GB2506580A

GB2506580A - Pin tumbler lock resistant to picking

Info

Publication number: GB2506580A
Application number: GB1214577.7A
Authority: GB
Inventors: Christopher Eric Belcher
Original assignee: UAP Ltd
Current assignee: UAP Ltd
Priority date: 2012-08-15
Filing date: 2012-08-15
Publication date: 2014-04-09
Anticipated expiration: 2032-08-15
Also published as: GB201214577D0; GB2506580B

Abstract

A key operated pin tumbler lock comprises a plurality of spring loaded pin stacks that each comprise a driver pin 26, 27, and a key pin 28, 29; the height of the key pins are selected from a biting range and matched with a driving pin such that all pin tumbler stacks are of equal length/height. A plurality of the driving pins 26 have a shoulder and reduced diameter at the end abutting the key pin adjacent the shear line. A further driving pin 27 has no shoulder, the pin having a constant diameter and is shorter than the other driving pins. The driving pin 27 is preferably located at the first tumbler position upon entry of the key and acts as a control pin matched with the longest key pin29 resisting rotation during a bumping attack and providing time for the other pins to fall back wherein the reduced smaller diameter pin ends pass the shear line and prevent further rotation. A second shoulder may be included having a third intermediate diameter with a conical tapered end (36, fig.5).

Description

IMPROVEMENTS TO KEY-OPERATED PIN TUMBLER LOCKS

The present invention relates to key-operated pin tumbler locks and to a set of driver pins for use in such a lock.

In a conventiona' key-operated pin tumbler lock, as shown schematically in Figs. i and 2, an outer shell or cylinder i of the lock has a cylindrical hole housing a rotatable plug or core 2. To open the lock, the core 2 must rotate relative to the shell 1 in order to operate a cam or lever mechanism (not shown) that controls withdrawal of a latch or bolt (not shown). A keyway 3 is formed in the core 2 to allow a key 4 to be inserted into the core 2, as shown in Fig. 2. Communicating with the keyway 3 is a series of identical bores, typically five or six in number although only four are shown in Figs. I and 2, that are drflled at right an&es to the keyway 3 through the core 2 and into the shell 1 these bores form a series of pin chambers 5. Within each of the pin chambers is located a pin stack that comprises a spring-loaded driver pin 6 stacked over a key pin 7. A ledge or other detent 8 within the core2 is used to prevent the pin stack from falling out of the bore.

When a key 4 is inserted into the keyway 3, notches 9 on the blade of the key 4 act on the key pins 7 and push them, against the force of the spring-loaded driver pins 6, into the pin chambers 5. If the correct key 4 is fully inserted into the keyway 3, the notches 9 between teeth 10 on the key 4 align with the key pins 7 and are of such a depth in the key 4 that the interfaces 11 between all of the key pins 7 and the driver pins 6 are aligned along a line 12 where the shell I and the core 2 meet. This line 12 is called the shear line as should all of the interfaces 11 align with it, as shown in Fig. 2, the core 2 can be rotated within the shell I by the body of the key 4 to open the lock.

However, when there is no key 4 in the keyway 3 or when a wrong key is inserted into the keyway 3, the interfaces 11 between the key pins 7 and their associated driver pins 6 do not align and the driver pins 6 stradffle the shear line so that the core 2 cannot be turned r&ative to the shell I, as shown in Fig. 1.

It will be appreciated that although the pin chambers 5 are identical and the detents 8 are located at the same level within the core 2, the heights of the pin stacks in each of the pin chambers 5 are different from one another. Locks vary because the combinations of heights of the pin stacks in every lock are deliberately designed to be different. The bitting of the lock refers to the particular combination of pin stack heights in the lock. Though each manufacturer is different, usually there is a range of as many as nine or ten possible heights of key pins 7 in a bitting range as the heights of the driver pins 6 in any given lock are usually identical. The heights in this range increase incrementally. Bittings can, therefore, be represented as a code that is usually a series of integers, for example 316482, where each integer can be translated from a key code chart or from a bitting code list issued by the lock manufacturer into a pin height. It will be appreciated that the bitting code for a lock instructs a locksmith how a key is to be cut for that lock. Each digit in the bitting code corresponds to a different cut or notch 9 on the key and represents the depth at which the key must be cut, In addition, the position of the number in the sequence indicates the location of the cut on the key blank. Depending on the maker, the bitting sequence can be from bow-to-tip along the blade of the key, the bow being the larger, handle portion of the key, or can be from tip-to-bow. Conventional locks typically have between four and six pin stacks and the bitting code will, therefore, have a corresponding number of digits.

Lock bumping is a known technique for opening a conventional pin tumbler lock of the type described above and is usually employed by locksmiths for opening locks when the correct key has been ost. 1-lowever, recently criminals have started to take advantage of the technique using a specially-made bump key'. Such a key can be used to open all locks of the same type and typicafly comprises a key similar to that used for the locks in question but with identically sized notches that will interact with all of the key pins 7 of the lock. The bitting of these notches is usually at the greatest depth of the bitting range for the lock. In order to bump' the lock a bump key is inserted into in the lock one notch out along the keyway 3 so that it protrudes slightly from the lock. A sudden quick impact is then applied to the key to force it deeper into the keyway 3 while at the same time a rotational force is applied to the key in a direction that wo&d open the lock.

The impact transmits an instantaneous force to all of the key pins 7 of the lock which in turn transmit the force to their associated driver pins 6 that move upwards against the force of their spring loading 13. The energy transmitted to the pin stacks is usually sufficient to cause the driver pins 6 to separate from their key pins 7 and for a fl-action of a second to isolate the driver pin 6 in the shefi I and the key pin 7 in the core 2, as shown in Fig. 3.

As all the pin stacks are activated at the same time, during the fl-action of a second when the pins 6 and 7 are apart it is possible to rotate the core 2 in the shell ito open the lock.

One known countermeasure to prevent lock bumping is to employ a damping oil or gel which is used to fill the core 2. The oil or gel damps the transmission of forces within the pin stacks so that there is no separation between the key pin 7 and the driver pin 6. This means that the lock cannot be opened. The problem with this countermeasure, however, is that solvents can be employed by criminals to destroy the damping effect of the oil or gel prior to bumping.

The object of the present invention is to provide a pin tumbler lock which uses mechanical means rather than chemical means to obviate or substantially mitigate its susceptibility of being opened by a lock bumping technique.

According to a first aspect of the present invention there is provided a key-operated pin tumbler lock comprising a shell; a core rotatably disposed relative to the shell and defining a keyway and a plurality of bores that communicate with the keyway through the core and that, in a locked position of the lock, communicate with corresponding bores formed in the shell to define a plurality ofpin chambers; and a plurality of spring-loaded pin stacks that each comprise a driver pin and a key pin with an interface therebetween, that are respectively located within the plurality of pin chambers and that are moveable within the pin chambers against the spring-loading by a key inserted in the keyway through a keyhole into a position wherein the core can be rotated relative to the shell, the key pins each having a height randomly selected from a bitting range; and wherein the height of each driver pin is matched to its respective key pin so that pin stacks all have the same height; wherein a plurality of the driver pills each define a shou'der part-way along its length in order that a part of each of these driver pins adjacent its respective key pin defines a diameter smaller than a greater diameter of a part of each of these driver pins adjacent the spring-loading; and wherein another driver pin other than those comprising said plurality of driver pins is shorter than any of the driver pins of said plurality of driver pins.

Preferably, the key pin associated with said other driver pin has a length in the higher end of the bitting range.

Preferably also, the shoulders of said plurality of driver pins align when there is no key inserted in the keyhole of the lock. Advantageously, the shoulders of said plurality of driver pins align when there is no key inserted in the keyhole of the lock.

Preferably also, the length of said one driver pin is equal to the lengths of the parts of the driver pins having the smaller diameter.

Preferably aho, said one driver pin and its associated key pin are located in one of the first three pin chambers along the keyway counting from the keyhole end.

According to a second aspect of the present invention there is provided a set of driver pins for a key-operated pin tumbler lock in accordance with the first aspect of the present invention.

According to a third aspect of the present invention there is provided a set of driver pins for a key-operated pin tumbler lock wherein all bar one of the driver pins defines a shoulder part-way along its length in order that a part of the driver pin defines a diameter smaller than a greater diameter of a part of the driver pin and wherein said one driver pin is shorter than the other driver pins.

Preferred additional features of the various aspects of the invention are described in the dependent claims appended hereto.

Embodiments of the various aspect of the present invention will now be described by way of example with reference to the accompanying drawings, in which:-Figs. 1 and 2 are schematic longitudinal cross sections of a conventional key-operated pin tumbler lock mechanism showing respectively the lock in a locked position and the lock with a key inserted that unlocks the lock; Fig. 3 is a diagram of a longitudinal cross section of a pin chamber of a the ock mechanism shown in Figs. I and 2 showing what happens during a lock bumping attack; Fig. 4 is a schematic longitudinal cross section of an embodiment of key-operated pin tumbler lock mechanism in accordance with the first aspect of the present invention showing the lock in a locked position; Fig. 5 is a perspective view of a set of ten driver pins in accordance with the second aspect of the present invention for use with key pins having a bitting range between 1 and 10 inclusive; Figs. 6a and 6b are side elevations to an enlarged scale of two of the driver pins forming part of the set shown in Fig. 5 with their respective key pins; and Fig. 7 is a diagram similar to Fig. 3 of a lock mechanism in accordance with the present invention.

An embodiment of key-operated cylindrical pin tumbler lock in accordance with the invention and as shown in Fig. 4, is similar to a conventional pin tumbler lock as described above with reference to Figs. 1 and 2 in that it comprises a shell 20, a core 21 defining a shear line 22 with the shell 20, a keyway 23 formed in the core 21 and a plurality of bores disposed at right ang'es to the longitudinal axis of the core 21 along the length of the keyway 23 that form pin chambers 24. Where the lock differs is in the construction of the pin stacks 25 that are located in the pin chambers 24.

Each of the pin stacks 25 comprises a driver pin 26 or 27 and a key pin 28 or 29 respectively with an interface 30 therebetween that is moveable within its respective pin chamber against a spring-loading 31 by a key (not shown) inserted into the keyway 23. Unlike conventional locks, wherein the driver pins are all identical, in the present invention the height of each driver pin 26, 27 is matched to its respective key pin 28, 29 so that all of the pin stacks 25 have the same height but the key pins 28 each have a height randomly selected from a bitting range. Hence, the driver pins 26, 27 have a height that is comp'ementary to the bitting height of the key pins 28, 29. In addition, each of the driver pins 26, other than one driver pin 27, defines a shoulder 32 part-way along its length. As shown in more detail in Figs. 5 and 6b, the shoulder 32 divides the pin 26 into a part 33 adjacent the respective key pin 28 that has a diameter dl smaller than a part 34 adjacent the spring loading 31 that has a greater diameter d2. The driver pins 26 that have key pins 28 at the middle and lower end of the bitting range preferably each define a second shoulder 35 adjacent its interface 30 with its respective key pin 28. The end of the pin 26 in contact with the key pin 28 thereby comprises a cap 36 that has a diameter d3 that is greater than the smaller diameter dl but smaller than the greater diameter d2. The end is preferably chamfered so that the cap 36 is conical.

The aforesaid one driver pin 27 may be termed a control driver pin and is different to the other pins 26 as it is cylindrical with the same diameter d4 along the whole of its length. The diameter d4 is the same as the greater diameter d2 of the pins 26. The pin 27 is also shorter than the driver pins 26 such that the key pin 29 associated with this driver pin 27 has a length in the higher end of the bitting range. in most embodiments of the invention the bitting range will comprises ten differing key pin lengths from which the lengths of the pin stacks 25 in any given lock will be selected. With such a bitting range, the key pin 29 associated with the control driver pin 27 has an 8, 9 or 10 bitting range length, the length of the other key pins 28 being smaller. In afi cases, the shoulders. 32 of the driver pins 26 align with one another when there is no key inserted in the lock. In addition, the shoulders 32 of the driver pins 26 align with the interface between the control driver pin 27 and its associated key pin 29. This means that the length 11 of the control driver pin 27 is the same as the length 12 of the part 34 of the other driver pins 26, the length 12 being the same in all of the driver pins 26, the length of the narrow part 33 of the pins 26 differing. The interface 30 between the control driver pin 27 and its associated key pin 29 is also located at or irnrnediat&y adjacent the shear line 22 of the lock. This means that the shoulders 32 of the driver pins 26 which are aligned with one another when there is no key inserted in the ock are also located at or immediately adjacent the shear line 22.

Fig. 5 shows a set often driver pins 26,27 adapted for use with a set of key pins 28,29 having a bitting range between land 10 inclusive. In this set, the control driver pin 27 will be matched with a key pin having a bitting height of 10. The driver pins labelled 26A, 26B and 26C are matched with key pins 28 having bitting heights of 9, 8 and 7 respectively and are not provided with caps 36. The only reason for this is that the parts 33 of these pins are not sufficiently long to enable a cap 36 to be provided. The remaining six pins 26 are provided with caps 36 and are matched with key pins 28 having bitting heights between I and 6 respectively.

In a representative embodiment of the invention, the lengths 1/and 12 maybe of the order of 5 mm, the minor diameter dl of the pins 26 maybe of the order of 1.7 mm and the major diameter d2 of the pins 26 and the diameter d4 of the pin 27 may be of the order of 2.7 mm. This is represented in Figs 6a and 6b, wherein Fig. 6a is a drawing of a control driver pin 27 and its respective key pin 29 and Fig. 6b is a drawing of a driver pin 26 and its respective key pin 27. In this embodiment the difference in diameter between parts 33 and 34 of the pins 26 is around 1.0 mm, corresponding to an annular difference of 0.5 mm. The diameter c13 of the cap 36 is of the order of 2.3 mm. Hence, the difference in diameter between part 33 and the cap 36 of the pins 26 is around 0.6 mm, corresponding to an annular difference of 0.3 mm. Preferably, the cap 36 is chamfered at 45° to form a cone.

In use in a key-operated pin tumbler lock such as that shown in Fig. 4, there are preferably at east six pin chambers 24. The contr6l driver pin 27 and its associated key pin 29 should be located in one of the first three pin chambers 24 counted along the keyway 23 from its end defining a keyhole 37. Preferably, the control driver pin 27 is located in the first pin chamber 24 adjacent the keyhole 37, as shown in Fig. 4. This is because it has been found that during lock bumping, the pin stacks in the pin chambers 24 closer to the keyhole 37 are the first to move and in the present invention, as described in more detail below, it is preferable for the control driver pin 27 and its associated key pin 29 to be if not the first, among the first, of the pin stacks to move. In some embodiments, in particular in those with at least six pin chambers 24, two and at most three contr& driver pins 27 may be provided.

During lock bumping as described above, it is the received wisdom that the instantaneous force applied to each driver pin causes it to separate from its key pin for a fraction of a second as the pin stacks move upwards against the force of the spring loading allowing the core to be rotated in the shell to open the lock. However, it has been found experimentally by the applicant that this is usually not the case. With particular reference to Figs. 3 and 7 both of which reference numbers are referred to in the following description separated by forward slash I', the applicant has found that when an instantaneous force is applied to each driver pin 6/26. 27 by the bump key, the driver pins 6/26, 27 and their associated key pins 7/28, 29 move uniformly and act as a single pin against the bias of their spring loading 13/31. This happens in the present invention as well as in conventional key-operated pin tumbler locks. In both cases, the clearance between the pin chambers and the pin stacks is small but such that the rotational force applied by the key is able to rotate the core 2/21 relative to the shell 1/20 by a small amount that misaligns the bores in the shell 1/20 and the core 2/21 and reduces the effective diameter of the pin chamber at the shear line 12/22, as shown in Figs. 3 and 7. When the interface 11/30 between the driver pins 6/26, 27 and the key pins 7/28, 29 coincides with the shear line 12/22, the rotational force being applied to the core 2/21 in some cases may be sufficient to stop further movement of the key pins 7/28, 29. However, the applicant has found that in most cases both driver pins 6/26, 27 and key pins 7/28, 29 continue to move as a single entity against the force of the spring-loading 13/31 until the force of the spring-loading 13/31 is sufficient to stop and then reverse movement of the pins 6/26, 27 and 7/28, 29 back along the pin chambers. However, on the return travel of the pins 6/26, 27 -j_o -and 7/28. 29 along the pin chambers, the pins 6/ 26, 27 and 7/28, 29 have lost most of the energy imparted by the impact.

In a conventional key-operated pin tumbler lock such as that represented in Figs ito 3, the reduction in the effective diameter of the pin chambers (see Fig. 3) coupled with this loss of energy is usually sufficient to slow the driver pins 6 so that the driver pins 6 separate from their key pins 7 for a fraction of a second isolating the driver pin 6 in the shell 1 and the key pin 7 in the core 2. However, during this fraction of a second when all of the pins 6 and 7 are apart because all of the pin stacks have been activated at the same time, the rotational force being applied to the key is sufficient to the rotate the core 2 in the shell I sufficiently so that the ock can be opened.

In contrast, in the present invention, as shown in Fig. 7. the reduction in the effective diameter of the pin chambers 24 whilst it may slow the driver pins 26 does not stop the narrower parts 33 of the driver pins 26 adjacent their respective key pins 28 from penetrating into the bore defining the pin chamber 24 in the core 21 and straddling the shear line 22. This prevents the core 21 from further rotation relative to the shell 20 and thereby thwarts lock bumping.

The function of the control driver pin 27 and its associated key pin 29 will now be described. The control driver pin 27 is always the driver pin 27 with the shortest length so that its associated key pin 29 is the longest of the key pins in the lock. When a bump key is inserted into the lock and impacted, the key pin 28 immediately crosses the shear line 22 and moves with the control driver pin 27 against the force of the spring loading 31 as do the other control pins 26 and key pins 28. The control driver pin 27, key pin 29 combination has a greater mass than that of the other pin stacks 25 as the driver pin is not narrowed. The length of the key pin 29 and the great mass of the control driver pin 27, key pin 29 combination ensures that the control pin 27. key pin 29 pin stack takes the longest time to return to a position wherein the interface 30 is again adjacent the shear line 22. Also, location of -11 -the control driver pin 27 and its associated key pin 29 in a pin chamber 24 close to the keyhole 37 makes it the first or one of the first pin stacks 25 to move. The time period during which the key pin 29 straddles the shear line 22 provides a delayduring which most ifnot all of the other pin stacks 25 can move against the force of their spring-loading 31 and be returned to a position wherein the key pins 28 have re-entered the core 21. All through this time period, the key pin 29 has only allowed a small rotation of the core 21 relative to the shell 20, as shown in Fig. 7, dependent on the clearance between the pins 27, 29 and the walls of the pin chamber 24. Typically, this clearance is of the order of 0.0254mm so that the core 2 can relative to the shell 20 to misalign the bores by around 0.0508 mm. Unlike a conventional key-operated pin tumbler lock, this misalignment is not sufficient to prevent the narrow parts 33 of the pins 26 from penetrating into the bores of the core 21 on their return down the pin chamber 24 during lock bumping. as shown in Fig. 7. These narrow parts 33 of the pins 26 then prevent further rotation of the core 21 relative to the shell 20 even if the control driver pin 27 and the key pin 29 are isolated in the shell 20 and the core 21 respectively. The control driver 27 and its key pin 29 have served their purpose at this point by providing the time necessary for the other driver pins 26 to return down the pin chambers 24 and locate in the bores of the core 21. If a rotation force is still appfied to the core 21, the pins 26 rotate slight'y pushing the ends of the narrow parts 33 towards the walls of the bores in the core 21. The caps 36 on the longer pins 26 can now come into play. These do not prevent the narrow parts of the driver pins 26 from penetrating the bores in the core 21. They do, however, frustrate an attempt to pick the lock by making it difficult to push the pins 26 into the shell using a lock pick tool acting on the respective key pins 28. Rotation of the pins 26 with the caps 36 within the pin chamber caused by the force being applied to the core 21 causes the caps 36 to bind against the wall of the bore in the core 21 thereby jamming the pins 26 and preventing their movement up the pin chamber 24.

As it is the size and shape of the driver pins 26, 27 that are at the heart of the present invention rather than the structure of the lock mechanism -12 -itself, it will be appreciated that a set of driver pins in accordance with the second aspect of the present invention may be retro-fitted to any suitable lock mechanism to reduce the risk of it being bumped. Preferably, such a lock and locks in accordance with the first aspect of the present invention are provided with a lock combination that comprises a wide range of driver pin 26 heights as these are the hardest locks to bump.

Claims

-j_3 -CLAIMS1. A key-operated pin tumbler lock comprising a shell; a core rotatably disposed relative to the shell and defining a keyway and a plurality of bores that communicate with the keyway through the core and that, in a locked position of the lock, communicate with corresponding bores formed in the shell to define a plurality of pin chambers; and a plurality of spring-loaded pin stacks that each comprise a driver pin and a key pin with an interface therebetween, that are respectively located within the plurality of pin chambers and that are moveable within the pin chambers against the spring-loading by a key in serted in the keyway through a keyhole in to a position wherein the core can be rotated relative to the shell, the key pins each having a height randomly selected from a bitting range; and wherein the height of each driver pin is matched to its respective key pin so that pin stacks all have the same height; wherein a plurality of the driver pins each define a shoulder part-way along its length in order that a part of each of these driver pins adjacent its respective key pin defines a diameter smaller than a greater diameter of a part of each of these driver pins adjacent the spring-loading; and wherein another driver pin other than those comprising said plurality of driver pins is shorter than any of the driver pins of said plurality of driver pins.
2. A lock as claimed in Claim 1, wherein the key pin associated with said other driver pin has a length in the higher end of the bitting range.
3. A lock as claimed in Claim 1 or Claim 2, wherein the bitting range comprises ten differing key pin lengths.

-j_4 -
4. A lock as claimed in Claim 3. wherein the key pin associated with said other driver pin has an 8, 9 or 10 bitting range length.
5. A lock as daimed in any of Claims Ito 4, wherein the shoulders of said plurality of driver pins align when there is no key inserted in the keyhole of the lock.
6. A lock as claimed in Claim 5, wherein the aligned shoulders of the driver pins are located at or immediately adjacent the shear line when there is no key inserted in the keyhole of the lock.
7. A lock as claimed in any of Claims I to 6, wherein the interface between said other driver pin and its associated key pin is located at or immediately adjacent the shear line when there is no key inserted in the keyhole of the lock.
8. Aock as claimed in ally of Claims Ito 7, wherein the lengths of the parts of the driver pins having the smaller diameter are equal.
9. A lock as claimed in Claim 8, wherein the length of said other driver pin is equal to the lengths of the parts of the driver pins having the smaller diameter.
10. A lock as claimed in any of Claims Ito 9, comprising at least five pin chambers.
11. A lock as claimed in any of Claims Ito 10. comprising at least six pin chambers and wherein said other driver pin and its associated key pin are located in one of the first three pin chambers along the keyway counting from the keyhole end.

-i_s -
12. A lock as claimed in any of Claims I to 11, wherein said other driver pin and its associated key pin are located in the first pin chamber along the keyway counting from the keyhole end.
13. A lock as claimed in any of Claims ito 12, wherein at least one of the driver pins of said plurafity of driver pins defines a second shoulder whereby an end of the pin in contact with the key pin has a diameter greater than said smaller diameter but smaller than said greater diameter.
14. A lock as claimed in Claim 13, wherein said end of said at least one pin in contact with the key pin has a conical shape.
15. A lock as daimed in Claim 13 or Claim 14, wherein the driver pins associated with key pins having a bitting range between 1 and 6 inclusive define said second shoulders.
16. A set of driver pins for a key-operated pin tumbler lock as claimed in any of Claims lto 15.
17. A set of driver pins for a key-operated pin tumbler ock wherein aH bar one of the driver pins defines a shoulder part-way along its length in order that a part of the driver pin defines a diameter smaller than a greater diameter of a part of the driver pin and wherein said one driver pin is shorter than the other driver pins.
18. A set of driver pins as claimed in Claim 17, wherein the engths of the parts of the driver pins having the smaller diameter are equal.
19. A set of driver pins as claimed in Claim 18, wherein the length of said one driver pin is equal to the lengths of the parts of the driver pins having the smaller diameter.

-16 -
20. A set of driver pins as claimed in any of Claims 17 to 19, wherein at least one of the driver pins other than said one driver pin defines a second shoulder whereby an end of the pin adjacent the part defining the smaller diameter has a diameter greater than said smaller diameter but smaller than said greater diameter.
21. A set of driver pins as claimed in Claim 20, wherein said end or ends of the pins has or have a conical shape.
22. A set of driver pins as claimed in Claim 20 or Claim 2i, wherein the driver pins of the set that associate with key pins having a bitting range between I and 6 indusive define said second shoulders.
23. A key-operated tumbler lock substantiafly as described herein with reference to Figs. 4 to? of the accompanying drawings.
24. A set of driver pins for a key-operated tumbler lock substantially as described herein with reference to Figs. 5, 6a and 6b of the accompanying drawings.