' _A lthough the latch is applicable to extending tripo ladders, ^• automatic firearms and the like, the latch will be 5 described .with particular reference to applications where t latch must exert a clamping action which can be applied and released frequently. One example of such an application is surveyor's rod which consists of .two or more mutually connec sliding calibrated sections which when extended for taking 10 readings rely upon a clamping latch to lock the overlapping lengths of the rod sections in a reproducible position. .Ove the lifetime of the rod thousands of operations of the latch occur. No slip is permissable between the sections when latched -closed and rod is in extended position. 15 Broadly the invention provides a latch of the over centre type adapted to mutually lock a first and second inte engaged, axially slidable members in an axially overlapping disposition by utilising a first pivot axis on the first mem and a second parallel pivot axis on the second member charac ^' 20 terised in that the latch straddles the two members, is con¬ nected to the first member by a pair of pin and slot connec¬ tions at the first pivot axis, is adapted to engage fulcrum means at a second axis in the second member and extends from the zone of the two axes like a lever whereby the latch is 25 til able between a release position wherein the members are slidable and a locked position where the distance between the two axes changes and in doing so locks the members together and a latch arrester holds the latch closed.

More particularly the invention provides a latch of

30

C...?I

the over centre type adapted to mutually lock a first and second interengaged, axially slidable members in an axially overlapping disposition comprising:

(a) a pair of transversely disposed latch-supporting pivots extending from the first member along a first common transverse axis;

(b) a pair of latch fulcrum pivots extending from the second member along a second common transverse axis which is parallel to the first transverse axis : (c) a latch arrester extending from the first member which constitutes with the other two pairs of pivots a trian¬ gular disposition of parts; (d) a latch body having a face and a pair of sides which extend to or past the latch fulcrum pivots; each side having recess means to engage the associated fulcrum - - pivot and an elongated slot to engage the associated lat support pivot and at least one of the sides having an edge protrusion, which protrusion must be snapped past the latch arrester to complete the latch action, the arrangement being that for adjustment the first member together with the captive latch body is free to slide relative to the second member, but when the members reac a desired overlap position, the first recess means in ea latch side engages the associated fulcrum pivot and the body becomes tiltable about the latter, like ends of the slots engage the latch-supporting pivots drawing the fir member in a locking direction, the protrusion is snapped past the latch arrester which urges the first member together with the latch-support pivots further in the

locking direction, such further movement being permitt ' by travel of the pivots in the slots and causing an ov centre clamping action, such action being irreversible except by snapping the latch body past the protrusion. Each pair of pivots may be constituted by a single pin the ends of which project from the sides of the rod so to provide a pair of supports on a common axis. Thus the p may be conveniently pressed in the holes drilled in the members. Conversely the latch has projections which engage recesses or dimples in the members. Such connections are cheap and convenient for application to a surveyor's rod wh specific latching sites are required but when any overlap position.is required instead, the second member has a longi tudinal groove in each sidewall thereof and the latch has a pair of projections which slide in the grooves.

One embodiment of the invention is now described w reference to the single figure of the accompanying drawing which is a perspective view of a triple member surveyor's ^' r with two latches, one shown in the open position, the other being shown in the closed position.

The first member 2 is a pvc box extrusion with a p of top guides 4 and a pair of bottom guides 6. This constr tion is common to the second and third members 8, 10. The members interengage to form an extendible surveying rod wit a three-part tape scale (not shown) located between guides The lowermost member 10 has its base end protected by a met channel plate 12 secured to the sides of the member by a pi 14 and stabilised by pressed out lugs which engage internal in the member 10.

P

The latch consists of a sheet metal latch body with a face 16 and a pair of parallel sides 18. Each side has an elongated slot 20 which is parallel to the face 16 and which receives the projecting ends of a transverse latch body supporting pin 22. The pin is a press fit through a hole in the first member. The sides each have a hooked recess 24 for the purpose of engaging the projecting ends of a fulcrum pin 26 which is a press fit in the second member.

The sides each have an angular recess 28 which engag the projecting ends of a latch arresting pin 30 which is retained in the first member. The recess 28 meets the inclin loading edge 32 at an obtuse peak 34 providing a protrusion t be snapped past the arresting pin. The snap distance can be judged by the projection of corner 34 above the arc R-R locus of radius pin 22 and near face of pin 30.

The latch works as follows:

The latch body to the top is shown in the release position with the recesses 24 in the body, desengaged from the fulcrum pin ends 26, the latch face lying clear of the guides 4 and the support pin ends 22 lying at the forward ends of slots 20. As the latch closes the slots draw top member 2 rearwards. The inclined edges 32 engage arrester pi 30 which drives the member 2 rearwards to complete the lockin action. The completion of the locking action is made possibl by the movement of the pins 22 towards the opposite ends of t slots 20. Peaks 34 snap over the arrester pins. The latch body lies flat parallel to guides 4 with the rear ends of the slots 20 against pin 22. The locking action is exerted between guides 6 of member 2 and guides 4 of member 8. When the latch is in the locking position the distance between pin

ends 26 and pin ends 22 increases thus tending to separate members 2 and 8. The separating force locks the guides and holds the members in the predetermined overlap position. In order to open the latch the user grips the latch body and tilts it about pin ends 26.

When the latches are engaged and the rod is in use the vertical compression axial load on the lower latch, for example, passes from the upper member 2 through pin 22 to the latch body, thence through 26 to the adjacent member 8. Self closing moment induced from pins 26 and 22 is arrested by pin 30 against recess 28.

Axial forces in the opposite direction (tension) ar defeated from applying a-self opening turning moment between pins 22 and 26 by provision of a load relieving slot 20 abou pin 22. Additionally the geometry of angular recess 28 maintains pin 30 captive, as is pin 26 in the latch body. T resultant force "F" passes between pins 26 and 30 (in adjace members) and there is no turning moment on the latch body- tending to cause unassisted release. The small clearance between pin ends 30 and near face of recess 28 (relative to pin.22) is created by the captive angle of the recess 28 relative to the tangent to radius about pin 26, and the provision of a snap close action over corner 34 with respect of radius about pin 22. This clearance may be kept to a minimum by the offset of pins 22 and 30 from the longitudinal axes of the rod.

We have found the advantages of the above embodiment to be as follows:

1. A standard three part rod may be used as a one, two or three part item or further extensions may be adde

because the parts are easily assembled and disassembled; ^• 2. The locking action is fast and reliable and operable by a user wearing gloves; 3. The latch contains no threaded parts, springs or sliding register pins which fare badly on site; 4.. Sliding members cannot be "false" latched out of position. Any "part latching" is readily apparent, obviating possibility of a false height reading. 5. The latch closes to the rod enabling the rod to be placed flush against a wall during measurement. In another embodiment the latch clamps by pulling th extrusions together in contrast to the embodiment above where latch clamps the extrusions by pushing them apart. In this specification, the term "pin" means any

-projection which forms an axis about which the latch works. Normally a single pin is utilised which projects beyond the width of the extrusions and the ends of the pin are the surfaces which are engaged by the latch.