvement may be confirmed by a calculation, as set out below, in which certain assumptions have been madefor the sake of simplicity. Taking an arrangement in which the fork frequency is 300 HZ. and the inertia weight is such that it may be permitted to sag under its own weight by 0.02 cm., assume that an RMS force of 100 grams is shaking the support. The force is equal to mass acceleration. Assume that the mass of the inertia weight is also 100 grams. The acceleration a during one quarter cycle second i Low-1 cm/ ec/sec 1200 S 100 The distance moved in 1 s1 1 m6 second (t) re s a t X -0.00034 cm.

where F is the fork frequency and F is the support frequency. In the above example F is 300 and F is 35 so that the ratio ofreduction of the force acting on the support is which is equal "to 0.0136. This ratio multiplied by the force gives 0.0136 100=1.36 'grarns compared with 1.7 grams given by the earlier calculationin which certain assumptions (which are not strictly correct) were made in the interests of simplicity. However, both calculations provide a good indication of the order of the reduction. 7 Where the fork is oscillating at a comparatively low frequency, e.g. 30 c/s., the inertial force and centrifugal force will both be very small and the main force tending to cause vibration to be transmitted to the base 11 will result from out-of-balance tine forces which are nearly always present in some degree. This will tend to act in the direction of the arrows 22 since the tines are vibrating in that direction, and in that case it may be desirable to modify the spring 18 or its method of mounting so that the spring 18, or at least a major part of it, lies between the tines and parallel to them. Preferably the spring is exactly in the middle of the space between the tines.

Such an arrangement is shown in FIGURE 4 in which an inertia weight 24 is attached substantially at the nodal point to a tuning fork 23. The weight is mounted on one end of a spring indicated by reference 25 having a portion 26 which lies between, parallel to and equally spaced from the two tines. The continuing portion 27 of the spring lies at right angles to the portion 26 and the other end of the spring is attached to a support 28 attached by screws 29 to a base, not shown. As before, the support 28 may be made of nylon or other material having vibration absorbing properties. As an alternative, the element 28 may be a lug which is integral with the spring portion 27 and is bent over at right-angles to form a fixing lug.

FIGURES 5 and 5a show an arrangement in which the inertia weight according to the invention is used in conjunction with a compound reed oscillator consisting of a strip 32 (FIGURES 5 and 5a) which is formed with two oppositely directed U shaped slots 51 and 52 (FIG- URE 5a) in order to leave two centraltongues 53' and 54 which constitute secondary reeds. The strip is then bent to a U form as shown in FIGURE 5, leaving the ends, respectively 33 and 34, of the secondary reeds projecting. The secondary reed 33 has a magnet 35 attached to its free end while the secondary reed 34 has a magnet 36 attached to its free end, the' magnets which is bent over toprovide means for fixing to a base with screws 41.

FIGURE 6 shows how the invention may be applied to a single reed. In the construction shown the reecl 42 is actually in one piece with the spring by which the inertia Weight is supported and, as will be seen from the drawing, the reed 42 has the inertia weight, which is in two parts 43a and 43b attached to it. The reed then. continues as the supporting spring with a right-angled bend, the part 44 beyond the right-angled bend being attached to a base 45 by means of screws 46. Two magnets (shown dotted), respectively 47 and 48, arecarried on the base 45 and are respectively surrounded by signal and drive

coils

49 and 50.

I claim: v I 1. An arrangement for mounting a tuning fork member of an electromechanical oscillator on a supporting base comprising an inertia weight attached to said member at the nodal point of said member, a spring capable of being deflected in a first direction parallel to the longitudinal direction of the oscillating member and in a second direction perpendicular to the said first direction and parallel to the plane in which the oscillating member oscillates, the spring being attached at one end thereof to the inertia Weight, andmeans attached to the other end of the spring by which the arrangement is mounted on the base.

2. An arrangement as claimed in claim 1 in which the inertia weight lies between the tines of the fork, the said one end of the spring is attached to a face formed on the Weight at right angles to the longitudinal direction of the tines, and the spring is formed with a right-angled bend. at a point part way along its length.

3. An arrangement as claimed in claim 1 in which the inertia weight lies between the tines of the fork, the said one end of the spring is attached to the weight so that the portion thereof nearest the weight lies parallel to the longitudinal direction of the tines, and the spring is formed with a right-angled bend part way along its length.

4. An arrangement as claimed in claim 1 in which the tuning fork member is a compound reed made from a strip of material formed with two oppositely directed U shaped slots defining two central tongues which constitute secondary reeds, the strip being bent into the form of a 5 6 tuning fork to leave the ends of the secondary reeds pro- References Cited jecting from the curved portion of the fork. FOREIGN PATENTS 5. An arrangement as claimed in claim 1 in which the spring is formed by an extension of the reed, the inertia 11480980 5/1967 Franceweight being attached to the strip forming the reed and spring part Way along its length, whereby the part on 5 RICHARD WILKINSON Pnmary Examme one side of the inertia weight forms the reed and the EDITH C. SIMMONS, Assistant Examiner part on the other side of the inertia weight forms the US. Cl. X.R.

spring. 31025