US 3213645 A
Description (OCR text may contain errors)
Oct. 26, 1965 D. PEASE 3,213,645
TORQUE LIMITING MECHANISM Filed Sept. 3, 1963 Tan INVENTOR- FIG. 2 DONALD 1.. PEASE A TTORNE Y United States Patent 3,213,645 TORQUE LIMITING MECHANISM Donald L. Pease, Marion, N.Y., assignor to Xerox Corporation, Rochester, N.Y., a corporation of New York Filed Sept. 3, 1963, Ser. No. 306,045 1 Claim. (Cl. 64-30) This invention relates to improvements in clutch mechanisms. More specifically, the invention relates to a clutch mechanism for transmitting substantially uniform torque.
In the process of xerography, for example, as disclosed in Carlson Patent 2,297,691, issued October 6, 1942, a Xerographic plate comprising a layer of photoconductive insulating material on a conductive backing is given a uniform electric charge over its surface and is then exposed to the subject matter to be reproduced, usually by conventional projection techniques. This exposure discharges the plate areas in accordance with the radiation intensity that reaches them and thereby creates an electrostatic latent image on or in the photoconductive layer. Development of the latent image is effected with an electrostatically charged, finely divided material, such as an electroscopic powder, that is brought into surface contact with the photoconductive layer and is held thereon electrostatically in a pattern corresponding to the electrostatic latent image. Thereafter, the developed xerographic powder image is usually transferred to a support surface to which it may be fixed by any suitable means.
The cycle above described is typical for forming xerographic reproductions on a support surface and may be carried out manually, or automatically, as is done with mechanized equipment which performs the cycle repeatedly. In either case, however, after transfer of a powder image from a plate to a support surface, there usually remains on the plate a residual of unremoved developing powder usually in image configuration and referred to in the art as a residual powder image. Before the plate can be reused for a subsequent cycle it is necessary to remove the residual image to prevent ghost images from subsequently reproducing.
In US. Patent 3,099,856, issued August 6, 1963, to Eichorn et al., there is disclosed an improved apparatus for cleaning residual powder from a xerographic plate. As disclosed in this patent, removal of residual powder is eflfected by rubbing against the drum surface, to be cleaned a web of fibrous material such as, for example, paper toweling, cheesecloth, flannel, and cotton fiber impregnated with a resin of urea formaldehyde.
In the type of apparatus disclosed in the above referenced patent, the web of material is taken from a supply spool and transported around a cleaning roll onto a takeup roll. The take-up roll is driven to permit uniform winding of the used web material onto the take-up roll. These inexpensive and disposable webs of fibrous material are advanced into pressure and rubbing or Wiping contact with the xerographic plate and are gradually and continually advanced to present a clean surface to the plate whereby complete removal of residual powder from the plate is effected. It has been found that a web of very thin tissue paper is a very suitable inexpensive material that can be disposed of after use. However, this latter material can be readily torn, as can the light weight cardboard tube forming the take-up spool on which it wound. Accordingly, it is necessary to drive the shaft carrying the take-up spool in a manner to maintain the web of material at slight tension, but the torque applied to this shaft must be limited to limit the tension on the web within limits to prevent tearing of the web or the cardboard take-up spool.
The principal object of the invention is to improve ice clutch mechanisms for applying a uniform torque to a web take-up shaft.
Another object of the invention is to improve clutch mechanisms for transmitting a drive torque in which the maximum torque transmitted may be predetermined.
For a better understanding of the invention as well as other objects and further features thereof, reference is had to the following detailed description of the invention to be read in connection with the accompanying drawings, wherein:
FIG. 1 is an enlarged view of the clutch mechanism for driving the take-up roll of the web cleaner apparatus;
FIG. 2 is a view similar to FIG. 5 but with the assembly rotated 90; and,
FIG. 3 is a vector force diagram of the clutch mechanism.
General Referring now to the subject matter of the invention, there is shown specifically in FIGS. 1 and 2, a preferred embodiment of a slip clutch mechanism constructed in accordance with the invention. As shown, the slip clutch mechanism includes the gear 451 which is journaled on the shaft SH13 for rotation relative to the shaft. The gear 451, which is a driven gear, is driven in the direction shown by the arrows in FIGS. 1 and 2, that is, in a counter-clockwise direction, when viewed from the left end of the shaft in these figures.
The lefthand face of the gear 451, as seen in FIGS. 5 and 6, contacts a thrust bearing 452 which butts against a retaining ring 454 secured in a suitable annular groove formed in the shaft SH13, the thrust bearing being restrained from rotating with respect to the shaft by the lug 453 thereon engaging the retaining ring.
Both the gear 451 and the thrust bearing 452 are formed of Delrin, a trade name of E. I. duPont de Nemours & Co. for a thermoplastic acetal resin A friction clutch element, in the form of a flanged cam 455, faced with cork 458 secured thereon as by cement, is normally biased into frictional contact with the righthand face of gear 451 by means of a torsion spring 461 which loosely encircles the flanged cam 455 and a second clutch element or flanged cam 465, the latter being secured to shaft SH13 by a pin 471 which extends through this flanged cam and the shaft.
Both of the flanged cams 455 and 465 are provided with cam or ramp portions 456 and 466, respectively, which are formed, in the embodiment shown, at an angle of to the horizontal, although any suitable cam angle may be used to obtain the desired result as indicated in the force diagram described in detail hereinafter. These flanged cams are positioned on the shaft with the ramp portions in contact with each other for a purpose to be described in detail hereinafter.
The torsion spring 461, in the embodiment shown, is formed of a length, equal to two turns plus 102 of a third turn when in the free position, but is put under a predetermined load when the clutch mechanism is assembled on the shaft, in order to load the friction faces together, that is, the cork 458 and the righthand face of gear 451. The torsion spring is wound in the direction such that when the spring is assembled, it is in a wound and loaded condition With opposite ends of the torsion spring hooked into suitable recesses formed in the flanged cams. The torsion spring thus assembled, tends to rotate the flange cams with respect to one another, which in turn causes them to expand axially. Since the flanged cam 465 is pinned to the shaft and the flanged cam 455 is captured by the gear 451, thrust bearing 452 and the retaining ring 454, a normal force is created between the gear 451 and the cork 458 secured to flanged cam 455. These are the friction surfaces previously described.
During operation, the gear 451 is driven in the direction previously described, while the shaft SH13 is retarded by the webwhich is'being held and driven by fric tional contact. The gear 451 through frictional contact with the cork 458 secured to the flanged cam 455 tends to rotate this flanged cam in such a direction, counterclockwise as viewed from the lefthand end of shaft SH13 in FIGS. 1 and 2, to wind up the torsion spring. However, this torsion spring, itself, which has previously been placed under a load during assembly, is trying to unwind. In this arrangement, the flanged cam 455 is acted upon by the torque of the torsion spring 461 minus the torque transmitted by gear 451 and, through the cam action of the ramp portions of the flanged cams, this action is converted into a normal force. This normal force times the coefficient of friction between the cork and the gear is the actual friction force on the gear 451 which in turn equals the torque of the gear. With this arrangement, there is a feedback of forces built into the clutch mechanism which tends to even out changes in the coefficient of friction.
To prevent frictional grab or excessive wear between the cork 458 and the righthand face of the gear 451, the face of the cork 458 in contact with the gear is provided with radially formed grooves 459 to permit lubrication of these friction elements. For a better understanding of the operation of the slip clutch mechanism, reference is made to the vector diagram of the forces in this mechanism shown in FIG. 3, and the following description:
f is the coeflicient of friction for the surfaces, i.e. the gear 451 and cork 458.
N is the normal force F is the friction force which is equal to fN F is the spring force or preset torque of the torsion spring 461.
is the cam angle from the horizontal (1) At time zero, gear 451 not being driven, P is zero because the gear 451 is not driven.
(2) Applying a torque to gear 451, through gear 431, will cause it to slip when the applied torque force equals the friction force Ff.
(3) At this time, with gear 451 rotating, flanged cam 455 is now forced against flanger cam 465 by a force F which is equal to s' f) Tan 0 Thus, this force is less than at time zero, i.e.,
F Ff Tan 0 5 Tan 0 since F; at time zero equals 0.
(4) If the coeflicient of friction 1 increases due to lack of lubrication or because of dirt between the cork 458 and gear 451 or because of other causes, F also increases since F =F Thus, the force F which is equal As the coeflicient of friction increases, for any of the reasons previously given, the driving force D increases, but since F F;) D E F (Tan 0 f D can only increase a small amount because if for any reason 1 increases to a large value,
s Fr Tan 0 will approach zero.
In operation, if the coeflicient of friction suddenly increases and the driving force increases, the torsion spring will begin to wind up further than itsoriginal load position which will cause flanged cam 455 to turn relative to fixed flanged cam 465 against the frictional force between cork 458 and gear 451 to cause flanged cam 455 to move toward flanged cam 465, because of the angle of the ramp. This action will allow further sliding action between the gear 451 and the cork 458 on flanged cam 455 thereby decreasing the friction force between the cork and gear 451 and thereby decreasing the drive torque to shaft SH13.
Thus, as the torque transmitted from gear 451 to shaft SH13 approaches the preset value of the torque set up in the torsion spring 461 when the clutch mechanism is assembled, equilibrium is reached. With this arrangement, the torque applied to shaft SH13 should never exceed the torque preset in the device by torsion spring 461.
With this type of clutch mechanism, the torque applied to the take-up shaft can be limited to prevent tearing of the web or damage to the cardboard take-up roll by the use of a spring of the desired output torque.
Another advantage of this clutch mechanism is that no adjustment of the mechanism is required due to wear of the parts. As the clutch elements Wear, the torsion spring and the flanged cams will take-up this wear with a very small loss in delivered torque to the take-up shaft.
The torque applied to the shaft through this clutch arrangement is sufiicient to permit the gear 451 to drive the take-up shaft to rewind the web material onto the take-up roll as it is advanced by the cleaning roll cooperating with the drum surface, while still permitting this clutch arrangement to slip whereby the desired range of tension on the web material is maintained. In this manner the web material is advanced only by the action of the cleaning roll and not by the take-up roll.
While the invention has been described with reference to the structure disclosed herein, it is not confined to the details set forth, since it is apparent that various modifications can be made to the clutch mechanism. This application is, therefore, intended to cover such modifications or changes as may come within the purposes of the improvements or the scope of the following claim.
What is claimed is:
A torque limiting mechanism including:
a shaft journaled for rotation,
a gear journaled on said shaft for rotation relative to said shaft,
retaining means secured to said shaft adjacent one side of said gear to axially align said gear in one direction on said shaft,
a drive element,
a driven element,
said drive element and said driven element each ineluding a hollow stepped cylinder having an enlarged portion and a reduced portion, the free end of the reduced portion being truncated to provide a cam surface at one end thereof,
a clutch plate mounted on the end of said drive element opposite said cam surface,
said drive element being rotatably mounted on said shaft with said clutch plate in contact with the side of said gear opposite said retaining means,
5 6 said driven element being secured to said shaft adjacent engagement With said gear to transmit a predetersaid drive element with the cam surface of said mined torque as determined by the load on said driven element in intimate contact with the mating preloaded torsion spring. cam surface of said drive element, and a preloaded torsion spring encircling the reduced cylin- 5 References Cited y the Examine! drical portions of said drive element and said driven UNI STATES PATENTS element with one end of sald torsion spring bemg 1,256,947 2/18 Stampem secured to the enlarged portion of said drlve element 1,322,119 11/19 Kiwul 64*29 and the opposite end of said torsion spring being 2,151,724 3/39 wengel et aL secured to the enlarged portion of said driven ele- 1O ment to normally bias said drive element into driven ROBERT C. RIORDON, Primary Examiner.