Low gradient spring motor
US 2899193 A
Abstract available in
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Description (OCR text may contain errors)
Aug. 11, 1959 E. E. FOSTER 2,899,193
LOW GRADIENT SPRING MOTOR Filed July 2, 1956 ATTORNEYS.
United States Patent Ofiice J 2,899,193 Patented Aug. 11', 19 59 LOW GRADIENT SPRING MOTOR Edwin E. Foster, Austin, Tex. Application July 2, 1956, Serial No. 595,290 6 Claims. 01. 267-1 This invention relates to low gradient spring motors and more particularly to motors of the ribbon spring type and a method of making the same.
The commonest types "of ribbon spring motors are thoseusing a so-called clock spring which is a straight or substantially straight ribbon spring wound down on a rotatable spindle and tending to unwind from the spindle to turn it. Springs of this type provide a relatively small number of turns for a given length of spring and have a very steep gradient so that thetorque they deliver to the spindle decreases very rapidly as the spring unwinds.
Among efforts to provide a more efficient spring motor is that disclosed in the patent-to Fornelius No. 2,063,799. This motor employs a spring biased so that it tends to coil in one direction, being normally in equilibrium in the wound condition. This means that the radius of bias of the spring increases from the innermost coil to the outermost coil. In the motor disclosed in the aforementioned Fornelius patent, such aspring is backwound on a drum so that it is bent oppositely to its bias. Motors of this type provide a larger number of useful turns and a much flatter gradient than a clock spring.
It has also been proposed to utilize in a spring motor of the Fornelius patent type, :a spring with constant bias such as that disclosed in the patent to Lermont No. 2,609,192, or a spring with a reverse bias such as that disclosed in my prior Patent No. 2,609,191. A spring of the latter type has a bias which is the reverse of thatof the Fornelius spring, namely of decreasing radius from the innermost coil to the outermost. These springs tend to flatten the gradient still further and in this respect are very satisfactory but are still relatively large and require a long ribbon spring fora given number of turns.
As shown in the Fornelius patent, and as used in all commercial applications with which applicant is familiar, the spindle or drum on which the spring is backwound is made relatively large to avoid overstressing the spring. Obviously if the spring is backwound on a drum of relatively large diameter the amount of reverse curvature per unit length imparted to the spring will be less than if it is backwound on a spring of relatively small diameter. When such a large diameter drum is used it means not only that the spindle or drum itself will be relatively bulky but that it will have a long circumference and will require a relatively long length of spring to produce one turn of the drum as the spring is unwound therefrom.
It has been generally understood in the art that use of a drum of large diameter on which the spring 'is back wound decreases the spring gradient, and that use of a smaller diameter drum results in an increase in spring gradient when a spring of the type described above is used. Spring designs have, therefore, been a compromise between drum size and spring gradient to produce a commercially acceptable motor. I
According to the present invention, I have discovered that a spring motor having a large ratio of turns to spring length and a very flatgradient can be produced by so relating the change of bias of the spring throughout its 2 1' length to the drum diameter that each increment-of the spring is equally stressed when it is backwound on the drum. Preferably, each increment of the spring is stressed to its elastic limit to obtain maximum use of the, spring material and to insure equal stressing of all parts of the spring.
This construction results in the use of a relatively small drum on which the spring is backwound as compared to conventional designs. According to presently accepted spring theory, the spring motor of the present invention should have a steeper gradient than spring motors of the type described above. Actual tests' have shown, however, that the gradient is actually flatter than the gradients obtained with the prior types of spring motors. I am presently unable to explain this action completely, except that it must result from the fact that each increment of my present spring is equally stressed when it is backwound.
It is, therefore, one object of the present invention to provide a spring motor which is extremely compact and eflicient and which provides a large number of turns relative to the length of spring used.
Another object is to provide a spring motor in which a ribbon spring is backwound on a spindle and is so biased initially that successive increments in the spring are equally stressed when they are backwound.
Still another object is to provide a method of making a spring motor in which a coiled ribbon spring is backwound on a spindle of such size that successive increments of the spring are stressed beyond the elastic limit of the material.
Another object of this invention is to provide a spring motor having a coiled ribbon spring adapted to be backwound on a drum the biasing of the spring and the diameter of the drum being such that successive increments in the spring are equally stressed when they are backwound.
The above and other objects and features of the-invention will be more readily apparent from the following description when read in connection with the accompanying drawing, in which:
Figure 1 is a plan view of a spring motor embodying the invention in its unwound condition;
Figure 2 is a similar view showing the motor in fully wound condition;
Figure 3 is a horizontal section on the line 3-3 of Figure 2;
Figure 4 is a graph showing typical spring characteristics;
Figures 5, 6 and 7 are partial elevations of the spindle with different amounts of spring wound thereon; and
Figures 8 and 9 are views similar to Figures 1 and 2 respectively of an alternative construction.
The motor of Figures 1 to 3 comprises a housing formed by top and bottom plates 10 and 11 joined by side plates 12 and 13. A spindle 14 is mounted between the plates 10 and 11 for free rotation and a similar spindle 15 is mounted between the plates 10 and 11 on a shaft 15 which is to be driven.
The spindle 14 serves as a storage spindle for a ribbon spring 16 which is so set and tempered that it is biased to form itself in a coil on the spindle, as shown in Figure 1. The inner end of the coil may be attached to the spindle by cutting away one side of the spindle, as indicated at 17, and riveting or otherwise fastening the end of the spring to the cut-out portion, as shown. The opposite or outer end of the spring is secured to the spindle 15 by forming a slot 18 in the spindle 15 into which the end of the spring is inserted and anchored by a pin or screw 19. With this method of anchoring the end of the spring, the outer surface of the spindle 15 has a smooth substantially uninterrupted cylindrical surface so that the spring will not tend to be kinked as it is wound on this spindle.
To limit winding of the spring and to prevent damage thereto a pivoted follower arm 21 may be provided in the casing resting against the outer surface of the spring, as coiled on the storage spindle 14. When the spring is unwound from the storage spindle, as shown in Figure 2, the arm 21 will engage the cut-out 17 in the spindle 14 and prevent further rotation of the spindle 14. 7
As shown in Figures 1 and 2, the end of the spring is so connected to the spindle 15 that when the spindle 15 is turned counter-clockwise in a spring winding operation, the spring will be backwound on the spindle. In other words, the spring will be bent counter to its normal bias so that it tends to unwind from the spindle 15 and to wind on the spindle 14. These two forces are combined to turn the spindle 15 with a relatively high torque and with maximum efficient use of the spring material.
The tendency of the spring to coil on the storage spindle 14 creates a tension in the spring which acts through the radius of the strip of spring extending between the two spindles from the axis of the spindle 15 to exert a turning moment on the spindle 15. The extent of this tensile force will depend on the bias in the active part of the spring just winding on the storage spindle 14 and with a spring of the type employed according to the present invention has a very slight drooping characteristic due to the varying bias of the spring throughout its length and to the changes in radius at the two spindles.
The major force tending to turn the spindle 15 results from the tendency of the spring to unwind from the spindle 15 and this force is controlled by varying the bias of the spring to produce a force which is constant throughout the full length of the spring to produce a flat gradient. The operation of the spring is illustrated more clearly in Figures 5, 6 and 7, and spindle 15 being shown empty in Figure 5, approximately one-half full in Figure 6, and full in Figure 7. When the spindle is empty, as shown in Figure 5, the spring will be bent through a relatively sharp angle as it winds on the spindle and the outer turns of the spring as it exits on the storage coil are so biased that the spring, if held at its point of tangency to the spindle 15 and released from the spindle 14, would tend to assume a position as indicated by the dotted line. When wound on the spindle, the spring is thus deflected through the angle alpha from its normal biased position. Similarly, when the spring is partially backwound on the spindle 15, as shown in Figure 6, it will be deflected through a somewhat smaller angle due to the larger effective size of the spindle, but its bias is at a somewhat smaller radius so that the total deflection from the normal biased position will be through the same angle alpha. When the spring is completely backwound on the spindle 15, it will be bent around the spindle through a smaller angle due to the greater eifective radius of the spindle, but is biased on a still smaller radius so that the total deflection from the normal biased position is still through the same angle alpha. Thus, regardless of the portion of the spring which is working at any particular time, the spring is stressed to the same extent by being deflected from its normal biased position through the same angle and will exert the same unwinding force and the same torque on the spindle 15.
While the spring can be so set and tempered as to produce the required variation in bias throughout its length, one convenient and very desirable way to accomplish this purpose is to overstress the spring during initial backwinding. Thus, it is possible to start with almost any type of bias in the spring which will cause it to coil on the storage spindle 14 and by utilizing a working spindle 15 of sufiiciently small diameter relative to the thickness and initial bias of the spring to overstress the spring when it is first backwound. The spring will thus be bent beyond its elastic limit and will take a permanent set 7 which will cause each increment in the length of the spring to be biased up to its elastic limit when it is subsequently backwound. This method of treating the spring will result in the correct variation in the bias to produce the same angular deflection in each increment in the length of the spring, as shown in Figures 5, 6 and 7, and will further result in each increment of the spring being stressed to the full elastic limit of the spring material so that maximum power is obtained from a spring of a given size. I have found that by using a spring in this manner maximum power and maximum number of turns can be obtained from a spring of a given size with a very satisfactory fatigue life for most, if not all, purposes.
The characteristic of a spring, as shown in Figures 1 and 2, is indicated in Figure 4 in comparison with a conventional clock spring and a spring of the type disclosed in the Fornelius patent. The graph of Figure 4 resulted from using a spring .008 inch thick, .75 inch wide, by 15 feet long, with a spindle 15 one inch in diameter and a storage spindle 14 also one inch in diameter. The solid line in Figure 4 illustrates the actual test results attained by the use of this spring motor. It will be observed that the torque in the fully wound condition is slightly higher than the torque available after forty turns of unwinding, but the characteristic is relatively flat and is extremely satisfactory for most commercial purposes, such, for example, as the driving of a movie camera. As compared to the motor of the present invention, a conventional clock motor using a spring of the same dimensions results in the dotted line 23. It will be seen that this spring is actually useful through only ten or twelve turns and that its gradient falls ofi so sharply that it becomes ineffective after this relatively small number of turns.
The dot-dash line 24 of Figure 4 illustrates a comparable characteristic of a spring motor of the type shown in the Fornelius patent utilizing a spring of the same width and thickness, but approximately 25 feet long in order to obtain 40 turns on a take-up or working drum of about two inches in diameter. This size of drum follows in general the proportions employed in current design practices as well as those indicated in the Fornelius patent. It will be seen that the characteristic curve of this motor has a far greater droop than the curve of the present motor, exerting less torque than the present motor after forty turns, in addition to requiring the use of substantially greater length of spring. It was also found that by utilizing a spring biased in accordance with the patent to Lermont, No. 2,609,192, or applicants prior patent, No. 2,609,191, a characteristic generally similar to but slightly steeper than the curve 22 of the present motor could be obtained, but it was necessary to utilize approximately 25 feet of spring due to the increased drum size. As pointed out above, reduction of the drum size to a point insufiicient to overstress the spring results in a still steeper curve.
By comparison with known types of motors, as indicated generally in Figure 4, the present motor is far more compact and less expensive due to the smaller sizes of the parts and the substantially decreased length of spring required. At the same time, the present motor provides characteristics better than those obtainable with the best type of prior art motors known to applicant and which are highly satisfactory for substantially all commercial purposes.
The same results can be achieved with a motor, as shown in Figures 8 and 9, which is substantially identical to the motor of Figures 1 to 3, except that no storage spindle is employed. This motor comprises a housing which may be generally cup-shaped with a flat bottom 25 and a circumferential flange 26. The housing may be covered by a closure plate, not shown, to hold the spring in place. A single spindle 27, similar to the take-up or working spindle 15 of Figures 1 and 2 is rotatably mounted in the housing and is, connected to a shaft to be driven. A coil spring 28, similar to the spring 16, is loosely mounted in the housing beside the spindle 27 and will normally coil upon itself in a relatively tight coil, as shown in Figure 8.
When the spindle 27 is turned in the housing to backwind the spring thereon, as shown in Figure 9, the spring will simply be uncoiled from the normal storage coil and the normal storage coil will be prevented from swinging around the spindle by engagement with the sides 26 of the housing. The spring in this instance works in exactly the same way as explained in connection with Figures 1 through 7 and provides exactly the same characteristics.
While two embodiments of the invention have been shown and described herein, it will be understood that they are illustrative only and not to be taken as a definition of the scope of the invention, reference being had for this purpose to the appended claims.
What is claimed is:
1. A low gradient spring motor comprising a housing, a cylindrical spindle rotatably mounted in the housing, a ribbon spring secured at one end to the spindle and being biased lengthwise so that when released it tends to coil on itself at the side of the spindle, the spring being backwound on the spindle when the spindle is turned in one direction in the housing and tending to unwind from the spindle and return to the coil, the bias of the spring varying lengthwise thereof so that when it is backwound on the spindle successive increments of the spring will be deflected from their normal biased positions through the same angle.
2. A low gradient spring motor comprising a housing, a cylindrical spindle rotatably mounted in the housing, a ribbon spring secured at one end to the spindle and being biased lengthwise so that when released it tends to coil on itself at the side of the spindle, the spring being backwound on the spindle when the spindle is turned in one direction in the housing and tending to unwind from the spindle and return to the coil, the bias of the spring varying lengthwise theneof so that when it is backwound on the spindle successive increments of the spring will be deflected to the full elastic limit of the material.
3. The spring motor of claim 1 including a freely rotatable spindle in the housing on which the coil is supported.
4. A low gradient spring motor comprising a ribbon spring biased lengthwise so that when released it will form a coil, a spindle to which the outer end of the spring is secured, means restraining said coil from turning freely about said spindle so that relative turning of said spindle and said restraining means causes the spring to be backwound on said spindle, and the bias of the spring varying lengthwise thereof so that when it is backwound successive increments of the spring will be deflected from their normal biased positions through the same angle.
5. A low gradient spring motor comprising a ribbon spring biased lengthwise so that when released it will form a coil, a spindle to which the outer end of the spring is secured, means restraining said coil from turning freely about said spindle, so that relative turning of said spindle and said restraining means causes the spring to be backwound on said spindle, and the bias of the spring varying lengthwise thereof so that when it is backwound, successive increments of the spring will be deflected to the full elastic limit of the material.
6. The method of making a low gradient spring motor comprising forming a ribbon spring with a longitudinal bias such that when released, it will form a tight coil upon itself, backwinding the spring spirally on a spindle of such a size relative to the thickness and bias of the spring that each increment in the length of the material of the spring will be stressed beyond its elastic limit, and then allowing the spring to return to the forwardly wound condition so that said spring will be deflected through equal angles throughout its length upon subsequent backwinding on a spindle of substantially the same size.
References Cited in the file of this patent UNITED STATES PATENTS 1,164,489 Fornelius Dec. 14, 1915 2,063,799 Fornelius Dec. 8, 1936 2,622,700 Geyer Dec. 23, 1952 2,647,743 Cook Aug. 4, 1953 FOREIGN PATENTS 232,211 Switzerland May 15, 1944