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You are an expert at summarizing long articles. Proceed to summarize the following text: 
This application claims the benefit of U.S. Provisional Application Ser. No. 60/055,980, filed on Aug. 18, 1997, which provisional application is incorporated by reference herein. 
    
    
     TECHNICAL FIELD 
     The invention relates to combination locks. More particularly, the invention relates to combination locks that accommodate combination changes by lock users. 
     BACKGROUND 
     Combination locks are quite common, and all mechanical ones operate in the same basic fashion. A graduated dial, typically scaled from 1 to 100, rotates a spindle that in turn rotates a drive cam. A bushing supports a series of tumblers, one of which is a drive tumbler connected to the drive cam. The drive cam drives the drive tumbler so that, when a user selects a series of combination numbers with the dial, the driven tumblers are aligned to allow operation of a door handle or lock hasp or the like. In door locks, each tumbler has a gate notch in its periphery situated such that when the notches of all of the tumblers are aligned, a stop on a door handle can travel, allowing operation of the door handle. Most combination locks have only one operative combination that is not easily changed. As a result, if the combination is compromised, a new lock with a different combination would have to be purchased, or the lock would have to be disassembled to change the combination. 
     In response to a desire for more flexibility and security in combination locks, changeable combination locks have been developed. Most changeable combination locks have some arrangement allowing realignment of the drive system and the tumbler gate notches to alter the combination. For example, one prior art changeable combination lock uses two-part tumblers. Inner tumblers carry teeth on their peripheries that engage teeth on the inner peripheries of outer tumblers. The outer tumblers also carry gate notches on their outer peripheries that allow use of a door handle when aligned. The inner tumblers can be disengaged from the outer tumblers to allow relative rotation for resetting the combination. To change the combination, a user throws the lock bolt out and then inserts a wire into the lock to allow relative movement of the inner and outer tumblers. The lock spindle is then pressed inward to disengage the inner tumblers from the outer tumblers, at which point the combination can be changed. This arrangement includes many steps and parts that increase the complexity of the lock. Also, the requirement of extra tools, such as a wire, to change the combination is inconvenient to the user. 
     Another prior art changeable combination lock includes a wave or disc spring mounted between an inner wall of a safe door and the immediately adjacent tumbler. The spring biases the tumblers against the drive cam. To change the combination of the lock, the user first removes the back panel of the lock, dials the combination to align the notches, and then holds the door handle in place to hold the notches in alignment. While holding the handle, the user pushes against the drive tumbler to disengage the drive tumbler from the drive cam. Still holding the handle and pushing against the drive tumbler, the user rotates the dial of the lock by a desired increment so that the drive cam engages the drive tumbler at a new point, thus changing the alignment point of all the tumblers by the increment the dial has been rotated. The handle and drive tumbler can then be released, and the lock can be used with the new combination. This lock is easier to use, but requires the user to do too many things at once and does not provide an easy way to disengage the drive cam and drive tumbler. 
     In view of the prior art, there is a need for a changeable combination lock that is simple in its construction and easy to use. The lock should allow the user to perform relatively few simple steps to change the combination of the lock. 
     SUMMARY OF THE INVENTION 
     My new changeable combination lock uses a clutch between the lock spindle and the drive tumbler for easy disengagement. An actuator of the clutch extends into and can protrude from the rear panel of the door or lock for easy access and operation of the clutch by a user from outside the door and without additional tools. The clutch severs the drive connection between the spindle and the drive tumbler so that the user can rotate the knob and spindle relative to the drive tumbler to change the combination. Because the clutch actuator is accessed through the rear panel of the door, the user does not need to remove the panel to change the combination. 
     More particularly, a first embodiment of my new lock can use a spring-biased combination changer. In this embodiment, the user opens the safe, leaving the handle in an open position to hold the tumbler notches in alignment. The user then presses a button in or protruding from the inside of the door and rotates the combination lock dial by a desired amount. After the user releases the button, the safe can be used with the new combination. This embodiment of my lock is advantageous over the prior art locks because the user need only press a button to disengage the drive cam from the driver rather than removing the back panel of the safe door and pressing on the drive tumbler itself. This embodiment of my lock is also an improvement over locks using multi-part tumblers because my tumblers are very simple. 
     Alternatively, my invention includes a thumbscrew that holds the drive cam in a disengaged state. Thus, to change the combination of my new lock, the user dials the combination, holds the door handle down, disengages the clutch with the thumbscrew, rotates the dial by a desired amount, reengages the clutch, and can then operate the lock with the new combination. This is a marked improvement over the prior art locks that require the user to remove the rear panel of the door and then hold a spring-biased drive tumbler in a state of disengagement while holding the door handle down and rotating the lock dial. Also, as with the first described embodiment, my lock is an improvement over locks using multi-part tumblers because my tumblers are very simple. 
     While my new lock can be used in any suitable device, it is intended for use in safe doors. My lock is particularly suited for use in insulated steel shell safes. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a front perspective view of a safe door including the changeable combination lock of the invention. 
     FIG. 2 is a rear perspective view of the safe door of FIG. 1. 
     FIG. 3 is a rear perspective view of the safe door of FIG. 1 with the rear panel removed. 
     FIG. 4 is a cross section of the safe door of FIG. 1 taken along line 4--4 where the door includes a first embodiment of the lock of the invention. 
     FIG. 5 is a detail of a cross section of the safe door of FIG. 1 taken along line 4--4 where the door includes a second embodiment of the lock of the invention. 
     FIG. 6 is a cross section of the safe door of FIG. 1 taken along line 6--6 showing the handle and its associated components in the safe door. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As seen in FIGS. 1-3, a door 1 for a steel shell safe (not shown) includes a front door plate 2, a jamb 3 attached to the back of door plate 2, and a hinge 4. A combination lock 5, a key-operated lock 6, and a handle 7 can be mounted on the door 1. Insulative material 8 fills the space between the front door plate 2 and the jamb 3. The preferred insulation 8 is concrete heavily laden with water. 
     As is known in the art, combination lock 5 includes a rotatable knob 10 mounted on the exterior of the door 1. With particular reference to FIGS. 4 and 5, the knob 10 is preferably insert molded onto a forward end of a spindle 11 that extends through the door 1. The inner or rearward region of the spindle 11 is in turn connected to a series of tumbler discs 12 via a driver 13. The tumbler discs 12 are rotatably mounted coaxially with the spindle 11 on the back of the door 1. The driver 13 retains, engages, and drives the tumbler discs 12 about the combination lock spindle 11 so that they can be operated to selectively lock and unlock the safe. The tumblers 12 carry gate notches 14 in their peripheries that are aligned when the combination is dialed. 
     The handle 7, as shown in FIG. 6, is attached to a spindle 30 that operates a live bolt system 31 mounted on the interior surface of the door 1. When the gate notches 14 are aligned and the handle 7 is rotated, the handle spindle 30 also rotates causing the live bolts 32 of the live bolt system to move into or out of a locked position. 
     The combination lock spindle 11 and handle spindle 30 are each supported by respective bushings 40, 42 in pass-through tube portions 80, 82 that pass from the back of front door plate 2 to the jamb 3. The tube portions 80, 82 preferably have supporting ribs 43 arranged around their outer surfaces and extending into the insulation 8 for extra support. The inner surface 44 of a forward region of the combination lock bushing 40 provides a running fit for the knob 10. 
     Inner end or rearward portions 50, 51 of the bushings 40, 42, respectively, extend from a forward region of the door 1 to a rearward region of the door 1. The rearward portions 50, 51 preferably extend beyond the rear wall 9 of the jamb 3 to form stub or sleeve portions 52, 53. The outer surfaces 47, 48 of stubs 52, 53 are preferably substantially cylindrical and support components of the combination lock 5 and live bolt system 31. 
     Stub portion 52 rotatably supports a drive tumbler 12a and additional driven tumblers 12b, 12c, 12d on its outer surface. The drive cam 13 carries thereon a number of teeth 90 that engage corresponding teeth 91 provided on the drive tumbler 12a such that the drive cam 13 rotates the drive tumbler 12a. The mutually engaging teeth 90, 91 can be of any suitable form, but are preferably saw teeth or pegs with corresponding holes. A disengagement mechanism or clutch 100 is interposed in the drive train such that it can selectively disengage the drive tumbler 12a from the drive provided by the combination lock spindle 11. 
     In a second embodiment of the invention, the drive cam 13 is mounted on the end of the spindle 11 for conjoint rotation therewith. The drive tumbler 12a is longitudinally slidable and is preferably biased against the drive cam 13 by a spring 114, such as a conical spring. The disengagement mechanism 100 is a cup 130 that preferably sits over and has a larger diameter than the drive cam 13. The cup 130 engages the drive tumbler 12a with teeth 94 that mesh with corresponding teeth 95 on the drive tumbler 12a. Thus, the teeth 94, 95 are mutually engaging teeth of the cup 130 and the drive tumbler 12a. In the preferred form of this embodiment, the cup 130 and the drive tumbler 12a are always in a state of engagement and rotate conjointly. The cup carries a button 131 that extends into the rear panel 120, preferably with its rearwardmost surface flush with the rear surface of the rear panel 120 for easy access. The button 131 can also be designed to protrude beyond the rear panel 120 of the lock, allowing easier access to the button 131 but increasing the risk that the button 131 will be depressed accidentally. When depressed, the button transmits force to the cup 130 which transmits the force to the drive tumbler 12a. Application of enough force to overcome the bias on the drive tumbler 12a causes the drive tumbler 12a to disengage the drive cam 13. When the drive tumbler 12a and the drive cam 13 are disengaged, the drive cam 13 can rotate freely with respect to the drive tumbler 12a and the cup 130. 
     To change the combination, the user dials in the combination with the lock knob 10 and opens the safe with the handle 7. Leaving the handle 7 down to hold the drive and driven tumblers 12 in place, the user presses the button 131 to disengage the drive tumbler 12a from the drive cam 13 and then rotates the lock knob 10 by a desired increment as indicated on the scale 101. The drive cam 13 rotates relative to the drive tumbler 12a so that the alignment position of the tumbler gate notches corresponds to the old combination plus the increment dialed by the user. The user then releases the button 131 to reengage the drive tumbler 12a and drive cam 13 and can then operate the safe with the new combination. The entire process is done without using any tools and without removing the rear panel 120 of the combination lock 5. 
     The number of teeth on the drive cam 13 need not be the same as the number of teeth on the drive tumbler 12a. One of these parts should carry a number of teeth that is a whole number factor of the maximum number of the dial scale 101. In other words, if the scale 101 has numbers from 1-100, the number of teeth on one of the drive tumbler 12a or the drive cam 13 should be a factor of 100. The teeth 90, 91, 94, 95 of all parts of this embodiment can be of any suitable type, but are preferably pegs that meet corresponding holes. 
     In a second embodiment of the invention, the disengagement mechanism 100 is the drive cam 13, which comprises an upper drive cam 13a and a lower drive cam 13b. A sleeve 15 on the upper drive cam 13a engages the outer surface of the interior end of the combination lock spindle 11 so that the upper drive cam 13a rotates with the spindle 11. Preferably, the outer surface of the spindle 11 and the inner surface of the sleeve 15 have square cross sections to better ensure conjoint rotation of these parts. 
     The lower drive cam 13b carries the teeth 90 that engage and drive the drive tumbler 12a via corresponding teeth 91 on the drive tumbler 12a. Thus, the teeth 90 and the teeth 91 are mutually engaging teeth of the lower drive cam 13b and the drive tumbler 12a. The lower drive cam 13b also carries a set of teeth 92 engaging corresponding teeth 93 on a lower portion of the upper drive cam 13a. Thus, the teeth 92 and the teeth 93 are mutually engaging teeth of the upper and lower drive cams 13a, 13b. A bearing or bushing 16 permits relative rotation between the lower drive cam 13b and the lock spindle 11, though the lower drive cam 13b can simply slide against the lock spindle 11 if a bearing or bushing 16 is not used. 
     Upper drive cam 13a carries in its center a coaxial adjusting screw 111, preferably a thumb screw that can be adjusted without tools. One end of the screw 111 is mounted in a threaded bore 112 in the interior end of the spindle 11, the other end of the screw extending into and/or protruding from the rear panel 120 of the lock 5 or the door 1. The spindle end of the screw 111 carries threads 115 that mesh with the threads of the spindle bore 112, causing relative axial movement between the screw 111 and the spindle 11 when the screw 111 is rotated relative to the spindle 11. Shoulders or snap rings 113 on the screw on either side of the upper drive cam 13a prevent longitudinal movement of the screw 111 relative to the upper drive cam 13a. 
     The drive cams 13a, 13b and the thumb screw 111 effectively form the clutch 100 in the knob/drive tumbler drive train, with the screw 111 acting as an actuator for the clutch. Unscrewing the screw 111 moves the upper drive cam 13a out of engagement with the lower drive cam 13b, permitting relative rotation between the spindle 11/upper drive cam 13a and the lower drive cam 13b/drive tumbler 12a. 
     In operation of the second embodiment, the user first dials the combination of the lock 5 using the knob 10, then operates the handle 7 to open the door 1. Leaving the handle 7 down, the user rotates the screw 111 to disengage the upper cam 13a from the lower cam 13b. The user then rotates the lock knob 10 by a desired increment to change all of the numbers of the combination by the increment as shown on the scale 101 of the knob 10. The user then screws the upper cam 13a back into engagement with the lower cam 13b and operates the safe with the new combination. The entire process requires no tools and can be done without removing the rear panel 120 of the combination lock 5 or door 1. 
     The number and size of the increments available to the user is fixed upon manufacture, but can be varied in the design of the drive cam assembly by changing the number of teeth 92, 93 on the upper and lower drive cams 13a, 13b. Preferably, the size of the increments is a whole number factor of the scale 101 of the safe dial. For example, if the scale were 1-100, 20 teeth would yield increments of 5. The number of teeth 92, 93 on the upper and lower drive cams 13a, 13b does not need to be the same. In such a design, the number of teeth on the drive cam with more teeth determines the increment size. The teeth 90, 91, 92, 93 of all parts of this embodiment can be of any suitable type, but are preferably saw teeth or pegs that fit in corresponding holes. 
     Parts list 
     1 Door 
     2 Front door plate 
     3 Jamb 
     4 Hinge 
     5 Combination lock 
     6 Key lock (key-operated day lock) 
     7 Handle 
     8 Insulative material 
     9 Rear wall of jamb 
     10 Knob or dial of combination lock 
     11 Lock spindle (combination lock spindle) 
     12 Tumblers (tumbler discs) 
     12a Drive tumbler 
     12b-d Driven tumblers 
     13 Driver (tumbler driver) or drive cam 
     13a Upper drive cam 
     13b Lower drive cam 
     14 Gate notch 
     15 Drive cam sleeve 
     16 Bearing or bushing of lower drive cam 
     30 Handle spindle 
     31 Live bolt system 
     32 Live bolts 
     40 Combination lock bushing 
     42 Handle bushing 
     43 Ribs 
     44 Inner surface of combination lock bushing 
     47 Outer surface of combination lock bushing 
     48 Outer surface of handle bushing 
     50 Inner end portion of combination lock bushing 
     51 Inner end portion of handle bushing 
     52 Stub or sleeve portion of combination lock bushing 
     53 Stub or sleeve portion of handle bushing 
     80 Tube portion of combination lock bushing 
     82 Tube portion of handle bushing 
     90 Teeth of drive cam engaging drive tumbler 
     91 Teeth on drive tumbler engaging drive cam 
     92 Teeth of lower drive cam engaging upper drive cam 
     93 Teeth of upper drive cam engaging lower drive cam 
     94 Cup teeth engaging drive tumbler 
     95 Drive tumbler teeth engaging cup 
     100 Disengagement mechanism or clutch 
     101 Scale of lock knob or dial 
     111 Screw of clutch in upper drive cam 
     112 Threaded bore hole in combination lock spindle 
     113 Shoulders or snap rings on screw 
     114 Spring biasing drive tumbler 
     115 Clutch screw threads 
     120 Rear panel of combination lock or door 
     130 Cup over drive cam and engaging drive tumbler 
     131 Button on clutch

Summary:
A combination of the inventive combination lock can be easily and conveniently changed by the user. A clutch selectively disengages the drive tumbler from the lock spindle. With the drive tumbler disengaged and the tumblers in an aligned position, the user rotates the lock knob by a desired increment, thereby changing all of the numbers by the increment. In one embodiment, the clutch is operated by a cup mounted around the drive cam and engaging the drive tumbler, a spring biasing the drive tumbler against the cup and the drive cam, and the cup carrying a button that, when pressed, moves the drive tumbler out of engagement with the drive cam. In another embodiment, the clutch is operated by a thumb screw projecting through a rear panel of the combination lock that moves an upper part of the drive cam out of engagement with a lower part of the drive cam, allowing relative rotation of the lock dial and the drive tumbler.