GB2483888A

GB2483888A - Mortise latch

Info

Publication number: GB2483888A
Application number: GB201015966A
Authority: GB
Inventors: Robin Graham
Original assignee: Zoo Hardware Ltd
Current assignee: Zoo Hardware Ltd
Priority date: 2010-09-23
Filing date: 2010-09-23
Publication date: 2012-03-28
Anticipated expiration: 2030-09-23
Also published as: GB2483888B; GB201015966D0

Abstract

A mortise latch comprising a casing 3 and a bolt 9 which is movable between an extended position in which the bolt projects from the casing and a retracted position in which the bolt is at least partially retracted into the casing. The mortise latch further comprises a follower 11 which is operatively connected to the bolt and is operable to move the bolt from the extended position to the retracted position. The latch further comprises at least one means arranged to apply a force to the follower to resist movement of the follower in a direction which moves the bolt from the extended position to the retracted position, and an adjustment means which is operable to vary the force applied by the force applying means. The means to apply a force may comprise two discrete settings, one corresponding to the absence of force on the follower. The means for applying a force may comprise resilient means, which may be springs. The means for applying a force may be accessible from the casing exterior.

Description

Mortise Latch The present invention relates to a mortise latch.

A mortise latch is designed to be fined into a pocket (mortise) that is cut into the side of a door and is usually positioned in the side of the door opposite the hinged side and arranged to interact with a strike plate and socket recessed into the door frame.

A mortise latch typically comprises a casing, a latch bolt, a follower for receiving a spindle and a deadbolt mechanism. The latch bolt is usually linearly moveable into and out from the casing and is configured to be urged outwardly from the casing by a resilient spring, although external pressure on the latch bolt can be applied to force the latch bolt back into the casing, against the action of the spring.

The follower interacts with the latch bolt via one or more follower arms which are arranged to urge the latch bolt into the casing when the follower is rotated. Rotation of the follower is facilitated by a spindle which is inserted through the follower and which acts as a point of attachment for a handle or door knob. When the handle or door knob is rotated, the spindle and, hence, the follower rotates which causes the follower arm to rotate, thereby drawing the latch bolt into the casing.

As with all conventional locks, the latch bolt is tapered on one side and straight on the other such that contact of the strike plate with the tapered side urges the latch bolt into the casing. When seated within the socket of the strike plate, the straight edge of the latch bolt is substantially parallel to and adjacent the inner side waIl of the strike plate which therefore holds the door in the closed position. When it

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is desired to open the door, the door handle is turned which draws the latch bolt into the casing so that the latch bolt is clear of the strike plate thereby releasing the door.

When the latch bolt is in its rest position, i.e. it extends out from the casing, and where a door handle is provided, it is arranged to extend substantially S horizontally. However, a problem with conventional mortise latches is that the handle tends to sag and this is especially the case when heavy premium door handles, which are increasingly in use, arc attached to the spindle. Sagging door handles are considered unsightly and so are undesirable. In addition, sagging of the handle can cause the latch to be urged into the casing thereby compromising the latch's ability to retain the door in a closed position. One proposed solution to this problem is disclosed in 0B2358668 which describes an arrangement in which a spring acting on the follower of the look can be changed to a stronger or weaker spring depending upon the requiremen1sof the lock. In this arrangement, the spring can be accessed from the backside of the lock without having to open the casing. However, such an IS arrangement requires that the lock be removed from the door to gain access to the spring and is overly complicated with too many individual components that are easily misplaced and/or broken.

An object of the present invention is to provide an improved mortise latch that overcomes the above mentioned problems.

According to an aspect of the present invention, there is provided a mortise latch comprising a casing, a bolt movable between an extended position in which the bolt projects from the casing and a retracted position in which the bolt is at least partially retracted into the casing, and a follower which is operatively connected to the bolt and is operable to move the bolt from the extended position to the retracted position, the Jock further comprising at least one means arranged to apply a force to the follower to resist movement of the follower in a direction which moves the bolt from. the extended position to the retracted position, an adjustment means operable to vary the force applied by the means arranged to apply a force characterised by the adjustment means being operable to vary the force in discrete amounts.

Advantageously, a mortise latch according to the present invention permits the force required to be applied to the follower to move the bolt from the extended position to the retracted position to be changed quickly without having to change components. A mortise latch according to the present invention therefore provides a convenient way to quickly tailor the characteristics of the mortise latch as required.

The means for applying a force may comprise two settings, one of which corresponds to the absence of the application of a force to the follower.

The means for applying a force may comprise a resilient means. The resilient IS means may be a spring. Movement of the follower in a direction which moves the bolt from the extended position to the retracted position may compress or stretch the resilient means.

There may be two resilient means. Movement of the follower in a direction which moves the bolt from the extended position to the retracted position may compress both resilient means, stretch both resilient means or compress one resilient means and stretch the other resilient means.

The adjustment means may be operable to select whether one or both resilient means are compressed or stretched by movement of the follower. The adjustment means may be accessible from the casing exterior.

One of the resilient means may be arranged to urge the bolt toward the extended position. The adjustment means may be operable to select whether or not the other resilient means is compressed or stretched by movement of the follower. The adjustment means may be operable to control whether or not one of the resilient means acts on the follower.

The mortise latch may further comprise a member which is movable by the follower and one of the resilient means may be connectable to the member so that movement of the member caused by movement of the follower causes the resilient means to be compressed or stretched. The adjustment means may be operable to connect the resilient means to or disconnect the resilient means from the member. The adjustment means may comprise a lug which is movable relative to the member and whose movement relative to the member connects the resilient means to or disconnects the resilient means from the member.

The member may define a space and have an aperture which is shaped and configured to permit the lug to pass into and out from the space when the lug is in a particular orientation and to prevent the lug from passing out from the member when in a different orientation and the resilient means may be connected to the member when the lug is prevented from passing out from the member.

The lug may be mounted on a shaft that extends into the member and which may be rotated relative to the member and moved linearly with respect to the member.

Rotation or linear movement of the shaft relative to the member may cause the lug to move relative to the member. The shaft may comprise a head which has a groove that cooperates with the end of a tool, such as a screwdriver, to permit the tool to move the shaft relative to the member. The shaft head may be accessible from. the casing exterior. The resilient means may urge the lug against a part of the shaft.

Another resilient means may be mounted on the shaft and arranged between the member and the casing to resist linear movement of the shaft relative to the casing.

In order that the invention may be more clearly understood an embodiment thereof will now be described, by way of example, with reference to the accompanying drawings of which: Fig. 1. is perspective view of the internal structure of a mortise latch according to the invention; Fig. 2 is a perspective view of the casing of the mortise latch shown in Fig. 1; Fig. 3 is a perspective view of a cover for the mortise latch shown in Fig. 1; Fig. 4 is a perspective view of a movable block housing of the mortise latch shown in Fig. 1; Fig. 5 is an enlarged perspective view of a shaft in the mortise latch shown inFig.l; Fig. 6 is an enlarged perspective view of a lug which may be mounted on the shaft shown Fig. 5; Fig. 7 is a perspective view of a part of the deadbolt mechanism mounted in the mortise latch shown in Fig. 1; Fig. 8 is an enlarged perspective view of a slide plate mounted in the mortise latch shown in Fig. 1; and Fig. 9 is an enlarged perspective view of a conveying member mounted in the mortise latch shown in Fig. I; Referring to the drawings, there is shown a mortise latch I generally made from stainless steel comprising a casing 3 having a front plate 5, the casing 3 being arranged to be closed by a cover plate 7. The mortise latch 1 further comprises a latch bolt assembly 9, a follower 11 and a deadbolt assembly 13, each of which is mounted in the casing 3.

The latch bolt assembly 9 comprises a latch bolt head 15 which is mounted on a spindle 17. The latch bolt head 15 is urged out from the casing 3 through an aperture 19 by a coil spring 21 which is mounted on the spindle 17 between the latch bolt head and a spindle guide 23. The spindle guide 23 is mounted to the casing 3 and serves not only to provide an abutment against which the spring 21 may be compressed but also to help guide the spindle 17 when undergoing linear movement. A substantially square spindle plate 25 is slidably mounted. to the spindle 17 on the opposite side of the spindle guide 23 to that of the latch bolt spring 21.

The latch bolt assembly 9 further comprises a stainless steel block member or housing 27 which is slidably mounted in the casing 3 for linear movement within the casing and substantially parallel to the casing 3 and which is guided by a block guide 29. The block housing 27 has a protrusion 31 which extends out from the housing 27 beyond the spindle 21 and which has a semi-circular groove 18 in which the spindle 17 is seated, thereby acting as a further guide for the spindle 17. The protrusion 31 is positioned between the spindle plate 25 and the spindle guide 23 and abuts the spindle plate 25 such that the latch bolt head ISis prevented from being urged too far from the casing 3 by the latch bolt spring 21. The protrusion 31 additionally serves to draw the latch bolt head 15 into the casing 3 when the housing 27 is moved linearly away from the front plate 5.

A cylindrical recess 30 is formed in the back of the housing 27 which serves as a receptacle for the end of a block spring 200. The block spring 200 bears upon the housing 27 and the cover 7 when attached. to the casing 3 and serves to urge the housing 27 toward the front plate 5, thereby resisting movement of the housing 27, and hence the latch bolt head 15, away from the front plate 5 by the follower 11.

A shaft 33 extends into the casing 3 from the front plate 5 and also extends into the block housing 27 via an aperture 35. A second spring 37, which serves to alter the resistance to movement of the housing 27, is slidably mounted on the shaft 33 and contained within the housing 27. One end of the spring 37 is attached to the block guide 29 and the other end abuts a lug 39 which is also slidably mounted on the shaft 33. The shaft 33 has a shaft head which is exposed via a circular aperture in the front plate 5 and which has a groove which is adapted to receive a flat head screwdriver.

The shaft 33 is thicker toward the shaft head end and thinner toward its free end, the thinner section being substantially square in cross section. The lug 39 is mounted on the thinner section of the shaft 33 and, since the thinner section has a substantially square cross section, cannot rotate relative to the shaft 33. The lug 39 is shaped and configured such that it abuts the end of the thicker section of the shaft 33 and provides a surface against which the spring 37 is pressed, thereby urging the lug 39 against the end of the thicker section.

The lug 39 is substantially circular in cross section and has a diameter similar to that of the thicker section of the shaft 33. The lug 39 has a substantially square cut out at its centre which is similar in cross section to the thinner section of the shaft to enable the lug 39 to be slidably mounted on the shaft 33. The lug 39 additionally has two projections 41, 42 on opposite sides respectively of its circular core section. The aperture 35 in the block housing 27 is shaped and configured to permit the lug 39, the thicker section of the shaft 33 and the housing spring 37 to pass therethrough.

However, the dimensions of part of the aperture 35 are less than the distance between the two extremities of the lug projections 41, 42 such that, in a particular orientation, the lug 39 is prevented from passing into or out from the block housing 27 interior.

A block stop 44 extends from the casing 3 and provides an abutment for the block housing 27 which is urged against the block stop 44 by the latch bolt spring 21 via the spindle plate 25 and housing protrusion 31. A third spring 46 is slidably mounted on the thicker section of the shaft 33 between the block stop 44 and a protruding lip 48 of the shaft head. The shaft spring 46 urges the shaft 33 outwardly from the casing 3 but the shaft 33 is prevented from being completely ejected from the casing 3 by the shaft head lip 48 which abuts the front plate 5. The point at which the shaft head lip abuts the front plate 5 is chosen such that the shaft head surface is substantially flush with the outer facing side of the front plate 5. The shaft head lip 48 has a plurality of male projections 49 which cooperate with correspondingly shaped female receptacles on the front plate 5 so as to prevent rotation of the shaft 33 when in its rest position. The shaft 33 may be moved substantially perpendicularly to the front plate S by applying pressure to the shaft head in a direction toward the casing but the shaft 33 is prevented from being inserted completely into the casing interior by compression of the shaft spring 46 which abuts the block stop 44.

An aperture 70 is fonTled at the backside of the cover 7 of the mortise latch I and is positioned such that it aligns substantially with the central longitudinal axis of the spindle 17 when the cover 7 is attached to the casing 3. The aperture 70 is shaped and configured to permit a tool, such as a screwdriver, to: be inserted into the casing 3 and cover 7 interior. The free end of the spindle 17 which faces the aperture 70 has a groove formation which may be configured to receive a flat head screwdriver or a Phillips head screwdriver as desired. An appropriate screwdriver may therefore be inserted into the casing 3, seated in the groove of the spindle end and used to urge the latch bolt head 15 further out from the casing 3. When the latch bolt head 15 is clear of the bolt aperture 19, the screwdriver can be used to rotate the spindle 17, and thus the latch bolt head 15 which is fixed to the spindle 17, thereby changing the direction in which the tapered side of the latch bolt head 15 faces.

A second spindle plate 72 is fixed to the spindle 17 by a pin toward the free end of the spindle 17 and is spaced apart from the first spindle plate 25. A coil spring 74 is mounted on the spindle 17 between the two spindle plates 25, 72. Since the first spindle plate 25 is slidably mounted on the spindle 17 and can therefore move relative to the spindle 17, when the spindle 17 is urged further out from the casing 3 with a screwdriver, the first spindle plate 25, which abuts the block housing protrusion 31, S cannot move any further and so the second spindle plate 72 is moved closer to the first spindle plate 25, thereby causing the spring 74 to compress. Thus, when the latch bolt head rotation operation is complete and the screwdriver and, hence, pressure is released from the spindle 17, the spring 74 urges the spindle 17 and latch bolt head 15 back into the casing 3 thereby returning it to the rest position. This rotation mechanism permits the mortise latch 1 to be used on a door that opens in either direction.

The follower 11 comprises a substantially cylindrical follower core 12 having a square-sectioned bore for receipt of a door handle spindle (not shown) which is pivotally mounted in the casing 3. Two follower arms 50, 52 are pivotally mounted on opposite sides respectively of the follower core 12 and extend toward the spindle 17 and block housing 27 in a direction substantially parallel to the casing 3. The end of one of the follower arms 50 extends between the spindle guide 23 and spindle plate 25 and its width is substantially equal to the distance between the spindle guide 23 and spindle plate 25 when in the rest position i.e. the latch bolt extends from the casing 3.

The end of the other follower arm 52 extends between the spindle guide 23 and the housing protrusion 31 and its width is substantially equal to the distance between the guide 23 and the protrusion 31 when in the rest position. Thus, movement of the first follower arm 50 independently of the second follower aim 52 gives rise to linear movement of the spindle 17 and latch bolt head 15 whereas movement of the second follower ann 52 independently of the first gives rise to linear movement of the block housing 27 and, consequently, the spindle 17 and latch bolt head 15 via the protrusion 31 and spindle plate 25. The end of the second follower arm 52 is tapered on its side which bears upon the housing protrusion 31 to ensure that a larger surface area is always in contact with the protrusion 31 than, for example, a non-tapered side. This gives rise to a smoother more even movement of the block housing 27 and, hence, spindle 17 and latch bolt head 15 when the follower core 12 is rotated.

The first follower arm 50 has a tongue 60 which extends out from the core section of the ann 50 toward the bottom of the lock 1. The tongue 60 has a circular aperture 62 which receives the hooked end 63 of a link member 64 which links the follower arm 50 to the bolt 100 of the deadbolt mechanism 13. The other end of the link member 64 is pivotally attached to the bolt 100 via a screw 101 which extends through a cut-out in the link member and into the bolt. The link member 64 is therefore able to slide along the screw via the cut-out.

The bolt 100 comprises a substantially rectangular piece of stainless steel which extends across the width of the casing 3. An end 102 of the bolt 100, which may extend from the casing 3 via an appropriately sized aperture in the front plate 5, is thicker than the remaining portion 104 of the bolt 100 due to its requirement to resist forced access to a restricted area. The internal portion 104 of the bolt 100 that remains in the casing 3 has a longitudinal cut-out 106 which extends along its length.

The casing has a pillar 108 which extends through the cut-out 106 and serves to guide the bolt 100 along a substantially linear path when it is urged from the casing 3. The length of the cut-out 106 and the point at which the pillar 108 abuts either end of the cut-out 106 corresponds to the two extreme positions of the bolt 100 in the withdrawn and extended positions respectively.

The upper side of the inner bolt 104 has two tapered projections 110, 1 12 that extend upwardly toward the follower core 12. As with the cut-out 106, the distance between the two projections 1 10, 112 is roughly equal to the distance between the two extreme positions of the bolt 100. Both projections 110, 112 cooperate with a slide plate 114 which is generally L-shaped and whose longer face is substantially parallel to the casing 3. The shorter face 115 is substantially perpendicular to the casing 3 and has a square hole 116 into which either projection 110, 112 may be inserted depending upon the position of the bolt 100. One of the two projections 110, 112 is seated within the square hole 116 when the bolt 100 is in one of its two extreme positions.

The slide plate 114 is slidably mounted in the casing 3 and can move in a direction substantially parallel to the front plate 5. The slide plate 114 has a pair of cut-outs 117, 118 through which extend the first pillar 108 and a second pillar 120 which both serve to restrict movement of the slide plate 114 to linear movement and which also serve to limit the extreme positions between which the slide plate 114 can move. The pillars 108, 120 also serve to prevent movement of the bolt 100 into or out from the casing 3 when the slide plate 114 is engaged with one of the bolt projections 110, 112. A further projection 210 which extends from the casing 3 is positioned adjacent to the slide bolt 114 and provides an abutment for the slide bolt 114. This projection 210 serves as an additional stop which prevents the bolt 100 from moving into the casing when in an extended position whereby the bolt projection 110 is engaged with the square hole 116 of the slide plate 114.

The slide plate 114 is urged in a downward direction toward the bottom of the lock I by a torsion spring 122 which is wrapped around a third pillar 124. One end of the torsion spring bears against the inner side of the front plate 5 and the other end bears against the top of the slide plate 114. Thus, when the slide plate 114 is moved in an upward direction toward the follower core 11, it is urged back down to its rest position by the torsion spring 122 when pressure is released from the plate 114. The bottom edge of the slide plate 114 is tapered to form a V-shape with the apex of the V at the centre point of the bottom edge.

An inverted U-shaped cut-out 130 is formed in the bottom edge of the bolt 100 and is positioned such that the tapered bottom edge of th.e slide plate 1t4is-adjacent the cut-out 130 throughout the movement of the bolt 100 between its two extreme positions. The tapered bottom edge and the U-shaped cut-out 130 cooperate with a conveying member 132 which is slidably mounted in the casing 3. The conveying member 132 comprises a U-shaped block 134 from which extends a conveying arm 136. Two guide pins 138, 140 are positioned on the end of the U-shaped block 134 and a single guide pin 142 is positioned on the end of the conveying arm 136. The guide pins 138, 140, 142 extend into aligned curved slots 144, 146 in the casing 3 and cover 7 which forces the conveying member 132 along an arcuate path. Two tapered edges 157, 158 extend from the U-shaped cut-out 130 to accommodate the arcuate path of the conveying member 132 when it moves from one side of the curved slots to the other.

The conveying member 132 is positioned such that the conveying arm 136 bears upon the tapered edge of the slide plate 114 and the side walls of the U-shaped cut-out 130 of the bolt 100 when the conveying member 132 is moved from one side of the curved slots to the other. Due to the tapered edge of the slide plate 114, movement of the conveying member 132 along its arcuate path urges the slide plate 114 in an upward direction against the action of the torsion spring 122 which releases one of the projections 110, 112 from the square hole 116 thereby freeing the bolt 100.

The conveying arm 136 also bears upon one of the side walls of the U-shaped cut-out 1.30 depending upon the direction of travel which causes the bolt 100, now freed, to move in the direction of travel of the conveying member 132. Therefore, movement of the conveying member 132 in either direction causes the bolt 100 to move from one extreme position toward the other.

As the conveying member 132 is moved from one side of the curved slots to the other, the tapered edge of the slide plate 114 permits the torsion spring 122 to urge IS the slide plate 114 back to its rest position where its square hole 116 re-engages with a correspondingly positioned projection 110 or 112. A tapered side of the projection 110, 112 bears upon the edge of the square hole 116 causing the bolt 100 to move beyond the interface between the conveying member 132 and. the U-shaped cut-out 130. The interaction between the tapered projections 110, 112 and the square hole 116 of the slide plate 114 therefore gives the bolt 100 an added push beyond that of the conveying member 132, Two aligned keyhole shaped apertures are formed in the casing 3 and cover 7 respectively and provide an aperture through which a euro cylinder lock may be inserted. The actuating arm of the euro cylinder lock (not shown) is positioned such that it is seated within the recess of the U-shaped block 138 of the conveying member 132. Movement of the actuating arm by a key causes the conveying member 132 to move along its areuate path, guided by the guide pins 138, 140, 142. This in turn activates the sliding plate 114 which is urged upwardly thereby releasing the bolt projection 110 or 112 (depending upon the bolt position) from the square hole 116 of the slide plate 114. Further movement of the actuating arm causes the conveying member 132 to bear upon the sidewall of the U-shaped cut-out 130 in the bolt 100 which causes the bolt to move with the conveying member 132 from one extreme position to the other depending upon the direction of motion of the actuating arm. The deadbolt can therefore be extended or retracted by turning the key of the euro cylinder lock, as appropriate.

In use, the mortise latch I is fitted* * to a pocket in the front edge of a doorand a square spindle is inserted through the bore of the follower 11 via appropriately sized and positioned holes in the sides of the door. A door handle is then attached to the spindle either side of the door. Turning the handle causes the follower 11 to rotate which, in turn, causes the follower arms 50, 52 to rotate. The follower arms 50, 52 simultaneously bear upon the spindle plate 25 and the housing protrusion 31 respectively which causes the latch bolt head 15 and spindle 17 to be drawn into the casing through rotation of the follower arms 50, 52 against the action of the spindle spring 21 and the block spring 200. Releasing the handle removes the force acting against the spindle spring 21 and block spring 200 which are then free to act on the housing 27 and latch bolt head 15 to urge the bolt head 15 back to the extended position.

When it is desired to change the force that must be overcome by the follower ii to move the latch bolt head 15 to the retracted position, the shaft 33 is pushed into the casing 3 by an appropriate screwdriver. This pushing action causes the spring 37 in the housing 27 to be compressed by the lug 39 which abuts the thicker part of the shaft 33. In thi.s orientation, the lug 39 is able to pass through the housing aperture 35 and into the housing 27. Rotation of the shaft 33 in a clockwise direction causes the lug 39 to rotate until the distance between the two lug projections 41, 42 is greater than the diameter of the housing aperture 35. Thus, when the shaft 33 is released, the shaft spring 46 urges the shaft 33 back to its rest position (flush with the front plate 5) leaving behind the lug 39 which is urged against the inner side wall of the housing 27 by the housing spring 37. Since the lug 39 is retained in place it acts as an abutment for the housing spring 37 which would normally pass through the housing aperture 35 when the lug 39 is not engaged with the housing 27, so that, when the follower 11 is rotated and the follower arms 50, 52 act upon the housing 27, the housing spring 37 is compressed. Therefore, engagement of the housing spring 37 with the housing 27 serves to resist movement of the latch bolt head 15 by the follower 11 which increases the force that must be overcome by the follower 11 to move the latch bolt head 15 to the retracted position. The mortise lock I is therefore switchable between two discrete settings, one of which corresponds to the absence of the application of a force by the housing spring 37 on the follower 11.

When it is desired to change back to the other resistance setting, the shaft 33 is once again pushed into the casing 3 and rotated in a counter clockwise direction to change the orientation of the lug 39 so that it may pass back through the housing aperture 35 thereby disengaging the housing spring 37 from the housing 27. In this position, the movement of the housing 27 is no longer resisted by the housing spring 37 which may pass through the housing aperture 35.

It is of course to be understood that the above embodiment has been described by way of example only and that many variations are possible without departing from the scope of the invention.

Claims

CLAIMS1. A mortise latch comprising a casing, a bolt movable between an extended position in which the bolt projects from the casing and a retracted position in which the bolt is at least partially retracted into the casing, and a follower which is operatively connected to the bolt and is operable to move the bolt from the extended position to the refracted position, the latch further comprising at least one means arranged to apply a force to the follower to resist movement of the follower in a direction which moves the bolt from the extended position to the refracted position characterised by an adjustment means: operable to vary the force applied by the force applying means.
2. A mortise latch as claimed in claim 1, wherein the means arranged to apply a force comprises two settings, one of which corresponds to the absence of the application of a force to the follower. C)

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3. A mortise latch as claimed in claim 1 or claim 2, wherein the means for applying a force comprises a resilient means.
4. A mortise latch as claimed in claim 3, wherein the resilient means is a spring.
5. A mortise latch as claimed in claim 3 or claim 4, wherein movement of the follower in a direction which moves the bolt from the extended position to the refracted position compresses or stretches the resilient means.
6. A mortise latch as claimed in any of claims 3 to 5, wherein there are two resilient means.
7. A mortise latch as claimed in claim 6, wherein movement of the follower in a direction which moves the bolt from the extended position to the retracted position compresses both resilient means, stretches both resilient means or compresses one resilient means and stretches the other resilient means.
8. A mortise latch as claimed in claim 7, wherein the adjustment means is operable to select whether one or both resilient means are compressed or stretched by movement of the follower.
9. A mortise latch as claimed in any preceding claim, wherein the adjustment means is accessible from the casing exterior.
10. A mortise latch as claimed in any of claims 6 to 9, wherein one of the resilient means is arranged to urge the bolt toward the extended position.
11. A mortise latch as claimed in claim 10, wherein the adjustment means is operable to r select whether or not the other resilient means is compressed or stretched by movement of the C') follower. o

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12. A mortise latch as claimed in any of claims 6 to It, wherein the adjustment means is (4 operable to control whether or not one of the resilient means acts on the follower.
13. A mortise latch as claimed in any of claims 6 to 12, wherein the mortise latch fhrther comprises a member which is movable by the follower, and wherein one of the resilient means is connectable to the member so that movement of the member caused by movement of the follower causes the resilient means to be compressed or stretched.
14, A mortise latch as claimed in claim 13, wherein the adjustment means is operable to connect the resilient means to or disconnect the resilient means from the member.
15. A mortise latch as claimed in claim 14, wherein the adjustment means comprises a lug which is movable relative to the member and whose movement relative to the member connects the resilient means to or disconnects the resilient means from the member.
16. A mortise latch as claimed in claim 15, wherein the member defines a space and has s an aperture which is shaped and configured to permit the lug to pass into and out from the space when the lug is in a particular orientation and to prevent the lug from passing out from the member when in a different orientation and the resilient means is connected to the member when the lug is prevented from passing out from the member,
17. A mortise latch as claimed in claim 15 or claim 16, wherein the adjustment means further comprises a shaft, the lug being mounted on the shaft which extends into the member and which may be rotated relative to the member and moved linearly with respect to the member, rotation or linear movement of the shaft relative to the member causing the lug to a) o move relative to the member.
18. A mortise latch as claimed in claim 17, wherein the resilient means urges the lug against a part of the shaft.
19. A mortise latch as claimed in claim 17 or claim 18, wherein another resilient means is mounted on the shaft and arranged between the member and the casing to resist linear movement of the shaft relative to the casing.