This invention relates to ventilator shutters for roof units and the like.

A known shutter for this purpose comprises a pair of semi-circular shutter blades hinged together on a diameter of the circle and movable from a gravity located e.g. co-planar condition to a generally Wee position by the fan generated pressure, so as to allow air flow past the blades.

The problem is that when in the closed position the shutter blades may be lifted by pressure fluctuations: for example by ωind, and in ωindy weather they are noisy. They rattle. The object of the invention is to solve this problem.

According to the invention a gravity shutter comprises at least one blade movable in opposite directions betuieen open and closed positions, a helically directed guide, and means provided on said blade cooperating uiith the guide so as to allow a bodily lifting movement of at least a part of the blade accompanied by an angular movement of the blade due to the helix, and catch means effective between said blade and an associated housing, said catch being engaged and disengaged by the helical movement.

One embodiment of the invention is now more particularly described ωith reference to the accompanying drawings ωherein:-

Figure 1 is a sectional elevation of a roof unit;

Figure 2 is a section plan view of the same taken on the line 2-2 of Figure 1 :

Figures 3 - 9 are fragmentary vieωs of components and sub-assemblies.

Turning first to Figure 1 , the roof unit comprises a housing 10 mounting a fan having blades 12 to assist ventilation flow in the direction of the arrow A Figure

1. A domed cover 14 is supported on struts 16 and has a skirt 18 surrounding the terminal portion 20 of the duct, so that ventilation flow may egress in the direction of the arrows B.

The roof unit is sealed by a gravity shutter 22.

Turning now to Figure 2, the gravity shutter in this embodiment is seen to be generally circular and comprises two portions 24 26 hinged on a diametrically extending pin 28 which projects beyond the diameter of the shutter and is received in guide tracks 30 as explained later in this specification. These guide tracks are fastened to two of the support struts 16 at opposite ends of the hinge diameter.

Two further struts 16 lie at opposite ends of another diameter, perpendicular to the first mentioned one and these used to mount catches, generally indicated by the reference numeral 32, as also described later herein.

The individual shutter blades may be stiffened by ribs 34 and provided with a peripheral shallow skirt 35 (Figure 1) for stiffness and for improved sealing.

One guide track 30 is shown in enlarged scale in Figure 3 and in side elevation in Figure 4. Conveniently it may be made as a plastics moulding or a diecasting. In this embodiment the track comprises a short vertically extending portion 36 opening to a longer inclined track portion 38. The track is of suitable width to receive the end portion of the hinge rod 28. Figure 3 shows the same hinge rod in two extreme positions of movement at 28A and 28B.

The catch assembly 32 is more particularly shown in Figure 5 (side view), Figure 6 (elevation) and Figure 9 (plan view not showing the pawl or its mounting) all on an enlarged scale.

The catch assembly comprises a pawl 40 hinged on pin

42 carried by brackets 44 fixed to the shutter blade 22 near the periphery. The paωl has a stop face 46 to cooperate with abutment 48 carried by the bracket.

The catch assembly also comprises a rack 50 fixed to the strut 16. The rack has a series of parallel downwardly inclined faces 52 and a series of generally horizontal faces 54. The rack teeth are formed projecting from the face of a plate 56 which is used to attach the catch component to the strut 15, and the plate is provided ωith a rib 58 parallel to the rack leaving a channel 60 between the rack and the rib as seen in Figure 9. The channel 60 is wider than the paωl 40, and Figure 6 illustrates (in broken line) the paωl 40 in two different positions as will be explained hereinafter.

Figure 1 shows the shutter in the closed sealed position on the ventilation duct. It will be seen that the pawls engage with the catches and effectively this holds the shutter flat against rattle or movement for example in wind.

When the fan is brought into operation, air pressure builds up on the underside of the shutter, but the catches initially prevent the outer edges of the shutter from moving, but the guide 30 allows the centre to move. So the initial movement is generally as shown in Figure 7 which takes the hinge pin 28 from one end of the track to the other and lifts the centre of the shutter blade so as to take the blades to an inverted shallow Vee position as shown in that figure. However, the inclination of the track portions 38, which essentially is in opposite directions at the two ends of the hinge pin, takes the hinge pin angularly about a centre point. The two track portions can be considered to be helical. So this twists the shutter as a whole about the centre point, and the effect of this will be seen from Figure 2 where the angular direction of movement to the Figure 7 position is shown by the arrow D. The pawls are slid laterally from the engaged rack teeth and enter the channels 60. The air pressure can now swing the shutter blades upwardly, i.e. in the direction of the arrow E Figure 8 so that the

blaαes fly to a Vee position anc tne air can De exnausted.

The opening of the snutter blaαes reduces the pressure on them anc this may allow the centre portion to return to its starting position, in ωhicn case the reverse helical movement aligns the catch parts ready for re-engagement when the fan is switched off.

However, if the centre portion has not completely returned when the fan is switched off, and if the pawls re-encounter the catches whilst the hinge pin is in the Figure 3 position as indicated by the reference 2BB, then the pawls will be engaged with the channels 60 and slice along tπose channels, and as the ninge pin 2b tnen returns to the Figure 3 anc reference 2BA position, tne shutter blades will turn angularly and clockwise as shown in Figure 2 so that the pawls will re-enter the racks by a lateral movement.

Observation of embodiments constructed according to this description and drawings shows that a combination of the two different re-engagement steps occurs in most cases, that is to say part of the helical movement has been completed before the pawls re-engage.

The stop faces 46. 46 prevent the pawls moving to positions where gravity would not be effective to return them to the engaging position.