US3783660A

US3783660A - Pick resistant lock

Info

Publication number: US3783660A
Authority: US
Inventors: V Gill
Original assignee: Unican Security Systems Ltd
Current assignee: Unican Security Systems Ltd
Priority date: 1972-04-20
Filing date: 1972-04-20
Publication date: 1974-01-08
Anticipated expiration: 1991-01-08

Abstract

A pick resistant lock having a housing and a rotatable spindle therewithin, each having a registrable aperture. Interposed between the housing and spindle is a ring that is rotatable relative to said housing a limited distance and has an aperture registrable with said housing and spindle apertures. A tumbler is disposed in said registrable apertures and is biased to a normal position in which said spindle and ring must move together. When the spindle is torqued to pick the lock, the spindle and ring will rotate said limited distance to permit the apertures therein, and the tumbler, to move out of register with said housing aperture and thus prevent the unlocking of the lock. The tumbler is further provided with a chamfered end so that it cannot be felt when the tumbler moves into said housing aperture.

Description

o United States Patent 1 [111 3,73,000 Gill Jan. 8, 1974 PICK RESISTANT LOCK Primary ExaminerR0bert L. Wolfe 75 Inventor: Vincent A. Gill, Montclair, NJ.

[73] Assignee: Unican Security Systems, Ltd,

Montreal, Canada [57] ABSTRACT [22] Filed: Apr. 20; 11972 A pick resistant lock having a housing and a rotatable 21 A L N 245 73 spindle therewithin, each havlng a registrable aper- 1 0 6 ture. interposed between the houslng and spindle is a ring that is rotatable relative to said housing a limited [52] US. Cl. 70/363, 70/419 di tance and has an aperture registrable with said [51] Int. Cl EOSb 27/08, EOSb 63/00 housing and spindle apertures. A tumbler is disposed [5 Field Of ar 364 in said registrable apertures and is biased to a normal 70/364 378 position in which said spindle and ring must move together. When the spindle is torqued to pick the lock, References Cited the spindle and ring will rotate said limited distance to UNITED STATES PATENTS permit the apertures therein, and the tumbler, to move 3,045,468 7/1962 Welch..'; 70/364 A out of register with said housing aperture and thus 047 4 3 2 1912 'Aguenbmun 70 3 4 A prevent the unlocking Of the lOCk. The tumbler lS fur- 3,656,328 4/1972 Hughes 70/276 ther Provided with a Chamfefed end 50 that it Cannot 1,6l9,252 3/1927 George 70/363 be felt when the tumbler moves into said housing ap- 3,541,819 ll/l97O Kerr 70/363 ertul'e.

3,3l9,443 5/1967 Perlick 70/42l 3 Claims, 6 Drawing Figures FALSE 60 a 2 -6 SHEAR PLANE-- E SHEAR PLANE E 6 TRUE PAIENTEDJAM elm 3,783,660

sum 1 OF 2 n ,TRuE SHEAR PLANE FALSE SHEAR PLANE! E TRUE SHEAR PLANE PATENTED 8W4 3,783,660

1/2 FALSE ISHEAR 4'6 TRUZ I SHEAR PLANE PICK RESISTANT Loc BACKGROUND OF THE INVENTION Field of the Invention This invention relates to looks and, more particularly, to locks containing one or more pick resistant features.

Normal (non-master) locks have a relatively rotatable spindle and housing with normally aligned apertures and a tumbler in said apertures bridging the spindie and housing to prevent relative rotation. The key moves the tumbler to free the spindle and thus permit unlocking. A master lock interposes a ring between said spindle and housing to provide an additional master key unlocking position in which the spindle and ring are held together by the tumbler but both are free to rotate relative to the housing.

SUMMARY OF THE INVENTION The pick resistant lock of this invention comprises a spindle rotatably mounted within a housing with a ring positioned about the spindle between the housing and spindle; The housing, ring and spindle each have an opening therein, which openings are normally aligned when the lock is locked.

This invention makes use of a three-pin set of tumbler pins. A first tumbler pin of each set is normally positioned in an associated opening in the housing with its outer end at a false shear plane between the ring and housing and its inner end abutting a standard compression spring disposed within said housing opening. A second tumbler pin of each set is normally positioned in an associated opening in the ring with its inner end normally in contact with the outer end of the first tumbler pin at the false shear plane and its outer end normally within the' associated spindle opening. Further, a third thumbler pin of each set is normally positioned in an associated opening in the spindle with an inner end in contact with the second tumbler pin and an outer end accessible to a key. Thus, in the normal condition the spindle is free to rotate with the ring along the unobstructed false shear plane.

However, means areprovided for allowing only a limited rotation of the ring relative to the housing. The extent of such limited rotation is sufficient to misalign the ring openings with the housing cavities, but insufficient to open the look. When the spindle isrotated as far as permitted, the openings of the ring will be misaligned with the openings in the housing so that the tumbler pins ,of each set cannot be pushed toward the associated spring to advance the inner end of the third tumbler pin to the true shear plane, which is required to open the lock. Thus, the lock cannot be picked.

In a further aspect of this invention, means are included for insuring substantially unobstructed axial movement of the third tumbler pin of each set at least part of the way into the associated openings in the ring when the lock is in its normal locked position. In this manner, the picker is prevented from sensing or feeling the arrival of the inner end of each third tumbler pin at the true shear plane. Thus, even if the picker initially attempts to push the third tumbler pins to the true shear plane before applying torque, it will be substantially impossible for him to sense at what point such pins actually are at the true shear plane.

' These and other aspects and advantages of this invention shall be more clearly described below with reference to the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a cross-sectional view of a cylindrical key lock embodying this invention;

FIG. 2 is a cross-sectional view taken along line 2-2 of FIG. 1;

FIG. 3 is a cross-sectional view similar to FIG. 1 but showing a key inserted therein:

FIG. 4 is a cross-sectional view of a flat key lock embodying this invention;

FIG. 5 is a cross-sectional view taken along line 5-5 of FIG. 4; and

FIG. 6 is a cross-sectional view similar to FIG. 4 but showing a key inserted therein.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring. to FIG. 1, a pick resistant lock 10 of this invention is shown. Look 10 includes a cylindrical housing 11 defined by a cylindrical shell 12 and an annular header 20. Shell 12 is preferably fabricated of a suitable metal and has an interior cylindrical chamber 14 therein. Chamber 14 extends between a pair of opposing opened ends ]l6 and 18, respectively. Header 20, also preferably fabricated of metal, is mounted within chamber 114 at end 18 thereof and extends in a direction parallel to the axis of shell 12 from end 18 of chamber 14 to a point within chamber 14 approximately

intermediate ends

16 and 18 thereof. Header 20 is fixed in position relative to shell 12 by a pin 22 which is inserted through aligned

openings

24 and 26 defined in shell 12 and header 20, respectively.

Header 20 further includes a restraining pin 28 which is fitted within a corresponding aperture 29 in the header and which has a forward end 30 extending a predetermined distance from an outer end 32 of header 20. The purpose of pin 28 will be described in more detail below. Header 20 further includes a plurality of elongate cavities 34 each extending substantially parallel to the longitudinal axis of shell 12. The cavities 34 are preferably spaced uniformly about the header. Lock 10 further includes a ring 36 (FIGS. 1 and 2) which is rotatably mounted about a spindle 44 which is positioned for rotation within chamber 14 with an inner end 38 abutting outer end 32 of header 20 and an outer end 48 of the ring abutting a peripheral flange 46 of the spindle. Ring 36 further includes a plurality of openings 40 each extending through the ring in a direction substantially parallel to the axis of the housing shell when the ring is disposed within the shell about spindle 44. Openings 40 are equal in number to, and are registrable with cavities 34 in header 20.

It is preferred that header 20 contain seven cavities 34 and with seven corresponding openings 40 being formed in ring 36 wherein the cavities and openings are each spaced at approximately 45 intervals relative to each other about the header and ring, respectively. This leaves a gap of approximately between one adjacent pair of cavities 34 and one adjacent pair of openings 40. In accordance with this invention, a slot 42 is provided in end 38 of ring 36 at a location on said ring within the above mentioned 90 gap thereon. Spindle 44 is positioned in chamber 14 with end 30 of pin 23 received within slot 42, the length of pin 28 being sufficient for this purpose. It is important that the diameter of pin 28 be less than the width of slot 42. Preferably,

the diameter of pin 28 is approximately about one-half the width of slot 42.

Thus, pin 28 and milled slot 42 on ring 36 provide a means for permitting limited rotation of ring 36 about its axis and relative to housing 11. More specifically, ring 36 may be rotated in either direction, but only until a side of slot 42 engages pin 28. For purposes of future explanation, the interface between end 38 of ring 36 and end 32 of heading 20 shall be referred to as the false shear plane" of lock 10.

Spindle 44 is preferably circularly cylindrical in shape having a main shaft 45 and peripheral flange 46 which is also preferably annular in shape. The diameter of flange 46 is slightly less than the inner diameter of shell 12, whereas the spindle shaft has a diameter slightly less than the inner diameter of header 20. In this manner, spindle 44 is normally rotatable within chamber 14 relative to housing shell 12 and ring 36. In the future, the interface between flange 46 and ring 36 shall be referred to as the true shear plane of lock 10.

In accordance with this invention, flange 46 of spindle 44 includes a plurality of apertures 50, preferably seven, extending therethrough in a direction substantially parallel to the axis of shell 12. Apertures 50 are spaced identically as openings 40 in ring 36 and are normally aligned with said openings.

Look further includes a plurality of tumbler pin sets equal in number to the number of cavities 34 in header 20. Each tumbler pin set includes a first tumbler pin 60 which is normally positioned entirely within a corresponding cavity 34 with an inner end 62 thereof abutting spring 58 and an outer end 64 thereof normally located at the false shear plane. Each tumbler pin set additionally includes a second tumbler pin 66 which is nonnally positioned in a corresponding opening of ring 36 with an inner end 68 thereof abutting outer end 64 of first tumbler pin 60, and an outer end 70 thereof disposed within an aligned aperture 50 of spindle flange 46. Lastly, each tumbler pin set includes a third tumbler pin 72 which is positioned within said aligned aperture 50 of spindle flange 46 with an inner end 74 thereof abutting outer end 70 of second tumbler pin 66, and an outer end 76 thereof located near end 16 of chamber 14. Suitable biasing means such as springs 58 are employed to urge the tumbler pin sets to their normal positions. However, other biasing means, such as gravity or magnetics, may be employed.

Thus, in the normal locked position of lock 10, the outer end 64 of each first tumbler pin 60, as well as the inner end 68 of each second tumbler pin 66 are located at the false shear plane so that ring 36 may be rotated relative to housing 11 (Le. header and shell 12). However, as pointed out above, such rotation may be effected only to the extent that pin 28 is free from engagement with the side walls of slot 42 of ring 36. Further, and in the normal position of lock 10, spindle 44 may not be rotated freely relative to ring 36, inasmuch as second tumbler pins 66 are disposed within apertures 50 of spindle flange 46 as well as openings 40 of ring 36. Thus, in attemptingto pick the lock the spindle 44 will be torqued and this will cause simultaneous rotation of the spindle and ring 36. However, rotation of spindle 44 will be limited due to the cooperation of pin 28 in milled slot 42 of ring 36. The significance of the above limited rotation will be explained below.

Referring to FIG. 3, the manner of opening lock 10 with a conventional cylindrical key 78 will be described. Key 78 includes an annular shaft 80 which is dimensioned for receipt within annular slot 54 defined between shell 12 and spindle 44.

Key 78 further includes a plurality of grooves 88 respectively associated with the plurality of tumbler pin sets. The grooves are provided in the outer surface of key shaft 80 at a location near the forward end of the key with the relative spacing between the grooves being identical as that between the tumbler pin sets so as to be alignable with said sets when the key is inserted in lock 10. In addition, key shaft 80 is provided with an indexing flange 82 which, when disposed in indexing slot 56, properly locates each groove 88 relative to its associated tumbler pin assembly. Lastly, a flange 84 on the inside of key shaft 80 fits into keyway 52 on spindle 44.

When shaft 80 of key 78 is inserted into slot 54, end 90 of key 78 engages spindle flange 46, whereas a floor 92 of each groove 88 will engage the outer end 76 of the aligned third tumbler pin 72. It will be realized, therefore, that the depth of each groove 88 between end 90 of the key and floor 92 of the groove necessarily determine the extent to which the three tumbler pins of each set will be pushed inwardly through the aligned cavity 34, opening 40 and aperture 50 toward the associated spring 58.

As is conventional in most locks, the tumbler pins of each set preferably have different aggregate lengths so that each set must be moved inwardly a different distance before positioning the inner end 74 of the third tumbler pin of each set at the true shear plane. The plurality of grooves 88 of key 78 are thus dimensioned accordingly to bring the inner end of each third tumbler pin into position at the true shear plane when the key is inserted in the lock. Then, a torque exerted on key 78 will cause rotation of the key and the spindle because flange 84 is in spindle keyway 52. Rotation of the spindle is not in any way limited since the spindle is free of ring 36 and housing 20 at the true shear plane.

As mentioned before, a person trying to pick a lock usually first attempts to apply torque to the spindle of the lock. He then operates each tumbler to move them to the true shear plane whereupon the spindle is free to rotate to open the lock. In accordance with the present invention, a lock picker applying torque to spindle 44, such as by inserting a pin in the key slot 52 in the spindle and applying torque to the spindle, will actually succeed in simultaneously rotating both spindle 44 and ring 36 a limited distance until pin 28 engages a side wall of ring 36. At this point, openings 40 in ring 36 will be out of alignment with cavities 34 of header 20 so that a subsequent attempt to push the tumbler pins inwardly will be prevented Only a very slight rotation of spindle 44 and ring 36 by the picker will cause a sufficient misalignment between openings 40 and cavities 34 to prevent picking. As it is very difficult to pick a lock without torquing the spindle, the lock 10 is highly pick resistant. However, if an attempt to pick the lock is made, thus leaving the spindle out of alignment with housing 20, the spindle may easily be realigned for subsequent key operation of the lock.

This invention also seeks to prevent a lock picker from feeling the arrival of the inner end of each third tumbler pins 72 at the true shear plane if he elects to attempt to push the pins inwardly before applying torque to spindle 44. Thus, in accordance with this invention, inner ends 74 of third tumbler pins 72 are chamfered so that the outer diameter of each pin 72 at end 74 is substantially less than the diameter of openings 40 in ring 36.

In this manner, the picker will, at best, be able to detect when an edge 73, representing the innermost edge of the non-chamfered portion of each pin 72, arrives at the true shear plane. At this point, the front end 74 of each third tumbler pin 72 is disposed well within the opening 40 of ring 36 and not at the true shear plane. Thus, the lock cannot be picked in this manner either.

Preferably, the inner and outer edges of ring 36 at the periphery of holes are also chamfered so as to further ensure the non-engagement of third tumbler pins 72 with ring 36.

It is to be noted, as mentioned above, that the feature of chamfering the inner end of the outermost tumbler pin of each set may be applied to any present day lock, in addition to lock of this invention. For example, the outer tumbler pin of each tumbler pin set in a conventional non-master lock may have its inner end chamfered so as to prevent a picker from sensing the engagement of such end with the shell at the shear plane.

Reference is now had to FIGS. 4-6 wherein another embodiment of a lock of this invention, lock 100 is shown. Lock 100 is specifically designed for operation with a conventional flat key (such as a flat key 102 partially shown in FIG. 6).

Referring now to FIGS. 4 and S in detail, it will be seen that lock 100 includes a housing 103 defined by a cylindrical shell 104 which is hollowed out to form an inner cylindrical chamber 106. Chamber 106 is preferably circularly cylindrical and is preferably positioned eccentric to the longitudinal axis of shell 104. Further, chamber 106 extends between a pair of opposing ends thereof (only outer end 108 of chamber 106 being shown). Chamber 106 is thus bounded peripherally by an inner side wall 110 of shell 104. Shell 104 further in cludes an annular groove 112 formed in inner wall 110 at a location near end 108 of chamber 106. Groove 112 is dimensioned to receive an annular flange 114 of a conventional flat key lock spindle 116 which will be more completely described below.

In accordance with thisinvention, an elongate cylindrical ring 1 18 is inserted within chamber 106 with its outer side wall 120 rotatably fitted against inner side wall 110 of shell 104. Hereinafter, the interface between ring 1 18 and shell 104 shall be referred to as the flase shear plane. v

Spindle 116 is inserted within chamber 106 through ring 118 with an outer side wall 122 of the spindle being loosely fitted against ring 118 for rotation of the spindle in the chamber relativeto the ring.,As stated above, spindle 116 preferably includes an annular flange 114 which is receivedwithin annular groove 112 in shell 104. The interface between spindle side wall 122 and ring 118 shall be referred to hereinafter as the true shear plane. I

. As with lock 10, lock 100 includes a plurality of cavities 126 defined in shell 104 which are alignable with a corresponding plurality of openings 128 in ring 118 and with a corresponding plurality of apertures 130 in spindle 116. In a flat key lock, cavities126, openings 128 and apertures 130 each-extend elongately in a direction perpendicular to the longitudinal axis of chamber 106 of shell 104Yand are spaced from one another along the longitudinal axis of the lock. Further, a conventional key slot 132 (FIG. 5) is provided through the spindle in a direction along the longitudinal axis of the spindle.

In accordance with the principles of this invention, a set of tumbler pins are provided for each alignable set of cavities 126, openings 128 and apertures 130. More specifically, a first tumbler pin 134 is normally positioned within a corresponding cavity 126 with an outer end 136 thereof in contact with a conventional tumbler spring 138, also positioned within cavity 126, and an inner end 139 of the pin normally positioned at the false shear plane. A second tumbler pin 140 is nonnally positioned in a corresponding opening 128 of ring 118 with an outer end 142 of the second tumbler pin at the false shear plane and an inner end 144 of the second tumbler pin within a corresponding and aligned aperture 130 of spindle 116 thereby preventing relative rotation between ring 118 and spindle 116. Lastly, a third tumbler pin 146 of each set is positioned within a corresponding aperture 130 of spindle 116 with an outer end 148 thereof in contact with the second tumbler pin and an inner end 150 of the third tumbler pin disposed within key slot 132 at a location thereat preferably intersecting the longitudinal axis of spindle 116.

In accordance with this invention, a segment 152 (FIG. 5) of outer side wall 120 of ring 118 is slotted to receive a limit pin 156 protruding through an opening 158 in inner side wall 110 of shell 104. The cooperation of pin 156 and slot 152 is entirely similar to the cooperation between pin 28 and slot 42 of lock 10. In other words, ring 118 may be rotated relative to shell 104 only until pin 156 engages either side of slot 152. Thus, a picker applying torque to the spindle to rotate it will succeed to a limited extent sufficient to misalign openings 130 with cavities 126. This prevents the picker from manually pushing in the tumbler pins to position the inner ends of the third tumbler pins at the true shear plane.

Also, and as with lock 10, the inner ends 148 of the third tumbler pins 146 are preferably chamfered so that a picker may not feel" when such ends arrive at the true shear plane. Thus, if the picker pushes the tumbler pins inwardly, he might push too far bringing the inner end of a third tumbler. pins into an aligned opening 130 so that ring 118 will be caused to rotate along with spindle 116 thereby again limiting rotation until pin 156 engages a side wall of 'slot 152. If he doesnt push far enough, the second tumbler pin will cause simultaneous rotation of spindle 116 and ring 118.

The method of opening lock 100 with a flat key 102 shall be described with reference to FIG. 6. As is conventional, flat key 102 includes a plurality of notches 160 cut beneath an upper edge 162 thereof. The spacing between notches 160 is identical to that between the tumbler pin sets. When key 102 is inserted within key slot 132, the front end of the key (not shown) will engage seriatim the third tumbler pins of each set forcing each set of tumbler pins toward springs 138. When the keyis fully inserted in slot 132, each third tumbler pin will be nested at its inner end within corresponding notch 160 of the key. The notches are cut of a depth such that the outer end of each third tumbler pin will be positioned at the true shear plane. Thus, a

torque exerted on key 102 will cause spindle 116 to rotate freely within chamber 106 and relative to ring 118 to open the lock.

Although the invention has been described with reference to a number of specifically illustrated embodiments, the invention is not to be so limited. Rather, the invention is deemed to include obvious modifications and changes to the above described embodiments.

For example, a cylindrical key lock may be fabricated with a housing shell and header formed as an integral unit. Thus, reference in the claims to a housing shall be deemed to include an integral shell/header unit, as well as separate shell'and header parts (lock 10). Further, the term housing is deemed to include a shell alone, as in the case of lock 100.

Other obvious modifications and changes are contemplated is this invention, the precise scope of which is to be defined by the following claims.

What is claimed is:

l. A pick resistant lock comprising:

a. a hollow housing having an elongate cavity formed therein;

b. a spindle rotatably mounted within said housing and including an aperture extending therethrough at a location thereon for alignment with said cavity;

0. ring means rotatably mounted about the spindle between the spindle and the housing and including an opening extending therethrough at a location thereon for alignment with said cavity and said aperture, the ring means and spindle being normally positioned with said cavity, aperture and opening in alignment, a true shear plane being defined at an interface between the spindle and ring means and a false shear plane being defined at an interface between the ring means and housing;

(1. a tumbler assembly comprising a first pin positioned in said cavity, said first pin having an outer end normally positioned at said false shear plane, a second pin normally positioned in said opening with an inner end thereof normally in contact with said first pin at said false shear plane and an outer end thereof normally within said aperture, a third pin normally positioned in said aperture with an inner end thereof normally in contact with said second pin, the third pin being accessible to a key from without the housing, the second pin normally preventing relative rotational movement between the spindle and ring, and means for biasing said tumbler'assembly to its normal position;

e. means for allowing only a limited rotation of the ring means relative to the housing, which limited rotation is sufficient in extent to effect a misalignmerit of the openings in the ring with the cavities in the housing; and

f. means for enabling substantially unrestrained axial movement of said third pin at least part of the way into the opening of said ring means.

2. A pick resistant lock comprising:

a. a hollow housing having an elongate cavity formed therein;

c. ring means rotatably mounted about the spindle between the spindle and the housing and including an opening extending therethrough at a location thereon for alignment with said cavity and said aperture, the ring means and spindle being normally positioned with said cavity, aperture and opening in alignment, a true shear plane being defined at an interface between the spindle and ring means and a false shear plane being defined at an interface between the ring means and housing;

d. a tumbler assembly comprising a first pin positioned in said cavity, said first pin having an outer end normally positioned at said false shear plane, a second pin normally positioned in said opening with an inner end thereof normally in contact with said first pin at said false shear plane and an outer end thereof normally within said aperture, a third pin normally positioned in said aperture with an inner end thereof normally in contact with said second pin, the third pin being accessible to a key from without the housing, the second pin normally preventing relative rotational movement between the spindle and ring, and means for biasing said tumbler assembly to its normal position;

e. means for allowing only a limited rotation of the ring means relative to the housing, which limited rotation is sufficient in extent to effect a misalignment of the openings in the ring with the cavities in the housing; and

f. wherein the cavity, aperture and opening each extend elongately in a direction parallel to the longitudinal axis of the housing.

3. The lock of claim 1, wherein the enabling means includes the inner end of said third tumbler pin being chamfered to an outer diameter substantially less than the diameter of said opening.

Claims

1. A pick resistant lock comprising: a. a hollow housing having an elongate cavity formed therein; b. a spindle rotatably mounted within said housing and including an aperture extending therethrough at a location thereon for alignment with said cavity; c. ring means rotatably mounted about the spindle between the spindle and the housing and including an opening extending therethrough at a location thereon for alignment with said cavity and said aperture, the ring means and spindle being normally positioned with said cavity, aperture and opening in alignment, a true shear plane being defined at an interface between the spindle and ring means and a false shear plane being defined at an interface between the ring means and housing; d. a tumbler assembly comprising a first pin positioned in said cavity, said first pin having an outer end normally positioned at said false shear plane, a second pin normally positioned in said opening with an inner end thereof normally in contact with said first pin at said faLse shear plane and an outer end thereof normally within said aperture, a third pin normally positioned in said aperture with an inner end thereof normally in contact with said second pin, the third pin being accessible to a key from without the housing, the second pin normally preventing relative rotational movement between the spindle and ring, and means for biasing said tumbler assembly to its normal position; e. means for allowing only a limited rotation of the ring means relative to the housing, which limited rotation is sufficient in extent to effect a misalignment of the openings in the ring with the cavities in the housing; and f. means for enabling substantially unrestrained axial movement of said third pin at least part of the way into the opening of said ring means.

2. A pick resistant lock comprising: a. a hollow housing having an elongate cavity formed therein; b. a spindle rotatably mounted within said housing and including an aperture extending therethrough at a location thereon for alignment with said cavity; c. ring means rotatably mounted about the spindle between the spindle and the housing and including an opening extending therethrough at a location thereon for alignment with said cavity and said aperture, the ring means and spindle being normally positioned with said cavity, aperture and opening in alignment, a true shear plane being defined at an interface between the spindle and ring means and a false shear plane being defined at an interface between the ring means and housing; d. a tumbler assembly comprising a first pin positioned in said cavity, said first pin having an outer end normally positioned at said false shear plane, a second pin normally positioned in said opening with an inner end thereof normally in contact with said first pin at said false shear plane and an outer end thereof normally within said aperture, a third pin normally positioned in said aperture with an inner end thereof normally in contact with said second pin, the third pin being accessible to a key from without the housing, the second pin normally preventing relative rotational movement between the spindle and ring, and means for biasing said tumbler assembly to its normal position; e. means for allowing only a limited rotation of the ring means relative to the housing, which limited rotation is sufficient in extent to effect a misalignment of the openings in the ring with the cavities in the housing; and f. wherein the cavity, aperture and opening each extend elongately in a direction parallel to the longitudinal axis of the housing.